Disruption of IRS-2 causes type 2 diabetes in mice

Obesity and mild hyperinsulinemia found in neuropeptide Y-Y1 receptor-deficient mice

To elucidate the role of neuropeptide Y (NPY)-Y1 receptor (Y1-R) in food intake, energy expenditure, and other possible functions, we have generated Y1-R-deficient mice (Y1-R-/-) by gene targeting. Contrary to our hypothesis that the lack of NPY signaling via Y1-R would result in impaired feeding and weight loss, Y1-R-/- mice showed a moderate obesity and mild hyperinsulinemia without hyperphagia. Although there was some variation between males and females, typical characteristics of Y1-R-/- mice include: greater body weight (females more than males), an increase in the weight of white adipose tissue (WAT) (approximately 4-fold in females), an elevated basal level of plasma insulin (approximately 2-fold), impaired insulin secretion in response to glucose administration, and a significant changes in mitochondrial uncoupling protein (UCP) gene expression (up-regulation of UCP1 in brown adipose tissue and down-regulation of UCP2 in WAT). These results suggest either that the Y1-R in the hypothalamus is not a key molecule in the leptin/NPY pathway, which controls feeding behavior, or that its deficiency is compensated by other receptors, such as NPY-Y5 receptor. We believe that the mild obesity found in Y1-R-/- mice (especially females) was caused by the impaired control of insulin secretion and/or low energy expenditure, including the lowered expression of UCP2 in WAT. This model will be useful for studying the mechanism of mild obesity and abnormal insulin metabolism in noninsulin-dependent diabetes mellitus.

Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling

The major substrates for the type I insulin-like growth factor (IGF-I) receptor are Shc and insulin receptor substrate (IRS) proteins. In the current study, we report that IGF-I induces a sustained tyrosine phosphorylation of Shc and its association with Grb2 in SH-SY5Y human neuroblastoma cells. The time course of Shc tyrosine phosphorylation parallels the time course of IGF-I-stimulated activation of extracellular signal-regulated kinase (ERK). Transfection of SH-SY5Y cells with a p52 Shc mutant decreases Shc tyrosine phosphorylation and Shc-Grb2 association. This results in the inhibition of IGF-I-mediated ERK tyrosine phosphorylation and neurite outgrowth. In contrast, IGF-I induces a transient tyrosine phosphorylation of IRS-2 and an association of IRS-2 with Grb2. The time course of IRS-2 tyrosine phosphorylation and IRS-2-Grb2 and IRS-2-p85 association closely resembles the time course of IGF-I-mediated membrane ruffling. Treating cells with the phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 blocks IGF-I-induced membrane ruffling. The ERK kinase inhibitor PD98059, as well as transfection with the p52 Shc mutant, has no effect on IGF-I-mediated membrane ruffling. Immunolocalization studies show IRS-2 and Grb2, but not Shc, concentrated at the tip of the extending growth cone where membrane ruffling is most active. Collectively, these results suggest that the association of Shc with Grb2 is essential for IGF-I-mediated neurite outgrowth, whereas the IRS-2-Grb2-phosphatidylinositol 3'-kinase complex may regulate growth cone extension and membrane ruffling.

The role of PI 3-kinase in insulin action

This review focuses on the recent advances made in our understanding of the mechanism by which insulin induces the activation of PI 3-kinase(s) whose role is to generate 3-phosphoinositide lipids which are the second messenger of the insulin signalling pathway. The mechanism by which these signalling molecules induce the activation of downstream signalling pathways leading to the activation of protein kinase B (PKB, also known as Akt) and other kinases is also discussed. PKB is likely to be a major mediator of many of the physiological responses of a cell to insulin and likely physiological cellular targets of this enzyme are highlighted.

Calorie restriction increases cell surface GLUT-4 in insulin-stimulated skeletal muscle

Reduced calorie intake [calorie restriction (CR); 60% of ad libitum (AL)] leads to enhanced glucose transport without altering total GLUT-4 glucose transporter abundance in skeletal muscle. Therefore, we tested the hypothesis that CR (20 days) alters the subcellular distribution of GLUT-4. Cell surface GLUT-4 content was higher in insulin-stimulated epitrochlearis muscles from CR vs. AL rats. The magnitude of this increase was similar to the CR-induced increase in glucose transport, and GLUT-4 activity (glucose transport rate divided by cell surface GLUT-4) was unaffected by diet. The CR effect was specific to the insulin-mediated pathway, as evidenced by the observations that basal glucose transport and cell surface GLUT-4 content, as well as hypoxia-stimulated glucose transport, were unchanged by diet. CR did not alter insulin's stimulation of insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase (PI3K) activity. Muscle abundance of IRS-2 and p85 subunit of PI3K were unaltered by diet, but IRS-1 content was lower in CR vs. AL. These data demonstrate that, despite IRS-1-PI3K activity similar to AL, CR specifically increases insulin's activation of glucose transport by enhancing the steady-state proportion of GLUT-4 residing on the cell surface.

