DNA damage-inducible transcript 4 is an innate guardian for human squamous cell carcinoma and an molecular vector for anti-carcinoma effect of 1,25(OH)₂ D₃.

Cutaneous squamous cell carcinoma (SCC) is one of the most common non-melanoma skin cancers worldwide. While its exact tumorigenesis mechanisms is far from well-established and less satisfied therapeutic strategy can be clinically used nowadays. In this study, we intended to investigate the role of DNA damage-inducible transcript 4 (DDIT4) in human SCC. Firstly, we identified DDIT4 is significantly suppressed in human SCC tissue and cultured A431 cell line, and reduced DDIT4 accelerates keratinocytes proliferation but impedes the autophagy flux through mTORC1 pathway by affecting the downstream S6 Kinase1, 4E-BP1, Beclin1 and LC3 II/I. While 1,25(OH)₂ D₃ enhanced DDIT4 expression and activated autophagy and inhibit mTORC1 to take the effect of anti-proliferation and activating autophagy. Further, formation of direct vitamin D receptor (VDR)-DDIT4 transcription complex was verified by ChIP-qPCR, which showed the molecular mechanism of how 1,25(OH)₂ D₃ promotes DDIT4 transcription. Thirdly, xenograft tumor-bearing mice model treated by gradient concentrations of 1,25(OH)₂ D₃ revealed the obvious anti-carcinoma effect of 1,25(OH)₂ D₃ in vivo and DDIT4 acted the molecular vector of 1,25(OH)₂ D₃ through mTORC1. Lastly, elevated DDIT4 expression was verified in human actinic keratoses tissue, and chronic long-term ultraviolet (UV) irradiation on mouse disclosed UV could promote DDIT4 expression inside epidermis. Conclusively, our research suggested a novel molecular mechanism about the human SCC tumorigenesis and the pharmacological mechanism about how 1,25(OH)₂ D₃ take its anti-carcinoma role on human SCC, as well as a striking paradoxes that how UV irradiation plays the tumorigenesis effect but synchronously take a protective role in the early stage of SCC carcinogenesis.

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