Preclinical exploration of combined glucagon inhibition and liver-preferential insulin for treatment of diabetes using in vitro assays and rat and mouse models.

AIMS/HYPOTHESIS: Normalisation of blood glucose in individuals with diabetes is recommended to reduce development of diabetic complications. However, risk of severe hypoglycaemia with intensive insulin therapy is a major obstacle that prevents many individuals with diabetes from obtaining the recommended reduction in HbA1c. Inhibition of glucagon receptor signalling and liver-preferential insulin action have been shown individually to have beneficial effects in preclinical models and individuals with diabetes (i.e. improved glycaemic control), but also have effects that are potential safety risks (i.e. alpha cell hyperplasia in response to glucagon receptor antagonists and increased levels of liver triacylglycerols and plasma alanine aminotransferase activity in response to glucagon receptor antagonists and liver-preferential insulin). We hypothesised that a combination of glucagon inhibition and liver-preferential insulin action in a dual-acting molecule would widen the therapeutic window. By correcting two pathogenic mechanisms (dysregulated glucagon signalling and non-physiological distribution of conventional insulin administered s.c.), we hypothesised that lower doses of each component would be required to obtain sufficient reduction of hyperglycaemia, and that the undesirable effects that have previously been observed for monotreatment with glucagon antagonists and liver-preferential insulin could be avoided. METHODS: A dual-acting glucagon receptor inhibitor and liver-preferential insulin molecule was designed and tested in rodent models (normal rats, rats with streptozotocin-induced hyperglycaemia, db/db mice and mice with diet-induced obesity and streptozotocin-induced hyperglycaemia), allowing detailed characterisation of the pharmacokinetic and pharmacodynamic properties of the dual-acting molecule and relevant control compounds, as well as exploration of how the dual-acting molecule influenced glucagon-induced recovery and spontaneous recovery from acute hypoglycaemia. RESULTS: This molecule normalised blood glucose in diabetic models, and was markedly less prone to induce hypoglycaemia than conventional insulin treatment (approximately 4.6-fold less potent under hypoglycaemic conditions than under normoglycaemic conditions). However, compared to treatment with conventional long-acting insulin, this dual-acting molecule also increased triacylglycerol levels in the liver (approximately 60%), plasma alanine aminotransferase levels (approximately twofold) and alpha cell mass (approximately twofold). CONCLUSIONS/INTERPRETATION: While the dual-acting glucagon receptor inhibitor and liver-preferential insulin molecule showed markedly improved regulation of blood glucose, effects that are potential safety concerns persisted in the pharmacologically relevant dose range.

Activation of insulin receptors and IGF-1 receptors in COLO-205 colon cancer xenografts by insulin and insulin analogue X10 does not enhance growth under normo- or hypoglycaemic conditions.

AIMS/HYPOTHESIS: Recent studies with normal rats and mouse allograft models have reported that insulin and insulin analogues do not activate the IGF-1 receptor in vivo, and that this characteristic therefore cannot be responsible for the increased incidence of mammary tumours observed for the insulin analogue X10 in chronic toxicity studies with Sprague Dawley rats. This is in clear contrast to reports of insulin and insulin analogues in vitro. Clarification of this is important for understanding the mechanisms behind possible growth-promoting effects of insulin analogues, and will have implications for the development of novel insulin analogues. METHODS: We established a xenograft model in BALB/c nude mice with the human colon cancer cell line COLO-205, which expresses human insulin and IGF-1 receptors, and explored the acute and chronic effects of treatment with supra-pharmacological doses of human insulin, insulin analogue X10 and human IGF-1. With a novel antibody, acute IGF-1 receptor activation was also examined in various tissues from normal rats treated with human insulin, insulin analogue X10 or human IGF-1. Finally, the effects of pharmacologically relevant doses of human insulin and insulin analogue X10 on receptor activation and growth of COLO-205 xenograft were explored in BALB/c nude mice with alloxan-induced hyperglycaemia. RESULTS: In normal rats and in BALB/c nude mice bearing a COLO-205 cell xenograft, treatment with supra-pharmacological doses of human insulin, insulin analogue X10 or human IGF-1 resulted in activation of insulin receptors as well as IGF-1 receptors. Treatment of diabetic nude mice with pharmacologically relevant doses of human insulin or insulin analogue X10, which decreased blood glucose from hyperglycaemic levels to the normoglycaemic range, did not increase IGF-1 receptor activation. Furthermore, repeated treatment with supra-pharmacological as well as pharmacological doses of human insulin or insulin analogue X10 did not influence the growth of COLO-205 xenografts. CONCLUSIONS/INTERPRETATION: This study demonstrates that activation of IGF-1 receptors in cancer cells by insulin and insulin analogues cannot be considered as a purely in vitro phenomenon. It does occur in vivo in animal models, although only after treatment with supra-pharmacological doses. Furthermore, treatment with insulin or insulin analogue X10 did not influence the growth of COLO-205 xenografts under normo- or hypoglycaemic conditions. Further studies are needed before a conclusion can be reached on whether IGF-1 receptor activation by insulin analogues correlates with increased growth in vivo.

