Transgenic mice overexpressing alpha2A-adrenoceptors in pancreatic beta-cells show altered regulation of glucose homeostasis

Secondary diabetes mellitus in pheochromocytomas and paragangliomas

Secondary diabetes mellitus (DM) in secretory pheochromocytomas and paragangliomas (PPGLs) is encountered in up to 50% of cases, with its presentation ranging from mild, insulin resistant forms to profound insulin deficiency states, such as diabetic ketoacidosis and hyperglycemic hyperosmolar state. PPGLs represent hypermetabolic states, in which adrenaline and noradrenaline induce insulin resistance in target tissues characterized by aerobic glycolysis, excessive lipolysis, altered adipokine expression, subclinical inflammation, as well as enhanced gluconeogenesis and glucogenolysis. These effects are mediated both directly, upon adrenergic receptor stimulation, and indirectly, via increased glucagon secretion. Impaired insulin secretion is the principal pathogenetic mechanism of secondary DM in this setting; yet, this is relevant for tumors with adrenergic phenotype, arising from direct inhibitory actions in beta pancreatic cells and incretin effect impairment. In contrast, insulin secretion might be enhanced in tumors with noradrenergic phenotype. This dimorphic effect might correspond to two distinct glycemic phenotypes, with predominant insulin resistance and insulin deficiency respectively. Secondary DM improves substantially post-surgery, with up to 80% remission rate. The fact that surgical treatment of PPGLs restores insulin sensitivity and secretion at greater extent compared to alpha and beta blockade, implies the existence of further, non-adrenergic mechanisms, possibly involving other hormonal co-secretion by these tumors. DM management in PPGLs is scarcely studied. The efficacy and safety of newer anti-diabetic medications, such as glucagon-like peptide 1 receptor agonists and sodium glucose cotransporter 2 inhibitors (SGLT2is), as well as potential disease-modifying roles of metformin and SGLT2is warrant further investigation in future studies.

Knockdown of Adra2a Increases Secretion of Growth Factors and Wound Healing Ability in Diabetic Adipose-Derived Stem Cells

Studies showed that compared to normal adipose-derived stem cells (ASCs), ASCs from type 2 diabetic (T2D) mice were less effective in treating diabetic cutaneous wounds. However, the mechanisms remain unclear. Our transcriptomic profiling comparison showed that the expression of α2A-adrenergic receptor (Adra2a) was significantly increased in ASCs from T2D mice (T2D ASCs). Therefore, the purpose of this study was to investigate whether the elevated Adra2a is involved in the diminished wound-healing capabilities of T2D ASCs. RNA-seq was used to compare the transcriptomic profiles of T2D and normal ASCs. The differential genes were verified by real-time RT-qPCR. Clonidine was used to active Adra2a, and lentivirus-mediated RNAi was used to knockdown Adra2a. The secretion and expression of growth factors were detected by enzyme-linked immunosorbent assay (ELISA) and real-time RT-qPCR, respectively. The cAMP and PKA activity were also detected. Wound healing abilities of various ASCs were assessed in T2D mouse excisional wound models. The results showed Adra2a agonist clonidine decreased the expression and secretion of growth factors, cAMP content, and activity of PKA in ASCs, while Adra2a knockdown T2D ASCs showed the opposite effects. Adra2a knockdown T2D ASCs also showed increased wound-healing capabilities compared to untreated T2D ASCs. Altogether, T2D increased Adra2a expression, which may subsequently decrease the expression and secretion of growth factors and eventually diminish the wound-healing capabilities of T2D ASCs. Adra2a knockdown can restore the secretion of growth factors in T2D ASCs and then accelerate the wound healing, which may provide a new possibility in the treatment of diabetic wounds.

Peripheral Innervation in the Regulation of Glucose Homeostasis

Precise regulation of circulating glucose is crucial for human health and ensures a sufficient supply to the brain, which relies almost exclusively on glucose for metabolic energy. Glucose homeostasis is coordinated by hormone-secreting endocrine cells in the pancreas, as well as glucose utilization and production in peripheral metabolic tissues including the liver, muscle, and adipose tissue. Glucose-regulatory tissues receive dense innervation from sympathetic, parasympathetic, and sensory fibers. In this review, we summarize the functions of peripheral nerves in glucose regulation and metabolism. Dynamic changes in peripheral innervation have also been observed in animal models of obesity and diabetes. Together, these studies highlight the importance of peripheral nerves as a new therapeutic target for metabolic disorders.

Roles of Epigenetic Modifications in the Differentiation and Function of Pancreatic β-Cells

Diabetes, a metabolic disease with multiple causes characterized by high blood sugar, has become a public health problem. Hyperglycaemia is caused by deficiencies in insulin secretion, impairment of insulin function, or both. The insulin secreted by pancreatic β cells is the only hormone in the body that lowers blood glucose levels and plays vital roles in maintaining glucose homeostasis. Therefore, investigation of the molecular mechanisms of pancreatic β cell differentiation and function is necessary to elucidate the processes involved in the onset of diabetes. Although numerous studies have shown that transcriptional regulation is essential for the differentiation and function of pancreatic β cells, increasing evidence indicates that epigenetic mechanisms participate in controlling the fate and regulation of these cells. Epigenetics involves heritable alterations in gene expression caused by DNA methylation, histone modification and non-coding RNA activity that does not result in DNA nucleotide sequence alterations. Recent research has revealed that a variety of epigenetic modifications play an important role in the development of diabetes. Here, we review the mechanisms by which epigenetic regulation affects β cell differentiation and function.

Effects of dexmedetomidine on glucose homeostasis in healthy cats

OBJECTIVES

Alpha(α)2-agonist administration has been documented to increase blood glucose concentrations in many species. The aim of this study was to further describe the effect of dexmedetomidine on glucose and its regulatory hormones in healthy cats.

METHODS

A randomized crossover study using eight healthy cats with a 14 day washout period was used to assess the effect of dexmedetomidine (10 μg/kg IV) and saline on glucose, cortisol, insulin, glucagon and non-esterified fatty acid (NEFA) concentrations at 0, 20, 60, 120 and 180 mins post-administration. Glucose:insulin ratios were calculated for each time point.

RESULTS

Within the dexmedetomidine group, significant differences (P <0.05) were detected: increased median (range) blood glucose concentrations at 60 mins (11.55 mmol/l [5.9-16.6 mmol/l]) and 120 mins (12.0 mmol/l [6.1-13.8 mmol/l]) compared with baseline (6.05 mmol/l [4.8-13.3 mmol/l]); decreased glucagon concentrations at 120 mins (3.8 pmol/l [2.7-8.8 pmol/l]) and 180 mins (4.7 pmol/l [2.1-8.2 pmol/l]) compared with baseline (11.85 pmol/l [8.3-17.2 pmol/l]); decreased NEFA concentrations at 60 mins (0.281 mmol/l [0.041-1.357 mmol/l]) and 120 mins (0.415 mmol/l [0.035-1.356 mmol/l]) compared with baseline (0.937 mmol/l [0.677-1.482 mmol/l]); and significantly larger (P <0.05) glucose:insulin ratios at 60 mins compared with baseline. Insulin and cortisol concentrations were not significantly changed after dexmedetomidine administration.

CONCLUSIONS AND RELEVANCE

Feline practitioners should be aware of the endocrine effects associated with the use of α2-agonists, particularly when interpreting blood glucose concentrations. The transient effects of dexmedetomidine on glucose homeostasis are unlikely to significantly affect clinical practice.

Evidence that the multiflorine-derived substituted quinazolidine 55P0251 augments insulin secretion and lowers blood glucose via antagonism at α2 -adrenoceptors in mice

AIMS

To investigate the mechanism of action of 55P0251, a novel multiflorine-derived substituted quinazolidine that augments insulin release and lowers blood glucose in rodents, but does not act via mechanisms addressed by any antidiabetic agent in clinical use.

