Uterine leiomyomas, or fibroids, are the most
common tumors of women in the United
States, probably occurring in the majority of
women by the time they reach menopause and
becoming clinically significant in about one-
third of these women. Despite their prevalence,
little attention has been directed toward the
causation and pathogenesis of fibroids until
recent years because of the rarity of their malig-
nant transformation. Regardless of their gener-
ally benign neoplastic character, uterine fibroids
are responsible for significant morbidity in a
large segment of the female population. The
clinical effects of these tumors are related to
their local mass effect, resulting in pressure
upon adjacent organs, excessive uterine bleed-
ing, or problems related to pregnancy, includ-
ing infertility and repetitive pregnancy loss
(Haney 2000). As a consequence of these local
pressure effects and bleeding, uterine fibroids
rank as the major reason for hysterectomy in
the United States, accounting for approxi-
mately one-third of all hysterectomies (Wilcox
et al. 1994), or about 200,000 hysterectomies
per year (Gambone et al. 1990).
Although the cause or causes of fibroids are
unknown, the scientific literature now contains
a sizeable body of information pertaining to
the epidemiology, genetics, hormonal aspects,
and molecular biology of these tumors. In this
review we have analyzed and summarized the
data available, with the goal of achieving a bet-
ter understanding of the factors related to the
etiology and pathogenesis of fibroids.
In considering the development of uterine
leiomyomas, it is convenient to subdivide the
factors that may be related to tumorigenesis
into four categories: predisposing or risk fac-
tors, initiators, promoters, and effectors. Risk
factors are characteristics associated with a
condition, generally identified by epidemio-
logic studies. Knowledge of such predisposing
factors may provide clues to the etiology of
these tumors as well as to preventive measures.
The initiators of fibroids are unknown; how-
ever, a few of the theories of initiation offered
in the literature are briefly reviewed in this
article. The occurrence of genetic aberrations
in fibroid tumors is considered. Despite the
abundance of cytogenetic investigations,
uncertainty remains as to the primary or sec-
ondary nature of these genetic changes and
their impact on the initiation and/or promo-
tion of these tumors. The role of growth pro-
moters of fibroids seems to belong in large
part to the ovarian hormones estrogen and
progesterone, and the clinical and laboratory
evidence for their involvement are cited.
Finally, the developing literature pertaining to
various growth factors as the effectors of estro-
gen and progesterone-induced stimulation is
Risk Factors Associated with
Although we have considered and discussed
these risk factors, or predisposing factors, in
isolation, there is in fact often an overlap or
interaction between one or more, for exam-
ple, obesity, diet, and exercise (Table 1).
Second, we can only speculate upon the
mechanistic link between these risk factors
and fibroid tumorigenesis. Although the
impact of many of these factors has often
been attributed to their effects upon estrogen
and progesterone levels or metabolism, prov-
ing this association is difficult, and other
mechanisms may well be involved. Finally,
there are limitations to the analysis of risk
factors, as few epidemiologic studies have
been conducted, and reports can easily be
biased because of the high prevalence of
asymptomatic cases (Schwartz and Marshall
There is a suggestion of slightly increased risk
of fibroids associated with early menarche,
although the risk has often not been statisti-
cally significant (Cramer et al. 1995; Parazzini
et al. 1988; Samadi et al. 1996). Recently, a
significant inverse association between risk of
fibroids and age at menarche was reported;
that is, compared with women who were 12
years of age at menarche, those who were ≤ 10
years of age at menarche were at increased risk
[relative risk (RR) 1.24], whereas women who
were age ≥ 16 years of age at menarche were at
lower risk (RR 0.68) (Marshall et al. 1998a).
Sato et al. (2000b) found that women with
uterine leiomyomas more often exhibited an
early normal menstrual cycle pattern, and con-
cluded that early menstrual regularity may
enhance leiomyoma growth in early reproduc-
tive life. The early onset of menstrual cycles
may increase the number of cell divisions that
the myometrium undergoes during the repro-
ductive years, resulting in an increased chance
of mutation in genes controlling myometrial
proliferation (Marshall et al. 1998a).
Several studies have shown an inverse relation-
ship between parity and the risk of fibroids
(Lumbiganon et al. 1996; Parazzini et al.
1996a; Ross et al. 1986; Samadi et al. 1996). A
relative risk of fibroids among parous women
of 0.5, compared with nulliparae (Parazzini
et al. 1988), and a progressive decline in risk
relative to the number of births have been
reported (Lumbiganon et al. 1996; Marshall
et al. 1998a; Parazzini et al. 1996a; Ross et al.
1986; Sato et al. 2000a). An explanation that
has been sometimes cited in the literature
(Parazzini et al. 1996a; Ross et al. 1986) for
these findings is that pregnancy reduces the
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
Etiology and Pathogenesis of Uterine Leiomyomas: A Review
Gordon P. Flake,1 Janet Andersen,2and Darlene Dixon1
1Comparative Pathobiology Group, Laboratory of Experimental Pathology, National Institute of Environmental Health Sciences,
Research Triangle Park, North Carolina, USA; 2Department of Pathology, School of Medicine, SUNY at Stony Brook, Stony Brook,
New York, USA
Address correspondence to D. Dixon, NIEHS, PO
Box 12233, MDC2-09, Research Triangle Park, NC
27709 USA. Telephone: (919) 541-3814. Fax: (919)
541-7666. E-mail: firstname.lastname@example.org
The authors thank C. Swartz, R. Newbold, and J.
Johnson for their critical review of the manuscript
and their suggestions. We are especially indebted to
D. Baird for her review and contributions to this
The authors declare they have no conflict of interest.
Received 24 May 2002; accepted 25 October 2002.
Uterine leiomyomas, or fibroids, represent a major public health problem. It is believed that these
tumors develop in the majority of American women and become symptomatic in one-third of these
women. They are the most frequent indication for hysterectomy in the United States. Although the
initiator or initiators of fibroids are unknown, several predisposing factors have been identified,
including age (late reproductive years), African-American ethnicity, nulliparity, and obesity.
Nonrandom cytogenetic abnormalities have been found in about 40% of tumors examined.
Estrogen and progesterone are recognized as promoters of tumor growth, and the potential role of
environmental estrogens has only recently been explored. Growth factors with mitogenic activity,
such as transforming growth factor-β3,basic fibroblast growth factor, epidermal growth factor, and
insulin-like growth factor-I, are elevated in fibroids and may be the effectors of estrogen and prog-
esterone promotion. These data offer clues to the etiology and pathogenesis of this common condi-
tion, which we have analyzed and summarized in this review. Key words: estrogen, fibroids,
genetics, growth factors, progesterone, risk factors. Environ Health Perspect 111:1037–1054
(2003). doi:10.1289/ehp.5787 available via http://dx.doi.org/ [Online 13 November 2002]
time of exposure to unopposed estrogens,
whereas nulliparity or reduced fertility may be
associated with anovulatory cycles character-
ized by long-term unopposed estrogens. The
alternative possibility exists that uterine
fibroids are actually the cause of the infertility,
rather than the consequence of it; however, the
diminished relative risk of fibroids associated
with parity remains essentially the same after
exclusion of women with a history of infertility
(Marshall et al. 1998a).
An increase with age in the prevalence of
fibroids during the reproductive years has been
demonstrated by several epidemiologic studies
(Marshall et al. 1997; Ross et al. 1986; Velebil
et al. 1995; Wilcox et al. 1994). Studies that
define cases by pathologic diagnosis, thus
restricting cases to those having surgery (Ross
et al. 1986), have shown a rapid increase in
fibroid diagnoses among women in their for-
ties. Whether the risk of new fibroids actually
increases rapidly in women during their forties
is not known. The observed increase could
also result from increased growth of, or
increased symptomatology from, already exist-
ing fibroids, as well as from a greater willing-
ness of women in the later reproductive years
to have gynecologic surgery. If the likelihood
of fibroid development and growth actually
accelerates during the late reproductive years,
hormonal factors associated with peri-
menopause may be important modulators;
alternatively, the apparent increase in the late
reproductive years may simply represent the
cumulative culmination of 20–30 years of
stimulation by estrogen and progesterone.
A reduced risk of fibroids requiring surgery in
postmenopausal patients (Parazzini et al.
1988; Ross et al. 1986; Samadi et al. 1996)
could be due to tumor shrinkage in the
absence of hormonal stimulus following the
menopause. Sectioning of uteri at 2-mm
intervals revealed a similar incidence of
leiomyomas in pre- and postmenopausal
patients (74 and 84%, respectively) although
the postmenopausal leiomyomas were smaller
and fewer (Cramer and Patel 1990). The esti-
mated risk in postmenopausal patients could
be reduced by selection bias because of a ten-
dency toward a more conservative nonsurgical,
clinical approach in postmenopausal women
(Parazzini et al. 1988).
Several studies have found an association
between obesity and an increased incidence of
uterine leiomyomas. In a prospective study
from Great Britain (Ross et al. 1986), the risk
of fibroids increased approximately 21% for
each 10-kg increase in body weight; similar
results were obtained when the body mass
index (BMI) was analyzed rather than weight.
In a case–control study from Thailand
(Lumbiganon et al. 1996), a 6% increase in
risk was observed for each unit increase in
BMI. Similarly, a large prospective study of
registered nurses in the United States (Marshall
et al. 1998b) found an increased fibroid risk
with increasing adult BMI, as well as an
increased risk associated with weight gain since
age 18 years. A case–control study from Japan
(Sato et al. 1998) likewise reported that
women with occult obesity (BMI < 24.0 and
percent body fat ≥ 30%) or women with
upper-body fat distribution (> 0.80 waist-to-
hip ratio) were at significantly higher risk. In a
study from Boston, Massachusetts (Shikora
et al. 1991), 51% of the hysterectomy- or
myomectomy-confirmed patients with leio-
myomata were overweight, and 16% were
severely obese; the authors compared their
patients with a national study group of women
in the United States included in The National
Health and Nutrition Survey (Abraham and
Johnson 1980; Flegal et al. 1998; Van Itallie
1985), quoting comparison figures of 25%
overweight and 7.2% severely obese. However,
it should be noted that the latter study
(Shikora et al. 1991) had no control group of
its own, used the percent of desirable body
weight as the yardstick rather than BMI, and
included fibroid patients from a slightly later
time period when the prevalence of obesity was
increasing generally in the United States. In
contrast to these studies, there are two reports
(Parazzini et al. 1988; Samadi et al. 1996) in
which no association was found between the
incidence of leiomyomas and BMI. Disparate
reports of overweight prevalence may relate to
definitional criteria, the method of measure-
ment, and choice of comparison groups
(Troiano and Flegal 1999).
This apparent association between obesity
and an increased risk of fibroids may be related
to hormonal factors associated with obesity,
but other pathologic pathways might also be
involved. Several relevant hormonal associa-
tions with obesity are known. A significant
increase occurs in the conversion of circulating
adrenal androgens to estrone by excess adipose
tissue. The hepatic production of sex
hormone–binding globulin is decreased, result-
ing in more unbound physiologically active
estrogen. Because almost all circulating estro-
gens postmenopausally are derived from
metabolism of circulating androgens by
peripheral tissues, including fat, these two
mechanisms probably have more impact in
postmenopausal than premenopausal women
(Glass 1989). In obese premenopausal women,
decreased metabolism of estradiol by the 2-
hydroxylation route reduces the conversion of
estradiol to inactive metabolites, which could
result in a relatively hyperestrogenic state
(Schneider et al. 1983).
The potential role of diet in the genesis of
fibroids has received little attention in the liter-
ature. In a case–control study in Italy
(Chiaffarino et al. 1999), a moderate associa-
tion was found between the risk of uterine
myomas and the consumption of beef, other
red meat, and ham, whereas a high intake of
green vegetables seemed to have a protective
effect. Unfortunately, no estimate of the total
caloric intake was obtained, and no attempt
was made to estimate the amount of fat in the
diet for cases and controls, although one might
assume that a higher intake of beef would be
associated with a greater amount of fat in the
diet. Despite the limitations of the study, the
results are interesting and raise a number of
issues. Because fibroids are known to be hor-
monally responsive tumors, are the dietary risks
noted above (Chiaffarino et al. 1999) sec-
ondary to the effects of various food groups
upon the bioavailability of estrogen or proges-
terone? Is the protective effect of a high intake
of green vegetables related to the fiber, some
other undetermined component, such as a vita-
min, or a corresponding reduction of fat in the
diet? What role, if any, do phytoestrogens play?
