Disruption of IRS-2 causes type 2 diabetes in mice

Battle for the bulge: directing small molecules to DNA and RNA defects

Small molecules were tailored to specifically bind bulged DNA by complementing the geometry and nucleotide size of the bulge site. The prospect of generating small molecules that influence the secondary structure of DNA and RNA holds great promise for clinical applications.

Bases Genéticas do Diabetes Mellitus Tipo 2

A patogênese do diabetes mellitus tipo 2 (DM2) é complexa, associando fatores genéticos e fatores ambientais. A hiperglicemia é secundária à combinação de defeitos tanto na sensibilidade à insulina quanto na disfunção das células beta-pancreáticas. Vários estudos estabeleceram claramente a importância dos fatores genéticos na predisposição ao DM2. No momento, conhecemos alguns genes implicados em formas monogênicas de diabetes (MODY, diabetes mitocondrial). No entanto, nas formas mais comuns da doença de caráter poligênico, conhecemos apenas poucos genes que são associados à doença de uma forma reprodutível nos diferentes grupos populacionais estudados. Cada um destes poligenes apresenta um papel isolado muito pequeno, atuando na modulação de fenótipos associados ao diabetes. Nestas formas tardias poligênicas de DM2 é evidente a importância dos fatores ambientais que modulam a expressão clínica da doença. Nesta revisão abordamos os avanços mais relevantes das bases genéticas do DM2.

Vias de Sinalização da Insulina

A insulina é um hormônio anabólico com efeitos metabólicos potentes. Os eventos que ocorrem após a ligação da insulina são específicos e estritamente regulados. Definir as etapas que levam à especificidade deste sinal representa um desafio para as pesquisas bioquímicas, todavia podem resultar no desenvolvimento de novas abordagens terapêuticas para pacientes que sofrem de estados de resistência à insulina, inclusive o diabetes tipo 2. O receptor de insulina pertence a uma família de receptores de fatores de crescimento que têm atividade tirosina quinase intrínseca. Após a ligação da insulina o receptor sofre autofosforilação em múltiplos resíduos de tirosina. Isto resulta na ativação da quinase do receptor e conseqüente fosforilação em tirosina de um a família de substratos do receptor de insulina (IRS). De forma similar a outros fatores de crescimento, a insulina usa fosforilação e interações proteína-proteína como ferramentas essenciais para transmitir o sinal. Estas interações proteína-proteína são fundamentais para transmitir o sinal do receptor em direção ao efeito celular final, tais como translocação de vesículas contendo transportadores de glicose (GLUT4) do pool intracelular para a membrana plasmática, ativação da síntese de glicogênio e de proteínas, e transcrição de genes específicos.

Interleukin-4-mediated protection of primary B cells from apoptosis through Stat6-dependent up-regulation of Bcl-xL

Apoptosis is an integral aspect of B lymphocyte development and homeostasis and is regulated by the engagement of antigen costimulatory and cytokine receptors. Although it is well established that interleukin 4 (IL-4) is a potent anti-apoptotic cytokine for B lymphocytes, little is known about the IL-4-induced molecular events regulating cell survival. Stat6 is rapidly activated after IL-4 stimulation, but its role in B lymphocyte apoptosis has not been explored. In this report we demonstrate that Stat6 is a critical signaling molecule for IL-4 in protecting primary B cells from passive and Fas-induced cell death. We show that expression of the Bcl-2 family member, Bcl-xL, is induced maximally by IL-4 and anti-IgM/IL-4 in a Stat6-dependent manner. Additionally, we demonstrate that bcl-xL transcription is likely to be directly activated through a Stat6 binding site in the bcl-xL-flanking region. Finally, reconstitution of Stat6-deficient splenic B cells with Bcl-xL was able to protect those cells from Fas-induced cell death. These results suggest that the anti-apoptotic activity of IL-4 in B cells is mediated through the activation of Stat6 and subsequent transcription of Bcl-xL.

In vivo phosphorylation of insulin receptor substrate 1 at serine 789 by a novel serine kinase in insulin-resistant rodents

Insulin resistance is a key pathophysiologic feature of obesity and type 2 diabetes and is associated with other human diseases, including atherosclerosis, hypertension, hyperlipidemia, and polycystic ovarian disease. Yet, the specific cellular defects that cause insulin resistance are not precisely known. Insulin receptor substrate (IRS) proteins are important signaling molecules that mediate insulin action in insulin-sensitive cells. Recently, serine phosphorylation of IRS proteins has been implicated in attenuating insulin signaling and is thought to be a potential mechanism for insulin resistance. However, in vivo increased serine phosphorylation of IRS proteins in insulin-resistant animal models has not been reported before. In the present study, we have confirmed previous findings in both JCR:LA-cp and Zucker fatty rats, two genetically unrelated insulin-resistant rodent models, that an enhanced serine kinase activity in liver is associated with insulin resistance. The enhanced serine kinase specifically phosphorylates the conserved Ser(789) residue in IRS-1, which is in a sequence motif separate from the ones for MAPK, c-Jun N-terminal kinase, glycogen-synthase kinase 3 (GSK-3), Akt, phosphatidylinositol 3'-kinase, or casein kinase. It is similar to the phosphorylation motif for AMP-activated protein kinase, but the serine kinase in the insulin-resistant animals was shown not to be an AMP-activated protein kinase, suggesting a potential novel serine kinase. Using a specific antibody against Ser(P)(789) peptide of IRS-1, we then demonstrated for the first time a striking increase of Ser(789)-phosphorylated IRS-1 in livers of insulin-resistant rodent models, indicating enhanced serine kinase activity in vivo. Taken together, these data strongly suggest that unknown serine kinase activity and Ser(789) phosphorylation of IRS-1 may play an important role in attenuating insulin signaling in insulin-resistant animal models.

Impaired ('diabetic') insulin signaling and action occur in fat cells long before glucose intolerance--is insulin resistance initiated in the adipose tissue?

This review postulates and presents recent evidence that insulin resistance is initiated in the adipose tissue and also suggests that the adipose tissue may play a pivotal role in the induction of insulin resistance in the muscles and the liver. Marked impairments in insulin's intracellular signaling cascade are present in fat cells from type 2 diabetic patients, including reduced IRS-1 gene and protein expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities. In contrast, upstream insulin signaling in skeletal muscle from diabetic subjects only shows modest impairments and PKB/Akt activation in vivo by insulin appears normal. However, insulin-stimulated glucose transport and glycogen synthesis are markedly reduced. Similar marked impairments in insulin signaling, including reduced IRS-1 expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities are also seen in some (approximately 30%) normoglycemic individuals with genetic predisposition for type 2 diabetes. In addition, GLUT4 expression is markedly reduced in these cells, similar to what is seen in diabetic cells. The individuals with reduced cellular expression of IRS-1 and GLUT4 are also markedly insulin resistant and exhibit several characteristics of the Insulin Resistance Syndrome.Thus, a 'diabetic' pattern is seen in the fat cells also in normoglycemic subjects and this is associated with a marked insulin resistance in vivo. It is proposed that insulin resistance and/or its effectors is initiated in fat cells and that this may secondarily encompass other target tissues for insulin, including the impaired glucose transport in the muscles.

