Angiotensin II exerts glucose-dependent effects on Kv currents in mouse pancreatic beta-cells via angiotensin II type 2 receptors

The renin-angiotensin system as a target to solve the riddle of endocrine pancreas homeostasis

Local renin-angiotensin system (RAS) in the pancreas is linked to the modulation of glucose-stimulated insulin secretion (GSIS) in beta cells and insulin sensitivity in target tissues, emerging as a promising tool in the prevention and/or treatment of obesity, diabetes, and systemic arterial hypertension. Insulin resistance alters pancreatic islet cell distribution and morphology and hypertrophied islets exhibit upregulated angiotensin II type 1 receptor, which drives oxidative stress, apoptosis, and fibrosis, configuring beta cell dysfunction and diminishing islet lifespan. Pharmacological modulation of RAS has shown beneficial effects in diet-induced obesity model, mainly related to the translational potential that angiotensin receptor blockers and ECA2/ANG (1-7)/MAS receptor axis modulation have when it comes to islet preservation and type 2 diabetes prevention and/or treatment. This review describes the existing evidence for different approaches to blocking RAS elements in the management of insulin resistance and diabetes and focuses on islet remodeling and GSIS in rodents and humans.

Short-term high glucose culture potentiates pancreatic beta cell function

The exposure of pancreatic islets to high glucose is believed to be one of the causal factors of the progressive lowering of insulin secretion in the development of type 2 diabetes. The progression of beta cell failure to type 2 diabetes is preceded by an early positive increase in the insulin secretory response to glucose, which is only later followed by a loss in the secretion capacity of pancreatic islets. Here we have investigated the electrophysiological mechanisms underlying the early glucose-mediated gain of function. Rodent pancreatic islets or dispersed islet cells were cultured in medium containing either 5.6 (control) or 16.7 (high-glucose) mM glucose for 24 h after isolation. Glucose-stimulated insulin secretion was enhanced in a concentration-dependent manner in high glucose-cultured islets. This was associated with a positive effect on beta cell exocytotic capacity, a lower basal K_ATP conductance and a higher glucose sensitivity to fire action potentials. Despite no changes in voltage-gated Ca²⁺ currents were observed in voltage-clamp experiments, the [Ca²⁺]_I responses to glucose were drastically increased in high glucose-cultured cells. Of note, voltage-dependent K⁺ currents were decreased and their activation was shifted to more depolarized potentials by high-glucose culture. This decrease in voltage-dependent K⁺ channel (Kv) current may be responsible for the elevated [Ca²⁺]_I response to metabolism-dependent and independent stimuli, associated with more depolarized membrane potentials with lower amplitude oscillations in high glucose-cultured beta cells. Overall these results show that beta cells improve their response to acute challenges after short-term culture with high glucose by a mechanism that involves modulation not only of metabolism but also of ion fluxes and exocytosis, in which Kv activity appears as an important regulator.

Xanthine-derived KMUP-1 reverses glucotoxicity-activated Kv channels through the cAMP/PKA signaling pathway in rat pancreatic β cells

Hyperglycemia-associated glucotoxicity induces β-cell dysfunction and a reduction in insulin secretion. Voltage-dependent K⁺ (Kv) channels in pancreatic β-cells play a key role in glucose-dependent insulin secretion. KMUP-1, a xanthine derivative, has been demonstrated to modulate Kv channel activity in smooth muscles; however, the role of KMUP-1 in glucotoxicity-activated Kv channels in pancreatic β-cells remains unclear. In this study we examined the mechanisms by which KMUP-1 could inhibit high glucose (25 mM) activated Kv currents (IKv) in pancreatic β-cells. Pancreatic β-cells were isolated from Wistar rats and IKv was monitored by perforated patch-clamp recording. The peak IKv in high glucose-treated β-cells was ∼1.4-fold greater than for normal glucose (5.6 mM). KMUP-1 (1, 10, 30 μM) prevented high glucose-stimulated IKv in a concentration-dependent manner. Reduction of high glucose-activated IKv was also found for protein kinase A (PKA) activator 8-Br-cAMP (100 μM). Additionally, KMUP-1 (30 μM) current inhibition was reversed by the PKA inhibitor H-89 (1 μM). Otherwise, pretreatment with the PKC activator or inhibitor had no effect on IKv in high glucose exposure. In conclusion, glucotoxicity-diminished insulin secretion was due to IKv activation. KMUP-1 attenuated high glucose-stimulated IKv via the PKA but not the PKC signaling pathway. This finding provides evidence that KMUP-1 might be a promising agent for treating hyperglycemia-induced insulin resistance.

