Stimulatory Effect Of Insulin Research Articles

Anti-inflammatory and insulin secretory activity in experimental type-2 diabetic rats treated orally with magnesium.

Abstract Background Diabetes mellitus causes low-grade chronic inflammation which leads to the development of long-term complications. Oral magnesium (Mg) intake amongst other effects was reported to reduce the levels of inflammatory markers. This study investigated the anti-inflammatory and insulin secretory activities in experimental type-2 diabetic rats (n=32) orally treated with Mg. Methods Experimental type-2 diabetic rats were induced with high fat diet and alloxan (50 mg/kg, single i.p.) for over 10 weeks prior to the experimental procedures. Male Wistar rats were divided into 4 equal groups: control, untreated experimental diabetics, and experimental diabetics treated orally with either metformin (Met) (250 mg/kg), or Mg (250 mg/kg), respectively, for 14 days. The blood glucose (BG) levels were monitored before experimental induction of diabetes and thereafter on days 1, 7, 10, and 14, respectively. Serum insulin, C-reactive protein (CRP), interleukin-6 (IL-6), and lipid profile were assessed using laboratory kits while pancreatic beta cell function (BCF) and insulin resistance were estimated using homeostasis model assessment equations. Results Significant increase in the BG level was observed in all experimental diabetic groups on day 1 compared to controls. On day 14, BG, BCF, triglyceride, cholesterol, and low-density lipoprotein levels were increased while the high-density lipoprotein level was reduced in untreated diabetics compared to other groups. Insulin and insulin resistance were increased in all groups compared to control. Serum insulin and IL-6 were reduced while CRP was elevated in diabetic treated groups (Met and Mg) compared to untreated diabetics. Conclusions This study shows a hypoglycemic, lipid regulatory, insulin stimulatory, and anti-inflammatory effect of oral Mg treatment in experimental type-2 diabetic rats.

Acute exercise alters skeletal muscle mitochondrial respiration and H2O2 emission in response to hyperinsulinemic-euglycemic clamp in middle-aged obese men.

Obesity, sedentary lifestyle and aging are associated with mitochondrial dysfunction and impaired insulin sensitivity. Acute exercise increases insulin sensitivity in skeletal muscle; however, whether mitochondria are involved in these processes remains unclear. The aim of this study was to investigate the effects of insulin stimulation at rest and after acute exercise on skeletal muscle mitochondrial respiratory function (JO2) and hydrogen peroxide emission (JH2O2), and the associations with insulin sensitivity in obese, sedentary men. Nine men (means ± SD: 57 ± 6 years; BMI 33 ± 5 kg.m2) underwent hyperinsulinemic-euglycemic clamps in two separate trials 1–3 weeks apart: one under resting conditions, and another 1 hour after high-intensity exercise (4x4 min cycling at 95% HRpeak). Muscle biopsies were obtained at baseline, and pre/post clamp to measure JO2 with high-resolution respirometry and JH2O2 via Amplex UltraRed from permeabilized fibers. Post-exercise, both JO2 and JH2O2 during ADP stimulated state-3/OXPHOS respiration were lower compared to baseline (P<0.05), but not after subsequent insulin stimulation. JH2O2 was lower post-exercise and after subsequent insulin stimulation compared to insulin stimulation in the rest trial during succinate supported state-4/leak respiration (P<0.05). In contrast, JH2O2 increased during complex-I supported leak respiration with insulin after exercise compared with resting conditions (P<0.05). Resting insulin sensitivity and JH2O2 during complex-I leak respiration were positively correlated (r = 0.77, P<0.05). We conclude that in obese, older and sedentary men, acute exercise modifies skeletal muscle mitochondrial respiration and H2O2 emission responses to hyperinsulinemia in a respiratory state-specific manner, which may have implications for metabolic diseases involving insulin resistance.

