Insulin signal transduction pathways

Reverter JL, Lucas A, Salinas I, Audi L, Foz M, Sanmarti A: 1992. Suppressive therapy with levothyroxine for solitary thyroid nodules. Clin Endocrinol (Oxf) 36:25-28. Ridgway EC: 1992, Clinician’s evaluation of a solitary thyroid nodule. J Clin Endocrinol Metab 74:231-235. Rojeski MT, Gharib H: 1985. Nodular thyroid disease: evaluation and management. N Engl J Med 313:428-436.

Ross DS: 1991. Evaluation of the thyroid nodule. J Nucl Med 32:2181-2192.

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Vander JB, Gaston EA, Dawber TR: 1968. The significance of nontoxic thyroid nodules: final report of a 15-year study of the incidence of thyroid malignancy. Ann Intern Med 69:537-540. Van Herle AJ, Rich P, Ljung B-ME, Ashcraft MW, Solomon DH, Keeler EB: 1982. The thyroid nodule. Ann Intern Med 96:221232.

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Insulin Signal Transduction Pathways Michael J. Quon, Atul J. Butte, and Simeon I. Taylor

Insulin initiates its pleiotropic effects by activating the insulin receptor tyrosine kinase to phosphorylate several intracellular proteins. Recent studies have demonstrated that phosphotyrosine residues bind specifically to proteins that contain src homology 2 (SH2) domains, and that this interaction mediates the regulation of multiple intracellular signaling pathways. This article reviews recent progress in elucidating the detailed pathways that lead from the insulin receptor to the ultimate biologic actions of insulin. (Trends Endocrinol Metab 1994;5:369376) Insulin plays a key role in promoting growth, differentiation, and metabolism. Many of insulin’s actions have been characterized in great detail: for example, recruitment of glucose transporters to the plasma membrane, regulation of enzymes such as glycogen synthase by phosphorylation/dephosphorylation mechanisms, and the regulation of gene expression. Like other polypeptide hormones, the pleiotropic effects of insulin are initiated by the binding of insulin to its receptor at the cell surface. The ability of the insulin receptor to act as a tyrosine kinase appears to be crucial in insulin signaling (Kahn et al. 1993, Taylor et al. 1992). Until very recently,

most studies

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action

have fo-

steps in the signaling

pathways.

How-

ever, in the past several years, considerable progress

has been made in elucidat-

ing the middle link

steps of pathways

insulin-stimulated

phorylation tions

of insulin

upon

of physiologic

insulin

receptor

been identified. are capable elucidated

tyrosine

to the ultimate

number

biologic

target

tyrosine

ac-

cells.

substrates

A

of the

kinase

have

Some of these substrates

of interacting

with recently

signal transduction

pathways

factors

This convergence

has made it possible to

biochemically physiologic

define

from

and cytokines.

insulin

the insulin

insulin

view, we summarize

effects.

signaling

receptor

molecular

to

In this re-

recent studies that

have advanced our understanding mechanisms

The mature insulin receptor tetrameric sisting

is a hetero-

cell surface glycoprotein

con-

of two a- and two p-subunits

joined by disulfide bonds (Figure 1) (Czech 1985). After the insulin receptor was cloned and sequenced, it was apparent that it belonged to a family of ligand-activated tyrosine kinases related to the protein encoded by the V-SIC oncogene (Ullrich et al. 1985, Ebina et al. 1985, Ullrich and Schlessinger 1990). The extracellular a-subunits contain an insulin-binding domain. The transmembrane B-subunits anchor the receptor in the plasma membrane and possess tyrosine-specific protein kinase activity, which is greatly enhanced when insulin binds to the a-subunits. In addition, the psubunit contains tyrosine residues that are themselves phosphorylated in response to insulin binding (Kasuga et al. 1982). Autophosphorylation of tyrosine residues at positions 1158, 1162, and 1163 is the earliest known event in insulin signaling required to mediate insulin action (Kahn et al. 1993). Although still somewhat controversial, the preponderance of evidence suggests that receptor tyrosine kinase activity and autophosphorylation are necessary to mediate most, if not all, of the actions of insulin (for review, see Kahn et al. 1993). Indeed, the observation that naturally occurring mutations in the tyrosine kinase domain of the insulin receptor cause insulin resistance in vivo is consistent with this hypothesis (Taylor et al. 1992).

that phos-

of other growth

pathways Michael J. Quon, Atul J. Butte, and Simeon I. Taylor are at the Diabetes Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.

of insulin

cused either on the first steps or the last

Insulin Receptor Structure and Function

of the

of postreceptor

events in insulin signal transduction.

