EXPRESSION OF 25-HYDROXYVITAMIN D-3-24-HYDROXYLASE MESSENGER-RNA IN CULTURED HUMAN KERATINOCYTES

It is well documented that 1 alpha,25-dihydroxyvitamin D-3 (1 alpha,25 [OH]D-2(3)), the most active vitamin D metabolite, inhibits epidermal keratinocyte proliferation and promotes differentiation. 1 alpha,25(OH )(2)D-3 can be produced in keratinocytes from 25-hydroxyvitamin D-3 by the enzyme 25-hydroxyvitamin D-3-1 alpha-hydroxylase (1-OHase). Hydro xylation of 1 alpha,25(OH)(2)D-3 by 25-hydroxyvitamin D-3-24-hydroxyla se (24-OHase), the first step in the catabolic pathway of 1 alpha,25(O H)(2)D-3 could significantly reduce the intracellular concentration of 1 alpha,25(OH)(2)D-3. Therefore, the expression of 24-OHase could hav e a critical regulatory role in 1 alpha,25(OH)(2)D-3-dependent gene ex pression. As a first step to examine this possibility, the steady stat e level of 24-OHase mRNA in cultured human keratinocytes (CHK) was inv estigated. 24-OHase mRNA was not detected in control CHK. 1 alpha,25(O H)(2)D-3 caused a dose- and time-dependent increase in 24-OHase mRNA l evel. The highest accumulation of 24-OHase mRNA was observed in CHK tr eated with 0.1-1 mu M 1 alpha,25(OH)(2)D-3 treatment. 1 beta,25-dihydr oxyvitamin D-3, the stereoisomer of 1 alpha,25(OH)(2)D-3, failed to in duced 24-OHase mRNA expression significantly. In addition to 24-OHase mRNA, a 1.0-kb mRNA hybridized strongly with both rat and human 24-OHa se cDNA probes. The origin of this 1.0-kb message is unknown at presen t, however, it was regulated by 1 alpha,25(OH)(2)D-3. These results de monstrate that 1 alpha,25(OH)(2)D-3 up-regulated the expression of 24- OHase mRNA, and this may be an important first step in the initiation of catabolism of 1 alpha,25(OH)(2)D-3 in human keratinocytes.