Beneficial Effects of Tephrosia purpurea Ethanolic Seed Extract on Lipids and Membrane Bound Enzymes in Experimental Diabetic Rats

: Objective: Traditionally, Tephrosia purpurea Linn. (Fabaceae) is known to possess medicinal values in treating diabetes and other endocrine disorders. In this experiment, authors evaluated the beneficial role of Tephrosia purpurea Ethanolic seed extract standard (normal) versus streptozotocin induced diabetic male albino wistar rats and studied the membrane stabilizing effects of TpEt in diabetic male albino rats. Methods: Male albino wistar rats (n=30) were used and divided into five groups, each having six animals. Diabetes mellitus was induced by giving streptozotocin intraperitoneally and blood glucose of above 250 mg/dl was considered diabetes. Blood glucose, glycosylated hemoglobin (HbA1c), plasma insulin, lipid profile and activities of membrane bound enzymes in erythrocytes and tissues were estimated. Results: Increased concentrations of blood glucose, glycosylated hemoglobin, and decreased plasma insulin, and accompanied by alterations in lipid profile and membrane bound enzymes activities in erythrocytes and tissues were noticed in experimental diabetic rats. Upon oral management of TpEt (300mg/kg body weight) for 45 days, the blood glucose and other alerted parameters in diabetic rats showed significant improvements. Conclusion: This study clearly showed Tephrosia purpurea seeds to possess membrane stabilizing effect in experimental diabetic rats probably due to the presence of flavonoids and isoflavonoids which could be responsible for its anti-lipid peroxidative and insulin stimulatory effects. Further studies are needed to separate and depict bioactive principle from the seeds of the same plant.Key words: Blood glucose, Diabetes, Glycosylated hemoglobin (HbA1c), Lipid profile membrane bound enzymes, Plasma insulin, Streptozotocin (STZ), Tephrosia purpurea ethanolic seed extract (TpEt).

Nypa fruticans Wurmb. Vinegar's Aqueous Extract Stimulates Insulin Secretion and Exerts Hepatoprotective Effect on STZ-Induced Diabetic Rats.

Background: An aqueous extract (AE) of vinegar made from Nypa fruticans Wurmb. can improve postprandial glucose levels in normoglycaemic rats. The aim of this study was to evaluate its antihyperglycaemic activity further using in vivo and in vitro approaches. Methods: AE was administered to streptozotocin (STZ)-induced diabetic rats twice daily at three doses (1000, 500, and 250 mg/kg b.w.) for 12 days p.o. Several biochemical analyses and a histological study of the pancreas and liver were performed, accompanied by a cell culture assay. Results: As compared to diabetic control (DC), AE at the doses of 500 and 1000 mg/kg b.w. caused significant reduction (p < 0.05) of blood glucose, total cholesterol and triglycerides levels, with positive improvement of serum insulin levels. Interestingly, immunohistochemical staining of the pancreas suggested no β-cell regeneration, despite significant increase in insulin production. AE-treated groups, however, showed overall restoration of the hepatic histoarchitecture of STZ-induced liver damage, suggesting a possible hepatoprotective effect. The pancreatic effect of AE was further studied through RIN-5F cell culture, which revealed a positive stimulatory effect on insulin release at a basal glucose concentration (1.1 mM). Conclusion: Nypa fruticans Wurmb. vinegar’s aqueous extract exerts its antihyperglycaemic activity, at least in part, through insulin stimulatory and hepatoprotective effects.

Hormonal Regulation of Glycine Metabolism And Its Potential Role in Diabetes Susceptibility

Type II diabetes (T2D) is characterized by insulin resistance, β cell dysfunction and impaired glucose tolerance. Multiple large‐scale metabolomics studies have demonstrated that diabetes is accompanied by alterations in amino acid metabolism. Interestingly, in both humans and animal models, circulating glycine levels are strongly reduced in obesity, glucose intolerance and T2D. These observations identify glycine as an important biomarker of T2D, and raise the possibility that altered glycine metabolism may play an active role in the development of the disease. The goals of our study are to determine if altered glycine metabolism plays a role in the etiology of diabetes, and to characterize the regulatory pathways that control plasma glycine levels. Glycine cleavage system (GCS), a multimeric enzyme complex, represents the only biochemical route that degrades glycine in human body. Human genetic evidences indicate that loss‐of‐function mutations of GCS cause hyperglycinemia and neurodevelopmental disorders resulting in early death. Given the key role of GCS in control of circulating level of glycine, we hypothesize that during progression of T2D, GCS‐mediated glycine catabolism may be altered in main metabolic organs including liver. Here, we show that in hepatoma cell lines both insulin and glucocorticoids strongly stimulate transcription of glycine decarboxylase (GLDC) gene that encodes the rate‐limiting component of GCS. We also show that the insulin responsive transcription factor sterol‐regulatory element binding protein‐1c (SREBP‐1c) regulates GLDC gene transcription and likely mediates the stimulatory effect of insulin. SREBP‐1c is known to mediate insulin's stimulatory effect on lipogenesis genes even in the presence of hepatic insulin resistance. These observations strengthen a working model that insulin‐mediated stimulation of GLDC, even in insulin resistant states, increases the activity of the glycine cleavage system and contributes to the low levels of glycine observed in T2D.Support or Funding InformationMichigan Diabetes Research and Training Center Pilot/Feasibility Grant