In a study of premenopausal vegetarian and
nonvegetarian women (Goldin et al. 1982;
Gorbach and Goldin 1987), the vegetarians
Review|Flake et al.
VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
Table 1. Risk factors associated with leiomyomas.
Age (late reproductive years)
Hormone replacement therapy
Marshall et al. 1998a
Parazzini et al. 1996a
Marshall et al. 1997
Ross et al. 1986
Baird et al. 1998
Lumbiganon et al. 1996
Samadi et al. 1996
Parazzini et al. 1996b
Marshall et al. 1998a
Schwartz et al. 1996
Chiaffarino et al. 1999
Saxena et al. 1987
Ezem and Otubu 1981
excreted 3-fold more estrogen in their feces, had
lower urinary estrogen excretion, and exhibited
15–20% reduced plasma estrogen levels. This
reduction is apparently related to the increased
fecal excretion of the estrogen fraction normally
excreted in the bile, resulting in diminished
enterohepatic circulation of estrogens. There
are several possible explanations for the greater
fecal excretion of estrogens in vegetarians,
including a) the greater bulk of undigested and
nonabsorbed fiber that may shield the estrogens
from bacterial deconjugation and reabsorption;
b) some characteristic of the vegetarian diet that
decreases the ability of the intestinal flora to
deconjugate biliary estrogen conjugates, a nec-
essary step for their reabsorption; or c) an effect
related to lower dietary fat levels that might
diminish estrogen absorption. In Goldin’s
study (Goldin et al. 1982), the vegetarians con-
sumed less total fat and more dietary fiber than
did the omnivores. Rose et al. demonstrated
that both high-fiber diets (Rose et al. 1991) and
low-fat diets (Rose et al. 1987) will reduce
serum estrogen levels, probably by altering the
fecal flora and reducing the enterohepatic circu-
lation of estrogens. Regardless of the relative
importance of dietary fat and fiber, such studies
have established that modulation of the diet can
influence estrogen metabolism in pre-
menopausal women, which may in turn influ-
ence the risk for fibroids. Likewise, a 17%
reduction in plasma estradiol concentration was
accomplished in postmenopausal women who
participated in a low-fat diet intervention
program (Prentice et al. 1990).
In recent years plant derivatives known as
phytoestrogens have gained attention in both
the lay and scientific press. Phytoestrogens are
diphenolic compounds that become converted
into estrogenic substances in the gastrointesti-
nal tract (Ginsburg and Prelevic 2000).
Although these compounds are present in
some 300 plants, the quantities present in most
are trivial compared with the concentrations in
soy and flax; in most populations the major
dietary source of phytoestrogens is thought to
be soy (Tham et al. 1998). These substances
generally act as weak estrogens, but they may
also have antiestrogenic effects, depending
upon their concentration, the concentration of
endogenous estrogens, and individual charac-
teristics such as gender and menopausal status
(Ginsburg and Prelevic 2000; Tham et al.
1998); in addition, the effect is probably not
identical in different organs (Adlercreutz and
Mazur 1997). In this regard, some investiga-
tors have suggested that phytoestrogens may
act as “natural” selective estrogen receptor (ER)
modulators (SERMs, such as tamoxifen)
(Ginsburg and Prelevic 2000; Nikov et al.
2000). The observed antiestrogenic effects of
phytoestrogens may be partially explained by
their competition with endogenous estradiol
for ERs (Abramowicz 2000). Prediction of the
effects of phytoestrogens is uncertain because
there are so many variables involved. Despite
their weak estrogenic activity, however, phyto-
estrogens could conceivably have a significant
clinical impact, as their concentrations in the
body may exceed those of the endogenous
estrogens (Adlercreutz et al. 1982).
The possibility of a relationship between exercise
and the occurrence of fibroids has been
addressed by comparing prevalences among a
large group of former college athletes and
nonathletes (Wyshak et al. 1986). Former
nonathletes were found to be 1.4 times more
likely than former athletes to develop benign
uterine tumors. In addition to differences in the
degree of physical activity, however, an athletic
lifestyle may have been associated with long-
term differences in diet and relative leanness
and, in turn, with reduced conversion of andro-
gens to estrogens in adipose tissue (Frisch et al.
1985; Wyshak et al. 1986).
There has been a general acceptance in the
literature that uterine fibroids are more preva-
lent in black women than white women. The
reference often cited is an early study
(Witherspoon and Butler 1934) that had
reported that 89.9% of the fibroid patients
seen at Charity Hospital in New Orleans,
Louisiana, were African American, whereas the
total gynecologic admissions were only slightly
higher among African Americans than whites.
Although this disparity has now been substan-
tiated in a few more current studies, the mag-
nitude of the difference has been less than the
factor of 3–9 times sometimes cited (Buttram
1986; Vollenhoven et al. 1990). For instance,
in one study (Baird et al. 1998), 73% of black
women and 48% of white women had uterine
fibroids by ultrasound examination. In a study
of hysterectomy specimens, (Kjerulff et al.
1996), 89% of the black women and 59% of
the white women had leiomyomas, which in
black women were often larger, more numer-
ous, and more symptomatic, and had devel-
oped at a younger age. In a recent report
(Marshall et al. 1997), 95,061 premenopausal
nurses with no history of uterine leiomyoma
were followed prospectively and had an inci-
dence rate of leiomyoma approximately 2–3
times greater among black women than among
white women. Although there was a higher
prevalence of risk factors, including a higher
mean BMI, among black women in this latter
study, these factors could not account for the
excessive rate of uterine leiomyomata among
premenopausal black women.
Although the basis for the higher prevalence
among black women is unknown, ethnic differ-
ences have been found in circulating estrogen
levels while on control diets, and differences in
estrogen metabolism have been noted. In
control groups of healthy, premenopausal
women placed on a high-fat, low-fiber diet sim-
ilar to their usual diet, African-American
women had significantly higher serum levels of
estrone, estradiol, and free estradiol than
Caucasian women. When subsequently placed
on a low-fat, high-fiber diet, both groups
responded with a significant lowering of their
estrogen levels (Woods et al. 1996). In addi-
tion, significantly lower 2-hydroxyestrone
urinary metabolite ratios have been found in
African-American women than in Caucasian
women (Taioli et al. 1996), which could also
contribute to greater estrogen exposure, as
2-OHE1 metabolites are devoid of peripheral
biologic activity, whereas 16α-OHE1 is estro-
genic. Whether the difference in estrogen
metabolism might be due to genetic or environ-
mental factors is unknown.
Fewer data are available regarding the
prevalence of uterine fibroids in Hispanics and
Asians. In a study of premenopausal nurses in
the United States (Marshall et al. 1997), the
incidence rates among these two groups, deter-
mined by ultrasound or hysterectomy, were
similar to those of the white women (rate per
1,000 woman-years = Hispanic 14.5, Asian
10.4, white 12.5, in contrast to black 37.9).
In summary, we conclude that the preva-
lence of myomas is high among both blacks
and whites, and probably also high among
Hispanics and Asians, in the United States.
The prevalence is relatively higher among
African Americans than other ethnic groups
based upon ultrasound data, and, more impor-
tantly, the clinical prevalence (symptomatic
cases) is higher among African Americans
because of a higher frequency of multiple
lesions and greater size of the fibroids (Baird
et al. 1998; Marshall et al. 1997). The issue of
clinical prevalence versus total prevalence is an
important distinction from an etiologic stand-
point, as it indicates that the initiating causes
of fibroids may require consideration separate
from those factors that could promote their
growth to clinically significant proportions.
Knowledge of the prevalence of uterine fibroids
in other countries could provide clues to the
importance of diet, environmental factors, and
ethnicity, but unfortunately, few such studies
exist in the literature. Sato et al. (Sato et al.
2000b) in Japan stated that “uterine leio-
myomas are the most common pelvic tumors”
but provided no data of the actual prevalence
among their patients. Others (Ezem and Otubu
1981) have cited a 68% incidence of uterine
fibromyomata among their hysterectomy cases
in Nigeria. A study from Malaysia (Ravindran
and Kumaraguruparan 1998) listed fibroids as
the main indication for hysterectomy in their
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
series (47.6% of cases). Similarly, other
investigators have implicated fibroid uterus as
the main indication for hysterectomy in
northern France (66.7% of cases) (Debodinance
Although no firm statistical conclusions
can be drawn, these reports suggest that uterine
fibroids occur commonly in women in many
parts of the world.
Several studies have revealed a reduced risk of
fibroids associated with current smoking, but
not past smoking (Lumbiganon et al. 1996;
Parazzini et al. 1988; Parazzini et al. 1996b;
Ross et al. 1986; Samadi et al. 1996; Wyshak
et al. 1986). In one study current smokers had
a 50% reduced risk of uterine myomas requir-
ing surgery (Parazzini et al. 1996b). In another
(Ross et al. 1986) the reduction in risk among
smokers was dose dependent; women who
smoked 10 cigarettes per day had an 18%
decreased risk compared with nonsmokers,
whereas smokers of 20 cigarettes per day had a
risk approximately 33% lower than that of
nonsmokers. In contrast to these results,
another survey (Marshall et al. 1998b) found
no indication of reduced risk in smokers.
The inverse correlation between smoking
and fibroids has been commonly attributed to
an antiestrogenic effect of cigarette smoking,
suggested by other epidemiologic associations
of smoking, including a reduced risk of
endometrial cancer, earlier natural menopause,
and increased osteoporosis. The pathophysiol-
ogy of this apparent antiestrogenic effect is not
entirely clear, however, because the levels of
estrone and total estradiol are often similar in
postmenopausal smokers and nonsmokers
(Baron et al. 1990), and investigation of hor-
monal levels in premenopausal smokers has
yielded inconsistent results (Longcope and
Johnston 1988; MacMahon et al. 1982;
Westhoff et al. 1996; Zumoff et al. 1990). On
the other hand, several derangements of steroid
metabolism have been identified in smokers.
Increased 2-hydroxylation of estradiol occurs
in smokers, resulting in decreased bioavailabil-
ity at estrogen target tissues (Michnovicz et al.
1986). Nicotine inhibition of aromatase
reduces the conversion of androgens to estrone
(Barbieri et al. 1986). Significantly higher
serum levels of sex hormone–binding globulin
have been found, resulting in less unbound
physiologically active estrogen (Daniel et al.
1992). Increased androstenedione and cortisol
levels have been noted in postmenopausal
smokers, suggestive of increased adrenal activ-
ity; elevated androgens may be significant, as
some evidence exists that androgens can inhibit
estrogen-mediated effects in the rat uterus
(Cassidenti et al. 1992; Hung and Gibbons
1983). These studies indicate that the hor-
monal metabolic effects of smoking are
probably multifactorial. In addition, smokers
as a group consistently exhibit lower body
weights than nonsmokers, possibly because of a
lower efficiency of calorie storage and/or an
increased metabolic rate (Wack and Rodin
1982). A lower body weight associated with a
reduced risk of fibroids might be expected to
be another indirect contributing mechanism
through which smoking exerts an effect, but in
three studies (Lumbiganon et al. 1996;
Parazzini et al. 1996b; Ross et al. 1986), the
effect of smoking was not changed by correc-
tion for BMI (Schwartz et al. 2000a).
Reports in the literature present inconsistencies
with regard to the effect of oral contraceptive
(OC) use upon the growth of myomas. An
early report suggested that OCs may play a role
in the development or growth of leiomyomata
(John and Martin 1971). Some have found no
association between the occurrence of fibroids
and the use of OCs (Parazzini et al. 1992;
Samadi et al. 1996); however, others have
reported a reduction in risk of fibroids with
OC use (Ratech and Stewart 1982; Ross et al.