Inactivation of muscle insulin and IGF-I receptors and insulin responsiveness

PURPOSE OF REVIEW

This review will outline the recent advances in the area of insulin-stimulated skeletal muscle glucose uptake and its effect on whole body glucose homeostasis, using gene-deletion and transgenic mouse models.

RECENT FINDINGS

Insulin resistance is often the first abnormality detected in cases of type 2 diabetes, and is seen at the level of the peripheral tissues especially muscle. Both the insulin receptor and the insulin-like growth factor I receptor are capable of stimulating glucose uptake into skeletal muscle. One model involves the gene deletion of muscle glucose transport protein 4, which leads to severe insulin resistance and hyperglycemia, and a second model using a transgenic approach abrogates the function of the insulin-like growth factor I receptor and the insulin receptor resulting in severe insulin resistance and progression to diabetes. Both models demonstrate that abrogation of the insulin-like growth factor I receptor and the insulin receptor or a common signalling pathway must be inhibited to cause sufficient insulin resistance to lead to type 2 diabetes; with either glucotoxicity or lipotoxicity being involved in the progression from severe to resistance to full-blown type 2 diabetes.

SUMMARY

Thus, abrogation of insulin-stimulated glucose uptake in skeletal muscle, at least in mice, may lead to severe insulin resistance and diabetes.

Cloning of the mouse insulin receptor substrate-3 (mIRS-3) promoter, and its regulation by p53

The insulin receptor susbtrate-3 (IRS-3) is a member of a family of intermediate adapter proteins that function as major intracellular targets for phosphorylation by the activated insulin and IGF-I receptors. Among the four IRS proteins identified so far, IRS-3 exhibits a rather peculiar expression pattern during both the embryonic development and adult life, suggesting a different mechanism of regulation of its expression. In this study, we cloned the 5' flanking region of the mIRS-3 gene and analyzed its promoter activity. The mIRS-3 promoter is inhibited by wild-type p53, and this effect is completely abolished by cotransfection of a dominant negative p53. Tumor-derived p53 mutants show variable, but lower suppressing capability than wt p53. In addition, treatment with doxorubicin inhibits endogenous expression of mIRS-3 mRNA in C2C12 and 3T3-L1 cells. The DNA region spanning from nucleotides -287 and -178 in the mIRS-3 promoter is responsible for a 32.2% reduction of the mouse double minute 2 (MDM2) promoter activity, suggesting its involvement in the p53-mediated inhibitory effect. In conclusion, our study demonstrates that the mIRS-3 promoter is regulated by p53 at the transcriptional level. The inhibition of mIRS-3 promoter by wild-type p53, and its de-repression by tumor-derived p53 mutants, appears to be similar to that previously reported for the IGF-I receptor promoter, suggesting a common role of these two genes in p53-mediated cell growth and differentiation.

Knockout models are useful tools to dissect the pathophysiology and genetics of insulin resistance

OBJECTIVE

The development of type 2 diabetes is linked to insulin resistance coupled with a failure of pancreatic beta-cells to compensate by adequate insulin secretion.

DESIGN

Here, we review studies obtained from genetically engineered mice that provide novel insights into the pathophysiology of insulin resistance.

RESULTS

Knockout models with monogenic impairment in insulin action have highlighted the potential role for insulin signalling molecules in insulin resistance at a tissue-specific level. Polygenic models have strengthened the idea that minor defects in insulin secretion and insulin action, when combined, can lead to diabetes, emphasizing the importance of interactions of different genetic loci in the production of diabetes. Knockout models with tissue-specific alterations in glucose or lipid metabolism have dissected the individual contributions of insulin-responsive organs to glucose homeostasis. They have demonstrated the central role of fat as an endocrine tissue in the maintenance of insulin sensitivity and the development of insulin resistance. Finally, these models have shown the potential role of impaired insulin action in pancreatic beta-cells and brain in the development of insulin deficiency and obesity.

Short-term leptin-dependent inhibition of hepatic gluconeogenesis is mediated by insulin receptor substrate-2

Leptin has both insulin-like and insulin-antagonistic effects on glucose metabolism. To test whether leptin interferes directly with insulin signaling, we perfused isolated rat livers with leptin (0.1, 0.5, 5, and 25 nmol/liter), leptin + insulin (5 nmol/liter + 10 nmol/liter), insulin (10 nmol/liter), or vehicle (control). Leptin reduced L-lactate-(10 mmol/liter)-stimulated glucose production by 39-66% (P < 0.006 vs. control) and phosphoenolpyruvate carboxykinase (PEPCK) activity by 22-52% (P < 0.001). Physiological leptin concentrations (0.1-5 nmol/liter) stimulated the tyrosine phosphorylation (pY) of insulin receptor substrate-2 (IRS-2) (280-954%; P < 0.05) and its associated phosphatidylinositol-3 kinase activity (122-621%; P < 0.003). Leptin (0.5-25 nmol/liter) inhibited IRS-1 pY and its associated phosphatidylinositol-3 kinase activity (20-89%; P < 0.03) but stimulated janus kinase-2 pY (272-342%; P < 0.001). Leptin also down-regulated its short receptor isoform in a time- and concentration-dependent manner (28-54%; P < 0.05). Exposure to leptin + insulin additively reduced glucose production and PEPCK activity (approximately 50%; P < 0.001 vs. control) and doubled IRS-2 pY (P < 0.01 vs. insulin). However, leptin + insulin decreased IRS-1 pY by 57% (P < 0.01 vs. insulin). Insulin alone (P < 0.01), but not leptin, increased autophosphorylation of nonreceptor tyrosine kinases (pp59(Lyn) + pp125(Fak)). In conclusion, leptin both alone and in combination with insulin reduces hepatic glucose production by decreasing the synthesis of the key enzyme of gluconeogenesis, PEPCK, which results mainly from the stimulation of the IRS-2 pathway.

Deficient expression of insulin receptor substrate-1 (IRS-1) fails to block insulin-like growth factor-I (IGF-I) stimulation of brain growth and myelination

To determine whether insulin receptor substrate-1 (IRS-1) is essential in mediating insulin-like growth factor-I (IGF-I) stimulation of brain growth and myelination in vivo, we cross-bred IGF-I transgenic (Tg) mice with IRS-1 null mutant (IRS-1(-/-)) mice and examined brain growth and expression of myelin-specific proteins in mice that overexpress IGF-I with or without IRS-1 expression. We found that while IGF-I overexpression stimulates a dramatic increase in brain weight (43%) by 7-8 weeks of age in the absence of IRS-1, it stimulates a greater increase (50%) with intact IRS-1 expression. To evaluate myelination we investigated IGF-I-stimulated expression of myelin basic protein (MBP) and proteolipid protein (PLP) in the cerebral cortex CTX and brainstem, and found similar increases in each region in IRS-1(-/-) and wild type mice. In studies using mixed glial cultures derived from IRS-1(-/-) mice, IGF-I also increased the abundance of MBP and PLP mRNA. To assess possible alternate mediators of IGF-I actions, we examined IRS-2 and IRS-4 and found that the abundance of each is increased in the CTX of IRS-1(-/-) mice and IGF-I Tg mice. Our results suggest that IRS-1 is not essential in IGF-I promotion of oligodendrocyte development and myelination, and that IRS-2 and IRS-4 may compensate for the loss of IRS-1 expression and function in the cells of oligodendrocyte lineage. Nonetheless, the finding that IGF-I stimulates brain growth less well in the absence of IRS-1 suggests that IRS-1-mediated signaling may be more central to IGF-I action in cells other than glia and oligodendrocytes.

