Insulin sensitisation in the treatment of Type 2 diabetes

Type 2 diabetes is reaching epidemic proportions worldwide, fueled by the increasing prevalence of obesity as many populations adopt a western lifestyle. Secondary complications affecting both the microvascular and macrovascular systems are responsible for premature mortality in Type 2 diabetes, with two thirds or more dying of cardiovascular disease. Two interacting metabolic defects, insulin resistance and beta-cell dysfunction are present in Type 2 diabetes. It is now recognised that insulin resistance is central to a cluster of metabolic abnormalities--called the insulin resistance syndrome--that are responsible for the excess of cardiovascular disease. Older antidiabetic agents such as the sulfonylureas, metformin and insulin are more effective than lifestyle modification in reducing microvascular complications of Type 2 diabetes, but overall do not reduce cardiovascular risk. Metformin, although no more effective as a glucose-lowering agent than sulfonylureas or insulin, does significantly reduce cardiovascular disease, probably as a result of its weak insulin-sensitising action. The newly-marketed thiazolidinedione insulin-sensitising antidiabetic agents also improve multiple biomarkers of cardiovascular risk, suggesting that novel approaches to insulin sensitisation will not only provide effective long-term glycaemic control but improve cardiovascular outcomes in Type 2 diabetes. Multiple therapeutic targets within the insulin signalling cascade are being explored, together with follow-up compounds to the first generation thiazolidinediones. These initiatives, together with developments in beta(3)-adrenoceptor agonists, 11 beta-hydroxysteroid dehydrogenase Type 1 inhibitors and modulators of the glucagon-like peptide 1 axis, all of which also potentially enhance insulin sensitivity, are critically evaluated.     

Insulin sensitisation in the treatment of Type 2 diabetes

Type 2 diabetes is reaching epidemic proportions worldwide, fuelled by the increasing prevalence of obesity as many populations adopt a western lifestyle. Secondary complications affecting both the microvascular and macrovascular systems are responsible for premature mortality in Type 2 diabetes, with two thirds or more dying of cardiovascular disease. Two interacting metabolic defects, insulin resistance and β-cell dysfunction are present in Type 2 diabetes. It is now recognised that insulin resistance is central to a cluster of metabolic abnormalities – called the insulin resistance syndrome – that are responsible for the excess of cardiovascular disease. Older antidiabetic agents such as the sulfonylureas, metformin and insulin are more effective than lifestyle modification in reducing microvascular complications of Type 2 diabetes, but overall do not reduce cardiovascular risk. Metformin, although no more effective as a glucose-lowering agent than sulfonylureas or insulin, does significantly reduce cardiovascular disease, probably as a result of its weak insulin-sensitising action. The newly-marketed thiazolidinedione insulin-sensitising antidiabetic agents also improve multiple biomarkers of cardiovascular risk, suggesting that novel approaches to insulin sensitisation will not only provide effective long-term glycaemic control but improve cardiovascular outcomes in Type 2 diabetes. Multiple therapeutic targets within the insulin signalling cascade are being explored, together with follow-up compounds to the first generation thiazolidinediones. These initiatives, together with developments in β₃-adrenoceptor agonists, 11βhydroxysteroid dehydrogenase Type 1 inhibitors and modulators of the glucagon-like peptide 1 axis, all of which also potentially enhance insulin sensitivity, are critically evaluated.     

Insulin resistance and PPAR insulin sensitizers

Drugs that reverse insulin resistance are of importance as insulin resistance is frequently associated with type 2 diabetes. The three peroxisome proliferator-activated receptors (PPARs) PPARalpha, PPAR90 and PPARgamma are essential for the actions of the many insulin sensitizers. PPARalpha activation enhances free fatty acid oxidation and potentiates anti-inflammatory effects, while PPARgamma is essential for normal adipocyte differentiation and proliferation, as well as fatty acid uptake and storage. Thiazolidinediones (TZDs) are selective ligands of PPARgamma and act as insulin sensitizers. TZDs also suppress free fatty acids via the inhibition of lipolysis in adipose tissue. Insulin sensitizers currently under development include partial PPARgamma agonists and antagonists, and dual PPARalpha/PPARgamma agonists. Given that TZDs show anti-inflammatory, anti-oxidant and antiprocoagulant properties in addition to their insulin sensitizing and antilipotoxic properties, a case may be made for initiating TZD therapy early in the treatment of type 2 diabetes, particularly in those patients at risk of cardiovascular disease. TZDs may also be an important therapeutic option in the treatment of metabolic syndrome.     

