Retinoids as chemopreventive agents.

Retinoids are promising agents for cancer chemoprevention. The myriad effects of retinoids on biological processes including development, differentiation, homeostasis, carcinogenesis and apoptosis are mediated through their molecular targets, the retinoid and rexinoid receptors. Tissue specific expression patterns, ligand specificities, receptor numbers, their distinct functions and functional redundancy make retinoid signaling highly complex. The cross-talks of these receptors with cell surface receptors signaling pathways, as well as their interactions with multiple co-activators and co-repressors further add to the complexity of the pleiotropic effects of retinoids. Elucidation of retinoid signaling pathways and indepth understanding of the mechanisms that underlie the anti-proliferative and apoptotic action of retinoids has paved the way for designing synthetic retinoids for effective chemoprevention and therapy of cancer. Development of receptor selective synthetic retinoids is a major focus of molecular retinoid development. Other new avenues encompass identification of novel retinoid regulated genes, orphan-receptor ligands/functions, novel retinoid mechanisms involving receptor-independent apoptosis inducing activity and synergistic combinations with other agents for cancer prevention and therapy. This review focuses on recent advances in the understanding of molecular mechanisms underlying the action of retinoids and retinoid molecular targeting studies designed primarily to develop retinoids with reduced toxicity, while maintaining or enhancing activity in context of chemoprevention. The clinical efficacy of retinoid based chemoprevention trials is discussed.