Stimulation of insulin secretion in clonal BRIN-BD11 cells by the imidazoline derivatives KU14r and RX801080

Investigation of morin-induced insulin secretion in cultured pancreatic cells

Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetes. Insulin secretion has been demonstrated to increase following the administration of morin. The present study is designed to investigate the potential mechanism(s) of morin-induced insulin secretion in the MIN6 cell line. First, we identified that morin induced a dose-dependent increase in insulin secretion and intracellular calcium content in MIN6 cells. Morin potentiated glucose-stimulated insulin secretion (GSIS). Additionally, we used siRNA for the ablation of imidazoline receptor protein (NISCH) expression in MIN6 cells. Interestingly, the effects of increased insulin secretion by morin and canavanine were markedly reduced in Si-NISCH cells. Moreover, we used KU14R to block imidazoline I₃ receptor (I-3R) that is known to enhance insulin release from the pancreatic β-cells. Without influence on the basal insulin secretion, KU14R dose-dependently inhibited the increased insulin secretion induced by morin or efaroxan in MIN6 cells. Additionally, effects of increased insulin secretion by morin or efaroxan were reduced by diazoxide at the dose sufficient to open K_ATP channels and attenuated by nifedipine at the dose used to inhibit L-type calcium channels. Otherwise, phospholipase C (PLC) is introduced to couple with imidazoline receptor (I-R). The PLC inhibitor dose-dependently inhibited the effects of morin in MIN6 cells. Similar blockade was also observed in protein kinase C (PKC) inhibitor-treated cells. Taken together, we found that morin increases insulin secretion via the activation of I-R in pancreatic cells. Therefore, morin would be useful to develop in the research and treatment of diabetic disorders.

Comparing BRIN-BD11 culture producing insulin using different type of microcarriers

This research was conducted to examine the growth profile, growth kinetics, and insulin-secretory responsiveness of BRIN-BD11 cells grown in optimized medium on different types of microcarriers (MCs). Comparisons were made on modified polystyrene (Hillex(®) II) and crosslinked polystyrene Plastic Plus (PP) from Solohill Engineering. The cell line producing insulin was cultured in a 25 cm(2) T-flask as control while MCs based culture was implemented in a stirred tank bioreactor with 1 L working volume. For each culture type, the viable cell number, glucose, lactate, glutamate, and insulin concentrations were measured and compared. Maximum viable cell number was obtained at 1.47 × 10(5) cell/mL for PP microcarrier (PPMCs) culture, 1.35 × 10(5) cell/mL Hillex(®) II (HIIMCs) culture and 0.95 × 10(5) cell/mL for T-flask culture, respectively. The highest insulin concentration has been produced in PPMCs culture (5.31 mg/L) compared to HIIMCs culture (2.01 mg/L) and T-flask culture (1.99 mg/L). Therefore overall observation suggested that PPMCs was likely preferred to be used for BRIN-BD11 cell culture as compared with Hillex(®) II MCs.

Rhes expression in pancreatic beta-cells is regulated by efaroxan in a calcium-dependent process

The monomeric G-protein Rhes has been described to be present in pancreatic beta-cells, and a putative role in the control of insulin release has been proposed. Here, we show that treatment of beta-cells with the imidazoline insulin secretagogue efaroxan resulted in a concentration- and time-dependent increase in the expression of Rhes, which peaked after 4h of efaroxan exposure; thereafter, Rhes mRNA levels decreased. Marked stereoselectivity was displayed, with (-)-efaroxan (the selectively insulinotropic enantiomer) being much more effective than (+)-efaroxan at raising Rhes transcript levels. The mechanism by which Rhes gene expression is activated in beta-cells appears to require the influx of extracellular calcium and de novo protein synthesis, and is not directly associated with the release of insulin. The present results confirm our earlier proposal that Rhes is an imidazoline-regulated transcript in pancreatic beta-cells. Studies to understand the role of Rhes as a regulator of beta-cell function are, thus, warranted.

Essential role of the imidazoline moiety in the insulinotropic effect but not the KATP channel-blocking effect of imidazolines; a comparison of the effects of efaroxan and its imidazole analogue, KU14R

AIMS/HYPOTHESIS

Imidazolines are a class of investigational antidiabetic drugs. It is still unclear whether the imidazoline ring is decisive for insulinotropic characteristics.

MATERIALS AND METHODS

We studied the imidazoline efaroxan and its imidazole analogue, KU14R, which is currently classified as an imidazoline antagonist. The effects of both on stimulus secretion-coupling in normal mouse islets and beta cells were compared by measuring KATP channel activity, plasma membrane potential, cytosolic calcium concentration ([Ca2+]c) and dynamic insulin secretion.

RESULTS

In the presence of 10 mmol/l but not of 5 mmol/l glucose, efaroxan (100 micromol/l) strongly enhanced insulin secretion by freshly isolated perifused islets, whereas KU14R (30, 100 or 300 micromol/l) was ineffective at both glucose concentrations. Surprisingly, the insulinotropic effect of efaroxan was not antagonised by KU14R. KATP channels were blocked by efaroxan (IC50 8.8 micromol/l, Hill slope -1.1) and by KU14R (IC50 31.9 micromol/l, Hill slope -1.5). Neither the KATP channel-blocking effect nor the depolarising effect of efaroxan was antagonised by KU14R. Rather, both compounds strongly depolarised the beta cell membrane potential and induced action potential spiking. However, KU14R was clearly less efficient than efaroxan in raising [Ca2+]c in single beta cells and whole islets at 5 mmol/l glucose. The increase in [Ca2+]c induced by 10 mmol/l glucose was affected neither by efaroxan nor by KU14R. Again, KU14R did not antagonise the effects of efaroxan.

