Pioglitazone acutely influences glucose-sensitive insulin secretion in normal and diabetic human islets

Metabolic dysfunction and insulin sensitizers in acute and chronic disease

INTRODUCTION

The concept of insulin resistance has been a major topic for more than 5 decades. While there are several treatments that may impact insulin resistance, this pathology is uniquely addressed by mitochondrially directed thiazolidinedione (TZD) insulin sensitizers. Understanding of this mechanism of action and consideration of 'insulin resistance' as a consequence of metabolic inflammation allows a new paradigm for approaching chronic diseases.

AREAS COVERED

We review evolving understanding of the mitochondrial pyruvate carrier (MPC) as a mitochondrial mechanism of action of the TZD insulin sensitizers and discuss how reprogramming of mitochondrial metabolism impacts pleotropic pharmacology in multiple tissues. Additional lines of investigation are proposed.

EXPERT OPINION

A change in paradigm can facilitate rethinking of insulin sensitizers in clinical trials, specifically beyond the treatment of frank type 2 diabetes. There should be broader clinical evaluation of insulin sensitizers in combination with weight loss and lifestyle approaches across diseases/syndromes associated with insulin resistance. Finally, 'connecting all the dots' to unwind the interconnectedness of cell biology involved in the syndromes impacted by metabolic dysfunction and the efficacy of TZD insulin sensitizers may also uncover new molecular targets. New studies should facilitate the discovery and development of novel pharmacologic agents.

On the causal relationships between hyperinsulinaemia, insulin resistance, obesity and dysglycaemia in type 2 diabetes

Hundreds of millions of people are affected by hyperinsulinaemia, insulin resistance, obesity and the dysglycaemia that mark a common progression from metabolic health to type 2 diabetes. Although the relative contribution of these features and the order in which they appear may differ between individuals, the common clustering and seemingly progressive nature of type 2 diabetes aetiology has guided research and clinical practice in this area for decades. At the same time, lively debate around the causal relationships between these features has continued, as new data from human trials and highly controlled animal studies are presented. This 'For debate' article was prompted by the review in Diabetologia by Esser, Utzschneider and Kahn ( https://doi.org/10.1007/s00125-020-05245-x ), with the purpose of reviewing established and emerging data that provide insight into the relative contributions of hyperinsulinaemia and impaired glucose-stimulated insulin secretion in progressive stages between health, obesity and diabetes. It is concluded that these beta cell defects are not mutually exclusive and that they are both important, but at different stages.

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

Functional loss of pancreatic islets in type 2 diabetes: How can we halt it?

The loss of beta-cell functional mass is a necessary and early condition in the development of type 2 diabetes (T2D). In T2D patients, beta-cell function is already reduced by about 50% at diagnosis and further declines thereafter. Beta-cell mass is also reduced in subjects with T2D, and islets from diabetic donors are smaller compared to non-diabetic donors. Thus, beta-cell regeneration and/or preservation of the functional islet integrity should be highly considered for T2D treatment and possibly cure. To date, the available anti-diabetes drugs have been developed as "symptomatic" medications since they act to primarily reduce elevated blood glucose levels. However, a truly efficient anti-diabetes medication, capable to prevent the onset and progression of T2D, should stop beta-cell loss and/or promote the restoration of fully functional beta-cell mass, independently of reducing hyperglycemia and ameliorating glucotoxicity on the pancreatic islets. This review provides a view of the experimental and clinical evidence on the ability of available anti-diabetes drugs to exert protective effects on beta-cells, with a specific focus on human pancreatic islets and clinical trials. Potential explanations for the lack of concordance between evidence of beta-cell protection in vitro and of persistent amelioration of beta-cell function in vivo are also discussed.

Improving glycaemic control in type 2 diabetes: Stimulate insulin secretion or provide beta-cell rest?

