Palmitate activates insulin signaling pathway in pancreatic rat islets

Palmitic acid is an intracellular signaling molecule involved in disease development

Emerging evidence shows that palmitic acid (PA), a common fatty acid in the human diet, serves as a signaling molecule regulating the progression and development of many diseases at the molecular level. In this review, we focus on its regulatory roles in the development of five pathological conditions, namely, metabolic syndrome, cardiovascular diseases, cancer, neurodegenerative diseases, and inflammation. We summarize the clinical and epidemiological studies; and also the mechanistic studies which have identified the molecular targets for PA in these pathological conditions. Activation or inactivation of these molecular targets by PA controls disease development. Therefore, identifying the specific targets and signaling pathways that are regulated by PA can give us a better understanding of how these diseases develop for the design of effective targeted therapeutics.

Palmitoleate inhibits insulin transcription by activating the ERK1/2 pathway in rat pancreatic β-cells

The aim of the present study was to evaluate the effects of palmitoleate on insulin secretion and insulin mRNA levels, and to investigate the transcriptional regulation of insulin. INS-1 rat insulinoma cells were treated with palmitoleate in the presence of high glucose, and the amount of secreted insulin was measured via radioimmunoassay. Reverse transcription-quantitative polymerase chain reaction was performed to evaluate the mRNA levels of insulin and pancreatic and duodenal homeobox 1 (PDX1) under palmitoleate treatment. The levels of PDX1, peroxisome proliferator-activated receptor gamma (PPARG), extracellular signal-regulated kinase (ERK)1/2 and phosphorylated ERK1/2 were measured using western blot analysis. Low concentrations of palmitoleate significantly induced insulin secretion (P=0.024), whereas the mRNA levels of insulin and PDX1 were markedly reduced. However, the inhibitory effects were reversed with the addition of U0126, suggesting that the ERK1/2-mediated pathway may be the underlying mechanism responsible for palmitoleate-induced downregulation of insulin mRNA. Exposure of INS-1 cells to high glucose significantly increased the phosphorylation of ERK1/2 (P=0.039), which was further enhanced by palmitoleate (P=0.025). Exposure of INS-1 cells to high glucose significantly decreased PPARG (P=0.001), which was further decreased by the addition of palmitoleate. U0126 was able to reverse the palmitoleate-induced effects. In conclusion, the present study suggested that palmitoleate may induce insulin secretion and inhibit insulin mRNA expression in pancreatic β-cells.

Evidence for the involvement of GPR40 and NADPH oxidase in palmitic acid-induced superoxide production and insulin secretion

G protein coupled receptor 40 (GPR40) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex have been shown to be involved in the fatty acid amplification of glucose-stimulated insulin secretion (GSIS). The effect of palmitic acid on superoxide production and insulin secretion by INS-1E cells and the possible involvement of GPR40 and NADPH oxidase in these processes were examined in this study. Cells were incubated during 1 h with palmitic acid in low and high glucose concentrations, a GPR40 agonist (GW9508) and inhibitors of NADPH oxidase (diphenyleneiodonium, DPI) and PKC (calphostin C). GW9508 induced superoxide production at 2.8 and 5.6 mM glucose concentrations and stimulated insulin secretion at 16.7 mM glucose concentration involving both PKC and NADPH oxidase activation. Palmitic acid induced superoxide production through NADPH oxidase and GPR40-dependent pathways and the stimulation of insulin secretion in the presence of a high glucose concentration was reduced by knockdown of GPR40 using siRNA. Our results suggest that palmitic acid induces superoxide production and potentiates GSIS through NADPH oxidase and GPR40 pathways in pancreatic ? cells.

NAD(P)H oxidase participates in the palmitate-induced superoxide production and insulin secretion by rat pancreatic islets

Nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase complex has been shown to be involved in the process of glucose-stimulated insulin secretion (GSIS). In this study, we examined the effect of palmitic acid on superoxide production and insulin secretion by rat pancreatic islets and the mechanism involved. Rat pancreatic islets were incubated during 1 h with 1 mM palmitate, 1% fatty acid free-albumin, 5.6 or 10 mM glucose and in the presence of inhibitors of NAD(P)H oxidase (DPI--diphenyleneiodonium), PKC (calphostin C) and carnitine palmitoyl transferase-I (CPT-I) (etomoxir). Superoxide content was determined by hydroethidine assays. Palmitate increased superoxide production in the presence of 5.6 and 10 mM glucose. This effect was dependent on activation of PKC and NAD(P)H oxidase. Palmitic acid oxidation was demonstrated to contribute for the fatty acid induction of superoxide production in the presence of 5.6 mM glucose. In fact, palmitate caused p47(PHOX) translocation to plasma membrane, as shown by immunohistochemistry. Exposure to palmitate for 1 h up-regulated the protein content of p47(PHOX) and the mRNA levels of p22(PHOX), gp91(PHOX), p47(PHOX), proinsulin and the G protein-coupled receptor 40 (GPR40). Fatty acid stimulation of insulin secretion in the presence of high glucose concentration was reduced by inhibition of NAD(P)H oxidase activity. In conclusion, NAD(P)H oxidase is an important source of superoxide in pancreatic islets and the activity of NAD(P)H oxidase is involved in the control of insulin secretion by palmitate.