Vitamin D is well characterized for its role in mineral homeostasis and maintenance of normal skeletal architecture. Vitamin D has been demonstrated to exert anti-inflammatory effects in a variety of disease states including diabetes, arthritis and inflammatory bowel disease. In these diseases poly[adenosine diphosphate (ADP)-ribose] polymerase (PARP) inhibitors have also proved effective as anti-inflammatory agents. Here we present data demonstrating that the active metabolite of vitamin D, 1α,25-dihydroxy-vitamin D3, is a PARP inhibitor. UV irradiation-mediated PARP activation in human keratinocytes can be inhibited by treatment with vitamin D, 7-dehydrocholesterol or 1α,25-dihydroxyvitamin D3. Inhibition of cytochrome P450 reversed the PARP inhibitory action of vitamin D and 7-dehydrocholesterol, indicating that conversion to 1α,25-dihydroxyvitamin D3 mediates their PARP inhibitory action. Vitamin D may protect keratinocytes against over-activation of PARP resulting from exposure to sunlight. PARP inhibition may contribute to the pharmacological and anti-inflammatory effects of vitamin D.

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