Regulation of transmembrane signalling system during senescence of human epidermal keratinocytes.

Cultured normal human epidermal keratinocytes show an increase in doubling time with increasing passage number and finally the cells show growth arrest. This is known as senescence of cultured cells. However, the mechanisms governing senescence of cells remain to be clarified. Stimulation from outside the cell and the response to the stimulation are important essential initial events for cell function. Alteration of intracellular Ca2+ is one of the essential responses of cells to stimulation from outside. Thus we studied whether cultured normal human epidermal keratinocytes show a modulation of the alteration of intracellular Ca2+ during senescence. Epinephrine and histamine induced transient increases in intracellular Ca2+ in the third to eighth passages in normal human epidermal keratinocytes. With increase in the passage number the responsiveness (the number of responding cells per examined cells) decreased particularly beyond the sixth passage. The attenuation of the responses was more obvious with epinephrine than with histamine. All-trans-retinoic acid and 1,25-dihydroxyvitamin D3 did not augment the responsiveness to epinephrine of normal human epidermal keratinocytes. We expected that such an essential and immediate reaction would be confined to living cells, and that during senescence cells would show reduced responsiveness. Some of the changes during senescence of cultured keratinocytes may be due to the attenuation of the responsiveness to stimulation through the cell membrane.