A framework for the in vitro evaluation of cancer-relevant molecular characteristics and mitogenic potency of insulin analogues.

Epidemiological and laboratory studies raise the possibility of a link between clinically prescribed insulin analogues and increased cancer risk. Accordingly, there is a regulatory mandate for cancer-related pre-clinical safety evaluation during insulin analogue development, but currently, there is no standardized framework for such in vitro evaluation. We tested human insulin; the super-mitogenic insulin, X10 and insulin-like growth factor I, in four cancer cell lines with a range of insulin-like growth factor-I receptor (IGF-IR)/IR (insulin receptor) ratios (HCT 116, HT-29, COLO 205 and MCF7) and related these to IGF-IR and IR expression in 17 human adenocarcinomas. All cell types were IR-A isoform dominant. We determined IGF-IR/IR signalling pathway endpoints in dose- and time-varying experiments, and performed mitogenic dose-response equivalent assays to derive EC50 values, and correlated these with IGF-IR/IR ratios. We superimposed relative EC50 values onto data from the literature in a meta-analysis. The IGF-IR/IR ratios varied from <1 to 12 in the selected cell lines; similar pattern ranges were observed in human adenocarcinomas. The three ligands demonstrated differential IR/IGF-IR and Akt phosphorylation, which correlated with cell-specific IGF-IR/IR ratios. Mitogenic profiles of X10 mimicked those for insulin-like growth factor I (IGF-I) and correlated with IGF-IR/IR ratios. The meta-analysis, adding data from five additional studies, supported the hypothesis that ligand mitogenic potency, relative to human insulin, increases with increasing cell-specific IGF-IR/IR ratio. This study established a framework for the in vitro evaluation of cancer-relevant bioassays for comparisons of insulin analogues, and specifically consolidated earlier studies that determination of the cell-specific IGF-IR/IR ratio is crucial for the interpretation of ranking relative biological activities.

Surface-expressed insulin receptors as well as IGF-I receptors both contribute to the mitogenic effects of human insulin and its analogues.

There is a medical need for new insulin analogues. Yet, molecular alterations to the insulin molecule can theoretically result in analogues with carcinogenic effects. Preclinical carcinogenicity risk assessment for insulin analogues rests to a large extent on mitogenicity assays in cell lines. We therefore optimized mitogenicity assay conditions for a panel of five cell lines. All cell lines expressed insulin receptors (IR), IGF-I receptors (IGF-IR) and hybrid receptors, and in all cell lines, insulin as well as the comparator compounds X10 and IGF-I caused phosphorylation of the IR as well as IGF-IR. Insulin exhibited mitogenicity EC(50) values in the single-digit nanomolar to picomolar range. We observed correlations across cell types between (i) mitogenic potency of insulin and IGF-IR/IR ratio, (ii) Akt phosphorylation and mitogenic potency and (iii) Akt phosphorylation and IR phosphorylation. Using siRNA-mediated knockdown of IR and IGF-IR, we observed that in HCT 116 cells the IR appeared dominant in driving the mitogenic response to insulin, whereas in MCF7 cells the IGF-IR appeared dominant in driving the mitogenic response to insulin. Together, our results show that the IR as well as IGF-IR may contribute to the mitogenic potency of insulin. While insulin was a more potent mitogen than IGF-I in cells expressing more IR than IGF-IR, the hyper-mitogenic insulin analogue X10 was a more potent mitogen than insulin across all cell types, supporting that the hyper-mitogenic effect of X10 involves the IR as well as the IGF-IR. These results are relevant for preclinical safety assessment of developmental insulin analogues.