MATERIALS AND METHODS

Using male mice, we determined the effects of 55P0251 on glucose tolerance, insulin secretion from isolated islets and blood oxygen saturation, including head-to-head comparison of 55P0251 to its inverted enantiomer 55P0250, as well as to other anti-hyperglycaemic multiflorine derivatives discovered in our programme.

RESULTS

55P0251 was clearly superior to its inverted enantiomer in the glucose tolerance test (area under the curve: 11.3 mg/kg 55P0251, 1.19 ± 0.04 min*mol/L vs 55P0250, 1.80 ± 0.04 min*mol/L; P < .0001). For insulin release in vitro, this superiority became visible only under concomitant adrenergic background stimulation (glucose-stimulated insulin release, fmol*islet^-1 *30 min^-1 : without α₂ -adrenoceptor agonist: 500 μmol/L 55P0251, 390 ± 34, vs 55P0250, 459 ± 40, nonsignificant; with α₂ -adrenoceptor agonist: 250 μmol/L 55P0251, 138 ± 9, vs 55P0250, 21 ± 6; P < .0001). Since receptor binding assays suggested antagonism at α_2A -adrenoceptors as a potential mechanism of action, we measured oxygen saturation in capillary blood from the tail as a surrogate of vasoconstriction, which supported α₂ -antagonistic action in vivo (90 mg/kg 55P0251, 83 ± 3%, vs 55P0250, 57 ± 3%; P < .0001). Lack of association between glucose-lowering activities and α_2A -adrenoceptor binding affinity arising from comparison of multiflorine derivatives was attributed to differences in their pharmacokinetic properties.

CONCLUSIONS

Our findings suggest that 55P0251 and related multiflorine derivatives are to be categorized as α₂ -adrenoceptor antagonists with potential to lower blood glucose by blocking α_2A -adrenoceptors on pancreatic β cells.

Admission Hyperglycemia as a Predictor of Mortality in Acute Heart Failure: Comparison between the Diabetics and Non-Diabetics

Background: To investigate the impact of admission hyperglycemia (HGL) on in-hospital death (IHD) and 1-year mortality in acute heart failure (AHF) patients with or without diabetes mellitus (DM). Methods: Among 5625 AHF patients enrolled in a nationwide registry, 5541 patients were divided into four groups based on the presence of admission HGL and diabetes mellitus (DM). Admission HGL was defined as admission glucose level > 200 mg/dL. IHD and 1-year mortality were compared. Results: IHD developed in 269 patients (4.9%), and 1-year death developed in 1220 patients (22.2%). DM was a significant predictor of 1-year death (24.8% in DM vs. 20.5% in non-DM, p < 0.001), but not for IHD. Interestingly, admission HGL was a significant predictor of both IHD (7.6% vs. 4.2%, p < 0.001) and 1-year death (26.2% vs. 21.3%, p = 0.001). Admission HGL was a significant predictor of IHD in both DM and non-DM group, whereas admission HGL was a significant predictor of 1-year death only in non-DM (27.8% vs. 19.9%, p = 0.003), but not in DM group. In multivariate analysis, admission HGL was an independent predictor of 1-year mortality in non-DM patients (HR 1.32, 95% CI 1.03–1.69, p = 0.030). Conclusion: Admission HGL was a significant predictor of IHD and 1-year death in patients with AHF, whereas DM was only a predictor of 1-year death. Admission HGL was an independent predictor of 1-year mortality in non-DM patients with AHF, but not in DM patients. Careful monitoring and intensive medical therapy should be considered in AHF patients with admission HGL, regardless of DM.

Role of the sympathetic nervous system in regulation of the sodium glucose cotransporter 2

BACKGROUND

The sympathetic nervous system (SNS) regulates glucose metabolism in various organs including the kidneys. The sodium glucose cotransporter 2 (SGLT2) mediates glucose reabsorption in renal proximal tubules and its inhibition has been shown to improve glucose control, cardiovascular and renal outcomes. We hypothesized that SNS-induced alterations of glucose metabolism may be mediated via regulation of SGLT2.

METHOD

We used human renal proximal tubule cells to investigate the effects of noradrenaline on SGLT2 regulation. Mice fed a high-fat diet were oral gavaged with dapagliflozin and the expression of noradrenaline and tyrosine hydroxylase was measured in the kidney and heart.

RESULTS

Noradrenaline treatment resulted in a pronounced increase in SGLT2 and interleukin (IL)-6 expression in HK2 cells and promoted translocation of SGLT2 to the cell surface. In vivo, dapagliflozin treatment resulted in marked glucosuria in high-fat diet-fed mice. SGLT2 inhibition significantly reduced high-fat diet-induced elevations of tyrosine hydroxylase and noradrenaline in the kidney and heart. We also aimed to assess the levels of hypertension-related cytokines in the kidneys of our mice treated with and without dapagliflozin. Excitingly, we demonstrate that SGLT2 inhibition with dapagliflozin promoted a trend towards reduced tumour necrosis factor-alpha and elevated IL-1β protein levels in the kidney.

CONCLUSION

Our in-vitro and in-vivo studies provide first evidence for an important cross-talk between the SNS and SGLT2 regulation that may not only account for SNS-induced alterations of glucose metabolism but potentially contribute to cardiovascular and renal protection observed with SGLT2 inhibitors.

Sympathetic innervation is essential for metabolic homeostasis and pancreatic beta cell function in adult rats

Obesity is associated with an imbalance in the activity of the autonomic nervous system (ANS), specifically in the organs involved in energy metabolism. The pancreatic islets are richly innervated by the ANS, which tunes the insulin release due to changes in energy demand. Therefore, changes in the sympathetic input that reach the pancreas can lead to metabolic dysfunctions. To evaluate the role of the sympathetic ends that innervate the pancreas, 60-day-old male Wistar rats were subjected to sympathectomy (SYM) or were sham-operated (SO). At 120 day-old SYM rats exhibited an increase in body weight, fat pads and metabolic dysfunctions. Decreases in the HOMA-IR and reductions in insulin release were observed both in vivo and in vitro. Furthermore, the SYM rats exhibited altered pancreatic islet function in both muscarinic and adrenergic assays and exhibited high protein expression of the alpha-2 adrenergic receptor (α2AR). Because α2AR has been linked to type 2 diabetes, these findings demonstrate the clinical implications of this study.

Endogenous α2A-Adrenoceptor-Operated Sympathoadrenergic Tones Attenuate Insulin Secretion via cAMP/TRPM2 Signaling

In pancreatic β-cells, pharmacological concentrations of catecholamines, including adrenaline, have been used to inhibit insulin release and explore the multiple mechanisms involved. However, the significance of these signaling pathways for physiological adrenergic functions in β-cells is largely unknown. In the process of glucose-induced insulin secretion, opening of background current through nonselective cation channels (NSCCs) might facilitate membrane depolarization by closure of the ATP-sensitive K⁺ channels. Here, we examined whether physiological insulinostatic adrenaline action is mediated via the transient receptor potential melastatin 2 (TRPM2) channel, a type of NSCC, in β-cells. Results showed that physiological concentrations of adrenaline strongly suppressed glucose-induced and incretin-potentiated cAMP production and insulin secretion and inhibited NSCCs current and membrane excitability via the α2A-adrenoceptor in wild-type mice; however, insulin secretion was not attenuated in TRPM2-knockout (KO) mice. Administration of yohimbine, an α2-adrenoceptor antagonist, failed to affect glucose tolerance in TRPM2-KO mice, in contrast to an improved glucose tolerance in wild-type mice receiving the antagonist. The current study demonstrated that a physiological concentration of adrenaline attenuates insulin release via coupling of α2A-adrenoceptor to cAMP/TRPM2 signaling, thereby providing a potential therapeutic tool to treat patients with type 2 diabetes.

Is it time to think about the sodium glucose co-transporter 2 sympathetically?

