Regulation of insulin signal transduction pathway by a small-molecule insulin receptor activator

Current Insights on the Use of Insulin and the Potential Use of Insulin Mimetics in Targeting Insulin Signalling in Alzheimer's Disease

Alzheimer's disease (AD) and type 2 diabetes (T2D) are chronic diseases that share several pathological mechanisms, including insulin resistance and impaired insulin signalling. Their shared features have prompted the evaluation of the drugs used to manage diabetes for the treatment of AD. Insulin delivery itself has been utilized, with promising effects, in improving cognition and reducing AD related neuropathology. The most recent clinical trial involving intranasal insulin reported no slowing of cognitive decline; however, several factors may have impacted the trial outcomes. Long-acting and rapid-acting insulin analogues have also been evaluated within the context of AD with a lack of consistent outcomes. This narrative review provided insight into how targeting insulin signalling in the brain has potential as a therapeutic target for AD and provided a detailed update on the efficacy of insulin, its analogues and the outcomes of human clinical trials. We also discussed the current evidence that warrants the further investigation of the use of the mimetics of insulin for AD. These small molecules may provide a modifiable alternative to insulin, aiding in developing drugs that selectively target insulin signalling in the brain with the aim to attenuate cognitive dysfunction and AD pathologies.

Structure-based survey of ligand binding in the human insulin receptor

The insulin receptor is a membrane protein responsible for regulation of nutrient balance and therefore an attractive target in the treatment of diabetes and metabolic syndrome. Pharmacology of the insulin receptor involves two distinct mechanisms, (1) activation of the receptor by insulin mimetics that bind in the extracellular domain and (2) inhibition of the receptor tyrosine kinase enzymatic activity in the cytoplasmic domain. While a complete structural picture of the full-length receptor comprising the entire sequence covering extracellular, transmembrane, juxtamembrane and cytoplasmic domains is still elusive, recent progress through cryoelectron microscopy has made it possible to describe the initial insulin ligand binding events at atomistic detail. We utilize this opportunity to obtain structural insights into the pharmacology of the insulin receptor. To this end, we conducted a comprehensive docking study of known ligands to the new structures of the receptor. Through this approach, we provide an in-depth, structure-based review of human insulin receptor pharmacology in light of the new structures.

Developing Insulin and BDNF Mimetics for Diabetes Therapy

Diabetes is a global public health concern nowadays. The majority of diabetes mellitus (DM) patients belong to type 2 diabetes mellitus (T2DM), which is highly associated with obesity. The general principle of current therapeutic strategies for patients with T2DM mainly focuses on restoring cellular insulin response by potentiating the insulin-induced signaling pathway. In late-stage T2DM, impaired insulin production requires the patients to receive insulin replacement therapy for maintaining their glucose homeostasis. T2DM patients also demonstrate a drop of brain-derived neurotrophic factor (BDNF) in their circulation, which suggests that replenishing BDNF or enhancing its downstream signaling pathway may be beneficial. Because of their protein nature, recombinant insulin or BDNF possess several limitations that hinder their clinical application in T2DM treatment. Thus, developing orally active "insulin pill" or "BDNF pill" is essential to provide a more convenient and effective therapy. This article reviews the current development of non-peptidyl chemicals that mimic insulin or BDNF and their potential as anti-diabetic agents.

Glucose consumption assay discovers coptisine with beneficial effect on diabetic mice

