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Quesada,
I, E Fuentes, MC Viso-León, B Soria, C Ripoll and A Nadal.
2002 Low doses of the endocrine disruptor bisphenol-A and
the native hormone 17ß-estradiol rapidly activate transcription
factor CREB. FASEB
16: 1671-1673.
One
of the centerpiece findings of research into endocrine disruption
is the ability of EDCs
to turn genes on and off at inappropriate times, that is,
to alter the pattern of gene expression. During normal development,
the right sequence of gene activation can be as important
as whether or not genes are there in the first place.
In
this paper, Quesada et al. identify a new
pathway for the ubiquitous contaminant, bisphenol
A (BPA) to interfere with normal gene expression,
and to do so at extremely low levels of exposure.
Moreover, the pathway they have discovered is one that is
involved in a wide array of important control mechanisms
for normal development and function, in people. Affected genes
include key elements of the way that long-term memory is stored,
that brain development is controlled, and that weight is regulated.
They
have discovered that bisphenol A activates a new type of estrogen
receptor located on the surface of the cell membrane. Attention
until now has focused on bisphenol A's interaction with the
classic nuclear estrogen receptor, located within the cell.
The
experiments carried out by the research team indicate that
bisphenol A is just as powerful as the natural human estrogen,
17ß-estradiol,
at activating this cell membrane estrogen receptor. This equipotency
undermines one recurring argument against the possible impact
of "weak" estrogen mimics like BPA, which are far
less powerful at binding with the nuclear estrogen receptor
than is estradiol.
What
did they do? Quesada et al. worked with
a line of pancreas cells isolated to live in test tube conditions.
This sort of preparation is the standard way for cell scientists
to explore biochemical pathways.
In
the experiment, they exposed the cells to different substances,
including bisphenol A, and looked to see whether a specific
transcription
factor,
called cyclic AMP response element binding protein, or CREB,
had been altered into the form it takes to activate genes.
To
detect the transformation into the activated form (called
phosphorylated CREB, or p-CREB), they exposed the cells to
an antibody known to react only to p-CREB, not unphosphorylated
CREB, and determined the number of cells that responded positively.
What
did they find? At a concentration of only 1 nanoMolar
(approximately 0.23 parts per billion) both estradiol and
BPA increased the percentage of cells with p-CREB by 35% after
only 5 minutes' exposure (graph below). |
| Quesada
et al. then used a series of biochemical tools to
understand more of the mechanism by which these compounds
achieved their effect:
- First
(graph to right), they repeated the experiment but left
calcium out of the solution in which the cells were bathed.
For both estradiol and BPA, this decreased the amount of
phosphorylization to well below the control. They took this
to indicate that part of the process of CREB activation
requires an influx of calcium into the cell (consistent
with previous information about CREB).
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Red
dot indicates significant difference from control
Adapted from Queseda et al. |
- They
then added (graph to left) another twist to the experiment,
adding a chemical (ICI 182,780) known to interfere with
estradiol (and BPA) binding to the nuclear estrogen receptor.
This had no effect on the outcome. If the activation of
CREB were dependent upon the classic intracellular estrogen
receptor, the chemical would have eliminated the response.
But the addition of ICI did not diminish either estradiol's
or BPA's impact.
- Finally,
they replaced simple estrogen with estrogen bound to a molecule
(horse radish peroxidase, HRP) which is unable to pass through
the cell membrane. Estradiol still had the same effect as
before, indicating that its mechanism must be through a
cell membrane receptor with estrogen's binding taking place
outside the cell on the cell membrane, rather than inside
the cell.
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What
does it mean? Quesada et al.'s work opens
up a new chapter, if not an entirely new book, on the role
of bisphenol A in interfering with normal gene expression.
Four points are key:
- It
involves a new type of estrogen receptor, located on the
surface of the cell membrane.
- It
finds bisphenol A to be just as powerful as estradiol in
binding with this new receptor.
- Binding
and alteration of gene expression occurs at extremely low
levels, low parts per billion, well within the range of
bisphenol A found in people today.
- By
activating the transcription
factor CREB, bisphenol A has the potential of altering
gene expression in several profoundly important systems
that are vital for normal development and functioning.
Among the systems that may be affected:
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Research
has established that CREB is involved in the molecular
basis of long-term memory. Studies with a variety of
animals, as diverse as sea slugs and mice, have shown
that interfering with CREB activity impairs long-term
memory formation. Enhancing it strengthens memory. These
results with bisphenol A raise the question: what
happens when the memory formation process is stimulated
at the wrong time?
CREB
plays another role in neurological function, controlling
key aspects of how the brain develops. Normal brain
development involves an interplay of neuron growth and
neuron death. Nerve cells grow and make connections
with other nerve cells. Some of those cells, however,
die in development, in a process called pre-programmed
cell death (apoptosis). Apoptosis is a natural and necessary
part of brain development. In the developing brain,
target cells that develop connections with growing neurons
exude a chemical signal called NGF.
Neurons that receive the signal live. Those that don't
die. That's the way the brain ensures the right connections
are formed and unnecessary neurons don't stay around.
When a cell receives the NGF signal, it prevents the
process of apoptosis from starting. And CREB is involved
in the response to NGF. It turns out that NGF causes
CREB to be converted into its active (phosphorylated)
form, which leads to gene expression. So Quesada
et al.'s results raise a series of developmental
questions: What happens to a developing brain when CREB
is phosphorylated not by the normal and natural signaling
pathways, but instead by an endocrine disrupting contaminant?
Finally,
in an entirely different realm, CREB has emerged as
an important signal in the molecular events that convert
precursor adipocyte
cells
to adipocyte cells. Quesada et al.'s
findings raise the possibility that bisphenol A exposure
pushes adipocyte formation faster and farther than would
happen without exposure. Indeed, previous
experiments have already suggested this, without
knowledge of the signaling mechanism. In combination,
these studies implicate bisphenol A in the molecular
and cellular basis of obesity: how many fat cells a
person has, and how much fat they contain, is the central
determinant of obesity.
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| These
experiments were carried out in mouse pancreas cells in cell
culture, not in living mice, and the proposed mechanism for
bisphenol A's effect is new. They clearly require replication
and further study to understand the full implications of the
results for human health. Yet at the same time, they add to
a rapidly growing body of data that plausibly connects bisphenol
A to an extraordinary array of important health endpoints, at
levels that are now known to occur in people. Efforts
to reduce human exposures should commence immediately. |
Background
on BPA and sources of exposure
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