Transcriptional and functional regulation of cell cycle and UV response by PPARβ in human skin epidermal cells.

Solar radiation is the main source of human exposure to UV rays, which is the major carcinogen in skin cancers by inducing DNA damage. Skin cells repair these damages by activating the DNA damage response (DDR) to safeguard genome integrity, thereby preventing skin cancers. Peroxisome proliferator-activated receptor beta (PPARβ), a druggable transcription factor, is involved in the development of UV-dependent skin cancers, although its role is not mechanistically elucidated. We showed previously that PPARβ knockout (KO) mice are less prone to UV-induced skin cancers. Here, we report that PPARβ directly regulates gene expression programs associated with cell cycle and DNA repair pathways in normal human epidermal keratinocytes (NHEK). The loss of function of PPARβ in human keratinocytes led to a downregulation in the expression of key cell cycle regulators, including cyclins and cyclin-dependent kinases (CDKs). Simultaneously, it upregulated the expression of p21 protein, a known CDK inhibitor. These molecular alterations resulted in a significant reduction in cell proliferation and induced cell cycle arrest at the G2/M phase. Moreover, the absence of functional PPARβ disrupted the expression and activation of the ataxia telangiectasia and Rad3-related (ATR) pathway, a critical component of the cellular response to UV-induced DNA damage. The alterations in the ATR pathway likely contributed to an increased apoptotic response of NHEK to UV radiation. Using a mouse melanoma model, we demonstrated that the depletion of PPARβ decreases tumorigenicity of melanoma cells and delays tumor formation. Our data suggest that PPARβ inhibition could be considered as a therapeutic target for the prevention of UV-induced skin cancers, by regulating cell proliferation, attenuating DDR, and eliminating skin cells with high UV-induced mutational burden.