Type 2 diabetes (T2D) is characterized by a gradual decline in pancreatic beta cell function that determines the progressive course of the disease. While beta-cell failure is an important contributor to hyperglycaemia, chronic hyperglycaemia itself is also detrimental for beta-cell function, probably by inducing prolonged secretory stress on the beta cell as well as through direct glucotoxic mechanisms that have not been fully defined. For years, research has been carried out in search of therapies targeting hyperglycaemia that preserve long-term beta-cell function in T2D, a quest that is still ongoing. Current strategies aim to improve glycaemic control, either by promoting endogenous insulin secretion, such as sulfonylureas, or by mechanisms that may impact the beta cell indirectly, for example, providing beta-cell rest through insulin treatment. Although overall long-term success is limited with currently available interventions, in this review we argue that strategies that induce beta-cell rest have considerable potential to preserve long-term beta-cell function. This is based on laboratory-based studies involving human islets as well as clinical studies employing intensive insulin therapy, thiazolidinediones, bariatric surgery, short-acting glucagon-like peptide (GLP)-1 receptor agonists and a promising new class of diabetes drugs, sodium-glucose-linked transporter (SGLT)-2 inhibitors. Nevertheless, a lack of long-term clinical studies that focus on beta-cell function for the newer glucose-lowering agents, as well as commonly used combination therapies, preclude a straightforward conclusion; this gap in our knowledge should be a focus of future studies.

BLX-1002 restores glucose sensitivity and enhances insulin secretion stimulated by GLP-1 and sulfonylurea in type 2 diabetic pancreatic islets

BLX-1002 is a novel thiazolidinedione with no peroxisome proliferator-activated receptor (PPAR) activity that has been shown to improve glycemia in type 2 diabetes without weight gain. We previously found that BLX-1002 selectively augments glucose-sensitive (but not basal) insulin secretion in normal mouse β-cells. We have now extended these observations to other insulin secretagogues and to diabetic rat islets. To this end, dynamics of insulin secretion stimulated by glucose, GLP-1, and the sulfonylurea tolbutamide were examined in pancreatic islets from nondiabetic Wistar and type 2 diabetic Goto-Kakizaki rats ex vivo. BLX-1002 restored normal glucose-sensitive insulin secretion in otherwise "glucose-blind" islets from GK rats, but did not affect basal or glucose-stimulated secretion in normal Wistar rat islets. The stimulatory effect of BLX-1002 on insulin secretion at high glucose required Ca(2+) and involved phosphatidylinositol 3-kinase (PI3K) activity. Consistent with its effects on insulin secretion, BLX-1002 also augmented insulin secretion and cytoplasmic-free Ca(2+) concentrations ([Ca(2+)]i) stimulated by high glucose, GLP-1, and tolbutamide in islets from GK, but not Wistar, rats. The inactive analog BLX-1237 had no effects. In conclusion, our findings suggest that BLX-1002 potentiates insulin secretion by different stimuli in diabetic β-cells only, in a Ca(2+)-dependent manner and involving PI3K.

Molecular mechanism by which pioglitazone preserves pancreatic beta-cells in obese diabetic mice: evidence for acute and chronic actions as a PPARgamma agonist

Pioglitazone preserves pancreatic beta-cell morphology and function in diabetic animal models. In this study, we investigated the molecular mechanisms by which pioglitazone protects beta-cells in diabetic db/db mice. In addition to the morphological analysis of the islets, gene expression profiles of the pancreatic islet were analyzed using laser capture microdissection and were compared with real-time RT-PCR of db/db and nondiabetic m/m mice treated with or without pioglitazone for 2 wk or 2 days. Pioglitazone treatment (2 wk) ameliorated dysmetabolism, increased islet insulin content, restored glucose-stimulated insulin secretion, and preserved beta-cell mass in db/db mice but had no significant effects in m/m mice. Pioglitazone upregulated genes that promote cell differentiation/proliferation in diabetic and nondiabetic mice. In db/db mice, pioglitazone downregulated the apoptosis-promoting caspase-activated DNase gene and upregulated anti-apoptosis-related genes. The above-mentioned effects of pioglitazone treatment were also observed after 2 days of treatment. By contrast, the oxidative stress-promoting NADPH oxidase gene was downregulated, and antioxidative stress-related genes were upregulated, in db/db mice treated with pioglitazone for 2 wk, rather than 2 days. Morphometric results for proliferative cell number antigen and 4-hydroxy-2-noneal modified protein were consistent with the results of gene expression analysis. The present results strongly suggest that pioglitazone preserves beta-cell mass in diabetic mice mostly by two ways; directly, by acceleration of cell differentiation/proliferation and suppression of apoptosis (acute effect); and indirectly, by deceleration of oxidative stress because of amelioration of the underlying metabolic disorder (chronic effect).

