Chronic Effects of Palmitate Overload on Nutrient-Induced Insulin Secretion and Autocrine Signalling in Pancreatic MIN6 Beta Cells

Abstract

BackgroundSustained exposure of pancreatic β cells to an increase in saturated fatty acids induces pleiotropic effects on β-cell function, including a reduction in stimulus-induced insulin secretion. The objective of this study was to investigate the effects of chronic over supply of palmitate upon glucose- and amino acid-stimulated insulin secretion (GSIS and AASIS, respectively) and autocrine-dependent insulin signalling with particular focus on the importance of ceramide, ERK and CaMKII signalling.Principal FindingsGSIS and AASIS were both stimulated by >7-fold resulting in autocrine-dependent activation of protein kinase B (PKB, also known as Akt). Insulin release was dependent upon nutrient-induced activation of calcium/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) as their pharmacological inhibition suppressed GSIS/AASIS significantly. Chronic (48 h, 0.4 mM) palmitate treatment blunted glucose/AA-induced activation of CaMKII and ERK and caused a concomitant reduction (∼75%) in GSIS/AASIS and autocrine-dependent activation of PKB. This inhibition could not be attributed to enhanced mitochondrial fatty acid uptake/oxidation or ceramide synthesis, which were unaffected by palmitate. In contrast, diacylglycerol synthesis was elevated suggesting increased palmitate esterification rather than oxidation may contribute to impaired stimulus-secretion coupling. Consistent with this, 2-bromopalmitate, a non-oxidisable palmitate analogue, inhibited GSIS as effectively as palmitate.ConclusionsOur results exclude changes in ceramide content or mitochondrial fatty acid handling as factors initiating palmitate-induced defects in insulin release from MIN6 β cells, but suggest that reduced CaMKII and ERK activation associated with palmitate overload may contribute to impaired stimulus-induced insulin secretion.

Activation of Extracellular Signal-Regulated Protein Kinases 1 and 2 (ERK1/2) by Free Fatty Acid Receptor 1 (FFAR1/GPR40) Protects from Palmitate-Induced Beta Cell Death, but Plays no Role in Insulin Secretion

“…A comparable anti-apoptotic role for GPR40/FFAR1 has been suggested previously using the unsaturated fatty acid oleate which also activated ERK1/2 in NIT-1 cells [24]. In contrast to these observations another study associated ERK1/2 phosphorylation with insulin secretion and chronic stimulation of GPR40/FFAR1 with beta cell death [14]. An explanation for the differences could be the use of high concentrations of the MEK inhibitor U0126, which has been found to induce side effects such as increases in the cellular ratios of ADP/ATP and AMP/ATP [25].…”

“…Activation of ERK1/2, especially by glucose, has been linked to stimulation of insulin secretion [14]. In contrast, our study showed that a concentration of PD98059 (10 µmol/L), which was sufficient to reverse the effect of glucose on ERK1/2 phosphorylation, significantly augmented glucose-induced insulin secretion in INS-1E cells [13].…”

“…The MEK1 inhibitor PD98059 did not affect glucose-induced insulin secretion in INS-1E cells [13]. In contrast, inhibition of glucose-dependent ERK1/2 phosphorylation upon chronic exposure to palmitate was linked to a concomitant reduction in glucose-induced insulin secretion in Min6 cells [14]. …”

Inhibition of Pancreatic β-Cell Ca2+/Calmodulin-dependent Protein Kinase II Reduces Glucose-stimulated Calcium Influx and Insulin Secretion, Impairing Glucose Tolerance

“…Perturbations in CaMKII signaling are observed following palmitate treatment of insulinoma ␤-cells (52). Therefore, changes in CaMKII signaling may contribute to aberrant ␤-cell function following HFD feeding, during the progression of diabetes (52,53).…”

Differential Activation of ER Stress Signal Pathways Contributes to Palmitate-Induced Hepatocyte Lipoapoptosis

“…Up to 30% of the population is afflicted by NAFLD and 5-10% of these patients develop non-alcoholic steatohepatitis (NASH), which can progress to cirrhosis and hepatocellular carcinoma [2,3]. Both obesity and insulin resistance are major risk factors for NASH and are characterized by an increase in circulating free fatty acids (FFAs) [4][5][6]. The excess serum FFAs in the context of insulin resistance are transported into hepatocyte by the fatty acid transporter.…”