The vitamin D endocrine system plays important but inadequately understood roles in skin homeostasis. 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] is the biologically active ligand for the vitamin D receptor (VDR). VDR knockout (KO) mice lack a functional receptor, whereas ligand knockout mice lack 25-hydroxyvitamin-D₃ 1α-hydroxylase (1αOHase), the enzyme required for 1,25(OH)₂D₃ synthesis. VDRKO mice have a hair follicle cycling defect resulting in alopecia and high susceptibility to chemically induced skin tumorigenesis, whereas 1αOHaseKO mice have normal follicle function. These models indicate that VDR functions independently of 1,25(OH)₂D₃ in hair follicle cycling.

The goal of this dissertation was to better understand the roles of VDR and 1,25(OH)₂D₃ in skin tumorigenesis and to elucidate how VDR activity is regulated in the skin. We demonstrate that 1αOHaseKO mice are resistant to chemically-induced tumorigenesis, indicating that 1,25(OH) ₂D₃ is not required for tumor protection. We also show that VDR protects against ultraviolet light-induced skin tumorigenesis, the relevant cause of skin cancer in humans. Proliferation assays indicate that UV inhibits short-term growth of wildtype, but not VDRKO, keratinocytes. Thymine dimer immunoblots show that VDR promotes DNA repair, and TUNEL assays indicate that VDR facilitates UV-induced apoptosis.

The differences between receptor and ligand knockout mice suggest that the actions of VDR and 1,25(OH)₂D₃ are uncoupled in the skin. Using reporter gene assays and RT-PCR, our research is the first to provide cell-based mechanistic evidence that VDR exerts activity in the absence of the 1,25(OH)₂D₃ ligand. The magnitude of 1,25(OH)₂D₃-independent VDR activity is on par with traditional, 1,25(OH)₂D₃-induced activity and occurs only in keratinocytes. Mammalian two-hybrid assays show that 1,25(OH) ₂D₃-independent activity of VDR stems from strong interaction between VDR and its partner, the retinoid X receptor, and is potentiated by the epidermal growth factor signaling cascade. We also show that constitutively active forms of certain Ca²⁺/calmodulin-dependent kinases enhance 1,25(OH)₂D₃-mediated transcription by increasing phosphorylation of VDR, promoting interaction between VDR and coactivators, and increasing the intrinsic transcriptional activity of steroid receptor coactivators. Cumulatively, our studies support a novel, 1,25(OH)₂D₃-independent role for VDR in keratinocytes that may be potentially exploited as a pharmacological target to treat hyperproliferative skin diseases.