Inter- and intra-individual variability of active glucagon-like peptide 1 among healthy adults.

Objective: To determine whether sex, age, and body mass index are correlated with active glucagon-like-peptide 1 concentrations and to investigate glucagon-like-peptide 1 reproducibility during repeated oral glucose tolerance tests. Methods: Sixty-one healthy volunteers underwent four 2-hour repeated oral glucose tolerance tests approximately 1 week apart. Because this randomized same-subject crossover trial was designed to investigate effects of non-nutritive sweeteners, participants received 355 mL (12 ounces) of water or a beverage containing non-nutritive sweeteners 10 minutes prior to each oral glucose tolerance test. Blood samples were collected 10 minutes before, and 0, 10, 20, 30, 60, 90, and 120 minutes following ingestion of 75 grams of glucose. Results: Basal active glucagon-like-peptide 1, peak glucagon-like-peptide 1, and glucagon-like-peptide 1 area-under-the-curve were higher in men than women (all p ≤0.04), adjusting for body mass index and age. Fasting and stimulated active glucagon-like-peptide 1 results were highly reproducible with little within-subject variability (between-subjects to within-subject variability ratio 4.2 and 3.5 for fasting glucagon-like-peptide 1 and glucagon-like-peptide 1 area-under-the-curve). Conclusion: Men had higher active glucagon-like-peptide 1 concentrations than women. In contrast to considerable inter-individual variability of basal and stimulated active glucagon-like-peptide 1 concentrations, intra-individual variability was low, consistent with tight physiological regulation.

Different in vivo and in vitro transformation of intestinal stem cells in mismatch repair deficiency.

Mutations in mismatch repair (MMR) genes result in microsatellite instability (MSI) and early onset of colorectal cancer. To get mechanistic insights into the time scale, sequence and frequency of intestinal stem cell (ISC) transformation, we quantified MSI and growth characteristics of organoids of Msh2-deficient and control mice from birth until tumor formation and related them to tissue gene expression. Although in Msh2-deficient organoids MSI continuously increased from birth, growth characteristics remained stable at first. Months before tumor onset, normal Msh2-deficient tissue contained tumor precursor cells forming organoids with higher MSI, cystic growth and growth rates resembling temporarily those of tumor organoids. Consistently, Msh2-deficient tissue exhibited a tumor-like gene signature. Normal Msh2-deficient organoids showed increased inheritable transient cyst-like growth, which became independent of R-spondin. ISC transformation proceeded faster in vitro than in vivo independent of the underlying genotype but more under MMR deficiency. Transient cyst-like growth but not MSI was suppressed by aspirin. In summary, as highlighted by organoids, molecular alterations continuously proceeded long before tumor onset in MMR-deficient intestine, thus increasing its susceptibility for ISC transformation.

Long-term immunosuppression after solitary islet transplantation is associated with preserved C-peptide secretion for more than a decade.

We report on two patients with type 1 diabetes (T1D) after solitary islet transplantation in 2001. They received steroid-sparing immunosuppression (daclizumab, sirolimus, and tacrolimus according to the Edmonton protocol). Both patients became insulin independent for 2 years: Patient A, a 42-year-old female with a 12-year history of T1D, received two islet infusions; patient B, a 53-year-old female with a 40-year T1D history, received one islet infusion. Pretransplant, both had undetectable C-peptide concentrations and frequent and severe hypoglycemia. Pretransplant, hemoglobin A1c (HbA1c) was 7.8% and 8.8% and insulin requirements were 0.47 and 0.33 units/kg/day, respectively. Posttransplant, C-peptide levels remained detectable while immunosuppression was continued, but decreased over time. Insulin was re-started 2 years posttransplant in both patients. Since patient A's glycemia and insulin requirements trended toward pretransplant levels, immunosuppression was discontinued after 13 years. This resulted in a sudden cessation of C-peptide secretion. Patient B continues on immunosuppression, has better HbA1c, and half the insulin requirement compared to pretransplant. Both patients no longer experience severe hypoglycemia. Herein, we document blood glucose concentrations over time (>30 000 measurements per patient) and ß cell function based on C-peptide secretion. Despite renewed insulin dependence, both patients express satisfaction with having undergone the procedure.

