B-CELL RECEPTOR CROSS-LINKING PREVENTS FAS-INDUCED CELL-DEATH BY INACTIVATING THE IL-1-BETA-CONVERTING ENZYME PROTEASE AND REGULATING BCL-2BCL-X EXPRESSION/

In the A20 cell line, we examined the mechanisms that modulate the fas -mediated apoptotic pathway through the B cell receptor. As in other s ystems, Fas signaling activates cysteine proteases, leading to specifi c proteolysis of poly(ADP-ribose) polymerase (PARP) and protein kinase C (PKC) delta. We describe that PKC-epsilon and PKC-zeta proteins are two new IL-1 beta-converting enzyme (ICE) substrates; we found that I CE activation and its proteolytic effects are inhibited by surface IgG (sIgG) cross-linking, Apoptosis induced by Fas ligation is consequent ly abrogated after sIgG engagement, and slgG signaling therefore inter feres with the apoptotic signal upstream of ICE protease activation. S ince tile PKC inhibitor bisindolymaleimide I completely abolishes the protective effect of the sIgG signal, a member of tile PKC family is p robably responsible for the prevention of ICE cascade activation. Dire ct activation of PKC by PMA partially mimics the protective effect of slgG cross-linking against Fas-mediated death in A20 cells, Neverthele ss, PMA inhibits neither ICE activation nor the subsequent proteolysis of ICE substrates, suggesting that the PKC responsible for ICE inacti vation is a non-PMA-sensitive PKC, In this system, Fas ligation also t riggers Bcl-2/Bcl-x down-regulation, an effect inhibited by sIgG cross -linking, the cysteine protease inhibitor acetyl-Tyr-Val-Ala-Asp-chlor omethyl ketone, and PMA treatment. In A20 cells, Fas signaling may thu s trigger both ICE activation and Bcl-x and Bcl-2 down-regulation, The se results indicate that sIgG signaling gives rise to two pathways aft er PKC activation, one presumably promoted by non-PMA-sensitive PKC, w hich inactivates the ICE cascade, and another produced by PMA-sensitiv e PKC, which maintains normal Bcl-2/Bcl-x levels.