Allantoin activates imidazoline I-3 receptors to enhance insulin secretion in pancreatic β-cells
© Tsai et al.; licensee BioMed Central Ltd. 2014
Received: 18 May 2014
Accepted: 28 August 2014
Published: 31 August 2014
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© Tsai et al.; licensee BioMed Central Ltd. 2014
Received: 18 May 2014
Accepted: 28 August 2014
Published: 31 August 2014
Imidazoline I3 receptors (I-3R) can regulate insulin secretion in pancreatic β-cells. It has been indicated that allantoin ameliorates hyperglycemia by activating imidazoline I2 receptors (I-2R). Thus, the effect of allantoin on I-3R is identified in the present study.
We used male Wistar rats to screen allantoin’s ability for lowering of blood glucose and stimulation of insulin secretion. Chinese hamster ovary-K1 cells transfected with imidazoline receptors (NISCH-CHO-K1 cells) were also applied to characterize the direct effect of allantoin on this receptor. Additionally, KU14R as specific antagonist was treated to block I-3R in rats and in the cultured pancreatic β-cells named Min 6 cells.
In rats, allantoin decreased blood sugar with an increase in plasma insulin. Also, allantoin enhanced calcium influx into NISCH-CHO-K1 cells in a way similar to agmatine, an I-R agonist. Moreover, KU14R dose-dependently blocked allantoin-induced insulin secretion both in Min 6 cells and in Wistar rats.
Allantoin can activate I-3R to enhance insulin secretion for lowering of blood sugar in Wistar rats. Thus, allantoin may provide beneficial effects as a supplement for diabetic patients after clinical trials.
Allantoin is one of the active principles contained in yam (Dioscorea spp.). Yam is widely used in the drug industry, and Dioscorea rhizome contains ureides, including allantoin, that prevent inflammation and ulcers. Herbs from Dioscoreaceae have been used to improve diabetic disorders. In Chinese traditional medicine, Shan-Yaw (Dioscorea opposita) improves insulin resistance, and the effect also observed in animals. Allantoin is contained in this herb, and we have identified the plasma glucose- lowering action of allantoin in diabetic rats.
The antihyperglycemic action of allantoin in type-1- like diabetic rats involves its activation of imidazoline I-2 receptors (I-2Rs). Imidazoline receptors (I-Rs) have many functions. Due to the presence of agmatine as endogenous ligand, the functions of I-Rs subtypes were widely investigated: I-1 receptor is known to regulate blood pressure; I-3R mediates insulin secretion; and I-2R is mentioned to reduce blood glucose[11, 12]. The I-R expressed on pancreatic β-cells has been classified as I-3 site showing different properties from the I-1 and/or I-2 sites[14, 15]. The insulin-secreting action of efaroxan, an I-3R ligand, is mediated by calcium influx through the closure of ATP-sensitive K+ (KATP) channel. However, the effect of allantoin on I-3R for insulin secretion is still unknown.
Compounds with guanidine-like structures, including metformin, can bind to I-Rs. I-R activation promotes glycemic control[18–20]. An increase in insulin secretion via the activation of I-3R located in pancreatic β-cells also seems helpful for glycemic control[21, 22]. Allantoin has the ability to activate I-2R, but the effect of allantoin on I-3R remained obscure. Thus, the present study investigated whether allantoin can bind I-R and explored its effects on I-3R both in vivo and in vitro.
The male Wistar rats weighing from 280 to 330 g were purchased from the Animal Center of National Cheng Kung University Medical College. The rats were free access to food and water in the house under a 12-h light/dark cycle. The animal experiments were approved and conducted in accordance with local institutional guidelines for the care and use of laboratory animals. All experiments conformed to the Guide for the Care and Use of Laboratory Animals as well as the guidelines of the Animal Welfare Act.
The effect of allantoin on postprandial blood glucose was performed as described previously. The rats were separated into three groups and eight animals in each group. After fasting for 12 h, blood samples obtained from tail vein were used to determine the plasma glucose level at the basal level (0 min). Then, two groups of animals received the injection of allantoin at 0.5 or 1 mg/kg into tail vein. Another group received a similar injection of vehicle at same volume to use as the control. At 30 min later, all rats received oral intake of glucose solution (1 g/kg body weight). Blood samples were obtained at desired time after the oral glucose challenge to determine the plasma glucose level. The insulin level in each sample was also estimated using an immunoassay kit (Mercodia, Uppsala, Sweden).
The Mus musculus insulinoma cell line Min 6 (BCRC No. CRL-11506) and Chinese hamster ovary-K1 (CHO-K1) cells (BCRC No. CCL-61) were purchased from the Culture Collection and Research Center of the Food Industry Institute (Hsin-Chiu City, Taiwan). Following a previous method[24, 25], Min 6 cells were maintained in RPMI 1640 medium, while CHO-K1 cells were maintained in F-12 K medium supplemented with 10% fetal bovine serum. The cells were sub-cultured once every three days by trypsinization (Gibco), and the culture medium was refreshed every 2–3 days.