Structure of a beta-alanine-linked polyamide bound to a full helical turn of purine tract DNA in the 1:1 motif

Polyamides composed of N-methylpyrrole (Py), N-methylimidazole (Im) and N-methylhydroxypyrrole (Hp) amino acids linked by beta-alanine (beta) bind the minor groove of DNA in 1:1 and 2:1 ligand to DNA stoichiometries. Although the energetics and structure of the 2:1 complex has been explored extensively, there is remarkably less understood about 1:1 recognition beyond the initial studies on netropsin and distamycin. We present here the 1:1 solution structure of ImPy-beta-Im-beta-ImPy-beta-Dp bound in a single orientation to its match site within the DNA duplex 5'-CCAAAGAGAAGCG-3'.5'-CGCTTCTCTTTGG-3' (match site in bold), as determined by 2D (1)H NMR methods. The representative ensemble of 12 conformers has no distance constraint violations greater than 0.13 A and a pairwise RMSD over the binding site of 0.80 A. Intermolecular NOEs place the polyamide deep inside the minor groove, and oriented N-C with the 3'-5' direction of the purine-rich strand. Analysis of the high-resolution structure reveals the ligand bound 1:1 completely within the minor groove for a full turn of the DNA helix. The DNA is B-form (average rise=3.3 A, twist=38 degrees ) with a narrow minor groove closing down to 3.0-4.5 A in the binding site. The ligand and DNA are aligned in register, with each polyamide NH group forming bifurcated hydrogen bonds of similar length to purine N3 and pyrimidine O2 atoms on the floor of the minor groove. Each imidazole group is hydrogen bonded via its N3 atom to its proximal guanine's exocyclic amino group. The important roles of beta-alanine and imidazole for 1:1 binding are discussed.

Phosphoenolpyruvate carboxykinase overexpression selectively attenuates insulin signaling and hepatic insulin sensitivity in transgenic mice

The ability of insulin to suppress gluconeogenesis in type II diabetes mellitus is impaired; however, the cellular mechanisms for this insulin resistance remain poorly understood. To address this question, we generated transgenic (TG) mice overexpressing the phosphoenolpyruvate carboxykinase (PEPCK) gene under control of its own promoter. TG mice had increased basal hepatic glucose production (HGP), but normal levels of plasma free fatty acids (FFAs) and whole-body glucose disposal during a hyperinsulinemic-euglycemic clamp compared with wild-type controls. The steady-state levels of PEPCK and glucose-6-phosphatase mRNAs were elevated in livers of TG mice and were resistant to down-regulation by insulin. Conversely, GLUT2 and glucokinase mRNA levels were appropriately regulated by insulin, suggesting that insulin resistance is selective to gluconeogenic gene expression. Insulin-stimulated phosphorylation of the insulin receptor, insulin receptor substrate (IRS)-1, and associated phosphatidylinositol 3-kinase were normal in TG mice, whereas IRS-2 protein and phosphorylation were down-regulated compared with control mice. These results establish that a modest (2-fold) increase in PEPCK gene expression in vivo is sufficient to increase HGP without affecting FFA concentrations. Furthermore, these results demonstrate that PEPCK overexpression results in a metabolic pattern that increases glucose-6-phosphatase mRNA and results in a selective decrease in IRS-2 protein, decreased phosphatidylinositol 3-kinase activity, and reduced ability of insulin to suppress gluconeogenic gene expression. However, acute suppression of HGP and glycolytic gene expression remained intact, suggesting that FFA and/or IRS-1 signaling, in addition to reduced IRS-2, plays an important role in downstream insulin signal transduction pathways involved in control of gluconeogenesis and progression to type II diabetes mellitus.

Overexpression of insulin receptor substrate-1, but not insulin receptor substrate-2, protects a T cell hybridoma from activation-induced cell death

The insulin receptor substrate (IRS) family of signaling molecules is expressed in lymphocytes, although their functions in these cells is largely unknown. To investigate the role of IRS in the protection of T cells from activation-induced cell death (AICD), we transfected the T cell hybridoma A1.1, which is IL-4 responsive but lacks expression of IRS family members with cDNA encoding IRS1 or IRS2. Stimulation of these clones with immobilized anti-CD3-induced expression of CD69 to the same level as the parental A1.1 cells. However, the A1.1 IRS1-expressing cells were markedly resistant to AICD, while the A1.1 IRS2-expressing cells were not. Inhibition of phosphatidylinositol 3'-kinase in the A1.1 IRS1-expressing cells did not abrogate their resistance to AICD. Fas mRNA was induced similarly by anti-CD3 in A1.1, A1.1 IRS1-expressing, and A1.1 IRS2-expressing cells. However, induction of Fas ligand (FasL) mRNA and functional FasL protein was delayed and decreased in IRS1-expressing cells, but not in IRS2-expressing cells. The induction of transcription from a 500-bp FasL promoter and a minimal 16-mer early growth response element linked to luciferase was also impaired in the IRS1-expressing cells. These results suggest that overexpression of IRS1, but not IRS2, protects A1.1 cells from AICD by diminishing FasL transcription through a pathway that is independent of the tyrosine phosphorylation of IRS1 and phosphatidylinositol 3'-kinase activity.

Role of the insulin receptor substrate 1 and phosphatidylinositol 3-kinase signaling pathway in insulin-induced expression of sterol regulatory element binding protein 1c and glucokinase genes in rat hepatocytes

The mechanism by which insulin induces the expression of the sterol regulatory element binding protein 1c (SREBP-1c) and glucokinase genes was investigated in cultured rat hepatocytes. Overexpression of an NH(2)-terminal fragment of IRS-1 that contains the pleckstrin homology and phosphotyrosine binding domains (insulin receptor substrate-1 NH(2)-terminal fragment [IRS-1N]) inhibited insulin-induced tyrosine phosphorylation of IRS-1 as well as the association of IRS-1 with phosphatidylinositol (PI) 3-kinase activity, whereas the tyrosine phosphorylation of IRS-2 and its association with PI 3-kinase activity were slightly enhanced. The equivalent fragment of IRS-2 (IRS-2N) prevented insulin-induced tyrosine phosphorylation of both IRS-1 and IRS-2, although that of IRS-1 was inhibited more efficiently. The insulin-induced increases in the abundance of SREBP-1c and glucokinase mRNAs, both of which were sensitive to a dominant-negative mutant of PI 3-kinase, were blocked in cells in which the insulin-induced tyrosine phosphorylation of IRS-1 was inhibited by IRS-1N or IRS-2N. A dominant-negative mutant of Akt enhanced insulin-induced tyrosine phosphorylation of IRS-1 (but not that of IRS-2) and its association with PI 3-kinase activity, suggesting that Akt contributes to negative feedback regulation of IRS-1. The Akt mutant also promoted the effects of insulin on the accumulation of SREBP-1c and glucokinase mRNAs. These results suggest that the IRS-1-PI 3-kinase pathway is essential for insulin-induced expression of SREBP-1c and glucokinase genes.