The role of renal proximal tubule transport in the regulation of blood pressure.

The electrogenic sodium/bicarbonate cotransporter 1 (NBCe1) on the basolateral side of the renal proximal tubule plays a pivotal role in systemic acid-base homeostasis. Mutations in the gene encoding NBCe1 cause severe proximal renal tubular acidosis accompanied by other extrarenal symptoms. The proximal tubule reabsorbs most of the sodium filtered in the glomerulus, contributing to the regulation of plasma volume and blood pressure. NBCe1 and other sodium transporters in the proximal tubule are regulated by hormones, such as angiotensin II and insulin. Angiotensin II is probably the most important stimulator of sodium reabsorption. Proximal tubule AT1A receptor is crucial for the systemic pressor effect of angiotensin II. In rodents and rabbits, the effect on proximal tubule NBCe1 is biphasic; at low concentration, angiotensin II stimulates NBCe1 via PKC/cAMP/ERK, whereas at high concentration, it inhibits NBCe1 via NO/cGMP/cGKII. In contrast, in human proximal tubule, angiotensin II has a dose-dependent monophasic stimulatory effect via NO/cGMP/ERK. Insulin stimulates the proximal tubule sodium transport, which is IRS2-dependent. We found that in insulin resistance and overt diabetic nephropathy, stimulatory effect of insulin on proximal tubule transport was preserved. Our results suggest that the preserved stimulation of the proximal tubule enhances sodium reabsorption, contributing to the pathogenesis of hypertension with metabolic syndrome. We describe recent findings regarding the role of proximal tubule transport in the regulation of blood pressure, focusing on the effects of angiotensin II and insulin.

Tamoxifen resistance and metastasis of human breast cancer cells were mediated by the membrane-associated estrogen receptor ER-α36 signaling in vitro.

The drug resistance and tumor metastasis have been the main obstacles for the longer-term therapeutic effects of tamoxifen (TAM) on estrogen receptor-positive (ER+) breast cancer, but the mechanisms underlying the TAM resistance are still unclear. Here, we demonstrated that the membrane-associated estrogen receptor ER-α36 signaling, but not the G protein-coupled estrogen receptor 1 (GPER1) signaling, might be involved in the TAM resistance and metastasis of breast cancer cells. In this study, a model of ER+ breast cancer cell MCF-7 that involves the up-regulated expression of ER-α36 and unchanged expression of ER-α66 and GPER1 was established via the removal of insulin from the cell culture medium. The mechanism of TAM resistance in the ER+ breast cancer cell line MCF-7 was investigated, and the results showed that the stimulating effect of insulin on susceptibility of MCF-7 to TAM was mediated by ER-α36 and that the expression level of ER-α36 in TAM-resistant MCF-7 cells was also significantly increased. Both TAM and estradiol (E2) could promote the migration of triple negative (ER-α66-/PR-/HER2-) and ER-α36+/GPER1+ breast cancer cells MDA-MB-231. The migration of MDA-MB-231 cells was inhibited by the down-regulated intracellular expression of ER-α36 by transient transfection of specific small interfering RNA, whereas no effect of GPER1 down-regulation was observed. Meanwhile, the effect of TAM on the migration of ER-α36-down-regulated MDA-MB-231 cells was also reduced. Furthermore, it was found that TAM enhanced the distribution of integrin β1 on the cell surface but did not affect the expression of integrin β1 in MDA-MB-231 cells. Collectively, these data suggested that ER-α36 signaling might play critical roles in acquired and de novo TAM resistance and metastasis of breast cancer, and ER-α36 might present a potential biomarker of TAM resistance in the clinical diagnosis and treatment of ER+ breast cancer.

