Amelioration by KRP-297, a new thiazolidinedione, of impaired glucose uptake in skeletal muscle from obese insulin-resistant animals

Understanding PPARγ and Its Agonists on Trophoblast Differentiation and Invasion: Potential Therapeutic Targets for Gestational Diabetes Mellitus and Preeclampsia

The increasing incidence of pregnancy complications, particularly gestational diabetes mellitus (GDM) and preeclampsia (PE), is a cause for concern, as they can result in serious health consequences for both mothers and infants. The pathogenesis of these complications is still not fully understood, although it is known that the pathologic placenta plays a crucial role. Studies have shown that PPARγ, a transcription factor involved in glucose and lipid metabolism, may have a critical role in the etiology of these complications. While PPARγ agonists are FDA-approved drugs for Type 2 Diabetes Mellitus, their safety during pregnancy is not yet established. Nevertheless, there is growing evidence for the therapeutic potential of PPARγ in the treatment of PE using mouse models and in cell cultures. This review aims to summarize the current understanding of the mechanism of PPARγ in placental pathophysiology and to explore the possibility of using PPARγ ligands as a treatment option for pregnancy complications. Overall, this topic is of great significance for improving maternal and fetal health outcomes and warrants further investigation.

Deletion of skeletal muscle Akt1/2 causes osteosarcopenia and reduces lifespan in mice

Aging is considered to be accelerated by insulin signaling in lower organisms, but it remained unclear whether this could hold true for mammals. Here we show that mice with skeletal muscle-specific double knockout of Akt1/2, key downstream molecules of insulin signaling, serve as a model of premature sarcopenia with insulin resistance. The knockout mice exhibit a progressive reduction in skeletal muscle mass, impairment of motor function and systemic insulin sensitivity. They also show osteopenia, and reduced lifespan largely due to death from debilitation on normal chow and death from tumor on high-fat diet. These phenotypes are almost reversed by additional knocking out of Foxo1/4, but only partially by additional knocking out of Tsc2 to activate the mTOR pathway. Overall, our data suggest that, unlike in lower organisms, suppression of Akt activity in skeletal muscle of mammals associated with insulin resistance and aging could accelerate osteosarcopenia and consequently reduce lifespan.

Tissue-specific control of mitochondrial respiration in obesity-related insulin resistance and diabetes

The tissue-specific role of mitochondrial respiratory capacity in the development of insulin resistance and type 2 diabetes is unclear. We determined mitochondrial function in glycolytic and oxidative skeletal muscle and liver from lean (+/?) and obese diabetic (db/db) mice. In lean mice, the mitochondrial respiration pattern differed between tissues. Tissue-specific mitochondrial profiles were then compared between lean and db/db mice. In liver, mitochondrial respiratory capacity and protein expression, including peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), was decreased in db/db mice, consistent with increased mitochondrial fission. In glycolytic muscle, mitochondrial respiration, as well as protein and mRNA expression of mitochondrial markers, was increased in db/db mice, suggesting increased mitochondrial content and fatty acid oxidation capacity. In oxidative muscle, mitochondrial complex I function and PGC-1α and mitochondrial transcription factor A (TFAM) protein levels were decreased in db/db mice, along with increased level of proteins related to mitochondrial dynamics. In conclusion, mitochondrial respiratory performance is under the control of tissue-specific mechanisms and is not uniformly altered in response to obesity. Furthermore, insulin resistance in glycolytic skeletal muscle can be maintained by a mechanism independent of mitochondrial dysfunction. Conversely, insulin resistance in liver and oxidative skeletal muscle from db/db mice is coincident with mitochondrial dysfunction.

Overexpression of monocyte chemoattractant protein-1 in adipose tissues causes macrophage recruitment and insulin resistance

Adipose tissue expression and circulating concentrations of monocyte chemoattractant protein-1 (MCP-1) correlate positively with adiposity. To ascertain the roles of MCP-1 overexpression in adipose, we generated transgenic mice by utilizing the adipocyte P2 (aP2) promoter (aP2-MCP-1 mice). These mice had higher plasma MCP-1 concentrations and increased macrophage accumulation in adipose tissues, as confirmed by immunochemical, flow cytometric, and gene expression analyses. Tumor necrosis factor-alpha and interleukin-6 mRNA levels in white adipose tissue and plasma non-esterified fatty acid levels were increased in transgenic mice. aP2-MCP-1 mice showed insulin resistance, suggesting that inflammatory changes in adipose tissues may be involved in the development of insulin resistance. Insulin resistance in aP2-MCP-1 mice was confirmed by hyperinsulinemic euglycemic clamp studies showing that transgenic mice had lower rates of glucose disappearance and higher endogenous glucose production than wild-type mice. Consistent with this, insulin-induced phosphorylations of Akt were significantly decreased in both skeletal muscles and livers of aP2-MCP-1 mice. MCP-1 pretreatment of isolated skeletal muscle blunted insulin-stimulated glucose uptake, which was partially restored by treatment with the MEK inhibitor U0126, suggesting that circulating MCP-1 may contribute to insulin resistance in aP2-MCP-1 mice. We concluded that both paracrine and endocrine effects of MCP-1 may contribute to the development of insulin resistance in aP2-MCP-1 mice.