In situ phosphorylation of Akt and ERK1/2 in rat mammary gland, colon, and liver following treatment with human insulin and IGF-1.

High doses of insulin and the insulin analog AspB10 have been reported to increase mammary tumor incidence in female rats likely via receptor-mediated mechanisms, possibly involving enhanced IGF-1 receptor activation. However, insulin and IGF-1 receptor functionality and intracellular signaling in the rat mammary gland in vivo is essentially unexplored. The authors investigated the effect of a single subcutaneous dose of 600 nmol/kg human insulin or IGF-1 on Akt and ERK1/2 phosphorylation in rat liver, colon, and mammary gland. Rat tissues were examined by Western blotting and immunohistochemistry by phosphorylation-specific antibodies. Insulin as well as IGF-1 caused Akt phosphorylation in mammary epithelial cells, with myoepithelial and basal epithelial cells being most sensitive. IGF-1 caused stronger Akt phosphorylation than insulin in mammary gland epithelial cells. Phosphorylation of ERK1/2 was not influenced by insulin or IGF-1. Rather, in liver and mammary gland P-ERK1/2 appeared to correlate with estrous cycling, supporting that ERK1/2 has important physiological roles in these two organs. In short, these findings supported that the rat mammary gland epithelium expresses functional insulin and IGF-1 receptors and that phosphorylation of Akt as well as ERK1/2 may be of value in understanding the effects of exogenous insulin in the rat mammary gland and colon.

Unique expression pattern of the three insulin receptor family members in the rat mammary gland: dominance of IGF-1R and IRR over the IR, and cyclical IGF-1R expression.

Supra-pharmacological doses of the insulin analog X10 (AspB10) increased the incidence of mammary tumors in female Sprague-Dawley rats in chronic toxicity studies, most likely via receptor-mediated mechanisms. However, little is known about the expression of the insulin receptor family in the rat mammary gland. Using laser micro-dissection, quantitative RT-PCR and immunohistochemistry, we examined the expression of IR (insulin receptor), IGF-1R (IGF-1 receptor), IRR (insulin receptor-related receptor), ERα (estrogen receptor alpha), ERß (estrogen receptor beta) and PR (progesteron receptor) in young, virgin, female Sprague-Dawley rats and compared to expression in reference organs. The mammary gland displayed the highest expression of IRR and IGF-1R. In contrast, low expression of IR transcripts was observed in the mammary gland tissue with expression of the IR-A isoform being 5-fold higher than the expression of the IR-B. By immunohistochemistry, expression of IR and IGF-1R was detected in all mammary gland epithelial cells. Expression of ERα and PR was comparable between mammary gland and ovary, whereas expression of ERß was lower in mammary gland than in the ovary. Finally, expression of IGF-1R and PR in the mammary gland varied during the estrous cycle. These findings are important for the understanding of carcinogenic effects of insulin analogs in the rat mammary gland, and relevant for development of refined short-term models for preclinical safety assessment of insulin analogs.

Increased insulin receptor binding and increased IGF-1 receptor binding are linked with increased growth of L6hIR cell xenografts in vivo.

