Expression of 25-Hydroxyvitamin D 3 -24-Hydroxylase mRNA in Cultured Human Keratinocytes

Abstract

Abstract

It is well documented that 1α,25-dihydroxyvitamin D₃ (1α,25[OH]₂D₃), the most active vitamin D metabolite, inhibits epidermal keratinocyte proliferation and promotes differentiation. 1α,25(OH)₂D₃ can be produced in keratinocytes from 25-hydroxyvitamin D₃ by the enzyme 25-hydroxyvitamin D₃-1α-hydroxylase (1-OHase). Hydroxylation of 1α,25(OH)₂D₃ by 25-hydroxyvitamin D₃-24-hydroxylase (24-OHase), the first step in the catabolic pathway of 1α,25(OH)₂D₃ could significantly reduce the intracellular concentration of 1α,25(OH)₂D₃. Therefore, the expression of 24-OHase could have a critical regulatory role in 1α,25(OH)₂D₃-dependent gene expression. As a first step to examine this possibility, the steady state level of 24-OHase mRNA in cultured human keratinocytes (CHK) was investigated. 24-OHase mRNA was not detected in control CHK. 1α,25(OH)₂D₃ caused a dose- and time-dependent increase in 24-OHase mRNA level. The highest accumulation of 24-OHase mRNA was observed in CHK treated with 0.1-1 μM 1α,25(OH)₂D₃. The level of 24-OHase mRNA reached a plateau 12-24 hr after 1α,25(OH)₂D₃ treatment. 1β,25-dihydroxyvitamin D₃, the stereoisomer of 1α,25(OH)₂D₃, failed to induce 24-OHase mRNA expression significantly. In addition to 24-OHase mRNA, a 1.0-kb mRNA hybridized strongly with both rat and human 24-OHase cDNA probes. The origin of this 1.0-kb message is unknown at present, however, it was regulated by 1α,25(OH)₂D₃. These results demonstrate that 1α,25(OH)₂D₃ up-regulates the expression of 24-OHase mRNA, and this may be an important first step in the initiation of catabolism of 1α,25(OH)₂D₃ in human keratinocytes.