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??

Substrates of the Insulin Receptor Tyrosine Kinase

Within the last few years, a number

of cellular substrates for the insulin receptor tyrosine kinase have been identified and characterized (Roth et al. 1992). These include insulin receptor substrate 1 (IRS-l) (Sun et al. 1991), SHC (Pelicci et al. 1992, Kovacina and Roth 1993), pp120/ectoATPase (Margolis et al. 1993), pp62 (Sung et al. 1994), and two distinct proteins called pp60, which can be distinguished by their ability to bind either phosphatidylinsositol 3-kinase (PI 3kinase) or Ras GTPase-activating protein (GAP) (Lavan and Lienhard 1993, Zhang and Roth 1992). The substrates with

369

w

(aminoacids)

1

a

(-27-7)

n (155.312)

3 (191-298) II

4(298-

Glycine-centered repeats (313-428) Major immunogenic domain (450-601)

348)

s(348-396) 6 (396-468)

??

10 (650-717) (718-729)

E(717

u

-729)

12 (729-821) 13 (821.867) 14 (867-921) /

Catalytic Loop (1131-I137) Tyr-1166,1162,1163

of the vari-

1_(468-510) L(510-594)

1 1

2(594-650)

Exon 11

The precise

role and relative importance

ous insulin receptor substrates in insulin signal transduction remain to be determined. However, the existence of multiple substrates provides a potential mechanism for divergence in insulin-signaling pathways with different branches leading to different biologic responses to insulin.

Glycine-centered repeats (i-154) Cys rich

homology 2 (SH2) domains.

-16 (955-978) 17 (978.1059) ~(1059-1096) ~(1096-1150) 20 (1150-1193) 21(1193-1238) 22 (1238-1343)

Figure 1. Map of strucuml domains in the human insulin receptor. The various structural domains of the proreceptor are depicted on the left side of this figure. For comparison, the corresponding exons of the insulin receptor gene are depicted on the right side of the drawing.

Role of SH2 Domains in Insulin Signaling

SH2 domains are protein domains containing -100 amino acids that share homology with a particular noncatalytic region of the SYC protooncogene product. Various proteins involved in growth factor signaling, such as PI 3-kinase, phospholipase Cy (PLCy), growth factor receptorbound protein 2 (GRBZ), SHZ-containing phosphotyrosinephosphatases (SHPTP), and Ras GTPase-activating protein (GAP) contain one or more SH2 domains. SH2 domains bind phosphotyrosine residues in the context of specific flanking amino acid sequences (Figure 2; Songyang et al. 1993). The protein-protein interactions mediated by SH2 domains are an important feature of signaling by receptor tyrosine kinases (Pawson and Gish 1992, Koch et al. 1991). When growth factor receptors are activated by ligand binding, the subsequent autophosphorylation of tyrosine residues enables binding of these receptors to specific SH2 domains. Recently, the crystal structure of the src SH2 domain has been elucidated (Waksman et al. 1993). The structural data revealed two well-defined pockets that complex tightly with peptides containing phosphotyrosine and flanking amino acids in a particular motif. The motif of the amino acids flanking the phosphotyrosine appears to determine the specificity of binding to various SH2

apparent

molecular masses of -60 kD are distinct from nonreceptor tyrosine kinases src, fyn, and yes, which have similar molecular weights. Interestingly, many of these proteins are also substrates for phosphorylation by other receptor kinases such as the receptors for insulinlike growth factor 1 (IGF-I), epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), and nerve growth factor (NGF). IRS-l is among the best characterized of the known substrates for the insulin

are transfected or microinjected with exogenous IRS-l, the cells acquire the ability to respond to the mitogenic actions of insulin (Wang et al. 1993, Chuang et al. 1993). In addition, microinjection of anti-IRS-l antibodies into fibroblasts overexpressing insulin receptors interferes with insulin-stimulated mitogenesis (Rose et al. 1994). These results suggest an essential role for IRS- 1 in mediating insulin-specific mitogenic effects. Interestingly, a number of variant sequences in the IRS-l gene have been reported to be increased in preva-

receptor (Myers and White 1993). IRS-l is found in most tissues of the body, including the insulin-responsive tissues that contribute to glucose homeostasis such as muscle, adipose tissue, and liver. IRS-l has an apparent molecular mass of 185 kD and contains at least 20 potential tyrosine phosphorylation sites (Sun et al. 1991). Several lines of evidence demonstrate the functional significance of IRS-l in insulin signaling. When cells normally deficient in IRS-l

lence among patients with non-insulindependent diabetes (Almind et al. 1993, Imai et al. 1994). Functional characterization of these mutations may reveal a role for IRS- 1 in glucose metabolism and the pathophysiology of diabetes.

