JL, M-C Orgebin-Crist, J-J Lareyre and CC Nelson. 2003. Disruption
of androgen regulation in the prostate by the environmental contaminant
Health Perspectives doi:10.1289/ehp.5919
cancer strikes more men in the United States than any other
cancer and it has become far more common a disease over the past
several decades. While treatment has advanced significantly, reducing
mortality rates, progress to understand the causes of prostate cancer
have moved forward slowly.
new paper opens up an important new front in the search for causes,
demonstrating that the ubiquitous organochlorine contaminant hexachlorobenzene
disrupts normal development of the male reproductive tract by interfering
action. Many other contaminants share the same mechanisms of action
of HCB and thus are also implicated by these results.
this paper, Ralph et al. use cell experiments in
vitro and also experiments with mice to show that low levels
of HCB enhance the responsiveness of prostate cells to androgen
but that high levels of HCB suppress it.
did they do? Ralph et al. carried out two different
sets of experiments,
working with prostate cells in vitro they varied exposure
levels to HCB. The cells had been modified using standard molecular
methods to produce a readily-detectable enzyme when the cell's
androgen system was activated.
they exposed male mice to HCB throughout gestation and lactation,
and studied the effects of the exposures on different tissues,
including the prostate and the epididymides.
The mice also had been altered genetically so that androgen activation
would be readily apparent and easy to localize to specific tissues.
Three HCB dosage levels were used: low (5 mg/kg/day), medium (10
mg/kg/day) and high (20 mg/kg/day). [These doses... 5 - 20 parts
per million), are far above the part per billion range used in
the in vitro work and significantly above exposures that most
people would experience.]
did they find? In their in vitro work, they first
established the normal pattern of responsiveness of the cells to
dihydrotestosterone, or DHT.
higher levels of DHT, Ralph et al. observe an increase in the
activity level of their androgen response assay. The assay was
based upon an gene that, when activated by an androgen, produced
a unique protein, luciferase, that could be detected with great
et al. used this first graph to determine the amount of
DHT to include in subsequent experiments: a level of DHT capable
of producing half of the maximal response, which in the figure above
can be seen to be approximately 2.5 nM DHT (it's a logarithmic scale).
then ran a series of experiments varying the amount of HCB with
that background level of DHT present.
red line is the level of response obtained by 2.5 nM DHT without
any HCB present.
levels of HCB exposure around 1 nM (parts per billion), Ralph
et al. saw up to a doubling of the androgenic response
in the presence of DHT.
at very high levels, the androgenic response was repressed.
is a striking example of a "non-monotonic
dose-response curve," in which low levels can produce larger
effects than high dose levels.
Ralph et al.'s experiments also revealed that while the
androgen receptor had to be present for HCB's effect to be observed,
HCB itself did not bind to the androgen receptor. Its effect was
being mediated via some other biochemical pathway.
work with mice involved two different parallel experiments, both
with mice that had been altered genetically so that androgen activity
could be readily observed and quantified. The first of these used
a strain of mice which a novel reporter
responsive to androgen within the prostate. The second experiment
used mice with a reporter gene in the epididymides.
addition to measuring the response of these reporter genes to different
levels of HCB, Ralph et al. also measured prostate, liver,
testes epididymides and overall body weights as well as other morphological
characteristics, examined several tissues histologically, and measured
serum hormone levels.
suggested by the in vitro experiments, the effects of HCB
were significant for a number of these variables, and in several
low dose effects were the opposite of high dose effects.
Ralph et al. conclude that their data indicate
"that HCB agonized androgen action at low doses but antagonized
it at high concentrations:"
were no overt signs of toxicity from the exposure levels used. One
of the strongest signs of overt toxicity caused by many contaminants
at relatively high levels is weight loss. In contrast, the low and
mid-dose animals in these experiments gained weight. Indeed,
the low dose animals gained roughly 15% weight compared to controls.
many effects reported:
with medium and high levels of HCB exposure showed reductions
in reporter gene activity within the prostate and the epididymides.
dose treatments resulted in an increase in prostate weight compared
to controls, while high doses reduced prostate weight.
dose treatments increased the frequency of one marker for early
sexual maturity whereas no effect was seen at the high dose level.
dose treatment increased epididymides weight; no effect was induced
by high dose treatment.
does it mean? Ralph et al. conclude that their
evidence of HCB acting as an endocrine disruptor in mice and
demonstrates its potential to impact the human androgen axis.
HCB can interfere with the transcriptional activity of androgen-regulated
genes and the downstream effects, thus amplifying its potential
endocrine-disrupting impact. The fact that HCB may affect the
androgen-signaling pathway in a different manner depending on
the dose should reinforce the concept that environmental
xenobiotics, though present at low doses, may pose a threat
to human health."
results provide no conclusive links to prostate cancer. Rather,
they reinforce the plausibility of links between exposure to this
organochlorine and androgen-related development errors in mammals,
demonstrably in mice, by extension in humans.
Ralph et al. observe, androgens, specifically DHT, are
profoundly important to the proper development of the male reproductive
tract, including the prostate.
contaminant that profoundly inteferes with normal control of that
development, as their results compelling demonstrate for HCB, must
necessarily be considered a candidate for prostate dysfunction in
humans, especially given the widespread exposure of people to HCB.
The challenge is that the experiments to prove these effects in
humans would be unethical, and epidemiological studies remain fraught
with many biases that increase the likelihood of false
additional concern is the fact that the standard treatment for prostate
cancer is based on medical interventions to suppress androgen responsiveness
in prostate tumors. If HCB enhances prostate androgen responsiveness
in humans as it does in vitro and in mice, it may work
against prostate cancer treatment.
patterns in their work were particularly interesting:
doses regularly caused effects at odds with high dose impacts.
These low-dose effects undermine
current methods used to protect public health from contaminants.
HCB was altering responsiveness to the androgen DHT, HCB did not
bind directly with the androgen receptor. Ralph et al.
propose that the response is mediated by HCB's known ability to
interact with the AhR receptor.