Accumulation of intracellular lipid by pancreatic islet β-cells has been proposed to inhibit normal glucose-regulated insulin secretion (‘glucolipotoxicity’). In the present study, we determine whether over-expression in rat islets of the lipogenic transcription factor SREBP1c (sterol-regulatory-element-binding protein-1c) affects insulin release, and whether changes in islet lipid content may be reversed by activation of AMPK (AMP-activated protein kinase). Infection with an adenovirus encoding the constitutively active nuclear fragment of SREBP1c resulted in expression of the protein in approx. 20% of islet cell nuclei, with a preference for β-cells at the islet periphery. Real-time PCR (TaqMan®) analysis showed that SREBP1c up-regulated the expression of FAS (fatty acid synthase; 6-fold), acetyl-CoA carboxylase-1 (2-fold), as well as peroxisomal-proliferator-activated receptor-γ (7-fold), uncoupling protein-2 (1.4-fold) and Bcl2 (B-cell lymphocytic-leukaemia proto-oncogene 2; 1.3-fold). By contrast, levels of pre-proinsulin, pancreatic duodenal homeobox-1, glucokinase and GLUT2 (glucose transporter isoform-2) mRNAs were unaltered. SREBP1c-transduced islets displayed a 3-fold increase in triacylglycerol content, decreased glucose oxidation and ATP levels, and a profound inhibition of glucose-, but not depolarisation-, induced insulin secretion. Culture of islets with the AMPK activator 5-amino-4-imidazolecarboxamide riboside decreased the expression of the endogenous SREBP1c and FAS genes, and reversed the effect of over-expressing active SREBP1c on FAS mRNA levels and cellular triacylglycerol content. We conclude that SREBP1c over-expression, even when confined to a subset of β-cells, leads to defective insulin secretion from islets and may contribute to some forms of Type II diabetes.
Abbreviations used: ACC1, acetyl-CoA carboxylase-1; AICAR, 5-aminoimidazole-4-carboxamide ribonucleoside; AMPK, AMP-activated protein kinase; Bcl2, B-cell lymphocytic-leukaemia proto-oncogene 2; DMEM, Dulbecco's modified Eagle's medium; eGFP, enhanced green fluorescent protein; EMSA, electrophoretic mobility shift assay; FAS, fatty acid synthase; FCS, fetal calf serum; GK, glucokinase; GLUT2, glucose transporter isoform-2; GSIS, glucose-stimulated insulin secretion; KRBH, Krebs–Ringer bicarbonate Hepes buffer; MOI, multiplicity of infection; PDX1, pancreatic duodenal homeobox-1; PPARγ, peroxisomal proliferator-activated receptor γ; PPI, pre-proinsulin; RT-PCR, reverse-transcriptase PCR; SREBP1c, sterol-regulatory-element-binding protein-1c; SREBP CA, constitutively active nuclear form of SREBP1c; TG, triacylglycerol; TRITC, tetramethylrhodamine β-isothiocyanate; UCP2, uncoupling protein-2.