otic enzyme PC, together with the proportion of pyruvate flux via PC, were much lower in human compared with rodent islets.71 In addition, human islets exhibited lower ACL protein and activity.71 Human islets were characterized by elevated succinyl-CoA:3ketoacid-CoA transferase and acetoacetyl-CoA synthetase, which form mitochondrial acetoacetate and permit the formation of cytosolic acyl-CoA respectively. Knockdown of SCOT or AAS in INS-1 832/13 cells impairs GSIS.23,72 This led to the suggestion that a pathway 480-44-4 chemical information exists involving acetoacetate synthesis, leading to the production of cytosolic acyl-CoAs71, which can act as signaling molecules for insulin exocytosis.73 Importantly, this pathway may be more active in human compared with rodent -cells due to lower pyruvate carboxylation as a result of lowered PC expression and activity compared PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19819037 with rodent islets.71 A Role for Pyruvate in the Acute Augmentation of GSIS through Inhibition of the PDHKs PDHK4 is relatively insensitive to suppression by the pyruvate analog dichloroacetate.74 Thus, an increased PDHK4/PDHK2 activity ratio could limit the extent to which an increased intracellular pyruvate generation promotes PDC flux. PDHK1 is also relatively insensitive to inactivation by pyruvate. This may in part contribute to the enhanced acute response of insulin secretion to stimulation by glucose in -cells where PDHK1 is suppressed.14 Roles for PDC, Citrate Cataplerosis and ACL in Mediating Effects of Chronic Exposure to High Glucose on -Cell Gene Expression As described above, the operation of one pyruvate cycling pathway requires malate export from the mitochondria and NADP+ dependent decarboxylation of malate to pyruvate by cME1. It has been reported that mouse -cells lack ME1 activity and, although adenoviral-mediated overexpression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not affect GSIS in mouse islets.79 However, other studies have identified ME activity and mRNA in C57BL/6 mouse islets and MIN-6 cells, as well as impaired ME activity in streptozotocin-treated mouse islets and increased ME activity in obese agouti-L mouse islets.80 Furthermore, ME siRNA inhibited ME activity and reduced GSIS and lowered NADPH.80 A further study,81 using both INS-1 832/13 cells and glucose-responsive INS-1 832/3 cells, demonstrated that siRNAmediated suppression of either mME or cME lowered GSIS, the latter in association with decreased NADPH. However, while adenovirus-mediated delivery of siRNAs specific to cME and mME to isolated rat islets suppressed the targets transcripts, it failed to alter GSIS.81 Furthermore, islets isolated from MEcnull MOD1 mouse islets are also characterized by normal glucose- and KCl-stimulated insulin secretion.81 These findings suggest that citrate/isocitrate may have another, as yet unidentified role in GSIS. Recently, Wellen et al. examined the effect of suppressing ACL expression in several different mammalian cell types, including cultured adipocytes. It was observed that siRNA suppression of ACL reduced global histone acetylation. Moreover, histone acetylation depended on glucose, with FA unable to substitute. This observation is consistent with a requirement for a cytosolic or nuclear pool of acetyl-CoA. Suppressing ACL in differentiating adipocytes has metabolic consequences: it prevented the expression of the major adipocyte glucose transporter, GLUT4, with reduced H3 and H4 histone acetylation found at the promoter of the Glut4-e