Antitumoral effects of calcitriol in basal cell carcinomas involve inhibition of hedgehog signaling and induction of vitamin D receptor signaling and differentiation

Activation of the Hedgehog (Hh)-signaling pathway due to deficiency in the Hh receptor Patched1 (Ptch) is the pivotal defect leading to formation of basal cell carcinoma (BCC). Recent reports provided evidence of Ptch-dependent secretion of vitamin D(3)-related compound, which functions as an endogenous inhibitor of Hh signaling by repressing the activity of the signal transduction partner of Ptch, Smoothened (Smo). This suggests that Ptch-deficient tumor cells are devoid of this substance, which in turn results in activation of Hh-signaling. Here, we show that the application of the physiologically active form of vitamin D(3), calcitriol, inhibits proliferation and growth of BCC of Ptch mutant mice in vitro and in vivo. This is accompanied by the activation of the vitamin D receptor (Vdr) and induction of BCC differentiation. In addition, calcitriol inhibits Hh signaling at the level of Smo in a Vdr-independent manner. The concomitant antiproliferative effects on BCC growth are stronger than those of the Hh-specific inhibitor cyclopamine, even though the latter more efficiently inhibits Hh signaling. Taken together, we show that exogenous supply of calcitriol controls the activity of 2 independent pathways, Hh and Vdr signaling, which are relevant to tumorigenesis and tumor treatment. These data suggest that calcitriol could be a therapeutic option in the treatment of BCC, the most common tumor in humans.