Rational drug design and PPAR agonists

Insulin resistance is a characteristic biological abnormality associated with type 2 diabetes, and is a key component of the metabolic syndrome, a condition in which an altered glucose control is associated with dyslipidemia, hypertension, and obesity. Thiazolidinediones (TZDs), a new class of oral drugs used for the treatment of type 2 diabetes, reduce insulin resistance via an action on peroxisome proliferator-activated receptors. Although the current use of TZDs is largely limited to the treatment of patients with diabetes, as recommended in the package insert, it is foreseeable that as the metabolic syndrome becomes a better understood clinical condition their use may be extended to the treatment of this cluster of disorders. The aim of the present article is to review the mechanism of action of TZDs, discuss the rationale for their use in the clinical setting, and provide an update on novel pharmacologic agents that, although not yet available for the treatment of diabetes, are likely to further enrich the repertoire of antidiabetic drugs in the very near future.

The dual PPARalpha/gamma agonist, ragaglitazar, improves insulin sensitivity and metabolic profile equally with pioglitazone in diabetic and dietary obese ZDF rats

In 6- and 10-week-old obesity-prone (fa/fa) Zucker diabetic fatty (ZDF) rats, effects of prevention and intervention therapies, respectively, were compared between PPARalpha/gamma agonist, ragaglitazar (RAGA) and separate PPARgamma and alpha agonists, pioglitazone (PIO) and bezafibrate (BF). In a separate study, lean (+/+) ZDF rats fed highly palatable chow to induce dietary obesity and insulin resistance were treated similarly. To test insulin-secretory capacity, all animals underwent a hyperglycaemic clamp. Insulin sensitivity was improved equally by RAGA and PIO in fa/fa rats subjected to both prevention and intervention treatments (e.g., prevention HOMA-IR: -71 and -72%, respectively), as was hyperglycaemia (both -68%). BF had no effect on either parameter in any study. Plasma lipids were markedly reduced (by 48-77%) by RAGA in all studies, equivalent to PIO, but to a greater extent than BF. RAGA improved beta-cell function (HOMA-beta) more than three-fold with prevention and intervention therapies, whereas PIO showed improvement only in intervention therapy. Consistent with improved insulin sensitivity, glucose infusion rate during the clamp was 60% higher in RAGA-treated animals subjected to prevention therapy, but there was little additional insulin-secretory response, suggesting that insulin secretion was already maximal.Thus, RAGA and PIO equally improve metabolic profile in ZDF rats, particularly when administered early in the course of diabetes. They also improve beta-cell function, although this is better demonstrated through indices incorporating fasting insulin and glucose concentrations than through the hyperglycaemic clamp technique in this model.

Novel strategies for the pharmacological management of type 2 diabetes

Type 2 diabetes is characterized by high concentrations of glucose in the blood, which is caused by decreased secretion of insulin from the pancreas and decreased insulin action. This condition is prevalent worldwide and is associated with morbidity and mortality secondary to complications such as myocardial infarction, stroke and end-stage renal disease. The importance of tight control of blood glucose in either preventing or delaying the progression of complications is recognized. Currently, there are many therapeutic options to treat hyperglycemia in type 2 diabetes. However, tight control is difficult to achieve and is often associated with side-effects. Recent advances in understanding insulin secretion, action and signaling have led to the development of new pharmacological agents. In this article, we review new molecules that are promising candidates for the future management of diabetes, focusing on their mechanism of action, efficacy, safety profile and potential benefits compared with pharmacological agents that are available currently.

Novel peroxisome proliferator-activated receptor ligands for Type 2 diabetes and the metabolic syndrome

As the incidence of Type 2 diabetes has reached near epidemic proportions, the quest for novel therapies to combat this disorder has intensified dramatically. In recent years, the peroxisome proliferator-activated receptor (PPAR) family has received tremendous attention as perhaps an ideal target class to address the multiple metabolic anomalies associated with the diabetic state. This review focuses on a variety of novel PPAR approaches currently being investigated for Type 2 diabetes or the metabolic syndrome, including the highly potent selective PPAR agonists, PPAR combination agonists and alternative PPAR ligands.