Effects of short-term sitagliptin treatment on immune parameters in healthy individuals, a randomized placebo-controlled study.

Sitagliptin, a dipeptidyl-peptidase 4 (DPP-4) inhibitor, improves blood glucose control in patients with type 2 diabetes by blocking cleavage of glucagon-like peptide 1 (GLP-1). In type 2 diabetes patients sitagliptin use is associated with an increase in minor infections, and in new-onset type 1 diabetes patients the ability of sitagliptin to dampen autoimmunity is currently being tested. DPP-4, also known as CD26, is expressed on leucocytes and can inactivate many chemokines important for leucocyte migration, as well as act as a co-stimulatory molecule on T cells. Therefore, this study was conducted to test whether sitagliptin is immunomodulatory. In this randomized, placebo-controlled trial, healthy volunteers were given sitagliptin or placebo daily for 28 days, and blood was drawn for immune assays. No significant differences were observed in the percentage of leucocyte subsets within peripheral blood mononuclear cells (PBMCs), plasma chemokine/cytokine levels or cytokines released by stimulation of PBMCs with either lipopolysaccharide (LPS) or anti-CD3. Individuals taking sitagliptin displayed increases in the percentage of cells expressing higher levels of CD26 at early time-points compared to placebo controls, but these differences resolved by day 28 of treatment. Therefore, in healthy volunteers, treatment with sitagliptin daily for 28 days does not overtly alter systemic immune function.

Consequences of stopping and restarting leptin in an adolescent with lipodystrophy.

BACKGROUND/AIMS: Lipodystrophy encompasses a group of rare disorders characterized by deficiency of adipose tissue resulting in hypoleptinemia, and metabolic abnormalities including insulin resistance, diabetes, dyslipidemia, and nonalcoholic steatohepatitis. Leptin replacement effectively ameliorates these metabolic derangements. We report effects of leptin discontinuation and resumption in a child with acquired generalized lipodystrophy. METHODS: Intermittent treatment with leptin with follow-up over 5 years. RESULTS: Pretreatment metabolic abnormalities included insulin resistance, hypertriglyceridemia and steatohepatitis. Leptin was started at the age of 10 years. After 2 years, the family requested discontinuation of leptin due to lack of visible physical changes. Nine months later, worsened metabolic abnormalities and arrest of pubertal development were observed. Leptin was restarted, followed by improvements in metabolic parameters. Laboratory changes (before vs. 6 months after restarting leptin) were: fasting glucose from 232 to 85 mg/dl, insulin from 232 to 38.9 µU/ml, HbA(1c) from 7.5 to 4.8%, triglycerides from 622 to 96 mg/dl, ALT from 229 to 61 U/l, AST from 91 to 18 U/l, and urine protein:creatinine ratio from 5.4 to 0.3. Progression of puberty was observed 1 year after restarting leptin. CONCLUSION: Initial leptin therapy likely prevented progression of metabolic abnormalities. Treatment discontinuation led to rapid metabolic decomposition and pubertal arrest. Reintroduction of leptin reversed metabolic abnormalities and allowed normal pubertal progression.

Rising incidence and challenges of childhood diabetes. A mini review.

Approximately 215,000 people younger than 20 yr of age, or 1 in 500 children and adolescents, had diabetes in the United States in 2010--and the incidence is rising. We still have insufficient knowledge about the precise mechanisms leading to the autoimmune mediated ß-cell destruction in Type 1 diabetes, and the ß-cell failure associated with insulin resistance in Type 2 diabetes. Long-term complications are similar: micro- and macrovascular disease occurs prematurely and presents an enormous burden on affected individuals, often as early as in middle age. In Type 1 diabetes, technological advances have clearly improved blood glucose management, but chronic peripheral over-insulinization remains a problem even with the most advanced systems. Thus, in Type 1 diabetes our research must focus on 1) finding the stimulus that ignites the immune response and 2) developing treatments that avoid hyperinsulinemia. In Type 2 diabetes in youth, the challenges start much earlier: most young patients do not even benefit from existing therapies due to non-compliance. Therefore, prevention of Type 2 diabetes and improvement of compliance, especially with non-pharmacological interventions, are the greatest challenges.