domains (Figure 2). For example, the SH2 domains in PI 3-kinase preferentially bind to YMXM motifs, whereas the SH2 domain in GRB2 preferentially binds to YVNI motifs. Thus, a mechanism exists to determine the specificity of SHZ-binding interactions.

Phosphorylation of tyrosine residues in protein substrates by the insulin receptor kinase presumably enables interactions with downstream signaling molecules. For example, phosphorylated IRS1 can bind to proteins containing src

Although most tyrosine-phosphorylated growth factor receptors are able to interact directly with SH2 domains (Panayotou and Waterfield 1993), this type of direct interaction with SH2 domains seems to be less important in the case of the

370

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TEM Vol. 5, No. 9, 1994

insulin receptor. Nevertheless, in vitro activation of PI 3-kinase can occur through

interaction

between

a YXXM

motif in the COOH terminus of the insulin receptor and the SH2 domains of PI-3 kinase (Van Horn et al. 1994). This suggests that in addition to the wellestablished interactions of insulin receptor substrates (for example, IRS-l and SHC) with SHZ-containing proteins, insulin signaling through SHZ-containing proteins may also occur through a more direct pathway similar to other growth factor receptors. As alluded to previously, insulin receptor signaling to SHZ-containing proteins can occur via substrates of the insulin receptor such as IRS-1 and SHC. When these substrates are phosphorylated by the receptor kinase, they are able to interact with signaling molecules containing SH2 domains. A number of putative tyrosine phosphorylation sites on IRS-l occur in YMXM, YXXM, or other motifs that are predicted to interact with SH2 domains. Recently, eight sites of insulin-mediated tyrosine phosphorylation on IRS-l were identified by radiosequencing. Furthermore, these tyrosine phosphorylation sites were located in amino acid sequences with appropriate specificities to bind SH2 domains from PI 3-kinase, GRBZ, or SHPTPZ (Figure 3; Sun et al. 1993). For example, phosphorylated tyrosines at positions 608 and 939 (YMXM motifs) bind preferentially to SH2 domains from the 85kD regulatory subunit of PI-3 kinase (p85a); the SH2 domain of GRB2 specifically binds to the phosphorylated tyrosine at position 895 (YVNI motif); and the amino terminal SH2 domain in SHPTPZ binds specifically to the phosphorylated tyrosine at position 1172

GRB2 N

SH3

SH3

Y

V

N

I

Y

M

X

M

X

M

E D

L

~8% (Pl3K) N

SH3

SH2

C

I

V E

N

SH3

SH2

C

Y

M L I

PLCy SH3

SH2

C

Y

L I

V SH3

N

C

Y

I

V

v

I L

P v I

L Ras GAP SH3

N

N

SH2

SH2

SH3

C

Y

A

A

S

C

Y

A M

A

S P

Figure 2. Some of the SHZ/SH3-containing proteins that have been signaling. The specific phosphotyrosine motifs that interact with the are indicated (adapted from Pawson and Gish 1992, Songyang et al. 1993, and Cooper and Kashishian 1993). For proteins with more than specificity of each domain is shown separately

(YIDL motif). The presence of multiple tyrosine phosphorylation motifs and the specificity of these sites for binding different SH2 domains provide a potential mechanism for divergence in insulin signaling. For example, interaction of a specific SHZ-containing protein with a particular phosphotyrosine site may be involved in metabolic signaling, whereas interaction of another SHZ-containing protein with a different phosphotyrosine site may be involved in mitogenic signaling. Like IRS- 1, some of the other insulin

Binding interactions with SH2 domains provide a means to regulate SH2containing proteins. In the case of SHPTPZ, PLCy, and Ras GAP, binding of growth factor receptors to the SH2 domains of these proteins causes an activa-

receptor substrates listed previously are known to interact directly with SH2containing proteins such as PI 3-kinase

tion of their enzymatic activity. In the case of PI 3-kinase, binding the SH2 domain of the regulatory subunit p85a

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C

or GRBZ, which have been implicated in mediating cell growth. For example, SHC has been shown to interact directly with GRB2 after being phosphorylated by the insulin receptor (Sasaoka et al. 1994).