According to the previous report, CHO-K1 cells were transiently transfected with human nischarin (NISCH) gene, also known as human imidazoline receptor antisera-selective (IRAS) protein, and an expression vector (Origene, Rockville, MD, USA) using the TurboFect transfection reagent (Thermo Fisher Scientific, USA). At 24 h later, the transfected cells were used to treat with allantoin or agmatine at indicated concentrations.
To identify the direct effect of allantoin on insulin secretion, we used Min 6 cells to investigate the in vitro secretion as described previously. The Min 6 cells were prepared at 1 × 105 cells per well in a 12-well plate containing 1 ml of DMEM before the experiment. Then, cells were incubated with KU14R (an I-3R antagonist) (Sigma-Aldrich, St. Louis, MO, USA) at effective concentrations or with same volume of vehicle, as the control, for 30 min. All cells were treated with allantoin at the indicated concentrations for 1 h. After the collection of media to store at -20°C, insulin levels in the media were estimated using an immunoassay kit (Mercodia).
The cells were lysed with ice-cold lysis and the protein was extracted following a previous report. Then, each sample at 30 micrograms was separated by 10% sodium dodecylsulfate polyacrylamide gel electrophoresis. The blots were then transferred to a polyvinyl difluoride membrane (Millipore, Billerica, MA, USA). After blocking with 10% skim milk for 1 h, immunoblots were developed with the primary antibody specific for NISCH and DDK (Origene, Rockville, MD, USA). The blots were subsequently hybridized using horseradish peroxidase-conjugated goat anti-rabbit or anti-mouse IgG (Calbiochem, San Diego, CA, USA) and developed by a chemiluminescence kit (PerkinElmer). The optical densities of the bands (37 kD) were identified through Gel-Pro analyzer software 4.0 (Media Cybernetics, Silver Spring, MD, USA).
The intracellular calcium concentrations were determined using the fluorescent probe fura-2 as described in our previous report. In brief, the NISCH-CHO-K1 cells were placed in a buffered physiological saline solution (PSS) containing 5 mM fura-2. The cells were incubated for 1 h under 5% CO2 in O2 aeration at 37.8°C. After washing, the cells were incubated for another 30 min in PSS. The NISCH-CHO-K1 cells were inserted into a temperature-controlled (37°C) cuvette containing 2 ml of PSS and indicated doses of allantoin or agmatine for 1 h. The fluorescence was determined by a fluorescence spectrofluorometer (Hitachi F-2000). The intracellular calcium [Ca2+]i was calculated from the ratio R = F340/F380 by the following formula: [Ca2+]i = KdB (R - Rmin)/(Rmax - R), where Kd is 225 nM, F is fluorescence, and B is the ratio of the fluorescence of the free dye to that of the Ca2+-bound dye measured at 380 nm. Rmax and Rmin were determined in separate experiments by using digoxin to equilibrate [Ca2+]i with ambient [Ca2+] (Rmax), and the addition of 0.1 mM MnCl2 and 1 mmol/L EGTA (Rmin). Background autofluorescence was measured in unloaded cells and was subtracted from all measurements.
KU14R (an I-3R antagonist) (Sigma-Aldrich, St. Louis, MO, USA) at the effective dose (4 or 8 mg/kg) or same volume of vehicle was used to treat the rats as described previously[29, 30] for 30 minutes before the injection of allantoin (0.1, 1 or 2.5 mg/kg). Blood samples collected from the femoral vein at indicated times were centrifuged and the plasma glucose was measured in an automatic analyzer (Quik-Lab, Ames; Miles Inc., Elkhart, IN, USA). Plasma insulin was estimated from each sample using an immunoassay kit (Mercodia, Uppsala, Sweden).
Data are expressed as the mean ± SEM of each group. Means of the two groups were compared by Student’s t-test using the software Microsoft EXCEL. The differences were analyzed by the unpaired t-test and considered significant at P < 0.05.
Normal Wistar rats were used to investigate the effects of allantoin on insulin secretion and basal blood glucose. The basal blood glucose was lowered by allantoin in a dose-dependent manner (Figure 1B). Plasma insulin was raised by allantoin in a same manner (Figure 1C). These results show that allantoin may lower blood glucose through an increase of plasma insulin.
We tested the ability of allantoin to bind with imidazoline receptor. After incubation with allantoin, calcium influx was significantly raised in NISCH-CHO-K1 cells, similar to the increase stimulated by agmatine (Figure 2C). After comparison, allantoin induced calcium influx was less effective than agmatine.
