Biphasic effect of 1,25‐dihydroxyvitamin D 3 on primary mouse epidermal keratinocyte proliferation

1,25‐Dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ] has been proposed as a physiologic regulator of keratinocyte growth and differentiation. Utilizing a proliferative serum‐free culture system, we have found that at physiologic (picomolar) concentrations this hormone stimulated proliferation of primary mouse epidermal keratinocytes; at higher (nanomolar to micromolar) doses, growth was inhibited by 1,25(OH) 2 D 3 . We investigated the nature of the signal transduction mechanism underlying the response to 1,25(OH) 2 D 3 and observed little or no effect of either low or high concentrations of the hormone on cytosolic calcium levels or Fos expression. Furthermore, the protein kinase C inhibitor, Ro 31‐7549, had very little effect on the growth inhibition induced by a high dose (1 μM) of 1,25(OH) 2 D 3 . This lack of rapid signal transduction events was consistent with the inability of a short (4‐hour) exposure to 1,25(OH) 2 D 3 to initiate a complete growth‐inhibitory response as measured using [ 3 H]thymidine incorporation. Our results indicate that physiologic concentrations of 1,25(OH) 2 D 3 are required for optimal keratinocyte growth. Furthermore, we found no evidence of rapid effects of 1,25(OH) 2 D 3 and suggest that in mouse epidermal keratinocytes, the response to this hormone is mediated by a slow transduction pathway, such as that activated by the intracellular 1,25(OH) 2 D 3 receptor (VDR). © 1995 Wiley‐Liss, Inc.