15-Deoxy-?12, 14-prostaglandin J2 (15d-PGJ2), a natural metabolite of prostaglandin D2 (PGD2), has potent pro-apoptotic activity in cancer cells. Although 15d-PGJ2 is the highest affinity natural ligand for peroxisome proliferator-activated receptor ??(PPAR?), the actual molecular target of 15d-PGJ2 for inducing apoptosis remains to be identified and characterized. Recently, we discovered a novel apoptotic pathway in cancer cells, in which orphan nuclear receptor TR3 translocates from the nucleus to the mitochondrion to induce cytochrome c (cyt c) release and apoptosis. In our preliminary studies, we observed that TR3 must heterodimerize with retinoid X receptor ? (RXR?) to translocate from the nucleus to the cytoplasm, where TR3 binds mitochondrial Bcl-2 to induce cyt c release and apoptosis. In addition, we discovered that 15d- PGJ2 binds RXR? and induces RXR? translocation from the nucleus to the cytoplasm, resulting in cotranslocation of TR3. These exciting findings lead us to propose that 15d-PGJ2 is a natural RXR? ligand, which acts by inducing RXR? cytoplasmic activity, and that 15d-PGJ2 promotes cancer cell apoptosis by inducing RXR?/TR3 cytoplasmic localization and mitochondrial targeting. The major objectives of this multidisciplinary research application are to study how binding of 15d-PGJ2 to RXR? regulates RXR? nuclear export and TR3-dependent apoptosis and on the basis of these findings develop efficient synthetic analogs that selectively modulate RXR? cytoplasmic action (rexporters).
Four Aims are proposed to accomplish our goals: 1. To determine the role of RXRs in mediating the apoptotic effects of 15d-PGJ2 in vitro and in vivo using RXR? stable transfection and RNAi approaches. 2. To investigate the molecular mechanisms by which 15d-PGJ2 binding modulates receptor dimerization and nuclear export of RXR? and its heterodimers, as well as how TR3 phosphorylation by Akt regulates RXR??TR3 dimerization and nuclear export. 3. To examine the effect of 15d-PGJ2 binding on the interaction of the RXR?/TR3 heterodimer and Bcl-2 and its impact on mitochondrial activities. 4. To design and synthesize 15d-PGJ2 analogs with enhanced RXR selectivity and activity using computational approaches coupled with the results of bioevaluation. Results obtained from our studies will establish 15d-PGJ2 as a natural RXR? ligand regulating a novel RXR signaling pathway, enhance our understanding of the molecular mechanism by which 15d-PGJ2 induces apoptosis, and lead to the identification of more effective synthetic rexporters with cancer preventive value.
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