Differential regulation of insulin receptor substrate-2 and mitogen-activated protein kinase tyrosine phosphorylation by phosphatidylinositol 3-kinase inhibitors in SH-SY5Y human neuroblastoma cells

Insulin-like growth factor I (IGF-I) is a potent neurotropic factor promoting the differentiation and survival of neuronal cells. SH-SY5Y human neuroblastoma cells are a well characterized in vitro model of nervous system growth. We report here that IGF-I stimulated the tyrosine phosphorylation of the type I IGF receptor (IGF-IR) and insulin receptor substrate-2 (IRS-2) in a time- and concentration-dependent manner. These cells lacked IRS-1. After being tyrosine phosphorylated, IRS-2 associated transiently with downstream signaling molecules, including phosphatidylinositol 3-kinase (PI 3-K) and Grb2. Treatment of the cells with PI 3-K inhibitors (wortmannin and LY294002) increased IGF-I-induced tyrosine phosphorylation of IRS-2. We also observed a concomitant increase in the mobility of IRS-2, suggesting that PI 3-K mediates or is required for IRS-2 serine/threonine phosphorylation, and that this phosphorylation inhibits IRS-2 tyrosine phosphorylation. Treatment with PI 3-K inhibitors induced an increased association of IRS-2 with Grb2, probably as a result of the increased IRS-2 tyrosine phosphorylation. However, even though the PI 3-K inhibitors enhanced the association of Grb2 with IRS-2, these compounds suppressed IGF-I-induced mitogen-activated protein kinase activation and neurite outgrowth. Together, these results indicate that although PI 3-K participates in a negative regulation of IRS-2 tyrosine phosphorylation, its activity is required for IGF-IR-mediated mitogen-activated protein kinase activation and neurite outgrowth.

IRS pleckstrin homology domains bind to acidic motifs in proteins

Using a yeast two-hybrid system, we identified several proteins that interact with the PH domains in IRS-1 and IRS-2, including Lon protease, myeloblast protein, and nucleolin. Although the roles of these molecules in insulin action are not yet known, each protein contained an acidic motif that interacted with the PH domain of IRS-2. However, only the acidic motif in nucleolin bound to IRS-1, suggesting that the PH domain in IRS-1 and IRS-2 are not identical. Moreover, synthetic peptides based on the acidic motif in Lon protease and myeloblast protein inhibited the binding of nucleolin to the PH domain of IRS-2 but not to the PH domain of IRS-1, confirming the selectivity of these PH domains. The ability to bind acidic motifs may be a specific function of the PH domain in IRS proteins, because the PH domains in betaARK, phospholipase Cgamma, or spectrin did not bind nucleolin. In 32D cells, nucleolin bound to both IRS-1 and IRS-2, and expression of the acidic motif of nucleolin inhibited insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2. These results suggest that the binding of acidic motifs to the PH domain of IRS-1 and IRS-2 disrupts coupling to the activated insulin receptor. Our results are consistent with the hypothesis that the PH domain in the IRS proteins may ordinarily bind acidic peptide motifs in membrane proteins or other acidic membrane elements that couple IRS proteins to activated membrane receptors.