Disturbances in glucose homeostasis are a key feature of the metabolic syndrome and type 2 diabetes. Renal glucose reabsorption is an important factor in glycaemic control. Glucose reabsorption in the proximal tubules is mediated by the sodium glucose co-transporter 2. The capacity for glucose reabsorption is increased in type 2 diabetes and contributes significantly to hyperglycaemia and impaired glucose control. Understanding the mechanisms underpinning the regulation of the sodium glucose co-transporter 2 is therefore of high clinical relevance. However, despite recent advances in the field and the availability of pharmacological inhibitors of this glucose transporter for the treatment of type 2 diabetes, the mechanisms that regulate sodium glucose co-transporter 2 expression are not fully understood. The sympathetic nervous system is an important modulator of glucose homeostasis, and sympathetic hyperactivity is a characteristic feature of obesity, the metabolic syndrome and type 2 diabetes. Sympathetic inhibition either achieved pharmacologically or by renal sympathetic denervation has been associated with improved glucose control. Importantly, sympathetic nerves innervate the proximal tubules of the kidney where they have been shown to regulate the expression of other transporters such as the sodium hydrogen exchanger 3. This review aims to explore the evidence for the regulation of sodium glucose co-transporter 2-mediated glucose reabsorption by the sympathetic nervous system.

The expression and function of histamine H₃ receptors in pancreatic beta cells

BACKGROUND AND PURPOSE

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

EXPERIMENTAL APPROACH

The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H₃ receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H₃ receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting.

KEY RESULTS

Histamine H₃ receptors were expressed in pancreatic beta cells. A selective H₃ receptor agonist, imetit, and a selective inverse H₃ receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H3 receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H₃ receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Histamine H₃ receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.

Variation in the α(2A) adrenoceptor gene and the effect of dexmedetomidine on plasma insulin and glucose

OBJECTIVES

Sympathetic activation inhibits insulin secretion through activation of pancreatic α(2)A adrenoreceptors (α(2A)ARs). A common genetic α(2A)AR variant (rs553668) is associated with impaired insulin secretion. α(2A)R agonists would be expected to decrease insulin secretion, but their effects on glucose homeostasis in humans are poorly characterized. We examined the hypotheses that the selective α(2A)R agonist, dexmedetomidine, decreases plasma insulin levels and increases plasma glucose levels in humans and that these effects are modified by genetic α(2A)AR variants.

METHODS

Healthy, fasting, White (n=31) and Black (n=33) participants aged between 18 and 45 years received three sequential infusions of placebo (normal saline) at 30-min intervals, followed by three infusions of dexmedetomidine (0.1, 0.15, and 0.15 mcg/kg). Plasma insulin and glucose concentrations were measured at baseline and after the administration of placebo and dexmedetomidine. We genotyped ADRA2A rs553668 and rs2484516, which characterize haplotypes 4 and 4b, respectively.

RESULTS

Dexmedetomidine decreased fasting insulin concentrations by 37%, from a median value after placebo administration of 7.9 μU/ml (interquartile range: 6.0-12.6) to 4.9 μU/ml (interquartile range: 3.5-7.9; P<0.001). Plasma glucose concentrations increased from 76±6 to 79±7 mg/dl (P<0.001). The rs2484516 variant allele was associated with higher baseline insulin concentrations before (P=0.001) and after adjustment for potential confounders (P=0.014) and a greater decrease in insulin concentration after dexmedetomidine administration (P=0.016), which was no longer significant after adjustment for baseline concentrations and other confounders (P=0.58).

CONCLUSION

Low-dose dexmedetomidine decreased plasma insulin concentration and mildly increased plasma glucose concentration in healthy fasting individuals. The ADRA2A genetic variation may affect baseline insulin concentrations and thus the insulin decrease after dexmedetomidine administration.

Genetic association of adrenergic receptor alpha 2A with obesity and type 2 diabetes

OBJECTIVE

The sympathetic nervous system (SNS) is linked to glucose, lipid, and protein metabolism. The α2A -adrenergic receptor (ADRA2A) is involved in the SNS and mediates inhibition of insulin secretion and lipolysis. The association of ADRA2A single-nucleotide polymorphisms (SNPs) with obesity and/or type 2 diabetes (T2D) was investigated.

DESIGN AND METHODS

Genotyping was performed in a case-control study of 1,177 Swedish individuals, including lean and obese subjects with normal glucose tolerance (NGT) and T2D patients. ADRA2A mRNA expression was measured in pancreatic islets isolated from T2D patients and nondiabetic subjects.

RESULTS

SNP rs553668 was associated with T2D in men (odds ratio [OR] = 1.47; 95% confidence interval [CI] = 1.08-2.01; P = 0.015) but this association was lost after adjusting for age and for body mass index (BMI). Associations were also detected when comparing obese NGT and lean NGT subjects (OR = 1.49; 95% CI = 1.07-2.07; P = 0.017), and in obese (OR = 1.62; 95% CI = 1.06-2.49; P = 0.026), but not in lean T2D. In women, multiple logistic regression regarding SNP rs521674 demonstrated an increased OR of 7.61 (95% CI = 1.70-34.17; P = 0.008) for T2D when including age as a covariant. Correcting for BMI removed the significant association. When age was included in the model, association also found when obese T2D patients were compared with lean NGT subjects (P = 0.041). ADRA2A mRNA expression in human pancreatic islets was detectable, but with no statistically significant difference between the diabetic and the control groups.

CONCLUSIONS

ADRA2A genetic polymorphisms are mainly associated with obesity and possibly with T2D in a Swedish population.

Mechanisms of antihyperglycaemic action of efaroxan in mice: time for reappraisal of α2A-adrenergic antagonism in the treatment of type 2 diabetes?

AIMS/HYPOTHESIS

Inspired by recent speculation about the potential utility of α(2A)-antagonism in the treatment of type 2 diabetes, the study examined the contribution of α(2)-antagonism vs other mechanisms to the antihyperglycaemic activity of the imidazoline (±)-efaroxan.

METHODS

Effects of the racemate and its pure enantiomers on isolated pancreatic islets and beta cells in vitro, as well as on hyperglycaemia in vivo, were investigated in a comparative manner in mice.

RESULTS

In isolated perifused islets, the two enantiomers of efaroxan were equally potent in counteracting inhibition of insulin release by the ATP-dependent K(+) (K(ATP)) channel-opener diazoxide but (+)-efaroxan, the presumptive carrier of α(2)-antagonistic activity, was by far superior in counteracting inhibition of insulin release by the α(2)-agonist UK14,304. In vivo, (+)-efaroxan improved oral glucose tolerance at 100-fold lower doses than (-)-efaroxan and, in parallel with observations made in vitro, was more effective in counteracting UK14,304-induced than diazoxide-induced hyperglycaemia. The antihyperglycaemic activity of much higher doses of (-)-efaroxan was associated with an opposing pattern (i.e. with stronger counteraction of diazoxide-induced than UK14,304-induced hyperglycaemia), which implicates a different mechanism of action.

CONCLUSIONS/INTERPRETATION

The antihyperglycaemic potency of (±)-efaroxan in mice is almost entirely due to α(2)-antagonism, but high doses can also lower blood glucose via another mechanism. Our findings call for reappraisal of the possible clinical utility of α(2A)-antagonistic compounds in recently identified subpopulations of patients in which a congenitally higher level of α(2A)-adrenergic activation contributes to the development and pathophysiology of type 2 diabetes.

