Optimizing Pro-apoptotic Compounds to Treat Cancer

We have found that on treatment with apoptosis-inducing compounds, cancer cells first experience increased levels of the cyclin kinase inhibitor p21WAF1/CIP1, followed by cell-cycle arrest. Cells then undergo the intrinsic form of apoptosis, which involves the release of cytochrome c and caspase activation.

Recently, the Zhang group found that expression of the transcription factor TR3 (nur77/NGFI-B) is induced ins some cancer cells. TR3 then exits the nucleus to interact with mitochondrial membrane-bound Bcl-2. The interaction between TR3 and Bcl-2 transforms the latter from a cytoprotective protein to one that fosters apoptosis. To facilitate these mechanistic studies, we designed and synthesized the tritiated analog of the parent compound, antagonists that prevent apoptosis, and other analogs that are only capable of inducing cell-cycle arrest. Recent research with the Fontana group led to identification of small heterodimer partner (SHP) as these compounds’ target. SHP is a unique orphan nuclear receptor that lacks a DNA-binding domain and modulates the activity of other transcription factors. These compounds are the first ligands identified for this receptor. Apoptosis induced by the interaction of these compounds with SHP requires the activation of the transcription factor NF-kB.

Retinoic Acid Analogs

Our second area of interest is the design and synthesis of synthetic analogs of retinoic acid that show selectivity for one of the retinoid receptor subtypes, of which there are six. Retinoic acid is the carboxylic acid analog of vitamin A and functions as a hormone by regulating such diverse processes as morphogenesis, cell differentiation, and cell proliferation. The retinoid receptors function as dimeric transcription factors that directly interact with responsive elements in gene promoters and in the presence of their retinoid ligands induce gene transcription. They also function indirectly by modulating the activity of other transcription factors. Because of the plethora of these effects, many signaling pathways are impacted. Thus, our objective is the identification of receptor dimer-selective retinoids that target specific signaling pathways. This research is also a collaborative effort with the Zhang group. Our group was the first to report retinoid X receptor-selective agonists. We have also identified several RXR antagonists and a new scaffold for RXR agonists.