Disruption of the p70(s6k)/p85(s6k) gene reveals a small mouse phenotype and a new functional S6 kinase

Recent studies have shown that the p70(s6k)/p85(s6k) signaling pathway plays a critical role in cell growth by modulating the translation of a family of mRNAs termed 5'TOPs, which encode components of the protein synthetic apparatus. Here we demonstrate that homozygous disruption of the p70(s6k)/p85(s6k) gene does not affect viability or fertility of mice, but that it has a significant effect on animal growth, especially during embryogenesis. Surprisingly, S6 phosphorylation in liver or in fibroblasts from p70(s6k)/p85(s6k)-deficient mice proceeds normally in response to mitogen stimulation. Furthermore, serum-induced S6 phosphorylation and translational up-regulation of 5'TOP mRNAs were equally sensitive to the inhibitory effects of rapamycin in mouse embryo fibroblasts derived from p70(s6k)/p85(s6k)-deficient and wild-type mice. A search of public databases identified a novel p70(s6k)/p85(s6k) homolog which contains the same regulatory motifs and phosphorylation sites known to control kinase activity. This newly identified gene product, termed S6K2, is ubiquitously expressed and displays both mitogen-dependent and rapamycin-sensitive S6 kinase activity. More striking, in p70(s6k)/p85(s6k)-deficient mice, the S6K2 gene is up-regulated in all tissues examined, especially in thymus, a main target of rapamycin action. The finding of a new S6 kinase gene, which can partly compensate for p70(s6k)/p85(s6k) function, underscores the importance of S6K function in cell growth.

The pleckstrin homology and phosphotyrosine binding domains of insulin receptor substrate 1 mediate inhibition of apoptosis by insulin

Insulin and insulin-like growth factor 1 (IGF-1) evoke diverse biological effects through receptor-mediated tyrosine phosphorylation of insulin receptor substrate (IRS) proteins. We investigated the elements of IRS-1 signaling that inhibit apoptosis of interleukin 3 (IL-3)-deprived 32D myeloid progenitor cells. 32D cells have few insulin receptors and no IRS proteins; therefore, insulin failed to inhibit apoptosis during IL-3 withdrawal. Insulin stimulated mitogen-activated protein kinase in 32D cells expressing insulin receptors (32DIR) but failed to activate the phosphatidylinositol 3 (PI 3)-kinase cascade or to inhibit apoptosis. By contrast, insulin stimulated the PI 3-kinase cascade, inhibited apoptosis, and promoted replication of 32DIR cells expressing IRS-1. As expected, insulin did not stimulate PI 3-kinase in 32DIR cells, which expressed a truncated IRS-1 protein lacking the tail of tyrosine phosphorylation sites. However, this truncated IRS-1 protein, which retained the NH2-terminal pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains, mediated phosphorylation of PKB/akt, inhibition of apoptosis, and replication of 32DIR cells during insulin stimulation. These results suggest that a phosphotyrosine-independent mechanism mediated by the PH and PTB domains promoted antiapoptotic and growth actions of insulin. Although PI 3-kinase was not activated, its phospholipid products were required, since LY294002 inhibited these responses. Without IRS-1, a chimeric insulin receptor containing a tail of tyrosine phosphorylation sites derived from IRS-1 activated the PI 3-kinase cascade but failed to inhibit apoptosis. Thus, phosphotyrosine-independent IRS-1-linked pathways may be critical for survival and growth of IL-3-deprived 32D cells during insulin stimulation.

A muscle-specific insulin receptor knockout exhibits features of the metabolic syndrome of NIDDM without altering glucose tolerance

Skeletal muscle insulin resistance is among the earliest detectable defects in humans with type 2 diabetes mellitus. To determine the contribution of muscle insulin resistance to the metabolic phenotype of diabetes, we used the Cre-loxP system to disrupt the insulin receptor gene in mouse skeletal muscle. The muscle-specific insulin receptor knockout mice exhibit a muscle-specific > 95% reduction in receptor content and early signaling events. These mice display elevated fat mass, serum triglycerides, and free fatty acids, but blood glucose, serum insulin, and glucose tolerance are normal. Thus, insulin resistance in muscle contributes to the altered fat metabolism associated with type 2 diabetes, but tissues other than muscle appear to be more involved in insulin-regulated glucose disposal than previously recognized.

Impaired glucose tolerance in mice with a targeted impairment of insulin action in muscle and adipose tissue

Type 2 diabetes is a complex metabolic disorder characterized by peripheral insulin resistance and impaired beta cell function. Insulin resistance is inherited as a non-mendelian trait. In genetically predisposed individuals, resistance of skeletal muscle and adipose tissue to insulin action precedes the onset of clinical diabetes, and is thought to contribute to hyperglycaemia by leading to impaired beta cell function and increased hepatic glucose production. It is not clear whether beta cell and liver defects are also genetically determined. To test the hypothesis that insulin resistance in muscle and fat is sufficient to cause type 2 diabetes in the absence of intrinsic beta cell and liver abnormality, we generated transgenic mice that were insulin-resistant in skeletal muscle and adipose tissue. These mice developed all the prodromal features of type 2 diabetes but, despite the compounded effect of peripheral insulin resistance and a mild impairment of beta cell function, failed to become diabetic. These findings indicate the need for a critical re-examination of the primary site(s) of insulin resistance in diabetes.

