1α, 25-Dihydroxyvitamin D(3) and the vitamin D receptor regulates ΔNp63α levels and keratinocyte proliferation

1α, 25-dihydroxyvitamin D(3) (VD(3)), a secosteriod that has been explored as an anti-cancer agent, was also shown to promote cell survival. Its receptor, the Vitamin D Receptor (VDR), is a direct target of the proto-oncogene ΔNp63α, which is overexpressed in non-melanoma skin cancers. The interconnection between VDR/VD(3) signaling and ΔNp63α, led us to examine whether VDR/VD(3) signaling promotes keratinocyte proliferation by regulating ΔNp63α levels. Our data demonstrate that VDR regulates ΔNp63α expression at both the transcript and protein level. Interestingly, although low doses of VD(3) led to an increase in ΔNp63α protein levels and keratinocyte proliferation, high doses of VD(3) failed to increase ΔNp63α protein levels and resulted in reduced proliferation. Increased expression of ΔNp63α by low dose VD(3) was shown to be dependent on VDR and critical for the proliferative effects of VD(3). VD(3)-mediated increases in ΔNp63α protein levels occur via activation of both p38 MAPK and Akt kinases. Finally, analysis of samples from patients with squamous cell carcinoma (SCC), basal cell carcinoma and precursors to invasive SCC demonstrated a significant correlation between p63 and VDR levels when compared with healthy normal skin control samples. Delineation of the mechanisms by which VD(3) exerts its effect on ΔNp63α and cell proliferation is critical for determining the future of VD(3) in cancer therapies.