The effects of troglitazone, an insulin-sensitizing agent, on the endothelial function in early and late type 2 diabetes: a placebo-controlled randomized clinical trial

Effects of pioglitazone on nitric oxide bioavailability measured using a catheter-type nitric oxide sensor in angiotensin II-infusion rabbit

Recently, peroxisome proliferator-activated receptor gamma (PPARgamma) ligands have been reported to increase nitric oxide (NO) bioavailability in vitro but not in vivo because of the difficulty of measuring plasma NO. Here, we investigated the effects of PPARgamma on plasma NO concentrations using the newly developed NO sensor in angiotensin II (Ang II)-infused rabbits. Male New Zealand rabbits were randomized for infusion with Ang II, either alone or in combination with pioglitazone (a PPARgamma agonist). Plasma NO concentration was measured using the catheter-type NO sensor placed in the aorta. We then infused N(G)-methyl-L-arginine (L-NMMA) and acetylcholine (ACh) into the aortic arch to measure the basal and ACh-induced plasma NO concentration. Vascular nitrotyrosine levels were examined by enzyme-linked immunoassay (ELISA). Both an immunohistochemical study and Western blotting were performed to examine the PPARgamma and gp91phox expression. The cotreatment with pioglitazone significantly suppressed the negative effects of Ang II, that is, the decreases in basal and ACh-induced NO production and the increase in vascular nitrotyrosine levels. Both the immunohistochemical study and Western blotting demonstrated that pioglitazone treatment enhaced PPARgamma expression and greatly inhibited Ang II-induced up-regulation of gp91phox. In conclusion, the PPARgamma agonist pioglitazone significantly improved NO bioavailability in Ang II-infused rabbits, most likely by attenuating nitrosative stresses.

Therapeutic regulation of endothelial dysfunction in type 2 diabetes mellitus

Endothelial dysfunction is universal in diabetes, being intimately involved with the development of cardiovascular disease. The pathogenesis of endothelial dysfunction in diabetes is complex. It is initially related to the effects of fatty acids and insulin resistance on 'uncoupling' of both endothelial nitric oxide synthase activity and mitochondrial function. Oxidative stress activates protein kinase C (PKC), polyol, hexosamine and nuclear factor kappa B pathways, thereby aggravating endothelial dysfunction. Improvements in endothelial function in the peripheral circulation in diabetes have been demonstrated with monotherapies, including statins, fibrates, angiotensin-converting enzyme (ACE) inhibitors, metformin and fish oils. These observations are supported by large clinical end point trials. Other studies show benefits with certain antioxidants, L-arginine, folate, PKC-inhibitors, peroxisome proliferator activated receptor (PPAR)-alpha and -gamma agonists and phosphodiesterase (PDE-5) inhibitors. However, the benefits of these agents remain to be shown in clinical end point trials. Combination treatments, for example, statins plus ACE inhibitors and statins plus fibrates, have also been demonstrated to have additive benefits on endothelial function in diabetes, but there are no clinical outcome data to date. Measurement of endothelial dysfunction in cardiovascular research can provide fresh opportunities for exploring the mechanism of benefit of new therapeutic regimens and for planning and designing large clinical trials.

Thiazolidinediones and the risk of edema: a meta-analysis

The use of thiazolidinediones (TZDs) in the management of type 2 diabetes mellitus (T2DM) has been associated with an increased risk of peripheral edema. A meta-analysis was performed to assess the overall risk for developing edema secondary to TZD. A systematic literature search was conducted using five electronic databases. All prospective, randomized, either placebo-controlled or comparative studies reporting the incidence of edema with TZD therapy were included. Odds ratios were generated by pooling estimates across the studies. The analysis included 26 studies consisting of 15,332 patients with T2DM. The pooled odds ratio for TZD induced edema was 2.26 (95% CI: 2.02-2.53). The results yielded a higher risk for developing edema with rosiglitazone (3.75 [2.70-5.20]) compared to pioglitazone (2.42 [1.90-3.08]). Concordant results persisted with calculations of the adjusted indirect estimate. This meta-analysis demonstrates at least a two-fold increase in the risk for developing edema with a TZD agent. The risk appears to be greater with rosiglitazone than with pioglitazone. Further studies are needed to explore this difference.

Thiazolidinediones and vascular damage

PURPOSE OF REVIEW

Although the thiazolidinediones were introduced for the treatment of hyperglycemia in type 2 diabetes, it became quickly apparent that these agents modulated many pathways related to vascular physiology and pathophysiology. Given the fact that cardiovascular disease is the leading cause of death in diabetes, it has become important to know whether these agents have vasculoprotective effects and if so whether these are associated with the prevention of cardiovascular disease.

RECENT FINDINGS

The thiazolidinedione class improves endothelial vasomotion, inhibits inflammatory and procoagulant processes and has powerful antiproliferative and antioxidant effects. Experimentally these agents retard atherosclerosis development in predisposed animals. Clinical studies demonstrate that they increase HDL cholesterol and LDL size, and may lower triglyceride levels. They modestly lower blood pressure, reduce microalbuminuria, arterial stiffness and reduce carotid wall thickening. These effects are generally independent of glucose lowering and in many instances have been shown to occur in nondiabetic subjects. A single clinical endpoint intervention trial of add-on pioglitazone treatment in type 2 diabetic patients with cardiovascular disease suggested on secondary analyses that the agent reduced cardiovascular events.

SUMMARY

The weight of the experimental, subclinical and clinical assessments of the effects of these agents supports the contention that they are vasculoprotective. In the final analysis their use in clinical practice to prevent cardiovascular disease will mostly depend on whether clinical trials consistently demonstrate that they reduced cardiovascular events.