Beyond fast food and slow motion: weighty contributors to the obesity epidemic.

Decreased physical activity and marketing-driven increased consumption of "junk" food, dubbed "The Big Two", are generally regarded as the most important contributors to the obesity epidemic. However, the full picture contains many more pieces of the puzzle. We address several additional issues and review current clinical developments in obesity research. In spite of dramatic advancements in our understanding of the adipose organ and its endocrine and immune products, the ultimate causes of the obesity epidemic remain elusive. Treatment is plagued by poor adherence to life style modifications, and available pharmacological options are marginally effective, often also associated with major side effects. Surgical treatments, albeit effective in decreasing body weight, are invasive and expensive. Thus, our approaches to finding the causes, improving the existing treatments, and inventing novel therapies must be manifold.

Pancreatic beta cell function persists in many patients with chronic type 1 diabetes, but is not dramatically improved by prolonged immunosuppression and euglycaemia from a beta cell allograft.

AIMS/HYPOTHESIS: We measured serum C-peptide (at least 0.167 nmol/l) in 54 of 141 (38%) patients with chronic type 1 diabetes and sought factors that might differentiate those with detectable C-peptide from those without it. Finding no differences, and in view of the persistent anti-beta cell autoimmunity in such patients, we speculated that the immunosuppression (to weaken autoimmune attack) and euglycaemia accompanying transplant-based treatments of type 1 diabetes might promote recovery of native pancreatic beta cell function. METHODS: We performed arginine stimulation tests in three islet transplant and four whole-pancreas transplant recipients, and measured stimulated C-peptide in select venous sampling sites. On the basis of each sampling site's C-peptide concentration and kinetics, we differentiated insulin secreted from the individual's native pancreatic beta cells and that secreted from allografted beta cells. RESULTS: Selective venous sampling demonstrated that despite long-standing type 1 diabetes, all seven beta cell allograft recipients displayed evidence that their native pancreas secreted C-peptide. Yet even if chronic immunosuppression coupled with near normal glycaemia did improve native pancreatic C-peptide production, the magnitude of the effect was quite small. CONCLUSIONS/INTERPRETATION: Some native pancreatic beta cell function persists even years after disease onset in most type 1 diabetic patients. However, if prolonged euglycaemia plus anti-rejection immunosuppressive therapy improves native pancreatic insulin production, the effect in our participants was small. We may have underestimated pancreatic regenerative capacity by studying only a limited number of participants or by creating conditions (e.g. high circulating insulin concentrations or immunosuppressive agents toxic to beta cells) that impair beta cell function.

p53 downregulates expression of the G1/S cell cycle phosphatase Cdc25A.

Overexpression of Cdc25A phosphatase is often observed in cancer and results in poor prognosis. Cdc25A mainly dephosphorylates and thereby activates Cyclin-dependent kinase 2 and thus induces progression in the cell cycle from G(1) to S phase. Here, we demonstrate that the tumor suppressor p53 downregulates expression from the Cdc25A gene. In a p53-inducible cell system, Cdc25A expression on the mRNA and protein level is downregulated upon p53 expression. Promoter-reporter assays show that this regulation is dependent on the Cdc25A promoter. Mutant p53 fails to reduce Cdc25A transcription. In contrast to p53, neither p63 nor p73 can repress Cdc25A transcription. The Cdc25A promoter displays no p53 binding site, and p53 does not bind directly to the promoter DNA as shown by chromatin immunoprecipitation assays. Previously, the contribution of p53 to G(1)/S arrest has been mostly linked to activating the expression of the Cdk inhibitor p21(WAF1/CIP1). By downregulating Cdc25A expression, p53 may impair transition from G(1) to S phase independently of p21(WAF1/CIP1). Therefore, the data suggest that, as long as p53 is intact, Cdc25A transcriptional downregulation might play a role in cancer prevention.