Many drugs with anti-diabetic effects regulate glucose consumption in peripheral tissues. Via cellular glucose consumption assays, we identified that coptisine, a main effective constituent from the plant Coptis chinensis, enhanced hepatic and skeletal muscle glucose consumption. We further explored its effects on glucose metabolism in diabetic animals to elucidate its mechanism of action. Our results showed that coptisine did not show cytotoxicity. Intragastric administration of coptisine for ten days in normal ICR mice markedly decreased fasting blood-glucose levels without significant effects on body weight. In alloxan-induced type 1 diabetic mice, intragastric administration of coptisine for 28 days decreased fasting and non-fasting blood-glucose levels as well. In type 2 diabetic KKAy mice, intragastric administration of coptisine for nine weeks improved glucose tolerance. It decreased fasting/non-fasting blood-glucose and fructosamine levels. Coptisine decreased low-density lipoprotein and total cholesterol levels, however, had no significant effect on triglyceride levels. Coptisine increased AMPK phosphorylation while decreasing Akt phosphorylation in HepG2 hepatic cells and C2C12 myotubes. Coptisine also reduced mitochondrial respiration in isolated and cellular mitochondria, suggesting that coptisine lowered cellular energy levels. In particularly, coptisine administration (10^-6 M) decreased the mitochondrial oxygen consumption rate (OCR) with a greater extracellular acidification rate (ECAR), resulting in an oxidative-to-glycolysis phosphorylation shifted for cellular energy generation. Our results demonstrate that coptisine acts as an enhancer of peripheral glucose consumption could improve glucose metabolism in diabetic animals. Coptisine may serve as a novel anti-diabetic agent and warrant further evaluation.

Structure and dynamics of the insulin receptor: implications for receptor activation and drug discovery

Recently, major progress has been made in uncovering the mechanisms of how insulin engages its receptor and modulates downstream signal transduction. Here, we present in detail the current structural knowledge surrounding the individual components of the complex, binding sites, and dynamics during the activation process. A novel kinase triggering mechanism, the 'bow-arrow model', is proposed based on current knowledge and computational simulations of this system, in which insulin, after its initial interaction with binding site 1, engages with site 2 between the fibronectin type III (FnIII)-1 and -2 domains, which changes the conformation of FnIII-3 and eventually translates into structural changes across the membrane. This model provides a new perspective on the process of insulin binding to its receptor and, thus, could lead to future novel drug discovery efforts.

Human biliverdin reductase-based peptides activate and inhibit glucose uptake through direct interaction with the kinase domain of insulin receptor

Insulin binding changes conformation of the insulin receptor kinase (IRK) domain and initiates glucose uptake through the insulin, IGF-1, phosphatidyl inositol 3-kinase (PI3K), and MAPK pathways; human biliverdin reductase (hBVR) is an IRK substrate and pathway effector. This is the first report on hBVR peptide-mediated IRK activation and conformational change. (290)KYCCSRK, which increased IRK V(max) without changing K(m), stimulated glucose uptake and potentiated insulin and IGF-1 stimulation in 4 cell lines. KYCCSRK in native hBVR was necessary for the hBVR and IRK cross-activation. Peptide treatment also activated PI3K downstream effectors, Akt and ERK, phosphorylation, and Elk transcriptional activity. In cells transfected with CMV-regulated EGFP-VP-peptide plasmid, C(292)→A mutant did not stimulate glucose uptake; K(296)→A decreased uptake and kinase activity. KEDQYMKMTV, corresponding to hBVR's SH2-binding domain, was a potent inhibitor of glucose uptake and IRK. The mechanism of action of peptides was examined using cells expressing IRK (aa 988-1263) activated by coexpressed KYCCSRK. Three active cys-mutants of IRK, with fluorophore coupled to cysteines, C(1056), C(1138), or C(1234), were examined for changes in fluorescence emission spectra in the presence of peptides. KYCCSRK and KEDQYMKMTV bound to different sites in IRK. The findings identify novel agents for activating or inhibiting insulin signaling and offer a new approach for treatment of type 2 diabetes and hypoglycemia.

Body composition and gene expression QTL mapping in mice reveals imprinting and interaction effects

BACKGROUND

Shifts in body composition, such as accumulation of body fat, can be a symptom of many chronic human diseases; hence, efforts have been made to investigate the genetic mechanisms that underlie body composition. For example, a few quantitative trait loci (QTL) have been discovered using genome-wide association studies, which will eventually lead to the discovery of causal mutations that are associated with tissue traits. Although some body composition QTL have been identified in mice, limited research has been focused on the imprinting and interaction effects that are involved in these traits. Previously, we found that Myostatin genotype, reciprocal cross, and sex interacted with numerous chromosomal regions to affect growth traits.