1986). Further, in the study by Ross et al., a
consistent decrease in the risk of fibroids was
noted with increasing duration of OC use
(approximate 17% reduction in risk with each
5 years of use); this apparent protective effect
was attributed to reduced exposure to unop-
posed estrogen due to the modifying effect of
progestogens (Ross et al. 1986). This study was
criticized, however, for indication bias (Ratner
1986), as fibroids had commonly been consid-
ered a contraindication to OC use, thus
resulting in a selected group for study.
These conflicting findings with regard to
the effect of OCs upon the growth of
myomas may relate to the differing content of
estrogen and the type of progestogen in each
specific OC preparation (Cramer 1992). In
fact, Ross et al. (1986) attempted to address
this issue by analyzing the estrogen and prog-
esterone content of each formulation.
Although no conclusions could be drawn
regarding the estrogens present, the authors
found that the higher the dose of the
progestogen norethisterone acetate, the lower
the incidence of fibroids, in preparations con-
taining the same quantity of the estrogen
ethinylestradiol. In contrast, all preparations
containing the progestogen ethynodiol diac-
etate were associated with an increased inci-
dence of fibroids, regardless of the quantity
present or the type or amount of the accom-
panying estrogen. The authors offered no
explanation for the latter finding and stated
that additional studies were needed for
A significantly elevated risk of fibroids has
been reported among women who first used
OCs in their early teenage years (13–16 years
of age) compared with those who had never
used them (Marshall et al. 1998a).
Hormone Replacement Therapy
Fibroids are expected to shrink after
menopause, but hormone replacement therapy
(HRT) may prevent this shrinkage and may
even stimulate growth. Two studies that were
conducted when estrogen was prescribed with-
out progestins reported elevated risk of fibroid
surgery (Romieu et al. 1991) or uterine
leiomyomata requiring hospitalization
(Ramcharan et al. 1981) among women taking
HRT. Addition of progestins does not appear
to reduce risk. One large (Polatti et al. 2000)
and several small (Colacurci et al. 2000; Fedele
et al. 2000; Sener et al. 1996; Ylostalo et al.
1996) clinical trials demonstrated increased
fibroid size during treatment with transdermal
estrogen when progesterone was included.
Similarly, injected estrogen plus progestin
resulted in an increase in the size and number
of myomas (Frigo et al. 1995). On the other
hand, in four studies (Clark and Johnson
2000; de Aloysio et al. 1998; Polatti et al.
2000; Sener et al. 1996) using oral HRT, little
change in tumor size was noted. In another
investigation oral HRT (using a heterogeneous
variety of treatment regimens including two
estradiol-patch patients) was accompanied by
an increase in volume of 17 myomas and a
decrease in size of 6 myomas, but the changes
were not statistically significant (Schwartz et al.
1996). Several of the oral HRT studies did not
include a control group of postmenopausal
women who were not on HRT; however, in
the two reports that did include control groups
(Clark and Johnson 2000; Schwartz et al.
1996), the myoma volume decreased over time
in the control group, although not significantly
in one study (Schwartz et al. 1996). Taken
together, these studies suggest that oral HRT
may not stimulate the growth of myomas or
may result in growth of some but not other
myomas. Although little data are available, the
two studies with control groups (Clark and
Johnson 2000; Schwartz et al. 1996) suggest
that oral HRT may inhibit normal menopausal
regression of fibroids.
The effect of HRT on fibroids in post-
menopausal women is obviously a complicated
issue resolvable only by future well-controlled
studies. Further emphasizing this point is the
assertion (Polatti et al. 2000) that an increase
in volume or number of uterine myomas dur-
ing HRT in postmenopause is likely not
related solely to the dose and route of adminis-
tration of the estrogen, but also to the type and
dosage of progestogen.
Tamoxifen is a partial estrogen agonist that
binds to ERs in receptive cells, thereby antago-
nizing the effects of estrogen by competitively
Review|Flake et al.
VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
binding to target organ receptors. Because
tamoxifen is effective adjuvant therapy for ER-
positive breast cancer, it might be expected to
induce regression of estrogen-responsive
uterine fibroids. Indeed, there are in vitro
studies indicating that tamoxifen does inhibit
estrogen-stimulated growth of Eker
rat–derived uterine leiomyoma cell lines
(Fuchs-Young et al. 1996). However, several
clinical studies have now reported the growth
or enlargement of uterine fibroids in breast
cancer patients undergoing tamoxifen therapy.
In some cases the expansion of tumor volume
has been sufficiently great to require hysterec-
tomy. Although these reports are anecdotal,
several have included postmenopausal patients
in whom fibroids typically regress rather than
enlarge. On the other hand, if tamoxifen were
efficacious in shrinking the size of fibroids in
some patients, one might expect to see anec-
dotal reports of such, but we were unable to
find any in the literature. These clinical
reports collectively seem to indicate that the in
vivo effect of tamoxifen, in both pre- and post-
menopausal patients at the dosage levels ordi-
narily used as therapy in breast cancer patients,
is either to stimulate the growth of uterine
fibroids or to exert no effect (Boudouris et al.
1989; Dilts et al. 1991; Kang et al. 1996; Le
Bouedec et al. 1995; Leo et al. 1994; Lumsden
et al. 1989a; Tomas et al. 1995; Ugwumadu
and Harding 1994). In a recent review
(Deligdisch 2000), tamoxifen for breast carci-
noma was reported to exert an estrogen-ago-
nist effect on the uterus in approximately 20%
of patients, who developed endometrial
polyps, glandular hyperplasia, adenomyosis,
and/or leiomyomata. A few cases of uterine
leiomyosarcoma developing in patients on
tamoxifen therapy have also been reported
(Chew et al. 1996; Kennedy et al. 1999;
McCluggage et al. 1996; Sabatini et al. 1999;
Silva et al. 1994). This apparent estrogenic
agonist effect of tamoxifen is further sup-
ported by the lack of shrinkage of uterine
leiomyomas by gonadotropin-releasing hor-
mone (GnRH) agonists when used in combi-
nation with tamoxifen (Lumsden et al.
Several inferences may be drawn from
these reports. First, the biologic actions of
tamoxifen are complex, and the information
gained from animal models and tissue culture
is not necessarily directly transferable to
humans. Second, the disparate effects of
tamoxifen in the breast and uterus exemplify
the mixed agonist/antagonist activity of
SERMs, which is apparently dictated by the
cell type and the promoter context of the ERs
for a given cell type (Hall et al. 2001).
A diverse group of exogenous compounds,
xenoestrogens, possesses the potential to
disrupt normal estrogenic function as a result
of either estrogenic agonist or antagonistic
effects. No common chemical structure is pre-
dictive of estrogenic activity, and such
substances may originate from dietary, indus-
trial, or pharmaceutical sources (Houston
et al. 2001). Although industrial chemicals
with estrogenic effects have come under recent
scrutiny, few studies have specifically
addressed this issue in regard to fibroid
tumorigenic effects, despite the known sensi-
tivity of uterine leiomyomas to estrogenic
stimulation (Hunter et al. 2000).
The pesticide dichlorodiphenyltrichloro-
ethane (DDT) and its analogs have been
shown to be estrogenic (Cecil et al. 1971).
Although banned in this country for more
than two decades, residues of organochlorine
pesticides remain detectable in mammalian
fat stores (Stellman et al. 1998), and some
DDT analogs such as methoxychlor are still
in common use in the United States
(Meadows 1996). In the only human stud-
ies, to our knowledge, of DDT and uterine
fibroids (Saxena et al. 1987), significantly
higher levels of DDT and its metabolites
were found in uterine leiomyomatous tissue
than in normal myometrium, and signifi-
cantly higher levels of DDT were reported in
the blood of women with uterine leiomy-
omas than in those without (Khare 1985).
In in vitro studies with Eker rat uterine
leiomyoma–derived cells, several organo-
chlorine pesticides, including 2,2-bis-(p-
kepone, endosulfan-α, methoxychlor, dield-
rin, toxaphene, and endosulfan-β acted as
ER agonists, upregulating progesterone
receptor expression and in some cases stimu-
lating proliferation of leiomyoma cells
(Hodges et al. 2000). Further, the mobiliza-
tion of organochlorines (stored in mam-
malian fat) that occurs during lactation
(Sonawane 1995) and fasting (Bigsby et al.
1997) could result in exposure levels several-
fold higher than those originally encoun-
tered in the environment (Hodges et al.
2000). Also of interest is the finding that the
more recently recognized ER-β binds two
xenoestrogens, methoxychlor and bisphenol
A, with considerably higher affinity than the
classic ER, ER-α (Enmark et al. 1997). In
view of the widespread use and exposure to
the organochlorine pesticides and other envi-
ronmental estrogens, a need clearly exists for
further investigation of a possible link to
fibroid pathogenesis. Studies with the potent
synthetic estrogen diethylstilbestrol have
clearly indicated that exogenous estrogen
exposure during critical stages of develop-
ment can result in permanent cellular and
molecular alterations (Newbold 1995),
including the formation of uterine leio-
myomas (Newbold et al. 2002).
Initiators of Tumorigenesis
Theories of Initiation
The most important aspect of the etiology of
fibroids—the initiator(s)—remains unknown.
Several theories have been advanced. One
hypothesis states that increased levels of estrogen
and progesterone result in an increased mitotic
rate that may contribute to myoma formation
by increasing the likelihood of somatic muta-
tions (Rein 2000). Another favors an inherent
abnormality in the myometrium of those who
develop fibroids, based upon the finding of sig-
nificantly increased levels of ER in the
myometrium of fibroid uteri (Richards and
Tiltman 1996). A predisposing genetic factor
has been suggested by others on the basis of eth-
nic and familial predilections (Marshall et al.
1997; Schwartz et al. 2000b).
Another interesting theory postulates that
the pathogenesis of uterine leiomyomas might
be similar to a response to injury (Stewart and
Nowak 1998) in a manner analogous to the
development of keloids (hypertrophic scars)
following surgery. One avenue of potential
injury might be ischemia associated with the
release of increased vasoconstrictive sub-
stances at the time of menses. Increased secre-
tion of prostaglandins and vasopressin by the
endometrium has been noted in patients with
dysmenorrhea (Emans et al. 1998), which
occurs in up to 70% of women by the fifth
year after menarche (Coupey 2000). Might
the smooth muscle cells of the myometrium
react to injury in a manner analogous to vas-
cular smooth muscle cells by undergoing a
transformation from a contractile phenotype
to a proliferative-synthetic phenotype?
Certainly, morphologic similarities exist, as
fibroids exhibit both an increased proliferative
rate (Dixon et al. 2002) and the synthesis of
extracellular fibrous matrix. After vascular
injury, basic fibroblast growth factor (bFGF)
is critical to smooth muscle proliferation, and
this factor is also overexpressed in leiomyomas
(Lindner and Reidy 1991; Mangrulkar et al.
1995). Finally, injury related to menses is
worthy of consideration in view of the univer-
sality of menstruation and the commonality
of fibroids. When we consider the various risk
factors, including those that have been attrib-
uted in the literature to increased exposure to
“unopposed estrogens,” such as early menar-
che and nulliparity, we observe that such
patients also experience more menstrual cycles
than their counterparts.
Of equal uncertainty in the genesis of
fibroids is the role of genetic and/or epigenetic
changes. The possibility of hereditary genetic
predisposition to fibroids cannot be excluded at
this time. On the other hand, evidence has
been presented, though limited in scope, that
karyotypic changes may occur secondarily
(Mashal et al. 1994) during the evolution or
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
aging of some fibroids. Regardless of whether
acquired karyotypic changes occur ab initio or
during clonal evolution of fibroids, we can
assume that preceding stimuli, conditions, or
injuries must be responsible for the induction of
genetic or epigenetic changes, and in this sense
acquired genetic changes may be regarded as
secondary. These changes are therefore discussed
in this section, not from the standpoint of pur-
ported initiators, but as possible potentiators or
effectors of currently unrecognized initiating
The Genetic Findings
There have been numerous studies and
reviews of the clonality and cytogenetics of
uterine leiomyomas (Gross and Morton
2001; Ligon and Morton 2000, 2001; Mark
et al. 1988; Nilbert and Heim 1990; Ozisik
et al. 1993b; Pandis et al. 1991). For the
purposes of this brief review, we have
attempted to summarize those features that
appear most salient.