Decreased insulin secretion in pregnant rats fed a low protein diet

Low protein (LP) diet during pregnancy leads to reduced plasma insulin levels in rodents, but the underlying mechanisms remain unclear. Glucose is the primary insulin secretagogue, and enhanced glucose-stimulated insulin secretion (GSIS) in beta cells contributes to compensation for insulin resistance and maintenance of glucose homeostasis during pregnancy. In this study, we hypothesized that plasma insulin levels in pregnant rats fed LP diet are reduced due to disrupted GSIS of pancreatic islets. We first confirmed reduced plasma insulin levels, then investigated in vivo insulin secretion by glucose tolerance test and ex vivo GSIS of pancreatic islets in the presence of glucose at different doses, and KCl, glibenclamide, and L-arginine. Main findings include (1) plasma insulin levels were unaltered on day 10, but significantly reduced on days 14-22 of pregnancy in rats fed LP diet compared to those of control (CT) rats; (2) insulin sensitivity was unchanged, but glucose intolerance was more severe in pregnant rats fed LP diet; (3) GSIS in pancreatic islets was lower in LP rats compared to CT rats in the presence of glucose, KCl, and glibenclamide, and the response to L-arginine was abolished in LP rats; and (4) the total insulin content in pancreatic islets and expression of Ins2 were reduced in LP rats, but expression of Gcg was unaltered. These studies demonstrate that decreased GSIS in beta cells of LP rats contributes to reduced plasma insulin levels, which may lead to placental and fetal growth restriction and programs hypertension and other metabolic diseases in offspring.

The serum protein responses to treatment with Xiaoke Pill and Glibenclamide in type 2 diabetes patients

AIM

The Xiaoke Pill containing Chinese herb extracts and Glibenclamide, is used in therapy for type 2 diabetes mellitus (T2DM), and is effective in reducing the risk of hypoglycemia and improving diabetes symptoms compared with Glibenclamide. We describe a quantitative proteomics project to measure the T2DM serum proteome response to the Xiaoke Pill and Glibenclamide.

METHODS

Based on a recently conducted 48-week clinical trial comparing the safety and efficacy of Glibenclamide (n = 400) and Xiaoke Pill (n = 400), after matching for age, sex, BMI, drug dose and whether hypoglycemia occurred, 32 patients were selected for the serum based proteomic analysis and divided into four groups (with/without hypoglycemia treated with Xiaoke Pill or Glibenclamide, n = 8 for each group). We screened the differential serum proteins related to treatments and the onset of hypoglycemia using the iTRAQ labeling quantitative proteomics technique. Baseline and follow-up samples were used.

RESULTS

The quantitative proteomics experiments demonstrated that 25 and 21 proteins differed upon treatment with the Xiaoke Pill in patients without and with hypoglycemia, respectively, while 24 and 25 proteins differed upon treatment with Glibenclamide in patients without and with hypoglycemia, respectively. The overlap of different proteins between the patients with and without hypoglycemia given the same drug treatment was much greater than between the patients given different drug treatments.

CONCLUSIONS

We conclude that the serum proteins response to the two different anti-diabetic drug treatments may serve as a sensitive biomarker for evaluation of the therapeutic effects and continue investigations into the mechanism.

Combination of Telmisartan and Linagliptin Preserves Pancreatic Islet Cell Function and Morphology in db/db Mice

OBJECTIVES

The present study aimed to investigate the synergistic action of telmisartan and linagliptin in ameliorating pancreatic islet functions and morphology in type 2 diabetes mellitus and to delineate the molecular signaling pathway involved.

METHODS

db/db mice were given telmisartan (3 mg/kg) or linagliptin (3 mg/kg) alone or in combination, daily for 8 weeks, and were studied in vivo by fasting and random blood glucose tests, oral glucose tolerance tests, and intraperitoneal insulin tolerance tests, as well as ex vivo by glucose-stimulated insulin secretion and morphology of pancreatic islets. The underlying signaling pathways were examined by Western blot, real-time quantitative polymerase chain reaction, and dihydroethidium staining analyses using mouse pancreatic islets and rat β-insulinoma cells.

