MOLECULAR MECHANISMS OF GLUCOCORTICOID HORMONE ACTION

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Understanding the mechanisms of regulation of gene expression is essential to comprehension of development and differentiation, and thus, cancer and other disorders that may arise when these processes go awry. A detailed understanding of steroid hormone-dependent regulatory mechanisms may lead to improvement of existing therapeutic regimens and the development of novel avenues for intervention, particularly in endocrine-dependent neoplasia. Our long term goal is to understand these mechanisms through which steroids control expression of target genes. Past research efforts have focused on the interaction of steroid receptors with DNA. Within this framework, new strategies and applications will address three specific questions. I. How do glucocorticoid and progesterone receptors differentially induce target genes? There are a few differences among the DNA binding domains of glucocorticoid, progesterone, mineralocorticoid, and androgen receptors. Our studies imply that glucocorticoid and progesterone receptors recognize target DNA sequence similarly, if not identically. This raises the question of whether response to these two hormones is controlled strictly by the site of receptor expression or whether more subtle factor also significantly contribute. Experiments proposed in this section address in detail the fundamental biological conundrum posed above. II. How well does receptor occupancy of target elements in vivo correlate with hormone responsiveness? We propose a novel, retroviral vector approach to assess the relative extent of receptor occupancy of its target site in vivo as a function of the biological activity of the hormone response element. The magnitude of hormone response will be altered by modifying the promoter. context, the sequence of the target site, or by stimulation of other cell signal transduction pathways. Additional aspects of steroid-dependent alterations in stimulation of other cell signal transduction pathways. Additional aspects of steroid- dependent alterations in chromatin structure and transcription initiation complex loading will also be assessed. III. How does the glucocorticoid receptor find target elements in the genome? Buried within the mammalian genome are a handful of target genes. Models in which receptor conducts a search for targets sites by diffusion or by sliding along the DNA do not satisfactorily account for the difficulties posed by the amount and density of DNA in the nuclear environment. We propose that the glucocorticoid receptor can search DNA by interdomain transfer. Analogous to Tarzan swinging vine-to-vine, this mechanism envisions receptors transferring between DNA sites without ever dissociating from DNA. A combination of molecular, kinetic and physical approaches will be employed to test this proposal.