Effect of increasing body condition on oxidative stress and mitochondrial biogenesis in subcutaneous adipose tissue depot of nonlactating dairy cows

With the onset of lactation, dairy cows with a body condition score >3.5 are sensitive to oxidative stress and metabolic disorders. Adipose tissue (AT) can adapt to varying metabolic demands and energy requirements by the plasticity of its size during lactation. In AT, angiogenesis is necessary to guarantee sufficient oxygen and nutrient supply for adipocytes. Cellular energy metabolism is reflected mainly by mitochondria, which can be quantified by the mitochondrial DNA copy number per cell. In the present study, we aimed to investigate the effect of overconditioning on angiogenesis and mitochondrial biogenesis in AT of nonlactating cows, irrespective of the physiological influences of lactation and pregnancy. Eight nonpregnant, nonlactating cows received a ration of increasing energy density for 15 wk, during which body weight and body condition increased substantially. Subcutaneous AT was biopsied every 8 wk, and blood was sampled monthly. The blood concentrations of indicators of oxidative stress increased continuously throughout the experimental period, possibly damaging mitochondrial DNA. Concomitantly, HIF-1α, a major marker for hypoxia, increased until wk 8, indicating insufficient angiogenesis in the rapidly expanding AT. Based on the observation that the number of apoptotic cells decreased with increasing hypoxia, the increasing mitochondrial DNA copy numbers might compensate for the hypoxia, reinforcing the production of oxidative stressors. Key transcription factors of mitochondrial biogenesis were largely unaffected. Thus, increased oxidative stress does not impair mitochondrial DNA.