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Diabetes, Vol 48, Issue 4 691-698, Copyright © 1999 by American Diabetes Association
Activation of protein kinase B and induction of adipogenesis by insulin in 3T3-L1 preadipocytes: contribution of phosphoinositide-3,4,5-trisphosphate versus phosphoinositide-3,4-bisphosphate
A Gagnon, CS Chen and A Sorisky
Department of Medicine, Ottawa Hospital Loeb Research Institute, University of Ottawa, Canada.
Ectopic expression of activated protein kinase B (PKB) induces the
differentiation of confluent 3T3-L1 preadipocytes into adipocytes. PKB is
regulated by the lipid products of phosphoinositide 3-kinase (PI 3-kinase),
phosphatidylinositol-3,4-bisphosphate [PI(3,4)P2], and
phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3]. However, the
relative contribution of each 3-phosphorylated phosphoinositide species in
activating PKB remains unclear. Treatment of intact 3T3-L1 preadipocytes
with synthetic 3-phosphorylated phosphoinositides revealed that only
PI(3,4)P2 stimulated PKB activity. PKB was also activated by insulin, in a
dose- and time-dependent manner. This activation was associated with an
isolated rise in PI(3,4,5)P3, without any detectable change in PI(3,4)P2,
demonstrating that this lipid was sufficient to activate PKB. Wortmannin
and LY294002, inhibitors of PI 3-kinase, reduced insulin-dependent
activation of PKB, whereas rapamycin, an inhibitor of p70 S6 kinase, had no
effect. Platelet-derived growth factor (PDGF), which is not adipogenic,
stimulated the production of both 3-phosphorylated phosphoinositide
species, and this was associated with a greater activation of PKB than that
observed with insulin. A low dose of PDGF (1 ng/ml), which increased the
production of only PI(3,4,5)P3 and mirrored the insulin effect, was unable
to induce adipocyte differentiation. In summary, insulin and PDGF differ
with respect to the accumulation of 3-phosphorylated phosphoinositides and
to PKB activation in 3T3-L1 preadipocytes, but these responses do not
themselves explain why insulin, but not PDGF, is adipogenic.

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Copyright © 1999 by the American Diabetes Association.
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