RESULTS

Here, we report on the identification of muscle, adipose, and morphometric phenotypic QTL (pQTL), translation and transcription QTL (tQTL) and expression QTL (eQTL) by applying a QTL model with additive, dominance, imprinting, and interaction effects. Using an F2 population of 1000 mice derived from the Myostatin-null C57BL/6 and M16i mouse lines, six imprinted pQTL were discovered on chromosomes 6, 9, 10, 11, and 18. We also identified two IGF1 and two Atp2a2 eQTL, which could be important trans-regulatory elements. pQTL, tQTL and eQTL that interacted with Myostatin, reciprocal cross, and sex were detected as well. Combining with the additive and dominance effect, these variants accounted for a large amount of phenotypic variation in this study.

CONCLUSIONS

Our study indicates that both imprinting and interaction effects are important components of the genetic model of body composition traits. Furthermore, the integration of eQTL and traditional QTL mapping may help to explain more phenotypic variation than either alone, thereby uncovering more molecular details of how tissue traits are regulated.

The response of glucose-excited neurones in the ventromedial hypothalamus to decreased glucose is enhanced in a murine model of type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) are associated with dysfunctional insulin signalling and impaired central glucose sensing. Glucose sensing neurones reside in key areas of the brain involved in glucose and energy homeostasis (e.g. ventromedial hypothalamus; VMH). We have recently shown that insulin attenuates the ability of glucose-excited (GE) neurones to sense decreased glucose. We hypothesise that this effect of insulin on VMH GE neurones is impaired during T2DM when insulin signalling is dysfunctional. To test our hypotheses, we used whole cell patch clamp recording techniques to evaluate the effects of insulin on VMH GE neurones in brain slices from wild-type and diabetic (db/db) mice. The effects of decreasing glucose from 2.5 to 0.1 mM on VMH GE neurones were similar in wild-type and db/db mice. However, decreasing glucose from 2.5 to 0.5 mM decreased the action potential frequency, membrane potential and input resistance of VMH GE neurones to a significantly greater extent in db/db versus wild-type mice. Furthermore, insulin (5 nM) blunted the effects of decreased glucose in wild-type, but not db/db mice. These differences in both glucose and insulin sensitivity between wild-type and db/db mice were completely ameliorated by the insulin sensitiser, Compound 2 (300 nM). These data are consistent with our hypothesis that impaired insulin signalling in T2DM sensitises VMH GE neurones to decreased glucose.

A novel hydroxyfuroic acid compound as an insulin receptor activator. Structure and activity relationship of a prenylindole moiety to insulin receptor activation

Background

Diabetes Mellitus is a chronic disease and many patients of which require frequent subcutaneous insulin injection to maintain proper blood glucose levels. Due to the inconvenience of insulin administration, an orally active insulin replacement has long been a prime target for many pharmaceutical companies. Demethylasterriquinone (DMAQ) B1, extracted from tropical fungus, Pseudomassaria sp., has been reported to be an orally effective agent at lowering circulating glucose levels in diabetic (db/db) mice; however, the cytotoxicity associated with the quinone moiety has not been addressed thus far.

Methods

A series of hydroxyfuroic acid compounds were synthesized and tested for their efficacies at activating human insulin receptor. Cytotoxicity to Chinese hamster ovary cells, selectivities over insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), and fibroblast growth factor (FGF) receptors were examined in this study.

Result and Conclusion

This study reports a new non-quinone DMAQ B1 derivative, a hydroxyfuroic acid compound (D-410639), which is 128 fold less cytotoxic as DMAQ B1 and as potent as compound 2, a DMAQ B1 synthetic derivative from Merck, at activating human insulin receptor. D-410639 has little activation potential on IGF-1 receptor but is a moderate inhibitor to EGF receptor. Structure and activity relationship of the prenylindole moiety to insulin receptor activation is discussed.