Insulin resistance and diabetes mellitus in transgenic mice expressing nuclear SREBP-1c in adipose tissue: model for congenital generalized lipodystrophy

Overexpression of the nuclear form of sterol regulatory element-binding protein-1c (nSREBP-1c/ADD1) in cultured 3T3-L1 preadipocytes was shown previously to promote adipocyte differentiation. Here, we produced transgenic mice that overexpress nSREBP-1c in adipose tissue under the control of the adipocyte-specific aP2 enhancer/promoter. A syndrome with the following features was observed: (1) Disordered differentiation of adipose tissue. White fat failed to differentiate fully, and the size of white fat depots was markedly decreased. Brown fat was hypertrophic and contained fat-laden cells resembling immature white fat. Levels of mRNA encoding adipocyte differentiation markers (C/EBPalpha, PPARgamma, adipsin, leptin, UCP1) were reduced, but levels of Pref-1 and TNFalpha were increased. (2) Marked insulin resistance with 60-fold elevation in plasma insulin. (3) Diabetes mellitus with elevated blood glucose (>300 mg/dl) that failed to decline when insulin was injected. (4) Fatty liver from birth and elevated plasma triglyceride levels later in life. These mice exhibit many of the features of congenital generalized lipodystrophy (CGL), an autosomal recessive disorder in humans.

Genetic engineering in mice: impact on insulin signalling and action

The expression of a number of genes encoding key players in insulin signalling and action, including insulin, insulin receptor (IR), downstream signalling molecules such as insulin receptor substrate-1 (IRS-1) and IRS-2, glucose transporters (GLUT4, GLUT2) and important metabolic enzymes such as glucokinase, has now been altered in transgenic or knockout mice. Such mice presented with phenotypes ranging from mild defects, revealing complementarity between key molecules or pathways, to severe diabetes with ketoacidosis and early postnatal death. Insulin action could also be improved by overproduction of proteins acting at regulatory steps. The development of diabetes by combining mutations, which alone do not lead to major metabolic alterations, validated the 'diabetogenes' concept of non-insulin-dependent diabetes mellitus. Genes encoding insulin-like growth factors (IGF-I and IGF-II) and their type I receptor (IGF-IR) have also been disrupted. It appears that although IR and IGF-IR are both capable of metabolic and mitogenic signalling, they are not fully redundant. However, IR could replace IGF-IR if efficiently activated by IGF-II. Studies with cell lines lacking IR or IGF-IR lend support to such conclusions. Concerning the issues of specificity and redundancy, studies with cell lines derived from IRS-1-deficient mice showed that IRS-1 and IRS-2 are also not completely interchangeable.

Affinity, stability and polarity of binding of the TATA binding protein governed by flexure at the TATA Box

The TATA binding protein (TBP), which plays a central role in gene regulation as an essential component of all three nuclear transcription systems, sharply kinks the TATA box at two sites and severely contorts the intervening DNA segment. DNA constructs with precisely localized flexure have been used to investigate the special repertoire of mechanisms and properties that arise from TBP interacting with the TATA box. DNA flexure precisely localized to the sites of TBP-mediated DNA kinking increases the affinity of TBP more than 100-fold; unexpectedly, this increase in affinity is achieved almost exclusively by increasing the stability of the TBP-DNA complex rather than the rate of its formation. In vitro transcription with RNA polymerase III provides a first demonstration that the orientation of TBP on the TATA box is governed by DNA deformability, its C-proximal repeat contacting the more flexible end of the TATA box. Exceptionally stable TBP-DNA complexes reach their orientational equilibrium very slowly; in these circumstances, assembly of stable ("committed") transcription initiation complexes can freeze far-from-equilibrium orientations of TBP on the TATA box, causing transcription polarity to be determined by a kinetic trapping mechanism.

