ATP-Citrate Lyase Reduction Mediates Palmitate-induced Apoptosis in Pancreatic Beta Cells

Abstract

Elevated extracellular lipids, such as the free fatty acid palmitate, can induce pancreatic beta cell endoplasmic reticulum (ER) stress and apoptosis, thereby contributing to the initiation and progression of type 2 diabetes. ATP-citrate lyase (ACLY), a key enzyme in cellular lipid production, was identified as a palmitate target in a proteomic screen. We investigated the effects of palmitate on ACLY activity and phosphorylation and its role in beta cell ER stress and apoptosis. We demonstrated that treatment of MIN6 cells, mouse islets and human islets with palmitate reduced ACLY protein levels. These in vitro results were validated by our finding that islets from high fat-fed mice had a significant decrease in ACLY, similar to that previously observed in type 2 diabetic human islets. Palmitate decreased intracellular acetyl-CoA levels to a similar degree as the ACLY inhibitor, SB-204990, suggesting a reduction in ACLY activity. ACLY inhibitors alone were sufficient to induce CCAAT/enhancer-binding protein homologues protein (CHOP)-dependent ER stress and caspase-3-dependent apoptosis. Similarly, even modest shRNA-mediated knockdown of ACLY caused a significant increase in beta cell apoptosis and ER stress. The effects of chemical ACLY inhibition and palmitate were nonadditive and therefore potentially mediated by a common mechanism. Indeed, overexpression of ACLY prevented palmitateinduced beta cell death. These observations provide new evidence that ACLY expression and activity can be suppressed by exogenous lipids and demonstrate a critical role for ACLY in pancreatic beta cell survival. These findings add to the emerging body of evidence linking beta cell metabolism with programmed cell death.

Ubiquitin C-terminal hydrolase L1 is required for pancreatic beta cell survival and function in lipotoxic conditions

“…Our new data, together with previous reports, demonstrate opposing actions of lipids and glucose on UCHL1 levels [7]. Other ubiquitin C-terminal hydrolase members and the ubiquitin precursor were also differentially regulated by palmitate and glucose, and this is consistent with our previous evidence for distinct lipotoxic and glucolipotoxic mechanisms [27,33]. High glucose alone had a significant effect on the ubiquitin landscape, generating a large increase in protein ubiquitination and a decrease in free ubiquitin.…”

“…To determine whether this increase in apoptosis was intrinsic to the islets, rather than the result of systemic effects, we examined isolated islets. Indeed, under basal conditions Uchl1 −/− islets displayed elevated ER stress markers, CHOP and BiP [27,33]. Importantly, Uchl1 −/− islets were more susceptible to ER stress caused by in vitro lipotoxicity, but not glucolipotoxicity (Fig.…”

“…Palmitic acid was complexed with 20% essentially fatty acid-free BSA as described [33]. All experiments were performed in the presence of 10% FBS.…”

“…Immunoblotting and immunoprecipitation Western blots were performed as described [33]. Protein was extracted from cells with lysis buffer (Cell Signaling Technology) containing protease inhibitors.…”

Circadian Control of Fatty Acid Elongation by SIRT1 Protein-mediated Deacetylation of Acetyl-coenzyme A Synthetase 1

“…Acetyl-CoA Measurements-We extracted and analyzed acetyl-CoA by modifying a previously reported method (26).…”