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Elsevier Science Inc., 1043-2760/94/$7.00

implicated in insulin various SH2 domains 1993, Hjermstad et al. one SH2 domain, the

causes activation of the catalytic subunit (~110). Binding to the SH2 domain of GRB2 facilitates the formation of signaling complexes with downstream elements such as SOS via interactions with the SH3 domains on GRBZ.

??

Role of SH3 Domains in Insulin Signaling

SH3 domains are regions of -50 amino acids that are homologous to a noncatalytic region (distinct from the SH2 domain) on the SK gene product. These domains are frequently found on pro-

371

IRS-1

3. IRS-l structure. Putative tyrosine phosphorylation sites are indicated. Phosphotyrosine sites confirmed by radiosequencing are indicated by boxes. Preferential binding sites for SH2 domains of PI 3-kinase, SHPTPZ, and GRB2 are noted. Adapted from Sun et al. (1993).

Figure

teins containing SH2 domains. The biochemistry of SH3 domains is not as well understood as SH2 domains. However, SH3 domains are known to bind to proline-rich sequences and may have a role in targeting proteins to specific subcellular locations (Bar-Sagi et al. 1993). In addition, SH3 domains may play a negative regulatory role in certain contexts, since mutation or deletion of SH3 domains in abl and SK gene products leads to activation of their transforming activity (for review, see Pawson and Gish 1992). Some SHZ/SH3-containing proteins contain catalytic domains and function as specific enzymes. Other SHZ/SH3-containing proteins have no known catalytic activity and appear to function as adapter molecules to form complexes of specific proteins. Examples of SHZ/SH3-containing proteins that have enzymatic activity include PLC-y and Ras GAP. PLC-), contains two SH2 domains and one SH3 domain. This enzyme catalyzes the cleavage of phosphatidylinositol-4,5-biphosphate (PIP, ) into diacylglycerol and inositol trisphosphate (IP,). These molecules act as second messengers that can stimulate protein kinase C activity di-

372

rectly in the case of diacylglycerol or by releasing Ca2+ in the case of IP,. Ras GAP also contains two SH2 domains and an SH3 domain. It acts to promote the conversion of Ras in the active GTPbound state to the inactive GDP-bound state. This enzyme is therefore a potential negative regulator of one of the central components of mitogenic signaling. In addition, there is some evidence that Ras GAP is a downstream effector of Ras (Lowy and Willumsen 1993). PI 3-kinase consists of a regulatory subunit without enzymatic activity (p85a, containing two SH2 domains and one SH3 domain), which binds to a catalytic subunit (~110) that specifically phosphorylates phosphatidylinositol at the 3’-OH of the inositol ring. This activity is thought to be relevant to mitogenesis, because it has been associated with cellular transformation by various oncogenes. In addition to the SH2/SH3 domains contained in PI 3-kinase, there are also two proline-rich regions on p85a capable of interacting with SH3 domains of other proteins (as well as the SH3 domain contained in p85a) (Pleiman et al. 1994). This provides an additional mechanism whereby PI 3-kinase can

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associate with other proteins and is also a potential means to form aggregates of PI 3-kinase. GRBZ, the gene product of the mammalian homologue of the Caerzorhabditis elegans gene Sem-5 and the Drosophila gene drk, is an example of an SH2/SH3containing protein that does not have any known intrinsic catalytic activity. GRB2 contains an SH2 domain flanked by two SH3 domains and appears to function as an adapter molecule coupling growth factor receptors with downstream signaling events. GRB2 is particularly important because it is on the pathway leading to activation of Ras. Genetic studies in C. elegans and Drosophila have shown that the Sem-S/drk protein is required for receptor tyrosine kinase activation of Ras (Clark et al. 1992, Simon et al. 1993, Olivier et al. genetic studies 1993). In Drosophila, have shown that a tyrosine kinase receptor known as sevenless is essential in the induction of R7 photoreceptor neurons. Furthermore, a protein called Son of sevenless (SOS) has been identified downstream of sevenless. Sos acts as a guanine nucleotide-releasing protein (GNRP) and is thus able to promote the exchange of GTP for GDP on Ras, resulting in activation of Ras. GRBZ/Sem-Sldrk acts as an adapter molecule by forming complexes between tyrosine kinase receptors