Insulin-sensitizing activities of tanshinones, diterpene compounds of the root of Salvia miltiorrhiza Bunge

In this study, the effects of the extract and four tanshinone compounds from the dried root of Salvia miltiorrhiza Bunge (Labiatae) on the tyrosine phosphorylation of the insulin receptor (IR) beta-subunit and the downstream signaling were examined in Chinese-hamster ovary cells expressing human insulin receptors (CHO/IR cells) as well as in 3T3-L1 adipocytes. In addition the translocation of the glucose transporter 4 was investigated in 3T3-L1 adipocytes. Total extract of Danshen (1-10 microg/ml) and the four tanshinones (10 microM) did not show any activity, but the total extract and the tanshinone I, IIA and 15, 16-dihydrotanshinone I except cryptotanshinone enhanced the activity of insulin (1 nM) on the tyrosine phosphorylation of the IR as well as the activation of the downstream kinases Akt, ERK1/2, and GSK3beta. In the adipocytes the same IR-downstream signaling and the translocation of glucose transporter 4 were demonstrated by the three tanshinones in the presence of insulin. These insulin-sensitizing activities of tanshinones may be useful for developing a new class of specific IR activators as anti-diabetic agents.

Establishment of new highly insulin-sensitive cell lines and screening of compounds to facilitate glucose consumption

To obtain compounds that promote glucose uptake in muscle cells, the novel cell lines A31-IS derived from Balb/c 3T3 A31 and C2C12-IS from mouse myoblast C2C12 were established. In both cell lines, glucose consumption was induced by insulin and suppressed by the addition of Akt-activating kinase inhibitor. The A31-IS cells highly express the insulin receptor beta chains, Glut4, and uncoupling protein-3, as compared to the parent Balb/c 3T3 A31 cells, and C2C12-IS cells highly express the insulin receptor beta chain as compared to its parent cell line. Using A31-IS cells, we screened our library compounds and obtained three compounds, DF-4394, DF-4451, and DG-5451. These compounds dose-dependently promoted glucose consumption in A31-IS cells and facilitated [3H]-2-deoxyglucose uptake in differentiated C2C12-IS cells. The compounds that we obtained from the library screening will be good candidates for improving insulin resistance in muscle cells.

Identification of novel orally available small molecule insulin mimetics

Oral hypoglycemic agents have great potential for the treatment of both type 1 and type 2 diabetes. Here we report the identification of novel, small-molecule, insulin mimetics that activate the insulin receptor (IR) in vivo and in vitro, stimulate the Akt and extracellular signal-regulated kinase pathways downstream of the IR, and mimic the ability of insulin to stimulate glucose uptake, glycogen synthesis, and lipid synthesis in 3T3-L1 adipocytes. However, the compounds do not mimic the mitogenic effect of insulin. In animals, these compounds have oral hypoglycemic effects in both normal C57BL6 mice and diabetic db/db mice. Quantitative structure activity relationship modeling on data from a library of 60 compounds has highlighted structural features that are important for IR agonist activity that can be used to guide design of second and third generation compounds with greater potency and specificity.

Glyceraldehyde 3-phosphate dehydrogenase is a cellular target of the insulin mimic demethylasterriquinone B1

This study was undertaken to identify cellular proteins that bind an orally active natural product insulin mimic. Phage display cloning was used with a biotinylated derivative of this molecule as bait. Among the proteins identified was glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which has recently been shown to affect insulin receptor signaling. Binding data support a role for human GAPDH as another target of the insulin mimic, which could explain its action as a selective insulin receptor modulator.

Insulin-mimetic and insulin-sensitizing activities of a pentacyclic triterpenoid insulin receptor activator