Nitric oxide activation of peroxisome proliferator-activated receptor gamma through a p38 MAPK signaling pathway

Both nitric oxide (NO*) and peroxisome proliferator-activated receptors (PPARs) protect the endothelium and regulate its function. Here, we tested for crosstalk between these signaling pathways. Human umbilical vein and hybrid EA.hy926 endothelial cells were exposed to S-nitrosoglutathione (GSNO) or diethylenetriamine NONOate (DETA NONOate). Electrophoretic mobility shift assays using PPAR-response element (PPRE) probe showed that NO* caused a rapid dose-dependent increase in PPARgamma binding, an effect that was confirmed in vivo by chromatin immunoprecipitation. Conversely, N(G)-monomethyl-L-arginine, a NOS inhibitor, decreased PPARgamma binding. NO*-mediated PPARgamma binding and NO* induction of cyclooxygenase-2 (COX-2), diacylglycerol (DAG) kinase alpha (DGKalpha), and heme oxygenase-1 (HO-1), genes with well-characterized PPRE motifs, were cGMP independent. NO* dose dependently activated p38 MAPK, and p38 MAPK inhibition with SB202190 or knockdown with siRNA was shown to block NO* activation of PPARgamma. Likewise, p38 MAPK and PPARgamma inhibitors or knockdown of either transcript all significantly blocked NO* induction of PPRE-regulated genes. PPARgamma activation by p38 MAPK may contribute to the anti-inflammatory and cytoprotective effects of NO* in the vasculature. This crosstalk mechanism suggests new strategies for preventing and treating vascular dysfunction.

Air pollution and inflammation in type 2 diabetes: a mechanism for susceptibility

BACKGROUND

Particulate air pollution has been associated with several adverse cardiovascular health outcomes, and people with diabetes may be especially vulnerable. One potential pathway is inflammation and endothelial dysfunction-processes in which cell adhesion molecules and inflammatory markers play important roles.

AIM

To examine whether plasma levels of soluble intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) and von Willebrand factor (vWF) were associated with particle exposure in 92 Boston area residents with type 2 diabetes.

METHODS

Daily average ambient levels of air pollution (fine particles (PM2.5), black carbon (BC) and sulphates) were measured approximately 500 m from the patient examination site and evaluated for associations with ICAM-1, VCAM-1 and vWF. Linear regressions were fit to plasma levels of ICAM-1, VCAM-1 and vWF, with the particulate pollutant index, apparent temperature, season, age, race, sex, glycosylated haemoglobin, cholesterol, smoking history and body mass index as predictors.

RESULTS

Air pollutant exposure measures showed consistently positive point estimates of association with the inflammatory markers. Among participants not taking statins and those with a history of smoking, associations between PM(2.5), BC and VCAM-1 were particularly strong.

CONCLUSIONS

These results corroborate evidence suggesting that inflammatory mechanisms may explain the increased risk of air pollution-associated cardiovascular events among those with diabetes.

Novel therapies targeting vascular endothelium

Endothelial dysfunction has been identified as a major mechanism involved in all the stages of atherogenesis. Evaluation of endothelial function seems to have a predictive role in humans, and therapeutic interventions improving nitric oxide bioavailability in the vasculature may improve the long-term outcome in healthy individuals, high-risk subjects, or patients with advanced atherosclerosis. Several therapeutic strategies are now available, targeting both the synthesis and oxidative inactivation of nitric oxide (NO) in human vasculature. Statins seem to be currently the most powerful category of these agents, improving endothelial function and decreasing cardiovascular risk after long-term administration. Other cardiovascular agents improving endothelial function in humans are angiotensin-converting enzyme inhibitors/angiotensin receptors blockers, which increase NO bioavailability by modifying the rennin-angiotensin-aldosterone system. Newer therapeutic approaches targeting endothelial dysfunction in specific disease states include insulin sensitizers, L-arginine (the substrate for endothelial NO synthase [eNOS]) as well as substances that target eNOS "coupling," such as folates or tetrahydrobiopterin. Although there are a variety of strategies to improve NO bioavailability in human endothelium, it is still unclear whether they have any direct benefit at a clinical level.

Pioglitazone improves endothelial function in non-diabetic patients with coronary artery disease

OBJECTIVE

To test the hypothesis that pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, will improve endothelial function in non-diabetic subjects with coronary artery disease, we conducted a prospective study to evaluate the effect of this medication on the brachial artery vasomotor function and circulating markers of endothelial activation.

METHODS

Baseline characteristics were collected. After initial endothelial function assessment, patients were treated with pioglitazone hydrochloride 30 mg daily. The medication was continued for 12 weeks and endothelial function was reassessed as well as the inflammatory markers. The study medication then was stopped, and all the tests were repeated 12 weeks later.

RESULTS

Seventeen subjects completed all three-study phases. Mean age was 58 (range: 36-77 years). Compared with the baseline, the endothelium-dependent vasodilation improved significantly with the treatment (p < 0.001) from 4.4 +/- 3.9 to 8.4 +/- 4.1%, a relative increase of 91%. After withdrawal of treatment, the endothelium-dependent vasodilation returned towards baseline values. There was no change in endothelium-independent vasodilatation (12.27 +/- 6.35 to 13.9 +/- 9.23%, to 12.42 +/- 5.35%, p = 0.177). The urine asymmetric dimethlyarginine levels decreased significantly with the treatment, but also returned to the initial values after the wash-out period (1.27 +/- 0.5 micromol/ml to 0.97 +/- 0.3 micromol/ml to 1.34 +/- 0.5 micromol/ml, p = 0.017). No difference in the lipid profile, C-reactive protein, erythrocyte sedimentation rate, or fibrinogen levels was seen.

CONCLUSION

Pioglitazone rapidly improves endothelial function in non-diabetic patients with coronary artery disease. This improvement is associated with a change in mean urinary asymmetric dimethylarginine levels, although a cause and effect cannot be determined from this investigation.

