Project 3: Molecular Targets of Rexinoid Action in Skin

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PROJECT SUMMARY. The incidence of non-melanoma skin cancer is increasing worldwide and it is especially high in the solid organ transplant population. These cancers are associated with a high morbidity in this population and, thus, it represents a significant public health burden. Systemic retinoids have proven to be effective for the chemoprophylaxis of non-melanoma skin cancers. They act by activating the transcription of RXR/RAR target genes, but their exact cancer chemopreventive mechanism is still not clear because retinoids have pleiotropic effects. The major limitation to the use of synthetic RAR agonists (acitretin, isotretinoin) at the doses required for skin cancer prevention, is poor tolerance due to headaches, musculoskeletal symptoms, hyperlipidemia, mucocutaneous inflammation and hepatotoxicity. The first clinically approved RXR agonist, bexarotene, is better tolerated than retinoids. However, hyperlipidemia induced by oral bexarotene is a major problem. Drs. Muccio and Atigadda (Project 2 and Core 2) have designed a selective RXR agonist, UAB30, which effectively prevents epithelial cancers (mammary, skin, etc.) but does not increase serum triglycerides in rodents or humans. While UAB30 clearly shows promise as a safe and effective chemopreventive drug, its mechanism of action at the molecular level is poorly understood. Our preliminary data indicate that treatment with UAB30 results in increased levels of ATRA in human skin epidermis. We propose that UAB30 potentiates the transcriptional activity of existing endogenous ATRA mediated by RXR/RAR heterodimers, which induce upregulation of genes responsible for the biosynthesis of ATRA. This leads to elevated levels of ATRA and further increase in transcriptional activity of RXR/RAR heterodimers. We also propose that the amplitude of upregulation of ATRA target genes serves as a good indicator of the potency of rexinoids for chemoprevention of non-melanoma skin cancer. Finally, we propose that evaluation of the UAB30 analogs based on their ability to induce ATRA signaling in epidermis can lead to development of the next generation of rexinoids with efficacy comparable to bexarotene but without its toxicity. These hypotheses will be tested by defining the molecular targets and studying the mechanism of action of UAB30 in epidermis using the dominant-negative mutant of RXRα and metabolic assays of retinoid metabolism (Aim 1); by characterizing the effect of UAB30 on skin cancer and markers of cell differentiation and proliferation using our novel model of human squamous cell carcinoma and mouse models of squamous cell carcinoma and basal cell carcinoma developed by Dr. Athar (Core 3); and by evaluating the next generation of UAB30 analogs for their potency in upregulation of ATRA target genes relative to UAB30 and bexarotene (Aim 3). These studies will provide novel insights into the mechanism of UAB30 action in epidermis, and are critical for developing the next generation of highly effective and safe rexinoids for chemoprevention of non-melanoma skin cancers in organ transplant recipients, and potentially for other malignancies and skin diseases.