1439
Association between insulin signaling and oxidative stress in serum and subcutaneous adipose tissue of overconditioned cows
Cows with higher BCS or greater BCS loss in early lactation have more problems to adapt to the needs of lactation and are more sensitive to oxidative stress. Mitochondrial (mt)DNA copy numbers, reflecting the abundance of mitochondria in a cell, can increase to compensate mtDNA damage. Moreover, reactive oxygen species produced through lipid-induced mitochondrial dysfunction impair insulin signaling. We hypothesized that decreasing insulin sensitivity in overconditioned cows will be associated with oxidative stress concomitant with increased numbers of mitochondria. Therefore, we aimed to investigate the association between oxidative stress (assessed by quantifying derivates of reactive oxygen species (dROM)) and mtDNA copies/cell in subcutaneous (sc) adipose tissue (AT) with variables describing insulin sensitivity (IS) in overconditioned cows independently from homeorhetic processes. Non-pregnant, non-lactating German Holstein cows (n=8) were gradually adapted to a high-energy ration (corn-grass-silage with increasing portion of corn-silage and increasing concentrate feed from 0% up to 60% of total dry matter intake). Over a period of 15 weeks, the average weight gain of the animals was 243 ± 33.3 kg. Blood samples were collected once per month and were analyzed for insulin, glucose and NEFA to calculate a surrogate marker for IS (RQUICKI). Adiponectin was measured in serum using an in-house developed ELISA. Derivates of reactive oxygen metabolites (dROM) were indirectly measured in serum using a photometric method with N,N,diethyl-1,4-phenylendiamine as chromogen. Every 8 weeks, scAT from tailhead was biopsied and snap frozen for genomic DNA isolation. The number of mtDNA copies/cell was measured by multiplex qPCR. Data (mean ± SEM) were analyzed using repeated measures ANOVA and Spearman correlations (SPSS). Decreasing insulin sensitivity throughout the experiment, indicated by decreasing RQUICKI values (P=0.001), were negatively correlated with dROM (r= -0.543, P=0.007) and mtDNA copies (r = -0.568, P=0.005). Moreover, adiponectin concentrations were decreased throughout the experiment (P<0.05) and tended to be negatively correlated (r =-0.381, P=0.067) with mtDNA copies. Increased oxidative stress seems to enhance insulin resistance. However, dROM was not associated with serum adiponectin which is known for its insulin sensitizing and lipolysis inhibiting effects. In contrast to reports about insulin resistance being related to reduced mitochondrial content in humans, increasing mtDNA copies in the present study seem to compensate mitochondrial damage caused by enhanced dROM.
Keywords:
insulin resistance, oxidative stress, mtDNA