Free, and Not Total, 1,25-Dihydroxyvitamin D Regulates 25-Hydroxyvitamin D Metabolism by Keratinocytes

Greater than 99% of the total circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D] is bound to proteins such as the vitamin D-binding protein (DBP) and albumin; in the normal human only 0.4% of the circulating 1,25-(OH)2D is free. Although it is often assumed that only the free concentration of 1,25-(OH)2D is available to cells, this has not been demonstrated. In particular, it is not clear whether the DBPs facilitate 1,25-(0H)2D entry into target cells or serve only to transport these metabolites within the circulation. To address this question, we evaluated one of the best characterized target tissue responses to 1,25-(OH)2D, namely its ability to inhibit its own production and induce that of 24,25-(OH)2D, using one of the most sensitive cells, the human foreskin keratinocyte. We incubated keratinocytes in the presence of 1,25-(OH)2D (from 10-11-10-8 M) in medium containing albumin (from 0.1–10%) or serum (from 0.1–10%) for 4 h [to inhibit the 25-hydroxyvitamin D (250HD) 1α-hydroxylase] or 16 h (to induce ...