Possible Links to Insulin Resistance: Differential Effects of Palmitate, Oleate and Oleate/Palmitate Mixture on mtDNA Damage, Mito-chondrial Dysfunction and Apoptosis in Rat L6 Skeletal Muscle Cells.

Recent studies suggested that the accumulation of muscular free fatty acids (FFA) might be responsible for the mitochondrial dysfunction and insulin resistance (IR) seen in type 2 diabetes mellitus (T2DM), and that the type of FFA is critical in this process. Additionally oxidative stress, which results from increased production of reactive oxygen/nitrogen species (ROS/RNS) leads to 1R and T2DM. We hypothesized that oxidative damage to mitochondrial DNA (mtDNA) is an early step involved in mitochondrial dysfunction and IR following exposure of skeletal muscle to FFA. Damage to mtDNA initiates processes which heighten production of ROS causing progressive mitochondrial and cellular dysfunction which ultimately leads to IR. To test this hypothesis, rat L6 myotubes were exposed to palmitate, which is the most prevalent saturated FFA; oleate, which makes up 80% of the circulating monounsaturated pool; and a mixture (oleate/palmitate; 2/1 ). Previously palmitate has been shown to induce IR in L6 myotubes whereas oleate improves insulin sensitivity. Moreover, palmitate-induced IR and apoptosis were positively related, while oleate did not induce apoptosis in L6 myotubes. The current studies were undertaken to investigate whether FFA-induced apoptosis and IR correlate with mtDNA damage and mitochondrial dysfunction in L6 myotubes. The results showed that only palmitate caused production of nitric oxide (NO), ROS, mtDNA damage and apoptosis in L6 myotubes. Incubation of L6 myotubes with similar concentrations of either oleate alone or oleate combined with palmitate did not stimulate production of NO. Oleate did not cause mtDNA damage in L6 myotubes and the addition of oleate to palmitate prevented palmitate-induced mtDNA damage, and palmitate-induced decrease in total ATP levels and cell viability. Addition of oleate to palmitate prevented palmitate-induced apoptosis. CONCLUSION: FFA-induced mtDNA damage is a key component contributing to IR in skeletal muscle probably by triggering mitochondrial dysfunction which leads to apoptosis. [ABSTRACT FROM AUTHOR]