Leucine stimulates insulin secretion via down-regulation of surface expression of adrenergic α2A receptor through the mTOR (mammalian target of rapamycin) pathway: implication in new-onset diabetes in renal transplantation

The amino acid leucine is a potent secretagogue, capable of inducing insulin secretion. It also plays an important role in the regulation of mTOR activity, therefore, providing impetus to investigate if a leucine-sensing mechanism in the mTOR pathway is involved in insulin secretion. We found that leucine-induced insulin secretion was inhibited by both the mTOR inhibitor rapamycin as well as the adrenergic α2 receptor agonist clonidine. We also demonstrated that leucine down-regulated the surface expression of adrenergic α2A receptor via activation of the mTOR pathway. The leucine stimulatory effect on insulin secretion was attenuated in diabetic Goto-Kakizaki rats that overexpress adrenergic α2A receptors, confirming the role of leucine in insulin secretion. Thus, our data demonstrate that leucine regulates insulin secretion by modulating adrenergic α2 receptors through the mTOR pathway. The role of the mTOR pathway in metabolic homeostasis led us to a second important finding in this study; retrospective analysis of clinical data showed that co-administration of rapamycin and clonidine was associated with an increased incidence of new-onset diabetes in renal transplantation patients over those receiving rapamycin alone. We believe that inhibition of mTOR by rapamycin along with activation of adrenergic α2 receptors by clonidine represents a double-hit to pancreatic islets that synergistically disturbs glucose homeostasis. This new insight may have important implications for the clinical management of renal transplant patients.

The rs553668 polymorphism of the ADRA2A gene predicts the worsening of fasting glucose values in a cohort of subjects without diabetes. A population-based study

AIMS

Single-nucleotide polymorphisms in the human ADRA2A gene have been associated with increased risk of Type 2 diabetes. The associations between the rs553668 polymorphism and fasting glucose concentrations both cross-sectionally and longitudinally after 6-year follow-up were evaluated in an adult Caucasian population-based cohort.

METHODS

From a cohort of 1658 individuals, after excluding patients with diabetes, those who died and those whose blood samples were not available for genotyping, data of 1345 individuals were analysed.

RESULTS

Subjects homozygous for the A allele showed significantly increased baseline fasting glucose values and a significant worsening of fasting glucose (β = 0.48; 95% CI 0.10-0.86) and insulin secretion (β =-20.75; -32.67 to -8.82 for homeostasis model assessment for β-cell function) at follow-up by using generalized estimating equations. Incidence of impaired fasting glucose and diabetes was almost twofold higher in subjects homozygous for the A allele (respectively: incident impaired fasting glucose 7.6-8.2, 16.1%, incident diabetes 1.7-2.3, 3.2% in GG, AG, AA carriers).

CONCLUSIONS

Our results suggested that the rs553668 polymorphism is associated with glucose worsening in subjects without diabetes at baseline.

GPCR mediated regulation of synaptic transmission

Synaptic transmission is a finely regulated mechanism of neuronal communication. The release of neurotransmitter at the synapse is not only the reflection of membrane depolarization events, but rather, is the summation of interactions between ion channels, G protein coupled receptors, second messengers, and the exocytotic machinery itself which exposes the components within a synaptic vesicle to the synaptic cleft. The focus of this review is to explore the role of G protein signaling as it relates to neurotransmission, as well as to discuss the recently determined inhibitory mechanism of Gβγ dimers acting directly on the exocytotic machinery proteins to inhibit neurotransmitter release.

Plasma glucose, insulin, free fatty acids, lactate and cortisol concentrations in dexmedetomidine-sedated dogs with or without MK-467: a peripheral α-2 adrenoceptor antagonist

Six healthy laboratory Beagles were treated IV with 10 μg/kg dexmedetomidine (DEX) or 10 μg/kg dexmedetomidine combined with 500 μg/kg MK-467 in the same syringe (DMK) in a randomised cross-over design with a 14 day washout. Blood was collected immediately before treatment and 35, 60 and 120 min post-injection through a central venous catheter. The plasma concentrations of glucose, insulin, non-esterified free fatty acids (NEFAs), lactate and cortisol were determined. A repeated-measures ANOVA test was used to compare treatments and effects for each sample time point. Significant differences between treatments were found for plasma glucose (P=0.037) and insulin (P=0.009). DEX significantly increased plasma glucose at 120 min, but reduced plasma insulin at 35 and 60 min. NEFA decreased for both treatments at 35 min. This reduction was transient for DMK, whereas it persisted during the follow up period for DEX. Plasma lactate concentrations increased at 35 and 60 min with DEX. Neither treatment altered plasma cortisol concentrations. The addition of MK-467 to dexmedetomidine prevented or abolished most metabolic changes in healthy Beagles.

Involvement of α2-adrenoceptor subtypes A and C in glucose homeostasis and adrenaline-induced hyperglycaemia

BACKGROUND AND AIMS

Insulin secretion is controlled by pancreatic α(2A)-adrenoceptors. Mice lacking α(2A)-adrenoceptors (α(2A)AR(-/-) mice) show hyperinsulinaemia, reduced blood glucose levels and improved glucose tolerance.

METHODS

In the present study, we used α(2AC)AR(-/-), α(2C)AR(-/-) and α(2A)AR(-/-) mice and a mouse line with adrenergic cell-specific expression of α(2A)-adrenoceptors (lacking these receptors in non-adrenergic cells), and their wild-type (WT) controls, to assess the glucoregulatory role of the α(2C)-adrenoceptor subtype in vivo. Glucose and insulin tolerance tests were performed and blood glucose and serum insulin levels were determined after fasting and glucose stimulation. Plasma catecholamines were also measured. In addition, the effect of pretreatment with (±)-propranolol was determined in α(2C)AR(-/-) mice.

RESULTS

α(2AC)AR(-/-) mice had a similar glucose and insulin phenotype as α(2A)AR(-/-) mice and mice with restored α(2A)-autoreceptors, suggesting that only deletion of postsynaptic α(2A)-adrenoceptors has major effects on glucose disposition. However, α(2AC)AR(-/-) mice were more sensitive to the glucose-lowering effect of insulin than WT mice. This was not observed in α(2A)AR(-/-) mice. The α(2C)AR(-/-) mice showed impaired glucose tolerance that was reversed by pretreatment with (±)-propranolol. No difference in insulin secretion was observed in α(2C)AR(-/-) mice compared with WT animals.

CONCLUSION

The results underline that depletion of postsynaptic pancreatic α(2A)-adrenoceptors has major effects on the regulation of glucose homeostasis in α(2AC)AR(-/-) and α(2A)AR(-/-) mice. Deletion of the α(2C) subtype leads to increased adrenaline secretion and has the potential to increase blood glucose levels via enhanced glycogenolysis.

Atypical antipsychotics and effects of muscarinic, serotonergic, dopaminergic and histaminergic receptor binding on insulin secretion in vivo: an animal model

The atypical antipsychotics (AAPs) have been associated with increased risk of type-2 diabetes. Evidence suggests direct, drug-related effects independent of weight gain and although mechanisms underlying this phenomenon are unclear, it has been suggested that the heterogeneous receptor binding profile of the AAPs may influence receptors implicated in glucose metabolism. This study aimed to clarify weight gain-independent mechanisms of AAP-induced changes in insulin secretion by deconstructing their binding profile with representative antagonists. Healthy rats were pretreated with a single subcutaneous dose of darifenacin 6 mg/kg (n=10), a selective M(3) muscarinic antagonist; ketanserin 2mg/kg (n=10), a 5HT(2A) antagonist; raclopride 0.3mg/kg (n=11) a selective D(2)/D(3) antagonist; terfenadine 20mg/kg (n=9) a selective H(1) antagonist; or, vehicle (n=11). Hyperglycemic clamps were employed following injection, providing an index of secretory capacity of pancreatic β-cells. Acute treatment with darifenacin and ketanserin significantly decreased insulin response to glucose challenge as compared to controls, which was confirmed in the darifenacin group by reduced C-peptide levels. Treatment with raclopride resulted in an increased insulin response and a strong tendency to increased C-peptide levels. H(1) blockade did not result in effects on insulin or C-peptide. Results suggest that the effects of antipsychotics on glucose dysregulation may be related to direct inhibitory effects of muscarinic (M(3)) and serotonergic (5HT(2)) antagonism on insulin secretion. Based on the expression of D(2)-like receptors in β-cells, which mediate inhibition of insulin secretion, we propose that prolonged D(2) blockade with antipsychotics may predispose to depletion of insulin stores and an eventual defect in pancreatic compensation.