PPAR与胰岛素抵抗

PPAR即过氧化物酶体增殖物激活受体 ,是核受体超家族成员之一 ,它可以促进脂肪细胞分化 ,在脂肪代谢中起重要作用。近年来随着对胰岛素增敏剂噻唑烷二酮 (TZD)类药物作用机制的深入研究 ,发现PPARγ是该类药物的主要功能受体 ,于是展开了对于PPAR与胰岛素抵抗之间关系的研究。TZD类药物激活PPARγ ,可以改善胰岛素抵抗 ,而在基因敲除的PPARγ+ / -中 ,却发现胰岛素敏感性增加。所以 ,PPAR激活与改善胰岛素抵抗之间不是简单的正相关关系。对二者关系的进一步明确 ,对于以PPAR为靶点寻找更加有效安全的治疗Ⅱ型糖尿病药物具有关键意义     

PPAR- γ agonist in treatment of diabetes: cardiovascular safety considerations

The peroxisome proliferator-activated receptors (PPARs) are nuclear fatty acid receptors, which contain a type II zinc finger DNA binding motif and a hydrophobic ligand binding pocket. These receptors are thought to play an important role in metabolic diseases such as obesity, insulin resistance, and coronary artery disease. Three subtypes of PPAR receptors have been described: PPARα, PPARδ/β, and PPARγ. PPARα is found in the liver, muscle, kidney, and heart. In the liver, its role is to up-regulate genes involved in fatty acid uptake, binding, β-oxidation and electron transport, and oxidative phosphorylation in subcutaneous fat but not in skeletal muscle. PPARδ/β is expressed in many tissues but markedly in brain, adipose tissue, and skin. PPARγ has high expression in fat, low expression in the liver, and very low expression in the muscle. The thiazolidinediones (TZD) are synthetic ligands of PPARγ. By activating a number of genes in tissues, PPARγ increases glucose and lipid uptake, increases glucose oxidation, decreases free fatty acid concentration, and decreases insulin resistance. Although, there is a rationale for the use of TZDs in patients with type 2 diabetes mellitus, clinical studies have produced conflicting data. While currently used TZDs are clearly associated with heart failure (HF) worsening; with regards to cardiovascular outcomes, pioglitazone seems to be related to a trend toward reduction in cardiovascular morbidity and mortality, whereas rosiglitazone may actually increase risk of cardiovascular events. We review the existing literature on TZDs and discuss role and cardiovascular safety of these agents for the contemporary treatment of diabetes. Other side effects of these agents i.e. increase in osteoporosis and possible risk of bladder cancer is also discussed.     

Clinical Trials Reports: Ongoing and planned clinical trials of antiarrhythmic drugs:Cardiovascular & Renal

The discovery that the insulin-sensitising thiazolidinediones (TZDs), specific peroxisome proliferator-activated receptor-γ (PPARγ) agonists, have antiproliferative, anti-inflammatory and immunomodulatory effects has led to the evaluation of their potential use in the treatment of diabetic complications and inflammatory, proliferative diseases in non-insulin-resistant, euglycaemic individuals. Apart from improving insulin resistance, plasma lipids and systemic inflammatory markers, ameliorating atherosclerosis and preventing coronary artery restenosis in diabetic subjects, currently approved TZDs have been shown to improve psoriasis and ulcerative colitis in euglycaemic human subjects. These data imply that the activation of PPAR-γ may improve cardiovascular risk factors and cardiovascular outcomes in both insulin-resistant diabetic and non-diabetic individuals. Through their immunomodulatory and anti-inflammatory actions, TZDs and other PPAR-γ agonists may prove to be effective in treating diseases unrelated to insulin resistance, such as autoimmune (e.g., multiple sclerosis), atopic (e.g., asthma, atopic dermatitis) and other inflammatory diseases (e.g., psoriasis, ulcerative colitis). Newer and safer selective PPAR-γ agonists are presently under development. Furthermore, of considerable interest is the recent discovery that a unique subset of currently prescribed antihypertensive angiotensin II Type 1 receptor antagonists has selective PPAR-γ-modulating activity. These discoveries pave the way for the development of drugs for treating chronic multigenic cardiovascular and metabolic diseases, for which therapy is presently insufficient or non-existent. The potential utility of the currently available TZDs rosiglitazone and pioglitazone and PPAR-γ-modulating angiotensin II Type 1 receptor antagonists in treating cardiovascular, metabolic and inflammatory diseases in insulin resistant and euglycaemic states is of immense clinical potential and should be investigated.     

