Hepatic and extrahepatic expression of serum amyloid A3 during lactation in dairy cows
Serum amyloid A3 (SAA3) is the predominant SAA isoform secreted by mammary epithelial cells in dairy cows; it is also expressed in bovine adipose tissue (AT). The adipokine SAA3 is linked to obesity and insulin resistance of AT and the respective inflammatory response, at least in mice. Dietary treatment with conjugated linoleic acids (CLA) reportedly also affects insulin sensitivity and inflammatory status in monogastrics. Both SAA3 and CLA thus seem to alter similar functions. Based on changes in insulin sensitivity and the inflammatory status throughout lactation, we hypothesized that the mRNA abundance of SAA3 in various tissues might be regulated as well and that CLA could be a modulator of SAA3 mRNA expression. In 2 trials, 21 pluriparous and 25 primiparous Holstein cows were fed 100 g/d of a CLA or a control fat supplement from d 1 to 182 or 105 postpartum, respectively. Biopsies from liver and subcutaneous (s.c.) AT from pluriparous cows and samples from 3 different visceral AT and 3 s.c. AT, muscle, mammary gland, and liver tissue from slaughtered primiparous cows were obtained. In an adipocyte cell culture system, cell samples were collected during differentiation of bovine preadipocytes at d 0, 2, 6, 8, 10, 12, and 13 relative to the onset of differentiation. The SAA3 mRNA abundance in tissues and in differentiating bovine preadipocytes was measured by real-time PCR. The presence of the SAA protein was confirmed by Western blotting. Treatment with CLA yielded only few and inconsistent effects on SAA3 mRNA abundance. In both trials, SAA3 mRNA peaked at d 1 postpartum in all tissues except in mesenteric AT, in which the change was not significant. The highest SAA3 mRNA expression was observed in the mammary gland, followed by omental AT. The SAA protein was present in the visceral and s.c. AT depots investigated. Adipocytes as one source of SAA3 were confirmed by the SAA3 mRNA profile in differentiating adipocytes. The longitudinal changes observed point to SAA3 being involved in the inflammatory situation around parturition.