Ronald M. Evans Professor of Biology, The Salk Institute Investigator, Howard Hughes Medical Institute e-mail: evans@salk.edu

     In our current work, we have utilized the yeast-two-hybrid screening approach to isolate nuclear receptor co-activators and co-repressors that may represent key molecules that underlie the hormonal switch. This work has resulted in two important discoveries. First, of three co-activators that we have characterized, all interact with each other and thus, by definition, form a co-activator complex. In addition, each of these co-activators has an intrinsic enzymatic activity that allows it to modify histone tails. This activity, referred to as histone acetylase transferase or HAT, represents an important link between chromatin remodeling and transcriptional activation. We are pursuing these studies by investigating how ligand binding to the receptor creates the interaction surface for binding the co-activator complex, and once bound, how does the localized concentration of HAT activity lead to transcriptional activation. The reciprocal process to activation is repression. In absence of ligand, many nuclear receptors are potent transcriptional repressors. Accordingly, we have isolated a separate set of co-factors (termed co-repressors) that mediate transcriptional silencing. The silencing complex also contains at least three proteins and includes a novel enzymatic activity referred to as histone deacetylase. In removing acetate groups from histone tails, the proteins can now bind more tightly to DNA and thus, give rise to a more repressive chromatin structure. Thus, critical aspects of hormonal signalling are mediated by the selective and targeted recruitment of enzymatic chromatin remodelers to the transcriptional template. We continue to actively characterize the co-activator and co-repressor complexes both in isolation of new proteins as well as understanding how the identified proteins exert their regulatory effects. Orphan receptors possess the structural features of known hormone receptors, but lack identified ligands. The search for hormonal activators for these newly discovered receptors has created an exciting area of research. Over the last several years we have identified five new distinct structural classes of nuclear receptor ligands for orphan receptors. These ligands include novel retinoids, novel prostaglandin derivatives, eicosanoids, unusual hydroxy lipids, and most recently, a molecular variant of para-amino-benzoic acid (PABA). In each case, the identification of a new orphan ligand in combination with an orphan receptor creates new opportunities to understand fundamental aspects of body physiology. For example, work on the peroxisome proliferator-activated receptors (PPARs) has led to three new ligands that are involved in energy metabolism specifically controlling fat burning, fat storage, and sugar utilization. This critical class of orphan nuclear receptor now appears to be a major target for a new generation of pharmaceutical compounds to control diabetes, obesity, and cardiovascular disease. This area is exciting now only because it identifies new vertebrate hormones, but also because it answers longstanding questions about cellular differentiation, embryo physiology, as well as offering new in-roads to human disease. 

      Nagy, L., Tontonoz, P., Alvarez, J.G.A., Chen, H. and Evans, R.M. (1998) Oxidized LDL regulates macrophage gene expression and foam cell formation through novel endogenous ligands of PPARg: 9-HODE and 13-HODE. Cell 93:229-240. 

      Tontonoz, P., Nagy, L., Alvarez, J.G.A., Thomazy, V.A. and Evans, R.M. (1998) A PPARg-mediated pathway for foam cell formation. Cell 93:241-252. 

      Nagy, L., Kao, H.-Y., Chakravarti, D., Lin, R., Hassig, C.A., Ayer, D.E., Schreiber, S.L. and Evans, R.M. (1997) Nuclear receptor repression mediated by a complex containing SMRT, Sin3, and histone deacetylase. Cell 89:373-380. 

      Chen, H., Lin, R.J., Chakravarti, D., Shiltz, L., Nash, A., Nagy, L., Privalsky, M.L., Nakatani, Y. and Evans, R.M. (1997) Nuclear receptor co-activator ACTR is a novel histone acetyltransferase and forms a multimeric activation complex with P/CAF and CBP/p300. Cell 90:569-580.