Heritability. Is there evidence of a genetic
predisposition to fibroids? This question has
been approached from four perspectives: ethnic
predisposition, twin studies, familial aggrega-
tion, and association with an inherited syn-
drome. The higher incidence of clinically
significant fibroids among African-American
women in the United States has been discussed
above. Two studies comparing monozygous
and dizygous twins may be cited. The first of
these reported a 2-fold higher correlation for
hysterectomy in monozygotic than dizygotic
twins (Treloar et al. 1992). Because leiomy-
omata represent the most common indication
for hysterectomy in the United States, this
finding in monozygous twins suggests a genetic
liability for fibroids. Because the study did not
report the actual incidence of leiomyomata,
however, it is recognized that heritable condi-
tions other than fibroids could contribute to
the observed correlation in twins (Gross and
Morton 2001). A more recent twin study
specifically addressed the risk of fibroids in
twins by examining hospital discharge diag-
noses from the Finnish Twin Cohort Study
and by performing transvaginal ultrasounds in
a random sample of these women (Luoto et al.
2000). The casewise concordance for hospital-
ization due to uterine fibroids was significantly
higher in monozygous twins than dizygous
twins, providing support for a genetic contri-
bution. On the other hand, by ultrasound
examination the risk ratio for fibroids in a
monozygous twin whose sister had been diag-
nosed with fibroids was only 1.1, the same as
for a dizygous twin; however, the authors
noted that the low participation rate decreased
the power of the study to detect potential dif-
ferences between the twins. The study con-
cluded that anthropometric and reproductive
factors, such as a higher BMI and nulliparity,
may play at least as large a role in pathogenesis
of fibroids as genetic factors.
Four studies of the familial clustering of
fibroids may be cited. The first was a German
study, reported in 1938 (Winkler and
Hoffmann 1938), in which fibroids were found
to be 4.2 times more common in first-degree
relatives of women with fibroids than those
without. Similar findings were noted in two
studies from Russia in which a higher incidence
of fibroids was found in first-degree relatives
(Vikhlyaeva et al. 1995) and sisters (Kurbanova
et al. 1989) of affected probands than in con-
trols. Last, in a study of 638 fibroid patients
and 617 controls in the Puget Sound area of
Washington State (Schwartz et al. 2000b),
fibroid patients again were found more likely
than the controls to report a history of fibroids
in a mother or sister (33.2% vs. 17.6%).
Furthermore, the odds ratio increased to 5.7 in
cases of early-onset fibroids, as might be
expected for a genetically influenced trait.
Unfortunately, these studies may be influenced
by reporting and detection bias. A woman hav-
ing clinical problems that could be attributed to
fibroids may be more likely to seek a diagnosis
if a close relative has had fibroids. A woman
who has been diagnosed may also be more
likely to learn about diagnoses among her
Finally, a rare inherited disorder known as
Reed’s Syndrome (Fisher and Helwig 1963;
Reed et al. 1973; Thyresson and Su 1981), or
multiple leiomyomatosis, is characterized by
the appearance of multiple leiomyomas in the
skin, uterus, or both. The family histories in
these cases suggest an autosomal dominant
inheritance with incomplete penetrance.
Recent reports of several families in England
and Finland with multiple uterine and cuta-
neous leiomyomata, and a subset of these with
papillary renal cell carcinoma, have indepen-
dently linked this disorder to a predisposition
gene in the region of chromosome 1q42.3-q43
(Alam et al. 2001; Kiuru et al. 2001;
Launonen et al. 2001). In follow-up studies of
this chromosomal region, mutations were
detected only in the fumarate hydratase gene
(Tomlinson et al. 2002)—a surprising finding,
as this enzyme is a component of the essential
energy-producing tricarboxylic acid cycle
(Rustin et al. 1997). Furthermore, the gene
appears to act as a classic tumor suppressor in
that loss of the wildtype allele was observed fre-
quently in the leiomyomata and renal cell can-
cers (Alam et al. 2001; Kiuru et al. 2001;
Launonen et al. 2001). Although this heredi-
tary syndrome is itself rare, the association with
inactivation of the fumarate hydratase gene is
of interest, as it is possible that other mecha-
nisms of transcriptional silencing of this gene
such as promoter hypermethylation could be
involved in the development of sporadic
leiomyomas (Kiuru et al. 2001).
Clonality. There is general acceptance in
the literature that these tumors are mono-
clonal. The underlying premise of these stud-
ies has been based on the Lyon hypothesis,
which assumes that only one X chromosome
is active in any female cell, the other X chro-
mosome remaining in an inactive state as a
Barr body, and that the X chromosome that is
inactivated (methylated) is determined ran-
domly. Thus, genetic loci known to be
located on the X chromosome can be studied
in these tumors for evidence of homogeneity
of expression in those patients identified as
heterozygous for a particular gene in their
normal, nontumor tissues.
The first studies of clonality used the
X-linked glucose 6-phosphate dehydrogenase
(G6PD) isozymes. After screening patients for
G6PD heterozygosity by analysis of red blood
cells, the resected fibroids and myometrium
were analyzed for the presence of one or both
electrophoretic types of G6PD. In two studies
(Linder and Gartler 1965; Townsend et al.
1970), both G6PD types (A and B) were
identified in almost all samples of myo-
metrium, whereas only one G6PD type (A or
B) was identified in each of the leiomyomas.
Furthermore, both A and B tumors were
often identified in the same patient, indicat-
ing independent origins of the individual
fibroids. These results suggested that the
tumors arose from single cells, although selec-
tive overgrowth of one cell type from a tumor
originally composed of both G6PD types can-
not be excluded. The major limitation of
these studies is the minor degree of G6PD
polymorphism in the population, as most
Caucasian females (> 99%) are homozygous
type B, and only 30% of African-American
females are heterozygous, and therefore only a
minority of cases would be informative by
studies of this gene.
More recently, clonality studies have
taken advantage of methylation-sensitive
restriction enzymes to discriminate between
active and inactive alleles of X-linked genes
known to be highly polymorphic (Vogelstein
et al. 1985). Tumors arising from single cells
should contain only one type of inactive
(methylated) allele, which will be amplified
exclusively following restriction-enzyme
digestion of the active (unmethylated) allele,
whereas tumors of multicellular origin should
contain some cells with one type of inactive
allele and other cells with a second type of
inactive allele, resulting in the amplification
of both alleles following digestion and poly-
merase chain reaction. This method has been
employed for analysis of both the X-linked
androgen receptor gene (Mashal et al. 1994)
and the X-linked phosphoglycerokinase gene
(Hashimoto et al. 1995). Both studies con-
cluded that the uterine fibroids examined
were monoclonal in origin.
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VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
One report has described chromosome 7
biclonality in four uterine leiomyomas (Ozisik
et al. 1993a), with the breakpoint regions in
two of these such that one clone could not pos-
sibly have originated from the other clone.
Taken in sum, however, the concept of mono-
clonal origin of most fibroids appears to be a
valid one, recognizing that some could be
biclonal in origin (Ozisik et al. 1993a) and
some are biclonal or oligoclonal because of
clonal evolution (Pandis et al. 1990), and that
monoclonality itself could be the result of
selective overgrowth of one clone from an orig-
inally polyclonal proliferation (Fey et al. 1992;
Vogelstein et al. 1987).
Cytogenetics. Most of the investigations of
leiomyomas seeking chromosomal aberrations
have used classic cytogenetic karyotyping, a
valuable tool because it is the only method that
allows one to survey the entire genetic consti-
tution of a tissue with a single assay. Standard
cytogenetic methodology with G-band analysis
can identify translocations, deletions, and
duplications, but does require the in vitro cul-
ture of leiomyoma cells to obtain metaphase
preparations. An alternative method that has
been employed in a few studies (Levy et al.
2000; Packenham et al. 1997) is comparative
genomic hybridization, which permits the
recognition of cytogenetic changes such as
deletions and amplifications without the need
for cell cultures of the tumor, although not
allowing for detection of balanced rearrange-
ments. Neither standard karyotyping nor com-
parative genomic hybridization permits the
detection of small, submicroscopic chromo-
somal abnormalities such as point mutations or
epigenetic changes such as methylation.
Most common cytogenetic changes.
Because the studies of tumor cytogenetics are
limited to tissue samples removed at surgery
and may be taken from larger fibroids, the
possibility exists that they may not be repre-
sentative of leiomyomas in general.
Nonetheless, based upon such samples,
approximately 40–50% of uterine fibroids are
reported to have nonrandom chromosomal
abnormalities (Table 2).
t(12;14). One of the most common of
these is a translocation between chromosomes
12 and 14, specifically t(12;14) (q14-q15;q23-
q24), which is present in about 20% of karyo-
typically abnormal leiomyomata (Ligon and
Morton 2000). This abnormality is of interest
for several reasons. First, the region q14-q15
on chromosome 12 is also commonly
rearranged in a variety of other mesenchymal
solid tumors, including angiomyxomas,
hemangiopericytomas, lipomas, and pul-
monary chondroid hamartomas, as well as
breast fibroadenomas, endometrial polyps, and
salivary gland adenomas. In addition, evidence
exists that a critical gene located in the
chromosome 12q14-q15 region may be
HMGIC (now designated HMGA2), a gene
encoding a member of the high-mobility group
(HMG) of proteins. These are DNA-binding
proteins that can induce conformational
changes in DNA, thereby indirectly regulating
transcription by influencing the access of other
DNA-binding proteins to target genes. The
HMGIC protein may play a role as a prolifera-
tion factor in growing tissues, particularly those
of mesenchymal origin; accordingly, expression
of this protein has been detected in leio-
myomata with 12q14-15 rearrangements, but
not in matched normal myometrium (Gattas
et al. 1999). In addition, the region on chro-
mosome 14 involved in this translocation,
q23-q24, is of particular interest because of its
specificity for fibroids compared with other
mesenchymal tumors in which HMGIC is
rearranged. The ER-β gene (ESR2) is located
in this region of chromosome 14 and would
seem to be a logical fusion partner with
HMGIC, as the growth of fibroids is responsive
to estrogen. More recently, ESR2 has been
mapped to a region approximately 2 Mb cen-
tromeric to the t(12;14) breakpoint, suggesting
that ESR2 is not involved with HMGIC.
However, this finding may not exclude the
possibility that ESR2 might be deregulated by
chromosomal translocation in view of its prox-
imity to the breakpoint (Pedeutour et al.
Evidence has also been presented that
RAD51L1 (formerly RAD51B), a member of
the RAD51 recombination repair gene family
(Albala et al. 1997; Shinohara et al. 1992), is
the chromosome 14 target gene and preferen-
tial fusion partner of HMGIC in uterine
leiomyomas with t(12;14) (Amant et al.
2001; Ingraham et al. 1999; Schoenmakers
et al. 1999; Takahashi et al. 2001). Although
the RAD51L1 protein has not yet been
shown to catalyze recombination reactions,
RAD51L1 appears to be an essential gene
(Shu et al. 1999) expressed in almost all
organs and tissues (Rice et al. 1997) and
probably plays a role in regulation of cell cycle
progression (Havre et al. 1998, 2000). In
view of the purported function of HMGIC in
regulation of cell proliferation (Reeves 2000)
and the probable role of RAD51L1 in cell
cycle regulation, it is reasonable to speculate
that the disruption of genomic structure asso-
ciated with the RAD51L1/HMGIC fusion
(Ingraham et al. 1999; Schoenmakers et al.