Subtotal pancreatectomy for hypoglycemia due to congenital hyperinsulinism: long-term follow-up of neurodevelopmental and pancreatic function.

OBJECTIVE: To evaluate neurodevelopmental status as well as endocrine and exocrine pancreatic function in children who have undergone subtotal pancreatectomy for hypoglycemia due to congenital hyperinsulinism. PATIENTS AND METHODS: Out of 15 identified patients, eight children (mean age 12.7 +/- 0.8 yr) participated in detailed psychometric testing and studies assessing glucose homeostasis, secretion of proinsulin, insulin, glucagon and C-peptide during a test meal. Additionally, a 24-h fast, glucagon challenge test, 72-h stool collection, and ultrasonography of the pancreatic remnant were performed. RESULTS: Five of the 15 initially identified children had seizure disorders, including two with mental retardation. Diabetes developed in two of 15 children. All eight children investigated in the present study had evidence for attentional control impairment and 50% had subnormal intellectual functioning. Two had symptomatic hypoglycemia during the 24-h fast, while one had an elevated fasting glucose concentration. Four children, including the latter patient, had proinsulin/insulin ratios resembling patients with type 2 diabetes. Exocrine pancreatic function was normal in all eight children. No correlation was found between pancreatic endocrine function and pancreatic remnant size, nor between multiple pre- and postoperative factors (i.e., age at diagnosis and surgery) and neurodevelopmental outcome. CONCLUSION: While severe mental retardation or diabetes occurred infrequently in our patient population compared with previous reports, all of the studied children had subtle anomalies in their cognitive performance tests and the majority had endocrine test results indicative of abnormal insulin secretion and stressed pancreatic beta cells. Although partial pancreatectomy remains the treatment of choice after medical therapy fails, improved therapeutic means are necessary to achieve better clinical outcome.

Matching organic libraries with protein-substructures.

We present a general approach which allows automatic identification of sub-structures in proteins that resemble given three-dimensional templates. This paper documents its success with non-peptide templates such as beta-turn mimetics. We considered well-tested turn-mimetics such as the bicyclic turned dipeptide (BTD), spiro lactam (Spiro) and the 2,5-disubstituded tetrahydrofuran (THF), a new furan-derivative which was recently developed and characterized. The detected geometric similarity between the templates and the protein patches corresponds to r.m.s.-values of 0.3 A for more than 80% of the constituting atoms, which is typical for active site comparisons of homologous proteins. This fast automatic procedure might be of biomedical value for finding special mimicking leads for particular protein sub-structures as well as for template-assembled synthetic protein (TASP) design.

Efficacy and safety of troglitazone in the treatment of lipodystrophy syndromes.

