Sex Differences in the Expression of Hepatic Drug
David J. Waxman and Minita G. Holloway
Division of Cell and Molecular Biology, Department of Biology, Boston University, Boston, Massachusetts
Received April 1, 2009; accepted May 29, 2009
Sex differences in pharmacokinetics and pharmacodynamics
characterize many drugs and contribute to individual differ-
ences in drug efficacy and toxicity. Sex-based differences in
drug metabolism are the primary cause of sex-dependent phar-
macokinetics and reflect underlying sex differences in the ex-
pression of hepatic enzymes active in the metabolism of drugs,
steroids, fatty acids and environmental chemicals, including
cytochromes P450 (P450s), sulfotransferases, glutathione
transferases, and UDP-glucuronosyltransferases. Studies in
the rat and mouse liver models have identified more than 1000
genes whose expression is sex-dependent; together, these
genes impart substantial sexual dimorphism to liver metabolic
function and pathophysiology. Sex differences in drug metab-
olism and pharmacokinetics also occur in humans and are due
in part to the female-predominant expression of CYP3A4, the
most important P450 catalyst of drug metabolism in human
liver. The sexually dimorphic expression of P450s and other
liver-expressed genes is regulated by the temporal pattern of
plasma growth hormone (GH) release by the pituitary gland,
which shows significant sex differences. These differences are
most pronounced in rats and mice, where plasma GH profiles
are highly pulsatile (intermittent) in male animals versus more
frequent (nearly continuous) in female animals. This review
discusses key features of the cell signaling and molecular reg-
ulatory mechanisms by which these sex-dependent plasma GH
patterns impart sex specificity to the liver. Moreover, the es-
sential role proposed for the GH-activated transcription factor
signal transducer and activator of transcription (STAT) 5b, and
for hepatic nuclear factor (HNF) 4?, as mediators of the sex-
dependent effects of GH on the liver, is evaluated. Together,
these studies of the cellular, molecular, and gene regulatory
mechanisms that underlie sex-based differences in liver gene
expression have provided novel insights into the physiological
regulation of both xenobiotic and endobiotic metabolism.
Individual differences in drug metabolism and pharmaco-
kinetics contribute to the individual-to-individual variation
that characterizes the responses to many drugs and other
foreign chemicals and presents a major challenge in clinical
pharmacology. Individual variation in the expression of ma-
jor drug-metabolizing enzymes (DMEs), including cyto-
chromes P450 (P450s), sulfotransferases, glutathione trans-
ferases, and UDP-glucuronosyltransferase, is associated
with substantial individual differences in the bioavailability
and clearance of drugs and other xenobiotics. Given the cru-
cial role of hepatic DMEs in regulating the pharmacological
and biological activity of drugs as well as steroid and other
endobiotics, it is important to understand the regulatory
features that lead to individual differences in the expression
of DMEs. Factors that contribute to interindividual differ-
ences in DME expression and drug metabolism include ge-
netic polymorphisms (Hines et al., 2008), prior or concomi-
tant exposure to drugs and environmental chemicals
(Urquhart et al., 2007), dietary factors (Moon et al., 2006;
Murray, 2007), pregnancy (Anderson, 2005a), diseased states
(Mann, 2006), epigenetic factors (Szyf, 2007), and endoge-
nous hormonal factors, which change with age and differ
between male and female subjects (Cotreau et al., 2005;
Genetic polymorphisms of DMEs influence the clinical out-
come of an estimated 20 to 25% of all drug therapies (In-
This work was supported in part by the National Institutes of Health
National Institute of Diabetes and Digestive and Kidney Diseases [Grant
Article, publication date, and citation information can be found at
ABBREVIATIONS: DME, drug-metabolizing enzyme; P450, cytochrome P450; GH, growth hormone; IGF, insulin-like growth factor; STAT, signal
transducer and activator of transcription; JAK, Janus kinase; HNF, hepatic nuclear factor.
Copyright © 2009 The American Society for Pharmacology and Experimental Therapeutics
Mol Pharmacol 76:215–228, 2009
Vol. 76, No. 2
Printed in U.S.A.
gelman-Sundberg, 2004; Eichelbaum et al., 2006). Several
key drug-metabolizing human P450 genes are highly poly-
morphic, with more than 350 distinct alleles identified for the
57 known human P450 genes. A well studied example is
CYP2D6, where genetic polymorphisms contribute to large
interindividual variability in the metabolism of many anti-
drugs (Ingelman-Sundberg et al., 2007). Interindividual dif-
ferences also arise from the induction of P450s and other
DMEs after exposure to lipophilic drugs and environmental
chemicals. For example, phenobarbital and other drugs can
increase the rates of metabolism of many drugs and chemi-
cals by increasing the expression of individual P450 genes
(Waxman and Azaroff, 1992). This P450 induction response
is mediated by ligand-activated transcription factors (nuclear
receptors) that function as sensors of foreign compounds and
can activate a large number of DME genes in liver and other
tissues (Waxman, 1999; Handschin and Meyer, 2003; Timsit
and Negishi, 2007), with the associated potential for drug
interactions (Jana and Paliwal, 2007; Sinz et al., 2008). The
xenobiotic-activated nuclear receptors constitutive andro-
stane receptor, pregnane X-receptor, and peroxisome prolif-
erator-activated receptor-? respond to a wide range of xeno-
chemicals and induce the expression of CYP2B, CYP3A, and
CYP4A genes, respectively. Aryl hydrocarbon receptor is a
receptor/transcription factor that induces CYP1 family mem-
bers and certain other DME genes upon binding polycyclic
aromatic hydrocarbons and other drugs and environmental
chemicals (Ramadoss et al., 2005). Hepatic expression of
DMEs can also be altered by pathophysiological conditions
such as diabetes, inflammation, alcohol consumption, and
protein-calorie malnutrition (Kim and Novak, 2007; Morgan
et al., 2008). In addition, circadian factors regulate the ex-
pression of certain DMEs (Gachon et al., 2006) and contrib-
ute to the dosing time-dependence of drug activity and tox-
icity (Levi and Schibler, 2007).
