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Address all correspondence and requests for reprints to: Morris F. White, Ph.D., Howard Hughes Medical Institute, Joslin Diabetes Center, 1 Joslin Place, Boston, Massachusetts 02215. E-mail: morris.white{at}joslin.harvard.edu
| Introduction |
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Genetic and physiological studies in diverse organisms from humans to
C. elegans provide strong evidence that the
insulin/IGF-signaling system is an evolutionarily conserved mechanism
integrating fuel metabolism with growth, development, reproduction, and
longevity (1). At a cellular level, insulin/IGF-signaling regulates
multiple processes, including carbohydrate and lipid metabolism, gene
transcription, DNA synthesis, anti-apoptosis, and cell proliferation.
Insulin and insulin-like growth factors (IGF-1 and IGF-2) bind to
members of the insulin receptor tyrosine kinase family, including the
type a and b insulin receptors (IRa and IRb) and the IGF-1
receptor (IGF1r). Insulin and IGF-1 are relatively specific agonists
for the insulin and IGF-1 receptors, respectively; however, IGF-2
provides cross-talk between these receptors as it binds with high
affinity to the IGF1r, and to IRa that predominates during fetal
development (Fig. 1
). In postnatal life,
insulin is an essential metabolic signal, whereas IGF-1 and IGF-2
promote cell proliferation, differentiation and survival; however, as
IGF-1 receptor signals promote pancreatic ß-cell survival and
expansion, the IGF1/2 might play important roles in the pathogenesis of
type 2 diabetes. Whereas the conventional view asserts that diabetic
complications arise from the deleterious effects of chronic
hyperglycemia, type 2 diabetes might be best understood as a global
disorder of insulin/IGF1 signal transduction that ultimately
dysregulates gene expression and cell function in a wide range of
tissues. The use of transgenic mice provides surprising new insights
into the pivotal role of insulin/IGF signal transduction mechanisms in
the pathogenesis of type 2 diabetes. Understanding the molecular basis
of insulin/IGF signaling might provide a common molecular platform to
rectify abnormal insulin action and insulin secretion.
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| Multisystem insulin resistance, an essential element in diabetes |
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Multisystem insulin resistance might be an essential element in the pathogenesis of type 2 diabetes, either exacerbating usual compensatory mechanisms or directly interfering with insulin secretion. Although the later possibility is generally ignored, the disruption of insulin receptors in pancreatic ß-cells reveals an unexpected role for insulin signaling during insulin secretion. Without insulin receptors, glucose-stimulated insulin secretion is reduced in ß-cells, and glucose intolerance develops with age, although diabetes does not occur (9, 10). Combined insulin resistance in ß-cells and hepatic/muscle might be an important component of type 2 diabetes. Thus, tissue specific insulin receptor disruption highlights the integrated nature of insulin-regulated metabolism.
| The IRS-proteins coordinate the insulin signaling cascade |
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IRS-proteins have a conserved amino terminus composed of adjacent
pleckstrin homology and phosphotyrosine-binding domains that mediate
coupling to activated receptor tyrosine kinases. By contrast, the
COOH-terminus of each IRS-protein contains a set of tyrosine
phosphorylation sites that act as on/off switches to recruit and
regulate various downstream signaling proteins. Each tyrosine
phosphorylation site is surrounded by a unique amino acid motif that
binds and activates specific effector proteins, including enzymes (PI
3-kinase; a phosphotyrosine phosphatase SHP2; and Src-like kinase, fyn)
and adapter proteins (Grb-2, nck, crk, and others) (Fig. 1
).
IRS-proteins are an important site of signal divergence. The activation
of the Erk-kinase cascade through Grb-2 binding promotes gene
transcription; this pathway might inhibit insulin signals by promoting
serine phosphorylation of IRS-proteins or the insulin receptor. The
association of SHP2 with IRS-1 during insulin stimulation promotes
phosphotyrosine dephosphorylation, which attenuates insulin signals by
reducing the association of Grb-2 and PI 3-kinase. By contrast,
activation of the type 1a PI 3-kinase during association with
IRS-proteins is implicated in many biological responses to insulin/IGF,
including glucose transport, DNA synthesis, and cell survival (Fig. 1
).
Interaction of a tyrosine phosphorylated IRS-protein with both SH2
domains of the regulatory subunit activates the associated catalytic
subunit, which generates various 3'-phospholipids, including
phosphatidylinositol (3, 4)bisphosphate
(PI-3,4-P2) and phosphatidylinositol (3, 4, 5)triphosphate (PI-3,4,5-P3); the levels of these
phospholipids are attenuated by the action of specific phosphatases,
including pTEN, which hydrolyzes the 3'-phosphate and SHIP2 which
hydrolyzes the 5'-phosphate from PI-3,4,5-P3
(Fig. 1
). Products of the PI 3-kinase recruit serine kinases to the
plasma membrane, including the phospholipid-dependent kinases (PDK1/2)
and protein kinase B (akt/PKB) (Fig. 1
). During colocalization, the
PDKs phosphorylate and activate akt/PKB, which might contribute
directly to the regulation of various biological responses (Fig. 1
).
