Effects of palmitate on ER and cytosolic Ca2+ homeostasis in beta-cells

There are strong links between obesity, elevated free fatty acids, and type 2 diabetes. Specifically, the saturated fatty acid palmitate has pleiotropic effects on beta-cell function and survival. In the present study, we sought to determine the mechanism by which palmitate affects intracellular Ca2+, and in particular the role of the endoplasmic reticulum (ER). In human beta-cells and MIN6 cells, palmitate rapidly increased cytosolic Ca2+ through a combination of Ca2+ store release and extracellular Ca2+ influx. Palmitate caused a reversible lowering of ER Ca2+, measured directly with the fluorescent protein-based ER Ca2+ sensor D1ER. Using another genetically encoded indicator, we observed long-lasting oscillations of cytosolic Ca2+ in palmitate-treated cells. In keeping with this observed ER Ca2+ depletion, palmitate induced rapid phosphorylation of the ER Ca2+ sensor protein kinase R-like ER kinase (PERK) and subsequently ER stress and beta-cell death. We detected little palmitate-induced insulin secretion, suggesting that these Ca2+ signals are poorly coupled to exocytosis. In summary, we have characterized Ca2+-dependent mechanisms involved in altered beta-cell function and survival induced by the free fatty acid palmitate. We present the first direct evidence that free fatty acids reduce ER Ca2+ and shed light on pathways involved in lipotoxicity and the pathogenesis of type 2 diabetes.