Molecular insights into insulin action and secretion

Tightly co-ordinated control of both insulin action and secretion is required in order to maintain glucose homeostasis. Gene knockout experiments have helped to define key signalling molecules that affect insulin action, including insulin and insulin-like growth factor-1 (IGF-1) receptors, insulin receptor substrate (IRS) proteins and various downstream effector proteins. beta-cell function is also a tightly regulated process, with numerous factors (including certain signalling molecules) having an impact on insulin production, insulin secretion and beta-cell mass. While signalling molecules play important roles in insulin action and secretion under normal circumstances, abnormal insulin signalling in muscle, adipose tissue, liver and pancreas leads to insulin resistance and beta-cell dysfunction. In particular, the signalling protein IRS-2 may have a central role in linking these abnormalities, although other factors are likely to be involved.

Identifying the links between obesity, insulin resistance and beta-cell function: potential role of adipocyte-derived cytokines in the pathogenesis of type 2 diabetes

A combination of insulin resistance and pancreatic beta-cell dysfunction underlies most cases of type 2 diabetes. While the interplay of these two impairments is believed to be important in the development and progression of type 2 diabetes, the mechanisms involved are unclear. A number of factors have been suggested as possibly linking insulin resistance and beta-cell dysfunction in the pathogenesis of type 2 diabetes mellitus. Pro-inflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) have deleterious effects on both glucose homeostasis and beta-cell function, and can disrupt insulin signalling pathways in both pancreatic beta cells and liver and adipose tissue. The anti-inflammatory activity of the thiazolidinedione anti-diabetic agents is potentially beneficial, given the possible role of pro-inflammatory cytokines in linking insulin resistance with beta-cell dysfunction.

Free fatty acids in obesity and type 2 diabetes: defining their role in the development of insulin resistance and beta-cell dysfunction

Plasma free fatty acids (FFA) play important physiological roles in skeletal muscle, heart, liver and pancreas. However, chronically elevated plasma FFA appear to have pathophysiological consequences. Elevated FFA concentrations are linked with the onset of peripheral and hepatic insulin resistance and, while the precise action in the liver remains unclear, a model to explain the role of raised FFA in the development of skeletal muscle insulin resistance has recently been put forward. Over 30 years ago, Randle proposed that FFA compete with glucose as the major energy substrate in cardiac muscle, leading to decreased glucose oxidation when FFA are elevated. Recent data indicate that high plasma FFA also have a significant role in contributing to insulin resistance. Elevated FFA and intracellular lipid appear to inhibit insulin signalling, leading to a reduction in insulin-stimulated muscle glucose transport that may be mediated by a decrease in GLUT-4 translocation. The resulting suppression of muscle glucose transport leads to reduced muscle glycogen synthesis and glycolysis. In the liver, elevated FFA may contribute to hyperglycaemia by antagonizing the effects of insulin on endogenous glucose production. FFA also affect insulin secretion, although the nature of this relationship remains a subject for debate. Finally, evidence is discussed that FFA represent a crucial link between insulin resistance and beta-cell dysfunction and, as such, a reduction in elevated plasma FFA should be an important therapeutic target in obesity and type 2 diabetes.

The evolution of beta-cell dysfunction and insulin resistance in type 2 diabetes

Insulin resistance and beta-cell dysfunction have important roles in the pathogenesis and evolution of type 2 diabetes. The development of precise methods to measure these factors has helped us to define the relationship between them and evidence is reviewed that changes in insulin sensitivity are compensated by inverse changes in beta-cell responsiveness such that the product of insulin sensitivity and insulin secretion (the disposition index) remains constant. While the disposition index promises to be a useful tool to predict individuals at high risk of developing type 2 diabetes, other factors that contribute to beta-cell dysfunction and mark disease onset and progression include impairments in proinsulin processing and insulin secretion, decreased beta-cell mass and islet amyloid deposition. Emerging data indicate that anti-diabetic agents, such as the thiazolidinediones that simultaneously target insulin resistance and beta-cell dysfunction, may have a beneficial impact on disease onset and progression. Several landmark clinical studies are underway to investigate if their initial promise is supported by data from large-scale trials.

From receptor to effector: insulin signal transduction in skeletal muscle from type II diabetic patients

Insulin resistance is a characteristic feature of type II diabetes mellitus and obesity. Although defects in glucose homeostasis have been recognized for decades, the molecular mechanisms accounting for impaired whole body glucose uptake are still not fully understood. Skeletal muscle constitutes the largest insulin-sensitive organ in humans; thus, insulin resistance in this tissue will have a major impact on whole body glucose homeostasis. Intense efforts are under way to define the molecular mechanisms that regulate glucose metabolism and gene expression in insulin-sensitive tissues. Knowledge of the human genome sequence, used in concert with gene and/or protein array technology, will provide a powerful means to facilitate efforts in revealing molecular targets that regulate glucose homeostasis in type II diabetes mellitus. This will offer quicker ways forward to identifying gene expression profiles in insulin-sensitive and insulin-resistant human tissue. This review will present our current understanding of potential defects in insulin signal transduction pathways, with an emphasis on mechanisms regulating glucose transport in skeletal muscle from people with type II diabetes mellitus. Elucidation of the pathways involved in the regulation of glucose homeostasis will offer insight into the causation of insulin resistance and type II diabetes mellitus. Furthermore, this will identify biochemical entry points for drug intervention to improve glucose homeostasis.

Pdx1 restores beta cell function in Irs2 knockout mice

The homeodomain transcription factor Pdx1 is required for pancreas development, including the differentiation and function of beta cells. Mutations in Pdx1 or upstream hepatocyte nuclear factors cause autosomal forms of early-onset diabetes (maturity-onset diabetes of the young [MODY]). In mice, the Irs2 branch of the insulin/Igf signaling system mediates peripheral insulin action and pancreatic beta cell growth and function. To investigate whether beta cell failure in Irs2(-/-) mice might be related to dysfunction of MODY-related transcription factors, we measured the expression of Pdx1 in islets from young Irs2(-/-) mice. Before the onset of diabetes, Pdx1 was reduced in islets from Irs2(-/-) mice, whereas it was expressed normally in islets from wild-type or Irs1(-/-) mice, which do not develop diabetes. Whereas male Irs2(-/-)Pdx1(+/+) mice developed diabetes between 8 and 10 weeks of age, haploinsufficiency for Pdx1 caused diabetes in newborn Irs2(-/-) mice. By contrast, transgenic expression of Pdx1 restored beta cell mass and function in Irs2(-/-) mice and promoted glucose tolerance throughout life, as these mice survived for at least 20 months without diabetes. Our results suggest that dysregulation of Pdx1 might represent a common link between ordinary type 2 diabetes and MODY.

Mouse models of insulin resistance

The hallmarks of type 2 diabetes are impaired insulin action in peripheral tissues and decreased pancreatic beta-cell function. Classically, the two defects have been viewed as separate entities, with insulin resistance arising primarily from impaired insulin-dependent glucose uptake in skeletal muscle, and beta-cell dysfunction arising from impaired coupling of glucose sensing to insulin secretion. Targeted mutagenesis and transgenesis involving components of the insulin action pathway have changed our understanding of these phenomena. It appears that the role of insulin signaling in the pathogenesis of type 2 diabetes has been overestimated in classic insulin target tissues, such as skeletal muscle, whereas it has been overlooked in liver, pancreatic beta-cells, and brain, which had been thought not to be primary insulin targets. We review recent progress and try to reconcile areas of apparent controversy surrounding insulin signaling in skeletal muscle and pancreatic beta-cells.