SIRT1-AMPK crosstalk is involved in high glucose-dependent impairment of insulin responsiveness in primary rat podocytes

Growing evidence indicates that in diabetes, high glucose concentrations affect podocyte metabolism and function. The crucial pathological feature of type 2 diabetes mellitus and metabolic syndrome is insulin resistance, often developed as a result of dysregulation of nutrient-responsible systems and disturbance of cellular homeostasis under diabetic conditions. Here, we report the involvement of the reciprocal interplay between deacetylase SIRT1 and protein kinase AMPK in podocyte high glucose–induced abolition of insulin-dependent glucose uptake, manifesting insulin resistance. Experiments were performed on primary rat podocytes cultured in standard or high glucose conditions. Immunodetection methods were used to determine SIRT1 protein level and AMPK phosphorylation degree. Insulin-stimulated changes in glucose uptake were used to determine podocyte responsiveness to insulin. SIRT1 activity was modulated by resveratrol, EX-527, or small interfering RNA targeting SIRT1. We have demonstrated that the absence of the stimulating effect of insulin on glucose uptake into primary rat podocytes after long-time exposition to high glucose concentrations, is a result of decreased SIRT1 protein levels and activity, associated with decreased AMPK phosphorylation degree, presumably underlying the induction of insulin resistance. Our findings suggest that the interplay between SIRT1 and AMPK is involved in the regulation of insulin action in podocytes.

Human Skeletal Muscle Glyoxalase‐1 Protein Expression is Attenuated with Type 2 Diabetes

INTRODUCTIONIndividuals with type 2 diabetes mellitus (T2DM) present with increased levels of advanced glycation endproducts (AGEs), which have been associated with progression and complications of the disease. AGEs are known to contribute to the disruption of insulin signaling in skeletal muscle and are primarily produced in vivo by the damaging glycolytic byproduct, methylglyoxal (MG). MG is effectively detoxified by the enzyme, glyoxalase‐1 (GLO1), whose protein expression has been previously shown to be inversely related to MG‐mediated protein‐damage and AGE formation. Despite the role of skeletal muscle insulin resistance in the pathogenesis of T2DM, GLO1 protein expression has not been fully elucidated in skeletal muscle. Additionally, the metabolic effects of insulin stimulation on GLO1 protein in human skeletal muscle has not been described.PURPOSETherefore, the purpose of this study was to examine skeletal muscle GLO1 protein content in individuals with T2DM compared to lean, healthy controls (LHC). Further, we sought to examine the effects of insulin stimulation on GLO1 protein expression through in vivo and in vitro approaches that included the hyperinsulinemic‐euglycemic clamp and primary human skeletal muscle cell culture experiments, respectively.METHODSSkeletal muscle biopsies from the vastus lateralis were obtained from 7 individuals with T2DM (age: 55 ± 4 yrs, BMI: 35.6 ± 2.4 kg/m2) and 10 LHCs (age: 27 ± 1 yrs, BMI: 22.3 ± 0.8 kg/m2) during basal and insulin stimulated states of the hyperinsulinemic (40mU/m2/min)‐euglycemic (5 mM) clamp. To further investigate the effect of insulin, primary human skeletal muscle cells were incubated with 300 pM insulin (a concentration similar to that experienced during the hyperinsulinemic‐euglycemic clamp) for 0, 30, and 180 mins. GLO1 protein content in human tissue and cell culture was assessed via western blot.RESULTSIndividuals with T2DM displayed a 5.6 fold reduction (p&lt;0.0001) in GLO1 compared to LHC at baseline. Clamp‐induced hyperinsulinemia had no significant effect on GLO1 protein expression in either group (p&gt;0.05). In cell culture, 30 minutes of insulin stimulation increased GLO1 protein expression by 37 % (p=0.0093). Interestingly, continued insulin stimulation for 180 mins returned GLO1 protein expression to levels comparable to baseline (0 min vs 180 min, p&gt;0.05).CONCLUSIONThese data suggest that the skeletal muscle of T2DM may be susceptible to MG‐mediated protein damage and accumulation of AGEs. Further, prolonged exposure to hyperinsulinemia may attenuate early insulin mediated increases in GLO1 expression profiles. Further study is need to elucidate the exact mechanisms of these observations.

