Receptor for advanced glycation endproducts signaling cascades are activated in pancreatic fibroblasts, but not in the INS1E insulinoma cell line: Are mesenchymal cells major players in chronic inflammation?

The receptor for advanced glycation endproducts (RAGE) is a pattern recognition receptor that plays an important role in natural immunity. It is suggested that mesenchymal cells are the major players during inflammation. Previously, we reported that advanced glycation end products (AGE), known to be one of the ligands of RAGE, inhibited glucose-induced insulin secretion from ex vivo pancreatic islets, although the mechanism responsible remains largely unknown. In the present study, we examined the cascades operating downstream from RAGE using the insulinoma cell line INS1E and primary-cultured pancreatic fibroblasts as in vitro models for parenchymal (ß) cells and mesenchymal cells, respectively. Phosphorylation of c-jun N-terminal kinase, inhibitor of nuclear factor κB kinase, and nuclear factor κB was stimulated by AGE or high mobility group binding 1 (HMGB1) in pancreatic fibroblasts, whereas no such effect was observed in INS1E cells. Expression of the Ccl5, Il-6, and Il-1b genes was increased by AGE/HMGB1 in fibroblasts, but not in INS1E cells. On the other hand, AGE inhibited the secretion of insulin from ex vivo pancreatic islets, and this effect was ameliorated by MK615, a Japanese apricot extract used as an anti-inflammatory agent. Glucose-induced insulin secretion from INS1E cells was not affected by direct administration of AGE/HMGB1, but was inhibited by fibroblast-conditioned medium. These results suggest that AGE suppresses glucose-induced insulin secretion from pancreatic islets through indirect mesenchymal RAGE signaling.

Advanced glycation end products impair glucose-induced insulin secretion from rat pancreatic ß-cells.

BACKGROUND: Advanced glycation end products (AGEs) are derivative compounds generated from non-enzymatic glycosylation and oxidation. In comparison with glucose-derived AGEs (Glu-AGEs), glyceraldehyde-derived AGEs (Glycer-AGEs) have stronger toxicity to living systems. In this study, we compared the effects of Glu-AGE and Glycer-AGE on insulin secretion. METHOD: Rat pancreatic islets were isolated by collagenase digestion and primary-cultured in the presence of 0.1 mg/ml bovine serum albumin (BSA) or 0.1 mg/ml Glu-AGE or Glycer-AGE-albumin. After 48 h of culture, we performed an insulin secretion test and identified the defects by a battery of rescue experiments [corrected]. Also, mRNA expression of genes associated with insulin secretion was measured. RESULTS: Insulin secretion induced by a high glucose concentration was 164.1 ± 6.0, 124.4 ± 4.4 (P < 0.05) and 119.8 ± 7.1 (P < 0.05) µU/3 islets/h in the presence of BSA, Glu-AGE, and Glycer-AGE, respectively. Inhibition of insulin secretion by Glu-AGE or Glycer-AGE was rescued by a high extracellular potassium concentration, tolbutamide and α-ketoisocaproic acid, but not by glyceraldehyde, dihydroxacetone, methylpyruvate, glucagon-like peptide-1 and acetylcholine. Glu-AGE or Glycer-AGE reduced the expression of the malate dehydrogenase (Mdh1/2) gene, which plays a critical role in the nicotinamide adenine dinucleotide (NADH) shuttle. CONCLUSION: Despite its reported cytotoxicity, the effects of Glycer-AGE on insulin secretion are similar to those of Glu-AGE.

Triphenyltin impairs insulin secretion by decreasing glucose-induced NADP(H) and ATP production in hamster pancreatic ß-cells.

Oral administration of triphenyltin chloride (TPT) (6 mg/100g body weight) inhibits insulin secretion by decreasing glucose-induced cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in pancreatic ß-cells of the hamster. To test the possibility that the abnormal level of the [Ca(2+)](i) induced by TPT administration could be due to a defect in the metabolic signal of glucose in the ß-cells, we tested the effects of TPT administration on the glucose-induced NAD(P)H and ATP production, and on the changes of membrane potential and [Ca(2+)](i) by glucose and high K(+) in the ß-cells. The [Ca(2+)](i) was measured in islet cells loaded with fura-2. TPT administration significantly reduced the NAD(P)H and ATP production, the depolarization of plasma membrane, and insulin secretion by 15 mM glucose in islet cells. TPT administration also reduced the insulin secretion by 10mM dihydroxyacetone and glyceraldehyde. However, TPT administration did not affect the increase of [Ca(2+)](i) and the insulin secretion by 30 mMK(+) or 100 µM tolbutamide, and the membrane potential by 30 mMK(+), and the insulin secretion by 10mM α-ketoisocaproic acid and 0.5mM formycin A, an analog of ATP in the presence of 15 mM glucose. These results suggested that the pathogenesis of TPT-induced hyperglycemia in hamster involves the reduction of [Ca(2+)](i) and insulin secretion in response to K(ATP) channel-dependent depolarization, which is related to the decrease of NAD(P)H and ATP production in pancreatic islet cells after glucose metabolism.