Beta cell expression of IGF-I leads to recovery from type 1 diabetes

Patients with type 1 diabetes are identified after the onset of the disease, when beta cell destruction is almost complete. beta cell regeneration from islet cell precursors might reverse this disease, but factors that can induce beta cell neogenesis and replication and prevent a new round of autoimmune destruction remain to be identified. Here we show that expression of IGF-I in beta cells of transgenic mice (in both C57BL/6-SJL and CD-1 genetic backgrounds) counteracts cytotoxicity and insulitis after treatment with multiple low doses of streptozotocin (STZ). STZ-treated nontransgenic mice developed high hyperglycemia and hypoinsulinemia, lost body weight, and died. In contrast, STZ-treated C57BL/6-SJL transgenic mice showed mild hyperglycemia for about 1 month, after which they normalized glycemia and survived. After STZ treatment, all CD-1 mice developed high hyperglycemia, hypoinsulinemia, polydipsia, and polyphagia. However, STZ-treated CD-1 transgenic mice gradually normalized all metabolic parameters and survived. beta cell mass increased in parallel as a result of neogenesis and beta cell replication. Thus, our results indicate that local expression of IGF-I in beta cells regenerates pancreatic islets and counteracts type 1 diabetes, suggesting that IGF-I gene transfer to the pancreas might be a suitable therapy for this disease.

Insulin secretory function is impaired in isolated human islets carrying the Gly(972)-->Arg IRS-1 polymorphism

Type 2 (non-insulin-dependent) diabetes results from decreased insulin action in peripheral target tissues (insulin resistance) and impaired pancreatic beta-cell function. These defects reflect both genetic components and environmental risk factors. Recently, the common Gly(972)-->Arg amino acid polymorphism of insulin receptor substrate 1 (Arg(972) IRS-1) has been associated with human type 2 diabetes. In this study, we report on some functional and morphological properties of isolated human islets carrying the Arg(972) IRS-1 polymorphism. Insulin content was lower in variant than control islets (94 +/- 47 vs. 133 +/- 56 microU/islet; P < 0.05). Stepwise glucose increase (1.7 to 16.7 mmol/l) significantly potentiated insulin secretion from control islets, but not Arg(972) IRS-1 islets, with the latter also showing a relatively lower response to glyburide and a significantly higher response to arginine. Proinsulin release mirrored insulin secretion, and the insulin-to-proinsulin ratio in response to arginine was significantly lower from Arg(972) IRS-1 islets than from control islets. Glucose utilization and oxidation did not differ in variant and wild-type islets at both low and high glucose levels. Electron microscopy showed that Arg(972) IRS-1 beta-cells had a severalfold greater number of immature secretory granules and a lower number of mature granules than control beta-cells. In conclusion, Arg(972) IRS-1 islets have reduced insulin content, impaired insulin secretion, and a lower amount of mature secretory granules. These alterations may account for the increased predisposition to type 2 diabetes in individuals carrying the Gly(972)-->Arg amino acid polymorphism of IRS-1.

Brain-derived neurotrophic factor (BDNF) regulates glucose and energy metabolism in diabetic mice

Neurotrophins are important regulators in the embryogenesis, development and functioning of nervous systems. In addition to the efficacy of brain-derived neurotrophic factor (BDNF) in neurological disorders, we have found that BDNF demonstrates endocrinological functions and reduces food intake and blood glucose concentration in rodent obese diabetic models, such as C57BL/KsJ-db/db mice. The hypoglycemic effect of BDNF was found to be stronger in younger db/db mice with hyperinsulinemia than in older mice. While BDNF itself did not alter blood glucose in normal mice and streptozotocin (STZ)-treated mice, BDNF enhanced the hypoglycemic effect of insulin in STZ-treated mice. These data indicate that BDNF needs endogenous or exogenous insulin to show hypoglycemic action. In addition, BDNF treatment enhanced energy expenditure in db/db mice. The efficacy of BDNF in regulating glucose and energy metabolism was reproduced through intracerebroventricular administration, suggesting that BDNF acted directly on the hypothalamus, the autonomic center of the brain.

Genetic manipulation of insulin signaling, action and secretion in mice. Insights into glucose homeostasis and pathogenesis of type 2 diabetes

Non-insulin-dependent diabetes mellitus (NIDDM) is a complex heterogeneous polygenic disease characterized mainly by insulin resistance and pancreatic beta-cell dysfunction. In recent years, several genetically engineered mouse models have been developed for the study of the pathophysiological consequences of defined alterations in a single gene or in a set of candidate diabetogenes. These represent new tools that are providing invaluable insights into NIDDM pathogenesis. In this review, we highlight the lessons emerging from the study of some of the transgenic or knockout mice in which the expression of key actors in insulin signaling, action or secretion has been manipulated. In addition to contributing to our knowledge of the specific roles of individual genes in the control of glucose homeostasis, these studies have made it possible to address several crucial issues in NIDDM that have remained controversial or unanswered for a number of years.

Insulin resistance, defective insulin receptor substrate 2-associated phosphatidylinositol-3' kinase activation, and impaired atypical protein kinase C (zeta/lambda) activation in myotubes from obese patients with impaired glucose tolerance

Impaired glucose tolerance (IGT) is characterized by insulin resistance. Recently, defects in the insulin-signaling cascade have been implicated in the pathogenesis of insulin resistance. To study insulin signaling in IGT, we used human skeletal muscle cells in primary culture from patients with IGT and control subjects. In these cultured myotubes, we assessed insulin-induced 2-deoxyglucose uptake and early steps of the metabolic insulin-signaling cascade. Myotubes in culture from patients with IGT had insulin-induced glucose uptake that was roughly 30-50% less than that from control subjects. This insulin resistance was associated with impaired insulin receptor substrate (IRS)-2-associated phosphatidylinositol 3' (PI3) kinase activation and IRS-2 tyrosine phosphorylation as well as significantly decreased protein kinase C (PKC)-zeta/lambda activation in response to insulin. IRS-1- associated PI3 kinase activation and insulin receptor autophosphorylation were comparable in the two groups. Protein expression levels for the insulin receptor, IRS-1, IRS-2, the p85 regulatory subunit of PI3 kinase, Akt, PKC-zeta/lambda, GLUT1, and GLUT4 were also similar in the two groups. In conclusion, myotubes from patients with IGT have impaired insulin-induced glucose uptake. This is associated with impaired IRS-2-associated PI3 kinase activation and PKC-zeta/lambda activation. Our results suggest that these defects may contribute to insulin resistance in IGT patients.