Investigational agents that protect pancreatic islet beta-cells from failure

Type 2 diabetes is associated with insulin resistance and reduced insulin secretion, which results in hyperglycaemia. This can then lead to diabetic complications such as retinopathy, neuropathy, nephropathy and cardiovascular disease. Although insulin resistance may be present earlier in the progression of the disease, it is now generally accepted that it is the deterioration in insulin-secretory function that leads to hyperglycaemia. This reduction in insulin secretion in Type 2 diabetes is due to both islet beta-cell dysfunction and death. Therefore, interventions that maintain the normal function and protect the pancreatic islet beta-cells from death are crucial in the treatment of Type 2 diabetes so that plasma glucose levels may be maintained within the normal range. Recently, a number of compounds have been shown to protect beta-cells from failure. This review examines the evidence that the existing therapies for Type 2 diabetes that were developed to lower plasma glucose (metformin) or improve insulin sensitivity (thiazolidinediones) may also have islet-protective function. Newer emerging therapeutic agents that are designed to increase the levels of glucagon-like peptide-1 not only stimulate insulin secretion but also appear to increase islet beta-cell mass. Evidence will also be presented that the future of drug therapy designed to prevent beta-cell failure should target the formation of advanced glycation end products and alleviate oxidative and endoplasmic reticulum stress.     

Investigational agents that protect pancreatic islet β-cells from failure

Type 2 diabetes is associated with insulin resistance and reduced insulin secretion, which results in hyperglycaemia. This can then lead to diabetic complications such as retinopathy, neuropathy, nephropathy and cardiovascular disease. Although insulin resistance may be present earlier in the progression of the disease, it is now generally accepted that it is the deterioration in insulin-secretory function that leads to hyperglycaemia. This reduction in insulin secretion in Type 2 diabetes is due to both islet β-cell dysfunction and death. Therefore, interventions that maintain the normal function and protect the pancreatic islet β-cells from death are crucial in the treatment of Type 2 diabetes so that plasma glucose levels may be maintained within the normal range. Recently, a number of compounds have been shown to protect β-cells from failure. This review examines the evidence that the existing therapies for Type 2 diabetes that were developed to lower plasma glucose (metformin) or improve insulin sensitivity (thiazolidinediones) may also have islet-protective function. Newer emerging therapeutic agents that are designed to increase the levels of glucagon-like peptide-1 not only stimulate insulin secretion but also appear to increase islet β-cell mass. Evidence will also be presented that the future of drug therapy designed to prevent β-cell failure should target the formation of advanced glycation end products and alleviate oxidative and endoplasmic reticulum stress.     

Insulin-sensitising agents

Current treatments for non-insulin-dependent diabetes mellitus (NIDDM; Type 2 diabetes) remain far from ideal. The presence of both hyperinsulinaemia and resistance to insulin action challenges the rationale of treatments that primarily boost insulin secretion. Novel therapeutic strategies focus mainly on increasing peripheral sensitivity to endogenous insulin, an approach that has the potential not only to treat but also to prevent Type 2 diabetes in high-risk individuals. Until recently, the most promising new class of agents appeared to be the thiazolidinedione derivatives. These agents are ligands for a specific subtype of peroxisome proliferator activated receptor (PPAR) and decrease plasma glucose levels while reducing circulating insulin and free fatty acid levels. At the time of writing (March 1998), the one approved thiazolidinedione had been withdrawn from the market in the UK after only nine weeks because of cases of hepatic dysfunction and failure occurring in patients in Japan and the USA. It is not yet clear whether this is a class effect and hence whether it will affect other similar agents in development. Reference is made to the current status of other agents in development.     