Glycated albumin suppresses glucose-induced insulin secretion by impairing glucose metabolism in rat pancreatic ß-cells.

BACKGROUND: Glycated albumin (GA) is an Amadori product used as a marker of hyperglycemia. In this study, we investigated the effect of GA on insulin secretion from pancreatic ß cells. METHODS: Islets were collected from male Wistar rats by collagenase digestion. Insulin secretion in the presence of non-glycated human albumin (HA) and GA was measured under three different glucose concentrations, 3 mM (G3), 7 mM (G7), and 15 mM (G15), with various stimulators. Insulin secretion was measured with antagonists of inducible nitric oxide synthetase (iNOS), and the expression of iNOS-mRNA was investigated by real-time PCR. RESULTS: Insulin secretion in the presence of HA and GA was 20.9 ± 3.9 and 21.6 ± 5.5 µU/3 islets/h for G3 (P = 0.920), and 154 ± 9.3 and 126.1 ± 7.3 µU/3 islets/h (P = 0.046), for G15, respectively. High extracellular potassium and 10 mM tolbutamide abrogated the inhibition of insulin secretion by GA. Glyceraldehyde, dihydroxyacetone, methylpyruvate, GLP-1, and forskolin, an activator of adenylate cyclase, did not abrogate the inhibition. Real-time PCR showed that GA did not induce iNOS-mRNA expression. Furthermore, an inhibitor of nitric oxide synthetase, aminoguanidine, and NG-nitro-L-arginine methyl ester did not abrogate the inhibition of insulin secretion. CONCLUSION: GA suppresses glucose-induced insulin secretion from rat pancreatic ß-cells through impairment of intracellular glucose metabolism.

Effects of habitual perilla (shiso) tea drinking on the incidence of diabetes mellitus in spontaneously diabetic Trii (SDT)rats.

The anti-diabetic effect of perilla (shiso) tea was evaluated in vivo. When shiso tea was given to model rats that spontaneously developed diabetes mellitus (DM), the development of DM was decelerated. In oral glucose tolerance tests, the disappearance of blood glucose in rats administered shiso tea was reinforced. These results suggest that habitual drinking of shiso tea is effective in preventing the onset of diabetes.

MK615 decreases RAGE expression and inhibits TAGE-induced proliferation in hepatocellular carcinoma cells.

AIM: To investigate the proliferative effect of advanced glycation end-products (AGEs) and the role of their cellular receptor (RAGE) on hepatocellular carcinoma (HCC) cells, and the inhibitory effects of MK615, an extract from Japanese apricot, against AGEs were also evaluated. METHODS: Two HCC cell lines, HuH7 and HepG2, were used. Expression of RAGE was investigated by polymerase chain reaction, Western blotting, and flow cytometry (FACS). The effect of MK615 on RAGE expression was also evaluated by FACS. The proliferative effects of a control (unglycated bovine serum albumin), glucose-derived AGEs (Glc-AGE), and glyceraldehyde-derived AGEs (Glycer-AGE), and the anti-proliferative effect of MK615 against AGEs, were evaluated using MTT assays. RESULTS: Expression of RAGE was confirmed at both the mRNA and protein levels in both HuH7 and HepG2. FACS revealed that the level of RAGE expression was higher in HuH7 than in HepG2. Treatment with 0.1 µg/mL MK615 decreased the expression level of RAGE from 24.3% to 3.7% in HuH7 and from 6.2% to 4.8% in HepG2. The growth indices for the control, Glc-AGE, and Glycer-AGE were 1.06 ± 0.08, 0.99 ± 0.04, and 1.38 ± 0.05, respectively, in HuH7 (P = 0.037), and were 1.03 ± 0.04, 1.04 ± 0.03, and 1.07 ± 0.05, respectively, in HepG2 (P > 0.05). When the cells were cultured simultaneously with Glycer-AGE and MK615, MK615 abrogated the proliferative effect of Glycer-AGE in HuH7. CONCLUSION: Only Glycer-AGE has a proliferative effect on HuH7, which expresses a higher level of RAGE. MK615 suppresses the proliferative effect of Glycer-AGE on HuH7 by decreasing the expression of RAGE.

Triphenyltin impairs a protein kinase A (PKA)-dependent increase of cytosolic Na+ and Ca2+ and PKA-independent increase of cytosolic Ca2+ associated with insulin secretion in hamster pancreatic beta-cells.

