Phosphorylation of perilipin is associated with indicators of lipolysis in Holstein cows

Perilipin is a regulatory protein that coats the lipid droplet in adipocytes. In the basal state, perilipin inhibits lipolysis by restricting access of hormone sensitive lipase (HSL) to the lipid droplet. In contrast, during stimulated lipolysis, phosphorylated perilipin interacts with HSL such that the catalytic activity of HSL on its acylglycerol substrate is enhanced. However, the regulation and function of perilipin in vivo has not been defined clearly across comparative animal models. Consequently, this study was undertaken to determine if changes in perilipin mRNA, protein, or phosphorylation state are associated with in vivo indicators of lipolysis in the dairy cow as a model of lipolysis induced by the marked metabolic demands of lactation. Semiquantitative western blotting and quantitative PCR were used to quantify total and phosphorylated HSL and perilipin in adipose tissue obtained from cows in early [5-14 days in milk (DIM), n=11] and mid (176-206 DIM, n=9) lactation. As expected, circulating NEFA and glycerol concentrations, and phosphorylated HSL were greater in early versus mid lactation, indicative of greater lipolytic activity in early lactation. Furthermore, phosphorylated, but not total perilipin abundance, was greater in early lactation when the metabolic demand for energy is greater than in mid lactation. Finally, the abundance of phosphorylated perilipin was positively correlated with circulating glycerol and NEFA concentrations during both early and mid lactation. Collectively, these data support the hypothesis that phosphorylated perilipin is a critical determinant of lipolytic activity stemming from the metabolic demands of lactation.