george, While looking for more info on the NBC news report, I ran across this nice summary of Ron Evan's Howard Hughes program:
Steroid Receptors in Development and Disease
Ronald M. Evans, Ph.D.-Investigator
Dr. Evans is also Professor at the Gene Expression Laboratory of the Salk
Institute for Biological Studies and Adjunct Professor of Biology and of
Biomedical Sciences at the University of California, San Diego. He received his
Ph.D. degree in microbiology and immunology from the University of
California, Los Angeles, School of Medicine. After postdoctoral training with James Darnell
at the Rockefeller University, he joined the faculty of the Salk Institute. Dr. Evans is a
member of the National Academy of Sciences. His research interests are in developmental
and inducible regulation of gene expression.
AN understanding of the mechanisms by which apparently distinct regulatory systems integrate to
modulate body function and behavior poses a central problem in molecular biology. Hence we have
focused our attention on the action of steroid, retinoid, and thyroid hormones in regulating cellular
and organ physiology. This field has undergone extraordinary development in the past several years,
as a consequence of the cloning and sequencing of the genes encoding the receptors for these
hormones and target cells.
These structurally related receptors constitute a superfamily of nuclear regulatory proteins capable of
modulating gene expression in a ligand-dependent fashion. One challenge is to define the molecular
properties of each receptor that determine its interactions with the transcription machinery. Another
challenge is to elucidate the contributions of individual regulatory systems to the integrated and
complex processes associated with cell growth, differentiation, and organ function.
In the Realm of the Retinoids
The cellular responsiveness to retinoids is conferred through two structurally and pharmacologically
distinct families of receptors: the retinoic acid receptors (RARs) and the retinoid X receptors
(RXRs). Each subfamily is composed of three genes, giving rise to a total of six retinoid receptor
gene products. One challenge is to understand the rationale for multiple receptors in each subfamily.
Do they serve interactive, independent, or redundant functions? Both RARs and RXRs are
ligand-dependent, but only the RARs respond to all-trans retinoic acid (atRA).
We showed that atRA can be converted by isomerization to the RXR ligand 9-cis RA. In work
partially supported by the National Institutes of Health, we demonstrated that the RXR serves as a
critical heterodimeric partner for the vitamin D receptor, the thyroid hormone receptor, and the
RARs. The capability of nuclear receptors to heterodimerize suggests the existence of an elaborate
network to process external hormonal signals coordinately. In this respect, RXR appears to play a
crucial role as a common denominator in nuclear receptor signaling.
In recent studies we identified a homologue of RXR in Drosophila as the product of the
ultraspiracle (usp) locus. Genetic and morphologic studies indicate that the USP protein is required
in multiple tissues and stages, with mutations resulting in complex reproductive, embryonic, and adult
phenotypes. In light of the proposed critical role of RXR in the formation of nuclear receptor
heterodimers in vertebrates, part of the observed USP pleiotropy may be due to the interaction of
USP with other factors important in Drosophila development. Indeed, we recently demonstrated
that the ecdysone receptor (EcR) forms a heterodimeric complex with USP, and it is this complex
that transmits the properties of the insect-molting hormone ecdysone.
Remarkably, we have shown that the EcR by itself is unable to bind hormone but requires USP for
this property. This is the first demonstration that formation of the heterodimer is required for
hormone binding. Furthermore, we demonstrated that the heterodimer also generates the
DNA-binding form of the receptor, indicating the importance of protein-protein interactions in
hormonal signaling. This is the first demonstration of true allostery by receptors in this family.
Based on these observations, we have attempted to transfer ecdysone responsiveness from insect
cells to mammalian cell lines. Should this be possible, we would be able to use insect hormones to
generate hormonally inducible systems in mammalian cells. The ability to transfer ecdysone
inducibility to mammalian systems will offer the possibility of generating a novel and simple inducible
system for gene transfer studies. Such a system would allow the activation of transfected genes
harboring ecdysone response elements in response to the insect hormone. The virtue of this system
is that it could allow high inducibility without activating endogenous receptors and, therefore, have
virtually no impact on the organism, with the exception of cells harboring the transfected gene.
In feasibility studies we have been able to transfer ecdysone responsiveness to cultured cells, using
cotransfected EcR-USP and USP expression vectors. The resulting cells are able to activate
ecdysone-responsive genes in a hormone-dependent fashion. We are attempting to expand the utility
of this system by generating transgenic mice expressing the EcR in a variety of embryonic and adult
tissues. In principle, these mice should be excellent recipients for transgenes harboring ecdysone
response elements: the progeny of these mice should be able to activate the transgene following the
addition of ecdysone to their diet. We are exploring this proposal.
Orphan Receptors-Orphan Ligands
An important advance in the characterization of this superfamily of regulatory proteins is the
delineation of a growing number of gene products (orphan receptors) that possess the structural
features of a hormone receptor but lack known ligands. The search for hormonal activators for these
newly discovered orphan receptors has created exciting areas of research on previously unknown
signaling pathways. The ability to identify novel hormonal systems has important implications in
physiology, as well as in human disease and its treatment.
Since receptors have been identified for all known nuclear-acting hormones, a question arises as to
the types of molecules that may activate orphan receptors. Products of intermediary metabolism act
as transcriptional regulators in bacteria and yeast, suggesting that they may serve similar functions in
higher organisms. A crucial biosynthetic pathway in higher eukaryotes is the mevalonate pathway,
which leads to the synthesis of cholesterol, bile acids, porphyrin, dolichol, ubiquinone, carotenoids,
retinoids, vitamin D, steroid hormones, and farnesylated proteins.
Farnesyl pyrophosphate (FPP) represents the last precursor common to all branches of this
pathway. As a result, FPP is required for such fundamental biological processes as membrane
biosynthesis, hormonal regulation, lipid absorption, glycoprotein synthesis, electron transport, and
cell growth. Because of the central role of FPP, it is logical that its concentration should be closely
regulated. This suggests that cells have developed strategies to sense and respond to changing levels
of FPP or its metabolites. One possible strategy is to use a transcription factor whose activity is
specifically regulated by an FPP-like molecule. Potential candidates for such a sensor include
members of the nuclear receptor superfamily, as these proteins are activated by
low-molecular-weight signaling molecules.
We describe a novel orphan nuclear receptor named FXR (farnesoid X receptor) that is activated
by farnesol and related molecules. FXR provides one of the first examples of a vertebrate
transcription factor that is regulated by an intracellular metabolite. These findings suggest the
existence of vertebrate signaling networks that are regulated by products of intermediary
metabolism. We refer to this novel regulatory paradigm as metabolite-controlled intracellular
(metacrine) signaling. |