1999; Takahashi et al. 2001) might result in
dysregulated cell growth.
del(7q). Another frequently encountered
karyotypic abnormality in fibroids is a deletion
of chromosome 7, del(7)(q22q32), which is pre-
sent in about 17% of karyotypically abnormal
fibroids (Ligon and Morton 2000). In some
series del(7q) has been the most common cyto-
genetic abnormality in fibroids (Nilbert and
Heim 1990; Ozisik et al. 1993b). Although
interstitial deletions and translocations involving
chromosome 7q have also been reported in
other benign tumors, such as lipomas and
endometrial polyps, the deletion is more com-
monly observed in fibroids than in any other
solid tumor. Because this region, 7(q22q32), is
physically large and gene-rich, pinpointing a
specific gene that could be implicated in the
genesis of fibroids has proven difficult. Recently,
however, the critical area on band 7q22 has
been narrowed to a 4-cM (centiMorgan) region
by allelotype analysis (van der Heijden et al.
1998). In the latter study loss of heterozygosity
in the leiomyomas was rare except in 7q22,
where a minimal deletion was observed in 34%
of the tumors, leading the authors to speculate
that this site probably harbors a novel tumor-
suppressor gene involved in the etiology of this
tumor (van der Heijden et al. 1998).
6p21. A third cytogenetic subgroup
consists of aberrations of 6p21, including dele-
tions, inversions, translocations, and insertions.
Interest in this region has been related in part
to the frequently observed alterations of band
6p21 in other benign mesenchymal tumors,
such as lipomas, and to the identification of
another high mobility group gene, HMGIY
(now designated HMGA1), in this region.
Rearrangements of 6p21 are much less com-
mon in fibroids than in these other tumors,
however, occurring with a frequency of < 5%.
Trisomy 12. A variety of other cytogenetic
abnormalities have been identified in leio-
myomata. The reporting of trisomy 12 in as
many as 12% of karyotypically abnormal
fibroids (Nilbert and Heim 1990; Vanni et al.
1992) raises the question of whether this
anomaly might reflect pathogenetic similari-
ties to t(12;14), acting to increase the gene
dosage of HMGIC. Many of the other abnor-
malities, such as ring chromosomes, occur less
frequently and often concomitantly with
other chromosomal changes and are therefore
thought to represent secondary abnormalities.
Correlations with tumor phenotype. No
indication of systematic histologic differences
between leiomyomas with normal karyotypes
and those with chromosomal aberrations were
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
Table 2. Leiomyoma-associated cytogenetic changes.
6p21 (del, inv, t, ins)
TGFβ3, HMGIC (HMGA2)
Ligon and Morton 2000
Ligon and Morton 2000
Nilbert and Heim 1990
Ligon and Morton 2000
aFrequency among those leiomyomas with abnormal karyotypes.
found in one study (Nilbert and Heim 1990);
however, there is some evidence from other
reports (Meloni et al. 1992; Pandis et al.
1990) that leiomyomas that are either cellular
with mitotic activity or atypical histologically
are more likely to demonstrate karyotypic
abnormalities or to show massive karyotypic
aberrations indicative of clonal evolution. In
a study of 114 myomas from 92 patients,
myomas > 6.5 cm demonstrated a signifi-
cantly higher proportion of abnormal kary-
otypes than myomas < 6.5 cm (75% vs. 34%)
(Rein et al. 1998). In the same study a rela-
tionship between particular karyotypes and
fibroid size was identified, with the largest
tumors carrying t(12;14) abnormalities and
the smaller tumors exhibiting chromosome 7
deletions, suggesting that chromosomal
abnormalities associated with individual
myomas may enhance myoma growth. A cor-
relation between the location of the fibroid
and the likelihood of a cytogenetic abnormal-
ity has also been reported (Brosens et al.
1998); submucous myomas presented signifi-
cantly fewer abnormal karyotypes (12%) than
did either the intramural (35%) or the sub-
serosal (29%) tumors, and furthermore, this
correlation remained significant regardless of
the diameter of the myoma.
Summary. Despite the large number of
cytogenetic studies, many unanswered ques-
tions remain. Are the chromosomal aberrations
primary to the genesis of these tumors or are
they secondary events? In one study chromo-
somal abnormalities were interpreted as sec-
ondary events because they were preceded by
monoclonality (Mashal et al. 1994); however,
the data are limited and additional studies are
needed for verification. Certain karyotypic
abnormalities such as the t(12;14) and the
del(7q) occur with sufficient frequency to war-
rant consideration as differing pathways lead-
ing to leiomyoma development, or at least to
consider that these sites may contain genes that
are important in the proliferation and differen-
tiation of smooth muscle cells. Because at least
one-half of fibroid tumors appear to be cytoge-
netically normal, there may exist an unidenti-
fied submicroscopic mutation in this
karyotypically normal subgroup or even in the
cytogenetically abnormal group as well.
Histologic subtypes such as the cellular and
atypical leiomyomas may ultimately be corre-
lated with certain karyotypic aberrations that
are either distinctive primary events or repre-
sent secondary changes of clonal evolution.
Finally, regarding heritability, a particular gene
or genes may one day be identified as predis-
posing to the development of leiomyomata, as
suggested by the familial clustering studies. If
so, it must be a very common gene, widespread
in the general population, in view of Cramer
and Patel’s finding of a 77% incidence of
leiomyomas in a thorough examination of
100 consecutive, nonselected hysterectomy
specimens (Cramer and Patel 1990).
Promoters: Evidence for the Role
of Estrogen and Progesterone
Estrogen has been traditionally proposed as the
primary promoter of uterine leiomyoma
growth. This supposition has been based in part
upon the clinical observations that fibroids
occur only after menarche, develop during the
reproductive years, may enlarge during preg-
nancy, and frequently regress following
menopause. Furthermore, because the risk of
fibroids is greater in nulliparous women who
might be subject to a higher frequency of
anovulatory cycles and obese women with
greater aromatization of androgens to estrone in
the fat, the concept of unopposed estrogens as
an underlying cause of uterine fibroids has
sometimes been proposed in the literature
(Cramer 1992; Parazzini et al. 1996a; Romieu
et al. 1991; Ross et al. 1986). Increased growth
of myomas among women taking tamoxifen or
receiving transdermal or injected estrogen-
replacement therapy further supports the
importance of estrogen. The estrogen hypothe-
sis has also been supported by clinical trials
evaluating the medical treatment of myomas
with GnRH agonists, the effective result of
which is hypoestrogenism accompanied by
regression of the fibroids (Friedman et al.
1989). As noted by Rein, however, distinguish-
ing the relative importance of estrogen versus
progesterone is difficult, as progesterone levels,
in a manner similar to those of estrogen, are
also cyclically elevated during the reproductive
years, are significantly elevated during preg-
nancy, and are suppressed after menopause
(Rein et al. 1995). Furthermore, regression of
uterine leiomyomata has been induced by treat-
ment with the antiprogesterone drug RU 486,
accompanied by reduction in the progesterone
receptor (PR) but not the ER in the tumors,
suggesting that the regression was attained
through a direct antiprogesterone effect
(Murphy et al. 1993). In addition patients
treated with leuprolide (a GnRH agonist capa-
ble of reducing the size of fibroids) who were
concomitantly given medroxyprogesterone
acetate demonstrated no significant reduction
in myoma or uterine volume (Carr et al. 1993;
Friedman et al. 1988). Indeed, clinical and lab-
oratory evidence to date would appear to indi-
cate that estrogen and progesterone may both
be important as promoters of myoma growth
We now consider further the impact of
sex steroids upon fibroid growth in two dia-
metrically opposed clinical situations,
namely, pregnancy with the associated
elevations of estrogen and progesterone, and
medical treatment with GnRH agonists
accompanied by reductions in these two
Pregnancy. A common clinical perception
prevails that myomas increase in size during
pregnancy (Buttram 1986). With the advent
of ultrasonographic studies, however, several
reports have noted that only a minority of
myomas (one-third or less) increase in size
during pregnancy, whereas the majority
remain stable or decrease in size (Aharoni et al.
1988; Rosati et al. 1992; Strobelt et al. 1994).
The larger the myoma, the greater the likeli-
hood of growth (Strobelt et al. 1994). Myoma
size can increase as a result of hypertrophy and
edema, while shrinkage of the tumor may
occur as a result of degenerative changes sec-
ondary to ischemia. A 10% complication rate
related to myomas has been reported during
pregnancy (Katz et al. 1989). The most com-
mon complication was the syndrome of
painful myomas, sometimes associated with
bleeding, and probably related to hemorrhagic
degeneration or infarction. Although the etiol-
ogy of the syndrome of painful myomas of
pregnancy is unclear, high concentrations of
progesterone, as in pregnancy, may play a role,
as similar changes of “red degeneration” have
been induced by high-dosage progestin ther-
apy (Goldzieher et al. 1966). Other reported
complications of myomas in pregnancy
include premature rupture of the membranes,
malpresentation, increased cesarean delivery
rate, and postpartum endomyometritis (Katz
et al. 1989). It has also been suggested that
fibroids are a more important feature in preg-
nancy now than in the past because many
women are delaying childbearing to their late
thirties, the time of greatest risk for fibroid
growth (Vollenhoven et al. 1990).
Gonadotropin-releasing hormone agonists
(luteinizing hormone–releasing hormone ago-
nists). GnRH analogs are therapeutic agents
derived from peptide substitutions of the hypo-
thalamic hormone luteinizing hormone–releas-
ing hormone (LHRH). These substitutions at
positions 6 and 10 in the amino acid structure
result in analogs that are 40–200 times more
potent than native LHRH (Vollenhoven et al.
1990). Although the initial response to these
agents is an elevation of serum gonadotrophin
levels and with it increased concentrations of
sex steroids, continuous administration results
in suppression of the pituitary–ovarian axis,
with decreased gonadotropin and sex steroid
levels. The mechanism of this suppression is
thought to be related to downregulation of the
pituitary LHRH receptors (Fraser 1988). The
hypoestrogenic state induced by these agents
results in reduction in size of the uterus itself as
well as many of the fibroids in the majority of
patients. A variety of theories have been pro-
posed for the pathophysiologic mechanism
leading to this shrinkage of fibroids, including
a reduction in uterine arterial blood flow
Review|Flake et al.
VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
(Shaw 1989), a combination of ischemic injury
and cellular atrophy (Colgan et al. 1993), a
reduction in cellularity (Upadhyaya et al.
1990), apoptosis (Higashijima et al. 1996),
and a reduction in the number of cycling cells
secondary to reduced levels of ER and PR
(Robboy et al. 2000; Vu et al. 1998).
Unfortunately, use of these agents as the
sole therapy for fibroids is limited by the rapid
enlargement of the myomas to near pretreat-
ment size following cessation of the GnRH
agonist therapy (Friedman et al. 1989) and by
the concern for potential bone resorption with
long-term administration of the drugs
(Friedman et al. 1990). However, GnRH
analogs have been used as preoperative therapy
to reduce the size of fibroids prior to hysterec-
tomy; this approach has resulted in reports of
significantly less blood loss at operation
(Lumsden et al. 1987) and increased feasibility
of vaginal rather than abdominal hysterectomy,
accompanied by shorter hospitalizations
(Stovall et al. 1991).
Estrogen and progesterone levels. Patients
with uterine leiomyomas have plasma estra-
diol and progesterone levels similar to those
of women without detectable myomas, as
indicated in three studies (Dawood and
Khan-Dawood 1994; Maheux et al. 1986;
Spellacy et al. 1972). An older report noted
that the urinary estrogens of approximately
one-third of the fibroid patients were ele-
vated with respect to their laboratory normal
range, but no control group was available for
comparison (Timonen and Vaananen 1959).