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and SOS. The SH2 domain of GRB2/Sem5/drk binds to specific tyrosine phosphorylated motifs on various tyrosine kinase receptors as well as insulin receptor substrates such as IRS-l and SHC (Sun et al. 1993, Skolnik et al. 1993a and b, Rozakis-Adcock et al. 1992). The two SH3 domains of GRB2ISem-51 drk bind to proline-rich motifs on SOS. SOS and the mammalian homologue mSos promote the activation of Ras by converting Ras in the inactive GDPbound state to the active GTP-bound state (Bar-Sagi 1994). A number of groups has recently demonstrated that complexes of growth factor receptors or insulin receptor substrates with GRB2 and mSos are able to signal Ras activation under physiologic conditions (Egan et al. 1993, Rozakis-Adcock et al. 1993, Gale et al. 1993, Li et al. 1993, Buday and Downward 1993, Skolnik et al. 1993a, Simon et al. 1993, Olivier et al. 1993, Bahensperger et al. 1993). The precise mechanism by which GRB2 activates SOS is not clear. However, the fact that receptorlGRB2lSoslRas complexes occur in insects, invertebrates, and mammals, and the observation that GRB2 and SOS can function in a species-independent fashion, suggests that this signaling pathway is highly conserved and functionally important. In the case of insulin signal-

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Figure 4. Insulin signal transduction pathways. The binding of insulin to the a-subunit of the insulin receptor results in autophosphorylation of tyrosine residues on the g-subunit of the insulin receptor and activation of the tyrosine kinase activity of the g-subunit. The activated insulin receptor tyrosine kinase phosphorylates various substrates that can then bind to downstream effector molecules via SH2 domains. The adapter molecule GRB2 is capable of forming complexes between phosphorylated receptor substrates and mSos via SH2 and SH3 domains. This substrate/GRBZ/mSos complex activates Ras, resulting in E&f-l activation and subsequent activation of the MAP kinase phosphorylation cascade. Metabolic and mitogenic effects of insulin are directly related to MAP kinase activation by phosphorylationl dephosphorylation events. One major area in insulin signaling that is still incompletely understood is the molecular signaling mechanisms of insulin-stimulated glucose transport.

ing, there are at least two pathways by which activation of Ras might occur because there are at least two insulin receptor substrates that bind GRB2 when phosphorylated

??

(that is, IRS-l

and SHC).

Role of Ras in Insulin Signaling

Ras is a well-characterized 21-kD GTPbinding protein and proto-oncogene product that plays a central role in regulating cellular growth and differentiation (for review, see Lowy and Willumsen 1993). When transfected fibtoblasts overexpress normal Ras, they acquire the ability to develop a transformed phenotype in response to insulin stimulation, which is similar to the transformation seen in unstimulated cells transfected with a constitutively active mutant Ras (Burgering et al. 1989). Furthermore, transfected fibroblasts overexpressing either normal

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Ras or insulin receptors acquire the ability to induce immediate early genes such as c-fos and c-jun in response to insulin (Burgering et al. 1991). Stimulation of these transfected cells with insulin (but not EGF or PDGF) is also associated with an increase in the amount of Ras in the active GTP-bound state. Thus, insulinstimulated Ras activation appears to be important for the action of insulin to regulate gene expression and stimulate mitogenesis (Maassen et al. 1992).

??

Downstream

from Ras

Using the yeast two-hybrid system, several groups have elegantly demonstrated that Ras can directly interact with Raf-1 (Van Aelst et al. 1993, Vojtek et al. 1993, Zhang et al. 1993). Raf-I is a serinethreonine kinase that triggers the mitogenactivated protein (MAP) kinase phos-

373

phorylation Although

cascade

(Howe et al. 1992).

the mechanism

whereby

activates Raf-1 is not clear, the interaction of Ras and Raf-1 provides a direct pathway for Ras to signal the induction of immediate early genes such as c-fos and c-jun as well as other downstream events in insulin signaling. Recent studies have also shown that signal transduction pathways involving the heterotrimeric G proteins and CAMP can also regulate the interaction between Ras, Raf-1, and subsequent activation of the MAP kinase phosphorylation cascade (Wu et al. 1993, Cook and McCormick 1993). Thus, Ras and Raf-1 represent a point of convergence for a number of diverse signaling pathways.