Sex-based differences1in pharmacokinetics and pharma-
codynamics are well recognized (Fletcher et al., 1994; Gandhi
et al., 2004; Franconi et al., 2007) and can be an important
source of individual differences in drug responses. Sex-based
differences in pharmacokinetics reflect differences in bio-
availability, distribution, metabolism, and/or excretion. Sex
hormones influence bioavailability through effects on gastro-
intestinal motility; estrogen inhibits gastric emptying. Sex
differences in pharmacokinetics can result from sex differ-
ences in distribution, which can be caused by differences in
body weight (lower in women), body fat (higher in women),
plasma volume (lower in women, but varies through the
menstrual cycle and during pregnancy), and organ blood flow
(higher in women). In addition, body-weight effects on glo-
merular filtration rate can contribute to lower renal clear-
ance in women in the case of drugs that are actively elimi-
nated via the kidney (Gandhi et al., 2004). However, sex
differences in metabolism are thought to be the primary
determinant of sex differences in pharmacokinetics. Male-
female differences in the metabolism of barbiturates were
documented in rats as early as 1932 (Nicholas and Barron,
1932) and were later found to characterize many drugs and
steroids, as shown in both rat and mouse models (Kato, 1974;
Skett, 1988). These sex differences in metabolism are now
known to reflect sex differences in the expression of specific,
individual hepatic P450 enzymes and other DMEs (Shapiro
et al., 1995). Sex-linked differences in P450-dependent drug
and steroid metabolism are also found in hamster (Teixeira
and Gil, 1991; Sakuma et al., 1994), dog (Lin et al., 1996; Hay
Kraus et al., 2000), chicken (Pampori and Shapiro, 1993), and
fish (Gray et al., 1991; Lee et al., 1998). Sex differences in
human hepatic P450-catalyzed drug metabolism are well
documented, but are less dramatic than in the rat (Tanaka,
1999; Parkinson et al., 2004; Scandlyn et al., 2008). This
review discusses some of the pharmacological consequences
of the observed sex differences in drug metabolism. The role
of hormones, in particular GH, in establishing and maintain-
ing these sex differences is discussed, and recent mouse and
rat model studies that elucidate the underlying cellular sig-
naling and molecular regulatory mechanisms by which pitu-
itary GH secretory profiles dictate sex-based differences in
DME expression in the liver are evaluated, focusing on fac-
tors that regulate the transcription of genes that code for
Sex Differences in Hepatic Drug Metabolism
Sex-based differences characterize the metabolism of many
drugs commonly used in humans (Gandhi et al., 2004; Ander-
son, 2005b; Schwartz, 2007); 6 to 7% of new drug applications
that include a sex analysis show at least 40% difference in
pharmacokinetics between men and women (Anderson,
2005b). For example, acetaminophen clearance rates are 22%
higher in men than in women because of a higher rate of
glucuronidation (Miners et al., 1983). The higher bioavail-
ability of aspirin in women than in men has been linked to
decreased conjugation with glycine and glucuronic acid
(Franconi et al., 2007). Men show a higher rate of clearance
of the benzodiazepines diazepam, chlordiazepoxide, and olan-
zapine (MacLeod et al., 1979; Roberts et al., 1979; Bigos et
al., 2008). Likewise, women display greater sensitivity to
diazepam, which can lead to impairment of psychomotor
skills (Palva, 1985). CYP3A4, the predominant cytochrome
P450 catalyst of oxidative metabolism in human liver, is
expressed at a higher protein and mRNA level in women
than in men (Hunt et al., 1992; Wolbold et al., 2003; Dicz-
falusy et al., 2008). Accordingly, certain CYP3A4 drug sub-
strates, including cyclosporine (Kahan et al., 1986), erythro-
mycin (Austin et al., 1980), and nifedipine (Krecic-Shepard et
al., 2000), show higher clearance rates in women. CYP3A4-
catalyzed hepatic microsomal N-dechloroethylation of the an-
ticancer drug ifosfamide is also more rapid in women, sug-
gesting that women could be more susceptible to the
neurotoxic side affects associated with this metabolic path-
way (Schmidt et al., 2001). CYP3A4 also metabolizes ste-
roids, such as cortisol, whose conversion to 6?-hydroxycorti-
sol is more rapid in women than in men and serves as a
biomarker for CYP3A4 metabolic activity (Inagaki et al.,
Higher CYP2B6 activity has been observed in women than
in men of Hispanic origin (Lamba et al., 2003). There is also
evidence for higher CYP2A6 activity in women (Sinues et al.,
1Although the term “gender differences” is often used in the medical liter-
ature to describe male-female differences, “sex” is the preferred term when
describing biologically determined physiological, or pathophysiological, differ-
ences between male and female members of a species. “Gender” is a social
construct that refers to an individual’s self-representation as masculine or
feminine, which, unlike “sex,” can be influenced by cultural factors (Gray,
Waxman and Holloway
2008), although that could, in part, reflect the use of estro-
gen-containing contraceptives, given the estrogen respon-
siveness of the human CYP2A6 gene (Higashi et al., 2007).
CYP1A2 substrates, including caffeine and the antipsychotic
agents olanzapine and clozapine, show higher clearance
rates in men (Schwartz, 2007). As a result, women show
greater improvement in psychotic symptoms but suffer a
greater number of adverse side effects after treatment with
these drugs. Men also show more rapid clearance of CYP2E1
substrates (e.g., chlorzoxazone) and certain CYP2D6 sub-
strates, including propanolol, metoprolol, dextromethorphan,
desipramine, and mirtazapine (Franconi et al., 2007;
(UGT2B15 activity with oxazepam as substrate) also charac-
terize male compared with female livers (Court et al., 2004).
Overall, women are more likely to experience adverse drug
reactions compared with men, due in part to sex differences
in metabolic activity (Rademaker, 2001), with notable effects
for cancer chemotherapeutic drugs (Mader, 2006; Wang and
Huang, 2007) and antiretroviral agents (Ofotokun, 2005).
Studies carried out in rats and mice have established that
sex-based differences in liver function also characterize many
phase II DMEs, including sulfotransferases (Klaassen et al.,
1998; Clodfelter et al., 2006; Kocarek et al., 2008), glutathi-
one transferases (Srivastava and Waxman, 1993; Clodfelter
et al., 2006; Knight et al., 2007), and UDP-glucuronosyltrans-
ferases (Takeuchi et al., 2004; Clodfelter et al., 2006; Buckley
and Klaassen, 2007). In addition to drugs and other xenobi-
otics, P450s and other DMEs metabolize endogenous sex
steroids (Waxman, 1988; Song and Melner, 2000; You, 2004;
Chouinard et al., 2008), suggesting that the sex differentia-
tion of drug metabolism reflects a need for sex-specific steroid
metabolism. The sexual dimorphism of liver gene expression
is not confined to DMEs; it is quite extensive and affects more
than 1000 individual genes in both rats and mice (Clodfelter
et al., 2006, 2007; Yang et al., 2006; Wauthier and Waxman,
2008). These include plasma lipoproteins (Oscarsson et al.,
1991; Rudling et al., 1992), pheromone binding proteins
(Johnson et al., 1995), regulators of fatty acid homeostasis
(Amador-Noguez et al., 2005; Cheung et al., 2007), nuclear
receptors (such as peroxisome proliferator-activated recep-
tor-? and prolactin receptor) (Robertson et al., 1990; Jalouli
et al., 2003), and other transcription factors, including sterol
regulatory element binding protein (Lahuna et al., 1997;
Ame ´en et al., 2004; Laz et al., 2007). The extent to which sex
differences occur in human liver is largely unknown; how-
ever, sex differences in genes such as these are likely to
contribute to the observed sex differences in hepatic patho-
physiology (Yokoyama et al., 2005), including differences in
susceptibility to hepatocellular carcinoma seen both in ani-
mal models and in the clinic (Rogers et al., 2007; Shimizu et
Gonadal and Pituitary Hormonal Determinants of Hepatic
Early studies carried out in rats demonstrated that sex-
based differences in P450 activities were abolished by cas-
tration, were re-established by testosterone replacement
(Kato and Onoda, 1970), and were reversed by treatment
with estradiol (Kramer et al., 1978). It was initially pre-
sumed that the gonadal hormones exerted their effects on
steroid and drug metabolism via direct actions on the liver.
