CASPASE-DEPENDENT CLEAVAGE OF SIGNALING PROTEINS DURING APOPTOSIS - ATURN-OFF MECHANISM FOR ANTI-APOPTOTIC SIGNALS

Caspases are activated during apoptosis and cleave specific proteins, resulting in the irreversible commitment to cell death. The signal tra nsduction proteins MEKK1, p21-activated kinase 2, and focal adhesion k inase are caspase substrates that contribute to the cell death respons e when cleaved. Thirty additional signaling proteins were screened for their ability to be cleaved during apoptosis. Twenty-two of these pro teins were not affected in Jurkat cells stimulated to undergo apoptosi s by Fas ligation, exposure to ultraviolet-C or incubation with etopos ide. Ras GTPase-activating protein was found to be a caspase substrate whose cleavage followed the same time course as that for activation o f caspase activity and the cleavage of MEKK1 and focal adhesion kinase . Four additional proteins, Cbl, Cbl-b, Raf-1, and Akt-1, were cleaved later in the apoptotic response. These signaling proteins were simila rly cleaved in U937 cells undergoing apoptosis, Cleavage of the protei ns was blocked by caspase inhibitors in Jurkat cells or in U937 cells expressing Bclx(L), demonstrating that the cleavage was dependent on c aspase activation. Cleavage of Raf-1 and Akt correlated with the loss of extracellular signal-regulated kinase and Akt activities in apoptot ic cells. Neither c-Jun N-terminal kinase nor p38 mitogen-activated pr otein kinase was cleaved in cells undergoing apoptosis, and the activa tion of the c-Jun N-terminal kinase and p38 mitogen-activated protein kinase pathways was not compromised in apoptotic cells. These results indicate that caspase-dependent cleavage of specific proteins induces the turn off of survival pathways, such as the extracellular signal-re gulated kinase and phosphatidylinositol-3 kinase/Akt pathways, that co uld otherwise interfere with the apoptotic response.