Insulin analogue X10 has a higher mitogenic potency than native human insulin in vitro and supra-pharmacological doses of insulin X10 increased the incidence of mammary tumours in rats. Compared to native human insulin, insulin X10 has increased binding affinity to the insulin receptor and the IGF-1 receptor, but it is not known whether either or both characteristics are important for stimulation of cell proliferation in vivo. The aim of this study was to explore how increased binding affinity to the insulin receptor or the IGF-1 receptor contributes to stimulation of cell proliferation in vivo. A mouse xenograft model was established with rat L6 myoblast cells transfected with the human insulin receptor (L6hIR cells) and effects of supra-pharmacological doses of native human insulin, insulin X10 or novel insulin analogues with increased binding affinity to either the insulin receptor or the IGF-1 receptor were examined. Treatment with insulin X10 and insulin analogues with increased binding affinity to either the insulin receptor or the IGF-1 receptor increased growth of L6hIR cell xenografts significantly compared to native human insulin. Thus, increased binding affinity to the insulin receptor and the IGF-1 receptor are each independently linked to increased growth of L6hIR cell xenografts in vivo.

Intraintestinal and Parenteral Administration of an Insulin Analogue Leads to Comparable Activation of Signaling Downstream of the Insulin Receptor in the Small Intestine.

BACKGROUND: Oral delivery of insulin was recently demonstrated to have therapeutic relevance in patients with diabetes. Insulin receptors are expressed in the gastrointestinal tract and can be activated by insulin in the bloodstream, but it is not known if the large amount of insulin in the intestinal lumen required for sufficient oral delivery will induce a different effect. The aim of this study was to compare the acute effect in the intestine of insulin administered in the intestinal lumen with that of insulin administered by a parenteral route. METHOD: Intraintestinal (ii) injection in the mid-jejunum of anaesthetized rats with insulin analogue 106 (I106), formulated with the absorption-enhancer sodium caprate, was used as an animal model of oral insulin administration. As control treatment, rats were treated with I106 by iv infusion according to algorithms which precisely mimicked the pharmacokinetic and pharmacodynamic properties of ii administered I106. Several fold more I106 was administered by ii injection than by iv infusion. Phosphorylated Akt (Ser473) was used as indicator of insulin-stimulated acute effects in the intestine. RESULTS: Treatment with I106 resulted in activation of Akt in the intestine, with no significant difference between the effects of ii or iv administration. CONCLUSION: The results from this rat model of orally administered insulin indicate that the unabsorbed insulin in the intestinal lumen after oral administration will not result in an enhanced acute effect in the intestine.

A-769662 activates AMPK beta1-containing complexes but induces glucose uptake through a PI3-kinase-dependent pathway in mouse skeletal muscle.

5'-AMP-activated protein kinase (AMPK) regulates several aspects of metabolism. Recently, A-769662 was shown to activate AMPK in skeletal muscle. However, no biological effects of AMPK activation by A-769662 in this tissue have been reported. We hypothesized that A-769662 would increase glucose uptake in skeletal muscle. We studied incubated soleus and extensor digitorum longus (EDL) muscles from 129S6/sv and C57BL/6 mice. Glucose uptake increased only in soleus from 129S6/sv when concentrations of A-769662 were 500 microM (approximately 15%, P < 0.05) and 1 mM (approximately 60%, P < 0.01). AMPK beta1- but not beta2-containing complexes were dose dependently activated by A-769662 in muscles from both genotypes (approximately 100% at 200 microM and 300-600% at 1 mM). The discrepancy between the A-769662-induced AMPK activation pattern and stimulation of glucose uptake suggested that these effects were unrelated. A-769662 increased phosphorylation of Akt in both muscles from both genotypes, with phosphorylation of T308 being significantly higher in soleus than in EDL in 129S6/sv mice (P < 0.01). In soleus from 129S6/sv mice, insulin receptor substrate 1-associated phosphatidylinositol 3 (PI3)-kinase activity was markedly increased with A-769662, and Akt phosphorylation and glucose uptake were inhibited by wortmannin while phosphorylation of acetyl-CoA carboxylase (S227) was unaffected. Thus, A-769662 activates beta1-containing AMPK complexes in skeletal muscle but induces glucose uptake through a PI3-kinase-dependent pathway. Although development of A-769662 has constituted a step forward in the search for AMPK activators targeting specific AMPK trimers, our data suggest that in intact muscle, A-769662 has off-target effects. This may limit use of A-769662 to study the role of AMPK in skeletal muscle metabolism.