However, an intact pituitary gland was later found to be
required for testosterone and estradiol to regulate hepatic
corticosterone metabolism, a sexually dimorphic metabolic
activity (Colby et al., 1973). Several other groups later con-
firmed the requirement of an intact pituitary gland for go-
nadal hormones to regulate sex-dependent drug metabolism
(Denef, 1974; Gustafsson and Stenberg, 1974; Lax et al.,
1974). Further investigations implicated GH, a 191-amino
acid protein hormone secreted by the anterior pituitary
gland, in mediating the effects of gonadal hormones on liver
drug and steroid metabolism. Wilson first observed that GH
treatment of male but not female rats suppressed total P450
protein and drug metabolic activity (Wilson, 1973; Wilson
and Bass, 1973). These effects of GH were later shown by
Kramer et al. (1978) and Rumbaugh and Colby (1980) to
mimic those of estradiol and were manifested in testosterone-
treated female rats, establishing a clear link between sex
steroids and the responsiveness of hepatic drug metabolism
to GH treatment.
A key finding was the subsequent discovery by Ede ´n (1979)
that GH was secreted by the pituitary gland in a sexually
dimorphic manner: in adult male rats, GH release into the
bloodstream occurs in discrete pulses every ?3.5 h, with little
or no GH detectable between pulses. In adult female rats,
however, GH secretion is more frequent, resulting in a more
continuous presence of GH in circulation, albeit at a much
lower level than peak male levels (Tannenbaum and Martin,
1976; Ede ´n, 1979). Mice also display sex differences in the
ultradian pattern of GH secretion, male mice having less
frequent GH pulses and longer baseline intervals compared
with female mice (MacLeod et al., 1991). These sex-depen-
dent patterns of pituitary GH release are set (“imprinted”) by
the action of gonadal hormones on the hypothalamus during
the neonatal period, are manifested at puberty, and continue
into adulthood (Jansson et al., 1985). The sexually dimorphic
expression of liver steroid metabolic activity was later shown
to be determined by the temporal pattern of GH stimulation:
continuous but not pulsatile GH treatment restored female
liver enzyme profiles in hypophysectomized female rats,
whereas estrogen treatment of intact male rats feminized
liver enzyme profiles in association with a feminization of the
plasma GH profile (Mode et al., 1982). Subsequent studies by
many laboratories demonstrated that 1) the differential ef-
fect of pulsatile versus continuous GH administration on
hepatic drug and steroid metabolism (mimicking male- and
female-like plasma GH patterns, respectively) is due to the
action of GH on individual P450s and other sex-specific liver
genes, and 2) these regulatory responses are largely deter-
mined at the level of gene transcription (Mode and Gustafs-
son, 2006; Waxman and O’Connor, 2006).
Humans also display plasma GH patterns that are sex-
dependent and gonadal hormone-regulated (Veldhuis, 1998;
Veldhuis and Bowers, 2003), with sex-dependent effects on
certain human DMEs, albeit much smaller than those seen in
rats and mice. CYP3A4 drug metabolic activity is increased
in acromegalic patients and in men treated with GH-releas-
ing hormone every 2 h (Watkins et al., 1993). In other stud-
ies, GH treatment of elderly men using a once-daily treat-
ment schedule led to induction of CYP1A2 and, to a lesser
extent, inhibition of CYP2C19 metabolic activity, but no ef-
fects were seen on CYP2D6 and CYP3A4 activity (Ju ¨rgens et
al., 2002). However, in a separate study that investigated the
Sex Differences in Liver DMEs
pattern-dependence of GH responses in GH-deficient male
and female patients, pulsatile GH was found to be more
effective than continuous GH treatment with respect to stim-
ulation of osteocalcin (a GH-responsive marker of bone for-
mation) and down-regulation of CYP1A2 metabolic activity,
whereas several other GH targets (serum IGF-1, serum IGF
binding protein-3, and hepatic CYP3A4 activity) were in-
creased more effectively using a continuous GH treatment
schedule (Jaffe et al., 2002). Thus, pulsatile and continuous
GH treatment can induce distinct responses of individual GH
target genes in humans, in a manner similar to that in the
rat and mouse models.
GH regulation of CYP3A4 can be recapitulated in primary
human hepatocyte cultures, where CYP3A4 protein and
mRNA can be induced by continuous GH treatment and
suppressed by intermittent (pulsatile) GH (Liddle et al.,
1998; Dhir et al., 2006). Moreover, as discussed elsewhere
(Waxman and O’Connor, 2006), in transgenic mouse lines
that contain the complete CYP3A4 gene, including 5? and 3?
flanking DNA and the associated cis regulatory sequences,
hepatic CYP3A4 expression shows a pattern of postnatal
developmental regulation and adult sex specificity indistin-
guishable from that of several endogenous female-specific
mouse Cyp3a gene products. Thus, hepatic CYP3A4 protein
and RNA are expressed in immature male transgenic mice
but are strongly down-regulated and become undetectable by
6 weeks of age, whereas in female mice, the CYP3A4 trans-
gene is expressed in both immature and adult livers. Fur-
thermore, continuous GH treatment of the transgenic male
mice substantially increased liver CYP3A4 protein and RNA
(Cheung et al., 2006). Thus, the human CYP3A4 gene con-
tains all of the DNA sequence elements required to respond
to the endogenous mouse hormonal environment, leading to a
pattern of postnatal developmental regulation, adult sexual
dimorphism, and plasma GH-responsiveness very similar to
that of the endogenous female-specific, GH-regulated mouse
Cyp3a genes. Sex-specificity and GH regulation have also
been observed for CYP2C18 and CYP2C19 in a transgenic
mouse liver model, where introduction of these tandem hu-
man genes together with flanking regulatory sequences re-
sults in hepatic expression that is male-specific and sup-
pressed by continuous GH treatment (Lo ¨fgren et al., 2008).
Further investigation will be required to ascertain the rele-
vance of these findings in transgenic mice to the expression of
these P450 genes in humans, which differ from mice in terms
of their plasma GH profiles and other physiological factors.
Sex differences in DME expression have been observed in
the kidney, where testosterone rather than GH is the proxi-
mal stimulus of male-specific gene expression. For example,
the male-specific expression of rat CYP4A2 is stimulated by
GH pulses in the liver but by testosterone in the kidney
(Sundseth and Waxman, 1992). This, together with similar
findings in studies of other DMEs in a mouse model (Hen-
derson and Wolf, 1991), indicates that, in the kidney, male-
specific DMEs are regulated by androgen receptor-dependent
mechanisms, rather than by GH-dependent mechanisms. It
is conceivable that a direct androgen-stimulatory mechanism
might be ineffective in hepatocytes, where androgen receptor
levels are generally low and high steroid metabolic activity
may lead to rapid and extensive inactivation of androgens.