Hypothalamic Phosphodiesterase 3B Pathway Mediates Anorectic and Body Weight-Reducing Effects of Insulin in Male Mice

Background: Insulin action in the hypothalamus plays a critical role in the regulation of energy homeostasis, yet the intracellular signaling mechanisms mediating insulin action are incompletely understood. Although phosphodiesterase 3B (PDE3B) mediates insulin action in the adipose tissue and it is highly expressed in the hypothalamic areas implicated in energy homeostasis, its role, if any, in mediating insulin action in the hypothalamus is unknown. We tested the hypothesis that insulin action in the hypothalamus is mediated by PDE3B. Methods: Using enzymatic assay, we examined the effects of peripheral or central administration of insulin on hypothalamic PDE3B activity in adult mice. Western blotting and immunohistochemistry also examined p-Akt and p-STAT3 levels in the hypothalamus. Effects of leptin on these parameters were also compared. We injected cilostamide, a PDE3 inhibitor, prior to central injection of insulin and examined the 12- to 24-hour food intake and 24-hour body weight. Finally, we examined the effect of cilostamide on insulin-induced proopiomelanocortin (Pomc), neurotensin (Nt), neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression in the hypothalamus by qPCR. Results: Peripheral or central injection of insulin significantly increased PDE3B activity in the hypothalamus in association with increased p-Akt levels but without any change in p-STAT3 levels. However, leptin-induced increase in PDE3B activity was associated with an increase in both p-Akt and p-STAT3 levels in the hypothalamus. Prior administration of cilostamide reversed the anorectic and body weight-reducing effects as well as stimulatory effect of insulin on hypothalamic Pomc mRNA levels. Insulin did not alter Nt, Npy and Agrp mRNA levels. Conclusion: Insulin induction of hypothalamic PDE3B activity and the reversal of the anorectic and body weight-reducing effects and stimulatory effect of insulin on hypothalamic Pomc gene expression by cilostamide suggest that activation of PDE3B is a novel mechanism of insulin signaling in the hypothalamus.

Super-resolution microscopy reveals the insulin-resistance-regulated reorganization of GLUT4 on plasma membranes.

GLUT4 (also known as SLC2A4) is essential for glucose uptake in skeletal muscles and adipocytes, which play central roles in whole-body glucose metabolism. Here, using direct stochastic optical reconstruction microscopy (dSTORM) to investigate the characteristics of plasma-membrane-fused GLUT4 at the single-molecule level, we have demonstrated that insulin and insulin resistance regulate the spatial organization of GLUT4 in adipocytes. Stimulation with insulin shifted the balance of GLUT4 on the plasma membrane toward a more dispersed configuration. In contrast, insulin resistance induced a more clustered distribution of GLUT4 and increased the mean number of molecules per cluster. Furthermore, our data demonstrate that the F5QQI motif and lipid rafts mediate the maintenance of GLUT4 clusters on the plasma membrane. Mutation of F5QQI (F5QQA-GLUT4) induced a more clustered distribution of GLUT4; moreover, destruction of lipid rafts in adipocytes expressing F5QQA-GLUT4 dramatically decreased the percentage of large clusters and the mean number of molecules per cluster. In conclusion, our data clarify the effects of insulin stimulation or insulin resistance on GLUT4 reorganization on the plasma membrane and reveal new pathogenic mechanisms of insulin resistance.

Insulin attenuates arsenic-induced neurite outgrowth impairments by activating the PI3K/Akt/SIRT1 signaling pathway

Arsenic neurotoxicity has a broad range of adverse effects on human health, which are induced in part by inhibition of neurite outgrowth. Insulin has been reported to promote neurite extension. The present study investigated whether insulin can protect neurons from impaired neurite outgrowth induced by arsenic, and examined the signaling pathway involved in this action. The study demonstrated that NaAsO2 caused inhibition of neurite outgrowth in differentiated SH-SY5Y cells indicating its neurotoxicity. This inhibitory effect of NaAsO2 was attenuated by insulin. It was found that blocking PI3K or Akt by selective inhibitors canceled the protective effect of insulin against NaAsO2-induced neurite outgrowth impairment suggesting the essential role of active PI3K and Akt in insulin’s protective action. Inhibition of GSK3, which mimics an effect of insulin stimulation, had no effect on the impairment of neurite outgrowth by NaAsO2 implying that the insulin protective action is probably not due to its mediation of GSK3 inhibition ability. Moreover, NaAsO2 decreased the Akt activity, as it caused reduction in Akt phosphorylation, and downregulated expression of SIRT1. Additionally, the reduction of these signals by NaAsO2 was attenuated by insulin. Taken together, these results show that insulin attenuates arsenic-induced neurite outgrowth impairment possibly via activation of PI3K/Akt/SIRT1 signaling, and arsenic may exert neurite outgrowth inhibition through a mechanism involving reduction of signaling molecules downstream from insulin, PI3K/Akt/SIRT1. Our findings raise the possibility of using insulin to combat arsenic neurotoxicity.