A novel high-affinity peptide antagonist to the insulin receptor.

In this publication we describe a peptide insulin receptor antagonist, S661, which is a single chain peptide of 43 amino acids. The affinity of S661 for the insulin receptor is comparable to that of insulin and the selectivity for the insulin receptor versus the IGF-1 receptor is higher than that of insulin itself. S661 is also an antagonist of the insulin receptor of other species such as pig and rat, and it also has considerable affinity for hybrid insulin/IGF-1 receptors. S661 completely inhibits insulin action, both in cellular assays and in vivo in rats. A biosynthetic version called S961 which is identical to S661 except for being a C-terminal acid seems to have properties indistinguishable from those of S661. These antagonists provide a useful research tool for unraveling biochemical mechanisms involving the insulin receptor and could form the basis for treatment of hypoglycemic conditions.

A Novel Strategy to Prevent Advanced Atherosclerosis and Lower Blood Glucose in a Mouse Model of Metabolic Syndrome.

Cardiovascular disease caused by atherosclerosis is the leading cause of mortality associated with type 2 diabetes and metabolic syndrome. Insulin therapy is often needed to improve glycemic control, but it does not clearly prevent atherosclerosis. Upon binding to the insulin receptor (IR), insulin activates distinct arms of downstream signaling. The IR-Akt arm is associated with blood glucose lowering and beneficial effects, whereas the IR-Erk arm might exert less desirable effects. We investigated whether selective activation of the IR-Akt arm, leaving the IR-Erk arm largely inactive, would result in protection from atherosclerosis in a mouse model of metabolic syndrome. The insulin mimetic peptide S597 lowered blood glucose and activated Akt in insulin target tissues, mimicking insulin's effects, but only weakly activated Erk and even prevented insulin-induced Erk activation. Strikingly, S597 retarded atherosclerotic lesion progression through a process associated with protection from leukocytosis, thereby reducing lesional accumulation of inflammatory Ly6Chi monocytes. S597-mediated protection from leukocytosis was accompanied by reduced numbers of the earliest bone marrow hematopoietic stem cells and reduced IR-Erk activity in hematopoietic stem cells. This study provides a conceptually novel treatment strategy for advanced atherosclerosis associated with metabolic syndrome and type 2 diabetes.

Caffeine-induced impairment of insulin action but not insulin signaling in human skeletal muscle is reduced by exercise.

We investigated the effects of caffeine ingestion on skeletal muscle glucose uptake, glycogen synthase (GS) activity, and insulin signaling intermediates during a 100-min euglycemic-hyperinsulinemic (100 microU/ml) clamp. On two occasions, seven men performed 1-h one-legged knee extensor exercise at 3 h before the clamp. Caffeine (5 mg/kg) or placebo was administered in a randomized, double-blind fashion 1 h before the clamp. During the clamp, whole-body glucose disposal was reduced (P < 0.05) in caffeine (37.5 +/- 3.1 micromol x min(-1) x kg(-1)) vs. placebo (54.1 +/- 2.9 micromol x min(-1) x kg(-1)). In accordance, the total area under the curve over 100 min (AUC(0--100 min)) for insulin-stimulated glucose uptake in caffeine was reduced (P < 0.05) by approximately 50% in rested and exercised muscle. Caffeine also reduced (P < 0.05) GS activity before and during insulin infusion in both legs. Exercise increased insulin sensitivity of leg glucose uptake in both caffeine and placebo. Insulin increased insulin receptor tyrosine kinase (IRTK), insulin receptor substrate 1-associated phosphatidylinositol (PI) 3-kinase activities, and Ser(473) phosphorylation of protein kinase B (PKB)/Akt significantly but similarly in rested and exercised legs. Furthermore, insulin significantly decreased glycogen synthase kinase-3alpha (GSK-3alpha) activity equally in both legs. Caffeine did not alter insulin signaling in either leg. Plasma epinephrine and muscle cAMP concentrations were increased in caffeine. We conclude that 1) caffeine impairs insulin-stimulated glucose uptake and GS activity in rested and exercised human skeletal muscle; 2) caffeine-induced impairment of insulin-stimulated muscle glucose uptake and downregulation of GS activity are not accompanied by alterations in IRTK, PI 3-kinase, PKB/Akt, or GSK-3alpha but may be associated with increases in epinephrine and intramuscular cAMP concentrations; and 3) exercise reduces the detrimental effects of caffeine on insulin action in muscle.

