Association between oxidative stress through excessive fat accumulation and the number of mitochondrial DNA copies in adipose tissue of dairy cows
In the transition period, overconditioned cows usually have more problems to adapt to the needs of lactation than leaner cows and are thus more prone to health problems. The mitochondrial DNA (mtDNA) copy number reflects the abundance of mitochondria in a cell and may change under different energy requirements and physiological conditions. Increased metabolic demands as well as increased mtDNA copies per cell are associated with elevated oxidative stress. However, the association between oxidative stress through excessive fat accumulation and mtDNA copy numbers in bovine adipose tissue (AT), being considered as a major contributor to systemic oxidative stress, has not been investigated so far. We hypothesized that the mtDNA copy number in AT will increase concomitant with oxidative stress (assessed by quantifying the derivates of reactive oxygen species (dROM)) during fat accretion in cows. Eight non-lactating, non-pregnant pluriparous German Holstein cows (Age: 4 - 6 years) were fed diets with increasing portions of concentrate feed during the first 6 weeks of the experiment until 60% were reached, which was maintained for 9 weeks. Within this period cows had an average body weight (BW) gain of 243 ± 33.3 kg. Blood samples were collected monthly and dROM were photometrically quantified in serum using N,N,diethyl-1,4-phenylendiamine as chromogen. Biopsies from the subcutaneous tailhead AT were taken every 8 weeks and immediately snap frozen for genomic DNA isolation. The number of mtDNA copies/cell was measured by a multiplex quantitative PCR using ß-globin as reference gene. Data (mean ± SEM) for mtDNA copies and dROM as well as for BW were analyzed using non-parametric tests or repeated measurement ANOVA, respectively. Correlations were calculated using the Spearman (r) correlation coefficient. Throughout the fat accumulation period mtDNA copies/cell and dROM increased 4-fold (329 ± 57.5 to 1385 ± 160; P= 0.002) and 2.5-fold (49.9 ± 9.24 to 125 ± 16.0 µg H2O2equivalents/mL; P=0.003), respectively. We observed a positive correlation between mtDNA copy numbers and BW (r= 0.596, P= 0.003) and dROM (r=0.550, P= 0.005). Increased mtDNA copies in AT might be an adaptation in response to oxidative stress that evolves from excessive fat accumulation in overconditioned cows. It is known, that mtDNA copy numbers increase as a compensatory response mechanism to mtDNA damage. Therefore, increased numbers of mitochondria and thus increased numbers of mtDNA copies per cell might then amplify the production of ROM leading to further mtDNA damage.
mtDNA, oxidative stress, fat accumulation