RESULTS

Telmisartan/linagliptin combination induced significantly better glucose homeostasis than the monotherapies. Posttreatment reactive oxygen species level was suppressed most significantly after the telmisartan/linagliptin combined therapy, whereas no significant change in peroxisome proliferator-activated receptor γ expressions was observed after treatments.

CONCLUSIONS

The telmisartan/linagliptin combination preserved pancreatic islet cell functions and morphology via reduction of oxidative stress but independent of the peroxisome proliferator-activated receptor γ pathway. Our data shed light on the therapeutic potential of using the telmisartan/linagliptin combination in the treatment of human type 2 diabetes mellitus and its related complications.

Effects of β(3)-adrenoceptor activation on expression of pancreatic adrenoceptors and angiotensin II receptors in ApoE(-/-) mice

Hyperlipidemia can be harmful to the pancreas and β3-adrenoceptor agonist can improve lipid metabolism disorder. We aimed to study the effects of β3-adrenoceptor activation on glucose, insulin and the expression of pancreatic adrenoceptors and angiotensin II receptors. Ten C57BL/6J mice at the age of 10 weeks served as normal control, and forty age-matched apolipoprotein E knockout (ApoE(-/-)) mice were randomly divided into hyperlipidaemia model group, low-dose and high-dose β3-adrenoceptor agonist group and β3-adrenoceptor antagonist group. After 26 weeks of high-fat diet, treatments were given for 12 weeks. Serum glucose and insulin levels in 48 weeks old mice were measured using an automatic biochemical detector. Quantitative rt-PCR and Western blot were used to analyze the mRNA and protein expression of α1A-, α2A-, β2-, β3-adrenoceptors and angiotensin II type 1 and type 2 receptors in pancreas. We found that β3-adrenoceptor agonist could decrease serum glucose and insulin levels in aged ApoE(-/-) mice (P<0.01) and down-regulate the expression of α1A-adrenoceptor and angiotensin II type 1 receptor which were significantly increased in model mice (P<0.05, P<0.01). Compared with the model mice, α2A-, β2-, β3-adrenoceptor and angiotensin II type 2 receptor expression were up-regulated in β3-adrenoceptor agonist treat mice (P<0.05, P<0.01). These results suggest that chronic β3-adrenoceptor activation regulated the expression of adrenoceptors and angiontensin II receptors towards contrary direction, which indicates that there are interactions between β3-adrenoceptor and subtypes of adrenoceptor and angiotensin II receptor, and these interactions may play a protective role in pancreas and improve glucose metabolism disorders.

The role of renin-angiotensin system in cellular differentiation: implications in pancreatic islet cell development and islet transplantation

In addition to the well-characterized circulating renin-angiotensin system (RAS), local RAS has been identified recently in diverse tissues and organs. The presence of key components of the RAS in local tissues is important for our understanding of the patho-physiological mechanism(s) of several metabolic diseases, and may serve as a major therapeutic target for cardiometabolic syndromes. Locally generated and physiologically active RAS components have functions that are distinct from the classical vasoconstriction and fluid homeostasis actions of systemic RAS and cater specifically for local tissues. Local RAS can affect islet-cell function and structure in the adult pancreas as well as proliferation and differentiation of pancreatic stem/progenitor cells during development. Differentiation of stem/progenitor cells into insulin-expressing cells suitable for therapeutic transplantation offers a desperately needed new approach for replacement of glucose-responsive insulin producing cells in diabetic patients. Given that the generation of functional and transplantable islet cells has proven to be difficult, elucidation of RAS involvement in cellular regeneration and differentiation may propel pancreatic stem/progenitor cell development and thus β-cell regeneration forward. This review provides a critical appraisal of current research progress on the role of the RAS, including the newly characterized ACE2/Ang-(1-7)/Mas axis in the proliferation, differentiation, and maturation of pancreatic stem/progenitor cells. It is thus plausible to propose that the AT1 stimulation could be a repair mechanism involving the AT2R as well as the ACE2/Ang-(1-7)/Mas axis in directing β-cell development in diabetic patients using genetic and pharmaceutical manipulation of the RAS.