The alpha2-5'AMP-activated protein kinase is a site 2 glycogen synthase kinase in skeletal muscle and is responsive to glucose loading.

The 5'AMP-activated protein kinase (AMPK) is a potential antidiabetic drug target. Here we show that the pharmacological activation of AMPK by 5-aminoimidazole-1-beta-4-carboxamide ribofuranoside (AICAR) leads to inactivation of glycogen synthase (GS) and phosphorylation of GS at Ser 7 (site 2). In muscle of mice with targeted deletion of the alpha2-AMPK gene, phosphorylation of GS site 2 was decreased under basal conditions and unchanged by AICAR treatment. In contrast, in alpha1-AMPK knockout mice, the response to AICAR was normal. Fuel surplus (glucose loading) decreased AMPK activation by AICAR, but the phosphorylation of the downstream targets acetyl-CoA carboxylase-beta and GS was normal. Fractionation studies suggest that this suppression of AMPK activation was not a direct consequence of AMPK association with membranes or glycogen, because AMPK was phosphorylated to a greater extent in response to AICAR in the membrane/glycogen fraction than in the cytosolic fraction. Thus, the downstream action of AMPK in response to AICAR was unaffected by glucose loading, whereas the action of the kinase upstream of AMPK, as judged by AMPK phosphorylation, was decreased. The fact that alpha2-AMPK is a GS kinase that inactivates GS while simultaneously activating glucose transport suggests that a balanced view on the suitability for AMPK as an antidiabetic drug target should be taken.

Synthesis and in vitro characterization of 1-(4-aminofurazan-3-yl)-5-dialkylaminomethyl-1H-[1,2,3]triazole-4-carboxylic acid derivatives. A new class of selective GSK-3 inhibitors.

A novel class of GSK-3 inhibitors with favorable water solubility was identified in a HTS screen. SAR studies identified bioisosteric structural moieties in this class of compounds. The compounds were tested in a GSK-3 inhibition assay at 100 microM ATP giving IC(50)'s in the range from 0.1 to 10 microM. The compounds are ATP competitive inhibitors. They modulate glycogen metabolism and stimulate the accumulation of intracellular beta-catenin in whole cell assays with EC(50)'s in the range from 2 to 18 microM and 4.5-44 microM, respectively. For selected compounds, only a 10-fold lower potency was obtained in cellular assays compared to the potency obtained for inhibition of the isolated enzyme, reflecting a good cell permeability of this compound class. At 10 microM of test compound a 3-fold stimulation of the glycogen synthesis in rat soleus muscle was obtained compared to the level of glycogen synthesis observed at 0.2 nM insulin. This stimulation of glycogen synthesis is comparable to the maximal stimulation by insulin itself.

Systematic evaluation of the metabolic to mitogenic potency ratio for B10-substituted insulin analogues.

