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Mol. Cell. Biol., 12 1997, 7386-7397, Vol 17, No. 12
PM Sharma, K Egawa, TA Gustafson, JL Martin and JM Olefsky
Activated insulin receptor (IR) interacts with its substrates, IRS-1,
IRS-2, and Shc via the NPXY motif centered at Y960. This interaction is
important for IRS-1 phosphorylation. Studies using the yeast two-hybrid
system and sequence identity analysis between IRS-1 and IRS-2 have
identified two putative elements, the PTB and SAIN domains, between amino
acids 108 and 516 of IRS-1 that are sufficient for receptor interaction.
However, their precise function in mediating insulin's bioeffects is not
understood. We expressed the PTB and SAIN domains of IRS-1 in HIRcB
fibroblasts and 3T3-L1 adipocytes utilizing replication- defective
adenoviral infection to investigate their role in insulin signalling. In
both cell types, overexpression of either the PTB or the SAIN protein
caused a significant decrease in insulin-induced tyrosine phosphorylation
of IRS-1 and Shc proteins, IRS-1-associated phosphatidylinositol 3-kinase
(PI 3-K) enzymatic activity, p70s6k activation, and p44 and p42
mitogen-activated protein kinase (MAPK) phosphorylation. However, epidermal
growth factor-induced Shc and MAPK phosphorylation was unaffected by the
overexpressed proteins. These findings were associated with a complete
inhibition of insulin- stimulated cell cycle progression. In 3T3-L1
adipocytes, PTB or SAIN expression extinguished IRS-1 phosphorylation with
a corresponding 90% decrease in IRS-1-associated PI 3-K activity. p70s6k is
a downstream target of PI 3-K, and insulin-stimulated p70s6k was inhibited
by PTB or SAIN expression. Interestingly, overexpression of either PTB or
SAIN protein did not affect insulin-induced AKT activation or insulin-
stimulated 2-deoxyglucose transport, even though both of these bioeffects
are inhibited by wortmannin. Thus, interference with the IRS- 1-IR
interaction inhibits insulin-stimulated IRS-1 and Shc phosphorylation, PI
3-K enzymatic activity, p70s6k activation, MAPK phosphorylation and cell
cycle progression. In 3T3-L1 adipocytes, interference with the IR-IRS-1
interaction did not cause inhibition of insulin-stimulated AKT activation
or glucose transport. These results indicate a bifurcation or
subcompartmentalization of the insulin signalling pathway whereby some
targets of PI 3-K (i.e., p70s6k) are dependent on IRS-1-associated PI 3-K
and other targets (i.e., AKT and glucose transport) are not. IR-IRS-1
interaction is not essential for insulin's effect on glucose transport, and
alternate, or redundant, pathways exist in these cells.
Copyright © 1997, American Society for Microbiology
Adenovirus-mediated overexpression of IRS-1 interacting domains abolishes insulin-stimulated mitogenesis without affecting glucose transport in 3T3-L1 adipocytes
Department of Medicine, University of California, San Diego, La Jolla 92093, USA.
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