Class I and Class II Sex-Specific Genes
Male-specific genes can be classified based on their re-
sponse to pituitary hormone ablation. Class I male-specific
genes require pituitary hormones (principally GH) for full
expression and are therefore decreased in expression after
hypophysectomy, whereas class II male-specific genes are
primarily regulated by the repressive actions of the female
GH pattern in both rats (Table 1) (Wauthier and Waxman,
2008) and mice (Table 2; V. Wauthier and D. J. Waxman,
unpublished observations). Thus, plasma GH pulses posi-
tively regulate class I male-specific P450 genes, whereas
class II male-specific P450 genes do not require GH; indeed,
they are expressed in liver at their highest levels when GH is
ablated by hypophysectomy (Table 1). Rat CYP2C11 is a
prototypic class I male-specific gene: it is induced after 4
weeks of age by the male, pulsatile GH secretion pattern,
whereas a more continuous, female-like plasma GH profile
abolishes CYP2C11 expression (Fig. 1 and Table 1). A GH-
free interpulse interval of at least 100 to 140 min is required
for full expression of CYP2C11 in male rat liver (Waxman et
al., 1991; Agrawal and Shapiro, 2001), whereas GH ablation
in male rats decreases CYP2C11 expression to 25 to 30% of
intact male levels (Morgan et al., 1985; Janeczko et al., 1990).
Class II male-specific genes include rat genes CYP2A2,
CYP2C13, and CYP3A2 (Yamazoe et al., 1986; Waxman et
al., 1988; McClellan-Green et al., 1989). These liver-ex-
pressed genes are not down-regulated by hypophysectomy;
i.e., they do not require male GH pulse stimulation. Most
strikingly, class II male-specific genes are up-regulated to
near normal adult male liver levels in hypophysectomized
female rats (Yamazoe et al., 1986; Waxman et al., 1988;
McClellan-Green et al., 1989), evidencing the strong suppres-
sive action of the female pituitary hormone profile (Pampori
Classification of sex-specific rat liver P450 genes and other genes
Genes are classified based on their response to hypophysectomy (Hx), pulsatile GH treatment of Hx rats, and continuous GH infusion in intact male rats. Male class II genes
are expressed at high levels in both male and female Hx rats; they are subject to partial repression by male GH pulses (Wauthier and Waxman, 2008) and nearly complete
repression by continuous GH treatment. Male class I genes, such as CYP2C11, are positively regulated by plasma GH pulses but are also repressed by continuous GH
treatment, hence their increase in expression in Hx female liver to a level similar to that found in Hx male liver.
Gene Class Prototypic Rat Genes
Loss of GH GH Treatment
Male HxFemale HxGH Pulses GH Continuous
CYP2A2, CYP2C13, CYP3A2
CYP2C12, HNF6, A1BG
1, increase in gene expression compared with untreated controls; 2, decrease in gene expression compared with untreated controls; –, no major changes in expression.
Waxman and Holloway
and Shapiro, 1999). Male GH pulses also repress the expres-
sion of class II male P450 genes (Agrawal and Shapiro, 2000)
in a process that is rapid (detected within 30 min), as re-
vealed by analysis of the effects of a single short-term GH
pulse on hepatic levels of the primary, unspliced transcripts
(hnRNAs) of CYP2A2 and CYP2C13 in hypophysectomized
male rat liver (Wauthier and Waxman, 2008). These latter
findings suggest that the transcription of class II male-spe-
cific genes is restricted to the GH-free interpulse interval in
adult male rat liver.
Female-specific genes can also be classified into sets of
class I and II genes based on their response to hypophysec-
tomy and GH replacement (Wauthier and Waxman, 2008).
The steroid sulfate 15?-hydroxylase CYP2C12 is a class I
female-specific gene; it requires continuous exposure to GH
to restore its expression in hypophysectomized rat liver (Mac-
Geoch et al., 1985; Pampori and Shapiro, 1996). It can also be
induced to near-female levels by continuous GH infusion in
intact male rats (Fig. 1B and Table 1). CYP2C12 is induced in
intact female but not male rat liver at puberty, at the time
when strong pituitary GH secretion commences, reflecting
the requirement for continuous GH stimulation for high level
expression (Fig. 1D). ADH4 and IGFBP1 are examples of
class II female-specific genes: they are strongly derepressed
(i.e., up-regulated) in both male and female liver after hy-
pophysectomy, with a loss of sex specificity, and they are
rapidly suppressed by physiological GH pulse replacement
(Wauthier and Waxman, 2008). Continuous GH infusion in
hypophysectomized female rats at doses that yield only ?3%
of the mean female plasma GH concentration (?40 ng/ml)
induces female P450 genes and suppresses both class I and II
male P450 genes (Pampori and Shapiro, 1996). The effective-
ness of such low GH concentrations is consistent with the low
Kdfor GH binding exhibited by GH receptor, ?2 ng/ml (Fuh
et al., 1992), which corresponds to only ?1% of the peak
plasma GH concentration in male rats (? 200 ng/ml).
Sex differences in the intrinsic responsiveness of the liver
to continuous GH are apparent, with the effects of continuous
GH on liver P450 expression being less complete and/or re-
quiring a higher GH dose in a male hypophysectomized rat
model compared with a female hypophysectomized rat model
(Pampori and Shapiro, 1999). These intrinsic sex differences
in continuous GH responsiveness can be recapitulated in
primary hepatocyte cultures (Thangavel et al., 2004). Like-
wise, male hepatocytes are more responsive than female
hepatocytes to GH pulses applied in culture (Thangavel et
al., 2006), which could reflect elevated levels of the JAK2-
STAT5 signaling inhibitors CIS and SOCS2 (Ram and Wax-
man, 1999; Ram and Waxman, 2000) in the female-derived
liver cells (Thangavel and Shapiro, 2007). Epigenetic mech-
anisms, such as DNA methylation and histone methylation,
could underlie these sex differences, reflecting a memory of
the sexual differentiation of intact liver in vivo. Alterna-
tively, these sex differences could be genetically determined
[e.g., by Y-chromosome-encoded genes, several of which show
strong expression in liver (Clodfelter et al., 2006)] and could
potentially modulate responsiveness to GH stimulation.
Requirement of STAT5b for the Sex-Dependent Actions of
GH activates multiple intracellular signaling pathways
(Herrington and Carter-Su, 2001), one of which involves
STAT5b, a GH-responsive transcription factor that has been
implicated in many of the sex-dependent actions of GH on the
liver (Waxman and O’Connor, 2006). GH signaling is initi-
ated at the cell surface, where GH binds to its receptor and
induces a conformational change that activates Janus kinase
2 (JAK2), a GH receptor-associated tyrosine kinase. The ac-
tivated JAK2 phosphorylates GH receptor on several intra-
cellular tyrosine residues, thereby forming docking sites for
STAT5b and other downstream signaling molecules that
have SH2 (phosphotyrosine-binding) domains. JAK2 subse-
quently phosphorylates the GH receptor-bound STAT5b on
tyrosine 699, which leads to STAT5b dimerization and trans-
location to the nucleus, where the active STAT5b dimer binds
DNA response elements containing the consensus sequence
TTC-NNN-GAA and activates gene transcription (Darnell,
1997; Herrington and Carter-Su, 2001). STAT5b is one of
Regulation of sex-specific genes in mouse liver by GH, STAT5b, and HNF4?
Data are based on Sakuma et al. (2002) and Holloway et al. (2006, 2007, 2008). Sex-specific mouse liver genes are classified based on the response to hypophysectomy (Hx),
as described for rat liver (Wauthier and Waxman, 2008), with the male class I genes further divided into subclasses A, B, and C based on the impact of Hx in female liver,
as shown in this table. The responses to Hx in male liver reflect ablation of the stimulatory effects (male class IA, IB, and IC genes) as well as the inhibitory effects (female
class II genes) of both STAT5b and male GH pulses, which activate STAT5b. In male mice, STAT5b deficiency and HNF4? deficiency have essentially the same impact on
sex-specific gene expression, suggesting that these two factors coregulate gene expression, as discussed in the text. “Mups” refers to several closely related Mup family genes.