Quantitative differences, however, have been
demonstrated between leiomyomas and
myometrium in the tissue concentrations of
ovarian hormones, their receptors, and a key
metabolizing enzyme. In one study, the con-
centration of 17β-estradiol was significantly
higher in leiomyomas than myometrium,
especially in the proliferative phase, whereas
no difference in the concentration of proges-
terone was found (Otubu et al. 1982). The
authors speculated that the higher levels of
estradiol in the leiomyomas could be related
to lower levels of the enzyme 17β-hydroxy-
steroid dehydrogenase, which accelerates the
conversion of estradiol to estrone. Other
investigators have also demonstrated higher
estradiol concentrations (Folkerd et al. 1984)
and more frequent expression or overexpres-
sion of aromatase activity in leiomyomata
than in matched myometrial samples
(Folkerd et al. 1984; Sumitani et al. 2000;
Yamamoto et al. 1984), leading these authors
to entertain the possibility that increased
androgen to estrogen conversion in fibroids
may potentiate their growth.
Estrogen and progesterone receptors. The
ER and PR literature comprises a rather
extensive and sometimes contradictory
collection of data that spans several decades of
research. Disparate results are probably attrib-
utable to the diversity of methodologies
employed (including assessment of the cytosol
alone versus the combined nuclear and
cytosolic fractions), the use of human versus
nonhuman tissues, the phase of the menstrual
cycle at the time of collection of specimens,
and the heterogeneity of myomas in the same
patient (Englund et al. 1998). In the absence
of experimental unanimity, the generaliza-
tions or conclusions that follow are therefore
based upon our assessment of the weight of
In the majority of the studies reviewed, the
concentrations of both the ERs and PRs were
greater in leiomyomata than the myometrium
(Andersen et al. 1995; Brandon et al. 1993,
1995; Buchi and Keller 1983; Eiletz et al.
1980; Englund et al. 1998; Kawaguchi et al.
1991; Lessl et al. 1997; Marugo et al. 1989;
Nisolle et al. 1999; Otsuka et al. 1989; Pollow
et al. 1978a; Puukka et al. 1976; Rein et al.
1990c; Sadan et al. 1987; Soules and McCarty
1982; Tamaya et al. 1979, 1985; Vij et al.
1990; Viville et al. 1997; Vollenhoven et al.
1994; Wilson et al. 1980). In addition, Sadan
et al. found the ER and PR to be elevated in
fibroids during all phases of the menstrual
cycle when compared with matched myo-
metria (Sadan et al. 1987). Interestingly, in one
study (Marugo et al. 1989) the ER and PR lev-
els were significantly higher in submucous than
subserosal leiomyomas, leading the authors to
speculate about different etiologies and types of
leiomyomas. The receptor concentrations were
independent of the size of the tumor in one
report (Sadan et al. 1987). Another investiga-
tion found marked variation in ER and PR
levels in different tumors from the same sub-
ject (Englund et al. 1998); such heterogeneity
may relate to the degree of hyalinization and
involution of individual tumors.
ER-α and ER-β. Because a second
subtype of the ER, designated ER-β, was not
discovered until 1996 (Kuiper et al. 1996;
Mosselman et al. 1996), the significance of
ER-β relative to that of the classic ER, ER-α,
has not been fully determined. Nuclear
expression of both ER-α and ER-β through-
out the entire myometrium has been demon-
strated immunohistochemically (Taylor and
Al-Azzawi 2000). One group (Pedeutour et
al. 1998) found ER-β mRNA in 14 of 15
leiomyomata, with no striking difference in
expression from the matched myometrial tis-
sues. Another group (Benassayag et al. 1999)
showed expression of both ER-α and ER-β
mRNA in leiomyomata, with the levels of
both receptors higher in most of the leio-
myomas than in the corresponding nonpreg-
nant myometria. Andersen noted that the
highest expression of ER-β in nonpregnant
myometrial and leiomyoma tissues is at the
beginning of the menstrual cycle, and the
lowest expression is at the early midluteal
phase; however, low levels of ER-β protein
were detected in these tissues, in contrast to
the more abundant expression in myometrial
tissue from pregnant women at term
(Andersen 2000). Despite the lack of consen-
sus regarding the quantitative levels of ER-β,
the possibility of a role for ER-β in leiomy-
omata cannot be ruled out at this time, as the
ER-β gene, ESR2, has been mapped to
14q22-24 (Enmark et al. 1997), close to the
breakpoint site of one of the more common
genomic rearrangements of fibroids.
Progesterone receptor-A and progesterone
receptor-B. Both forms of PR (PR-A and PR-B)
are expressed in leiomyomas and myometrium,
with the concentration of PR-A higher than
that of PR-B in both tissues (Viville et al.
1997). In one study PR-A levels were increased
in leiomyomata compared with the matched
myometrium (Brandon et al. 1993).
Interaction between estrogen, progesterone,
and their receptors. The interaction between
the two hormones and their respective recep-
tor levels has been the subject of numerous
studies and is of interest with regard to the
promotion of fibroid growth. Strong evidence
exists that the effect of estrogen is to increase
the levels of both ER and PR in the
myometrium, whereas the effect of proges-
terone is to decrease the level of the ER
(Hsueh et al. 1975; Katzenellenbogen 1980;
Thi et al. 1975). These conclusions are consis-
tent with the sequential presentation of these
two hormones during the menstrual cycle and
the predominant observations that in the
myometrium both ER and PR rise during the
follicular (proliferative) phase and then fall
during the luteal (secretory) phase of the men-
strual cycle (Adams et al. 1993; Buchi and
Keller 1983; Englund et al. 1998; Hsueh et al.
1975; Janne et al. 1975; Kawaguchi et al.
1991; Lessl et al. 1997; Marugo et al. 1989;
Rein et al. 1990c; Sadan et al. 1987; Schmidt-
Gollwitzer et al. 1979; Soules and McCarty
1982; Thi et al. 1975). Because PR levels also
fall during the luteal phase, some feel that
progesterone may downregulate its own recep-
tor (Englund et al. 1998); this conclusion was
also reached by Thi et al. (1975), who demon-
strated a fall in PR in the myometrium of
ovariectomized guinea pigs when given prog-
esterone (Thi et al. 1975). However, the alter-
native explanation that the fall in PR is related
to the fall in levels of estradiol during the
luteal phase is difficult to exclude (Englund
et al. 1998; Schmidt-Gollwitzer et al. 1979).
The majority of studies have reported the
occurrence of similar cyclic rises and falls in ER
and PR in uterine fibroids during the menstrual
cycle, although there is some controversy
regarding the degree, or the existence, of such a
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
fall in ER during the luteal phase. In one study,
ER expression occurred throughout the
menstrual cycle in leiomyomas (Kawaguchi
et al. 1991). Likewise, another investigation
showed that elevated levels of the ER in fibroids
continue throughout the cycle, suggesting that
leiomyomas may have lost a negative regulation
that is maintained in the myometrium and lim-
its the myometrial response to estrogen in the
beginning of the menstrual cycle (Andersen and
Barbieri 1995). On the other hand, it is clear
that these tumors are subject to hormonal mod-
ulation during the cycle, as mitotic activity is
reported to be significantly higher during the
secretory phase than during the proliferative
phase (Kawaguchi et al. 1989; Lamminen et al.
1992; Nisolle et al. 1999). These latter reports
are consistent with a study by Tiltman
(Tiltman 1985) that demonstrated a signifi-
cantly higher mitotic activity in the leiomyomas
of patients who received a progestin-only prepa-
ration. In lone contrast to these studies is an
earlier report that had noted no mitotic activity
in the myomas of patients given progestin ther-
apy (Goldzieher et al. 1966). When considered
in sum, however, these studies support the con-
cept of a mitogenic effect of progesterone in
Although these data show that proges-
terone plays an important role in the growth of
leiomyomas, it is also evident that some degree
of cell proliferation occurs continuously during
the menstrual cycle, as mitotic activity, albeit
of a lesser degree, is present during the follicu-
lar phase of the cycle as well (Kawaguchi et al.
1989; Lamminen et al. 1992). Although the
possibility of progesterone carryover effect
from the luteal phase cannot be excluded, this
suggests that estrogen may exert a mitogenic
effect as well, and there are some clinical data
(Ramcharan et al. 1981; Romieu et al. 1991)
as well as tissue culture work (Chen et al.
1973; Maruo et al. 2000) to support this sup-
position. In addition, we might reason that the
mitogenic effect of progesterone is dependent
upon prior exposure to estrogen, as estrogen
priming increases the concentration of PRs in
myomas. In summary the evidence available
suggests that during the follicular phase, estro-
gen upregulates ER and PR, thus setting the
stage for the luteal phase progesterone surge
associated with a heightened mitogenic effect
and subsequent downregulation of ER and PR.
Metabolism of estradiol. The metabolism
of estradiol involves a series of enzymatically
catalyzed oxidative transformations, which
may occur by several pathways. Because some
estradiol metabolites possess significant estro-
genic activity whereas others are virtually
devoid of activity, the levels of the specific
metabolizing enzymes and the predominant
pathways employed could play important
roles in fibroid tumorigenesis. Of interest,
therefore, is the demonstration of alterations
in two of these enzymes, 17β-hydroxysteroid
dehydrogenase and estradiol 4-hydroxylase, in
Regardless of the phase of the cycle, the prolif-
erative index of leiomyomas is significantly
higher than that of the myometrium (Dixon
et al. 2002; Kawaguchi et al. 1991; Maruo
et al. 2000). This finding is not surprising in
view of the elevated levels of both ERs and
PRs in leiomyomas throughout the menstrual
cycle. Because estradiol up-regulates both of
these receptors, the increased concentration of
estradiol in these tumors compared with that
in the myometrium (Otubu et al. 1982) could
be indicative of a pathogenetic link to the
development of leiomyomata. The demonstra-
tion of reduced activity in leiomyomas of the
enzyme 17β-hydroxysteroid dehydrogenase
(Eiletz et al. 1980; Pollow et al. 1978b), the
enzyme responsible for the conversion of estra-
diol to estrone, would seem to provide a plau-
sible explanation for the accumulation of
estradiol in these tumors (Otubu et al. 1982).
Although estrone is weakly estrogenic, it
exhibits a lower binding affinity for ERs than
estradiol, and it diffuses out of the cell more
rapidly than estradiol. In the myometrium,
the activity of this enzyme is maximal during
the early secretory phase because of upregula-
tion by progesterone (Tseng and Gurpide
1973), resulting in a diminished estradiol
effect during the second half of the cycle. In
leiomyomas, on the other hand, the reduced
activity of 17β-hydroxysteroid dehydrogenase
may allow for the accumulation of estradiol in
the cells during the secretory as well as the
proliferative phase of the cycle, thus resulting
in continual stimulation by estrogen, with up-
regulation of both the ERs and PRs, accompa-
nied by the associated growth-promoting
effects. Whether the enzymatic deficiency is a
quantitative or qualitative one, and regardless
of whether it is a primary or secondary devel-
opment in the genesis of fibroids, the reduced
activity of this enzyme could play a significant
role in the pathogenesis of these tumors.
Estradiol 4-hydroxylase. Both estradiol and
estrone may be metabolized by irreversible
hydroxylation at several sites, including the C-2
and C-4 positions (forming catechol estrogens)
and the C-6, C-15, and C-16 positions. These
various hydroxylated metabolites may have
quite different biologic properties. For example,
the C-2 metabolites (the predominant form in
humans) have limited or no activity, whereas
the C-4 and C-16 metabolites possess potent
estrogenicity (Martucci and Fishman 1993).
For this reason, it is of great interest that the
mean rate of 4-hydroxylation of estradiol is 8-
fold higher than that of 2-hydroxylation in
myomas, and further, that 4-hydroxylation is
substantially elevated in myomas compared
with surrounding myometrial tissue (Liehr et al.
1995). Because the dissociation rate of 4-
hydroxyestradiol from the ER complex is also
reduced compared with estradiol (Zhu and
Conney 1998), this catechol metabolite may
also function as a long-acting estrogen, suggest-
ing that overexpressed 4-hydroxylase activity
may play a role in the etiology of uterine
fibroids (Liehr et al. 1995).
