DECREASE OF CA2-ATPASE ACTIVITY IN HUMAN KERATINOCYTES DURING CALCIUM-INDUCED DIFFERENTIATION()

Ca2+ regulates keratinocyte differentiation by increasing intracellula r Ca2+ levels. Ca2+-ATPase in the Ca2+-induced differentiation of huma n keratinocytes was investigated by measuring Ca2+-ATPase mRNA, protei n, and activity levels. Human keratinocytes were grown in Keratinocyte Growth Medium containing 0.03, 0.1, or 1.2 mM Ca2+ and assayed on day s 2, 5, 7, 14, and 21. Ca2+-ATPase mRNA levels were found to be modest ly increased in 5-, 7-, and 14-day cultured cells as compared with 2-d ay cultured cells, hut levels fell below that of the 2-day cultured ce lls in the 21-day cultured cells. The Ca2+-ATPase mRNA levels were not affected by Ca2+ levels. A 135-kDa protein in human keratinocytes cro ss reacted with the monoclonal antibody against human erythrocyte Ca2-ATPase. The level of-this protein was decreased by Ca2+ and lost duri ng differentiation, in parallel with the loss of enzymatic activity. C a2+ influx of postconfluent 1.2 mM Ca2+-grown cells was higher than th at oi cells grown in lower Ca2+ concentrations. Ca2+ efflux from postc onfluent cells grown in 0.03 mM Ca2+ was less than that from cells gro wn in stronger Ca2+ concentrations. These results suggest that the los s of the plasma membrane Ca2+-ATPase with time in culture contributes to the rise in intracelluar Ca2+, thus promoting keratinocyte differen tiation. (C) 1997 Wiley-Liss, Inc.