Skin is an autonomous organ in synthesis, two-step activation and degradation of vitamin D 3: CYP27 in epidermis completes the set of essential vitamin D 3-hydroxylases

Abstract

The current understanding of the vitamin D 3 system shows skin as the unique site of vitamin D 3 production and liver is thought to be the main site of conversion to 25(OH)D 3. Skin is capable of activating 25(OH)D 3 via 1α-hydroxylation and the resulting 1α,25(OH) 2D 3 plays a role in epidermal homeostasis in normal and diseased skin. It also rapidly up-regulates the major vitamin D 3 metabolizing enzyme 24-hydroxylase at the mRNA level, which is an established indicator for 1α,25(OH) 2D 3-presence. We investigated the capability of primary human keratinocytes to produce 25(OH)D 3 and subsequent metabolites from vitamin D 3. Thus, by orchestrating the entire system of production, activation and inactivation, skin could be independent of other organs in supply of hormonally active vitamin D 3. First, we demonstrated substantial conversion of 3H-D 3 to 3H-25(OH)D 3 in primary human keratinocytes. 25-Hydroxylation was slow, followed first order rate kinetics and was not saturable under our experimental conditions. Then we showed expression of 25-hydroxylase mRNA and compared it to levels of 1α-hydroxylase and 24-hydroxylase. Pre-incubation with vitamin D 3 resulted in dose and time dependent up-regulation of 24-hydroxylase mRNA, whereas neither 1α-hydroxylase nor 25-hydroxylase expression was affected. Since both, D 3 and 25(OH)D 3 are lacking intrinsic 24-hydroxylase-inducing capacity, up-regulation had to be the consequence of a two-step activation process via 25-hydroxylation and subsequent 1α-hydroxylation. 24-Hydroxylase-activities closely followed the corresponding mRNA levels. When 1α,25(OH) 2D 3 itself or its precursor 25(OH)D 3 were used as inducing agents, 24-hydroxylase mRNA and enzyme activity followed a transient time course. In contrast, induction observed with physiological doses of D 3 remained high, even after a 20 h-time period. These differing characteristics may be explained by the slow but constant formation of 1α,25(OH) 2D 3 from a large reservoir of D 3 in the target cell, providing constant supplies for induction.