Association of monocyte chemoattractant protein-1 and vascular endothelial growth factor in subcutaneous and visceral adipose tissue of early lactating dairy cows
Adipose tissue (AT) is an endocrine organ, producing and secreting a wide range of adipokines, which are known to regulate metabolism and immune function. Metabolic adaptations during early lactation in dairy cows may be accompanied by changes in vascularization within AT to ensure nutrient supply for the adipocytes and/or to support the release of NEFA and glycerol through increased blood flow. Small concentrations of monocyte chemoattractant protein-1 (MCP-1) can stimulate AT vascularization through the vascular endothelial growth factor (VEGF). Based on the virtual absence of macrophages in bovine AT, we hypothesized that MCP-1 in bovine AT is rather related to vascularization than to recruiting monocytes. Therefore, we aimed to investigate MCP-1 together with VEGF in different bovine AT. Primiparous German Holstein cows (n = 25) from a feeding trial were allocated to 3 groups that were slaughtered on day 1 (n = 5), 42 and 105 (each n = 10) postpartum (p.p.). Subcutaneous (sc) AT (tailhead, withers, sternum) and visceral (vc) AT (omental, mesenteric, retroperitoneal) were sampled. Quantification of MCP-1 and VEGF was done by qPCR; mRNA abundance was summarized for sc and vc AT (means ± SEM). Differences throughout lactation were analyzed using the Kruskal-Wallis test, comparison between the sc and vc AT was done by Mann-Whitney-U test, and Spearman correlation was used to investigate the relation between the two variables (SPSS 21, P ≤ 0.05). From d 1 to d 105 p.p. the mRNA abundance of MCP-1 and VEGF decreased 1.6-fold (P = 0.008) and 2-fold (P = 0.002), respectively, irrespective of AT depot. Comparing different AT across all days, MCP-1 mRNA abundance was 2.5-times (P < 0.001) and VEGFA mRNA abundance was 1.4-times (P < 0.001) higher in vc than in sc AT. Together with the previous finding that hardly no macrophages were observed in bovine AT, the positive correlation between MCP-1 and VEGF (r = 0.297; P < 0.001) indicates that MCP-1 may play a role in vascularization of bovine AT. High levels of MCP-1 and VEGF at the onset of lactation might increase vascularization to improve the essential blood flow in bovine AT right after calving. The higher expression of both MCP-1 and VEGF in vc than in sc AT might be due to the higher metabolic activity of vc AT.
Keywords: VEGF, MCP-1, adipose tissue