High glucose represses β-klotho expression and impairs fibroblast growth factor 21 action in mouse pancreatic islets: involvement of peroxisome proliferator-activated receptor γ signaling

Circulating fibroblast growth factor 21 (FGF21) levels are elevated in diabetic subjects and correlate directly with abnormal glucose metabolism, while pharmacologically administered FGF21 can ameliorate hyperglycemia. The pancreatic islet is an FGF21 target, yet the actions of FGF21 in the islet under normal and diabetic conditions are not fully understood. This study investigated the effects of high glucose on islet FGF21 actions in a diabetic mouse model by investigating db/db mouse islet responses to exogenous FGF21, the direct effects of glucose on FGF21 signaling, and the involvement of peroxisome proliferator-activated receptor γ (PPARγ) in FGF21 pathway activation. Results showed that both adult db/db mouse islets and normal islets treated with high glucose ex vivo displayed reduced β-klotho expression, resistance to FGF21, and decreased PPARγ expression. Rosiglitazone, an antidiabetic PPARγ ligand, ameliorated these effects. Our data indicate that hyperglycemia in type 2 diabetes mellitus may lead to FGF21 resistance in pancreatic islets, probably through reduction of PPARγ expression, which provides a novel mechanism for glucose-mediated islet dysfunction.

High glucose activates the alternative ACE2/Ang-(1-7)/Mas and APN/Ang IV/IRAP RAS axes in pancreatic β-cells

The activation of the classical angiotensin (Ang)-converting enzyme (ACE)/Ang II/Ang II type 1 receptor (AT1R) axis of the renin-angiotensin system (RAS) has been associated with islet dysfunction and insulin resistance. Hyperglycaemia, hypertension and obesity, major components of metabolic syndrome, are all associated with increased systemic and tissue levels of Ang II. Whereas it is well established that Ang II, by binding to AT1R, impairs glucose-stimulated insulin secretion and insulin signaling, the contribution of alternative RAS axes to β-cell function remains to be fully elucidated. In this study, using the BRIN-BD11 rat insulinoma cell line, we i) examined the basal expression levels of components of classical and alternative RAS axes and ii) investigated the effects of normal (5.5 mM) and elevated (11, 15, 25 mM) glucose concentrations on their expression and/or enzymatic activity by means of reverse transcription quantitative PCR (RT-qPCR), immunoblot analysis and enzymatic activity assays. The results correlated with the insulin production and release. Essential components of all RAS axes were found to be expressed in the BRIN-BD11 cells. Components of the alternative RAS axes, ACE2, neutral endopeptidase 24.11, Mas receptor (Mas), aminopeptidases A (APA) and N (APN) and insulin-regulated aminopeptidase (IRAP) showed an increased expression/activity in response to high glucose. These alterations were paralleled by the glucose-dependent increase in insulin production and release. By contrast, components of the classical RAS axis, ACE, AT1R and Ang II type 2 receptor (AT2R), remained largely unaffected under these conditions. Glucose induced the activation of the alternative ACE2/Ang-(1-7)/Mas and APN/Ang IV/IRAP RAS axes simultaneously with the stimulation of insulin production/release. Our data suggest the existence of a functional link between the local RAS axis and pancreatic β-cell function; however, further studies are required to confirm this hypothesis.

Angiotensin II increased neuronal stem cell proliferation: role of AT2R

Angiotensin II (Ang II), known a potent vasoactive substance in the renin-angiotensin system in the brain, plays a critical role in systemic blood pressure control. However, increasing evidence indicated that the physiological role of Ang II go beyond its vasoactive effect. In the present study, we demonstrated that Ang II type-1 receptor (AT1R) and type-2 receptor (AT2R) were expressed in primary rat hippocampal neuronal stem cells (NSCs). Treatment of rat hippocampal NSCs with Ang II increased cell proliferation. Pretreatment of NSCs with specific AT2R, but not AT1R, antagonist significantly suppressed Ang II-induced cell proliferation. Furthermore, Ang II stimulated ERK and Akt phosphorylation in NSCs. Pretreatment of MEK inhibitor, but not PI3K inhibitor, inhibited Ang II-induced ERK phosphorylation as well as cell proliferation. In addition, stimulation of NSCs with Ang II decreased expression of K_V 1.2/K_V 3.1 channels and blocked K⁺ currents which lie downstream of ERK activation. Taken together, these findings underpin the role of AT2R as a novel target that regulates cell proliferation mediated by Ang II with implications for therapeutic intervention for regulation of neurogenesis.

