Abstract 4697: Metabolic regulation of caspase-2
Authors: Andersen Joshua L. , Thompson J. Will , Buchakjian Marisa R. , Moseley M. Arthur , Kornbluth Sally 
Published: June 2012

Abstract
Caspase-2 has been shown to act as an apical caspase, upstream of cytochrome c release, in a variety of cell death settings including chemotherapy-induced apoptosis. However, the mechanism governing caspase-2 activation in response to chemotherapeutics is poorly understood. We previously demonstrated that production of NADPH via the pentose phosphate pathway (PPP) stimulates caspase-2 suppression via a CaMKII-dependent phosphorylation on Ser135 (Nutt et al., 2005), and that as PPP activity declines, caspase-2 becomes dephosphorylated and activated to promote cell death. Our current efforts are focused on identifying the cellular factors that link metabolism to caspase-2 activation, with the hope that such factors could be exploited therapeutically in cancer.
In this study we show that inhibition of Sirt1 deacetylase overrides the anti-apoptotic effect of high PPP activity (stimulated by G6P) to cause caspase-2 activation and cell death. In an effort to more precisely understand the role of Sirt1 in caspase-2 activation, we developed a novel biotin-switch-based proteomics approach to identify Sirt1 substrates in an unbiased manner. This approach yielded a variety of high-confidence Sirt1 substrates, many of which are cytoplasmic proteins that primarily group into the areas of glycolysis/metabolism, oxidative stress, cytoskeletal dynamics, and apoptosis; all areas where Sirt1 has been implicated. We had previously shown that one of these substrates, 14-3-3ζ, binds to caspase-2 in a phosphorylation-dependent manner and protects Ser135 from dephosphorylation (Nutt et al., 2009). We show here that acetylation of 14-3-3ζ occurs in response to a drop in PPP activity and triggers 14-3-3ζ release from caspase-2, leading to caspase-2 activation. Conversely, we show that high PPP levels promote Sirt1 activity, which, in turn, promotes 14-3-3ζ binding to caspase-2 by antagonizing the acetylation of 14-3-3ζ. We have extended these results to a panel of breast tumor cell lines, in which inhibition of Sirt1 induces the rapid dissociation of caspase-2 from 14-3-3ζ, and results in a marked enhancement of the cells’ sensitivity to paclitaxel. Interestingly, we observe cytoplasmic localization of Sirt1 at high levels in these cells with very little to none in the nucleus, in contrast to a more nuclear Sirt1 distribution in normal breast tissue.
In conclusion, these data support a model in which Sirt1 promotes the pro-survival effect of high metabolic activity by directly regulating 14-3-3ζ-mediated inhibition of caspase-2, and suggest that as PPP activity declines, a consequent decrease in Sirt1 activity triggers caspase-2 activation. As Sirt1 is known to also modulate numerous transcription factors, its suppressive effect on caspase-2 may play a particularly key role under conditions in which transcription is not ongoing (e.g., paclitaxel-induced mitotic arrest) or when Sirt1 is excluded from the nucleus (breast and other tumor cell lines).
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4697. doi:10.1158/1538-7445.AM2011-4697