The hormone 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) elicits the programmed pattern of differentiation in epidermal keratinocytes. Based on data indicating a potential role of phospholipase D (PLD) in mediating keratinocyte differentiation, we investigated the effect of 1,25(OH) 2 D 3 on PLD expression. A 24-h exposure to 1,25(OH) 2 D 3 stimulated PLD-1, but not PLD-2, mRNA expression. This 1,25(OH) 2 D 3 -enhanced expression was accompanied by increased total PLD and PLD-1 activity. Time

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... rse studies indicated that 1,25(OH) 2 D 3 induced PLD-1 expression by 8 h, with a maximal increase at 20 -24 h. Exposure to 1,25(OH) 2 D 3 inhibited proliferation over the same time period with similar kinetics. Expression of the early (spinous) differentiation marker keratin 1 decreased in response to 1,25(OH) 2 D 3 over 12-24 h. Treatment with 1,25(OH) 2 D 3 enhanced the activity of transglutaminase, a late (granular) differentiation marker, by 12 h with a maximal increase after 24 h. In situ hybridization studies demonstrated that the highest levels of PLD-1 expression are in the more differentiated (spinous and granular) layers of the epidermis, with little expression in basal keratinocytes. Our results suggest a role for PLD expression/activity during keratinocyte differentiation.