Mechanisms by which oleate mobilizes intracellular calcium in insulin secreting cells

INS-1E beta cells were used to understand how oleate (OA) modulates [Ca 2+]i. OA binding to GPR40 and GPR120 receptors caused the release of endoplasmic reticulum (ER) [Ca2+]i via IP3 and ryanodine receptors, causing Ca2+ influx via L-type Ca2+ channels and Ca2+ release activated Ca2+ channels (CRACCs). [Ca2+]i release measured using fura-2 consisted of fast transient rises of [Ca2+] i, more prolonged plateaus or [Ca2+]i oscillations. Postulating sustained Ca2+ influx and IP3-mediated [Ca2+]i release, the full repertoire of observed [Ca2+]i changes could be readily simulated mathematically. Furthermore, [Ca2+]i release was mimicked by the GPR40 and GPR120 agonist, GW9508. The Ca2+ filling state of the ER store was critical, as unloading the store with the SERCA inhibitors thapsigargin or CPA abrogated OA action. We thus propose that the ER is a central regulator of fatty acid action in insulin secreting cells. It was necessary to probe the endogenous energy sources for SERCA-mediated Ca2+ uptake because while glucose influenced OA action, it was not mandatory. ER filling was partially dependent on glucose metabolism as well as lipolysis. Exposure of cells to an hour of glucose free media or in the presence of 11 mM glucose plus the lipolysis inhibitor orlistat significantly decreased but did not abolish OA action. This suggests that both glucose metabolism and triglyceride breakdown are involved in OA actions but the details of this mechanism remain unresolved. Glucose also modulated [Ca2+] i release triggered by GW9508, suggesting some metabolism likely modulates GPR40 and GPR120 receptor activation, ER store filling, or ER [Ca2+ ]i release. In sum, the regulation of ER [Ca2+] i store filling and release by OA is a complex process in 1NS-1E cells and appears to involve novel mechanisms including endogenous fatty acid energy sources for [Ca2+]i pumping, L-type Ca2+ channels and CRACCs to maintain standing Ca2+ influx. Defects in this pathway could contribute to lipotoxicity in diabetes mellitus.