Caspase-3 Protects Stressed Organs against Cell Death

Abstract

The ability to generate appropriate defense responses is crucial for the survival of an organism exposed to pathogenesis-inducing insults. However, the mechanisms that allow tissues and organs to cope with such stresses are poorly understood. Here we show that caspase-3-knockout mice or caspase inhibitor-treated mice were defective in activating the antiapoptotic Akt kinase in response to various chemical and environmental stresses causing sunburns, cardiomyopathy, or colitis. Defective Akt activation in caspase-3-knockout mice was accompanied by increased cell death and impaired survival in some cases. Mice homozygous for a mutation in RasGAP that prevents its cleavage by caspase-3 exhibited a similar defect in Akt activation, leading to increased apoptosis in stressed organs, marked deterioration of their physiological functions, and stronger disease development. Our results provide evidence for the relevance of caspase-3 as a stress intensity sensor that controls cell fate by either initiating a RasGAP cleavage-dependent cell resistance program or a cell suicide response. Executioner caspases mediate cell death during apoptosis (45). Of these, caspase-3 has the ability to cleave the majority of the caspase substrates (43), and its activity is required for the induction of cell death in response to many apoptotic stimuli (1). While executioner caspases are indispensable for apoptosis, there are situations when their activation does not lead to death. For example, healthy dividing cells can weakly activate caspase-3 in response to mild stresses (47). Caspase-3 also participates, in an apoptosisindependent manner, in T and B cell homeostasis (35,46), in microglia activation (6), in long-term depression (26), and in muscle (17), monocyte (44), embryonic stem cell (18), and erythroid cell (13) differentiation. However, it remains unclear how activation of caspase-3 under these conditions does not eventually lead to cell death (1, 24). Cells could have an intrinsic ability to tolerate low caspase activity by constitutively expressing antiapoptotic molecules, such as members of the inhibitors of the apoptosis protein family, or may stimulate antiapoptotic pathways in parallel to caspase activation (24). Alternatively, the caspases themselves might activate prosurvival pathways, in particular, when they are mildly stimulated. Indeed, there is evidence in cultured cells that caspase-3 mediates neuroprotection after preconditioning (30) and that caspase-3 activity turns on the antiapoptotic Akt kinase following partial cleavage of the RasGAP protein (47). Other caspase substrates that could potentially induce protective signals once cleaved include p27 kip1 (14), Lyn (28), synphilin-1 (19), and Rb (42), yet the physiological importance of these cleaved substrates has not been evaluated to date.In the present study, we have investigated the role played by caspase-3 and its substrate p120 RasGAP in the induction of the antiapoptotic Akt kinase in stressed tissues in vivo. MATERIALS AND METHODS Caspase-3-KO mice. B6.129S1...

RasGAP Shields Akt from Deactivating Phosphatases in Fibroblast Growth Factor Signaling but Loses This Ability Once Cleaved by Caspase-3

“…Fragment N is known to exert potent survival functions that depend on the activation of the Ras/PI3K/Akt pathway. Previous work has shown that fragment N expression in mammalian cell lines leads to Ras and Akt stimulation (23,26,63). Using the Xenopus oocyte, it was found that fragment N alone could not activate Akt on its own.…”

The caspase-3/p120 RasGAP module generates a NF-κB repressor in response to cellular stress

“…RasGAP, first identified as a Ras and Rho regulator (Trahey and McCormick, 1987), has been shown to have two caspase-3 cleavage sites used sequentially as cellular stress increases (Wen et al, 1998;Widmann et al, 1998;Yang et al, 2004;Yang and Widmann, 2001). The N-terminal fragment resulting from the first cleavage event on RasGAP (called fragment N) displays strong antiapoptotic properties by activating the Ras-PI3K-Akt pathway in vitro and in vivo (Khalil et al, 2012;Yang and Widmann, 2002). We initially reported that fragment-N-mediated Akt stimulation did not result in activation of its downstream effector NF-κB (Yang and Widmann, 2002).…”

“…Wild-type and D455 knock-in mice skin was subjected or not to a low dose (0.05 J/cm 2 ) of UV-B illumination. This UV-B dose has been shown previously to generate an anti-apoptotic Akt response that is dependent on caspase-3 and fragment N (Khalil et al, 2012). At 24 h after UV-B illumination, NF-κB activation was induced through topical treatment with anthralin.…”

“…Note that this mutation also prevents cleavage of RasGAP at the second caspase-3 recognition site (at position 157), presumably because the latter is only exposed when RasGAP is cleaved at the first site (Yang and Widmann, 2001). D455A knock-in mice cannot generate fragment N when they are subjected to various stress stimuli in different organs (Khalil et al, 2012). NF-κB activation in the skin was induced through topical treatment with anthralin, a drug used for the treatment of psoriasis.…”

“…In order to test whether fragment N generation affects NF-κB activity in vivo, we used RasGAP D455A/D455A knock-in mice (D455A knock-in mice) that cannot cleave RasGAP due to a mutation at the first caspase-3 cleavage site (Khalil et al, 2012). Note that this mutation also prevents cleavage of RasGAP at the second caspase-3 recognition site (at position 157), presumably because the latter is only exposed when RasGAP is cleaved at the first site (Yang and Widmann, 2001).…”