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Effects of hyperglycemia and long chain free fatty acids on myogenic regulatory factors

Myogenic regulatory factors (MRFs), MyoD and myogenin regulate transcription of muscle specific genes. MRF protein levels are reduced in muscle atrophy. We have observed significantly lower skeletal muscle weights in obese zucker rats (OZR) as compared to lean rats. Since OZR are both hyperglycemic and hyperlipidemic we tested the effects of both glucose and long chain free fatty acids (FFAs) on MRFs in vitro. For these experiments, C2C12 myotubes were incubated overnight (16 h) in conditions of excess glucose or FFA (palmitate). We performed an additional experiment in which myotubes were incubated for 72 h with high glucose to determine if glucose treatment was cumulative. Myogenin and MyoD protein content was 17% and 50% lower, respectively in myotubes after 16 h of FFA treatment than control cultures. Myogenin protein content was 48% and 40%, lower than control cultures after 16 h and 72 h of hyperglycemia treatment, respectively. MyoD protein content in myotubes was increased by 48% after 12 h of hyperglycemia, whereas 72 h of hyperglycemia reduced MyoD content by 40%, although this failed to reach significance (P=0.09). The data show that hyperlipidemia and hyperglycemia treatments alter MRF content in myotubes in vitro. This is consistent with the hypothesis that MRF dysregulation may have a role in reducing skeletal muscle mass with insulin resistance.