The Regulation of Lipid Deposition by Insulin in Goose Liver Cells Is Mediated by the PI3K-AKT-mTOR Signaling Pathway.

BackgroundWe previously showed that the fatty liver formations observed in overfed geese are accompanied by the activation of the PI3K-Akt-mTOR pathway and an increase in plasma insulin concentrations. Recent studies have suggested a crucial role for the PI3K-Akt-mTOR pathway in regulating lipid metabolism; therefore, we hypothesized that insulin affects goose hepatocellular lipid metabolism through the PI3K-Akt-mTOR signaling pathway.MethodsGoose primary hepatocytes were isolated and treated with serum-free media supplemented with PI3K-Akt-mTOR pathway inhibitors (LY294002, rapamycin, and NVP-BEZ235, respectively) and 50 or 150 nmol/L insulin.ResultsInsulin induced strong effects on lipid accumulation as well as the mRNA and protein levels of genes involved in lipogenesis, fatty acid oxidation, and VLDL-TG assembly and secretion in primary goose hepatocytes. The stimulatory effect of insulin on lipogenesis was significantly decreased by treatment with PI3K-Akt-mTOR inhibitors. These inhibitors also rescued the insulin-induced down-regulation of fatty acid oxidation and VLDL-TG assembly and secretion.ConclusionThese findings suggest that the stimulatory effect of insulin on lipid deposition is mediated by PI3K-Akt-mTOR regulation of lipogenesis, fatty acid oxidation, and VLDL-TG assembly and secretion in goose hepatocytes.

Dysregulation of muscle glycogen synthase in recovery from exercise in type 2 diabetes.

Insulin and exercise stimulate skeletal muscle glycogen synthase (GS) activity by dephosphorylation and changes in kinetic properties. The aim of this study was to investigate the effects of insulin, exercise and post-exercise insulin stimulation on GS phosphorylation, activity and substrate affinity in obesity and type 2 diabetes. Obese men with type 2 diabetes (n = 13) and weight-matched controls (n = 14) underwent euglycaemic-hyperinsulinaemic clamps in the rested state and 3 h after 60 min of cycling (70% maximal pulmonary oxygen uptake [VO2max]). Biopsies from vastus lateralis muscle were obtained before and after clamps, and before and immediately after exercise. Insulin-stimulated glucose uptake was lower in diabetic patients vs obese controls with or without prior exercise. Post exercise, glucose partitioning shifted away from oxidation and towards storage in both groups. Insulin and, more potently, exercise increased GS activity (fractional velocity [FV]) and substrate affinity in both groups. Both stimuli caused dephosphorylation of GS at sites 3a + 3b, with exercise additionally decreasing phosphorylation at sites 2 + 2a. In both groups, changes in GS activity, substrate affinity and dephosphorylation at sites 3a + 3b by exercise were sustained 3 h post exercise and further enhanced by insulin. Post exercise, reduced GS activity and substrate affinity as well as increased phosphorylation at sites 2 + 2a were found in diabetic patients vs obese controls. Exercise-induced activation of muscle GS in obesity and type 2 diabetes involves dephosphorylation of GS at sites 3a + 3b and 2 + 2a and enhanced substrate affinity, which is likely to facilitate glucose partitioning towards storage. Lower GS activity and increased phosphorylation at sites 2 + 2a in type 2 diabetes in the recovery period imply an impaired response to exercise.