Insulin stimulates the tyrosine dephosphorylation of docking protein p130cas (Crk-associated substrate), promoting the switch of the adaptor protein crk from p130cas to newly phosphorylated insulin receptor substrate-1

The docking protein p130(cas) (Crk-associated substrate) forms a stable complex with the adaptor protein CrkII in a tyrosine-phosphorylation-dependent manner. Insulin-induced tyrosine phosphorylation of insulin receptor substrates results in the redistribution of CrkII between p130(cas) and insulin receptor substrate-1. A decrease in the association between CrkII and p130(cas) in response to insulin stimulation was detected in CHO cells stably expressing insulin receptor or insulin receptor substrate-1, and in L6 rat myoblasts. Along with the decrease in the association of CrkII with p130(cas), the amount of tyrosine-phosphorylated insulin receptor substrate-1 co-precipitated with CrkII increased in all cell types studied. The insulin-induced decrease in the CrkII-p130(cas) association was further confirmed by Far Western Blot analysis with the Src homology 2 (SH2) domain of CrkII. Insulin regulates the association of CrkII with p130(cas) by tyrosine dephosphorylation of p130(cas) and co-ordinated tyrosine phosphorylation of insulin receptor substrate-1. Tyrosine-phosphorylated insulin receptor substrate-1 serves as a docking protein for multiple adaptor proteins and competes with p130(cas) for CrkII.

Phosphoinositide 3-kinase: the key switch mechanism in insulin signalling

Insulin plays a key role in regulating a wide range of cellular processes. However, until recently little was known about the signalling pathways that are involved in linking the insulin receptor with downstream responses. It is now apparent that the activation of class 1a phosphoinositide 3-kinase (PI 3-kinase) is necessary and in some cases sufficient to elicit many of insulin's effects on glucose and lipid metabolism. The lipid products of PI 3-kinase act as both membrane anchors and allosteric regulators, serving to localize and activate downstream enzymes and their protein substrates. One of the major ways these lipid products of PI 3-kinase act in insulin signalling is by binding to pleckstrin homology (PH) domains of phosphoinositide-dependent protein kinase (PDK) and protein kinase B (PKB) and in the process regulating the phosphorylation of PKB by PDK. Using mechanisms such as this, PI 3-kinase is able to act as a molecular switch to regulate the activity of serine/threonine-specific kinase cascades important in mediating insulin's effects on endpoint responses.

A beta-sheet peptide inhibitor of E47 dimerization and DNA binding

BACKGROUND

Many transcription factors are active only in their dimeric form, including the basic-helix-loop-helix (bHLH) family of transcription factors. The disruption of the dimer therefore presents a means of inhibiting the biological functions of such transcription factors. E47 is a homodimeric bHLH transcription factor with a four-helix bundle dimerization interface. Here, we investigate the concept of dimerization inhibition using peptides derived from the dimerization domain of E47.

RESULTS

We have synthesized several peptides corresponding to the E47 dimerization interface that inhibit E47 DNA-binding activity with IC50 values in the range of 3.6-120 mM. Interestingly, helix II; a peptide corresponding to the carboxy-terminal helix of the E47 dimerization interface, adopted a beta-sheet structure in solution, as shown using circular dichroism (CD), and inhibited the binding of E47 to DNA at equimolar concentrations. Size-exclusion chromatography, analytical ultracentrifugation and cross-linking experiments verified that this peptide prevented E47 dimerization. Furthermore, CD experiments provided evidence that helix II could induce a beta-sheet secondary structure upon the highly alpha-helical E47 bHLH domain.

CONCLUSIONS

This study is the first demonstration of dissociative inhibition in the bHLH class of transcription factors and also provides an example of beta-sheet induction in an alpha-helical protein. Future experiments will prove the structural determinants of the beta-sheet secondary structure in helix II and investigate the generality of the dissociative strategy in other transcription factor families.

Assessment of the roles of mitogen-activated protein kinase, phosphatidylinositol 3-kinase, protein kinase B, and protein kinase C in insulin inhibition of cAMP-induced phosphoenolpyruvate carboxykinase gene transcription

