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Mechanisms of vitamin D3 activation and cathelicidin response. Extrarenal metabolism of vitamin D3 by keratinocytes provides a system for rapid control of cathelicidin expression. Activation of 25D3 to 1,25D3 requires 2 hydroxylation steps that occur sequentially in the liver and kidney. However, keratinocytes also express CYP27B1, a 1α-hydroxylase that activates 1,25D3. CYP27B1 expression in keratinocytes is controlled by danger signals during skin infection and tissue damage.
