Elevated SNAP-25 is associated with fatty acid-induced impairment of mouse islet function
ABSTRACT The role of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in insulin secretion following chronic exposure to non-esterified fatty acids (NEFAs) has not been extensively investigated. Here, we show that synaptosome-associated protein of 25 kDa (SNAP-25) levels were predominantly elevated in the soluble fraction of mouse islets exposed to palmitate. This coincided with an impairment of insulin secretion to glucose and non-glucose secretagogues, consistent with a defect at a distal regulatory step in exocytosis. Removal of palmitate from the media restored both SNAP-25 protein levels and insulin secretion to control levels. We conclude that increased expression of SNAP-25 is associated with NEFA-induced impairment of insulin secretion in mouse islets.
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- "In mouse insulinoma MIN6 cells, long-term exposure to palmitate alters the expression of several proteins involved in β cell exocytosis (Lovis et al., 2008). Mice lacking syntaxin-1a exhibit a reduced number of docked granules and loss of first-phase insulin secretion but normal second-phase release (Ohara-Imaizumi et al., 2007), but our own data did not reveal any abnormalities in the expression or distribution of syntaxin-1 (Figure S3) or SNAP-25 (Zraika et al., 2004). There were likewise no major changes in the transcription of other exocytotic proteins/Ca 2+ channel subunits that would explain the functional data (Table S1). "
ABSTRACT: Long-term (72 hr) exposure of pancreatic islets to palmitate inhibited glucose-induced insulin secretion by >50% with first- and second-phase secretion being equally suppressed. This inhibition correlated with the selective impairment of exocytosis evoked by brief (action potential-like) depolarizations, whereas that evoked by long ( approximately 250 ms) stimuli was unaffected. Under normal conditions, Ca(2+) influx elicited by brief membrane depolarizations increases [Ca(2+)](i) to high levels within discrete microdomains and triggers the exocytosis of closely associated insulin granules. We found that these domains of localized Ca(2+) entry become dispersed by long-term (72 hr), but not by acute (2 hr), exposure to palmitate. Importantly, the release competence of the granules was not affected by palmitate. Thus, the location rather than the magnitude of the Ca(2+) increase determines its capacity to evoke exocytosis. In both mouse and human islets, the palmitate-induced secretion defect was reversed when the beta cell action potential was pharmacologically prolonged.Cell metabolism 12/2009; 10(6):455-65. DOI:10.1016/j.cmet.2009.09.011 · 16.75 Impact Factor
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- "Based on this observation, we suggest that in LP islets and in the presence of high glucose, NEFA may be preferentially used for insulin secretion. Considering that exogenous NEFA can acutely potentiate glucose-stimulated insulin secretion, possibly by providing additional acyl groups for long-chain acyl-CoA formation or complex lipid synthesis (Corkey et al. 2000), and since LP rats are chronically exposed to elevated serum NEFA levels, it is possible that the intracellular level of acyl-CoA may be elevated in the islets of these rats and could activate enzymes, such as protein kinase C (Alcazar et al. 1997) and synaptosomeassociated protein 25 (Zraika et al. 2004), that are involved in the insulin secretory and exocytotic machinery. "
ABSTRACT: A severe reduction in insulin release in response to glucose is consistently noticed in protein-deprived rats and is attributed partly to the chronic exposure to elevated levels of NEFA. Since the pancreatic and duodenal transcription factor homeobox 1 (PDX-1) is important for the maintenance of beta-cell physiology, and since PDX-1 expression is altered in the islets of rats fed a low protein (LP) diet and that rats show high NEFA levels, we assessed PDX-1 and insulin mRNA expression, as well as PDX-1 and p38/stress activated protein kinase 2 (SAPK2) protein expression, in islets from young rats fed low (6%) or normal (17%; control) protein diets and maintained for 48 h in culture medium containing 5.6 mmol/l glucose, with or without 0.6 mmol/l palmitic acid. We also measured glucose-induced insulin secretion and glucose metabolism. Insulin secretion by isolated islets in response to 16.7 mmol/l glucose was reduced in LP compared with control rats. In the presence of NEFA, there was an increase in insulin secretion in both groups. At 2.8 mmol/l glucose, the metabolism of this sugar was reduced in LP islets, regardless of the presence of this fatty acid. However, when challenged with 16.7 mmol/l glucose, LP and control islets showed a severe reduction in glucose oxidation in the presence of NEFA. The PDX-1 and insulin mRNA were significantly higher when NEFA was added to the culture medium in both groups of islets. The effect of palmitic acid on PDX-1 and p38/SAPK2 protein levels was similar in LP and control islets, but the increase was much more evident in LP islets. These results demonstrate the complex interrelationship between nutrients in the control of insulin release and support the view that fatty acids play an important role in glucose homeostasis by affecting molecular mechanisms and stimulus/secretion coupling pathways.British Journal Of Nutrition 12/2007; 96(6):1006-12. DOI:10.1017/BJN20061950 · 3.34 Impact Factor