The proliferation of animal cells is tightly regulated by polypeptide growth factors, which interact with their specific cell surface receptors and trigger a series of biochemical events that culminate in the rapid activation of a set of genes without requiring de novo protein synthesis. Among these """"""""immediate-early"""""""" genes activated by growth factors are those that encode transcription regulators, cytokines, kinases and phosphatases. The activities of immediate-early gene products are thought to mediate the biological responses to the growth factors. One such immediate-early gene is nur77, which encodes a transcription factor of the steroid/thyroid hormone receptor superfamily whose functions have been implicated in the regulation of steroidogenesis and apoptosis. In various cell types, nur77 can be activated by a variety of extracellular signals including hormones, mitogens, differentiation factors, and neurotransmitters, which cause disparate biological responses. In addition, nur77 is also inducible through physiologic and pharmacologic input in rodents. Thus, nur77 provides a particularly interesting and comprehensive model in which to study gene regulation in both cell culture systems and in mammalian experimental models. In this proposal, experiments are designed to address the mechanisms regulating immediate-early gene induction in cell culture systems and in the whole animal. First, several hypotheses concerning the mechanisms of immediate-early genes activation through cellular kinases, inactivation during cellular senescence, and the molecular basis of cell-type specificity will be tested. Second, the mechanisms of how JunD activates nur77 in PC12 cells upon stimulation by nerve growth factor and membrane depolarization will be examined. Third the pathways mediating physiologic and pharmacologic regulation of nur77 in rodents will be analyzed. Finally, the promoter elements identified to be important for activation in cell culture systems will be examined in the context of the whole animal using transgenic mice. It is hoped that these studies will lead to a better understanding of the mechanism of gene activation in both cultured cells and in the organism.
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