Variants of ADRA2A are associated with fasting glucose, blood pressure, body mass index and type 2 diabetes risk: meta-analysis of four prospective studies

AIMS/HYPOTHESIS

We quantified the effect of ADRA2A (encoding α-2 adrenergic receptor) variants on metabolic traits and type 2 diabetes risk, as reported in four studies.

METHODS

Genotype data for ADRA2A single nucleotide polymorphisms (SNPs) rs553668 and rs10885122 were analysed in >17,000 individuals (1,307 type 2 diabetes cases) with regard to metabolic traits and type 2 diabetes risk. Two studies (n = 9,437), genotyped using the Human Cardiovascular Disease BeadChip, provided 12 additional ADRA2A SNPs.

RESULTS

Rs553668 was associated with per allele effects on fasting glucose (0.03 mmol/l, p = 0.016) and type 2 diabetes risk (OR 1.17, 95% CI 1.04-1.31; p = 0.01). No significant association was observed with rs10885122. Of the 12 SNPs, several showed associations with metabolic traits. Overall, after variable selection, rs553668 was associated with type 2 diabetes risk (OR 1.38, 95% CI 1.09-1.73; p = 0.007). rs553668 (per allele difference 0.036 mmol/l, 95% CI 0.008-0.065) and rs17186196 (per allele difference 0.066 mmol/l, 95% CI 0.017-0.115) were independently associated with fasting glucose, and rs17186196 with fasting insulin and HOMA of insulin resistance (4.3%, 95% CI 0.6-8.1 and 4.9%, 95% CI 1.0-9.0, respectively, per allele). Per-allele effects of rs491589 on systolic and diastolic blood pressure were 1.19 mmHg (95% CI 0.43-1.95) and 0.61 mmHg (95% CI 0.11-1.10), respectively, and those of rs36022820 on BMI 0.58 kg/m(2) (95% CI 0.15-1.02).

CONCLUSIONS/INTERPRETATION

Multiple ADRA2A SNPs are associated with metabolic traits, blood pressure and type 2 diabetes risk. The α-2 adrenergic receptor should be revisited as a therapeutic target for reduction of the adverse consequences of metabolic trait disorders and type 2 diabetes.

G protein-coupled receptors: mutations and endocrine diseases

Over the past 20 years, naturally occurring mutations that affect G protein-coupled receptors (GPCRs) have been identified, mainly in patients with endocrine diseases. The study of loss-of-function or gain-of-function mutations has contributed to our understanding of the pathophysiology of several diseases with classic hypophenotypes or hyperphenotypes of the target endocrine organs, respectively. Simultaneously, study of the mutant receptors ex vivo was instrumental in delineating the relationships between the structure and function of these important physiological and pharmacological molecules. Now that access to the crystallographic structure of a few GPCRs is available, the mechanics of these receptors can be studied at the atomic level. Progress in the fields of cell biology, molecular pharmacology and proteomics has also widened our view of GPCR functions. Initially considered simply as guanine nucleotide exchange factors capable of activating G protein-dependent regulatory cascades, GPCRs are now known to display several additional characteristics, each susceptible to alterations by disease-causing mutations. These characteristics include functionally important basal activity of the receptor; differential activation of various G proteins; differential activation of G protein-dependent and independent effects (biased agonism); interaction with proteins that modify receptor function; dimerization-dependent effects; and interaction with allosteric modulators. This Review attempts to illustrate how natural mutations of GPCR could contribute to our understanding of these novel facets of GPCR biology.

α2-adrenoceptor regulation of blood glucose homeostasis

The α(2A)-adrenoceptor has been identified as an important regulator of blood glucose homeostasis. α(2A)-Adrenoceptors on pancreatic β-cells inhibit insulin secretion, and α(2A)-adrenoceptors on sympathetic nerves and on adrenomedullary chromaffin cells limit sympathoadrenal output. Recently, human α(2A)-adrenoceptor gene polymorphisms that influence α(2A)-adrenoceptor expression and function have been described. Increased α(2A)-adrenoceptor expression has been associated with impaired glucose-stimulated insulin secretion, elevated fasting blood glucose levels and an increased risk of type 2 diabetes. Accordingly, administration of α(2)-adrenoceptor agonists generally increases blood glucose levels, in spite of the ensuing sympatholysis that would be expected to lower blood glucose as a result of diminished α(1)- and β-adrenoceptor activation. α(2)-Adrenoceptor antagonists increase insulin secretion and reduce blood glucose levels by inhibiting tonically active α(2A)-adrenoceptors on pancreatic β-cells, but may also enhance sympathoadrenal output. In addition, α(2)-adrenoceptor antagonists potentiate the insulinotropic effect of sulphonylurea drugs, pointing to a potentially serious adverse drug interaction when the two classes of drugs are combined. The α(2)-adrenoceptor antagonist atipamezole is widely used in veterinary medicine, and sulphonylureas are prescribed for the treatment of type 2 diabetes in cats and dogs. Even if no dedicated α(2)-adrenoceptor antagonists are in clinical use in humans, some antipsychotic and antidepressant drugs are relatively potent α(2)-adrenoceptor antagonists. In the treatment of type 2 diabetes, α(2)-adrenoceptor agonists could possibly protect against sulphonylurea-induced hypoglycaemia, and α(2)-adrenoceptor antagonist drugs could improve insulin secretion. The potential usefulness of such drugs may vary between individuals, depending on α(2A)-adrenoceptor genetics, sympathetic tone and concomitant pathological conditions, such as cardiovascular disease and obesity.

Pharmacological modulation of dopamine receptor D2-mediated transmission alters the metabolic phenotype of diet induced obese and diet resistant C57Bl6 mice

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

Differential sensitivity to adrenergic stimulation underlies the sexual dimorphism in the development of diabetes caused by Irs-2 deficiency

The diabetic phenotype caused by the deletion of insulin receptor substrate-2 (Irs-2) in mice displays a sexual dimorphism. Whereas the majority of male Irs-2(-/-) mice are overtly diabetic by 12 weeks of age, female Irs-2(-/-) animals develop mild obesity and progress less rapidly to diabetes. Here we investigated β-cell function and lipolysis as potential explanations for the gender-related differences in this model. Glucose-stimulated insulin secretion was enhanced in islets from male null mice as compared to male WT whereas this response in female Irs-2(-/-) islets was identical to that of female controls. The ability of α(2)-adrenoceptor (α(2)-AR) agonists to inhibit insulin secretion was attenuated in male Irs2 null mice. Consistent with this, the expression of the α(2A)-AR was reduced in male Irs-2(-/-) islets. The response of male Irs-2(-/-) islets to forskolin was enhanced, owing to increased production of cAMP. Basal lipolysis was increased in male Irs-2(-/-) but decreased in female Irs-2(-/-) mice, concordant with the observation that adipose tissue is sparse in males whereas female Irs2 null mice are mildly obese. Adipocytes from both male and female Irs-2(-/-) were resistant to the anti-lipolytic effects of insulin but female Irs-2(-/-) fat cells were additionally resistant to the catabolic effects of beta-adrenergic agonists. This catecholamine resistance was associated with impaired generation of cAMP. Consequently, targets of cAMP-dependent protein kinase (PKA) which mediate lipolysis were not phosphorylated in adipose tissue of female Irs-2(-/-) mice. Our findings suggest that IRS-2 deficiency in mice alters the expression and/or sensitivity of components of adrenergic signaling.