??

Role of MAP Kinase Phosphorylation Cascade in the Mitogenic and Metabolic Actions of Insulin

Activated Raf-I directly phosphorylates and activates MAP/Erk kinase (MEK, formerly known as MAP kinase kinase) (Kyriakis et al. 1992), which in turn phosphorylates and activates MAP kinase (Ahn et al. 1992). Like the Ras/Raf-1 interaction, the MAP kinase phosphorylation cascade represents another area of potential signal convergence. In addition to Raf- 1, pathways involving the heterotrimeric G proteins can activate MEK kinase, another enzyme (distinct from Raf-1) with the ability to phosphorylate and activate MEK (Lange-Carter et al. 1993). Activated MAP kinase can phosphorylate transcription factors and induce immediate early genes such as c-fos and c-jun (Blenis 1993). Thus, the interactions between the insulin receptor, receptor substrates (IRS-l and SHC), and downstream effecters provide a potential signaling pathway to mediate insulin’s effects to regulate gene expression, cell growth, and differentiation (Figure 4). Similarly, the ability of MAP kinase to phosphorylate RSK S6 kinase results in phosphorylation of protein phosphatase 1 and subsequent dephosphotylation and activation of glycogen synthase. This is an example of how the MAP kinase phosphorylation cascade may be involved in mediating some of the metabolic effects of insulin (Sturgill and Wu 1991, Roach et al. 1991).

374

??

Ras

Signaling of Insulin-Stimulated Glucose Uptake

The effects of insulin on growth, differentiation, and gene expression can be clearly linked to the signaling pathways and molecules discussed earlier. Unfortunately, little is known about the signaling pathways responsible for insulinstimulated glucose uptake-one of the most important physiologic actions of insulin involved in the regulation of glucose homeostasis. Recently, the importance of the insulin receptor tyrosine kinase in mediating insulin-stimulated glucose transport was demonstrated in rat adipose cells transfected with normal or tyrosine-kinase-deficient insulin receptors (Quon et al. 1994). The role of other signaling molecules in insulinstimulated glucose uptake remains controversial. For example, In 3T3-Ll adipocytes, overexpression of a constitutively active mutant Ras (Lys-61) results in an increase in basal glucose transport activity (in the absence of insulin) that is similar to the maximal insulin-stimulated glucose transport activity observed in nontransfected cells (Kozma et al. 1993). However, others have reported that Ras, Raf-1, and MAP kinase pathways do not play a role in the acute effect of insulin to stimulate glucose transport (Berghe et al. 1994, Fingar and Birnbaum 1994). PI 3-kinase is another signaling protein that has recently been implicated in insulin-stimulated glucose transport in studies using a specific inhibitor of PI 3-kinase activity (Okada et al. 1994). One hopes that the elucidation of signaling pathways involved in insulin-stimulated glucose transport will progress as molecular methods for studying insulin-sensitive tissues such as muscle and adipose tissue are developed.

??

Conclusions

In the past several years, remarkable progress in understanding the molecular mechanisms of growth factor signal transduction has had important implications for understanding insulin signaling. The ability of insulin to affect many of the signaling proteins discussed in this review has been known for some time. However, the elucidation of contiguous biochemical pathways that directly link the initial event of insulin binding with both mitogenic and metabolic ac-

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tions of insulin represents a major advance. In addition to biochemical studies, genetic studies in Drosophila and C. elegans provide evidence that at least some of these pathways are physiologically relevant. The signaling pathways discussed in this review have potential sites of both signal divergence and convergence. The convergence of multiple pathways provides a potential mechanism for combinatorial specificity (that is, integration of multiple signals to achieve specific effects). However, the mechanisms by which various growth factors are able to signal distinct biologic functions despite the utilization of common pathways remains unclear. In addition, sites of potential signal divergence represent important areas for future study, as there remain processes such as insulin-stimulated glucose transport for which the molecular signaling events are only incompletely understood.

??

Acknowledgment

A.J.B. is a Howard Hughes Medical Institute-NIH Research Scholar.

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