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is induced by glucagon, acting through cAMP and protein kinase A, and this induction is inhibited by insulin. Conflicting reports have suggested that insulin inhibits induction by cAMP by activating the Ras/mitogen-activated protein kinase (MAPK) pathway or by activating the phosphatidylinositol 3-kinase (PI3-kinase), but not MAPK, pathway. Insulin activated PI3-kinase phosphorylates lipids that activate protein kinase B (PKB) and Ca2+/diacylglycerol-insensitive forms of protein kinase C (PKC). We have assessed the roles of these pathways in insulin inhibition of cAMP/PKA-induced transcription of PEPCK by using dominant negative and dominant active forms of regulatory enzymes in the Ras/MAPK and PKB pathways and chemical inhibitors of PKC isoforms. Three independently acting inhibitory enzymes of the Ras/MAPK pathway, blocking SOS, Ras, and MAPK, had no effect upon insulin inhibition. However, dominant active Ras prevented induction of PEPCK and also stimulated transcription mediated by Elk, a MAPK target. Insulin did not stimulate Elk-mediated transcription, indicating that insulin did not functionally activate the Ras/MAPK pathway. Inhibitors of PI3-kinase, LY294002 and wortmannin, abolished insulin inhibition of PEPCK gene transcription. However, inhibitors of PKC and mutated forms of PKB, both of which are known downstream targets of PI3-kinase, had no effect upon insulin inhibition. Dominant negative forms of PKB did not interfere with insulin inhibition and a dominant active form of PKB did not prevent induction by PKA. Phorbol ester-mediated inhibition of PEPCK transcription was blocked by bisindole maleimide and by staurosporine, but insulin-mediated inhibition was unaffected. Thus, insulin inhibition of PKA-induced PEPCK expression does not require MAPK activation but does require activation of PI3-kinase, although this signal is not transmitted through the PKB or PKC pathways.

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.

Insulin-like growth factor I (IGF-I)-stimulated pancreatic beta-cell growth is glucose-dependent. Synergistic activation of insulin receptor substrate-mediated signal transduction pathways by glucose and IGF-I in INS-1 cells

Nutrients and certain growth factors stimulate pancreatic beta-cell mitogenesis, however, the appropriate mitogenic signal transduction pathways have not been defined. In the glucose-sensitive pancreatic beta-cell line, INS-1, it was found that glucose (6-18 mM) independently increased INS-1 cell proliferation (>20-fold at 15 mM glucose). Insulin-like growth factor I (IGF-I)-induced INS-1 cell proliferation was glucose-dependent only in the physiologically relevant concentration range (6-18 mM glucose). The combination of IGF-I and glucose was synergistic, increasing INS-1 cell proliferation >50-fold at 15 mM glucose + 10 nM IGF-I. Glucose metabolism and phosphatidylinositol 3'-kinase (PI 3'-kinase) activation were necessary for both glucose and IGF-I-stimulated INS-1 cell proliferation. IGF-I and 15 mM glucose increased tyrosine phosphorylation mediated recruitment of Grb2/mSOS and PI 3'-kinase to IRS-2 and pp60. Glucose and IGF-I also induced Shc association with Grb2/mSOS. Glucose (3-18 mM) and IGF-I, independently of glucose, activated mitogen-activated protein kinase but this did not correlate with IGF-I-induced beta-cell proliferation. In contrast, p70(S6K) was activated with increasing glucose concentration (between 6 and 18 mM), and potentiated by IGF-I in the same glucose concentration range which correlated with INS-1 cell proliferation rate. Thus, glucose and IGF-I-induced beta-cell proliferation were mediated via a signaling mechanism that was facilitated by mitogen-activated protein kinase but dependent on IRS-mediated induction of PI 3'-kinase activity and downstream activation of p70(S6K). The glucose dependence of IGF-I mediated INS-1 cell proliferation emphasizes beta-cell signaling mechanisms are rather unique in being tightly linked to glycolytic metabolic flux.

Drug--DNA interactions

Significant progress has been made over the past few years in studies of drug-DNA interactions. Structure-based design strategies have yielded new DNA-binding agents with clinical promise. The hairpin polyamides represent the result of a design strategy with outstanding potential. One specific molecule of this class has now been proven to inhibit the expression of a specific gene in vivo. A new bisintercalating anthracycline antibiotic binds with high affinity to DNA, and appears to overcome a specific form of multidrug resistance. Progress in fundamental studies of drug binding to DNA continues, with detailed thermodynamic studies providing new insights into the forces that drive complex formation. New tools have been developed in order to characterize both the binding mode and the sequence specificity of drug binding to DNA, tools that will enable the fundamental aspects of these biologically important reactions to be understood in more detail.

Small-molecule-based strategies for controlling gene expression

A central goal in chemical biology is to gain control over biological pathways using small molecules, and the mRNA-synthesizing machinery is a particular important target. New advances in our understanding of transcriptional regulation suggests strategies to manipulate these pathways using small molecules.