Role of extracellular calcium in the regulation of 1,25-dihydroxyvitamin D₃ formation in cultured human keratinocytes

Cultured normal human keratinocytes (NHK) provide a useful experimental model for studies of processes occuring during terminal differentiation, since the extent of keratinocyte maturation can be manipulated experimentally by modulation of extracellular calcium concentration. When NHK are maintained in low calcium (0.06 mM) medium they proliferate but do not stratify. Raising the level of calcium to 1-2 mM results within a few hours in induction of keratinocyte differentiation. Results of the present study show that formation of 1,25-(OH)₂D₃ is higher in NHK grown at 0.06 mM than in NHK grown at 1.6 mM calcium concentration. After 2 h exposure of low calcium cultures to 1.6 mM calcium the 1,25-(OH)₂D₃ production starts to decrease. On the other hand, exposure of cells cultured in 1.6 mM calcium medium to 0.06 mM calcium concentration induced already within 4 h an increase in 1,25-(OH)₂D₃ formation which was not accompanied by a decrease in cornified envelope formation. Thereby, the present study demonstrated that calcium can regulate 1,25-(OH)₂D₃ formation independently of changes in keratinocyte differentiation.