Contrasting effects of nateglinide and rosiglitazone on insulin secretion and phospholipase C activation

Combination of nateglinide with thiazolidinediones in Type 2 diabetes

Insulin sensitivity and insulin secretion are reciprocally related such that insulin resistance is adapted by increased insulin secretion to maintain normal glucose and lipid homeostasis. Treatment of Type 2 diabetes should aim to restore and sustain the normal relationship between insulin sensitivity and secretion. Nateglinide is a rapid-onset, short-acting insulin-secretion enhancer that restores early-phase insulin secretion, reduces postprandial glucose excursions and prevents long-term hyperinsulinemia. Given its mechanism of action, it is evident that nateglinide would be more effective when used in combination with an insulin sensitizer, such as the thiazolidinediones. Thiazolidinediones do not stimulate insulin release and, therefore, are potentially suitable candidates for combination therapy with an insulin-secretion enhancer, such as nateglinide. Combination therapy of thiazolidinediones with nateglinide is effective, carries low risk of hypoglycemia and is suitable for patients with moderate renal impairment, although weight gain and edema are common side effects. Further studies are needed to determine whether nateglinide in combination with thiazolidinediones will help clinicians better achieve their treatment goals in targeting Type 2 diabetes. Moreover, comparative studies between nateglinide and medications targeting postprandial glycemia, such as dipeptidyl peptidase-4 inhibitors and glucagon-like peptide-1 analogues, are necessary. This article summarizes data concerning the mechanism of action, efficacy and safety of therapy with nateglinide and thiazolidinediones as monotherapy and in combination treatment, and aims at a better understanding of the substrate defects their synergy hopes to defy.

Serine-385 phosphorylation of inwardly rectifying K+ channel subunit (Kir6.2) by AMP-dependent protein kinase plays a key role in rosiglitazone-induced closure of the K(ATP) channel and insulin secretion in rats

AIMS/HYPOTHESIS

Rosiglitazone, an insulin sensitiser, not only improves insulin sensitivity but also enhances insulin secretory capacity by ameliorating gluco- and lipotoxicity in beta cells. Rosiglitazone can stimulate insulin secretion at basal and high glucose levels via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. We hypothesised that regulation of phosphorylation of the ATP-sensitive potassium (K(ATP)) channel might serve as a key step in the regulation of insulin secretion.

METHODS

Insulin secretory responses were studied in an isolated pancreas perfusion system, cultured rat islets and MIN6 and RINm5F beta cells. Signal transduction pathways downstream of PI3K were explored to link rosiglitazone to K(ATP) channel conductance with patch clamp techniques and insulin secretion measured by ELISA.

RESULTS

Rosiglitazone stimulated AMP-activated protein kinase (AMPK) activity and induced inhibition of the K(ATP) channel conductance in islet beta cells; both effects were blocked by the PI3K inhibitor LY294002. Following stimulation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a pharmacological activator, both AICAR-stimulated insulin secretion and inhibition of K(ATP) channel conductance were unaffected by LY294002, indicating that AMPK activation occurs at a site downstream of PI3K activity. The serine residue at amino acid position 385 of Kir6.2 was found to be the substrate phosphorylation site of AMPK when activated by rosiglitazone or AICAR.

CONCLUSIONS/INTERPRETATION

Our data indicate that PI3K-dependent activation of AMPK is required for rosiglitazone-stimulated insulin secretion in pancreatic beta cells. Phosphorylation of the Ser(385) residue of the Kir6.2 subunit of the K(ATP) channel by AMPK may play a role in insulin secretion.

Effects of short-term cinnamon ingestion on in vivo glucose tolerance

AIMS

Various spices display insulin-potentiating activity in vitro, and in particular, cinnamon spice and its phenolic extracts have been shown to exhibit these capabilities. In vivo study shows that cinnamon may have beneficial effects on glucose homeostasis; therefore the aim of this study was to further investigate this phenomenon in humans.

METHODS

Seven lean healthy male volunteers, aged 26 +/- 1 years, body mass index 24.5 +/- 0.3 kg/m(2) (mean +/- s.e.m.), underwent three oral glucose tolerance tests (OGTT) supplemented with either a 5 g placebo (OGTT(control)), 5 g of cinnamon (OGTT(cin)), or 5 g of cinnamon taken 12 h before (OGTT(cin12hpre)) in a randomized-crossover design.

RESULTS

Cinnamon ingestion reduced total plasma glucose responses (AUC) to oral glucose ingestion [-13% and -10% for OGTT(cin) (p < 0.05) and OGTT(cin12hpre) (p < 0.05), respectively], as well as improving insulin sensitivity as assessed by insulin sensitivity index measures based on Matsuda's model in both OGTT(cin) (p < 0.05) and OGTT(cin12hpre) (p < 0.05) trials compared with OGTT(control).

CONCLUSIONS

These data illustrate that cinnamon spice supplementation may be important to in vivo glycaemic control and insulin sensitivity in humans, and not only are its effects immediate, they also appear to be sustained for 12 h.

Accelerated recovery of 5-fluorouracil-damaged bone marrow after rosiglitazone treatment

Our preliminary data indicate that rosiglitazone may be myeloprotective. We investigated whether it can modify bone marrow recovery. Five-day pre-treatment with rosiglitazone significantly accelerated recovery of 5-fluorouracil-damaged bone marrow in mice. Frequency and femoral content of granulocyte-macrophage progenitors reached mean baseline faster in pre-treated groups than in 5-fluorouracil-treated controls. Consequently, neutropenia was milder. Five-day insulin pre-treatment had similar effects in vivo. Insulin supports in vitro hematopoiesis. The observed myeloprotection demonstrated the importance of insulin in vivo. Clinical use of insulin to moderate myelotoxicity is impractical but rosiglitazone, an insulin sensitizer, could offer hope. Although rosiglitazone tends to increase plasma insulin levels, the significant myeloprotection was partly due to direct effects on progenitors. In vitro rosiglitazone enhanced the survival of both murine progenitor and human mobilized blood stem cells in the presence of 5-fluorouracil, the effect of which was neutralized by a peroxisome-proliferator-activated receptor-gamma antagonist.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

Pioglitazone and sodium salicylate protect human beta-cells against apoptosis and impaired function induced by glucose and interleukin-1beta

Decreased functional beta-cell mass in type 1 and type 2 diabetes is due to beta-cell apoptosis and impaired secretory function suggested to be mediated, in part, by immune- and/or high-glucose-induced production of IL-1beta acting through the nuclear factor kappaB (NFkappaB)/Fas pathway. The aim of this study was to determine whether two drugs believed to block NFkappaB activation, the thiazolidinedione (glitazone) pioglitazone and the nonsteroidal antiinflammatory drug sodium salicylate, can protect human beta-cells against the toxic effects of IL-1beta and high glucose in vitro. Human islets were maintained in culture 2-4 d at 100 mg/dl (5.5 mm) glucose with or without (control) IL-1beta or at 600 mg/dl (33.3 mm) glucose. IL-1beta and 600 mg/dl glucose increased beta-cell apoptosis and abolished short-term glucose-stimulated insulin secretion. Both drugs protected partially against loss of glucose-stimulated insulin secretion and prevented completely increased apoptosis caused by IL-1beta or 600 mg/dl glucose. IL-1beta secretion from islets was increased by 4-d culture at 600 mg/dl, and this was blocked by pioglitazone. Both drugs prevented activation of beta-cell NFkappaB by high glucose. Pioglitazone and sodium salicylate thus protect human islets against the detrimental effects of IL-1beta and high glucose by blocking NFkappaB activation and may therefore be useful in retarding the manifestation and progression of diabetes.