The Cyp3a genes (last column) do not respond to GH pulse treatment or STAT5b deficiency; they also respond unusually slowly (?7 days) to continuous GH treatment in
intact male mice, indicating a unique regulatory mechanism. The response of Cyp2d9 to STAT5b deficiency in female mice is based on analysis of hepatocyte-specific
Stat5a/Stat5b-deficient mice (Holloway et al., 2007). .
Class IAClass IB Class IC Class IIOther
Prototypic mouse genes
Hx Male ? GH pulses
Male ? continuous GH
Cyp2d9Cyp7b1, Mups Cyp4a12, GstpCyp2a4, Cyp2b9 Cyp3a16, Cyp3a41,Cyp3a44
1, increase in gene expression compared with untreated controls; 2, decrease in gene expression compared with untreated controls; –, no major changes in expression.
Sex Differences in Liver DMEs
seven mammalian STAT transcription factors that share a
conserved SH2-domain and a carboxyl-terminal tyrosine
phosphorylation site (Grimley et al., 1999). STAT5a is closely
related to STAT5b, with ?90% sequence similarity, but is
apparently unable to compensate for the loss of STAT5b in a
gene knockout mouse model (Udy et al., 1997) and in humans
with deleterious mutations in the STAT5b gene (Rosenfeld et
al., 2007). STAT5a is essential for normal mammary gland
development (Liu et al., 1997), whereas STAT5b is required
for GH-stimulated pubertal and adult growth and for the
sex-dependent effects of GH on liver gene expression (Udy et
al., 1997; Teglund et al., 1998), as discussed below.
STAT5b is rapidly activated by each incoming plasma GH
pulse in adult male rat liver (Fig. 2), whereas in adult female
rats, liver STAT5b activity is maintained at a low but per-
sistent level by the more continuous plasma GH pattern
(Waxman et al., 1995; Choi and Waxman, 1999; Choi and
Waxman, 2000). Hepatic STAT5b activity seems to have a
similar sex dependence in mouse liver (Sueyoshi et al., 1999),
although differences can be expected based on the unique
features of the sexual dimorphic plasma GH profiles reported
for mice (MacLeod et al., 1991). The sexual dimorphism of
hepatic nuclear STAT5b activity, and its direct dependence
on male plasma GH pulse stimulation, suggested that
STAT5b might mediate the sex-dependent effects of GH on
male liver gene expression (Waxman et al., 1995). Indeed,
mice with a global disruption of the Stat5b gene display a loss
of male-specific liver gene expression and lose their male-
characteristic body growth rate profile (Udy et al., 1997;
Teglund et al., 1998). Of seven male-specific liver RNAs
investigated (Holloway et al., 2006), all were down-regulated
(class I genes; Table 2), whereas five of eight female-specific
RNAs were up-regulated in male liver in the absence of
STAT5b (class II genes; Table 2) (Holloway et al., 2006).
These findings are supported by a genome-wide transcrip-
tional profiling study, where 90% of 850 male-specific genes
were down-regulated and 61% of 753 female-specific genes
were up-regulated in male liver in the absence of STAT5b
(Clodfelter et al., 2006). In contrast, 90% of the sex-depen-
dent genes investigated were unaffected by the loss of
STAT5b in female liver, where STAT5b activity is much
lower than in male liver (Choi and Waxman, 1999), and
where STAT5a is required for expression of a subset of fe-
male-specific genes (Clodfelter et al., 2007). Thus, in male
liver, STAT5b maintains male liver gene expression through
its stimulatory effects on male-specific genes and its inhibi-
tory effects on female-specific genes. These effects of STAT5b
are likely to involve a combination of direct and indirect
regulatory mechanisms, as discussed below.
Potential Limitations of the Global STAT5b-Deficient
The near-global effect of Stat5b gene disruption on sex-
specific liver gene expression could be a direct result of im-
paired GH signaling due to the loss of STAT5b per se. How-
ever, indirect mechanisms involving the neuroendocrine
control of pituitary GH secretion could also explain the ob-
served demasculinization of male liver in the absence of
STAT5b. GH secretion by the anterior pituitary is controlled
by the action of two hypothalamic hormones: somatostatin,
which inhibits GH release, and GH-releasing hormone,
which stimulates GH secretion (Hartman et al., 1993; Fodor
et al., 2006). Somatostatin mRNA is down-regulated in the
hypothalamus of STAT5b-deficient mice, suggesting a role
for hypothalamic STAT5b in the feedback regulation of so-
matostatin neurons by GH (Bennett et al., 2005). Conse-
quently, the inhibition of pituitary GH release by somatosta-
tin could be impaired in STAT5b knockout mice in a manner
that feminizes circulating GH patterns and liver gene expres-
sion. A second possibility is that the loss of sex-specific liver
gene expression in STAT5b-deficient male liver is a conse-
quence of the down-regulation of Igf1, a STAT5b-dependent
gene (Davey et al., 2001), the deletion of which in liver
increases circulating GH levels as a result of the loss of a
feedback inhibitory mechanism that regulates pituitary GH
release (Sjo ¨gren et al., 1999; Wallenius et al., 2001). Indeed,
circulating GH levels are elevated in global STAT5b knock-
out mice (Udy et al., 1997), and this could contribute to the
feminization of male liver in the absence of STAT5b.
The impact of STAT5b deficiency on liver Cyp expression
has been investigated in hypophysectomized mice given ex-
ogenous pulses of GH to help distinguish direct effects of
STAT5b deficiency in the liver from indirect effects that the
loss of STAT5b in the hypothalamus (or decreased Igf1 ex-
1 2 3 4 5 6 7 8 9 1011
B Feminization of CYPs:
A GH Pulse Induction
2 P M
F-Hx + GH
1 2 3 4 5 6 7 8 9
M Hx 6 P 2 PF
M-Hx + GH
1 2 3 4 5 6 7 8
F M + GH M
Fig. 1. Regulation of rat liver CYP2C11 (male-specific) and CYP2C12
(female-specific) by plasma GH profile (A and B) and through postnatal
development (C and D). A and B, Northern blots probed for the indicated
liver mRNAs isolated from intact male (M) and female (F) rats, or from
hypophysectomized (Hx) rats given physiological GH pulses (P) by intra-
venous injection six or two times per day for 7 days (A; data from
Waxman et al., 1991). GH pulse treatment induces CYP2C11 in both
male (top) and female (bottom) rats. Intact male rats were infused with
GH continuously for 7 days using an osmotic mini-pump (B; data from
Sundseth and Waxman, 1992), which suppresses CYP2C11 and induces
CYP2C12. Tubulin RNA serves as a loading control. Each lane represents
an individual rat liver. C and D show rat hepatic mRNA levels for
CYP2C11 and CYP2C12 at various postnatal ages, as determined by
quantitative polymerase chain reaction (data from Laz et al., 2007).
