Lipotoxicity of the Pancreatic β-Cell Is Associated With Glucose-Dependent Esterification of Fatty Acids Into Neutral Lipids

Abstract

Prolonged exposure of isolated islets to supraphysiologic concentrations of palmitate decreases insulin gene expression in the presence of elevated glucose levels. This study was designed to determine whether or not this phenomenon is associated with a glucosedependent increase in esterification of fatty acids into neutral lipids. Gene expression of sn-glycerol-3-phosphate acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT), and hormone-sensitive lipase (HSL), three key enzymes of lipid metabolism, was detected in isolated rat islets. Their levels of expression were not affected after a 72-h exposure to elevated glucose and palmitate. To determine the effects of glucose on palmitate-induced neutral lipid synthesis, isolated rat islets were cultured for 72 h with trace amounts of [ 14 C]palmitate with or without 0.5 mmol/l unlabeled palmitate, at 2.8 or 16.7 mmol/l glucose. Glucose increased incorporation of [14 C]palmitate into complex lipids. Addition of exogenous palmitate directed lipid metabolism toward neutral lipid synthesis. As a result, neutral lipid mass was increased upon prolonged incubation with elevated palmitate only in the presence of high glucose. The ability of palmitate to increase neutral lipid synthesis in the presence of high glucose was concentration-dependent in HIT cells and was inversely correlated to insulin mRNA levels. 2-Bromopalmitate, an inhibitor of fatty acid mitochondrial ␤-oxidation, reproduced the inhibitory effect of palmitate on insulin mRNA levels. In contrast, palmitate methyl ester, which is not metabolized, and the medium-chain fatty acid octanoate, which is readily oxidized, did not affect insulin gene expression, suggesting that fatty-acid inhibition of insulin gene expression requires activation of the esterification pathway. These results demonstrate that inhibition of insulin gene expression upon prolonged exposure of islets to palmitate is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids. Diabetes 50:315-321, 2001A ccording to the lipotoxicity hypothesis, chronic exposure to elevated lipid levels impairs pancreatic ␤-cell function in type 2 diabetic patients (1,2). We (3) and others (4,5) have previously shown that prolonged (>1 day) culture of normal islets in the presence of supraphysiologic concentrations of palmitate decreases insulin content and impairs insulin gene expression only in the presence of elevated glucose levels. This occurs, at least in part, via decreased insulin gene promoter activity in HIT-T15 cells (3) and decreased binding of the transcription factor pancreas-duodenum homeobox-1 (PDX-1) to the insulin gene in islets (4). In Zucker diabetic fatty (ZDF) rats, it has been postulated that ␤-cell dysfunction is due to increased triacylglycerol (TAG) content in islets (6-8), which leads to increased production of nitric oxide (9) and ceramide synthesis (10). However, the ZDF rat is an extremely obese genetic model of type 2 diabetes bearing a mutation in the leptin receptor gene. It remains...

Soybean oil treatment impairs glucose-stimulated insulin secretion and changes fatty acid composition of normal and diabetic islets

“…3a) when compared with untreated normal animals. It has been reported that impaired GSIS observed in GK rats is associated with alterations in β-cell glucose metabolism, protein kinase C pathway, modulation of UCP-2 expression, acetylcholine and oxidative stress [11,26,[47][48][49]. However, it can also be associated with fatty acid content alterations.…”

“…It has been shown that palmitic acid (C16:0) is toxic to β-cells, even at moderate levels [19,25]. This fatty acid affects glucose-induced insulin gene expression, ceramide synthesis and activates mitochondrial apoptotic pathways [13,15,[26][27][28]. In contrast, the monounsaturated palmitoleic acid (C16:1) promotes β-cell proliferation at low glucose concentrations, improves β-cell function and counteracts the negative effects of palmitic acid [27].…”

Adverse physicochemical properties of tripalmitin in beta cells lead to morphological changes and lipotoxicity in vitro

“…INS1 beta cells (rat insulinoma, passage [9][10][11][12][13][14][15][16][17][18][19] were cultured in RPMI 1640 (Sigma, Poole, UK), 11 mmol/l glucose, 10% fetal bovine serum (FBS), 2 mmol/l L-glutamine, 50 μmol/l β-mercaptoethanol, 10 mmol/l HEPES, 1 mmol/l sodium pyruvate, 50 U/ml penicillin and 50 μg/ ml streptomycin (Gibco, Paisley, UK) in 5% CO 2 . COS7 cells (monkey kidney fibroblasts) were cultured in RPMI 1640, 11 mmol/l glucose, 10% FBS, 2 mmol/l L-glutamine, 50 U/ml penicillin and 50 μg/ml streptomycin (Gibco) in 5% CO 2 .…”

“…NEFA have also been shown to reduce cellular viability and induce beta-cell apoptosis [16]. NEFA, and particularly saturated NEFA such as palmitic acid [17][18][19], have been identified as toxic agents which activate signalling cascades that result in apoptotic beta-cell death [20,21].…”

Glucagon-like peptide-1 prevents beta cell glucolipotoxicity

“…However, chronic hyperglycaemia causes beta cell dysfunction characterised by reduced insulin biosynthesis [6] and increased levels of apoptosis (glucotoxicity) [7,8,9,10,11]. Similarly, acute exposure to NEFA potentiates glucose-induced insulin secretion by beta cells [12], whereas prolonged exposure to high concentrations of NEFA triggers beta cell apoptosis (lipotoxicity) [13,14,15,16,17]. A recent study showed that protein kinase B (PKB) activation can rescue MIN6 cells from oleate cytotoxicity [18].…”