Together with other PI 3-kinase-dependent serine kinases, akt/PKB
promotes glucose transport, protein synthesis, glycogen synthesis, cell
proliferation, and cell survival in various cells and tissues (11, 14, 15).
The regulation of PI 3-kinase by IRS-proteins is an important site of
signal redundancy and diversity. In cell-based assays, IRS-1 and IRS-3
activate PI 3-kinase more strongly than IRS-2, whereas IRS-4 barely
activates PI 3-kinase (16, 17). However, in murine liver, IRS-2 appears
to play a major role in the regulation of PI 3-kinase (12). In addition
to the diversity of the IRS-proteins, the PI 3-kinase itself exists as
multiple isoforms, owing to the differential expression and
dimerization of multiple regulatory subunits (p85
, p85ß,
p55PIK, p55
, or p50
) and catalytic subunits
(p110
, p110ß, or p110
). (18). Disruption in mice of p85
reveals the redundancy and selectivity at this important signaling
step. Unexpectedly, mice lacking p85
display increased insulin
sensitivity and hypoglycemia due to increased glucose transport in
skeletal muscle and adipocytes; insulin-stimulated PI3K activity
associated with IRS-proteins is mediated through p50
, which augments
the generation of PI-3,4,5-P3 in adipocytes.
Thus, IRS-proteins and PI 3-kinase provide an important coordinate step
in the regulation of insulin signaling cascade.
| IRS-proteins integrate growth and development |
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| IRS-proteins mediate the effects of insulin on peripheral carbohydrate metabolism |
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IRS-1 appears to exert its greatest effect on metabolism by regulating
insulin signals in muscle and adipose tissue as demonstrated by both
in vivo clamp experiments and in vitro tissue
culture studies, whereas it plays a lesser role in mediating insulins
effects on the liver metabolism (6, 10, 12, 20, 21, 22). To compare the
impact of irs1 or irs2 gene disruptions on basal
and insulin-stimulated carbohydrate and lipid metabolism in
vivo, we studied 18-h fasted chronically catheterized mice during
basal and euglycemic (
5 mM), hyperinsulinemic
(5
mU·kg-1·min-1)
clamp (90210 min) conditions. Both IRS1-/-
and IRS2-/- mice are markedly insulin resistant
compared with wild-type mice as reflected by the absence of insulin on
the rate of glucose appearance. Most of the decreased insulin
responsiveness in the IRS1-/- mice is related
to decreased insulin-stimulated peripheral glucose uptake, whereas
insulin resistance in IRS2-/- mice is
attributed to both decreased peripheral glucose uptake and unsuppressed
hepatic gluconeogenesis (Previs, S. F., J. Ren, D. Withers,
M. F. White, and G. I. Shulman, in preparation). Moreover,
insulin poorly suppresses lipolysis in IRS2-/-
mice. These data suggest important tissue specific roles for IRS1 and
IRS2 in mediating insulins effects on carbohydrate and lipid
metabolism in vivo, with IRS-2 being important in liver and
muscle, and IRS-1 having a major role in muscle.
To further refine these observations upon the interactions of IRS-1 and
IRS-2 with insulin receptor in the regulation of metabolism, we created
combined heterozygous mutations for the insulin receptor, IRS-1 and
IRS-2 (23). Insulin sensitivity was estimated by the relative activity
of PI 3-kinase in muscle and liver extracts, and compared against the
fed serum insulin levels (Fig. 2
). Mice
lacking one allele of each protein
[ir/irs1/irs2]+/- display significant hepatic
and skeletal muscle insulin resistance with marked hyperinsulinemia;
serum glucose levels are only slightly elevated (Fig. 2
). Less drastic
disruption of these signaling elements, as in
ir+/-, [ir/irs1]+/- or
[ir/irs2]+/- mice, attains better
insulin-stimulated activation of PI 3-kinase and normal serum glucose
levels with lower insulin levels (Fig. 2
). These genetic experiments
might reflect the balance between insulin secretion and insulin action
ordinarily encountered during compensation for peripheral insulin
resistance.
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| The insulin/IGF signaling system regulates ß-cell function |
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To delineate the signals that activate IRS-2 pathways in ß-cells, we
analyzed fetal igf1r-/- mice, or postnatal
igf1r+/- and
[igf1r+/-/irs2-/-]
mice. Islets are reduced in size in igf1r+/-
mice, and develop poorly in igf1r-/- mice as a
reduced number of
-cells and ß-cell fail to form typical islet
structures. The igf1r-/- mice die at birth, whereas igf1r± mice
survive with reduced ß-cell mass, and never develop diabetes;
however, combined with an irs2 disruption,
[igf1r+/-irs2-/-]
mice die at 30 days of age with severe hyperglycemia, owing to small
islets containing a reduced number of
-cell and nearly no ß-cells.
These results suggest provisionally that IGF1
IRS2 signaling pathway
might be critical for both the embryonic development and postnatal
growth of ß-cells and reveal an important interface between the
insulin and IGF signaling pathways.
| Conclusions |
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| Footnotes |
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Received April 6, 2000.
| References |
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