Abstract
Accumulating evidence suggests that glucolipotoxicity, arising from the combined actions of elevated glucose and free fatty acid levels, acts as a key pathogenic component in type II diabetes, contributing to β-cell dysfunction and death. Endoplasmic reticulum (ER) stress is among the molecular pathways and regulators involved in these negative effects, and ceramide accumulation due to glucolipotoxicity can be associated with the induction of ER stress. Increased levels of ceramide in ER may be due to enhanced ceramide biosynthesis and/or decreased ceramide utilization. Here, we studied the effect of glucolipotoxic conditions on ceramide traffic in INS-1 cells in order to gain insights into the molecular mechanism(s) of glucolipotoxicity. We showed that glucolipotoxicity inhibited ceramide utilization for complex sphingolipid biosynthesis, thereby reducing the flow of ceramide from the ER to Golgi. Glucolipotoxicity impaired both vesicular- and CERT-mediated ceramide transport through (1) the decreasing of phospho-Akt levels which in turn possibly inhibits vesicular traffic, and (2) the reducing of the amount of active CERT mainly due to a lower protein levels and increased protein phosphorylation to prevent its localization to the Golgi. In conclusion, our findings provide evidence that glucolipotoxicity-induced ceramide overload in the ER, arising from a defect in ceramide trafficking may be a mechanism that contributes to dysfunction and/or death of β-cells exposed to glucolipotoxicity.
Highlights
Glucolipotoxicity is defined as the condition in which the combined action of elevated glucose and free fatty acid (FFA) levels synergizes in exerting deleterious effects on pancreatic b-cell function and survival [1,2,3]
Western blot analysis of GRP78, a endoplasmic reticulum (ER) stress marker, showed that a 12 h co-treatment with 0.4 mM palmitate and 30 mM glucose induced about 60% increase in the amount of GRP78 compared to 5 mM glucose with or without 0.4 mM palmitate and 30 mM glucose in INS-1 cells (Fig. 1b), suggesting that glucolipotoxic conditions induce ER stress
To evaluate the effects of palmitate and high glucose concentrations on Cer utilization for the biosynthesis of SM and GSLs in INS-1 cells, we studied Cer metabolism using [3H]-sphingosine as a metabolic precursor as it is rapidly internalized in the cells and efficiently N-acylated to Cer, which in turn is converted to SM, glucosylceramide and complex GSLs
Summary
Glucolipotoxicity is defined as the condition in which the combined action of elevated glucose and free fatty acid (FFA) levels synergizes in exerting deleterious effects on pancreatic b-cell function and survival [1,2,3]. Chronic exposure of b-cells to supraphysiological levels of glucose and free fatty acids (FFAs) has been shown to be cytotoxic and cause b-cell dysfunction and failure [5]. Hyperglycaemia has been shown to potentiate the negative effects of high levels of saturated FFAs on pancreatic b-cells [1,2]. While palmitate and other saturated FFAs exhibit low toxicity at low glucose concentrations, they have been shown to synergize with elevated glucose concentrations to promote b-cell apoptosis, both in the b-cell line INS-1 and in human islets [10,11,12]
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