A SECRETED SERINE-PROTEASE CAN INDUCE APOPTOSIS IN PAM212 KERATINOCYTES

The epidermal keratinocyte cell line Pam212 undergoes spontaneous apop tosis in culture, providing an in vitro model for the early steps of e pidermal differentiation. Pam212 cells exhibit characteristics of basa l keratinocytes, committed for the transition to the spinous layer of the epidermis. Bcl-2 can regulate the differentiation of these cells b y negatively regulating several genes that have been implicated in apo ptosis. We show evidence that a serine protease activity, secreted by the Pam212 cells, could induce apoptosis in Pam212 and several other c ell lines. This activity might be regulated via the bcl-2 pathway. We suggest that this serine protease could either directly, via binding a nd/or cleavage of a serine protease-activated receptor, or indirectly, via the cleavage of an unknown protein, activate the signaling for ap optosis in Pam212 cells. Alternatively, this secreted serine protease could reenter the cell and start a proteolytic cascade reaction that l eads to cell death. This is based on the induction of apoptosis in sev eral cell lines by the partially purified serine protease activity, an d the minimal effect of protein synthesis inhibition on Pam212 apoptos is. We propose that in vivo, a two-step mechanism controls keratinocyt e apoptosis and differentiation. The basal cells of the epidermis cont ain all of the necessary proteins required for apoptosis, as well as t he repressor protein Bcl-2. As Bcl-2 levels go down, the cells commit to terminal differentiation. A serine protease, secreted from these ce lls, then induces the death process. This second step enables the cell s to undergo apoptosis and continue the process of terminal differenti ation.