CONCLUSIONS/INTERPRETATION

The presence of an imidazole instead of an imidazoline ring leads to virtually complete loss of the insulinotropic effect in spite of a preserved ability to block KATP channels. The imidazole compound is less efficient in raising [Ca2+]c; in particular, it lacks the ability of the imidazoline to potentiate the enhancing effect of energy metabolism on Ca2+-induced insulin secretion.

Acute and long-term effects of nateglinide on insulin secretory pathways

Acute and chronic effects of the insulinotropic drug nateglinide upon insulin release were examined in the BRIN-BD11 cell line. Nateglinide (10-400 microm) stimulated a concentration-dependent increase (P<0.05-P<0.001) in insulin release at a non-stimulatory (1.1 mm) glucose concentration. The insulinotropic response to 200 microm nateglinide was increased at 30 mm (P<0.01), but not 5.6-16.7 mm glucose concentrations. In depolarized cells, nateglinide (50-200 microm) evoked K(ATP) channel-independent insulin secretion (P<0.05-P<0.001) in the absence and presence of 5.6-30.0 mm glucose (P<0.001). Exposure for 18 h to 100 microm nateglinide abolished the acute insulinotropic effects of 200 microm nateglinide, tolbutamide or glibenclamide, but had no effect upon the insulinotropic effect of 200 microm efaroxan. While 18 h exposure to 100 microm nateglinide did not affect basal insulin release or insulin release in the presence of 16.7 mm glucose, 25 microm forskolin or 10 nm PMA, significant inhibition of the insulinotropic effects of 20 mm leucine and 20 mm arginine were observed. These data show that nateglinide stimulates both K(ATP) channel-dependent and-independent insulin secretion. The maintained insulinotropic effects of this drug with increasing glucose concentrations support the antihyperglycaemic actions of nateglinide in Type II diabetes. Studies of the long-term effects of nateglinide indicate that nateglinide shares signalling pathways with sulphonylureas, but not the imidazoline efaroxan. This may be significant when considering a nateglinide treatment regimen, particularly in patients previously treated with sulphonylurea.

Novel strategies for the pharmacological management of type 2 diabetes

Type 2 diabetes is characterized by high concentrations of glucose in the blood, which is caused by decreased secretion of insulin from the pancreas and decreased insulin action. This condition is prevalent worldwide and is associated with morbidity and mortality secondary to complications such as myocardial infarction, stroke and end-stage renal disease. The importance of tight control of blood glucose in either preventing or delaying the progression of complications is recognized. Currently, there are many therapeutic options to treat hyperglycemia in type 2 diabetes. However, tight control is difficult to achieve and is often associated with side-effects. Recent advances in understanding insulin secretion, action and signaling have led to the development of new pharmacological agents. In this article, we review new molecules that are promising candidates for the future management of diabetes, focusing on their mechanism of action, efficacy, safety profile and potential benefits compared with pharmacological agents that are available currently.

Effects of the imidazoline ligands efaroxan and KU14R on blood glucose homeostasis in the mouse

The putative imidazoline I(3) receptor antagonist 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) has been shown to block the effects of the atypical I(3) agonist efaroxan at the level of the ATP-sensitive K(+) (K(ATP)) channel in isolated pancreatic islet beta cells, but its effects in vivo are not known. We have therefore investigated the effects of KU14R on blood glucose and insulin level in vivo. When KU14R was administered before or after a hypoglycaemic dose of efaroxan, the fall in blood glucose was at least additive. When the antihyperglycaemic imidazoline ligand S22068 was administered after a dose of KU14R, it did not alter the hypoglycaemic response. In the mouse isolated vas deferens preparation, neither rauwolscine (at concentrations which competitively antagonised the inhibitory response to 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK14304)) nor KU14R affected inhibition produced by S22068. At 10(-4) M, KU14R had weak alpha(2)-adrenoceptor antagonist activity. We conclude that KU14R does not act as an antagonist of either efaroxan or S22068 at an imidazoline site in vivo.

Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist

1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.

Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell

Efaroxan induces membrane depolarization by interaction with the pore forming subunit of the ATP-sensitive potassium channel, Kir6.2. However, this effect is not responsible for its full secretory activity. In this study we have used an anti-idiotypic approach to generate antibodies that recognize additional proteins that may be regulated by efaroxan in pancreatic beta-cells. Using these antisera in an expression cloning strategy we have identified a monomeric GTP-binding protein, Rhes, as a potential target for regulation by imidazoline ligands. Rhes is shown to be expressed in beta-cells and its expression is regulated by efaroxan under conditions when a structurally related molecule, KU14R, is ineffective. The results reveal that beta-cells express Rhes and suggest that changes in the expression of this molecule may regulate the sensitivity of beta-cells to imidazoline secretagogues.