Thiazolidinediones and risk for atherosclerosis: pleiotropic effects of PPar gamma agonism

Despite advances in the development of anti-hyperglycaemic drugs and a greater focus on cardiovascular risk modification for patients with diabetes, cardiovascular disease remains the most common complication of type 2 diabetes. Since their initial availability in 1997, the thiazolidinediones have become one of the most commonly prescribed classes of medications for type 2 diabetes. In addition to glucose control, the thiazolidinediones have a number of pleiotropic effects on myriad traditional and non-traditional risk factors for cardiovascular disease, and hold promise with regard to modification of cardiovascular risk. In a recently reported large-scale clinical trial, pioglitazone was associated with improved cardiovascular outcomes in patients with type 2 diabetes and prevalent atherosclerotic disease. In this review, we summarise the experimental, preclinical and clinical data regarding the effects of the thiazolidinediones on cardiovascular risk factors and clinical outcomes.

Valsartan improves resting skin blood flow in type 2 diabetic patients and reduces poly(adenosine diphosphate-ribose) polymerase activation

OBJECTIVE

To examine the effect of a 12-week daily treatment with 160 mg of valsartan, an angiotensin II receptor blocker, on the microcirculation and macrocirculation of type 2 diabetic patients (T2DM) and healthy subjects.

METHODS

This was a prospective, randomized, double-blind, placebo-controlled crossover study. Thirteen T2DM with no severe complications and 13 healthy subjects completed the trial.

RESULTS

Treatment with valsartan in T2DM improved the resting forearm skin blood flow and increased the resting brachial artery diameter but had no effects on arterial blood pressure, large vessel vascular reactivity, or carotid intima-media thickness. Resting skin blood flow increased by 60% (2%-90%; median and 25th-75th percentiles) during valsartan treatment and by only 2% (-22% to 27%) during placebo treatment (P < .05). No changes were observed in the nondiabetic subjects. Immunostaining studies of forearm skin biopsy samples from T2DM and healthy subjects showed that valsartan reduced poly(adenosine diphosphate-ribose) polymerase (PARP) activity in 50% (6/12) of the subjects. PARP activity remained unchanged in placebo-treated subjects (P < .02). In addition, valsartan treatment increased CD31 staining in 33% (4/12) of the subjects, whereas no change was noted in sequential skin biopsy samples of placebo-treated subjects (P = .057). Valsartan had no effect on the biochemical markers of endothelial cell activation and other cytokines, including CAMs, interleukin 6, tumor necrosis factor alpha, C-reactive protein, adiponectin, and plasma activator inhibitor 1.

CONCLUSIONS

Valsartan increases the resting skin blood flow in T2DM, likely through reduction of PARP activity.

Thiazolidinediones

Our knowledge and understanding of the role played by peroxisome proliferator-activated gamma receptors in physiology and pathophysiology has expanded dramatically over the past 5 years. Originally described as having important functions in adipogenesis and glucose homeostasis, their pharmacologic agonists, the thiazolidinediones, were introduced as antihyperglycemic, insulin-sensitizing agents for the management of type 2 diabetes mellitus. However, it was to some degree inevitable that the thiazolidinediones would be rapidly recognized as having vasculoprotective properties beyond glycemic control that might also be beneficial. First, diabetic complications are vascular in nature, the earliest feature of these is endothelial dysfunction. Second, it is being increasingly appreciated that these complications develop through inflammatory and procoagulant pathways in which increased oxidative stress is considered a major etiologic mechanism, and which are closely linked to the presence of insulin resistance, visceral obesity, and hyperglycemia. Early appreciation that the thiazolidinediones have antioxidant, anti-inflammatory, anti-procoagulant, and antiproliferative properties in addition to their insulin-sensitizing, anti-lipotoxic properties created a marriage of investigative pathways that has not only led to a very large body of literature on the pleiotropic effects of thiazolidinediones, but also to the development of new understandings of the connections between insulin resistance, obesity, and hyperglycemia and the onset of vascular disease. Understandably, most of the focus has been directed at the macrovascular complications of diabetes, since these are the major causes of morbidity and mortality in this population. However, there is evidence that these agents may have benefits for the microvascular complications as well, and their potential role for cardiovascular disease prevention in non-diabetic patients with the metabolic syndrome is a logical extension of the work performed in diabetes. The recently reported results of the effects of pioglitazone versus placebo on cardiovascular events in patients with type 2 diabetes support the contention that these agents have vasculoprotective effects.

Short-term pioglitazone treatment improves vascular function irrespective of metabolic changes in patients with type 2 diabetes

To determine whether pioglitazone influences endothelial function directly, we examined in a randomized, crossover, placebo-controlled, double-blind trial the effects of 4 weeks of pioglitazone treatment in 20 male type 2 diabetic patients. We conclude that short-term pioglitazone treatment ameliorates endothelial dysfunction in conduit arteries irrespective of significant beneficial changes in plasma levels of insulin, FFA, adiponectin, or CRP in type 2 patients with diabetes. Pioglitazone, a PPARgamma agonist, not only improves insulin resistance and glycemic control but may also have additional beneficial vascular effects in patients with type 2 diabetes. Low-grade inflammation, free fatty acids, and adiponectin may play a role in modulation of vascular function. We studied the effect of 4 weeks of pioglitazone treatment on endothelial function, metabolic changes, and C-reactive protein in patients with type 2 diabetes. A randomized, crossover, placebo-controlled, double-blind trial was performed in which pioglitazone 30 mg once daily was administered to 20 patients with type 2 diabetes on oral antihyperglycemic agents for 4 weeks. Shear stress-induced flow-mediated dilation (FMD) of the brachial artery was used as outcome parameter for vascular function. Brachial artery endothelial function was significantly increased by pioglitazone treatment compared with placebo (FMD 5.4 +/- 0.5% versus 3.1 +/- 0.5%, P = 0.001). Endothelium-independent vasodilation was not different between the 2 study periods. Pioglitazone treatment reduced insulin, FFA, and C-reactive protein concentrations compared with placebo (18.3 +/- 2.4 versus 14.8 +/- 2.1 mU/L, P = 0.03; 641 +/- 46 versus 542 +/- 33 mumol/L, P = 0.04; and 3.5 +/- 0.6 mg/L versus 2.6 +/- 0.5 mg/L, P = 0.01; respectively). A significant increase in plasma adiponectin concentration (3.95 +/- 0.57 microg/mL versus 7.59 +/- 0.95 microg/mL, P = 0.002) was also observed. No correlations were found between these metabolic changes and the improvement of conduit artery endothelial function. Short-term pioglitazone treatment ameliorates endothelial dysfunction in conduit arteries irrespective of changes in insulin, FFA, adiponectin, or CRP in type 2 patients with diabetes.

