Acyl-CoA synthetase 5 induces apoptosis sensitisation in human hepatocytes by channelling Acyl-CoAs into sphingolipid metabolism

In the pathogenesis of nonalcoholic fatty liver disease, accumulation of lipids in hepatocytes and hepatocyte apoptosis are strongly implicated in disease progression from the potentially reversible condition of steatosis to severe acute and chronic liver injury. Acyl-CoA synthetase 5 (ACSL5), a member of the ACSL gene family that catalyzes the activation of long-chain fatty acids for lipid biosynthesis, is the only ACSL isoform located on mitochondria and involved in apoptosis. Up-regulation of ACSL5 by fatty acids is found to increase apoptosis susceptibility in human hepatocytes and thus was hypothesized to be a promoting factor in hepatocellular lipoapoptosis. Using liquid chromatography, tandem mass spectrometry (LC-MS/MS), RNA interference and proteinchemical techniques, we investigated the mechanisms selectively inducing apoptosis sensitisation in HepG2 cells by ACSL5. Access ACSL5 activity decreased HepG2 cell viability and increased susceptibility to TRAIL and TNFα, whereas knock down of ACSL5 reduced apoptosis susceptibility in oleic acid treated cells. Apoptosis sensitisation was accompanied by enhanced caspase-3/7 activity, but was not associated with up-regulation of DR4, DR5 or TNF-R1. By applying lipidomic techniques we analyzed the effect of ACSL5 on the partitioning of Acyl-CoAs to sphingolipids which are known to be important regulators of cell death and survival. Over-expression of ACSL5 in HepG2 cells increased synthesis of long-chain Acyl-CoA by 50%, and ceramide as well as sphingomyelin levels were enhanced 2 to 3 fold. These results indicate that steatosis-induced up-regulation of ACSL5 increased apoptosis susceptibility in human hepatocytes and that alterations in sphingolipid metabolism might contribute to ACSL5-mediated apoptotic effects. We propose that ACSL5 could play a role in promoting fatty acid-induced lipoapoptosis in heaptocytes which is currently discussed as important mechanism in fatty liver-related disorders.

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Acyl-CoA synthetase - fatty liver disease - lipoapoptosis - sphingolipid metabolism