BACKGROUND: Troglitazone promotes adipocyte differentiation in vitro and increases insulin sensitivity in vivo. Therefore, troglitazone may have therapeutic benefit in lipoatrophic diabetes. OBJECTIVE: To determine whether troglitazone ameliorates hyperglycemia and hypertriglyceridemia or increases fat mass in lipoatrophic patients. DESIGN: Open-labeled prospective study. SETTING: United States and Canada. PATIENTS: 20 patients with various syndromes associated with lipoatrophy or lipodystrophy. INTERVENTION: 6 months of therapy with troglitazone, 200 to 600 mg/d. MEASUREMENTS: Levels of hemoglobin A1c triglycerides, free fatty acids, and insulin; respiratory quotient; percentage of body fat; liver volume; and regional fat mass. RESULTS: In the 13 patients with diabetes who completed 6 months of troglitazone therapy, hemoglobin A1c levels decreased by a mean of 2.8% (95% CI, 1.9% to 3.7%; P < 0.001). In all 19 study patients, fasting triglyceride levels decreased by 2.6 mmol/L (230 mg/dL) (CI, 0.7 to 4.5 mmol/L [62 to 398 mg/dL]; P = 0.019) and free fatty acid levels decreased by 325 micromol/L (CI, 135 to 515 micromol/L; P = 0.035). The respiratory quotient decreased by a mean of 0.12 (CI, 0.08 to 0.16; P < 0.001), suggesting that troglitazone promoted oxidation of fat. Body fat increased by a mean of 2.4 percentage points (CI, 1.3 to 4.5 percentage points; P = 0.044). Magnetic resonance imaging showed an increase in subcutaneous adipose tissue but not in visceral fat. In one patient, the serum alanine aminotransferase level increased eightfold during the 10th months of troglitazone treatment but normalized 3 months after discontinuation of treatment Liver biopsy revealed an eosinophilic infiltrate, suggesting hypersensitivity reaction as a cause of hepatotoxicity. CONCLUSION: Troglitazone therapy improved metabolic control and increased body fat in patients with lipoatrophic diabetes. The substantial benefits of troglitazone must be balanced against the risk for hepatotoxicity, which can occur relatively late in the treatment course.

Role of insulin receptors and IGF receptors in growth and development.

Observations in targeted mouse mutants and patients with genetic abnormalities grant insight into the various and distinct roles of insulin-like growth factor receptors (IGF-Rs) and insulin receptors (IRs) during early development. While IGF-1Rs (mediating both IGF-1 and IGF-2 actions) are important for embryonic and fetal growth, IRs (mediating IGF-2 rather than insulin action) play a minor role. However, it is an oversimplification to conclude that IGF-1Rs mediate growth and IRs mediate metabolic responses. Mice lacking both IRs and IGF-1Rs are more severely growth retarded than mice lacking either receptor alone. The phenotype of combined deficiency of IRs and IGF-1Rs is similar to the phenotype caused by the absence of IGF-1 and IGF-2. This provides genetic proof that these two receptors account for the entirety of the growth promoting effects of IGF-1 and IGF-2. There is little evidence that hybrid insulin/IGF-1 receptors promote embryonic growth to a significant degree. The clinical presentation regarding severity of growth retardation and metabolic disturbances observed in animal models versus in humans may differ greatly and the reasons will be reviewed in detail.

Conservation of substructures in proteins: interfaces of secondary structural elements in proteasomal subunits.

It is observed that during divergent evolution of two proteins with a common phylogenetic origin, the structural similarity of their backbones is often preserved even when the sequence similarity between them decreases to a virtually undetectable level. Here we analyzed, whether the conservation of structure along evolution involves also the local atomic structures in the interfaces between secondary structural elements. We have used as study case one protein family, the proteasomal subunits, for which 17 crystal structures are known. These include 14 different subunits of Saccharomyces cerevisiae, 2 subunits of Thermoplasma acidophilum and one subunit of Escherichia coli. The structural core of the 17 proteasomal subunits has 23 secondary structural elements. Any two adjacent secondary structural elements form a molecular interface consisting of two molecular patches. We found 61 interfaces that occurred in all 17 subunits. The 3D shape of equivalent molecular patches from different proteasomal subunits were compared by superposition. Our results demonstrate that pairs of equivalent molecular patches show an RMSD which is lower than that of randomly chosen patches from unrelated proteins. This is true even when patch comparisons with identical residues were excluded from the analysis. Furthermore it is known that the sequential dissimilarity is correlated to the RMSD between the backbones of the members of protein families. The question arises whether this is also true for local atomic structures. The results show that the correlation of individual patch RMSD values and local sequence dissimilarities is low and has a wide range from 0 to 0.41, however, it is surprising that there is a good correlation between the average RMSD of all corresponding patches and the global sequence dissimilarity. This average patch RMSD correlates slightly stronger than the C(alpha)-trace RMSD to the global sequence dissimilarity.