Adipose tissue function in the insulin-resistance syndrome

Insulin resistance is often seen as a consequence of obesity and there are several possible links between adipose tissue function and insulin resistance determined in other organs such as skeletal muscle or liver. One such link is the regulation of NEFA (non-esterified fatty acid) delivery to the rest of the body. Simplistically, an expanded adipose tissue mass delivers more NEFA to the systemic circulation and these fatty acids compete for substrate utilization in skeletal muscle, which in turn reduces glucose utilization. This increases blood glucose concentration and provides the stimulus for increased insulin secretion and hyperinsulinaemia is a key feature of the insulin-resistance syndrome. However, there is abundant evidence that adipose tissue is exquisitely insulin sensitive and hyperinsulinaemia may therefore lead to a constant lipolytic inhibition in adipose tissue. Consequently, the main function of adipose tissue, to rapidly switch between fat uptake and fat release, will be hampered. Adipose tissue blood flow is the conveyor of signals and substrates to and from the adipose tissue. In healthy people adipose tissue blood flow is much enhanced by food intake, whereas in insulin-resistant subjects this response is blunted. This is another facet of unresponsiveness of adipose tissue in the insulin-resistance syndrome.

Metabolic and vascular abnormalities in subjects at risk for type 2 diabetes: the early start of a dangerous situation

Various groups at risk for type 2 diabetes have been identified, including individuals with family history of type 2 diabetes, obesity, prior gestational diabetes, polycystic ovary syndrome, metabolic syndrome, hypertension, dyslipidemia and particularly those with pre-diabetes (impaired glucose tolerance and/or impaired fasting glucose). To various degrees, all these groups have also been identified with significant vascular abnormalities that range from endothelial dysfunction and low-grade or sub-clinical inflammation to evident atherosclerosis. The mechanisms involved in establishing a link between the risk of type 2 diabetes and vascular dysfunction are multiple and complex. The presence in the circulation of various cytokines, hormones and substrates associated with increased visceral fat and insulin resistance, the frequent appearance of associated cardiovascular risk factors and/or the possibility of some genetically determined intrinsic vascular abnormalities are all explanatory mechanisms that are being evaluated in clinical research. Whereas the possibility of appreciating a significant reduction in cardiovascular outcomes in long-term prospective clinical trials in all these groups at risk for type 2 diabetes is still lacking, understanding these mechanisms and recognizing how various interventions may improve vascular health is a worthwhile area of research that may translate into important clinical strategies to reduce the burden of type 2 diabetes and cardiovascular disease.

Rapid Effects of Rosiglitazone Treatment on Endothelial Function and Inflammatory Biomarkers

Background— Antidiabetic thiazolidinediones (TZDs), like rosiglitazone or pioglitazone, improve endothelial function in patients with type 2 diabetes or metabolic syndrome, but it is currently unknown, whether these beneficial effects of TZDs depend on their metabolic action or may be caused by direct effects on the endothelium. Therefore, the present study examined whether short-term rosiglitazone treatment influences endothelium-dependent vasodilation as well as serum levels of vascular disease biomarkers in healthy, nondiabetic subjects.

Methods and Results— Short-term treatment (21 days) of healthy subjects (n=10) did not significantly change blood glucose levels or lipid profile. In contrast, rosiglitazone significantly increased flow-mediated, endothelium-dependent vasodilation already within the first day from 5.3±2.7% at baseline to 7.8±2.6%, further increasing it to 9.4±3.0% at day 21. In addition, the early improvement of endothelium-dependent vasodilation was paralleled by a rapid reduction of serum levels of the biomarkers C-reactive protein (CRP), serum amyloid A (SAA), and sE-selectin. Moreover, after drug withdrawal all markers remained suppressed for the whole follow-up period of 7 days. In contrast, rosiglitazone treatment did not significantly affect tumor necrosis factor (TNF)-α, interleukin (IL)-6, sICAM-1, sVCAM-1, and sCD40L levels.

Conclusions— Our study suggests a direct effect of TZD treatment on endothelial function and inflammatory biomarkers of arteriosclerosis, promoting the concept that TZDs, independent of their metabolic action, may exhibit protective effects in the vessel wall.