Akt/protein kinase B promotes organ growth in transgenic mice

One of the least-understood areas in biology is the determination of the size of animals and their organs. In Drosophila, components of the insulin receptor phosphoinositide 3-kinase (PI3K) pathway determine body, organ, and cell size. Several biochemical studies have suggested that Akt/protein kinase B is one of the important downstream targets of PI3K. To examine the role of Akt in the regulation of organ size in mammals, we have generated and characterized transgenic mice expressing constitutively active Akt (caAkt) or kinase-deficient Akt (kdAkt) specifically in the heart. The heart weight of caAkt transgenic mice was increased 2.0-fold compared with that of nontransgenic mice. The increase in heart size was associated with a comparable increase in myocyte cell size in caAkt mice. The kdAkt mutant protein attenuated the constitutively active PI3K-induced overgrowth of the heart, and the caAkt mutant protein circumvented cardiac growth retardation induced by a kinase-deficient PI3K mutant protein. Rapamycin attenuated caAkt-induced overgrowth of the heart, suggesting that the mammalian target of rapamycin (mTOR) or effectors of mTOR mediated caAkt-induced heart growth. In conclusion, Akt is sufficient to induce a marked increase in heart size and is likely to be one of the effectors of the PI3K pathway in mediating heart growth.

Activation of IRS-2-mediated signal transduction by IGF-1, but not TGF-alpha or EGF, augments pancreatic beta-cell proliferation

Transforming growth factor (TGF)-alpha- and epidermal growth factor (EGF)-induced signal transduction was directly compared with that of glucose and insulin-like growth factor-1 (IGF-1) in INS-1 cells. TGF-alpha/EGF transiently (<20 min) induced phosphorylation of extracellular-regulated kinase (Erk)-1/2 (>20-fold), glycogen synthase kinase (GSK)-3 (>10-fold), and protein kinase B (PKB) (Ser(473) and Thr(308)), but did not increase [(3)H]thymidine incorporation. In contrast, phosphorylation of Erk1/2, GSK-3, and PKB in response to glucose and IGF-1 was more prolonged (>24 h) and, though not as robust as TGF-alpha/EGF, did increase beta-cell proliferation. Phosphorylation of p70(S6K) was also increased by IGF-1/glucose, but not by TGF-alpha/EGF, despite upstream PKB activation. It was found that IGF-1 induced phosphatidylinositol 3-kinase (PI3K) association with insulin receptor substrate (IRS)-1 and -2 in a glucose-dependent manner, whereas TGF-alpha/EGF did not. The importance of specific IRS-2-mediated signaling events was emphasized in that adenoviral-mediated overexpression of IRS-2 further increased glucose/IGF-1-induced beta-cell proliferation (more than twofold; P < 0.05) compared with control or adenoviral-mediated IRS-1 overexpressing INS-1 cells. Neither IRS-1 nor IRS-2 overexpression induced a beta-cell proliferative response to TGF-alpha/EGF. Thus, a prolonged activation of Erk1/2 and PI3K signaling pathways is important in committing a beta-cell to a mitogenic event, and it is likely that this sustained activation is instigated by signal transduction occurring specifically through IRS-2.

Comparative thermodynamics for monomer and dimer sequence-dependent binding of a heterocyclic dication in the DNA minor groove

Phenylamidine cationic groups linked by a furan ring (furamidine) and related symmetric diamidine compounds bind as monomers in the minor groove of AT sequences of DNA. DB293, an unsymmetric derivative with one of the phenyl rings of furamidine replaced with a benzimidazole, can bind to AT sequences as a monomer but binds more strongly to GC-containing minor-groove DNA sites as a stacked dimer. The dimer-binding mode has high affinity, is highly cooperative and sequence selective. In order to develop a better understanding of the correlation between structural and thermodynamic aspects of DNA molecular recognition, DB293 was used as a model to compare the binding of minor-groove agents with AT and mixed sequence DNA sites. Isothermal titration calorimetry and surface plasmon resonance results clearly show that the binding of DB293 and other related compounds into the minor groove of AT sequences is largely entropy-driven while the binding of DB293 as a dimer into the minor groove of GC-containing sequences is largely enthalpy-driven. At 25 degrees C, for example, the AT binding has DeltaG degrees, DeltaH degrees and TDeltaS degrees values of -9.6, -3.6 and 6.0 kcal/mol while the values for dimer binding to a GC-containing site are -9.0, -10.9 and -1.9 kcal/mol (per mol of bound compound), respectively. These results show that the thermodynamic components for binding of compounds of this type to DNA are very dependent on the structure, solvation and sequence of the DNA binding site.

Variations in insulin secretion in carriers of gene variants in IRS-1 and -2

Associations between type 2 diabetes (and/or parameters contributing to glucose homeostasis) and genetic variation in the genes encoding insulin receptor substrate (IRS)-1 and -2 have been reported in several populations. Recently, it has been reported that the Gly(972)Arg variant in IRS-1 was associated with reduced insulin secretion during hyperglycemic clamps in German subjects with normal glucose tolerance. We have examined glucose-stimulated insulin secretion in relation to gene variants in the IRS-1 (Gly(972)Arg) and IRS-2 (Gly(1057)Asp) genes in two Dutch cohorts. Subjects with normal (n = 64) or impaired (n = 94) glucose tolerance underwent 3-h hyperglycemic clamps at 10 mmol/l glucose. All subjects were genotyped for the IRS-1 and IRS-2 variants by PCR-RFLP--based methods. We did not observe any significant difference in both first- and second-phase insulin secretion between carriers and noncarriers of both gene variants, nor was there evidence for an association with other diabetes-related parameters. We conclude that the common gene variants in IRS-1 and IRS-2 are not associated with altered glucose-stimulated insulin secretion in two populations from the Netherlands.

Increased insulin sensitivity in IGF-I receptor--deficient brown adipocytes

Immortalized brown adipocyte cell lines have been generated from fetuses of mice deficient in the insulin-like growth factor I receptor gene (IGF-IR(-/-)), as well as from fetuses of wild-type mice (IGF-IR(+/+)). These cell lines maintained the expression of adipogenic- and thermogenic-differentiation markers and show a multilocular fat droplets phenotype. IGF-IR(-/-) brown adipocytes lacked IGF-IR protein expression; insulin receptor (IR) expression remained unchanged as compared with wild-type cells. Insulin-induced tyrosine autophosphorylation of the IR beta-chain was augmented in IGF-IR--deficient cells. Upon insulin stimulation, tyrosine phosphorylation of (insulin receptor substrate-1) IRS-1 was much higher in IGF-IR(-/-) brown adipocytes, although IRS-1 protein content was reduced. In contrast, tyrosine phosphorylation of IRS-2 decreased in IGF-IR--deficient cells; its protein content was unchanged as compared with wild-type cells. Downstream, the association IRS-1/growth factor receptor binding protein-2 (Grb-2) was augmented in the IGF-IR(-/-) brown adipocyte cell line. However, SHC expression and SHC tyrosine phosphorylation and its association with Grb-2 were unaltered in response to insulin in IGF-IR--deficient brown adipocytes. These cells also showed an enhanced activation of mitogen-activated protein kinase (MAPK) kinase (MEK1/2) and p42/p44 mitogen-activated protein kinase (MAPK) upon insulin stimulation. In addition, the lack of IGF-IR in brown adipocytes resulted in a higher mitogenic response (DNA synthesis, cell number, and proliferating cell nuclear antigen expression) to insulin than wild-type cells. Finally, cells lacking IGF-IR showed a much lower association between IR or IRS-1 and phosphotyrosine phosphatase 1B (PTP1B) and also a decreased PTP1B activity upon insulin stimulation. However, PTP1B/Grb-2 association remained unchanged in both cell types, regardless of insulin stimulation. Data presented here provide strong evidence that IGF-IR--deficient brown adipocytes show an increased insulin sensitivity via IRS-1/Grb-2/MAPK, resulting in an increased mitogenesis in response to insulin.

