Protection of palmitic acid-mediated lipotoxicity by arachidonic acid via channeling of palmitic acid into triglycerides in C2C12

Background: Excessive saturated fatty acids have been considered to be one of major contributing factors for the dysfunction of skeletal muscle cells as well as pancreatic beta cells, leading to the pathogenesis of type 2 diabetes. Results: PA induced cell death in a dose dependent manner up to 1.5 mM, but AA protected substantially lipotoxicity caused by PA at even low concentration of 62 mu M, at which monounsaturated fatty acids including palmitoleic acid (POA) and oleic acid (OA) did not protect as much as AA did. Induction of cell death by PA was resulted from mitochondrial membrane potential loss, and AA effectively blocked the progression of apoptosis. Furthermore, AA rescued significantly PA-impaired glucose uptake and -signal transduction of Akt in response to insulin. Based on the observations that polyunsaturated AA generated competently cellular droplets at low concentration within the cytosol of myotubes compared with other monounsaturated fatty acids, and AA-driven lipid droplets were also enhanced in the presence of PA, we hypothesized that incorporation of harmful PA into inert triglyceride (TG) may be responsible for the protective effects of AA against PA-induced lipotoxicity. To address this assumption, C2C12 myotubes were incubated with fluorescent probed-PA analogue 4,4-difluoro-5, 7-dimethyl-4-boro-3a,4a-diaza-s-indacene-3-hexadecanoic acid (BODIPY FL C16) in the presence of AA and their subsequent lipid profiles were analyzed. The analyses of lipids on thin layer chromatograpy (TLC) showed that fluorescent PA analogue was rapidly channeled into AA-driven TG droplets. Conclusion: Taken together, it is proposed that AA diverts PA into inert TG, therefore reducing the availability of harmful PA into intracellular target molecules.