Waxman and Holloway
pression in the liver) may have on liver gene expression as a
result of changes in the hypothalamic regulation of pituitary
GH secretion. Whereas GH pulse treatment of hypophysec-
tomized mice restored male liver gene expression and male-
characteristic body growth rates in wild-type mice, these
responses were not seen in STAT5b knockout mice (Davey et
al., 1999; Holloway et al., 2006). This indicates a requirement
for STAT5b for GH pulses to stimulate sex-specific gene
expression in male liver. In other studies, hepatocyte-specific
deletion of Stat5b and the neighboring Stat5a gene using a
Cre transgene under the control of the albumin promoter
significantly decreased liver expression of male-specific P450
genes and other genes and concomitantly increased female-
specific liver gene expression in male mouse liver (i.e., a
pattern very similar to that of the global STAT5b knockout)
(Holloway et al., 2007). Although plasma GH levels were
elevated in individual hepatocyte STAT5a/STAT5b-deficient
male mice, the maintenance of normal male body growth
rates and the absence in these male mice of certain female-
specific, continuous GH-inducible liver gene transcripts (e.g.,
Cyp3a16; Holloway et al., 2007) indicates that the loss of
STAT5b, and not the increase in plasma GH levels, is respon-
sible for the observed demasculinization of liver gene expres-
sion. Targeted disruption of the Stat5a gene alone has a
greater impact on sex-specific gene expression in female liver
than in male liver (Clodfelter et al., 2007; Holloway et al.,
2007), supporting the conclusion that the loss of STAT5b per
se is responsible for the feminization of male liver gene
expression in these STAT5-deficient mouse models.
STAT5-Dependent Sex Differences in Postnatal Growth
In addition to the above-described affects of GH on liver
gene expression, pituitary GH release imparts sex-differ-
ences to long bone growth and overall body growth rates.
These sex differences become apparent at puberty, when the
sex differences in plasma GH patterns first emerge (Pampori
et al., 1991; Gabriel et al., 1992). The male-characteristic
pubertal body growth spurt is greatly reduced in global
Stat5b-disrupted mice, resulting in decreased growth (Udy et
al., 1997; Teglund et al., 1998), similar to that seen in
STAT5b-deficient humans (Hwa et al., 2005; Vidarsdottir et
al., 2006) and certain models of liver STAT5 deficiency
(Engblom et al., 2007). Other studies, however, suggest that
normal postnatal growth requires STAT5b signaling in one
or more extrahepatic tissues, such as bone and skeletal mus-
cle (Klover and Hennighausen, 2007). Further insight into
the role of STAT5 in postnatal growth is provided by mouse
models in which the intracellular cytoplasmic tail of GH
receptor (required for STAT5 signaling) has been deleted or
the membrane-proximal “Box 1” sequence of GH receptor
(which binds JAK2) has been mutated to abolish JAK2 bind-
ing and thereby eliminate GH signaling via STAT5 and other
molecules (Rowland et al., 2005; Lichanska and Waters,
2008). The requirement of glucocorticoids for postnatal
growth may also be explained by their role in coregulating a
subset of STAT5b-dependent transcripts (including some
sex-specific transcripts) via a glucocorticoid receptor-STAT5b
transcriptional complex (Engblom et al., 2007).
Impact of Liver-Specific Hnf4?-Deletion on Sex-Specific
HNF4? (NR2A1) is a member of the nuclear receptor su-
perfamily (Sladek, 1994; Gonzalez, 2008). It is essential for
normal liver development (Lemaigre and Zaret, 2004) and
regulates multiple liver functions, including lipid homeosta-
sis, lipoprotein production, and bile acid biosynthesis (Hay-
hurst et al., 2001; Hanniman et al., 2006; Inoue et al., 2006;
Miura et al., 2006). Many sex-specific hepatic genes require
HNF4? for full expression, as evidenced by the functional
binding sites for HNF4? found in the promoters of several
sex-specific P450 genes, including mouse Cyp2a4, mouse
Cyp3a16, and rat CYP2C12 (Yokomori et al., 1997; Sasaki et
al., 1999; Nakayama et al., 2001). In humans, a single nucle-
otide polymorphism at a putative HNF4? binding site in the
CYP2B6 gene has been linked to polymorphic gene expres-
sion (Lamba et al., 2003), and in primary human hepato-
cytes, HNF4? is required for the expression of CYP3A4,
CYP3A5, and CYP2A6 (Jover et al., 2001).
800900 1000 1100 1200 1300
Plasma GH (ng/ml)
A GH PeakB GH Trough Period
STAT5 EMSA ACTIVITY
800900 1000 1100 1200 1300
Fig. 2. Liver STAT5 is activated in direct response to each plasma GH pulse in adult male rats. Plasma samples obtained from intact rats every 15
min were assayed for GH. Samples were collected from 8 to 11 AM (A) or from 8 AM to 1 PM (B), with the collection end times of each group of n ?
4 rats corresponding to a peak (A) and a trough in plasma GH levels (B), as indicated. The rats were then killed (red arrow) and the livers were assayed
for STAT5 DNA-binding activity (blue bar on right). Data shown are from Tannenbaum et al. (2001).
Sex Differences in Liver DMEs
Liver-specific deletion of the Hnf4a gene (Hayhurst et al.,
2001) has a sex-dependent impact on gene expression in
liver, with ?1000 fewer genes responding to the loss of
HNF4? in female compared with male mice (Holloway et al.,
2008). The male-predominant impact of liver HNF4? defi-
ciency is even more striking when sex-specific genes are
considered (372 sex-specific genes are specifically affected by
the loss of HNF4? in male mouse liver, versus only 61 sex-
specific genes are specifically affected in female mouse liver).
The similarity between the response of sex-specific liver
genes to the loss of HNF4? compared with the loss of STAT5b
in male liver is striking, with parallel responses seen for 65%
of the sex-specific genes common to the wild-type mouse
strains used in both knockout studies (Table 2) (Clodfelter et
al., 2006; Holloway et al., 2008). Thus, in male mouse liver,
STAT5b and HNF4? positively regulate many male-specific
Cyp genes and at the same time suppress a subset of female-
specific genes. Moreover, the impact of STAT5b deletion and
HNF4? deletion on sex-specific gene expression is substan-
tially reduced in female compared with male liver. Thus,
STAT5b and HNF4? may coregulate sex-specific liver genes
by mechanisms that are primarily active in male liver.
STAT5b and HNF4? exhibit mutual signaling cross-talk
(Park et al., 2006); STAT5b is able to enhance HNF4? acti-
vation of select male-specific liver P450 genes, including
Cyp2d9 and CYP8B1 (Wiwi and Waxman, 2005). STAT5b
and HNF4? binding sites are found in the promoter regions
of several sex-specific Cyp genes, including Cyp2d9 and
Cyp2a4 (Yokomori et al., 1997; Wiwi et al., 2004). Moreover,
HNF6 (ONECUT1), a female-predominant transcription fac-
tor (Lahuna et al., 1997) that positively regulates the female-
specific genes CYP2C12 and A1BG (Delesque-Touchard et
al., 2000; Gardmo and Mode, 2006), shows increased 5?-
regulatory region binding of STAT5b and HNF4? in liver
cells stimulated with GH (Lahuna et al., 2000).
