Regulation of Protein Phosphatase 2A Activity by Caspase-3 during Apoptosis

Abstract

Although the available evidence suggests that whereas the caspase family plays a major role in apoptosis, they are not the sole stimulators of death. A random yeast two-hybrid screen of a lymphocyte cDNA library (using caspase-3 as the bait) found an interaction between caspase-3 and the regulatory subunit A␣ of protein phosphatase 2A. This protein was found to be a substrate for caspase-3, but not caspase-1, and could compete effectively against either a protein or synthetic peptide substrate.In Jurkat cells induced to undergo apoptosis with anti-Fas antibody, protein phosphatase 2A (PP2A) activity increased 4.5-fold after 6 h. By 12 h, the regulatory A␣ subunit could no longer be detected in cell lysates. There was no change in the amount of the catalytic subunit. The effects on PP2A could be prevented by the caspase family inhibitors acetyl-Asp-Glu-Val-Asp (DEVD) aldehyde or Ac-DEVD fluoromethyl ketone. The mitogen-activated protein (MAP) kinase pathway is regulated by PP2A. At 12 h after the addition of anti-Fas antibody, a decrease in the amount of the phosphorylated forms of MAP kinase was observed. Again, this loss of activated MAP kinase could be prevented by the addition of DEVD-cho or DEVD-fmk. These data are consistent with a pathway whereby induction of apoptosis activates caspase-3. This enzyme then cleaves the regulatory A␣ subunit of PP2A, increasing its activity. These data show that the activated PP2A will then effect a change in the phosphorylation state of the cell. These data provide a link between the caspases and signal transduction pathways.

TNFα mediates susceptibility to heat-induced apoptosis by protein phosphatase-mediated inhibition of the HSF1/hsp70 stress response

“…Thus, PPs appear as key linker molecules between the cellular effects of TNFa, the activation of HSF1/hsp70 stress responses and cellular susceptibility to apoptosis. Several points of evidence support this concept: (i) PPs PP1/PP2a and PP2b can be activated by TNFa, 29,39 (ii) activation of these PPs increases susceptibility to apoptotic cell death, 28,29 (iii) activation of PP1/PP2a and PP2b (calcineurin) as well as elevated intracellular calcium levels inhibit phosphorylation of HSF1 and thus activation of the HSF1/hsp70 stress response, [40][41][42] (iv) hsp70, which acts in a negative-feedback loop on HSF1 activation, 18 induces PP1/PP2a as well as PP2b and thus blocks continuing activation of HSF1 43,44 and (v) ERK and NF-kB signaling, which maintained a certain responsiveness of HSF1/hsp70 and also protected cells from apoptotic death, is a concurring mechanism to PPs, since their mutual inhibition has been repeatedly demonstrated. 45,46 In summary, the data presented provide insights into the interplay of signals essential for cell survival, stress resistance and apoptosis.…”

“…27 Some of these may involve the activation of protein phosphatases. 28,29 Signals involved in inflammatory responses of TNFa, such as NF-kB, suppress TNFa-mediated apoptosis (Figure 9b; arrow c) and thereby prevent a conflicting cellular response. 2,30 This concept of suppression of conflicting cellular responses is also applicable for the HSF1/hsp70 system.…”

Identification of PP1α as a Caspase-9 Regulator in IL-2 Deprivation-Induced Apoptosis

“…First, the phosphorylation status of a caspase substrate may decide whether the protein is cleaved or not. Second, caspases directly affect protein phosphatases (10). Third, caspases may be, directly or indirectly, controlled by protein phosphatases (11)(12)(13)(14).…”

Representational difference analysis of cDNA identifies novel genes expressed following preconditioning of the heart

“…Caspase 3 is a key enzyme in the apoptosis cascade. This protease is able to distinguish between the A subunit α and β isoforms, specifically cleaving the α isoform (PPP2r1a), indicating a unique functional role for the α isoform (Santoro et al, 1998). Several lines of evidence implicate caspase 3 in ischaemia/reperfusion induced apoptosis (Piot et al, 1999;Condorelli et al, 2001).…”

“…These include C-terminal Jun N-terminal kinase (JNK; Shanley et al, 2001), RAF-1 (Abraham et al, 2000), heat shock factor 2 (HSF2; Hong and Sarge, 1999), and caspase 3 (Santoro et al, 1998). JNK is a stress activated protein kinase which phosphorylates JUN, affecting transcription through AP1, and its activation can lead to the induction of apoptosis.…”