AJP - Cell Physiology, Vol 270, Issue 5 C1461-C1467, Copyright © 1996 by American Physiological Society
Comparative effects of insulin and isoproterenol on lipoprotein lipase in rat adipose cells
G. E. Chiappe de Cingalani, J. W. Goers, M. Giannotti and C. I. Caldiz
Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Medicas, La Plata, Argentina.
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.