Impact of thiazolidinedione therapy on atherogenesis

Thiazolidinediones (TZDs), which are synthetic ligands for peroxisome proliferator activated receptor g (PPARg) activation, have been introduced in clinical medicine to improve insulin resistance and glycemic control in patients with type 2 diabetes. The metabolic effects of TZDs are mediated by receptor-dependent activation of the PPARg-retinoid X receptor (RXR) complex and subsequent transcriptional activation of target genes. The PPARg1 isoform is also expressed in endothelial cells, vascular smooth muscle cells, and monocytes/macrophages in the vasculature. TZDs have been shown to have anti-atherosclerotic effects on these cells in vitro, which appear to be partially independent of the PPARg-RXR-mediated transcriptional effects. Direct anti-atherosclerotic effects of TZDs include increased nitric oxide bioavailability, decreased leukocyte/endothelial cell interaction, reduced vascular smooth muscle cell migration and proliferation, and cholesterol efflux from macrophages. So far, there are no data on the effects of TZDs on cardiovascular events, but studies using surrogate markers of vascular disease provide preliminary evidence that TZDs delay progression of atherosclerosis in different patient groups. TZDs interfere with key processes in atherogenesis and may, therefore, offer additional opportunities to improve cardiovascular risk beyond treatment of glycemic control in patients with type 2 diabetes.

Metabolic syndrome and endothelial dysfunction

The incidence of metabolic syndrome is rapidly increasing in the United States. Metabolic syndrome is associated with increased cardiovascular morbidity and mortality, and endothelial dysfunction is an early pathogenetic event in the metabolic syndrome. Endothelial dysfunction of either the coronary, the peripheral, or the cerebral vasculature is a predictor of vascular events and appears to be a marker of uncontrolled atherosclerotic risk that adds to the burden of the genetic predisposition to cardiovascular disease. Clinically and experimentally, endothelial dysfunction can be restored by several agents, including blockers/inhibitors of the renin-angiotensin-aldosterone system, as well as statins. Nevertheless, it would be premature, and most likely inappropriate, to use improvement of endothelial function as a surrogate end point to predict reduction in cardiovascular morbidity and mortality. However, a clear understanding of the mechanisms of endothelial dysfunction in the metabolic syndrome may allow the development of preventive and early therapeutic measures targeting cardiovascular disease.

Insulin resistance and the endothelium

Type 2 diabetes is a cardiovascular disease equivalent that is associated with accelerated atherosclerosis and significant mortality. However, the metabolic syndrome and prediabetes are associated with increased cardiovascular mortality, indicating that atherogenic vascular changes begin prior to the onset of overt diabetes. At the core of diabetes and the metabolic syndrome is insulin resistance (IR), which sets the stage for dyslipidemia, hypertension, and inflammation. Endothelial dysfunction is the first stage of the atherosclerosis process and results from exposure to cardiovascular risk factors, such as IR and diabetes. IR and atherosclerosis follow parallel paths as they progress in severity. Thiazolidinediones, angiotensin-converting enzyme inhibitors, angiotensin receptor-AT1 blockers, and statins are widely used in the treatment of diabetes. Emerging evidence indicates that these pharmacologic agents have added mechanisms of action, especially on the endothelium and in the prevention of diabetes.

Redox modulation of insulin signaling and endothelial function

Reactive oxygen and nitrogen species (ROS and RNS) recently emerged as critical signaling molecules in cardiovascular research. Several studies over the past decade have shown that physiological effects of vasoactive factors are mediated by these reactive species and, conversely, that altered redox mechanisms are implicated in the occurrence of metabolic and cardiovascular diseases. Oxidant stress occurs when ROS and/or RNS production exceeds the cell natural antioxidant systems, and pathological events ensue. Cardiovascular risk factors are associated with an imbalance of the redox equilibrium toward oxidative stress, leading to endothelial activation and proinflammatory processes implicated in atherogenesis and metabolic disorders. Recent studies indicate that insulin and insulin-sensitizing drugs activate antiinflammatory pathways that may limit oxidant stress in insulin target tissues. The main goal of this brief review is to discuss recent progress in the field of cellular redox signaling as it pertains to insulin modulation of vascular endothelial function in cardiovascular diseases.

The effect of statin therapy on endothelial function in type 2 diabetes without manifest cardiovascular disease

OBJECTIVE

Cardiovascular disease (CVD) is the most important cause of mortality in patients with type 2 diabetes and is preceded by endothelial dysfunction. Flow-mediated dilation (FMD) is a noninvasive technique for measuring endothelial dysfunction. We aimed to determine the effect of long-term statin therapy versus placebo on FMD in patients with type 2 diabetes without manifest CVD.

RESEARCH DESIGN AND METHODS

A randomized, placebo-controlled, double-blind trial was performed with 250 type 2 diabetic patients. Patients were given 0.4 mg cerivastatin or placebo daily. In August 2001, when cerivastatin was withdrawn from the market, the 0.4 mg cerivastatin was replaced by 20 mg simvastatin, without deblinding the study. The primary end point was the change in FMD, measured by B-mode ultrasound, after 2 years.

RESULTS

Determinants of baseline FMD were diabetes duration, common carotid intima-media thickness, and brachial artery diameter. FMD at baseline was 1.51% in the placebo group and 1.66% in the statin group and did not change significantly after 2 years.

CONCLUSIONS

The 2-year statin therapy had no effect on FMD in type 2 diabetes. Statin-induced improvement of cardiovascular risk in patients with type 2 diabetes may be mediated through mechanisms other than increased nitric oxide availability.

Diabetes enhances vulnerability to particulate air pollution-associated impairment in vascular reactivity and endothelial function

BACKGROUND

Epidemiological studies suggest that people with diabetes are vulnerable to cardiovascular health effects associated with exposure to particle air pollution. Endothelial and vascular function is impaired in diabetes and may be related to increased cardiovascular risk. We examined whether endothelium-dependent and -independent vascular reactivity was associated with particle exposure in individuals with and without diabetes.