Stimulatory effect of insulin on renal proximal tubule sodium transport is preserved in type 2 diabetes with nephropathy

Our previous study indicates that hyperinsulinemia in metabolic syndrome in the absence of nephropathy may promote hypertension by stimulating renal proximal tubule (PT) sodium transport via insulin receptor substrate (IRS) 2/phosphoinositide 3-kinase pathway. In the present study we showed that the stimulatory effect of insulin on the Na+-HCO3- cotransporter NBCe1 in isolated PTs was completely preserved in type 2 diabetic rats with overt nephropathy. Furthermore, the IRS2 expression and insulin-induced Akt phosphorylation in kidney cortex were preserved in these rats. By contrast, the IRS1 expression in kidney cortex was markedly reduced, which might be relevant to enhanced renal gluconeogenesis consistently reported in diabetes. The stimulatory effect of insulin on NBCe1 was preserved also in a human type 2 diabetic patient with advanced nephropathy. These results revealed that insulin can stimulate PT sodium transport even in type 2 diabetes with overt nephropathy. In addition to hypoglycemia, insulin-induced renal sodium retention might also play a role in increased cardiovascular risk associated with intensive glycemic control in type 2 diabetic patients with nephropathy.

Preserved Na/HCO3 cotransporter sensitivity to insulin may promote hypertension in metabolic syndrome

Hyperinsulinemia can contribute to hypertension through effects on sodium transport. To test whether the stimulatory effect of insulin on renal proximal tubule sodium transport is preserved in insulin resistance, we compared the effects of insulin on abdominal adipocytes and proximal tubules in rats and humans. Insulin markedly stimulated the sodium-bicarbonate cotransporter (NBCe1) activity in isolated proximal tubules through the phosphoinositide 3-kinase (PI3-K) pathway. Gene silencing in rats showed that while insulin receptor substrate (IRS)1 mediates the insulin effect on glucose uptake into adipocytes, IRS2 mediates the insulin effect on proximal tubule transport. The stimulatory effect of insulin on glucose uptake into adipocytes was severely reduced, but its stimulatory effect on NBCe1 activity was completely preserved in insulin-resistant Otsuka Long-Evans Tokushima Fatty (OLETF) rats and patients with insulin resistance. Despite widespread reduction of IRS1 and IRS2 expression in insulin-sensitive tissues, IRS2 expression in the kidney cortex was exceptionally preserved in both OLETF rats and patients with insulin resistance. Unlike liver, acute insulin injection failed to change the expression levels of IRS2 and sterol regulatory element-binding protein 1 in rat kidney cortex, indicating that regulatory mechanisms of IRS2 expression are distinct in liver and kidney. Thus, preserved stimulation of proximal tubule transport through the insulin/IRS2/PI3-K pathway may play an important role in the pathogenesis of hypertension associated with metabolic syndrome.

Insulin resistance and hypertension: new insights

Insulin resistance is associated with hypertension. Nakamura et al. demonstrate in rodents and humans with insulin resistance that while the stimulatory effect of insulin on glucose uptake in adipocytes, mediated via insulin receptor substrate 1 (IRS1), was severely diminished, its effect on salt reabsorption in the kidney proximal tubule, mediated via IRS2, was preserved. Compensatory hyperinsulinemia in individuals with insulin resistance may enhance salt absorption in the proximal tubule, resulting in a state of salt overload and hypertension.

The effect of insulin on circulating PCSK9 in postmenopausal obese women

BackgroundPCSK9 (proprotein convertase subtilisin/kexin type 9) promotes the degradation of the LDLR (LDL receptor) in hepatocytes, leading to an increase in plasma LDL-C (LDL cholesterol). Previous animal studies have shown that insulin stimulates PCSK9 transcription and observational human studies showed a positive correlation between plasma PCSK9 concentration and fasting insulinemia. ObjectiveThe purpose of this study was to investigate the effects of chronic and acute hyperinsulinemia on PCSK9 in a large cohort of human subjects as well as at a cellular level. MethodsThe in vivo effect of hyperinsulinemia on plasma PCSK9 concentration was studied using euglycemic–hyperinsulinemic clamps in 82 non-diabetic post-menopausal obese patients. We studied the in vitro effects of insulin stimulation on PCSK9 mRNA as well as on protein expression and secretion in HepG2 and Huh7 cells. ResultsAnalysis of the pre and post-clamp data revealed a 15.4% (p<0.001) lowering of plasma PCSK9 concentration after acute insulin induction. In vitro studies post-insulin stimulation showed that mRNA levels of PCSK9 reduced by 25% in HepG2 cells (p<0.027) and by 59% in Huh7 cells (p<0.01). Intracellular concentration of PCSK9 were 10% lower in HepG2 cells (p<0.05) and 35% lower in Huh7 cells (p<0.05). ConclusionsOur results show an inhibitory effect of acute hyperinsulinemia on PCSK9 in humans both in vitro and in vivo. This data may assist in evaluating PCSK9 levels in individuals on insulin therapy.