BACKGROUND: Insulin analogues comprising acidic amino acid substitutions at position B10 have previously been shown to display increased mitogenic potencies compared to human insulin and the underlying molecular mechanisms have been subject to much scrutiny and debate. However, B10 is still an attractive position for amino acid substitutions given its important role in hexamer formation. The aim of this study was to investigate the relationships between the receptor binding properties as well as the metabolic and mitogenic potencies of a series of insulin analogues with different amino acid substitutions at position B10 and to identify a B10-substituted insulin analogue without an increased mitogenic to metabolic potency ratio. METHODOLOGY/PRINCIPAL FINDINGS: A panel of ten singly-substituted B10 insulin analogues with different amino acid side chain characteristics were prepared and insulin receptor (both isoforms) and IGF-I receptor binding affinities using purified receptors, insulin receptor dissociation rates using BHK cells over-expressing the human insulin receptor, metabolic potencies by lipogenesis in isolated rat adipocytes, and mitogenic potencies using two different cell types predominantly expressing either the insulin or the IGF-I receptor were systematically investigated. Only analogues B10D and B10E with significantly increased insulin and IGF-I receptor affinities as well as decreased insulin receptor dissociation rates displayed enhanced mitogenic potencies in both cell types employed. For the remaining analogues with less pronounced changes in receptor affinities and insulin receptor dissociation rates, no apparent correlation between insulin receptor occupancy time and mitogenicity was observed. CONCLUSIONS/SIGNIFICANCE: Several B10-substituted insulin analogues devoid of disproportionate increases in mitogenic compared to metabolic potencies were identified. In the present study, receptor binding affinity rather than insulin receptor off-rate appears to be the major determinant of both metabolic and mitogenic potency. Our results also suggest that the increased mitogenic potency is attributable to both insulin and IGF-I receptor activation.

Molecular characterisation of long-acting insulin analogues in comparison with human insulin, IGF-1 and insulin X10.

AIMS/HYPOTHESIS: There is controversy with respect to molecular characteristics of insulin analogues. We report a series of experiments forming a comprehensive characterisation of the long acting insulin analogues, glargine and detemir, in comparison with human insulin, IGF-1, and the super-mitogenic insulin, X10. METHODS: We measured binding of ligands to membrane-bound and solubilised receptors, receptor activation and mitogenicity in a number of cell types. RESULTS: Detemir and glargine each displayed a balanced affinity for insulin receptor (IR) isoforms A and B. This was also true for X10, whereas IGF-1 had a higher affinity for IR-A than IR-B. X10 and glargine both exhibited a higher relative IGF-1R than IR binding affinity, whereas detemir displayed an IGF-1R:IR binding ratio of ≤ 1. Ligands with high relative IGF-1R affinity also had high affinity for IR/IGF-1R hybrid receptors. In general, the relative binding affinities of the analogues were reflected in their ability to phosphorylate the IR and IGF-1R. Detailed analysis revealed that X10, in contrast to the other ligands, seemed to evoke a preferential phosphorylation of juxtamembrane and kinase domain phosphorylation sites of the IR. Sustained phosphorylation was only observed from the IR after stimulation with X10, and after stimulation with IGF-1 from the IGF-1R. Both X10 and glargine showed an increased mitogenic potency compared to human insulin in cells expressing many IGF-1Rs, whereas only X10 showed increased mitogenicity in cells expressing many IRs. CONCLUSIONS: Detailed analysis of receptor binding, activation and in vitro mitogenicity indicated no molecular safety concern with detemir.

Dysregulation of glycogen synthase COOH- and NH2-terminal phosphorylation by insulin in obesity and type 2 diabetes mellitus.

CONTEXT: Insulin-stimulated glucose disposal is impaired in obesity and type 2 diabetes mellitus (T2DM) and is tightly linked to impaired skeletal muscle glucose uptake and storage. Impaired activation of glycogen synthase (GS) by insulin is a well-established defect in both obesity and T2DM, but the underlying mechanisms remain unclear. DESIGN AND PARTICIPANTS: Insulin action was investigated in a matched cohort of lean healthy, obese nondiabetic, and obese type 2 diabetic subjects by the euglycemic-hyperinsulinemic clamp technique combined with muscle biopsies. Activity, site-specific phosphorylation, and upstream signaling of GS were evaluated in skeletal muscle. RESULTS: GS activity correlated inversely with phosphorylation of GS site 2+2a and 3a. Insulin significantly decreased 2+2a phosphorylation in lean subjects only and induced a larger dephosphorylation at site 3 in lean compared with obese subjects. The exaggerated insulin resistance in T2DM compared with obese subjects was not reflected by differences in site 3 phosphorylation but was accompanied by a significantly higher site 1b phosphorylation during insulin stimulation. Hyperphosphorylation of another Ca(2+)/calmodulin-dependent kinase-II target, phospholamban-Thr17, was also evident in T2DM. Dephosphorylation of GS by phosphatase treatment fully restored GS activity in all groups. CONCLUSIONS: Dysregulation of GS phosphorylation plays a major role in impaired insulin regulation of GS in obesity and T2DM. In obesity, independent of T2DM, this is associated with impaired regulation of site 2+2a and likely site 3, whereas the exaggerated insulin resistance to activate GS in T2DM is linked to hyperphosphorylation of at least site 1b. Thus, T2DM per se seems unrelated to defects in the glycogen synthase kinase-3 regulation of GS.