The antagonist of I-3R, KU14R, influenced the action of allantoin markedly (Figure 3B). Insulin secretion increased by allantoin was inhibited by KU14R in a concentration-dependent manner. At the highest concentration, KU14R abolished the action of allantoin. This result indicates the activation of I-3R by allantoin.
Yam containing allantoin is a widely used nutrient. Allantoin ameliorates hyperglycemia in diabetic rats by activating I-2R. In the present study, we found that allantoin also can activate I-3R, linking its increase of insulin secretion to reduce blood glucose in Wistar rats. Additionally, this is the first report demonstrated that allantoin can activate I-R directly using the response in CHO -cells transfected with imidazoline receptor gene (NISCH-CHO-K1 cells). Moreover, we applied a pharmacological antagonist named KU14R at a dose sufficient to block I-3R, as described previously[31, 32], to block the actions of allantoin in both pancreatic β-cells (Min 6 cells) and Wistar rats, indicating the mediation of I-3R in insulin secretion induced by allantoin.
Allantoin is easily degraded in the intestinal tract and loses its activity after oral administration[34, 35]. Thus, we treated rats with allantoin using intravenous injection (iv). Allantoin (1 mg/kg, iv) attenuated the hyperglycemia in fasting rats challenged with glucose. Moreover, allantoin decreased blood sugar and increased blood insulin in normal rats in a dose-dependent manner. Thus, bolus injection of allantoin induces lowering of blood glucose in a way associated with the increased insulin secretion in rats.
Imidazoline receptors have been established[36–38], but research tools to study them are still not well developed. There is no ideal radioligand to perform ligand-receptor binding assay. Also, the antagonist specific for each subtype of imidazoline receptor is not sufficient. In the present study, we transfected the imidazoline receptor gene (NISCH) into CHO cells. Success of the transfection was confirmed using Western blotting analysis. Agmatine, a well-known ligand of I-Rs, induced an increase in calcium influx in these cells, indicating that the transfected CHO-cells were functional. Additionally, allantoin enhanced calcium influx into NISCH- expressing CHO-K1 cells in a manner similar to agmatine. The activation of imidazoline receptor by allantoin was thus confirmed. These results show that CHO -cells transfected with imidazoline receptor gene can be used to identify the direct effect of allantoin.
Allantoin can activate I-2R to lower blood sugar and to improve insulin sensitivity[39–42]. However, these results were observed in the absence of endogenous insulin. We applied Min 6 cells to investigate the effect of allantoin on I-3R in pancreatic cells and used glibenclamide as positive control. Glibenclamide has been applied to enhance insulin secretion through an induction of calcium influx in pancreatic β-cells[43, 44]. We observed that 1 μM allantoin increases calcium influx in Min 6 cells to a level similar to that produced by 1 μM glibenclamide. Glibenclamide is known to inhibit ATP-regulated potassium (KATP) channels in pancreatic β-cells, thereby causing calcium influx to result in the increase of intracellular calcium. In the present study, we identified that allantoin increases calcium influx in pancreatic cells in a manner similar to glibenclamide.
The binding site(s) of imidazolines in pancreatic β-cell has been distinguished with I-1 and I-2 receptors. I-1 sites were not expressed in β-cells and I-2 sites were mentioned as not reliable for the activity of imidazoline ligands. Thus, imidazolines induced insulin secretion after binding to a single site named I-3 site. In the present study, we found that an antagonist specific for I-3R (KU14R) inhibited allantoin-induced insulin secretion in Min 6 cells in a dose-dependent manner. Similar results were observed in rats showing the plasma insulin increasing action of allantoin blocked by KU14R. Thus, activation of I-3R is involved in the insulin secretion induced by allantoin.
Allantoin can activate I-1R in brain to produce antihypertension and decrease feeding behaviors. Allantoin can stimulate I-2R to lower blood sugar and improve insulin sensitivity in type 1-like diabetic rats[39–42]. In this study, allantoin activates I-3R to increase insulin secretion. Taken together, these findings indicate that allantoin could be useful for treating metabolic syndrome. However, more data are needed to confirm this hypothesis, especially from the clinical trials, in the future.
Allantoin can enhance insulin secretion by activating I-3R to lower blood glucose in rats. Thus, allantoin or the related compound(s) that activates I-3R could be developed for the supplementary treatment of diabetic disorders.
Imidazoline I3 receptor
Chinese Overy Hamster-K1
Imidazoline I2 receptor
Imidazoline I1 receptor
Mouse homologue of human imidazoline receptor antisera-selective
ATP-regulated potassium channels.
We thank Miss Mei-Jen Wang, Yang-Lian Yan and Pei-Ru Liao for their skillful assistance in conducting the experiments. The present study was supported in part by a grant ( CMFHR10302) from Chi-Mei Medical Center, Yong Kang, Tainan City, Taiwan.
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