Combination therapy with PPARgamma and PPARalpha agonists increases glucose-stimulated insulin secretion in db/db mice

Although peroxisome proliferator-activated receptor (PPAR)gamma agonists ameliorate insulin resistance, they sometimes cause body weight gain, and the effect of PPAR agonists on insulin secretion is unclear. We evaluated the effects of combination therapy with a PPARgamma agonist, pioglitazone, and a PPARalpha agonist, bezafibrate, and a dual agonist, KRP-297, for 4 wk in male C57BL/6J mice and db/db mice, and we investigated glucose-stimulated insulin secretion (GSIS) by in situ pancreatic perfusion. Body weight gain in db/db mice was less with KRP-297 treatment than with pioglitazone or pioglitazone + bezafibrate treatment. Plasma glucose, insulin, triglyceride, and nonesterified fatty acid levels were elevated in untreated db/db mice compared with untreated C57BL/6J mice, and these parameters were significantly ameliorated in the PPARgamma agonist-treated groups. Also, PPARgamma agonists ameliorated the diminished GSIS and insulin content, and they preserved insulin and GLUT2 staining in db/db mice. GSIS was further increased by PPARgamma and -alpha agonists. We conclude that combination therapy with PPARgamma and PPARalpha agonists may be more useful with respect to body weight and pancreatic GSIS in type 2 diabetes with obesity.

[New antilipemics: prospects]

The field of new lipid-lowering drug research is very active, with researchers, looking to make the currently available drugs more powerful and safer, and to develop new classes of drugs. Among the statins, development has gone the farthest for rosuvastatin and pitavastatin. Colesevelam is a new bile acid sequestrant with a better digestive tolerance. Among the new classes of drugs, the most promising molecules are the cholesterol absorption inhibitors--with ezetimibe as the first in line--and the PPAR-alpha and PPAR-gamma activators. Among the other classes, the acyl-CoA:cholesterol acyltransferase (ACAT) inhibitors, microsomal triglyceride transfer protein (MTP) inhibitors, cholesteryl ester transfer protein (CETP) inhibitors, and ileal bile acid transporter inhibitors, have to be mentioned. In most of the cases, those new compounds are being developed mainly as a combined treatment with statins. However, these combination therapies differ depending on the lipid abnormalities of the patient. The statin-ezitimibe and the statin-bile acid sequestrant combinations have been the most studied treatments in pure hypercholesterolaemia. On another hand, the statin-PPAR-alpha and -gamma activator combination were the first to be developed for patients with combined hyperlipidaemia or type 2 diabetes mellitus. However, the clinical benefit of ACAT or CETP inhibitors remains to be determined and the development of MTP inhibitors has been restricted so far, because of problems of digestive intolerance and hepatic steatosis. Finally, the discovery of new specific lipoprotein receptors, such as the ABCA1 and SRB1 receptors, means that we can work towards developing new potential targets for pharmacological intervention.

Tissue-specific actions of antidiabetic thiazolidinediones on the reduced fatty acid oxidation in skeletal muscle and liver of Zucker diabetic fatty rats

Fatty acid overload has been proposed as a cause of decreased responsiveness in the major insulin target tissues of the body such as muscle and liver tissue. We therefore investigated fatty acid oxidation in soleus muscle and liver isolated from Zucker diabetic fatty (ZDF) rats treated with thiazolidinediones, a new class of antidiabetic agents. 14CO2 production from [14C]palmitic (C16:0) acid was lower in the soleus muscle and liver of ZDF rats versus lean rats (P < .05). When administered orally to ZDF rats for 2 weeks, the thiazolidinediones troglitazone (300 mg/kg) and KRP-297 (10 mg/kg) increased palmitic acid oxidation in the soleus muscle of ZDF rats (P < .05). KRP-297, but not troglitazone, increased palmitic acid oxidation in the liver of ZDF rats (P < .05), and both troglitazone and KRP-297 inhibited triglyceride accumulation in the skeletal muscle of ZDF rats. Hepatic triglyceride accumulation in ZDF rats was inhibited by KRP-297, but not by troglitazone. A reduction of fatty acid oxidation in the liver of ZDF rats and an increase in response to KRP-297 were observed only when C16:0 and C18:0 fatty acids, not C8:0, were used as substrates. Thus, there were defects in fatty acid catabolic activity and triglyceride accumulation in the soleus muscle and liver of ZDF rats. These results indicate that KRP-297 has advantages over troglitazone in the amelioration of these lipid metabolic abnormalities in insulin resistance associated with obesity.