Aspects of Lipid Synthesis Unique to Brown Fat

Abstract

Chemical thermogenesis, necessary for extra-uterine adaptation, is partly mediated through oxidation of fatty acids in brown fat, requiring a rapid rate of trigly-ceride turnover. Previous studies from our laboratory indicated that brown fat contains glycerokinase, making it possible for brown fat to reutilize glycerol. Tri-glyceride synthesis in brown fat, thus, may not be completely dependent on glucose transport. The role of glycerol in triglyceride synthesis in brown fat has been further studied, using both in vitro and in vivo techniques. Explants of brown fat were incubated for periods up to 3 days, with either glucose or glycerol as substrate. Insulin was added at varying times, as were tracer quantities of acetate C14, glucose C14 or glycerol C14. Glycerol promoted a greater incorporation of acetate C14 into mono-, di- and triglycerides of brown fat thatn did glucose. Incorporation of glycerol C14 and glucose C14 into lipids of brown fat were both increased by insulin. IP injection of glycerol-2-H3 into newborn rats was followed by much greater incorporation into lipids of brown as compared to white fat. Net triglyceride synthesis was not demonstrable in vitro regardless of substrate or insulin, suggesting marked enhancement of triglyceride turnover by insulin. Increased turnover rate is also indicated by greater conversion of acetate C14 to C14O2 in the presence of insulin. These results suggest the following: (1) Glycerol can be utilized by brown fat as substrate for lipid synthesis; (2) transport of glycerol in brown fat is subject to regulation by insulin; and (3) the anti-lipolytic effect of insulin is not demonstrable in the in vitro system used in these studies.