Tumor necrosis factor related apoptosis inducing ligand triggers apoptosis in dividing but not in differentiating human epidermal keratinocytes

Using serial analysis of gene expression we have previously identified the expression of several pro-apoptotic and anti-apoptotic genes in cultured human primary epidermal keratinocytes, including tumor necrosis factor related apoptosis inducing ligand (TRAIL). TRAIL is a potent inducer of apoptosis in transformed and tumor cell lines, but usually not in other cells. Here we present a study on the effect of TRAIL on cultured keratinocytes. It is shown that differentiated and undifferentiated keratinocytes undergo apoptosis after addition of TRAIL to the medium as determined by morphologic and biochemical criteria, such as cellular shrinkage and activation of caspases. The sensitivity for TRAIL differs greatly between undifferentiated and differentiating keratinocytes, however, with undifferentiated cells being much more susceptible to apoptosis. Commitment to terminal differentiation in the absence of TRAIL does not in itself induce apoptosis. In contrast to the promyelocytic cell line HL60, internucleosomal DNA fragmentation is not observed in keratinocytes, as assessed by flow cytometric analysis and agarose gel electrophoresis. Interestingly, the prime effector of DNA fragmentation, DNA fragmentation factor of 40 kDa (DFF40), is expressed in keratinocytes, yet internucleosomal cleavage fails to occur. Our data indicate that programmed cell death during keratinocyte differentiation is distinct from receptor-mediated apoptosis in response to a death ligand.