Dominant-negative suppression of HNF-1 alpha results in mitochondrial dysfunction, INS-1 cell apoptosis, and increased sensitivity to ceramide-, but not to high glucose-induced cell death

Maturity onset diabetes of the young (MODY) 3 is a monogenic form of diabetes caused by mutations in the transcription factor hepatocyte nuclear factor (HNF)-1 alpha. We investigated the involvement of apoptotic events in INS-1 insulinoma cells overexpressing wild-type HNF-1 alpha (WT-HNF-1 alpha) or a dominant-negative mutant (DN-HNF-1 alpha) under control of a doxycycline-dependent transcriptional activator. Forty-eight h after induction of DN-HNF-1 alpha, INS-1 cells activated caspase-3 and underwent apoptotic cell death, while cells overexpressing WT-HNF-1 alpha remained viable. Mitochondrial cytochrome c release and activation of caspase-9 accompanied DN-HNF-1 alpha-induced apoptosis, suggesting the involvement of the mitochondrial apoptosis pathway. Activation of caspases was preceded by mitochondrial hyperpolarization and decreased expression of the anti-apoptotic protein Bcl-xL. Transient overexpression of Bcl-xL was sufficient to rescue INS-1 cells from DN-HNF-1 alpha-induced apoptosis. Both WT- and DN-HNF-1 alpha-expressing cells demonstrated similar increases in apoptosis when cultured at high glucose (25 mm). In contrast, induction of DN-HNF-1 alpha highly sensitized cells to ceramide toxicity. In cells cultured at low glucose, DN-HNF-1 alpha induction also caused up-regulation of the cell cycle inhibitor p27(KIP1). Therefore, our data indicate that increased sensitivity to the mitochondrial apoptosis pathway and decreased cell proliferation may account for the progressive loss of beta-cell function seen in MODY 3 subjects.

Insulin signaling after exercise in insulin receptor substrate-2-deficient mice

The period immediately after exercise is characterized by enhanced insulin action in skeletal muscle, and on the molecular level, by a marked increase in insulin-stimulated, phosphotyrosine-associated phosphatidylinositol (PI) 3-kinase activity. Because the increase in PI 3-kinase activity cannot be explained by increased insulin receptor substrate (IRS)-1 signaling, the present study examined whether this effect is mediated by enhanced IRS-2 signaling. In wild-type (WT) mice, insulin increased IRS-2 tyrosine phosphorylation (approximately 2.5-fold) and IRS-2-associated PI 3-kinase activity (approximately 3-fold). Treadmill exercise, per se, had no effect on IRS-2 signaling, but in the period immediately after exercise, there was a further increase in insulin-stimulated IRS-2 tyrosine phosphorylation (approximately 3.5-fold) and IRS-2-associated PI 3-kinase activity (approximately 5-fold). In IRS-2-deficient (IRS-2(-/-)) mice, the increase in insulin-stimulated, phosphotyrosine-associated PI 3-kinase activity was attenuated as compared with WT mice. However, in IRS-2(-/-) mice, the insulin-stimulated, phosphotyrosine-associated PI 3-kinase response after exercise was slightly higher than the insulin-stimulated response alone. In conclusion, IRS-2 tyrosine phosphorylation and associated PI 3-kinase activity are markedly enhanced by insulin in the immediate period after exercise. IRS-2 signaling can partially account for the increase in insulin-stimulated phosphotyrosine-associated PI 3-kinase activity after exercise.

Insulin signaling pathways in time and space

Despite remarkable progress in dissecting the signaling pathways that are crucial for the metabolic effects of insulin, the molecular basis for the specificity of its cellular actions is not fully understood. One clue might lie in the spatial and temporal aspects of signaling. Recent evidence suggests that signaling molecules and pathways are localized to discrete compartments in cells by specific protein interactions. Also, the rapid termination of tyrosine or lipid phosphorylation by phosphatases or serine kinases might tightly control the strength of a signaling pathway, thus determining its effect on growth, differentiation and metabolism.

Accurate assessment of beta-cell function: the hyperbolic correction

Only in the last decade did modeling studies predict, and knockout experiments confirm, that type 2 diabetes is a "2-hit" disease in which insulin resistance is necessarily accompanied by beta-cell defect(s) preventing the compensatory upregulation of insulin secretion. This long- delayed insight was associated with the development of a constant, the "disposition index," describing the beta-cell sensitivity-secretion relationship as a rectangular hyperbola. Shifts in insulin sensitivity are accompanied by compensatory alterations in beta-cell sensitivity to glucose. Insulin-sensitive subjects do not require a massive insulin response to exogenous glucose to maintain a normal blood glucose. But if their insulin sensitivity decreases by 80%, as in late pregnancy, they need a fivefold greater insulin response to achieve an identical disposition index. Women with gestational diabetes have an insulin response similar to that of normal volunteers; at first glance, this suggests similar islet function, but the utility of the disposition index is to normalize this response to the amplitude of third trimester insulin resistance, revealing severe beta-cell deficiency. The index is a quantitative, convenient, and accurate tool in analyzing epidemiologic data and identifying incipient impaired glucose tolerance. Separate major issues remain, however: the causes of insulin resistance, the causes of the failure of adequate beta-cell compensation in type 2 diabetes, and the nature of the signal(s) from insulin-resistant tissues that fail to elicit the appropriate beta-cell increment in sensitivity to glucose and other stimuli. The disposition index is likely to remain the most accurate physiologic measure for testing hypotheses and solutions to these challenges in whole organisms.

Clinical characterization of insulin secretion as the basis for genetic analyses

A strong genetic component of the beta-cell defect of type 2 diabetes is undisputed. We recently developed a modification of the classic hyperglycemic clamp to assess beta-cell function in response to various stimuli (10 mmol/l glucose, additional glucagon-like peptide [GLP]-1, and arginine). Subjects at risk for developing type 2 diabetes (impaired glucose-tolerant individuals, women with gestational diabetes, and individuals with a family history of type 2 diabetes) clearly showed a significantly decreased mean secretory response to all secretagogues compared with controls. We also showed that normal glucose-tolerant carriers of the Gly972Arg polymorphism in the insulin receptor substrate 1 have significantly reduced insulin secretion in response to glucose and arginine but not to GLP-1. More remarkably, however, the relative impairment of the different secretory phases varied greatly in the same individual, indicating a substantial heterogeneity of beta-cell dysfunction. Specific prominence of this heterogeneity may reflect a specific cellular defect of the beta-cell. In subjects sharing this pattern of heterogeneity, any underlying genetic variant may be enriched and thus more likely not only to be identified but also to be related to a pathophysiological mechanism. In conclusion, we believe that careful clinical characterization of beta-cell function (and dysfunction) is one way of identifying and understanding the genetic factors leading to the insulin secretory failure of type 2 diabetes.