Effectors: Growth Factors and
The growth-promoting effects of estrogen
and progesterone upon the myometrium and
uterine myomas may be mediated through
the mitogenic effects of growth factors pro-
duced locally by smooth muscle cells and
fibroblasts (Mangrulkar et al. 1995; Rein and
Nowak 1992). Growth factors are polypep-
tides or proteins that are secreted by a num-
ber of cell types, have a wide range of biologic
effects, and generally act over short distances
either in an autocrine or paracrine manner
(Pusztai et al. 1993). They are essential ele-
ments in controlling the proliferation rate of
cells, and overexpression of either the growth
factor or its receptor may contribute to
tumorigenesis. Growth factors exert most of
their effects on target cells by interaction with
specific cell-surface receptors, with subsequent
message transmission via signal transduction
systems in the cell. Even in the physiologic
state, the cellular responses evoked by growth
factors are complex and dependent upon a
number of variables, including the cell type,
the differentiation stage of the cell, other
stimuli acting simultaneously upon the cell
(e.g., two growth factors together may have a
different effect than either one alone), and the
tendency for most growth factor receptors to
interact with an entire family of growth
factors (Pusztai et al. 1993).
Evidence for Regulation of Growth
Factors by Estrogens and Progestins
The evidence is 2-fold. First, several studies
have demonstrated increases or decreases in
production of particular growth factors in tis-
sue culture cell lines or laboratory animals
in vivo when given estrogen or progesterone
(Charnock-Jones et al. 1993; Cullinan-Bove
and Koos 1993; Fujimoto et al. 1997; Hyder
et al. 1996; Presta 1988; Reynolds et al. 1998;
Rider et al. 1997; Takahashi et al. 1994).
Second, there is the indirect evidence that cer-
tain growth factors or their receptors are
reduced in leiomyoma tissues from patients
who are hypoestrogenic because of treatment
with GnRH agonists (Lumsden et al. 1988;
Rein et al. 1990b).
Although acknowledging this evidence
that growth factors may be regulated by the
sex steroids and simply play the role of sec-
ondary effectors in fibroid tumorigenesis, we
cannot exclude the alternative possibility that
Review|Flake et al.
VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
abnormal expression of a growth factor or its
receptor could represent a primary event in
the genesis of these tumors.
Growth Factors Identified in Fibroids
Several growth factors and their receptors
have now been identified in both myo-
metrium and leiomyomas. Those that have
received the most attention in the literature
include transforming growth factor (TGF)-β,
bFGF, epidermal growth factor (EGF),
platelet-derived growth factor (PDGF), vascu-
lar endothelial growth factor (VEGF), and
insulin-like growth factor (IGF) (Table 3).
Each will be considered briefly in summary
Transforming growth factor-β. The
TGF-β superfamily includes more than 30
structurally related polypeptide growth factors
(Miyazono 2000), which are multifunctional
cytokines that can act both as inhibitors and
stimulators of cell replication (Arici and Sozen
2000). Within this large family of related fac-
tors is the TGF-β subfamily, which is com-
posed of three major isoforms (Massague
1998) of particular interest with regard to
fibroids, because they are capable not only of
promoting mitogenesis but also of upregulat-
ing the synthesis of many components of the
extracellular matrix, leading to fibrosis (Lyons
and Moses 1990). Both of these features are
characteristic of uterine fibroids. Expression of
all three types of TGF-β, as well as TGF-β
receptors I–III, has been detected in human
myometrial tissue (Chegini et al. 1994; Tang
et al. 1997). One study (Arici and Sozen
2000) found that the TGF-β3 mRNA levels
in leiomyomas were 3.5-fold higher than in
the myometrium, and similarly, Nowak
(2000) found TGF-β3 expression to be ele-
vated in leiomyomas compared with matched
myometrium. In contrast, no significant dif-
ference was observed between fibroids and
myometrium in TGF-β1 mRNA abundance
(Vollenhoven et al. 1995). Although these
data suggest that TGF-β3 could be important
in uterine leiomyoma growth by stimulating
cellular proliferation and the production of
extracellular matrix, the effects of TGF-β may
be either stimulatory or inhibitory, depending
upon multiple factors, including the specific
target cell, the concentration of TGF-β, and
the presence of other growth-regulatory mole-
cules. In low concentrations, both TGF-β1
(Battegay et al. 1990) and TGF-β3 (Arici and
Sozen 2000) have elicited significant increases
in smooth muscle cell proliferation, whereas at
higher concentrations this effect has not been
observed. Mitogenesis induced in cultures of
aortic smooth muscle cells by TGF-β appears
to be mediated indirectly through stimulation
of autocrine secretion of PDGF, whereas
higher concentrations of TGF-β result in
downregulation of PDGF receptors (Battegay
et al. 1990). An observed striking increase of
TGF-β3 mRNA levels in luteal phase leio-
myoma samples compared with those in the
follicular phase suggests a pivotal role of prog-
esterone in the regulation of TGF-β3 expres-
sion (Arici and Sozen 2000). In contrast, no
variation was observed in one study in the
expression of TGFβ mRNAs and proteins in
myometrial tissue during the menstrual cycle
(Chegini et al. 1994), and other investigators
concluded that TGF-βs had no significant
effect on myometrial cell proliferation (Tang
et al. 1997).
In view of the probable role of this growth
factor in fibroid pathophysiology, it is of par-
ticular interest that the gene coding for
TGF-β3 is located near the 14q23-24 break-
points (Andersen 1998), one of the most
common translocation sites identified in cyto-
genetic studies of fibroids.
Basic fibroblast growth factor. bFGF
causes proliferation of smooth muscle cells,
including leiomyoma and myometrial cells
(Stewart and Nowak 1996), and also pro-
motes angiogenesis. This factor can also bind
to a component of the extracellular matrix
(Dixon et al. 2000; Mangrulkar et al. 1995).
In one study there was much stronger
immunohistochemical staining for bFGF in
fibroids than in the myometrium because of
the large amount of extracellular matrix in
uterine myomata; this finding led the
authors to conclude that large quantities of
bFGF are stored in the extracellular matrix
of these tumors (Mangrulkar et al. 1995). In
addition, increased expression of bFGF
mRNA was found in the leiomyomas com-
pared with the myometrium. Some
immunoreactivity for the FGF type 1 recep-
tor in the extracellular matrix of leiomyomas
has been demonstrated, although the cellular
staining for the receptor was greater in the
myometrium than in the leiomyomas
(Anania et al. 1997).
Thus, apparently both TGF-β3 and bFGF
are overexpressed in leiomyomas compared
with matched myometrium, and both factors
may contribute to the enhanced growth of
leiomyomas. Indeed, Stewart and Nowak feel
that these two factors may be central to the
pathogenesis of uterine leiomyomas (Stewart
and Nowak 1998).
Epidermal growth factor. EGF is mitogenic
for the cells of both myometrium and leio-
myomas in tissue cultures (Fayed et al. 1989).
Equally important, and possibly a unique fea-
ture of this factor, is its apparent upregulation
in fibroids by progesterone (Maruo et al. 2000).
The concentration of EGF mRNA in leio-
myomas is similar to that of the myometrium
during the follicular phase but significantly ele-
vated in leiomyomas during the luteal phase,
whereas the concentration in the myometrium
remains essentially unchanged (Harrison-
Woolrych et al. 1994). Because the mitotic
activity of leiomyomas is maximal during the
luteal phase of the cycle, this finding suggests
that the production of EGF may be one mecha-
nism through which progesterone stimulates
mitotic activity in fibroids.
The mRNA for the EGF receptor has been
detected in both myometrial and leiomyoma
cells (Yeh et al. 1991). Although the levels of
EGF receptors are not significantly higher in
leiomyomas than in the myometrium and do
not seem to fluctuate during the menstrual
cycle (Chegini et al. 1986; Hofmann et al.
1984; Lumsden et al. 1988), there is a sharp
reduction of EGF-receptor levels in the
leiomyomas but not in the myometrium of
women treated with GnRH agonists prior to
surgery (Lumsden et al. 1988). These data sug-
gest that the EGF receptors in fibroids are
more sensitive to regulation by the ovarian sex
steroids than those in the myometrium. More
importantly, because the reduction of EGF
receptor levels correlates with shrinkage of the
fibroids as a result of the GnRH-agonist
Environmental Health Perspectives • VOLUME 111 | NUMBER 8 | June 2003
Table 3. Potential effectors and their receptors implicated in leiomyoma pathobiology.a
Yes, low concentration
Yes, in conjunction
with EGF or IGF
Arici and Sozen 2000
Mangrulkar et al. 1995
Harrison-Woolrych et al. 1994
Fayed et al. 1989
Harrison-Woolrych et al. 1995
Boehm et al. 1990
Vollenhoven et al. 1993
Nowak et al. 1999
aLists whether the factor is elevated in leiomyomas compared with myometrium, elevated during the luteal phase, and/or
associated with mitogenic activity.
therapy, it suggests that the effects of sex
steroids on fibroid growth may be mediated, in
part, by EGF (Rein and Nowak 1992). In this
regard, it is of interest that in cultures of
leiomyoma cells, estradiol augmented the
expression of the EGF receptor, whereas prog-
esterone increased the expression of EGF, sug-
gesting to the authors that estradiol and
progesterone may act in combination to stimu-
late proliferation in fibroids through the induc-
tion of EGF and its receptor (Maruo et al.
Platelet-derived growth factor. PDGF is a
potent mitogen for vascular smooth muscle
cells and another of the heparin-binding
growth factors along with bFGF and VEGF.
Because of the capacity of these factors to bind
to heparin, they may become sequestered in
the extracellular matrix, which is typically
abundant in fibroids and may therefore serve as
a reservoir for these growth factors (Nowak
1999). The mRNA for PDGF is expressed in
leiomyomas, but the levels are similar to those
found in the myometrium (Boehm et al.
1990). On the other hand, significantly more
PDGF receptor sites per cell are seen in
leiomyomas than in the myometrium,
although the PDGF receptor binding affinity
in the tumor cells is lower than that of the
myometrium (Fayed et al. 1989).
Perhaps the most interesting aspect of
PDGF in leiomyomas, however, may not be its
growth factor role, acting in isolation, but
rather its action in conjunction with other
growth factors such as EGF and IGFs. For
example, when myometrial cells are treated
with both PDGF and EGF, there is a synergis-
tic decrease in DNA synthesis, whereas treat-
ment of leiomyoma cells with both factors
results in an additive increase in DNA synthe-
sis (Fayed et al. 1989). Insulin and PDGF
exert an additive effect upon DNA synthesis in
myometrial and leiomyoma cells (Fayed et al.
1989); previous studies using other cell systems
have found that target cells must have prior
exposure to a competence growth factor such
as PDGF before IGF stimulation will promote
movement through the cell cycle (Pledger et al.
1978; Stiles et al. 1979).
Vascular endothelial growth factor. Five
VEGF isoforms have been identified (Neufeld
et al. 1999). All but one (VEGF-121) contain
heparin-binding regions that can mediate
binding to the extracellular matrix (Hyder
et al. 2000), which may thus serve as a reser-
voir for this factor as with the other heparin-
binding factors bFGF and PDGF. Although
VEGF seems to be a highly specific mitogen
for vascular endothelial cells, VEGF mRNA
and VEGF protein expression have now been
identified in the smooth muscle cells of both
myometrium and leiomyomata (Dixon et al.
2000; Harrison-Woolrych et al. 1995), and
VEGF receptors have been demonstrated in
the smooth muscle cells of the myometrium
(Brown et al. 1997). Leiomyomata apparently
do not have significantly different levels of
VEGF mRNA than the myometrium, do not
exhibit differences in VEGF mRNA levels
between the proliferative and secretory phases
of the cycle, and show similar levels of VEGF
mRNA after treatment with a GnRH analog
(Harrison-Woolrych et al. 1995).
Despite these findings, and evidence that
VEGF is not mitogenic to smooth muscle cells
(Ferrara et al. 1992), interest remains in the
potential role of this factor in fibroid growth,
for several reasons. VEGF stimulates angio-
genesis, which is essential for actively growing
tumors, and VEGF is the most potent agent
known for increasing capillary permeability,
which could enhance the growth of fibroids by
increasing their nutrient supply. VEGF could
also have an indirect effect by inducing the
proliferation of endothelial cells, which them-
selves produce a number of growth factors. In
addition, VEGF acts synergistically with
fibroblast growth factor (FGF) (Hyder et al.
