Comparative effects of insulin and isoproterenol on lipoprotein
lipase in rat adipose cells.
Chiappe, Gladys E., De Cingolani, John W. F. Goers, Monica Giannotti,
and Claudia I. Caldiz.
Centro de Investigaciones Cardiovasculares, C[acute]atedra de
Fisiolog[acute]ia con Biof[acute]isica, Facultad de Ciencias
M[acute]edicas, Calle 60 y 120, (1900) La Plata, ARGENTINA and
*Department of Chemistry, California Polytechnic State University,
San Luis Obispo, California 93407, U.S.A.
APStracts 2:0438C, 1995.
The acute effects of insulin and isoproterenol on lipoprotein lipase
immunoreactive mass and enzyme activity were investigated in rat
adipose cells. Fat cells were pulse labeled for 1 h with [35S]
-methionine, after which subcellular fractions were prepared and
assayed for enzyme activity and immunoprecipitable [35S]-methionine
-labeled lipoprotein lipase. The results showed that about 80 % of
newly synthesized lipoprotein lipase is associated to plasma and
intracellular membranes and 20 % is secreted into the cell incubation
medium whereas enzyme activity was mainly associated with lipoprotein
lipase secreted into the medium. Following preincubation of
adipocytes for 1 h, 10 min incubation with 10-7 M insulin stimulates
the secretion of lipoprotein lipase activity from adipocytes as well
as the activity associated to cellular membrane fractions.
Conversely, 10 min incubation with 10-6 M isoproterenol decreased
enzyme activity in all fractions. In addition, insulin induced an
increase of membrane associated lipoprotein lipase mass with a loss
of lipoprotein lipase in the incubation medium of insulin treated
cells. When adipocytes were exposed to isoproterenol, there was a
loss of lipoprotein lipase mass from both adipocyte membranes and
incubation medium. Data are also presented which show that insulin
and isoproterenol stimulated phosphorylation of lipoprotein lipase.
Phosphorylation with a decrease in enzyme activity were also observed
in adipocytes incubated for 10 min in the presence of 10-6 M okadaic
acid. The results suggest that insulin stimulates the secretion of
active lipoprotein lipase from adipocytes as well as a rapid reuptake
of inactive secreted lipoprotein lipase whereas the decrease in
lipoprotein lipase activity induced by isoproterenol is due to enzyme
degradation. Since both hormones stimulate phosphorylation of
lipoprotein lipase, phosphorylation may play a role in the effect of
insulin increasing activity, secretion or reuptake of the enzyme as
well as in the effect of isoproterenol inducing degradation of
lipoprotein lipase in rat adipose cells.
Received 5 September 1995; accepted in final form 1 December
1995.
APS Manuscript Number C541-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 12 December 95