Modulation of hypovitaminosis D-induced islet dysfunction and insulin resistance through direct suppression of the pancreatic islet renin-angiotensin system in mice

AIMS/HYPOTHESIS

Vitamin D is necessary for normal insulin action and suppresses renin production. Increased renin-angiotensin system (RAS) activity causes islet damage, including reduced insulin secretion. We therefore sought to determine whether hypovitaminosis D-induced upregulation of islet RAS in vivo impairs islet cell function and increases insulin resistance, and whether pharmacological suppression of the RAS during continuing vitamin D deficiency might correct this.

METHODS

C57BL/6 mice were rendered vitamin D-deficient by diet, and glucose and insulin tolerance was assessed. The expression and translation of islet functional, and islet RAS, genes were measured and the effects of pharmacological renin suppression examined.

RESULTS

Mice with diet-induced hypovitaminosis D developed impaired glucose tolerance, increased RAS component expression and impaired islet function gene transcription. Treatment with pharmacological renin inhibition (aliskiren), without vitamin D status correction, reduced islet RAS over-reactivity, islet dysfunction and insulin resistance, and improved glucose tolerance.

CONCLUSIONS/INTERPRETATION

Upregulation of islet RAS genes can contribute to hypovitaminosis D-induced impairment of islet function and increase insulin resistance independently of vitamin D status. Thus, our findings support the use of RAS inhibitors in impaired glucose homeostasis or early diabetes. They also suggest that combining RAS inhibition with correction of hypovitaminosis D might be useful in treating impaired glycaemic control and also in type 2 diabetes prevention. However, the use of aliskiren in established diabetes is contraindicated due to the increased risk of side effects such as hyperkalaemia, so other more suitable RAS blockers need to be identified.

Improved Glucose-Stimulated Insulin Secretion by Selective Intraislet Inhibition of Angiotensin II Type 1 Receptor Expression in Isolated Islets of db/db Mice

Recent evidence supported the presence of a local renin-angiotensin system (RAS) in the pancreas, which is implicated in many physiological and pathophysiological processes. We utilized small interfering RNA (siRNA) to investigate the effects of angiotensin II type 1 receptor (AT1R) knockdown on glucose-stimulated insulin secretion (GSIS) in isolated islets of db/db mice and to explore the potential mechanisms involved. We found that Ad-siAT1R treatment resulted in a significant decrease both in AT1R mRNA level and in AT1R protein expression level. With downexpression of AT1R, notable increased insulin secretion and decreased glucagon secretion levels were found by perifusion. Simultaneously, significant increased protein levels of IRS-1 (by 85%), IRS-2 (by 95%), PI3K(85) (by 112.5%), and p-Akt2 (by 164%) were found by western blot. And upregulation of both GLUT-2 (by 190%) and GCK (by 121%) was achieved after AT1R inhibition by Ad-siAT1R. Intraislet AT1R expression level is a crucial physiological regulator of insulin sensitivity of β cell itself and thus affects glucose-induced insulin and glucagon release. Therefore, the characteristics of AT1R inhibitors could make it a potential novel therapeutics for prevention and treatment of type 2 diabetes.

The renin-angiotensin system: a target of and contributor to dyslipidemias, altered glucose homeostasis, and hypertension of the metabolic syndrome