Oral administration of triphenyltin chloride (TPT) (60 mg/kg body weight) inhibits the insulin secretion by decreasing the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by glucose-dependent insulinotropic polypeptide (GIP) in pancreatic beta-cells of the hamster. To test the possibility that the abnormal level of [Ca2+]i induced by TPT administration could be due to a defect in the cAMP-dependent cytoplasmic Na+ concentration ([Na+]i) in the beta-cells, we investigated the effects of TPT administration on the changes of [Na+]i induced by GIP, glucagon-like peptide-1 (GLP-1), or forskolin, an activator of adenylyl cyclase, and on the changes of [Na+]i or [Ca2+]i induced by 6-Bnz-cAMP, an activator of protein kinase A (PKA), and 8-pCPT-2'-O-Me-cAMP, an activator of Epac. The [Na+]i and [Ca2+]i were measured in islet cells loaded with sodium-binding benzofuran isophthalate (SBFI) and fura-2, respectively. In the presence of 135 mM Na+, TPT administration significantly reduced the rise in [Na+]i by 10 nM GLP-1, 10 microM forskolin, and 50 microM 6-Bnz-cAMP, but had not effect in a Na+-free medium. In the presence of 135 mM Na+, TPT administration also reduced the rise in [Ca2+]i by 8-pCPT-2'-O-Me-cAMP plus10 microM H-89, a inhibitor of PKA, and 6-Bnz-cAMP. Moreover, TPT administration significantly reduced the insulin secretion by 2 mM db-cAMP, GLP-1, GIP, and 8-pCPT-2'-O-Me-cAMP with and without H-89, and that by 6-Bnz-cAMP and forskolin. Our study suggested that TPT has inhibitory effects on the cellular Ca2+ response due to a reduced Na+ permeability through PKA-dependent mechanisms in hamster islet cells. Also TPT has the reduction of [Ca2+]i related to Na+-dependent insulin secretion after an activation of Epac.

Glucagon-like peptide-1 induces a cAMP-dependent increase of [Na+]i associated with insulin secretion in pancreatic beta-cells.

Glucagon-like peptide-1 (GLP-1) elevates the intracellular free calcium concentration ([Ca2+]i) and insulin secretion in a Na+-dependent manner. To investigate a possible role of Na ion in the action of GLP-1 on pancreatic islet cells, we measured the glucose-and GLP-1-induced intracellular Na+ concentration ([Na+]i), [Ca2+]i, and insulin secretion in hamster islet cells in various concentrations of Na+. The [Na+]i and [Ca2+]i were monitored in islet cells loaded with sodium-binding benzofuran isophthalate and fura 2, respectively. In the presence of 135 mM Na+ and 8 mM glucose, GLP-1 (10 nM) strongly increased the [Na+]i, [Ca2+]i, and insulin secretion. In the presence of 13.5 mM Na+, both glucose and GLP-1 increased neither the [Na+]i nor the [Ca2+]i. In a Na+-free medium, GLP-1 and glucose did not increase the [Na+]i. SQ-22536, an inhibitor of adenylate cyclase, and H-89, an inhibitor of PKA, incompletely inhibited the response. In the presence of both 8 mM glucose and H-89, 8-pCPT-2'-O-Me-cAMP, a PKA-independent cAMP analog, increased the insulin secretion and the [Na+]i. Therefore, we conclude that GLP-1 increases the cAMP level via activation of adenylate cyclase, which augments the membrane Na+ permeability through PKA-dependent and PKA-independent mechanisms, thereby increasing the [Ca2+]i and promoting insulin secretion from hamster islet cells.

Glucose intolerance induced by a high-fat/low-carbohydrate diet in rats effects of nonesterified fatty acids.

We examined the time course of effects of a high-fat/low-carbohydrate (HF/LC) diet on the impairment of glucose tolerance in rats, clarified whether insulin secretion and sensitivity were impaired by the HF/LC diet, and investigated the relationship between the increased nonesterified fatty acids (NEFA) after HF/LC diet feeding and insulin secretion and sensitivity. We found that glucose tolerance and the postglucose-loading insulin secretion were impaired after 3 and 7 d on the HF/LC diet. The glucose intolerance was accompanied by a rise in the fasting plasma NEFA level. When stimulated with 15 mmol/L of glucose, the insulin secretion was impaired in pancreatic islets from rats fed the HF/LC diet. Rats fed the HF/LC diet showed insulin resistance in vivo. The glucose-stimulated insulin secretion was inhibited in the islets following 24-h culture with palmitic acid. The 24-h infusion of palmitic acid decreased whole-body insulin sensitivity. In summary, at least 3 d on a HF/LC diet is needed to induce glucose intolerance in rats, and the impairment may be induced by decreased insulin secretion and sensitivity, which is related to the increase in the plasma NEFA level.