Nuclear receptors function to transduce hormonal signals into transcription responses. Many of these receptors require dimerization with the retinoid receptor X (RXR), suggesting extensive interaction of vitamin A with many hormonal signals. However, for receptors without known ligands (orphan receptors), critical questions are what kind of signaling pathways do orphan receptors represent and what roles do they play in vitamin A biology. This lab has recently characterized an orphan receptor family TR2/TR4 which are specifically elevated in developing germ cells and able to modulate retinoic acid (RA) signals in several experimental systems. Based upon our preliminary studies, it is hypothesized that this orphan receptor family members function to modulate vitamin A signaling pathways by both active and passive mechanisms. Additionally, it is hypothesized that the active silencing mechanism is mediated by its ligand binding domain which encodes a trans-repressive activity and involves an interacting protein RIP140 that cross-talks with retinoic acid receptors RARs and RXRs. The passive mechanism is mediated by forming their own homo- or hetero-dimers within this family, which competes with RAR/RXR for DNA target sites. This proposal will test these hypotheses by asking three questions in three aims. 1) What is the molecular basis of the active silencing activity of TR2 that involves the interaction with RIP140? 2) What underlies the ability of TR2 and TR4 (but not RXR) hetero-dimerization? 3) Does TR2 and TR4 dimers (homo- and hetero-) efficiently compete for DNA binding sites with receptor dimers involving RARs or RXRs. Molecular and biochemical approaches will be taken to address these questions. We hope to extend our understanding of nuclear receptor functions in general, to provide a molecular explanation for the functions of orphan receptors without known hormonal signals, and to understand specifically how TR2 family members cross-talk with vitamin A for specific gene regulation. Ultimately, these studies will provide a model to address how vitamin A interacts with specific cellular factors and exerts its functions, and how the efficacy of vitamin A, as a therapeutic agent or a nutrient, may be modulated by specific cellular factors.
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