1α,25-Dihydroxy-3-epi-vitamin D 3 a physiological metabolite of 1α,25-dihydroxyvitamin D 3: Its production and metabolism in primary human keratinocytes

Abstract

Recent studies of metabolism using pharmacological substrate concentrations of 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2D 3] in several tissues including primary cultures of human keratinocytes, bovine parathyroid cells and bone cells led to the identification of 1α,25-dihydroxy-3-epi-vitamin D 3 [1α,25(OH) 2-3-epi-D 3] as a major natural metabolite of 1α,25(OH) 2D 3. In the present study, we demonstrate that human keratinocytes incubated with 25-hydroxy[26,27- 3H] vitamin D 3 produce 1α,25(OH) 2-3-epi-D 3 along with 1α,25(OH) 2D 3. The production of 1α,25(OH) 2-3-epi-D 3 is also identified in human keratinocytes incubated with physiological substrate concentrations of 1α,25(OH) 2D 3. Unlike 24-hydroxylase, the major enzyme involved in the further metabolism of 1α,25(OH) 2D 3 in human keratinocytes, the enzyme(s) responsible for the production of 1α,25(OH) 2-3-epi-D 3 is constitutive and is not inhibited by ketoconazole. It is also noted that 1α,25(OH) 2-3-epi-D 3 is further metabolised in human keratinocytes into several as yet unidentified metabolites, the production of which is inhibited to a great extent by SDZ 89-443, an inhibitor of 24-hydroxylase. This finding indicates that the 24-hydroxylase like in the case of 1α,25(OH) 2D 3, also plays a major role in the metabolism of 1α,25(OH) 2-3-epi-D 3. The results obtained from the metabolism studies performed in parallel among 25OHD 3, 1α,25(OH) 2D 3 and 1α,25(OH) 2-3-epi-D 3 indicate that 1α,25(OH) 2-3-epi-D 3 and its metabolites exhibit higher metabolic stability. In summary, we demonstrate for the first time that 1α,25(OH) 2-3-epi-D 3 is a physiological metabolite of 1α,25(OH) 2D 3 in human keratinocytes. Also, 1α,25(OH) 2-3-epi-D 3 is further metabolised in human keratinocytes mainly through the activity of 24-hydroxylase. Furthermore, our finding of the relative metabolic stability of 1α,25(OH) 2-3-epi-D 3 and especially its metabolites when compared to 1α,25(OH) 2D 3 and its metabolites provides an important explanation for its previously observed potent inhibitory effect on keratinocyte growth in spite of its low affinity to vitamin D receptor.