The effects of insulin and isoproterenol on lipoprotein lipase mass and enzyme activity were investigated in rat adipocytes. Cells were pulse labeled for 1 h with [35S]methionine to measure immunoprecipitable lipoprotein lipase. The results showed that 80% of the newly synthesized enzyme was membrane associated and 20% was secreted into the cell incubation medium. Enzyme activity was mainly associated with lipoprotein lipase secreted into the medium. A 10-min incubation with 10(-7) M insulin stimulated the secretion of lipoprotein lipase activity and the activity associated with adipocyte membranes. Conversely, 10(-6) M isoproterenol decreased the activity in all fractions. In addition, insulin increased lipoprotein lipase mass associated with cell membranes and decreased that in the incubation medium, whereas isoproterenol induced a decrease in both cell membranes and medium. Insulin and isoproterenol stimulated phosphorylation of lipoprotein lipase. These findings suggest that insulin stimulates the secretion of active lipoprotein lipase and a reuptake of inactive secreted enzyme, and isoproterenol decreases the activity by enzyme degradation. Moreover, because both agents stimulate phosphorylation of lipoprotein lipase, phosphorylation may play a role in the effect of insulin increasing enzyme activity, in secretion or reuptake, and in the effect of isoproterenol inducing degradation of lipoprotein lipase.
Retention of glucose by N-linked oligosaccharide chains impedes expression of lipoprotein lipase activity: effect of castanospermine.