Five pentacyclic triterpenoids isolated from Campsis grandiflora were tested for insulin-mimetic and insulin-sensitizing activity. The compounds enhanced the activity of insulin on tyrosine phosphorylation of the IR (insulin receptor) beta-subunit in CHO/IR (Chinese-hamster ovary cells expressing human IR). Among the compounds tested, CG7 (ursolic acid) showed the greatest enhancement and CG11 (myrianthic acid) the least. We characterized the effect of CG7 further, and showed that it acted as an effective insulin-mimetic agent at doses above 50 mug/ml and as an insulin-sensitizer at doses as low as 1 mug/ml. Additional experiments showed that CG7 increased the number of IRs that were activated by insulin. This indicates that a major mechanism by which CG7 enhances total IR auto-phosphorylation is by promoting the tyrosine phosphorylation of additional IRs. CG7 not only potentiated insulin-mediated signalling (tyrosine phosphorylation of the IR beta-subunit, phosphorylation of Akt and glycogen synthase kinase-3beta), but also enhanced the effect of insulin on translocation of glucose transporter 4 in a classical insulin-sensitive cell line, 3T3-L1 adipocytes. The results of the present study demonstrate that a specific pentacyclic triterpenoid, CG7, exerts an insulin-sensitizing effect as an IR activator in CHO/IR cells and adipocytes. The enhancement of insulin activity by CG7 may be useful for developing a new class of specific IR activators for treatment of Type 1 and Type 2 diabetes.

Rin1 regulates insulin receptor signal transduction pathways

Rin1 is a multifunctional protein containing several domains, including Ras binding and Rab5 GEF domains. The role of Rin1 in insulin receptor internalization and signaling was examined by expressing Rin1 and deletion mutants in cells utilizing a retrovirus system. Here, we show that insulin-receptor-mediated endocystosis and fluid phase insulin-stimulated endocytosis are enhanced in cells expressing the Rin1:wild type and the Rin1:C deletion mutant, which contain both the Rab5-GEF and GTP-bound Ras binding domains. However, the Rin1:N deletion mutant, which contains both the SH2 and proline-rich domains, blocked insulin-stimulated receptor-mediated and insulin-stimulated fluid phase endocytosis. In addition, the expression of Rin1:delta (429-490), a natural occurring splice variant, also blocked both receptor-mediated and fluid phase endocystosis. Furthermore, association of the Rin1 SH2 domain with the insulin receptor was dependent on tyrosine phosphorylation of the insulin receptor. Morphological analysis indicates that Rin1 co-localizes with insulin receptor both at the cell surface and in endosomes upon insulin stimulation. Interestingly, the expression of Rin1:wild type and both deletion mutants blocks the activation of Erk1/2 and Akt1 kinase activities without affecting either JN or p38 kinase activities. DNA synthesis and Elk-1 activation are also altered by the expression of Rin1:wild type and the Rin1:C deletion mutant. In contrast, the expression of Rin1:delta stimulates both Erk1/2 and Akt1 activation, DNA synthesis and Elk-1 activation. These results demonstrate that Rin1 plays an important role in both insulin receptor membrane trafficking and signaling.

Therapeutic Actions of an Insulin Receptor Activator and a Novel Peroxisome Proliferator-Activated Receptor γ Agonist in the Spontaneously Hypertensive Obese Rat Model of Metabolic Syndrome X

Insulin resistance clusters with hyperlipidemia, impaired glucose tolerance, and hypertension as metabolic syndrome X. We tested a low molecular weight insulin receptor activator, demethylasterriquinone B-1 (DMAQ-B1), and a novel indole peroxisome proliferator-activated receptor gamma agonist, 2-(2-(4-phenoxy-2-propylphenoxy)ethyl)indole-5-acetic acid (PPEIA), in spontaneously hypertensive obese rats (SHROB), a genetic model of syndrome X. Agents were given orally for 19 days. SHROB showed fasting normoglycemia but impaired glucose tolerance after an oral load, as shown by increased glucose area under the curve (AUC) [20,700 mg x min/ml versus 8100 in lean spontaneously hypertensive rats (SHR)]. Insulin resistance was indicated by 20-fold excess fasting insulin and increased insulin AUC (6300 ng x min/ml versus 990 in SHR). DMAQ-B1 did not affect glucose tolerance (glucose AUC = 21,300) but reduced fasting insulin 2-fold and insulin AUC (insulin AUC = 4300). PPEIA normalized glucose tolerance (glucose AUC = 9100) and reduced insulin AUC (to 3180) without affecting fasting insulin. PPEIA also increased food intake, fat mass, and body weight gain (81 +/- 12 versus 45 +/- 8 g in untreated controls), whereas DMAQ-B1 had no effect on body weight but reduced subscapular fat mass. PPEIA but not DMAQ-B1 reduced blood pressure. In skeletal muscle, insulin-stimulated phosphorylation of the insulin receptor and insulin receptor substrate protein 1-associated phosphatidylinositol 3-kinase activity were decreased by 40 to 55% in SHROB relative to lean SHR. PPEIA, but not DMAQ-B1, enhanced both insulin actions. SHROB also showed severe hypertriglyceridemia (355 +/- 42 mg/dl versus 65 +/- 3 in SHR) attenuated by both agents (DMAQ-B1, 228 +/- 18; PPEIA, 79 +/- 3). Both these novel antidiabetic agents attenuate insulin resistance and hypertriglyceridemia associated with metabolic syndrome but via distinct mechanisms.