The renin-angiotensin system (RAS) is an important therapeutic target in the treatment of hypertension. Obesity has emerged as a primary contributor to essential hypertension in the United States and clusters with other metabolic disorders (hyperglycemia, hypertension, high triglycerides, low HDL cholesterol) defined within the metabolic syndrome. In addition to hypertension, RAS blockade may also serve as an effective treatment strategy to control impaired glucose and insulin tolerance and dyslipidemias in patients with the metabolic syndrome. Hyperglycemia, insulin resistance, and/or specific cholesterol metabolites have been demonstrated to activate components required for the synthesis [angiotensinogen, renin, angiotensin-converting enzyme (ACE)], degradation (ACE2), or responsiveness (angiotensin II type 1 receptors, Mas receptors) to angiotensin peptides in cell types (e.g., pancreatic islet cells, adipocytes, macrophages) that mediate specific disorders of the metabolic syndrome. An activated local RAS in these cell types may contribute to dysregulated function by promoting oxidative stress, apoptosis, and inflammation. This review will discuss data demonstrating the regulation of components of the RAS by cholesterol and its metabolites, glucose, and/or insulin in cell types implicated in disorders of the metabolic syndrome. In addition, we discuss data supporting a role for an activated local RAS in dyslipidemias and glucose intolerance/insulin resistance and the development of hypertension in the metabolic syndrome. Identification of an activated RAS as a common thread contributing to several disorders of the metabolic syndrome makes the use of angiotensin receptor blockers and ACE inhibitors an intriguing and novel option for multisymptom treatment.

A novel role for vitamin D: modulation of expression and function of the local renin-angiotensin system in mouse pancreatic islets

AIMS/HYPOTHESIS

The aim of this study was to demonstrate that hormonal vitamin D (calcitriol) modulates the local pancreatic islet renin-angiotensin system (RAS) whilst improving islet beta cell secretory function.

METHODS

Isolated islets cultured ex vivo under high- or low-glucose conditions and treated with or without calcitriol were examined for changes in RAS component activity and glucose-stimulated insulin secretion (GSIS). Isolated islets from vitamin D receptor knockout (VDR-KO) mice were compared with islets from wild-type (WT) mice for major RAS component expression and RAS protein production.

RESULTS

Isolated islets incubated ex vivo under high-glucose conditions showed increased expression and production of major RAS components; this was prevented and reversed by calcitriol in parallel with increases in GSIS. VDR-KO mice displayed increased RAS component mRNA expression and protein production as compared with WT mice, despite comparable glucose homeostasis.

CONCLUSIONS

Young mice with vitamin D receptor ablation showed abnormal increases in islet RAS components at mRNA and protein levels, despite unaltered glucose homeostasis. Calcitriol prevents and can correct induction of RAS component production under high-glucose conditions in parallel with the well-known effect of calcitriol on increasing islet beta cell secretory responses to glucose.

An update on the islet renin-angiotensin system

The traditional renin-angiotensin system (RAS) components have been studied extensively since the rate-limiting component of RAS, renin, was first characterized. The ongoing identification of various novel RAS components and signaling pathways continues to elaborate the complexity of this system. Regulation of RAS according to the conventional and contemporary views of its functions in various tissues under pathophysiological conditions is a main treatment strategy for many metabolic diseases. The local pancreatic RAS, first proposed to exist in pancreatic islets two decades ago, could regulate islet function and glycemic control via influences on islet cell mass, inflammation, and ion channels. Insulin secretion, the major function of pancreatic islets, is controlled by numerous factors. Among these factors and of particular interest are glucagon-like peptide-1 (GLP-1) and vitamin D, which may regulate islet function by directly binding receptors on islet beta cells. These factors may work with local RAS signaling in islets to protect and maintain islet function under diabetic and hyperglycemic conditions. In this concise review, the local islet RAS will be discussed with particular attention being paid to recent notable findings.

Diabetic microvascular complications: possible targets for improved macrovascular outcomes

The results of recent outcome trials challenge hypotheses that tight control of both glycohemoglobin and blood pressure diminishes macrovascular events and survival among type 2 diabetic patients. Relevant questions exist regarding the adequacy of glycohemoglobin alone as a measure of diabetes control. Are we ignoring mechanisms of vasculotoxicity (profibrosis, altered angiogenesis, hypertrophy, hyperplasia, and endothelial injury) inherent in current antihyperglycemic medications? Is the polypharmacy for lowering cholesterol, triglyceride, glucose, and systolic blood pressure producing drug interactions that are too complex to be clinically identified? We review angiotensin-aldosterone mechanisms of tissue injury that magnify microvascular damage caused by hyperglycemia and hypertension. Many studies describe interruption of these mechanisms, without hemodynamic consequence, in the preservation of function in type 1 diabetes. Possible interactions between the renin-angiotensin-aldosterone system and physiologic glycemic control (through pulsatile insulin release) suggest opportunities for further clinical investigation.