Adrenergic signaling polymorphisms and their impact on cardiovascular disease

This review examines the impact of recent discoveries defining personal genetics of adrenergic signaling polymorphisms on scientific discovery and medical practice related to cardiovascular diseases. The adrenergic system is the major regulator of minute-by-minute cardiovascular function. Inhibition of adrenergic signaling with pharmacological beta-adrenergic receptor antagonists (beta-blockers) is first-line therapy for heart failure and hypertension. Advances in pharmacology, molecular biology, and genetics of adrenergic signaling pathways have brought us to the point where personal genetic differences in adrenergic signaling factors are being assessed as determinants of risk or progression of cardiovascular disease. For a few polymorphisms, functional data generated in cell-based systems, genetic mouse models, and pharmacological provocation of human subjects are concordant with population studies that suggest altered risk of cardiovascular disease or therapeutic response to beta-blockers. For the majority of adrenergic pathway polymorphisms however, published data conflict, and the clinical relevance of individual genotyping remains uncertain. Here, the current state of laboratory and clinical evidence that adrenergic pathway polymorphisms can affect cardiovascular pathophysiology is comprehensively reviewed and compared, with a goal of placing these data in the broad context of potential clinical applicability.

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the alpha2-adrenergic receptors in beta cells

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the alpha2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic alpha2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.

Identification of known and novel pancreas genes expressed downstream of Nkx2.2 during development

BACKGROUND

The homeodomain containing transcription factor Nkx2.2 is essential for the differentiation of pancreatic endocrine cells. Deletion of Nkx2.2 in mice leads to misspecification of islet cell types; insulin-expressing beta cells and glucagon-expressing alpha cells are replaced by ghrelin-expressing cells. Additional studies have suggested that Nkx2.2 functions both as a transcriptional repressor and activator to regulate islet cell formation and function. To identify genes that are potentially regulated by Nkx2.2 during the major wave of endocrine and exocrine cell differentiation, we assessed gene expression changes that occur in the absence of Nkx2.2 at the onset of the secondary transition in the developing pancreas.

RESULTS

Microarray analysis identified 80 genes that were differentially expressed in e12.5 and/or e13.5 Nkx2.2-/- embryos. Some of these genes encode transcription factors that have been previously identified in the pancreas, clarifying the position of Nkx2.2 within the islet transcriptional regulatory pathway. We also identified signaling factors and transmembrane proteins that function downstream of Nkx2.2, including several that have not previously been described in the pancreas. Interestingly, a number of known exocrine genes are also misexpressed in the Nkx2.2-/- pancreas.

CONCLUSIONS

Expression profiling of Nkx2.2-/- mice during embryogenesis has allowed us to identify known and novel pancreatic genes that function downstream of Nkx2.2 to regulate pancreas development. Several of the newly identified signaling factors and transmembrane proteins may function to influence islet cell fate decisions. These studies have also revealed a novel function for Nkx2.2 in maintaining appropriate exocrine gene expression. Most importantly, Nkx2.2 appears to function within a complex regulatory loop with Ngn3 at a key endocrine differentiation step.

Overexpression of alpha2A-adrenergic receptors contributes to type 2 diabetes

Several common genetic variations have been associated with type 2 diabetes, but the exact disease mechanisms are still poorly elucidated. Using congenic strains from the diabetic Goto-Kakizaki rat, we identified a 1.4-megabase genomic locus that was linked to impaired insulin granule docking at the plasma membrane and reduced beta cell exocytosis. In this locus, Adra2a, encoding the alpha2A-adrenergic receptor [alpha(2A)AR], was significantly overexpressed. Alpha(2A)AR mediates adrenergic suppression of insulin secretion. Pharmacological receptor antagonism, silencing of receptor expression, or blockade of downstream effectors rescued insulin secretion in congenic islets. Furthermore, we identified a single-nucleotide polymorphism in the human ADRA2A gene for which risk allele carriers exhibited overexpression of alpha(2A)AR, reduced insulin secretion, and increased type 2 diabetes risk. Human pancreatic islets from risk allele carriers exhibited reduced granule docking and secreted less insulin in response to glucose; both effects were counteracted by pharmacological alpha(2A)AR antagonists.

ADRA2A polymorphism and smoking in a Turkish population

Adrenoceptors (ARs) consist of nine subtypes, which are involved in a wide spectrum of physiological functions and are the site of action for a considerable percentage of currently prescribed therapeutics. All AR subtypes (except α1D) can be polymorphic because of the genetic variations in the coding and non-coding regions. Sixteen sequence variations were identified in α-adrenergic 2A (ADRA2A) gene. Among them, ADRA2A C1291G polymorphism is one of the most important polymorphisms, which plays a major role in regulating neurotransmitter release, blood pressure, lipolysis, insulin secretion, and platelet aggregation. A C–G transversion results in an MspI restriction fragment length polymorphism located at 1291 bp upstream of the origin of transcription. Because Medline search showed no study showing the allelic frequencies, and no information is available on inter-individual variability of ADRA2A C1291G polymorphism in Turkish population, we genotyped 203 healthy Turkish subjects. Because of large genetic variation of the polymorphism, we aimed to find out the distribution of C1291G polymorphism in Turkish population. Furthermore, we evaluated the possible association between the C1291G polymorphism in the ADRA2A receptor gene and smoking. The frequencies for the 1291C and 1291G alleles were 64% and 36%, respectively. The genotype frequencies for C1291C, C1291G, and G1291G were 35.5%, 57.6%, and 6.9%, respectively, in Turkish population. The allelic frequencies (1291C and 1291G) and G1291G homozygous variant genotype were similar to those reported in different Caucasian populations; however, C1291C and C1291G genotypes were different. We also observed that the frequency of the G allele was slightly higher in smoker subjects and lower among controls. The ADRA2A G allele may play a role in the predisposition to smoking. There is a need for expanding genotype and haplotype studies because of its importance in various physiological disorders and to confirm the association of this polymorphism with smoking.

Identification of Glucose-Dependent Insulin Secretion Targets in Pancreatic β Cells by Combining Defined-Mechanism Compound Library Screening and siRNA Gene Silencing

Identification and validation of novel drug targets continues to be a major bottleneck in drug development, particularly for polygenic complex diseases such as type 2 diabetes. Here, the authors describe an approach that allows researchers to rapidly identify and validate potential drug targets by combining chemical tools and RNA interference technology. As a proof-of-concept study, the known mechanism Sigma LOPAC library was used to screen for glucose-dependent insulin secretion (GDIS) in INS-1 832/13 cells. In addition to several mechanisms that are known to regulate GDIS (such as cyclic adenosine monophosphate—specific phosphodiesterases, adrenoceptors, and Ca2+ channels), the authors find that several of the dopamine receptor ( DRD) antagonists significantly enhance GDIS, whereas DRD agonists profoundly inhibit GDIS. Subsequent siRNA studies in the same cell line indicate that knockdown of DRD2 enhanced GDIS. Furthermore, selective DRD2 antagonists and agonists also enhance or suppress, respectively, GDIS in isolated rat islets. The data support that the approach described here offers a rapid and effective way for target identification and validation. ( Journal of Biomolecular Screening 2008;128-134)

Reduced blood glucose levels, increased insulin levels and improved glucose tolerance in alpha2A-adrenoceptor knockout mice