BLX-1002, a novel thiazolidinedione with no PPAR affinity, stimulates AMP-activated protein kinase activity, raises cytosolic Ca2+, and enhances glucose-stimulated insulin secretion in a PI3K-dependent manner

BLX-1002 is a novel small thiazolidinedione with no apparent affinity to peroxisome proliferator-activated receptors (PPAR) that has been shown to reduce glycemia in type 2 diabetes without adipogenic effects. Its precise mechanisms of action, however, remain elusive, and no studies have been done with respect to possible effects of BLX-1002 on pancreatic beta-cells. We have investigated the influence of the drug on beta-cell function in mouse islets in vitro. BLX-1002 enhanced insulin secretion stimulated by high, but not low or intermediate, glucose concentrations. BLX-1002 also augmented cytoplasmic free Ca2+ concentration ([Ca2+](i)) at high glucose, an effect that was abolished by pretreatment with the Ca2+-ATPase inhibitor thapsigargin. In contrast, BLX-1002 did not interfere with voltage-gated Ca2+ channel or ATP-sensitive K+ channel activities. In addition, cellular NAD(P)H stimulated by glucose was not affected by the drug. The stimulatory effect of BLX-1002 on insulin secretion at high glucose was completely abolished by treatment with the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY-294002. Stimulation of the beta-cells with BLX-1002 also induced activation of AMP-activated protein kinase (AMPK) at high glucose. Our study suggests that BLX-1002 potentiates insulin secretion only at high glucose in beta-cells in a PI3K-dependent manner. This effect of BLX-1002 is associated with an increased [Ca2+](i) mediated through Ca2+ mobilization, and an enhanced activation of AMPK. The glucose-sensitive stimulatory impact of BLX-1002 on beta-cell function may translate into substantial clinical benefits of the drug in the management of type 2 diabetes, by avoidance of hypoglycemia.

Berberine acutely inhibits insulin secretion from beta-cells through 3',5'-cyclic adenosine 5'-monophosphate signaling pathway

Berberine, a hypoglycemic agent, has recently been shown to activate AMP-activated protein kinase (AMPK) contributing to its beneficial metabolic effects in peripheral tissues. However, whether berberine exerts a regulatory effect on beta-cells via AMPK or other signaling pathways and counteracts glucolipotoxicity remains uncertain. In the present study, the impact of berberine on beta-cell function was investigated in vivo and in vitro. In high-fat-fed rats, berberine treatment for 6 wk significantly decreased plasma glucose and insulin levels before and after an oral glucose challenge along with the reduction of body weight and improvement of blood lipid profile. In accordance with the in vivo results, berberine acutely decreased glucose-stimulated insulin secretion (GSIS) and palmitate-potentiated insulin secretion in MIN6 cells and rat islets. However, pretreated with berberine for 24 h augmented the response of MIN6 cells and rat islets to glucose and attenuated the glucolipotoxicity. Berberine acutely increased AMPK activity in MIN6 cells. However, compound C, an AMPK inhibitor, completely reversed troglitazone-suppressed GSIS, not berberine-suppressed GSIS. Otherwise, berberine decreased cAMP-raising agent-potentiated insulin secretion in MIN6 cells and rat islets. These results suggest that the activation of AMPK is required for troglitazone-suppressed GSIS, whereas cAMP signaling pathway contributes, at least in part, to the regulatory effect of berberine on insulin secretion.

Troglitazone acutely activates AMP-activated protein kinase and inhibits insulin secretion from beta cells

Changes in AMP-activated protein kinase (AMPK) activity contribute to the regulation of insulin secretion. Troglitazone has been shown to lower serum insulin levels and protect beta cell function. The aim of the present study was to examine the effects of troglitazone on AMPK activity and insulin secretion in beta cells. Isolated rat islets and MIN6 cells were treated for a short (1 h) or a long time (20 h) with troglitazone. One-hour troglitazone treatment activated AMPK and inhibited both glucose-stimulated insulin secretion (GSIS) and the response of insulin secretion to combined stimuli of glucose and palmitate. Long (20 h) treatment with troglitazone caused a sustained phosphorylation of AMPK and acetyl-CoA carboxylase, and increased GSIS after withdrawal of the drug. This study provided evidence that troglitazone activated AMPK in beta cells. In addition to the insulin-sensitizing effects in peripheral tissues, troglitazone also directly inhibits insulin hypersecretion by the elevated glucose and fatty acids, and thus protects beta cells from glucolipotoxicity.