Targeted gene mutations define the roles of insulin and IGF-I receptors in mouse embryonic development.

Insulin-like growth factors (IGFs) and their receptors regulate embryonic and post-natal growth. Genetic evidence derived from targeted mouse mutants indicates that both the insulin receptor (IR) and IGF-I receptors (IGF-IRs) are required for mouse embryonic growth. However, the roles of IRs and IGF-IRs are functionally distinct, with IGF-IRs mediating both IGF-I and IGF-II actions, and IRs mediating IGF-II, rather than insulin, action. The combined interactions of IGF-IRs and IRs with IGF-I and IGF-II account for the entirety of the growth effects of these two ligands, and provide the molecular basis for IGFs-mediated intrauterine growth and differentiation. Genetic ablation experiments of insulin receptor substrate-1 (IRS-1) and -2 (IRS-2), two important molecules in the IR and IGF-IR signaling pathways, are also beginning to shed light onto the mechanisms accounting for the specificity of IR and IGF-IR signaling. IRS-1-deficient mice are growth retarded, while IRS-2-deficient mice develop diabetes, indicating that the two molecules play a more specific role than previously recognized in IGF-IR and IR signaling.

Leptin secretion in Cushing's syndrome: preservation of diurnal rhythm and absent response to corticotropin-releasing hormone.

The normal inverse relationship between leptin and cortisol is lost in chronic hypercortisolism. We studied this apparent dysregulation in patients with Cushing's syndrome to investigate 1) the effect of chronic hypercortisolemia on the circadian rhythm of leptin secretion, 2) the response of leptin after administration of CRH, and 3) the short term effect of curative surgery on leptin. The preoperative morning leptin concentration was 54.2 +/- 8.1 ng/mL, and the nighttime value was 68.6 +/- 9.8 ng/mL, reflecting a mean rise of 32.8 +/- 7.6%, similar to the nocturnal increase observed in normal subjects. By contrast, cortisol's diurnal variation (21.8 +/- 1.7 vs. 16.9 +/- 1.1 mg/dL) was blunted. In women, but not men, body mass index correlated with leptin (P = 0.001). Preoperative ACTH and cortisol (both P < 0.0001), but not leptin levels increased after CRH. Ten days after surgery, basal cortisol values were subnormal (1.1 +/- 0.6 mg/dL), but leptin levels remained unchanged and did not increase after CRH. Body mass index and insulin also remained unchanged. Insulin, but not age, urinary free cortisol, or plasma cortisol correlated with leptin (P < 0.05). In summary, patients with Cushing's syndrome have moderately elevated leptin levels that maintain an intact circadian rhythm but do not respond to acute or subacute alterations of cortisol.

Phosphatidylinositol 3-kinase-dependent membrane association of the Bruton's tyrosine kinase pleckstrin homology domain visualized in single living cells.

Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) has been proposed to act as a second messenger to recruit regulatory proteins to the plasma membrane via their pleckstrin homology (PH) domains. The PH domain of Bruton's tyrosine kinase (Btk), which is mutated in the human disease X-linked agammaglobulinemia, has been shown to interact with PI(3,4,5)P3 in vitro. In this study, a fusion protein containing the PH domain of Btk and the enhanced green fluorescent protein (BtkPH-GFP) was constructed and utilized to study the ability of this PH domain to interact with membrane inositol phospholipids inside living cells. The localization of expressed BtkPH-GFP in quiescent NIH 3T3 cells was indistinguishable from that of GFP alone, both being cytosolic as assessed by confocal microscopy. In NIH 3T3 cells coexpressing BtkPH-GFP and the epidermal growth factor receptor, activation of epidermal growth factor or endogenous platelet-derived growth factor receptors caused a rapid (<3 min) translocation of the cytosolic fluorescence to ruffle-like membrane structures. This response was not observed in cells expressing GFP only and was completely inhibited by treatment with the PI 3-kinase inhibitors wortmannin and LY 292004. Membrane-targeted PI 3-kinase also caused membrane localization of BtkPH-GFP that was slowly reversed by wortmannin. When the R28C mutation of the Btk PH domain, which causes X-linked agammaglobulinemia, was introduced into the fluorescent construct, no translocation was observed after stimulation. In contrast, the E41K mutation, which confers transforming activity to native Btk, caused significant membrane localization of BtkPH-GFP with characteristics indicating its possible binding to PI(4,5)P2. This mutant, but not wild-type BtkPH-GFP, interfered with agonist-induced PI(4,5)P2 hydrolysis in COS-7 cells. These results show in intact cells that the PH domain of Btk binds selectively to 3-phosphorylated lipids after activation of PI 3-kinase enzymes and that losing such binding ability or specificity results in gross abnormalities in the function of the enzyme. Therefore, the interaction with PI(3,4,5)P3 is likely to be an important determinant of the physiological regulation of Btk and can be utilized to visualize the dynamics and spatiotemporal organization of changes in this phospholipid in living cells.

