Objective Caspase-2 is an initiator caspase involved in multiple apoptotic pathways, particularly in response to specific intracellular stressors (eg. DNA damage, ER stress). We recently reported that caspase-2 was pivotal for the induction of cell death triggered by excessive intracellular accumulation of long chain fatty acids, a response known as lipoapoptosis. The liver is particularly susceptible to lipid-induced damage, explaining the pandemic status of non-alcoholic fatty liver disease (NAFLD). Progression from NAFLD to non-alcoholic steatohepatitis (NASH) results, in part, from hepatocyte apoptosis and consequential paracrine-mediated fibrogenesis. We evaluated the hypothesis that caspase-2 promotes NASH-related cirrhosis. Design Caspase-2 was localized in liver biopsies from NASH patients. Its expression was evaluated in different mouse models of NASH, and outcomes of diet-induced NASH were compared in wild type (WT) and caspase-2 deficient mice. Lipotoxicity was modeled in vitro using hepatocytes derived from WT and caspase-2 deficient mice. Results We showed that caspase-2 is integral to the pathogenesis of NASH-related cirrhosis. Caspase-2 is localized in injured hepatocytes and its expression was markedly up-regulated in patients and animal models of NASH. During lipotoxic stress, caspase-2 deficiency reduced apoptosis, inhibited induction of pro-fibrogenic Hedgehog target genes in mice, and blocked production of Hedgehog ligands in cultured hepatocytes. Conclusion These data point to a critical role for caspase-2 in lipid-induced hepatocyte apoptosis in vivo, for the production of apoptosis-associated fibrogenic factors and in the progression of lipid-induced liver fibrosis. This raises the intriguing possibility that caspase-2 may be a promising therapeutic target to prevent progression to NASH.

While lysine acetylation in the nucleus is well characterized, comparatively little is known about its significance in cytoplasmic signaling. Here we show that inhibition of the Sirt1 deacetylase, which is primarily cytoplasmic in cancer cell lines, sensitizes these cells to caspase-2-dependent death. To identify relevant Sirt1 substrates, we developed a novel proteomics strategy, enabling the identification of a range of putative substrates, including 14-3-3ζ, a known direct regulator of caspase-2. We show here that inhibition of Sirtuin activity accelerates caspase activation and overrides caspase-2 suppression by nutrient abundance. Furthermore, 14-3-3ζ is acetylated prior to caspase activation, and supplementation of Xenopus egg extract with glucose-6-phosphate, which promotes caspase-2/14-3-3ζ binding, enhances 14-3-3ζ-directed Sirtuin activity. Conversely, inhibiting Sirtuin activity promotes 14-3-3ζ dissociation from caspase-2 in both egg extract and human cell lines. These data reveal a role for Sirt1 in modulating apoptotic sensitivity, in response to metabolic changes, by antagonizing 14-3-3ζ acetylation.

Background:Prior experiments described the metabolic regulation of caspase-2, but the underlying caspase-2-activating stimulus was unknown. Results: Metabolomics implicated an accumulation of long-chain fatty acid (LCFA) metabolites in caspase-2 activation. Conclusion: Caspase-2 is activated by an overabundance of saturated LCFAs and is required for cell death. Significance: These findings provide mechanistic insight into LCFA-induced apoptosis, highlighting a novel role for caspase-2 in this setting.