Alpha(2)-Adrenoceptors regulate insulin secretion and sympathetic output. In the present study, alpha(2A)-adrenoceptor knockout (alpha(2A)-KO) mice and their C57BL/6J wild-type (WT) controls were used to assess the glucoregulatory role of the alpha(2A)-adrenoceptor subtype in vivo. Fasting and glucose-stimulated blood glucose and plasma insulin levels were determined with or without (+/-)-propranolol (5 mg/kg) or atropine (10 mg/kg) pre-treatment. Intraperitoneal glucose (1 g/kg) and insulin (0.5 and 1.0 IU/kg) tolerance tests were performed. Fasting plasma glucagon and corticosterone levels were measured. Blood glucose levels (mean+/-S.E.M.) were lower in alpha(2A)-KO males (7.2+/-0.6 mM) and females (7.2+/-0.2 mM) than in WT males (9.8+/-0.3 mM) and females (9.1+/-0.3 mM). Plasma insulin levels were higher in alpha(2A)-KO males (2.2+/-0.5 microg/l) and females (1.7+/-0.3 microg/l) than in WT males (0.7+/-0.1 microg/l) and females (0.8+/-0.2 microg/l). These differences remained after pharmacological beta-adrenoceptor and muscarinic acetylcholine receptor inhibition. In spite of a tendency for slightly decreased insulin sensitivity in alpha(2A)-KO mice, glucose tolerance in alpha(2A)-KO mice was significantly better than in WT mice. However, glucose-stimulated insulin secretion was not increased in alpha(2A)-KO mice compared to WT controls. Plasma glucagon levels, but not corticosterone levels, were elevated in alpha(2A)-KO mice. These results suggest that lack of inhibitory pancreatic beta-cell alpha(2A)-adrenoceptor function results in hyperinsulinaemia, reduced blood glucose levels and improved glucose tolerance in alpha(2A)-KO mice, and demonstrate a key role for the alpha(2A)-adrenoceptor in adrenergic regulation of blood glucose and insulin homeostasis.

G-protein-coupled receptors and islet function-implications for treatment of type 2 diabetes

Islet function is regulated by a number of different signals. A main signal is generated by glucose, which stimulates insulin secretion and inhibits glucagon secretion. The glucose effects are modulated by many factors, including hormones, neurotransmitters and nutrients. Several of these factors signal through guanine nucleotide-binding protein (G protein)-coupled receptors (GPCR). Examples of islet GPCR are GPR40 and GPR119, which are GPCR with fatty acids as ligands, the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), the receptors for the islet hormones glucagon and somatostatin, the receptors for the classical neurotransmittors acetylcholine (ACh; M(3) muscarinic receptors) and noradrenaline (beta(2)- and alpha(2)-adrenoceptors) and for the neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP; PAC(1) and VPAC(2) receptors), cholecystokinin (CCK(A) receptors) and neuropeptide Y (NPY Y1 receptors). Other islet GPCR are the cannabinoid receptor (CB(1) receptors), the vasopressin receptors (V1(B) receptors) and the purinergic receptors (P(2Y) receptors). The islet GPCR couple mainly to adenylate cyclase and to phospholipase C (PLC). Since important pharmacological strategies for treatment of type 2 diabetes are stimulation of insulin secretion and inhibition of glucagon secretion, islet GPCR are potential drug targets. This review summarizes knowledge on islet GPCR.

Adrenergic drugs modify the level of noradrenaline in the insular cortex and alter extinction of conditioned taste aversion in rats

We compared the effect of conditioned taste aversion in rats by measuring the amount of sucrose that they drunk after conditioning, which differed according to whether rats had drunk the sucrose freely (SD: self drinking) during the conditioning session, or had been forced to drink it (IO: intra-oral administration through a chronically implanted cannula). The SD procedure delayed the extinction of conditioned taste aversion. Enhanced arousal, alertness, awareness or attention in the SD condition may have strengthened the memory of the taste. Brain noradrenergic networks are involved in such processes. We administered two noradrenergic drugs that produce opposite effects on noradrenaline release in the brain, methoxy-idazoxan, RX821002 (1mg/kg, i.p.), and guanfacine (0.12mg/kg, i.p.). We evaluated their effect (i) on the level of noradrenaline in the gustatory cortex using microdialysis, (ii) on glycaemia that is an essential factor of taste learning and (iii) on the comparative SD versus IO conditioned taste aversion protocol mentioned above. Injecting RX821001 increased the level of noradrenaline in the gustatory cortex up to two-fold of the baseline. This effect lasted 1h. The same dose of RX821002 did not elicit any alteration of glycaemia. It enhanced extinction of conditioned taste aversion in the SD group of rats. Injecting 0.12mg/kg of guanfacine produced the opposite effect. The noradrenaline level of the gustatory cortex decreased, but only down to 20% of the baseline. This decrease lasted 2h. Guanfacine increased glycaemia. Extinction of conditioned taste aversion was only marginally decreased by guanfacine in the SD group of rats. These results fit with Aston-Jones' point of view that the role of the noradrenergic coeruleo-cortical system may be to enhance arousal, alertness, awareness or attention to an event by a transient increase of cortical noradrenaline.

High-resolution quantitative trait locus analysis reveals multiple diabetes susceptibility loci mapped to intervals<800 kb in the species-conserved Niddm1i of the GK rat

Niddm1i, a 16-Mb locus within the major diabetes QTL in the diabetic GK rat, causes impaired glucose tolerance in the congenic NIDDM1I strain. Niddm1i is homologous to both human and mouse regions linked with type 2 diabetes susceptibility. We employed multiple QTL analyses of congenic F2 progeny selected for one recombination event within Niddm1i combined with characterization of subcongenic strains. Fine mapping located one hyperglycemia locus within 700 kb (Niddm1i4, P=5x10(-6)). Two adjacent loci were also detected, and the GK allele at Niddm1i2 (500 kb) showed a glucose-raising effect, whereas it had a glucose-lowering effect at Niddm1i3 (400 kb). Most proximally, Niddm1i1 (800 kb) affecting body weight was identified. Experimental data from subcongenics supported the four loci. Sorcs1, one of the two known diabetes susceptibility genes in the region, resides within Niddm1i3, while Tcf7l2 maps outside all four loci. Multiple-marker QTL analysis incorporating the effect of cosegregating QTL as cofactors together with genetically selected progeny can remarkably enhance resolution of QTL. The data demonstrate that the species-conserved Niddm1i is a composite of at least four QTL affecting type 2 diabetes susceptibility and that two adjacent QTL (Niddm1i2GK and Niddm1i3GK) act in opposite directions.

Identification of ADRA2A polymorphisms related to shear-mediated platelet function

alpha2A adrenergic receptor (ADRA2A) on platelets interacts with epinephrine, which has a key role in regulating platelet functions. There is familial clustering of inter-individual variations in the epinephrine-induced platelet aggregation, the molecular basis of which, however, has not been fully understood. In this study, we screened the sequence variations in the transcriptional region of ADRA2A gene and analyzed the relationship between the two common polymorphisms and platelet function using epinephrine/collagen cartridge in the platelet function analyzer-100 system, in a healthy Japanese male population (n=211). Among the identified 16 sequence variations including five novel variations, 1780GG genotype was associated with longer closure time which represents low platelet function under high shear-stress conditions (p=0.0478). We also observed enhanced effect of the combination of 1780GG and 2372AA genotypes on longer closure time (p=0.0319). These findings suggest that 1780A/G and 2372A/G polymorphisms are associated with platelet function in interactions with collagen/epinephrine.

Therapeutic potential of RNAi in metabolic diseases

Over the past years RNA interference (RNAi) has exploded as a new approach to manipulate gene expression in mammalian systems. More recently, RNAi has acquired interest as a potential therapeutic strategy. This review focuses on the potential therapeutic use of RNAi for metabolic diseases, the current understanding of RNAi biology, and how RNAi has been utilized to study the role of different genes in the pathogenesis of diabetes and obesity. Also reviewed are the in vivo proof-of-principle experiments that provide the preclinical justification for the development of RNAi-based therapeutics for diabetes and the key challenges that currently limit its application in the clinical setting.