2000), and it can release the angiogenic factor
bFGF from its storage on heparan sulfates of
the extracellular matrix (Jonca et al. 1997),
with the resulting combination of the two
angiogenic mitogens having a synergistic effect
on angiogenesis (Asahara et al. 1995; Goto
et al. 1993). Further, the resulting availability
of bFGF permits the expression of its mito-
genic effect upon the smooth muscle cells.
Insulin-like growth factor. The IGFs have
received considerable attention in the literature.
The family of IGFs consists of two IGFs (IGF-I
and IGF-II), two cell membrane receptors
(IGF-IR and IGF-IIR), and six IGF binding
proteins (Yu and Berkel 1999). Thus, the
actions of the IGFs are mediated through the
IGF receptors, primarily IGF-IR, and are regu-
lated by the IGF-binding proteins. The IGFs
are produced by most tissues of the body, are
abundant in the circulation, and have the
potential to act through endocrine, autocrine,
and paracrine mechanisms (Cohick and
Clemmons 1993). These factors are structurally
related to proinsulin and promote cellular pro-
liferation, differentiation, and cell survival
(Strawn et al. 1995; Yu and Berkel 1999).
Evidence exists for dissimilar roles of the two
IGFs, in that IGF-II appears to be primarily
responsible for the terminal differentiation of
skeletal muscle cells and the down-regulation of
IGF-I receptor gene expression, whereas IGF-I
is responsible for myogenesis (Rosenthal et al.
1994; Strawn et al. 1995). In most situations
the IGF binding proteins inhibit the actions of
IGFs by blocking their binding to the receptor;
in certain circumstances, however, these bind-
ing proteins may be able to enhance the action
of IGF-I by binding to it and preventing its
degradation, thereby increasing its bioavailabil-
ity in target tissues (Yu and Berkel 1999).
Several investigators have identified
mRNAs for IGF-I and IGF-II and their recep-
tors in both the myometrium and fibroid
tumors. IGF-I, but not IGF-II, was mitogenic
in leiomyoma cell cultures (Strawn et al.
1995). The levels of IGF-I mRNA were
reported higher in leiomyomas than in the
myometrium in two studies (Boehm et al.
1990; Hoppener et al. 1988), whereas two
other studies concluded that the levels were not
significantly different (Gloudemans et al.
1990; Vollenhoven et al. 1993). Increased
IGF-I peptide has been detected in some, but
not all, leiomyomata compared with
myometrium in immunohistochemical studies
(Dixon et al. 2000). The variation in relative
amounts of IGF-I mRNA reported in these
studies may have been due to the heterogeneity
that exists among fibroid tumors (Rein and
Nowak 1992). In three of these studies
(Boehm et al. 1990; Hoppener et al. 1988;
Vollenhoven et al. 1993) the mRNA levels of
IGF-II were higher in leiomyomas than in the
myometrium, whereas one study reported low
levels in both tissues (Gloudemans et al. 1990).
Giudice et al. (1993) found the IGF-I gene
expression to be most abundant in leiomy-
omata during the late proliferative phase of the
cycle, suggesting that estrogen upregulates this
growth factor in leiomyomas; on the other
hand, IGF-II gene expression did not vary with
the phase of the cycle.
Both IGFs can bind to the IGF-I receptor
with similar affinity, whereas the IGF-II
receptor preferentially binds IGF-II (Van der
Ven et al. 1997). The IGF-I receptor medi-
ates most of the biologic actions of both IGF-
I and IGF-II (Cohick and Clemmons 1993),
including the mitogenic, metabolic, and cell-
survival properties of IGFs through tyrosine
kinase signaling activity. The IGF-II/mannose
6-phosphate receptor appears to be a bifunc-
tional receptor serving as both a lysosomal
enzyme-targeting system and a suppressor of
the action of IGF-II by increasing its degrada-
tion (Nissley and Lopaczynski 1991; Oates
et al. 1998). The levels of IGF-I receptors in
leiomyomas have been reported to exceed
those of the myometrium in three studies
(Chandrasekhar et al. 1992; Tommola et al.
1989; Van der Ven et al. 1997), whereas
Chandrasekhar et al. found no difference in
the levels of the IGF-II receptors. The levels
of neither IGF-I nor IGF-II receptors seem to
vary with the stage of the menstrual cycle
(Giudice et al. 1993).
The conclusion of most of these studies has
been that IGF-I may play a mitogenic role in
the growth of uterine fibroids because of
increased levels of IGF-I receptors and overex-
pression of the growth factor itself. Lower levels
of the IGF binding protein-3 in leiomyomas
than in myometrium could also be significant,
as this would increase the bioavailability of free
Review|Flake et al.
VOLUME 111 | NUMBER 8 | June 2003 • Environmental Health Perspectives
bioactive IGF in fibroids (Vollenhoven et al.
Prolactin. Although initially identified as
a pituitary gland hormone, several studies
have demonstrated that prolactin is also pro-
duced by uterine tissues, including the
endometrium, myometrium, and uterine
leiomyomas (Daly et al. 1984; Maslar and
Riddick 1979; Walters et al. 1983). The sig-
nificance of prolactin production in leiomy-
omas is not yet well defined; however, interest
in this hormone has been stimulated by the
finding that prolactin acts as a mitogen for
vascular smooth muscle (Sauro and Zorn
1991). In addition, in one study of myome-
trial and leiomyoma explant cultures, fibroid
prolactin secretion was substantially greater
than myometrial prolactin secretion (Rein
et al. 1990a). On the other hand, Daly et al.
found that estrogen enhanced the secretion of
prolactin in fibroid tissue cultures, whereas
progesterone exhibited a suppressive effect
(Daly et al. 1984). Because leiomyomas are
mitotically active during the luteal phase, the
inhibition of leiomyoma prolactin production
by progesterone tends to cast some doubt
upon the role of this hormone in fibroid
growth. However, in a recent study, treat-
ment of leiomyoma and myometrial cell cul-
tures with a prolactin-neutralizing antibody
inhibited cell proliferation, leading the
authors to conclude that prolactin may be an
autocrine or paracrine growth factor for both
leiomyoma and myometrial cells (Nowak
et al. 1999). At this date, it would seem that
the prolactin story is unfinished, evolving,
and worthy of further study.
Summary of Growth Factors
From this brief review of the major growth
factors identified in fibroids thus far, we can
surmise that multiple growth factors are prob-
ably important in the pathogenesis of these
tumors. Different growth factors could play a
role at different stages of the disease
(Newbold et al. 2000). Many of the factors
may interact, sometimes resulting in a syner-
gistic effect, as demonstrated by the two
angiogenic mitogens VEGF and bFGF. In
other situations, the effect of one growth fac-
tor is dependent upon the presence of
another, exemplified by IGF-I acting as a pro-
gression factor in the cell cycle when compe-
tence factors such as PDGF and FGF are also
present (Cohick and Clemmons 1993), and
by the indirect mitogenic effect of TGF-β
resulting from the stimulation of PDGF
secretion (Battegay et al. 1990).
In this overview of the etiology and pathogen-
esis of uterine fibroids, we have attempted to
analyze the literature and present prevailing
evidence and opinions. Although research in
this area has been lacking in the past, much
has been learned about this extremely com-
mon public health problem during the last 20
years (McBride 1999; Newbold et al. 2000).
We briefly summarize some of these data in
the following conclusions:
• Risk factors for fibroids may achieve signifi-
cance through their contribution to either
the initiation or promotion phases of
tumorigenesis. Although their impact often
appears related to their effect upon estrogen
and progesterone, other mechanisms may be
involved. For example, early menarche
increases the overall estrogen exposure, but
also involves more menstruations with their
concomitant tissue damage. Two of the
more consistent risk factors that have been
identified are age (late reproductive years)
and African-American ethnicity. The effect
of age may reflect more opportunity for dys-
regulated cells to be produced or, alterna-
tively, a prolonged period for growth under
the hormonal influences of the reproductive
years. Why African-American women are at
higher risk for clinically significant fibroids
is not known, but apparent metabolic dif-
ferences could increase the estrogenic pro-
motional effect, such as the predilection for
the 16α hydroxylation of estradiol meta-
bolic pathway. Increased risk has also been
associated with early menarche, nulliparity,
and obesity, whereas decreased risk has been
found with increasing parity and smoking.
On the basis of clinical reports, tamoxifen
also appears to be a risk factor.
• Karyotypic abnormalities have been identi-
fied in approximately 40% of surgically
removed uterine leiomyomas. The most
common of these are the translocation
t(12;14) and the deletion of 7q; however,
these abnormalities do not exclude submicro-
scopic mutations of a more universal nature,
which will require molecular demonstration.
There may be more than one genetic path-
way to the formation of fibroids. Phenotypic
fibroid variants are probably related to chro-
mosomal differences, either from the outset
or as a result of clonal evolution.
• Estrogen and progesterone appear to be pro-
moters of fibroid growth, acting in concert.
Thus, estrogen upregulates both ERs and
PRs during the follicular phase, followed by
progesterone-induced mitogenesis during
the luteal phase. The deficiency of the estro-
gen-metabolizing enzyme 17β-hydroxy-
steroid dehydrogenase in fibroids may be
responsible for the accumulation of estradiol
in these tumors and its consequent growth-
promoting effects. Likewise, the overexpres-
sion of estradiol 4-hydroxylase seems highly
significant, as the resulting metabolite pos-
sesses long-acting estrogenic activity.
• The levels of several growth factors and their
receptors are increased in fibroids. TGF-β3
and bFGF may be especially important in the
pathogenesis of these tumors in view of their
combined mitogenic effect and promotion of
extracellular matrix production. EGF appears
to be significant, as it is the only character-
ized growth factor, other than TGF-β3, with
elevated expression during the luteal phase,
when leiomyoma mitotic activity is maximal.
IGF-I almost certainly plays an important
role because of its potent mitogenic capacity
and the overexpression of both the peptide
and its receptor in leiomyomas. Growth fac-
tors may be the mediators or effectors of sex
steroid upregulation, but a primary dysregu-
lation of one or more growth factors must
also be considered.
• Finally, the most important piece of the
fibroid puzzle, the initiator(s), remains
unsolved. Further elucidation of the genetic
and molecular changes will provide insights
into the pathobiology of these tumors and
may offer clues to initiating conditions
responsible for such changes. Considering
the extremely high incidence of fibroids, evi-
dently within all races and all geographic
areas of the world, we believe that the initiat-
ing conditions must be common to most or
all women. Because the presence of fibroids
offers no known advantages to affected
women, but rather considerable morbidity in
many cases, one is challenged to fathom the
evolutionary basis for the development of an
organ so prone to tumor formation, albeit
benign tumors. Perhaps there are conditions
existing today that significantly impact uter-
ine physiology that were not prevalent in
antiquity. There can be little doubt that
women in the past experienced fewer men-
strual periods because of their shorter life
spans, more demanding physical conditions,
and prolonged breastfeeding. Under such
circumstances the presumed reproductive
advantages offered by a hemochorial pla-
centa (Campbell and Cameron 1998) and
the occasionally menstruating uterus might
have been enjoyed, with only the exceptional
disadvantage of rarely developing uterine
leiomyomas. Other changes in modern
lifestyle, such as dietary shifts to a higher-fat,
lower-fiber diet and the potential impact of
environmental estrogens, could also be sig-
nificant because of increased estrogen
On the basis of our current state of
knowledge, we can only speculate upon the
initiators of this common condition. Future
research efforts may provide a better under-
standing, however, of the causes and mecha-
nisms of uterine fibroid tumorigenesis.
Insights resulting from elucidation of the
basic biology of these tumors might then be
successfully translated into preventative strate-
gies that will reduce the incidence and/or
morbidity of this disease.
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