Molecular Endocrinology 

Abstract: Estrogen receptors (ERs) act by regulating transcriptional processes. The classical mechanism of ER action involves estrogen binding to receptors in the nucleus, after which the receptors dimerize and bind to specific response elements known as estrogen response elements (EREs) located in the promoters of target genes. However, ERs can also regulate gene expression without directly binding to DNA. This occurs through protein-protein interactions with other DNA-binding transcription factors in the nucleus. In addition, membrane-associated ERs mediate nongenomic actions of estrogens, which can lead both to altered functions of proteins in the cytoplasm and to regulation of gene expression. The latter two mechanisms of ER action enable a broader range of genes to be regulated than the range that can be regulated by the classical mechanism of ER action alone. This review surveys our knowledge about the molecular mechanism by which ERs regulate the expression of genes that do not contain EREs, and it gives examples of the ways in which the genomic and nongenomic actions of ERs on target genes converge. Genomic and nongenomic actions of ERs that do not depend on EREs influence the physiology of many target tissues, and thus, increasing our understanding of the molecular mechanisms behind these actions is highly relevant for the development of novel drugs that target specific receptor actions.

Abstract: Estrogen rapidly activates the mitogen-activated protein kinases, Erk-1 and Erk-2, via an as yet unknown mechanism. Here, evidence is provided that estrogen-induced Erk-1/-2 activation occurs independently of known estrogen receptors, but requires the expression of the G protein-coupled receptor homolog, GPR30. We show that 17beta-estradiol activates Erk-1/-2 not only in MCF-7 cells, which express both estrogen receptor alpha (ER alpha) and ER beta, but also in SKBR3 breast cancer cells, which fail to express either receptor. Immunoblot analysis using GPR30 peptide antibodies showed that this estrogen response was associated with the presence of GPR30 protein in these cells. MDA-MB-231 breast cancer cells (ER alpha-, ER beta+) are GPR30 deficient and insensitive to Erk-1/-2 activation by 17beta-estradiol. Transfection of MDA-MB-231 cells with a GPR30 complementary DNA resulted in overexpression of GPR30 protein and conversion to an estrogen-responsive phenotype. In addition, GPR30-dependent Erk-1/-2 activation was triggered by ER antagonists, including ICI 182,780, yet not by 17alpha-estradiol or progesterone. Consistent with acting through a G protein-coupled receptor, estradiol signaling to Erk-1/-2 occurred via a Gbetagamma-dependent, pertussis toxin-sensitive pathway that required Src-related tyrosine kinase activity and tyrosine phosphorylation of tyrosine 317 of the Shc adapter protein. Reinforcing this idea, estradiol signaling to Erk-1/-2 was dependent upon trans-activation of the epidermal growth factor (EGF) receptor via release of heparan-bound EGF (HB-EGF). Estradiol signaling to Erk-1/-2 could be blocked by: 1) inhibiting EGF-receptor tyrosine kinase activity, 2) neutralizing HB-EGF with antibodies, or 3) down-modulating HB-EGF from the cell surface with the diphtheria toxin mutant, CRM-197. Our data imply that ER-negative breast tumors that continue to express GPR30 may use estrogen to drive growth factor-dependent cellular responses.

Abstract: Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors controlling the expression of genes involved in lipid homeostasis PPARs activate gene transcription in response to a variety of compounds including hypolipidemic drugs as well as natural fatty acids From the plethora of PPAR activators, Scatchard analysis of receptor-ligand interactions has thus far identified only four ligands These are the chemotactic agent leukotriene B4 and the hypolipidemic drug Wy 14,643 for the alpha-subtype and a prostaglandin J2 metabolite and synthetic antidiabetic thiazolidinediones for the gamma-subtype Based on the hypothesis that ligand binding to PPAR would induce interactions of the receptor with transcriptional coactivators, we have developed a novel ligand sensor assay, termed coactivator-dependent receptor ligand assay (CARLA) With CARLA we have screened several natural and synthetic candidate ligands and have identified naturally occurring fatty acids and metabolites as well as hypolipidemic drugs as bona fide ligands of the three PPAR subtypes from Xenopus laevis Our results suggest that PPARs, by their ability to interact with a number of structurally diverse compounds, have acquired unique ligand-binding properties among the superfamily of nuclear receptors that are compatible with their biological activity

Abstract: POMC, the precursor of ACTH, MSH, and beta-endorphin peptides, is expressed in the pituitary and in two sites in the brain, in the arcuate nucleus of the hypothalamus and the commissural nucleus of the solitary tract of the brain stem. Little is known regarding the functions of melanocortin (ACTH and MSH) peptides in the brain. We report here the detailed neuroanatomical distribution of the MC4-R mRNA in the adult rat brain. The melanocortin 3 receptor (MC3-R), characterized previously, was found to be expressed in arcuate nucleus neurons and in a subset of their presumptive terminal fields but in few regions of the brainstem. The highly conserved MC4-R is much more widely expressed than MC3-R and is pharmacologically distinct. MC4-R mRNA was found in multiple sites in virtually every brain region, including the cortex, thalamus, hypothalamus, brainstem, and spinal cord. Unlike the MC3-R, MC4-R mRNA is found in both parvicellular and magnocellular neurons of the paraventricular nucleus of the hypothalamus...