The Reciprocal Regulation of Lipoprotein Lipase Activity and Hormone-sensitive Lipase Activity in Rat Adipocytes

Abstract The regulation of lipoprotein lipase activity was studied in rat adipocytes. Incubation of fat cells for 60 min in buffer without glucose and insulin resulted in a 50% decrease in lipoprotein lipase activity. This decrease was prevented by glucose and insulin. Inhibition of fat cell protein synthesis by cycloheximide abolished the effect of glucose and insulin and caused a rapid decay of lipoprotein lipase activity (T½ = 24 min). Lipolytic concentrations of N6, O2'-dibutyryl cyclic adenosine 3',5'-monophosphate (dibutyryl cAMP) in the presence of glucose and insulin decreased fat cell lipoprotein lipase activity and also decreased the rate of l-leucine-1-14C incorporation into fat cell protein. A close correlation (r = .95) was found between changes in the rate of protein synthesis and changes in lipoprotein lipase activity caused by lipolytic agents, cycloheximide, and by the omission of glucose and insulin. Incubation with dibutyryl cAMP decreased fat cell ATP content. In the presence of glucose and insulin this occurred without an intracellular accumulation of fatty acid, and it is suggested that the decrease was due to consumption of ATP by the process of re-esterification of fatty acids. Thus the decrease in lipoprotein lipase activity caused by lipolytic hormones could be the last step in a series of events. Activation of lipolysis with re-esterification of some of the fatty acids results in an increased consumption of ATP, and therefore the ATP available for protein synthesis is limited. This leads to a decrease in the rate of protein synthesis and then, because of the rapid decay of this enzyme, to a decrease in lipoprotein lipase activity.