et al. report that low levels of bisphenol
A (BPA) combined with insulin can affect the formation of adipocytes
(fat cells). This finding provides a challenge to current interpretations
of the causes of the obesity epidemic now underway in the US and
many other countries around the world. It raises the possibility
that contaminant disruption of weight regulation may be a contributing
is about body fat, which is stored in the body in fat cells called
adipocytes. At the cellular level, obesity is caused either because
of an increase in the number of fat cells (called fat cell hyperplasia)
or an enlargement of fat cells with each cell carrying greater amounts
of fat (fat cell hypertrophy), or both.
paper suggests that exposure to BPA may increase body fat via both
fat cell hyperplasia and hypertrophy: In these tissue culture experiments
it both triggers the conversion of cells to adipocytes and increases
the quantity of stored fat.
much work remains to be done to determine whether this physiological
impact of BPA is relevant to weight regulation in people, what is
certain is that human exposure to BPA
is widespread through its use in dental procedures, in food storage
and in polycarbonate plastic. For example, polycarbonate baby bottles
heated by microwave leach BPA into milk fed to infants. The
possibility that BPA may be contributing to the world-wide epidemic
of obesity makes aggressive study of Masuno et al's hypothesis
did they do? Masuno et al. carried out their experiments
in a cell tissue culture line of mouse cells (called 3T3-L1 cells)
which have become a model system for exploring some of the factors
that affect the conversion rate to adipocytes (fat cells). This
conversion rate is now understood to be a key step in developing
obesity. The more rapid and extensive the conversion, the more likely
the past 25 years, work with 3T3-L1 cells and similar cell lines
has allowed a detailed characterization of the steps leading to
adipocyte formation, called adipogenesis. During adipogenesis, pre-adipocyte
cells go through a series of stages that involve characteristic
changes in shape and in the distribution and amount of fat within
them. Characteristic genes are also expressed at different stages,
and the presence of these genes can be used as biochemical markers
their experiments Masuno et al. combined different levels
of insulin and bisphenol A in a series of tests to tease apart the
impact on cellular differentiation of the compounds individually
and together. Their methods take advantage of prior work that has
characterized the morphological and biochemical markers that accompany
adipocyte formation. They used a parallel series of experiments
with compounds known to stimulate cell differentiation as positive
were particularly interested in the impact of exposure on the activity
levels of two enzymes that had been established by earlier studies
to be indicators of conversion of pre-adipocytes to adipocytes,
LPL (lipoprotein lipase) and GDPH (glycerol-3-phosphate dehydrogenase),
and on the accumulation of fat (triacylglycerol, or TG) within the
cells. Typically, LPL appears before GDPH activity, which is acommpanied
by TG build-up.
did they find? Masuno et al. first established
a dose-response relationship between BPA and LPL activity, in the
presence of insulin. Remember that the presence of LPL is a marker
for reaching the final stages of adipogenesis.
level of LPL activity in 3T3-L1 cells rises with increasing
BPA exposure in the presence of insulin.
then performed an additional series of experiments using 20 µg/ml
of BPA, comparing cell parameters for cells with different regimes
of BPA and insulin treatment. They confirmed that BPA triggered
the conversion of pre-adipocytes into adipocytes.
experiment ran for 11 days. During the first 2 days, cells were
either exposed, or not exposed (control group) to BPA. During the
latter 9 days, they were exposed only to insulin. Wasuno et
al. refer to these two different phases as the "trigger"
(BPA) and the "treatment" (insulin). The BPA-triggered
cells showed a 60% increase in LPL activity (compared to controls),
a 500% increase in GPDH, and a 150% increase in fat content. Microscopic
examination of the cells showed that the lipid droplets within the
cells had coalesced and become larger, consistent with the what
happens during adipogenesis. So the BPA exposure triggered
the formation of fat cells.
repeated a variation on this experiment, and determined that BPA
not only triggers the conversion, it stimulates the conversion process
once triggering has occurred. They did this by leaving
BPA in the trigger and adding BPA to the treatment. Thus during
the first 2 days, all cells were exposed to BPA (comparable to the
experimental group in the prior experiment). During the latter 9,
one group was exposed only to insulin (control) the other to both
BPA and insulin. The combination of insulin and BPA caused a 1,300%
increase in fat levels. GPDH activity was 3.3 times higher than
cultures treated only with insulin, and the percentage of lipid-positive
cells in the cultures was 83%. These results are much higher
than the effect of simply using BPA only as a trigger (e.g.,
1,300% vs. 150% increase in fat). Hence Masuno et al.
concluded that BPA in the treatment phase stimulated the conversion
process more effectively than insulin alone.
et al. report that these chemical and morphological impacts
are very similar to positive control experiments with compounds
known to stimulate adipogenesis (dexamethasone [DEX] and 1-methyl-3-isobutylxanthine
[MIX]). This is consistent with their interpretation that BPA also
et al. looked more closely at the behavior of the 3T3-L1
cells over time in response to a BPA trigger and BPA-insulin treatment,
comparing them to the positive controls (DEX, MIX and insulin in
the trigger and insulin in the treatment). DNA content of the BPA-insulin
group increased, although not as much as the positive controls.
Changes in fat content and LPL activity were similar between the
BPA-insulin group and positive controls, again consistent with the
interpretation that BPA is stimulating adipogenesis.
does it mean? Heretofore, the prevailing wisdom about the
causes of the world-wide obesity epidemic have focused almost exclusively
on life style issues: consumption of junk food, decreases in exercise,
etc. These remain plausible interpretations, and almost certainly
are contributing factors. But this research report by Masuno
et al. opens up an entirely new front: the disruption of
weight regulation (weight homeostasis) by hormone-disrupting contaminants.
In this case, BPA triggers and then stimulates two of the key biological
mechanisms underlying obesity: it increases the number of adipocytes
(fat cell hyperplasia), and enhances their fat storage (fat cell
steps lie between these initial observations and confirmation that
contaminant disruption of weight homeostasis contributes to obesity
in people, although there are a few hints elsewhere in the scientific
literature that Masuno's mechanism may be relevant. For example,
Howdeshell et al. had noted in an earlier
paper that BPA-treated mice matured heavier than controls, but
they had no insight as to mechanism.
the enormous health consequences of obesity and the diseases to
which it is linked, like diabetes, pursuing this line of research
now becomes imperative. What is intriguing is that as further
research confirms the plausible involvement of endocrine disrupting
compounds in interfering with weight homeostasis, then there are
simple and attainable interventions that can introduced on a precautionary
basis: reducing exposures.