The vitamin D receptor: a tumor suppressor in skin

Epidemiologic evidence supporting a major chemopreventive role for vitamin D in various malignancies is strong. Likewise the use of the active metabolite of vitamin D, 1,25(OH)(2)D(3), and its analogs to prevent and/or treat a wide variety of malignancies in animals is well established. The evidence has been less compelling for epidermal carcinogenesis perhaps because the same agent that produces vitamin D in the skin, UVB radiation (UVR), is also the same agent that results in most epidermal malignancies. However, recent studies indicate that the role of vitamin D and its receptor (VDR) in protecting against the development of epidermal tumors deserves a closer look. One such study found mice lacking the VDR were quite sensitive to epidermal tumor formation following the administration of the carcinogen DMBA. A more recent study showed that these mice were similarly more sensitive to tumor formation following UVR, results we have confirmed. The epidermis of the VDR null mouse is hyperproliferative with gross distortion of hair follicles, structures that may provide the origin for the tumors found in the skin following such treatment. Two interacting pathways critical for epidermal and hair follicle function, beta-catenin and hedgehog (Hh), result in epidermal tumors when they are activated abnormally. Thus, we considered the possibility that loss of VDR predisposes to epidermal tumor formation by activation of either or both beta-catenin and Hh signaling. We determined that all elements of the Hh signaling pathway are upregulated in the epidermis and utricles of the VDR null mouse, and that 1,25(OH)(2)D(3) suppresses the expression of these elements in normal mouse skin. In addition we observed that the transcriptional activity of beta-catenin was increased in keratinocytes lacking the VDR. These results lead us to the hypothesis that the VDR with its ligand 1,25(OH)(2)D(3) functions as a tumor suppressor with respect to epidermal tumor formation in response to UVR by regulating Hh and beta-catenin signaling.