METHODS AND RESULTS

Study subjects were 270 greater-Boston residents. We measured 24-hour average ambient levels of air pollution (fine particles [PM2.5], particle number, black carbon, and sulfates [SO4(2-)]) approximately 500 m from the patient examination site. Pollutant concentrations were evaluated for associations with vascular reactivity. Linear regressions were fit to the percent change in brachial artery diameter (flow mediated and nitroglycerin mediated), with the particulate pollutant index, apparent temperature, season, age, race, sex, smoking history, and body mass index as predictors. Models were fit to all subjects and then stratified by diagnosed diabetes versus at risk for diabetes. Six-day moving averages of all 4 particle metrics were associated with decreased vascular reactivity among patients with diabetes but not those at risk. Interquartile range increases in SO4(2-) were associated with decreased flow-mediated (-10.7%; 95% CI, -17.3 to -3.5) and nitroglycerin-mediated (-5.4%; 95% CI, -10.5 to -0.1) vascular reactivity among those with diabetes. Black carbon increases were associated with decreased flow-mediated vascular reactivity (-12.6%; 95% CI, -21.7 to -2.4), and PM2.5 was associated with nitroglycerin-mediated reactivity (-7.6%; 95% CI, -12.8 to -2.1). Effects were stronger in type II than type I diabetes.

CONCLUSIONS

Diabetes confers vulnerability to particles associated with coal-burning power plants and traffic.

Effect of a peroxisome proliferator-activated receptor-gamma agonist on myocardial blood flow in type 2 diabetes

OBJECTIVE

The relationship between coronary endothelial function and insulin resistance remains speculative. We sought to determine whether pioglitazone, an insulin-sensitizing peroxisome proliferator-activated receptor (PPAR)-gamma agonist, improves cardiac endothelial function in individuals with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Sixteen subjects with insulin-treated type 2 diabetes and without overt cardiovascular disease were randomly assigned to receive either 45 mg of pioglitazone or matching placebo for 3 months. Rest and adenosine-stimulated myocardial blood flow (MBF) were quantified with [(13)N]ammonia and positron emission tomography at baseline and study conclusion.

RESULTS

After 3 months, HbA(1c) levels dropped by 0.68% in the pioglitazone group and increased by 0.17% in the placebo group (P = 0.009 for difference between groups). Triglyceride (-93 vs. -39 mg/dl, P = 0.026) and HDL concentrations (+4.8 vs. -6.0 mg/dl, P = 0.014) improved significantly in the pioglitazone group compared with placebo. Despite these favorable changes, there was no demonstrable change in baseline MBF (-0.05 +/- 0.24 vs. -0.09 +/- 0.24 ml . min(-1) . g(-1), P = 0.45), adenosine-stimulated MBF (0.10 +/- 0.75 vs. 0.14 +/- 0.31 ml . min(-1) . g(-1), P = 0.25), or coronary flow reserve (0.45 +/- 1.22 vs. 0.35 +/- 0.72 ml . min(-1) . g(-1), P = 0.64) after 12 weeks of exposure to pioglitazone or placebo, respectively. Regression analysis revealed that lower glucose concentration at the time of the study was associated with higher coronary flow reserve (P = 0.012).

CONCLUSIONS

Pioglitazone treatment for 12 weeks in subjects with insulin-requiring type 2 diabetes had no demonstrable effect on coronary flow reserve despite metabolic improvements. Higher ambient glucose levels contribute to impaired vascular reactivity in individuals with diabetes.

The renin angiotensin system as a therapeutic target to prevent diabetes and its complications

The role of the RAAS in development and maintenance of blood pressure is well established. In addition, the deleterious effects of angiotensin II on the heart, vasculature, and kidneys have been clearly defined. There seems to be a close relationship between endothelial dysfunction, insulin resistance (a precursor to diabetes and coronary artery disease) and angiotensin II. The signaling pathways for insulin in the vascular wall interacts with the angiotensin signaling, giving rise to potential mechanisms for development of diabetes and resulting harmful effects. A large number of clinical trials using ACE inhibitors or ARBs have shown significant reduction in secondary endpoints in the development of new onset of diabetes. Ongoing prospective studies involving ARBs (eg, the Nateglinide and Valsartan Impaired Glucose Tolerance Outcomes Research trial) and ACE inhibitors (eg, the Diabetes Re-duction Assessment with Ramipril and Rosiglita-zone Medication trial) are testing the ability of certain agents to prevent type 2 diabetes. In the meantime, it is important to recognize insulin resistance and metabolic syndrome as entities that increase the risk for cardiovascular disease. In addition to lifestyle modifications, managing endothelial dysfunction and protecting the vasculature will help prevent diabetes and cardiovascular disease.

Endothelial-dependent vasodilation and incidence of type 2 diabetes in a population of healthy postmenopausal women

OBJECTIVE

Both postmenopausal state and diabetes are associated with endothelial dysfunction and are well-known risk factors for atherosclerosis. However, the relationship of endothelium-dependent vasodilation and diabetes has never been prospectively evaluated. This study provided the opportunity to assess the association between endothelial vasodilation function and the incidence of diabetes in a cohort of apparently healthy postmenopausal women.

RESEARCH DESIGN AND METHODS

We conducted a prospective cohort study that began in 1997 with 840 apparently healthy, nonobese, postmenopausal women, aged 53 +/- 6 years, initially with normal glucose tolerance at the oral glucose tolerance test. All participants were followed up for a mean period of 3.9 +/- 0.7 years (range 0.5-6.9). Endothelial function was measured as flow-mediated dilation (FMD) of the brachial artery, using high-resolution ultrasound.

RESULTS

There were no significant differences in demographic, blood pressure, and biochemical profiles among each tertile group at baseline or at follow-up review. During follow-up, 102 women developed type 2 diabetes. The adjusted relative risk (RR) for women with FMD </=4.3 (lowest tertile) was 5.87 (95% CI 4.34-8.10) versus women with FMD >/=5.6 (highest tertile reference). Each 1-unit decrease of FMD was associated with a significant 32% (22-48%) increase in the multiple-adjusted RR of incident diabetes.

CONCLUSIONS

These prospective data indicate a significant increase in the RR of diabetes with each unit decrease of FMD. This could suggest that an impaired endothelial function may play a fundamental role in diabetogenesis in postmenopausal women.

