Terminal differentiation of human keratinocytes and stratum corneum formation is associated with caspase-14 activation.

Programmed cell death of epidermal keratinocytes (KC) results in the formation of cornified cells, which constitute the outermost skin layer, the stratum corneum. Here we show by reverse transcription-polymerase chain reaction, western blot, and immunohistochemistry that epidermal KC express caspase-14, a member of the caspase family of pro-apoptotic proteases, in a tissue-specific manner. Caspase-14 protein abundance strongly increases during terminal differentiation of KC in vivo and in vitro. Under conditions that lead to stratum corneum formation caspase-14 cleavage products, which indicate proenzyme activation, appeared in the KC lysates. Cleavage of the enzyme was also detected in lysates from normal human epidermis and in extracts of stratum corneum. Our findings demonstrate that caspase-14 is activated during KC differentiation and strongly suggest that it is involved in the formation of the human skin barrier.J Invest Dermatol 115:1148-1151 2000

001: hepatocyte growth factor/scatter factor inhibits UVB induced apoptosis of human keratinocytes via the PI-3-kinase pathway

UVB-irradiation induces apoptosis in primary keratinocytes (KC) and KC-derived cell-lines A431 and HaCaT. Here we report on the inhibition of UV induced KC-apoptosis by hepatocyte growth factor/scatter factor (HGF/SF). The protective effect of HGF/SF for UVB-irradiated primary KC was observed at concentrations as low as 1 ng/ml HGF and was confirmed by demonstration of the inhibition of nucleosome-release and the activation of caspase-3. In contrast to the observation with primary KC HGF/SF had no effect on the survival of A431 and HaCaT cells after UVB-irradiation, despite the fact that we could demonstrate that these cells functionally express the HGF/SF receptor c-met. When blocking signalling pathways initiated by c-met, we found that the inhibition of the phosphatidylinositol-3-OH (PI-3) kinase by wortmannin or LY294002 led to a total inhibition of the anti-apoptotic effect of HGF/SF, whereas the blockade of the MAP-kinase pathway by PD90859 had no effect. This represents the first demonstration of an involvement of the PI-3 kinase pathway in the anti-apoptotic effect of HGF/SF. In conclusion, our data demonstrate that HGF/SF is able to rescue KC but not autonomously growing KC cell lines from apoptosis induced by UVB. Since in vivo HGF/SF is produced by mesenchymal cells, this mechanism may represent an important paracrine loop in the skin supporting the survival of KC after UV-injury.