However, other studies in rodents and humans have reported that high fat diet induced a coordinated downregulation in markers of mitochondrial biogenesis. Notably, studies in humans were limited to the INCB18424 assessment of mRNA expression, which are not necessarily reflective of protein levels. In this study, we did not detect any change in palmitate oxidation and citrate synthase activity with overfeeding, and in vivo mitochondrial capacity was not assessed. We chose to focus on skeletal muscle as the potential mediator of the reduction in peripheral insulin sensitivity that was observed. A potential limitation of this study is that we did not perform 2- step clamps with tracers to assess the effects of overfeeding on hepatic insulin sensitivity. Previous studies have reported hepatic insulin resistance in response to high fat overfeeding in rodents and humans. The increase in HOMA-IR that was observed in this study is supportive of this notion.
Moreover, timeline studies performed in rodents have shown that whilst hepatic insulin resistance is observed within 3 days of high fat diet, this is followed by insulin resistance in muscle only at 3 weeks. We highlight that the insulin dose in the hyperinsulinemic clamp that was chosen here will have completely suppressed hepatic glucose production, and that the muscle is the major site of glucose uptake at this dose in both rodents and humans. Interestingly, we also observed that despite a doubling in dietary fat intake, there was little evidence of an increase in fatty acid oxidation in muscle, besides an increase in CTP1b protein, the rate limiting step for entry of long chain fatty acyl-CoA into mitochondria. Rather our data support previous human studies and suggests that the immediate response to overfeeding an energy dense diet is to suppress adipose tissue lipolysis and oxidize glucose, at least in the fasting state, with transient decreases in NEFA and increases in lactate, skeletal muscle glycolytic enzyme activities and whole body glucose oxidation.
Few gender differences were detected in this study, but, we recognize that the cohort size may have been too small to detect some effects between men and women, especially for endpoints in muscle in which a reduced cohort was available for analysis. In conclusion, peripheral insulin sensitivity decreased without reducing the markers of mitochondrial content examined. Oxidative stress was however increased, and may have contributed to the decrease in insulin sensitivity that was observed. However, conclusions regarding the temporal sequence of events and whether these effects are a consequence of overfeeding or of weight gain cannot be drawn in the present study. We speculate that with prolonged overfeeding and sustained increases in ROS, oxidative stress-sensitive mitochondrial proteins involved in metabolism may be impaired, resulting in mitochondrial dysfunction and skeletal muscle lipid accumulation that generally characterize obesity, insulin resistance and type 2 diabetes. Grainy head transcription factors are crucial for many aspects of development.
For instance, Drosophila GRH regulates development of the epidermis and head skeleton, wound healing, neuroblast proliferation, early embryonic patterning, and tracheal-tube morphology.