Altered glucose homeostasis in alpha2A-adrenoceptor knockout mice

To elucidate the functions of alpha2-adrenoceptor subtypes in metabolic regulation, we determined plasma glucose and insulin levels and tissue uptake of the glucose analogue 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) in C57Bl/6J wild-type (WT) and alpha2A-adrenoceptor knockout (alpha2A-KO) mice at baseline and following alpha2-adrenoceptor agonist ((+)-4-(S)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole (dexmedetomidine)) and antagonist (4-[2-ethyl-2,3-dihydro-1H-inden-2-yl]-1H-imidazole (atipamezole)) administration. Basal glucose levels were 30% lower in alpha2A-KO mice than in WT mice. In WT mice, dexmedetomidine lowered insulin and elevated glucose levels, and atipamezole reduced glucose levels. In alpha2A-KO mice, neither drug affected the glucose or insulin levels. [18F]FDG uptake was investigated in plasma, heart, liver, kidney, pancreas, lung, fat, and skeletal muscle. Cardiac [18F]FDG uptake was a sensitive indicator of sympathetic function. Liver [18F]FDG uptake conformed to the plasma glucose levels. In alpha2A-KO mice, drug effects on [18F]FDG tissue uptake were absent. Thus, the alpha2A-adrenoceptor is the alpha2-adrenoceptor subtype primarily involved in the regulation of blood glucose homeostasis in vivo.

Pertussis toxin-sensitive pathway inhibits glucose-stimulated Ca2+ signals of rat islet β-cells by affecting L-type Ca2+ channels and voltage-dependent K+ channels

A role of pertussis toxin (PTX)-sensitive pathway in regulation of glucose-stimulated Ca2+ signaling in rat islet beta-cells was investigated by using clonidine as a selective agonist to alpha2-adrenoceptors which link to the pathway. An elevation of extracellular glucose concentration from 5.5 to 22.2 mM (glucose stimulation) increased the levels of [Ca2+]i of beta-cells, and clonidine reversibly reduced the elevated levels of [Ca2+]i. This clonidine effect was antagonized by yohimbine, and abolished in beta-cells pre-treated with PTX. Clonidine showed little effect on membrane currents including those through ATP-sensitive K+ channels induced by voltage ramps from -90 to -50 mV. Clonidine showed little effect on the magnitude of whole-cell currents through L-type Ca2+ channels (ICa(L)), but increased the inactivation process of the currents. Clonidine increased the magnitude of the voltage-dependent K+ currents (IVK). These clonidine effects on ICa(L) and IVK were abolished in beta-cells treated with PTX or GDP-betaS. These results suggest that the PTX-sensitive pathway increases IVK activity and decreases ICa(L) activity of islet beta-cells, resulting in a decrease in the levels of [Ca2+]i elevated by depolarization-induced Ca2+ entry. This mechanism seems responsible at least in part for well-known inhibitory action of PTX-sensitive pathway on glucose-stimulated insulin secretion from islet beta-cells.

Aged transgenic mice with increased glucocorticoid sensitivity in pancreatic beta-cells develop diabetes

Glucocorticoids are diabetogenic hormones because they decrease glucose uptake, increase hepatic glucose production, and inhibit insulin release. To study the long-term effects of increased glucocorticoid sensitivity in beta-cells, we studied transgenic mice overexpressing the rat glucocorticoid receptor targeted to the beta-cells using the rat insulin I promoter. Here we report that these mice developed hyperglycemia both in the fed and the overnight-fasted states at 12-15 months of age. Progression from impaired glucose tolerance, previously observed in the same colony at the age of 3 months, to manifest diabetes was not associated with morphological changes or increased apoptosis in the beta-cells. Instead, our current results suggest that the development of diabetes is due to augmented inhibition of insulin secretion through alpha(2)-adrenergic receptors (alpha(2)-ARs). Thus, we found a significantly higher density of alpha(2)-ARs in the islets of transgenic mice compared with controls, based on binding studies with the alpha(2)-AR agonist UK 14304. Furthermore, incubation of islets with benextramine, a selective antagonist of the alpha(2)-AR, restored insulin secretion in response to glucose in isolated islets from transgenic mice, whereas it had no effect on control islets. These results indicate that the chronic enhancement of glucocorticoid signaling in pancreatic beta-cells results in hyperglycemia and impaired glucose tolerance. This effect may involve signaling pathways that participate in the regulation of insulin secretion via the alpha(2)-AR.

Diabetes and insulin secretion: whither KATP?

The critical involvement of ATP-sensitive potassium (KATP) channels in insulin secretion is confirmed both by the demonstration that mutations that reduce KATP channel activity underlie many if not most cases of persistent hyperinsulinemia, and by the ability of sulfonylureas, which inhibit KATP channels, to enhance insulin secretion in type II diabetics. By extrapolation, we contend that mutations that increase beta-cell KATP channel activity should inhibit glucose-dependent insulin secretion and underlie, or at least predispose to, a diabetic phenotype. In transgenic animal models, this prediction seems to be borne out. Although earlier genetic studies failed to demonstrate a linkage between KATP mutations and diabetes in humans, recent studies indicate significant association of KATP channel gene mutations or polymorphisms and type II diabetes. We suggest that further efforts to understand the involvement of KATP channels in diabetes are warranted.

Role of mu-opioid receptors in insulin release in the presence of inhibitory and excitatory secretagogues

In mouse pancreatic islets incubated under static conditions, the inhibitory effects on glucose-evoked insulin release induced by adrenaline (1 microM), clonidine (2 microM) and UK 14,304 (brimonidine, 0.001-1 microM) were abolished by naloxone (30 nM). Only CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2), 0.1 microM), a very selective mu-opioid receptor antagonist, blocked the response to UK 14,304. Glucose-induced insulin secretion was attenuated by both beta-endorphin (0.01 microM) and endomorphin-1 (0.1 microM). Naloxone and CTOP prevented these inhibitory responses. The stimulatory effect of glibenclamide (1 microM) was also reduced by endomorphin-1. However, when islets were incubated in the presence of K(+) (30 mM), carbachol (100 microM) or forskolin (0.1 microM), neither the inhibitory effect induced by UK 14,304 was reversed by naloxone, nor endomorphin-1 altered the responses promoted by the excitatory agents. Thus, alpha(2)-adrenoceptor stimulation might inhibit glucose-induced insulin secretion by releasing endogenous opioids. Mu-Opioid receptor activation and opening of K(ATP) channels could be involved in the response.

A C-1291G polymorphism in the alpha2A-adrenergic receptor gene (ADRA2A) promoter is associated with cortisol escape from dexamethasone and elevated glucose levels

OBJECTIVES

The objective of the current study was to examine the potential impact of a C right arrow G substitution at position -1291 of the alpha2A-adrenergic receptor gene (ADRA2A) promoter on obesity and estimates of insulin, glucose, and lipid metabolism as well as circulating hormones, including salivary cortisol in 284 unrelated Swedish men born in 1944.

MAIN OUTCOME MEASURES

The subjects were genotyped by using PCR amplification of the promoter region of the ADRA2A gene followed by digestion with the restriction enzyme MspI.

RESULTS

The frequencies were 0.23 for allele C and 0.77 for allele G. The observed genotype frequencies were 45.8 and 54.2% for C/G and G/G, respectively. Heterozygotes (n=121) had significantly (P=0.009) higher salivary cortisol levels after 0.5 mg dexamethasone compared with G/G homozygotes (n=143). Fasting glucose was found to be significantly (P=0.017) higher in heterozygotes than in G/G homozygotes. The latter group had also a borderline significantly (P=0.080) higher mean diastolic blood pressure. These results were all adjusted for the potential confounding effect of body mass index (BMI) and waist-to-hip ratio (WHR). Other measurements such as BMI, WHR, abdominal sagittal diameter, total testosterone, insulin-like growth factor I, serum leptin, fasting insulin and serum lipids were not different across the ADRA2A genotype groups.

CONCLUSIONS

In conclusion, we have shown that an C --> G polymorphism at position -1291 of the ADRA2A gene is associated with a subnormal cortisol response to dexamethasone, elevated glucose levels and perhaps increased diastolic blood pressure. The pathophysiology could involve an altered density of the alpha2A-AR that destabilizes the sympathetic-hypothalamic-pituitary-adrenal systems in those with genetic vulnerability in the alpha2A-adrenergic receptor gene promoter.