Influence of Rosiglitazone on Flow-Mediated Dilation and Other Markers of Cardiovascular Risk in HIV-Infected Patients with Lipoatrophy

Background

Antiretroviral therapy for HIV infection is commonly complicated by lipoatrophy, insulin resistance and dyslipidaemia. In HIV-uninfected adults with insulin resistance or type 2 diabetes, thiazolidinediones can lower blood pressure and improve both insulin sensitivity and endothelial function. This study sought to investigate the effects of rosiglitazone on endothelial function and other markers of cardiovascular risk in patients with HIV-related lipoatrophy.

Methods

HIV-infected, lipoatrophic adults receiving anti-retroviral therapy were randomized to receive either rosiglitazone 4 mg or matched placebo, twice daily. Percentage flow-mediated forearm arterial dilation (FMD%) was measured at weeks 0, 12, 24 and 48, together with other markers of vascular risk (blood pressure, lipids, glycaemic parameters, adiponectin and leptin).

Results

Out of 64 enrolled adults, 44 (69%) attended all visits (23 rosiglitazone, 21 placebo). Relative to placebo, at week 48, rosiglitazone decreased systolic blood pressure (8 mmHg, P=0.03), insulin (3 μIU/ml, P=0.02), insulin resistance ( P=0.03) and leptin (0.6 ng/ml, P=0.02), whilst adiponectin was increased (3.3 μg/ml, P<0.0001). However, rosiglitazone increased total cholesterol (49.1 mg/dl, P=0.001), low-density lipoprotein cholesterol (23.5 mg/dl, P=0.01) and triglycerides (146 mg/dl, P=0.06). Mean baseline FMD% for the entire cohort was moderately impaired (4.5%). Compared with baseline, mean on-treatment FMD% increased by 0.8% with rosiglitazone and decreased by 0.3% with placebo (mean difference 1.1%, 95% CI -0.2 to 2.5, P=0.09).

Conclusions

Rosiglitazone has minimal effect on flow-mediated dilation in HIV-infected lipoatrophic adults. However, despite worsening of the lipid profile, the overall effect of rosiglitazone on the cardiovascular risk profile in these subjects was positive.

Endothelial cell dysfunction and the vascular complications associated with type 2 diabetes: assessing the health of the endothelium

Diabetes-associated vascular complications are collectively the major clinical problems facing patients with diabetes and lead to the considerably higher mortality rate than that of the general population. People with diabetes have a much higher incidence of coronary artery disease as well as peripheral vascular diseases in part because of accelerated atherogenesis. Despite the introduction of new therapies, it has not been possible to effectively reduce the high cardiovascular morbidity and mortality associated with diabetes. Of additional concern is the recognition by the World Health Organization that we are facing a global epidemic of type 2 diabetes. Endothelial dysfunction is an early indicator of cardiovascular disease, including that seen in type 2 diabetes. A healthy endothelium, as defined in terms of the vasodilator/blood flow response to an endothelium-dependent vasodilator, is an important indicator of cardiovascular health and, therefore, a goal for corrective interventions. In this review we explore the cellular basis for endothelial dysfunction in an attempt to identify appropriate new targets and strategies for the treatment of diabetes. In addition, we consider the question of biomarkers for vascular disease and evaluate their usefulness for the early detection of and their role as contributors to vascular dysfunction.

PPARgamma and atherosclerosis

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR)gamma regulates a number of cellular processes that affect glucose homeostasis, endothelial function and vessel wall inflammation, as well as protecting against cardiovascular complications that occur in diabetes. Thiazolidinediones are PPARgamma agonists that are in clinical use for the treatment of type 2 diabetes. Accumulating evidence indicates that thiazolidinediones may exert cardioprotective effects at each stage of atherogenesis.

SCOPE

This paper reviews preclinical and clinical evidence (identified from a search of MEDLINE databases) supporting a beneficial cardiovascular effect of thiazolidinediones and discusses the implications of these data for the optimal use of thiazolidinediones in clinical practice.

FINDINGS

In vitro animal model and clinical studies indicate that thiazolidinediones correct endothelial dys function, suppress chronic inflammatory processes, reduce fatty streak formation, delay plaque evolution and vessel wall thickening and enhance plaque stabilization and regression.

CONCLUSION

Thus, thiazolidinediones show potential as potent anti-inflammatory, antithrombotic agents that could both improve glucose levels and the long-term cardio vascular risk related to atherosclerosis in patients with type 2 diabetes.

Mechanisms, Significance and Treatment of Vascular Dysfunction in Type 2 Diabetes Mellitus