Rapid elevation of sodium transport through insulin is mediated by AKT in alveolar cells.

Alveolar fluid clearance is driven by vectorial Na+ transport and promotes postnatal lung adaptation. The effect of insulin on alveolar epithelial Na+ transport was studied in isolated alveolar cells from 18–19‐day gestational age rat fetuses. Equivalent short‐circuit currents (ISC) were measured in Ussing chambers and different kinase inhibitors were used to determine the pathway of insulin stimulation. In Western Blot measurements the activation of mediators stimulated by insulin was analyzed. The I SC showed a fast dose‐dependent increase by insulin, which could be attributed to an increased ENaC (epithelial Na+ channel) activity in experiments with permeabilized apical or basolateral membrane. 5‐(N‐Ethyl‐N‐isopropyl)amiloride inhibition of ISC was not affected, however, benzamil‐sensitive ISC was increased in insulin‐stimulated monolayers. The application of LY‐294002 and Akti1/2 both completely blocked the stimulating effect of insulin on I SC. PP242 partly blocked the effect of insulin, whereas Rapamycin evoked no inhibition. Western Blot measurements revealed an increased phosphorylation of AKT after insulin stimulation. SGK1 activity was also increased by insulin as shown by Western Blot of pNDRG1. However, in Ussing chamber measurements, GSK650394, an inhibitor of SGK1 did not prevent the increase in ISC induced by insulin. The application of IGF‐1 mimicked the effect of insulin and increased the ENaC activity. In addition, an increased autophosphorylation of the IGF‐1R/IR was observed after insulin stimulation. We conclude that insulin rapidly increases epithelial Na+ transport by enhancing the activity of endogenous ENaC through activation of PI3K/AKT in alveolar cells.

Evaluation of six plant species used traditionally in the treatment and control of diabetes mellitus in South Africa using in vitro methods

Context: Numerous plants are used by the local communities of South Africa for the treatment and management of type II diabetes.Objectives: For this study, we undertook a survey of the plants sold for the management of diabetes in the town of Newcastle, South Africa. Identified plants were subsequently evaluated for their in vitro antidiabetic activity.Materials and methods: Plants were identified through an interview with a herbalist at the market. Antidiabetic activity of extracts of purchased plants was evaluated using in vitro α-amylase and α-glucosidase activity, as well as islets of Langerhans excretory activity.Results: Senna alexandrina Mill. (Fabaceae), Cymbopogon citrates Stapf. (Poaceae), Cucurbita pepo L. (Cucuribitaceae), Nuxia floribunda Benth. (Stilbaceae), Hypoxis hemerocallidea Fisch. and Mey (Hypoxidaceae), and Cinnamomum cassia Blume (Lauraceae) were identified. The hexane extract of S. alexandrina (EC50 = 0.083 mg/ml), ethyl acetate extract of H. hemerocallidea (EC50 = 0.29 mg/ml), and methanol extracts of Cymbopogon citratus (EC50 = 0.31 mg/ml) and Cinnamomum cassia (EC50 = 0.12 mg/ml) had the highest α-amylase inhibitory activity, albeit lower than acarbose (EC50 = 0.50 mg/ml). All the plants had good α-glucosidase inhibitory activity (>50%) with the exception of some methanol (Cinnamomum cassia, N. floribunda, and Cymbopogon citratus) and acetone extracts (Cucurbita pepo and N. floribunda). Only the H. hemerocallidea acetone extract had an insulin stimulatory effect (2.5 U/ml at 8 μg/ml).Conclusion: All the evaluated plants demonstrated inhibitory activity against the specific GIT enzyme systems evaluated. Only H. hemerocallidea had insulin secretory activity, adding evidence to the traditional use of these purchased plants in the management of the type II diabetic post-prandial hyperglycemia.