A substantial fraction (31%) of the liver-expressed genes
that contain functional HNF4? binding sites within 10 kilo-
bases of the transcription start site, as determined by chro-
matin immunoprecipitation (Odom et al., 2007), require
HNF4? for expression in one or both sexes (Holloway et al.,
2008). The set of HNF4?-bound genes is 5-fold enriched for
genes that are positively regulated by HNF4? (Holloway et
al., 2008), supporting the conclusion that positive regulation
by HNF4? proceeds by a direct DNA-binding mechanism.
HNF4? protein and DNA-binding activity (but not RNA lev-
els) are ?5-fold higher in male compared with female rat
liver (C. A. Wiwi and D. J. Waxman, unpublished observa-
tions), which suggests a mechanism for the sex specificity
that characterizes certain HNF4?-dependent genes.
Mechanisms Whereby STAT5b Regulates Sex-Specific
Given the stimulatory effects of liver STAT5b on male-
specific gene expression established by the mouse knockout
studies discussed above, STAT5b may directly trans-activate
its male-specific gene targets (Fig. 3). This mechanism is
likely to apply to those male-specific genes that are rapidly
activated in liver in vivo in response to a single plasma GH
pulse, such as the Mup genes (Wauthier and Waxman, 2008).
Indeed, Mup genes bind STAT5 at multiple sites in male liver
chromatin, as determined by chromatin immunoprecipita-
tion, with no specific binding to these sites detectable in
female liver (Laz et al., 2009). STAT5 binding to liver chro-
matin is dynamic; i.e., it is directly induced by GH and cycles
on and off chromatin in response to each plasma GH pulse.
Male-specific STAT5 binding in liver in vivo is observed at
low-affinity STAT5 binding sites, including those associated
with Mup genes (Laz et al., 2009). The persistent but low
level of STAT5 typically found in female liver nuclei (Choi
and Waxman, 1999) is too low for efficient binding to occur at
these sites. In contrast, STAT5 binds to chromatin at similar
levels in male and female liver at several high-affinity
STAT5 sites implicated in the regulation of GH-responsive
genes such as IGF1 and SOCS2 (Laz et al., 2009), which do
not show marked sex specificity. Based on these findings, it
can be predicted that male-specific genes that are directly
regulated by STAT5b (e.g., Mup genes) will have low-affinity
STAT5 binding sites. These may potentially include noncon-
sensus STAT5 sites (Soldaini et al., 2000), which could com-
plicate efforts aimed at their discovery (Park and Waxman,
Two findings indicate that for a majority of liver male-
specific genes, the requirement for STAT5b for full expres-
sion in male liver is likely to involve an indirect mechanism.
Male GH pulses
STAT5b and HNF-4α
High concentration of active
STAT5b and HNF-4α
in male liver nucleus
Fig. 3. Model for direct regulation of sex-specific
genes by STAT5b and HNF4? in male liver. In the
model shown, GH pulse-activated STAT5 binds di-
rectly to and trans-activates its direct male-specific
genes targets, which may include the family of male-
specific Mup genes (Laz et al., 2009). HNF4? may
also contribute to the expression of these genes.
STAT5 and HNF4? may also directly bind to and
repress female-specific genes, as shown. Some of the
genes that serve as primary targets of STAT5 and
HNF4? may also mediate the effects of these tran-
scription factors on their downstream (i.e., indirect)
targets, as shown in Fig. 4.
Waxman and Holloway
First, most sex-specific genes are not activated when a single
physiological plasma GH pulse is given to hypophysecto-
mized animals (Wauthier and Waxman, 2008), even though
liver STAT5 activation is rapid (within 5–10 min) under
these conditions (Ram et al., 1996); second, continuous GH
infusion in male mice feminizes liver gene expression slowly
(over a period of days) (Holloway et al., 2006), whereas the
down-regulation of STAT5 signaling occurs rapidly (within a
few hours) (Ferna ´ndez et al., 1998; Gebert et al., 1999). In
one model of indirect regulation, a limited number of tran-
scriptional regulators or other signaling molecules serve as
the primary targets of GH pulse-activated STAT5b. These
include transcriptional activators and transcriptional repres-
sors, which regulate a larger set of secondary sex-specific
target genes through a hierarchical network that includes
both positive and negative regulatory mechanisms (Fig. 4). In
an alternative formulation of this model, activation of some
male-specific genes could require the coordinate binding of
STAT5b and a STAT5b-dependent transcriptional activator.
Several candidates for these primary transcriptional regula-
tors have been identified based on their early response to a
single plasma GH pulse given to hypophysectomized rats
(Wauthier and Waxman, 2008). Liver expression of these
primary response factors could be subject to postnatal devel-
opmental control that requires factors distinct from STAT5b.
This, in turn, could help explain the finding that liver STAT5
alone is not sufficient to induce an adult male pattern of liver
gene expression when it is activated precociously in the livers
of prepubertal rats given exogenous GH pulses (Choi and
Waxman, 2000). Indirect GH regulation of female-specific
genes is also likely, as suggested by studies of the GH-
regulated and female-predominant transcriptional activators
HNF6 (Onecut1), HNF3? (Foxa1), and HNF3? (Foxa2), which
can trans-activate female-specific genes, such as CYP2C12 and
A1BG (Delesque-Touchard et al., 2000; Gardmo and Mode,
2006) and Cyp2b9 (Hashita et al., 2008). Finally, three nuclear
factors are derepressed in STAT5b-deficient male liver and also
HNF4?-deficient male liver; all three factors (Cux2/Cutl2, Tox,
and Trim24) are female-specific transcriptional repressors that
could contribute to the global down-regulation of male-specific
genes seen in STAT5b-deficient male liver (Laz et al., 2007).
Although STAT5b is not generally thought of as a transcrip-
tional repressor, widespread derepression of female-specific
genes is observed in both hypophysectomized and STAT5b-
felter et al., 2006), as noted above. STAT5b could mediate the
inhibitory effects of plasma GH pulses on these female-specific
genes, and on class II male-specific genes in male liver (Wau-
thier and Waxman, 2008), in an indirect manner, as exempli-
fied by its inhibition of IGF binding protein-1 expression
through effects on the transcription factor FoxO1 (Ono et al.,
2007). Twice as many genes were repressed by STAT5b as were
induced in studies where adenoviral vectors were used to de-
liver a constitutively active form of STAT5b to the liver, sug-
gesting that STAT5b is a major mediator of the rapid suppres-
sive effects of GH (Vidal et al., 2007). It has been suggested that
even the rapid transcriptional effects of GH in the liver largely
proceed via STAT5b-independent pathways, given the lack of
phylogenetically conserved STAT5b binding sites immediately
upstream or within hepatic genes that are rapidly induced by
GH treatment (Vidal et al., 2007). However, the involvement of
far upstream STAT5 sites, not examined in that study but seen
in the case of Igf1 (Wang and Jiang, 2005; Laz et al., 2009),
cannot be excluded.
Hepatic STAT5a/STAT5b deficiency leads to aberrant ac-
tivation of two other STAT family members, STAT1 and
STAT3 (Cui et al., 2007), a response also seen in both male
and female livers of global STAT5b knockout mice (Udy et al.,
1997). Because each STAT protein regulates its own unique
set of target genes, the activation of STAT1 and STAT3 in
STAT5-deficient liver is likely to lead to changes in the ex-
pression of genes that are otherwise independent of STAT5b.