Phosphorylation-dependent translocation of glycogen synthase to a novel structure during glycogen resynthesis.

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood. It is well accepted that the rate-limiting enzymes in glycogenesis and glycogenolysis are glycogen synthase (GS) and glycogen phosphorylase (GPh), respectively. Both enzymes are regulated by reversible phosphorylation and by allosteric effectors. However, evidence in the literature indicates that changes in muscle GS and GPh intracellular distribution may constitute a new regulatory mechanism of glycogen metabolism. Already in the 1960s, it was proposed that glycogen was present in dynamic cellular organelles that were termed glycosomas but no such cellular entities have ever been demonstrated. The aim of this study was to characterize muscle GS and GPh intracellular distribution and to identify possible translocation processes of both enzymes. Using in situ stimulation of rabbit tibialis anterior muscle, we show GS and GPh intracellular redistribution at the beginning of glycogen resynthesis after contraction-induced glycogen depletion. We identify a new "player," a new intracellular compartment involved in skeletal muscle glycogen metabolism. They are spherical structures that were not present in basal muscle, and we present evidence that indicate that they are products of actin cytoskeleton remodeling. Furthermore, for the first time, we show a phosphorylation-dependent intracellular distribution of GS. Here, we present evidence of a new regulatory mechanism of skeletal muscle glycogen metabolism based on glycogen enzyme intracellular compartmentalization.

AMP kinase activation ameliorates insulin resistance induced by free fatty acids in rat skeletal muscle.

We examined whether acute activation of 5'-AMP-activated protein kinase (AMPK) by 5'-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR) ameliorates insulin resistance in isolated rat skeletal muscle. Insulin resistance was induced in extensor digitorum longus (EDL) muscles by prolonged exposure to 1.6 mM palmitate, which inhibited insulin-stimulated glycogen synthesis to 51% of control after 5 h of incubation. Insulin-stimulated glucose transport was less affected (22% of control). The decrease in glycogen synthesis was accompanied by decreased glycogen synthase (GS) activity and increased GS phosphorylation. When including 2 mM AICAR in the last hour of the 5-h incubation with palmitate, the inhibitory effect of palmitate on insulin-stimulated glycogen synthesis and glucose transport was eliminated. This effect of AICAR was accompanied by activation of AMPK. Importantly, AMPK inhibition was able to prevent this effect. Neither treatment affected total glycogen content. However, glucose 6-phosphate was increased after inclusion of AICAR, indicating increased influx of glucose. No effect of AICAR on the inhibited insulin-stimulated GS activity or increased GS phosphorylation by palmitate could be detected. Thus the mechanism by which AMPK activation ameliorates the lipid-induced insulin resistance probably involves induction of compensatory mechanisms overriding the insulin resistance. Our results emphasize AMPK as a promising molecular target for treatment of insulin resistance.

Use of transgenic models to understand effects of exercise on glucose metabolism.

This review will focus on the achievements obtained using transgenic animals in our understanding of exercise-induced insulin sensitivity as well as the regulation of glycogen storage in skeletal muscle, including the effects of acute exercise and insulin on the major determinants: glycogen synthesis and glucose uptake.