Caspase-dependent Cleavage of Signaling Proteins during Apoptosis

Abstract

Caspases are activated during apoptosis and cleave specific proteins, resulting in the irreversible commitment to cell death. The signal transduction proteins MEKK1, p21-activated kinase 2, and focal adhesion kinase are caspase substrates that contribute to the cell death response when cleaved. Thirty additional signaling proteins were screened for their ability to be cleaved during apoptosis. Twenty-two of these proteins were not affected in Jurkat cells stimulated to undergo apoptosis by Fas ligation, exposure to ultraviolet-C or incubation with etoposide. Ras GTPase-activating protein was found to be a caspase substrate whose cleavage followed the same time course as that for activation of 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. Apoptosis is regulated by a series of biochemical events that commit a cell to death. A common feature of cells undergoing apoptosis is the activation of caspases, a family of aspartic acid-directed proteases (1). Caspase substrates are rapidly being identified, but the general assumption is that caspases recognize a limited set of cellular proteins (2). Caspase-mediated proteolysis of specific proteins results in an irreversible commitment of cells to undergo apoptosis characterized by cytoplasmic shrinkage, membrane blebbing, nuclear condensation, and DNA fragmentation.Caspase-dependent cleavage can inactivate protein substrates. Examples include poly(ADP-ribose) polymerase, lamin, and focal adhesion kinase. Cleavage of poly(ADP-ribose) polymerase abolishes its DNA repair ability in cells undergoing apoptosis, nuclear lamin degradation contributes to nuclear condensation, and cleavage of focal adhesion kinase impairs the ability of cells to maintain matrix adherence (2). In contrast, there are examples, besides the caspases themselves (3), where cleavage actually activates the substrate; examples of such substrates include MEKK1, 1 p21-activated kinase, protein kinase C␦, and gelsolin. When cleaved, each of these proteins contributes to the apoptotic response (4 -9).Signal transduction pathways involving the mitogen-activated protein kinases (MAPKs) including the ERKs, JNKs, and p38/HOG1 kinase, have been shown to differentially contribute to pro-and anti-apoptotic pathways (10). In addition, the phosphatidylinositol 3-phosphate-regulated protein kinase Akt has been shown to have significant anti-apoptotic signaling properties (11). This is, at least in part, mediated by the ability of Akt to phosphorylate and inactivate BAD, a pro-apoptotic member of the Bcl family (12,13).In this study, we have surveyed a large set of proteins that are involved in pathways regulating cell growth, cell survival, or cell death, including members of the MAPK network. Among 30 signaling proteins tested, we have identified five new protease substrates, RasGAP, Raf1, Akt-1, Cbl, and Cbl-b, that were found to be cleaved in a caspase-dependent manner during the apoptotic resp...

Degradation of eukaryotic polypeptide chain initiation factor (eIF) 4G in response to induction of apoptosis in human lymphoma cell lines

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Anoikis: an emerging hallmark in health and diseases

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Human spindle checkpoint kinase Bub1 is cleaved during apoptosis

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