Effect of the insulin mimetic L-783,281 on intracellular Ca2+ and insulin secretion from pancreatic beta-cells

L-783,281, an antidiabetic fungal metabolite that has previously been shown to activate insulin signaling in CHO cells, was tested for its effect on intracellular Ca(2+) ([Ca(2+)](i)) and insulin secretion in single mouse pancreatic beta-cells. Application of 10 micromol/l L-783,281 for 40 s to isolated beta-cells in the presence of 3 mmol/l glucose increased [Ca(2+)](i) to 178 +/- 10% of basal levels (n = 18) as measured by fluo-4 fluorescence. L-767,827, an inactive structural analog of the insulin mimetic, had no effect on beta-cell [Ca(2+)](i). The L-783,281-evoked [Ca(2+)](i) increase was reduced by 82 +/- 4% (n = 6, P < 0.001) in cells incubated with 1 micromol/l of the SERCA (sarco/endoplasmic reticulum calcium ATPase) pump inhibitor thapsigargin and reduced by 33 +/- 6% (n = 6, P < 0.05) in cells incubated with 20 micromol/l of the L-type Ca(2+)-channel blocker nifedipine. L-783,281-stimulated [Ca(2+)](i) increases were reduced to 31 +/- 3% (n = 9, P < 0.05) and 48 +/- 10% (n = 6, P < 0.05) of control values by the phosphatidylinositol 3-kinase (PI3-K) inhibitors LY294002 (25 micromol/l) and wortmannin (100 nmol/l), respectively. In beta-cells from IRS-1-/- mice, 10 micromol/l L-783,281 had no significant effect on [Ca(2+)](i) (n = 5). L-783,281 also resulted in insulin secretion at single beta-cells. Application of 10 micromol/l L-783,281 for 40 s resulted in 12.2 +/- 2.1 (n = 14) exocytotic events as measured by amperometry, whereas the inactive structural analog had no stimulatory effect on secretion. Virtually no secretion was evoked by L-783,281 in IRS-1-/- beta-cells. LY294002 (25 micromol/l) significantly reduced the effect of the insulin mimetic on beta-cell exocytosis. It is concluded that L-783,281 evokes [Ca(2+)](i) increases and exocytosis in beta-cells via an IRS-1/PI3-K-dependent pathway and that the [Ca(2+)](i) increase involves release of Ca(2+) from intracellular stores.

Molecular mechanism of insulin-induced degradation of insulin receptor substrate 1

Insulin receptor substrate 1 (IRS-1) plays an important role in the insulin signaling cascade. In vitro and in vivo studies from many investigators have suggested that lowering of IRS-1 cellular levels may be a mechanism of disordered insulin action (so-called insulin resistance). We previously reported that the protein levels of IRS-1 were selectively regulated by a proteasome degradation pathway in CHO/IR/IRS-1 cells and 3T3-L1 adipocytes during prolonged insulin exposure, whereas IRS-2 was unaffected. We have now studied the signaling events that are involved in activation of the IRS-1 proteasome degradation pathway. Additionally, we have addressed structural elements in IRS-1 versus IRS-2 that are required for its specific proteasome degradation. Using ts20 cells, which express a temperature-sensitive mutant of ubiquitin-activating enzyme E1, ubiquitination of IRS-1 was shown to be a prerequisite for insulin-induced IRS-1 proteasome degradation. Using IRS-1/IRS-2 chimeric proteins, the N-terminal region of IRS-1 including the PH and PTB domains was identified as essential for targeting IRS-1 to the ubiquitin-proteasome degradation pathway. Activation of phosphatidylinositol 3-kinase is necessary but not sufficient for activating and sustaining the IRS-1 ubiquitin-proteasome degradation pathway. In contrast, activation of mTOR is not required for IRS-1 degradation in CHO/IR cells. Thus, our data provide insight into the molecular mechanism of insulin-induced activation of the IRS-1 ubiquitin-proteasome degradation pathway.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Shc and CEACAM1 interact to regulate the mitogenic action of insulin

CEACAM1, a tumor suppressor (previously known as pp120), is a plasma membrane protein that undergoes phosphorylation on Tyr(488) in its cytoplasmic tail by the insulin receptor tyrosine kinase. Co-expression of CEACAM1 with insulin receptors decreased cell growth in response to insulin. Co-immunoprecipitation experiments in intact NIH 3T3 cells and glutathione S-transferase pull-down assays revealed that phosphorylated Tyr(488) in CEACAM1 binds to the SH2 domain of Shc, another substrate of the insulin receptor. Overexpressing Shc SH2 domain relieved endogenous Shc from binding to CEACAM1 and restored MAP kinase activity, growth of cells in response to insulin, and their colonization in soft agar. Thus, by binding to Shc, CEACAM1 sequesters this major coupler of Grb2 to the insulin receptor and down-regulates the Ras/MAP kinase mitogenesis pathway. Additionally, CEACAM1 binding to Shc enhances its ability to compete with IRS-1 for phosphorylation by the insulin receptor. This leads to a decrease in IRS-1 binding to phosphoinositide 3'-kinase and to the down-regulation of the phosphoinositide 3'-kinase/Akt pathway that mediates cell proliferation and survival. Thus, binding to Shc appears to constitute a major mechanism for the down-regulatory effect of CEACAM1 on cell proliferation.

Apoptosis in the beta cells: cause or consequence of insulin secretion defect in diabetes?

Pancreatic beta-cell dysfunction and insulin resistance are two interrelated defects in the pathophysiology of type 2 diabetes. Defects in peripheral insulin action precede the development of glucose intolerance, as the pancreas compensates for insulin resistance by increasing insulin production and secretion. This may be achieved by enhancing cellular secretory capacity or by increasing beta-cell mass. Over time, the pancreatic secretion of insulin becomes inadequate for the extent of insulin resistance, and the levels of fasting and postprandial glucose rise leading to the onset of frank hyperglycemia, which leads to reduction in beta-cell function and survival through a process referred to as glucose toxicity. There is increasing evidence that apoptosis is the main mode of pancreatic beta-cell death not only in type 1 but also in type 2 diabetes. Recently, studies in knockout mice, human and rat islets, and pancreatic beta-cell lines demonstrated that defective insulin signaling in beta-cells might play an important pathophysiological role by affecting both secretory function and cell survival. The purpose of this review is to present recent advances in understanding of the interrelationship between molecular mechanisms underlying defects in insulin secretion and beta-cell survival in type 2 diabetes caused by impaired activation of insulin signaling pathways.

Regulation of cell size in growth, development and human disease: PI3K, PKB and S6K

It has generally been observed that cells grow to a certain size before they divide. In the last few years, the PI3K signal transduction pathway has emerged as one of the main signaling routes utilized by cells to control their increase in size. Here we focus on two components of this pathway, PKB and S6K, and briefly review the experiments that initially uncovered their roles in cell size control. In addition, we discuss a number of recent observations suggesting that the generic models used to describe this pathway to date may have been oversimplified. Indeed, recent observations in Drosophila and mouse support a more complex interaction between these signaling components in development. Finally, we have utilized two contemporary studies involving PKB- and S6K-deficient mice as a paradigm to underscore the importance of cell size and to accurately delineate the connections between signaling pathways for human disease, such as diabetes mellitus.