Chemico-Biological Interactions 110 (1998) 1–5
Indole-3-carbinol: Anticarcinogen or tumor
promoter in brassica vegetables?
Roderick H. Dashwood *
Department of En?ironmental Biochemistry, Uni?ersity of Hawaii, 1800 East West Road,
Honolulu, HI 96822, USA
Received 26 September 1997; received in revised form 25 November 1997;
accepted 26 November 1997
Indole-3-carbinol (I3C) is one of several compounds in brassica vegetables that demon-
strate anticarcinogenic effects in experimental animals. A review of Medline and CancerLit
databases indicated that interest in I3C, as a cancer chemopreventive agent, has increased
significantly in the past 5–10 years. Whereas most studies report inhibitory or protective
effects of I3C in vivo, a few provide clear evidence for promotion or enhancement of
carcinogenesis, depending upon the initiator, exposure protocol and species. In the absence
of detailed information on the inhibitory and in particular, promotional mechanisms, it
would seem advisable to proceed with caution before including I3C in extensive human
clinical trials. © 1998 Elsevier Science Ireland Ltd. All rights reserved.
Keywords: Indole-3-carbinol; Anticarcinogen; Promoter; Brassica; Cruciferae
Verhoeven et al. recently published, in this journal, a review of the mechanisms
underlying anticarcinogenicity by brassica vegetables . The 50-page ‘mini-review’,
with 220 references and nine tables, provided a welcome update of the chemopre-
* E-mail: firstname.lastname@example.org
0009-2797/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved.
R.H. Dashwood / Chemico-Biological Interactions 110 (1998) 1–52
ventive effects of brassica vegetables and their constituents in in vitro assays, animal
experiments and various human studies. Alterations in various phase I and phase II
biotransformation enzymes were cited as possible mechanisms by which brassica
vegetables might inhibit chemical carcinogenesis.
Although the review focused specifically on the ‘putative beneficial properties of
glucosinolate hydrolysis products and brassicas’, an interesting and thought-pro-
voking section was included on the possible adverse effects of various isothio-
cyanates and indoles derived from brassica vegetables. The section on adverse
effects contained sub-headings related to ‘possible mutagenicity and carcinogenic-
ity’, ‘toxicity’ and ‘goitrogenicity’, but an important omission was a section
dedicated to ‘tumor promotion and enhancement’. This issue is addressed in the
present commentary using indole-3-carbinol (I3C) as a case-in-point, since it raises
some important questions about how chemicals are selected for human clinical
Among various indoles, I3C has received particular interest as a possible cancer
chemopreventive agent and this is reflected in the increased number of citations to
I3C in the medical literature. Most of these papers have focused on the in vitro
inhibitory properties of I3C or its acid condensation products (dimers, trimers and
other oligomers), their anticarcinogenic activities in experimental animals, or their
beneficial effects in humans with respect to altering estrogen metabolizing pathways
. From the number of such reports on chemopreventive effects, the weight of
evidence might appear to support the recent move into human clinical trials of I3C
However, several studies have provided clear evidence for promotion or enhance-
ment of carcinogenesis by I3C and interestingly, a degree of initiator specificity to
the promotional activity. Exposure to I3C before, during and after carcinogen
exposure significantly enhanced dimethylhydrazine (DMH)-initiated colon tumori-
genesis in the rat , but inhibited the formation of colonic aberrant crypt foci
(ACF) induced by the heterocyclic amines 2-amino-1-methyl-6-phenylimidazo[4,5-
b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f ]quinoline (IQ) [5,6]. Al-
though suggestive, these findings are complicated by the fact that putative
preneoplastic lesions (i.e. ACF) were used as the end-point in the latter studies
versus frank tumors in the DMH experiment. Moreover, the DMH results do not
show clear promotion by I3C, since this term is restricted to effects solely during
In recent studies, however, pure post-initiation exposure to I3C reportedly
increased the size and number of glutathione S-transferase-placental form (GST-P)-
positive liver foci after 24 weeks, in rats given diethylnitrosamine (DEN) or DEN
plus two other initiating agents [7,8]. At 52 weeks, animals given I3C post-initiation
were reported as showing a ‘tendency’ for promotion in the liver and thyroid gland
tumors were increased significantly by I3C .
The clearest evidence for promotional activity by I3C has come from studies in
which rainbow trout were pre-initiated with aflatoxin B1(AFB1) in the diet. Thus,
whereas I3C pretreatment and co-treatment strongly inhibited AFB1-initiated hepa-
tocarcinogenesis, post-treatment with I3C was strongly promotional . The extent
R.H. Dashwood / Chemico-Biological Interactions 110 (1998) 1–53
of promotion increased linearly with time of dietary I3C post-initiation exposure
and promotion occurred even when I3C was administered intermittently, such as
Monday and Thursday of each week. As also defines a ‘true’ promoter, the increase
in tumor incidence was observed after various lag phases (1, 3 or 6 months)
between AFB1and I3C treatment .
The quantitiative aspects of this ‘ambivalent’ modulatory response by I3C also
have been defined using full sets of tumor dose-response curves. In the range of
likely human dietary exposures to I3C (?1500 ppm), the inhibitory and promo-
tional potencies of I3C were approximately equivalent [10,11]. When one considers
Ambivalent activity of indole-3-carbinol in carcinogenesis studiesa
ActionI3C dosing regimenSite Animal Reference
aFrom studies which examined either frank tumors or intermediate biomarkers, such as ACF in the
colon or GST-P-positive liver foci. The examples listed in the table were selected to convey the
ambivalent activity of I3C in vivo and do not represent the entirety of published works on the subject.
For more details, see [1,25].
b‘Initiation’ means I3C exposure during, or before and during, the carcinogen treatment.
c‘Post-initiation’ is used here to mean I3C exposure after, or during and after the carcinogen, although
by strict definition a modulator that increases tumors or intermediate biomarkers in these protocols
should be termed ‘promoter’ and ‘enhancer’, respectively.
R.H. Dashwood / Chemico-Biological Interactions 110 (1998) 1–54
that the data were generated from studies that collectively used ?15000 animals,
the above mentioned ‘weight of evidence’ for including I3C in clinical trials might
be called into question. These and other examples of the ambivalent activity of I3C
as an inhibitor or enhancer/promoter of carcinogenesis are summarized in Table 1.
Further work certainly is required in order to better define the possible risks
versus benefits of I3C exposure, particularly at higher exposure levels (?2000 ppm
in the diet). Detailed investigations of the mechanism(s) of inhibition and (in
particular) promotion are relatively scarce for I3C and many other ‘anticarcino-
gens’ and these will be important if a clear, rational argument is to be made for
including such compounds in clinical trials. This point was illustrated by recent
mechanism studies in the rat , in which I3C induced CYP1A1 and certain other
cytochromes P-450, but concurrently inhibited the activity and expression of
flavin-containing monooxygenases . Simultaneous changes in such enzyme
pathways has potentially deleterious consequences as regards carcinogen and drug
metabolism in vivo. The final caution provided by Larsen-Su and Williams is apt in
that, indeed, ‘‘testing for a similar phenomenon in humans would seem advisable
before wide-spread administration’’ .
The author would like to thank the referees of this paper for useful comments for
improvement, in particular the inclusion of Table 1. The authors’ original ACF
studies were supported by NIH grant CA65525.
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