Oleic, myristic and linoleic acid stimulate insulin secretion through free fatty acid receptor 1 from pancreatic β-cells

Objectives: Free fatty acids (FFA) cause a rise in cytosolic free Ca²⁺ ([Ca²⁺]i) and stimulate insulin release from pancreatic β-cells. The mechanism by which FFA exert their effects on insulin secretion has long been thought to involve the uptake and metabolism of the FFA. The G-protein coupled receptor GPR40 or free fatty acid receptor 1 (FFAR1) can be activated by various FFA. We therefore investigated whether FFAR1 is involved in the generation of the FFA-induced Ca²⁺ signal and insulin secretion from pancreatic β-cells (INS-1 cells).

Methods: [Ca²⁺]i was measured in single fura-2 loaded INS-1 cells and insulin secretion was assessed during static incubation. Downregulation of FFAR1 expression by siRNA transfection was carried out in INS-1 cells and FFAR1 expression analysis was accomplished by TaqMan-PCR.

Results: Stimulation of single INS-1 cells with the saturated FFA myristic acid, the monounsaturated FFA oleic acid, and the polyunsaturated FFA linoleic acid (100µM each) caused a rise in [Ca²⁺]i and stimulated insulin secretion in the presence of 5 mM glucose. The FFA-induced Ca²⁺ signal required mobilization of internal Ca²⁺ and Ca²⁺ influx through voltage-sensitive Ca²⁺ channels. Downregulation of FFAR1 by specific siRNA treatment lead to a significant inhibition of the FFA (100µM)-induced [Ca²⁺]i response. This was unlikely to be caused by non-specific events, since the arginine-vasopressin (100 nM)- and glucose (25 mM)-induced Ca²⁺ signals were unimpaired. The inhibitory action of FFAR1 downregulation on the FFA-induced Ca²⁺ signal was paralleled by the inhibition of insulin secretion, since downregulation of FFAR1 expression largely and significantly abolished the FFA (100µM)-induced stimulation of insulin secretion from INS-1 cells.

Conclusion: These results indicate that the FFA-stimulated Ca²⁺ signal and the augmentation of insulin release are predominantly mediated by activation of FFAR1 expressed in these cells.