Inverse Relationship Between Cytotoxicity of Free Fatty Acids in Pancreatic Islet Cells and Cellular Triglyceride Accumulation

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Abstract
Studies in Zucker diabetic fatty rats have led to the concept that chronically elevated free fatty acid (FFA) levels can cause apoptosis of triglyceride-laden pancreatic β-cells as a result of the formation of ceramides, which induce nitric oxide (NO)-dependent cell death. This “lipotoxicity” hypothesis could explain development of type 2 diabetes in obesity. The present study examines whether prolonged exposure to FFA affects survival of isolated normal rat β-cells and whether the outcome is related to the occurrence of triglyceride accumulation. A dose-dependent cytotoxicity was detected at 5–100 nmol/l of unbound oleate and palmitate, with necrosis occurring within 48 h and an additional apoptosis during the subsequent 6 days of culture. At equimolar concentrations, the cytotoxicity of palmitate was higher than that of oleate but lower than that of its nonmetabolized analog bromopalmitate. FFA cytotoxicity was not suppressed by etomoxir (an inhibitor of mitochondrial carnitine palmitoyltransferase I) or by antioxidants; it was not associated with inducible NO synthase expression or NO formation. An inverse correlation was observed between the percentage of dead β-cells on day 8 and their cellular triglyceride content on day 2. For equimolar concentrations of the tested FFA, oleate caused the lowest β-cell toxicity and the highest cytoplasmic triglyceride accumulation. On the other hand, oleate exerted the highest toxicity in islet non–β-cells, where no FFA-induced triglyceride accumulation was detected. In conditions without triglyceride accumulation, the lower FFA concentrations caused primarily apoptosis, both in islet β-cells and non–β-cells. It is concluded that FFAs can cause death of normal rat islet cells through an NO-independent mechanism. The ability of normal β-cells to form and accumulate cytoplasmic triglycerides might serve as a cytoprotective mechanism against FFA-induced apoptosis by preventing a cellular rise in toxic free fatty acyl moieties. It is conceivable that this potential is lost or insufficient in cells with a prolonged triglyceride accumulation as may occur in vivo.