The Role and Regulation of Caspase-2 in Lipoapoptosis
Non-alcoholic fatty liver disease (NAFLD) is now the most common liver disorder in Western countries. NAFLD is characterized by hepatic steatosis, or an intracellular accumulation of lipids in hepatocytes. An excess of intracellular lipids can cause toxicity, leading to hepatocyte cell death and progression of NAFLD to Non-alcoholic steatohepatitis (NASH). NASH is very severe and is additionally characterized by inflammation and fibrosis, leading to liver failure. In order to develop appropriate strategies to prevent the progression from NAFLD to NASH, it is essential to define the molecular mechanisms leading to hepatocyte cell death that promote this transition.
Since caspase-2 has been reported to respond to different types of intracellular metabolic stress to promote apoptosis, we investigated whether this enzyme promotes lipid-induced apoptosis, or lipoapoptosis, in tissue culture cells. We discovered that the enzymatic activity of caspase-2 is activated by saturated free fatty acids, and that expression of caspase-2 is necessary for lipoapoptosis in multiple hepatocyte cell lines. We also explored the role of caspase-2 in lipoapoptosis in vivo using a mouse model of NASH and patient biopsies. We found caspase-2 expression increases in the liver of both patients with NASH and in mice fed a methionine choline-deficient (MCD) diet, a model for NASH. In addition, we fed WT and caspase-2 knockout mice the MCD diet and observed a decrease in both apoptotic and fibrotic markers in caspase-2 knockout mice, indicating caspase-2 promotes lipoapoptosis in vivo. Finally, we investigated the molecular mechanism of lipotoxicity-induced caspase-2 activation, and identified a novel caspase-2 binding protein, TRAF3. TRAF3 knockdown was also able to reduce lipoapoptosis in cultured hepatocytes. Reducing the binding between these two proteins decreased the pro-apoptotic activity of caspase-2. In conclusion, we identified caspase-2 as a key protein in the regulation of lipoapoptosis both in vitro and in vivo, and discovered a novel regulator of caspase-2 activity. This work furthers our knowledge on the mechanisms leading to NAFLD progression, and may inform future studies in the development of treatment options to limit this progression.
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