Recent findings concerning thiazolidinediones in the treatment of diabetes

Thiazolidinediones (TZDs) are peroxisomal proliferator-activated receptor (PPAR)-gamma agonists. They increase insulin action through several mechanisms including: stimulation of the expression of genes that increase fat oxidation and lower plasma free fatty acid levels; increased expression, synthesis and release of adiponectin; and stimulation of adipocyte differentiation resulting in more and smaller fat cells. TZDs lower blood sugar comparably to sulfonylureas and metformin. The clinical use of TZDs is limited due to the long duration of time required before they reach their full blood sugar-lowering action (3-4 months) and adverse effects such as fluid retention, resulting in excessive weight gain and occasionally in peripheral and/or pulmonary oedema and congestive heart failure. Troglitazone, a TZD that has since been removed from the market because of hepatoxicity, has been demonstrated to decrease the progression from normal or impaired glucose tolerance to overt Type 2 diabetes mellitus. Pioglitazone, another TZD, marginally decreased the incidence of cardiovascular complications in patients with Type 2 diabetes mellitus (PROactive trial). Other, as yet, unapproved uses of TZDs include: non-alcoholic fatty liver disease, in which TZDs reduced hepatic fat accumulation and improved liver function tests; polycystic ovary syndrome, where TZDs improved ovulation, hirsutism and endothelial dysfunction; and lipodystrophies, where TZDs increased body fat (marginally) and decrease liver size. Lastly, because PPAR-alpha and -gamma agonists improve atherosclerotic vascular disease and insulin sensitivity, respectively, dual PPAR-alpha/gamma agonists, which are currently undergoing clinical trials, may be useful in treating patients with the metabolic syndrome.     

Effects of PPAR gamma agonists on cardiovascular function in obese, non-diabetic patients

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, and when activated by their ligands, they induce perixosome proliferation. Three receptors have been identified: PPAR gamma, PPAR delta, and PPAR alpha, all with different tissue expression. PPAR gamma is predominantly expressed in adipose tissue and regulates the formation of fat cells and their function. The effect of PPAR gamma activation is to enhance the action of insulin in insulin-sensitive tissue by increasing peripheral glucose disposal and decreasing hepatic glucose production. The thiazolidinediones (TZDs) are a class of medications used for treatment and possibly the prevention of type 2 diabetes, which are potent agonists for the PPAR gamma receptor. Because the thiazolidinediones target insulin resistance, these agents may improve many of the risk factors associated with obesity and insulin resistance including dyslipidemia, hypertension, impaired fibrinolysis, and atherosclerosis. The impact of the thiazolidinediones on cardiovascular mortality is currently unclear but it appears that the thiazolidinediones exert numerous non-glycemic effects that may improve cardiovascular outcomes. Several non-TZD PPAR gamma agonists and combined PPAR gamma/alpha effect on cardiovascular disease are also being evaluated. These drugs have anti-inflammatory and vascular properties and are currently the subject of numerous studies targeting the primary and secondary prevention of macrovascular disease in patients with diabetes and insulin resistance and might be developed as anti-atherogenic agents on the basis of their actions.     

PPARs:脂代谢调节与胰岛素增敏治疗药物的作用靶标

过氧化物酶体增殖物激活受体 (PPARs)是核受体超家族成员之一。PPARα、PPARγ可分别被氯贝特类和TZD类药物特异性激活 ,调节脂代谢、改善胰岛素抵抗 ;有研究表明PPARβ也参与脂肪代谢。因此 ,以PPARs为药物靶标 ,发现和优化单一或双重激动剂将为肥胖和 2型糖尿病的预防和治疗提供有效药物     

Insulin sensitiser drugs

Insulin resistance is the predominant early pathological defect in Type 2 diabetes. As well as being a risk factor for the development of Type 2 diabetes, insulin resistance is also associated with increased cardiovascular risk and other metabolic disturbances including visceral adiposity, hyperinsulinaemia, impaired glucose tolerance, hypertension and dyslipidaemia [1-4]. The newest approach to oral antidiabetic therapy is to target improvements in insulin sensitivity at muscle, adipose tissue and hepatic level. This results in improvements in glycaemic control and other features of the insulin resistance syndrome, with potential long-term benefits in preventing/delaying the onset of diabetic complications and macrovascular disease.