Effect of insulin on farnesyltransferase. Specificity of insulin action and potentiation of nuclear effects of insulin-like growth factor-1, epidermal growth factor, and platelet-derived growth factor.

We have previously demonstrated that insulin activates farnesyltransferase (FTase) and augments the amounts of farnesylated p21 (Goalstone, M. L., and Draznin, B. (1996) J. Biol. Chem. 271, 27585-27589). We postulated that this aspect of insulin action might explain the "priming effect" of insulin on the cellular response to other growth factors. In the present study, we show the specificity of the effect of insulin on FTase. Insulin, but not insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), or platelet-derived growth factor (PDGF), stimulated the phosphorylation of the alpha-subunit of FTase and the amounts of farnesylated p21. Even though all four growth factors utilized the Ras pathway to stimulate DNA synthesis, only insulin used this pathway to influence FTase. Insulin failed to stimulate FTase in cells expressing the chimeric insulin/IGF-1 receptor and in cells derived from the insulin receptor knock-out animals. Insulin potentiated the effects of IGF-1, EGF, and PDGF on DNA synthesis in cells expressing the wild type insulin receptor, but this potentiation was inhibited in the presence of the FTase inhibitor, alpha-hydroxyfarnesylphosphonic acid. We conclude that the effect of insulin on FTase is specific, requires the presence of an intact insulin receptor, and serves as a conduit for the "priming" influence of insulin on the nuclear effects of other growth factors.

Evidence that IRS-2 phosphorylation is required for insulin action in hepatocytes.

Insulin receptor substrates (IRSs) are tyrosine-phosphorylated following stimulation with insulin, insulin-like growth factors (IGFs), and interleukins. A key question is whether different IRSs play different roles to mediate insulin's metabolic and growth-promoting effects. In a novel system of insulin receptor-deficient hepatocytes, insulin fails to (i) stimulate glucose phosphorylation, (ii) enhance glycogen synthesis, (iii) suppress glucose production, and (iv) promote mitogenesis. However, insulin's ability to induce IRS-1 and gab-1 phosphorylation and binding to phosphatidylinositol (PI) 3-kinase is unaffected, by virtue of the compensatory actions of IGF-1 receptors. In contrast, phosphorylation of IRS-2 and generation of IRS-2/PI 3-kinase complexes are markedly reduced. Thus, absence of insulin receptors selectively reduces IRS-2, but not IRS-1 phosphorylation, and the impairment of IRS-2 activation is associated with lack of insulin effects. To address whether phosphorylation of additional IRSs is also affected, we analyzed phosphotyrosine-containing proteins in PI 3-kinase immunoprecipitates from insulin-treated cells. However, these experiments indicate that IRS-1 and IRS-2 are the main PI 3-kinase-bound proteins in hepatocytes. These data identify IRS-2 as the main effector of both the metabolic and growth-promoting actions of insulin through PI 3-kinase in hepatocytes, and IRS-1 as the main substrate mediating the mitogenic actions of IGF-1 receptors.