Thus, some of the genes that are up-regulated in both male
and female global STAT5b-deficient male mice (Clodfelter et
al., 2006) may be induced due to the increased activity of
STAT1 and STAT3, rather than the loss of inhibitory signal-
ing by STAT5b per se (Hosui and Hennighausen, 2008).
High concentration of active
STAT5b and HNF-4α
in male liver nucleus
STAT5b and HNF-4α
Male GH pulses
Fig. 4. Model for indirect regulation of
sex-specific genes by STAT5b and
HNF4? in male liver. In the model
shown, the effects of STAT5b and
HNF4? on sex-specific liver gene ex-
pression are mediated indirectly, by
male-specific transcriptional activa-
tors and repressors, which are pri-
mary targets of these two transcrip-
formulation of this model (not shown),
trans-activation of a secondary male-
specific target gene could involve di-
rect binding of STAT5b and the pri-
activator. Several candidate primary
target transcriptional regulators have
been identified (Wauthier and Wax-
Sex Differences in Liver DMEs
Epigenetic Regulation of GH-Responsive Sex-Specific
For many genes, a change in chromatin accessibility is an
important, underlying mechanism of transcriptional control.
Long-term silencing of gene expression by the conversion of
euchromatin to heterochromatin involves epigenetic events,
such as DNA CpG methylation, binding of sequence-specific
repressors linked to recruitment of heterochromatin protein
1 and interactions with noncoding RNAs (Kwon and Work-
man, 2008; Delcuve et al., 2009) and plays a major role in
tissue-specific gene expression. Support for a role of epige-
netic factors in the regulation of sex-specific liver gene ex-
pression comes from the discovery of sex- and GH-dependent
DNase I hypersensitive sites in the promoter regions of
CYP2C11 and Slp, which correlate with the male-specific
expression of these genes (Hemenway and Robins, 1987;
Stro ¨m et al., 1994), and from studies of female-specific hy-
persensitivity sites in the case of CYP2C12 (Endo et al.,
2005). These GH- and sex-regulated changes in chromatin
accessibility could be functionally linked to the sexual dimor-
phism of gene expression in the liver and may be a general
feature of sex-specific genes.
The slow feminization of gene expression in male liver
after continuous GH infusion (Holloway et al., 2006) may
involve large-scale chromatin remodeling that is ultimately
dependent on either STAT5b or HNF4?. The silencing of
male-specific genes after continuous GH treatment may re-
quire the local conversion of euchromatin to heterochroma-
tin, whereas the de-repression of many female-specific genes
that is seen in male liver deficient in STAT5b or HNF4?, or
after continuous GH infusion, may require the conversion of
heterochromatin to euchromatin (Fig. 5). The latter process
could be facilitated by HNF3? (FOXA2), which is more highly
expressed in female than male liver (Wiwi et al., 2004), and
whose related family member, HNF3? (FOXA1), serves as a
“pioneer factor” that binds to nucleosomal DNA at distant
enhancers and facilitates recruitment of other tissue-specific
factors (Lupien et al., 2008).
An unusually long period of continuous GH treatment (?7
days) is required to induce hepatic expression of the female-
specific genes Cyp3a16, Cyp3a41, and Cyp3a44 in adult male
mouse liver (Fig. 6) (Holloway et al., 2006, 2007). These three
female-specific genes can be induced in male liver by contin-
uous GH treatment, even in the absence of STAT5a and
STAT5b (Table 2) (Holloway et al., 2006, 2007), indicating a
unique regulatory mechanism. This could involve STAT5-
independent signaling pathways induced by GH, for example
extracellular signal-regulated kinase/mitogen-activated pro-
tein kinase signaling (Cesen ˜a et al., 2007), which requires a
Src-like tyrosine kinase, rather than JAK2 signaling (Row-
linson et al., 2008). All three Cyp3a genes are expressed in
both male and female liver before puberty, after which they
are selectively down-regulated in male liver (Sakuma et al.,
2002; Cheung et al., 2006). The silencing of these genes in
male mouse liver at puberty could be associated with forma-
tion of a “permanent” closed heterochromatin structure; ac-
cordingly, their delayed responses to a change in plasma GH
status may reflect molecular events required to reverse long-
term silencing via CpG demethylation, the reversal of het-
erochromatin protein 1 binding (Kwon and Workman, 2008;
Delcuve et al., 2009), and/or nucleosome displacement, pro-
cesses that may be dependent on hepatocyte division associ-
ated with continued body growth and an increase in liver
Studies carried out in rat and mouse models have estab-
lished that GH is the key hormonal factor that dictates sex
differences in the expression of a large number of liver gene
products, including many P450s and other DMEs. A similar
mode of regulation may characterize CYP3A4 and other
DMEs in humans, where clinically significant sex differences
in metabolism are increasingly being recognized. GH is pro-
posed to activate a complex hierarchical regulatory network
of transcription factors that exert both stimulatory and in-
hibitory effects on sex-specific DMEs and other liver-ex-
pressed genes. The transcription factors STAT5b and HNF4?
Female CYP in
adult female liver and in
continuous GH-treated male liver
TF binding sites
in adult male liver
Fig. 5. Epigenetic regulation of sex-specific P450 genes. Female-specific
Cyp genes are proposed to be repressed in male liver, and male-specific
Cyp genes are proposed to be repressed in female liver by packaging in
heterochromatin. Continuous GH is proposed to activate female-specific
genes, such as Cyp3a genes, by a mechanism that involves the local
conversion of heterochromatin to euchromatin, which enables the binding
of transcription factors (TF) that activate P450 gene expression. This
process could involve loss of DNA CpG methylation and/or loss of chro-
matin marks associated with repressed chromatin, such as histone H3
lysine 27 trimethylation, which is typically found in genes in a compact
chromatin structure and is associated with a stable, inactive heterochro-
matic state (Cheung and Lau, 2005).
Fig. 6. Time course for continuous GH feminization of P450 gene expres-
sion in adult male liver. Model shown is based on data presented in
Holloway et al. (2006); hypothetical regulatory interactions are indicated
by question marks. Female-specific repressors (Laz et al., 2007), are
rapidly induced in male liver by continuous GH treatment and are pro-
posed to down-regulate male-specific genes. The latter genes are proposed
to include one or more male-specific repressors, whose down-regulation
leads to de-repression of female-specific genes, such as Cyp2b9 and
Cyp2b10; these Cyp2b genes require several days of continuous GH
treatment for induction (Holloway et al., 2006). A distinctly longer time
frame (?7 days) is required for derepression leading to induction of the
female-specific Cyp3a genes, which may involve epigenetic mechanisms,
as discussed in the text and diagrammed in Fig. 5.
Waxman and Holloway
are both essential for the sexual dimorphism of an unexpect-
edly large number of liver-expressed genes. The actions of
these factors are likely to be mediated through the actions of
secondary target genes, including other transcription factors
and downstream signaling molecules. Further studies are
needed to identify and characterize these secondary regula-
tors, the mechanisms by which they are regulated by GH and
its sex-dependent plasma profiles, and their potential to con-
tribute to sex-dependent chromatin remodeling and epige-
netic events likely to be important in enforcing liver sex-
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Address correspondence to: David J. Waxman, Department of Biology,
Boston University, 5 Cummington Street, Boston, MA 02215. Email: djw@
Waxman and Holloway