A novel glucagon receptor antagonist inhibits glucagon-mediated biological effects

Glucagon maintains glucose homeostasis during the fasting state by promoting hepatic gluconeogenesis and glycogenolysis. Hyperglucagonemia and/or an elevated glucagon-to-insulin ratio have been reported in diabetic patients and animals. Antagonizing the glucagon receptor is expected to result in reduced hepatic glucose overproduction, leading to overall glycemic control. Here we report the discovery and characterization of compound 1 (Cpd 1), a compound that inhibits binding of 125I-labeled glucagon to the human glucagon receptor with a half-maximal inhibitory concentration value of 181 +/- 10 nmol/l. In CHO cells overexpressing the human glucagon receptor, Cpd 1 increased the half-maximal effect for glucagon stimulation of adenylyl cyclase with a KDB of 81 +/- 11 nmol/l. In addition, Cpd 1 blocked glucagon-mediated glycogenolysis in primary human hepatocytes. In contrast, a structurally related analog (Cpd 2) was not effective in blocking glucagon-mediated biological effects. Real-time measurement of glycogen synthesis and breakdown in perfused mouse liver showed that Cpd 1 is capable of blocking glucagon-induced glycogenolysis in a dosage-dependent manner. Finally, when dosed in humanized mice, Cpd 1 blocked the rise of glucose levels observed after intraperitoneal administration of exogenous glucagon. Taken together, these data suggest that Cpd 1 is a potent glucagon receptor antagonist that has the capability to block the effects of glucagon in vivo.

Small-molecule insulin mimetic reduces hyperglycemia and obesity in a nongenetic mouse model of type 2 diabetes

Adiposity positively correlates with insulin resistance and is a major risk factor of type 2 diabetes. Administration of exogenous insulin, which acts as an anabolic factor, facilitates adipogenesis. Recently nonpeptidal insulin receptor (IR) activators have been discovered. Here we evaluate the effects of the orally bioavailable small-molecule IR activator (Compound-2) on metabolic abnormalities associated with type 2 diabetes using a nongenetic mouse model in comparison with the effects of a novel non-thiazolidinedione (nTZD) peroxisome proliferator-activated receptor-gamma agonist. Both Compound-2 and nTZD alleviated fasting and postprandial hyperglycemia; accelerated glucose clearance rate; and normalized plasma levels of nonesterified fatty acids, triglycerides, and leptin. Unlike nTZD, which increased body weight gain, and total fat mass, which is a common feature for PPARgamma agonists, Compound-2 prevented body weight gain and hypertrophy of brown, and white adipose tissue depots and the development of hepatic steatosis in the mouse model of type 2 diabetes. The effect of the two compounds on proximal steps in insulin signal transduction pathway was analyzed in tissues. Compound-2 enhanced insulin-stimulated phosphorylation of IR tyrosine and/or Akt in the liver, skeletal muscle, and white adipose tissue, whereas nTZD potentiated the phosphorylation of IR and Akt in the adipose tissue only. In conclusion, small-molecule IR activators have unique features as insulin sensitizers and hold potential utility in the treatment of type 2 diabetes and obesity.