Endothelial dysfunction and increased arterial stiffness occur early in the pathogenesis of diabetic vasculopathy. They are both powerful independent predictors of cardiovascular risk. Advances in non-invasive methodologies have led to widespread clinical investigation of these abnormalities in diabetes mellitus, generating a wealth of new knowledge concerning the mechanisms of vascular dysfunction, risk factor associations and potential treatment targets. Endothelial dysfunction primarily reflects decreased availability of nitric oxide (NO), a critical endothelium-derived vasoactive factor with vasodilatory and anti-atherosclerotic properties. Techniques for assessing endothelial dysfunction include ultrasonographic measurement of flow-mediated vasodilatation of the brachial artery and plethysmography measurement of forearm blood flow responses to vasoactive agents. Arterial stiffness may be assessed using pulse wave analysis to generate measures of pulse wave velocity, arterial compliance and wave reflection. The pathogenesis of endothelial dysfunction in type 2 diabetes is multifactorial, with principal contributors being oxidative stress, dyslipidaemia and hyperglycaemia. Elevated blood glucose levels drive production of reactive oxidant species (ROS) via multiple pathways, resulting in uncoupling of mitochondrial oxidative phosphorylation and endothelial NO synthase (eNOS) activity, reducing NO availability and generating further ROS. Hyperglycaemia also contributes to accelerated arterial stiffening by increasing formation of advanced glycation end-products (AGEs), which alter vessel wall structure and function. Diabetic dyslipidaemia is characterised by accumulation of triglyceride-rich lipoproteins, small dense low-density lipoprotein (LDL) particles, reduced high-density lipoprotein (HDL)-cholesterol and increased postprandial free fatty acid flux. These lipid abnormalities contribute to increasing oxidative stress and may directly inhibit eNOS activity. Although lipid-regulating agents such as HMG-CoA reductase inhibitors (statins), fibric acid derivatives (fibrates) and fish oils are used to treat diabetic dyslipidaemia, their impact on vascular function is less clear. Studies in type 2 diabetes have yielded inconsistent results, but this may reflect sampling variation and the potential over-riding influence of oxidative stress, dysglycaemia and insulin resistance on endothelial dysfunction. Results of positive intervention trials suggest that improvement in vascular function is mediated by both lipid and non-lipid mechanisms, including anti-inflammatory, anti-oxidative and direct effects on the arterial wall. Other treatments, such as renin-angiotensin-aldosterone system antagonists, insulin sensitisers and lifestyle-based interventions, have shown beneficial effects on vascular function in type 2 diabetes. Novel approaches, targeting eNOS and AGEs, are under development, as are new lipid-regulating therapies that more effectively lower LDL-cholesterol and raise HDL-cholesterol. Combination therapy may potentially increase therapeutic efficacy and permit use of lower doses, thereby reducing the risk of adverse drug effects and interactions. Concomitant treatments that specifically target oxidative stress may also improve endothelial dysfunction in diabetes. Vascular function studies can be used to explore the therapeutic potential and mechanisms of action of new and established interventions, and provide useful surrogate measures for cardiovascular endpoints in clinical trials.

Peroxisome proliferator activated receptor gamma and its activation in the treatment of insulin resistance and atherosclerosis: issues and opportunities

Atherosclerosis remains a major complication of type 2 diabetes mellitus. Increasing data suggest insulin resistance, and its associated metabolic abnormalities, may underlie many of the cardiovascular complications seen among patients with insulin resistance and/or diabetes mellitus. This insight has also suggested that therapeutic approaches targeting insulin resistance may not only improve metabolism but also limit complications like atherosclerosis and the inflammation that contributes to it. Thiazolidinediones, agonists of the nuclear receptor peroxisome proliferator activated receptor gamma, are one such insulin-sensitizing therapeutic intervention in current use among patients with type 2 diabetes mellitus. The existing data regarding thiazolidinedione effects on the cardiovascular system are reviewed and considered, along with the future prospects for this emerging drug class.

Treatment of insulin resistance in diabetes mellitus

Insulin resistance is a condition in which the glycemic response to insulin is less than normal. The change in insulin sensitivity leads to several sets of responses. One set effects the beta cell and leads to its accelerated destruction and the development of diabetes mellitus. The other set generates a series of nontraditional cardiovascular risk factors that result in accelerated atherosclerosis. Both of these sets of responses may have impacts on other tissues such as the nervous system. Insulin resistance is probably the result of increased visceral adiposity with increased release of free fatty acids and cytokines and a decreased release of adiponectin. Treatment of insulin resistance and its associated abnormalities can be achieved by lifestyle modification which results in weight loss, by drugs that reverse the abnormal adipocyte effects, by drugs that improve insulin sensitivity at the level of the liver and by anti-inflammatory agents that block activation of the nuclear factor kappa B cascade.

Insulin resistance and cardiovascular disease: the role of PPARgamma activators beyond their anti-diabetic action

Over the past few years it has been recognised that insulin resistance (IR) is an independent risk factor for major cardiovascular events. In addition, IR is associated with other factors such as hypertension, dyslipidaemia and endothelial dysfunction, and this cluster of metabolic disorders contributes to the cardiovascular risk of patients with IR. Given the increasing number of patients with IR, the modulation of their cardiovascular risk is a major task in diabetology and vascular medicine. This review will focus on the role of IR as a cardiovascular risk factor and on the potential of activators of the nuclear transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) to modulate these risk factors associated with IR.

Endothelial dysfunction, inflammation, and insulin resistance: a focus on subjects at risk for type 2 diabetes

Subjects with obesity, family history of type 2 diabetes, polycystic ovary syndrome, previous gestational diabetes, dyslipidemia, hypertension, impaired glucose tolerance or impaired fasting glucose, and those with metabolic syndrome are at risk for the development of type 2 diabetes. Some of them are also at risk for cardiovascular disease. Some underlying abnormalities such as insulin resistance, endothelial dysfunction, and low-grade chronic inflammation are frequently present and closely associated in all these groups. The flow of substrates, hormones, and cytokines from visceral fat to skeletal muscle and to the endothelial cells, along with some genetic abnormalities that lead to impaired insulin action in the peripheral tissues and to impaired insulin-stimulated nitric oxide production in endothelial cells, may play a role in establishing these shared metabolic and vascular derangements. Weight loss, thiazolidinediones, and metformin improve vascular function in subjects at risk for type 2 diabetes and may prove to reduce cardiovascular events in these individuals.

Insulin resistance and the endothelium

There is increasing evidence of a parallel progression between insulin resistance and endothelial dysfunction, suggesting a close association between insulin action and the endothelium. Numerous studies have demonstrated that endothelial dysfunction occurs early in the insulin-resistant state and is predictive of future cardiovascular events. Similarly, insulin resistance has been associated with the metabolic syndrome, which also increases the risk of adverse cardiovascular outcomes. Approaches that improve endothelial dysfunction, such as treatment with statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, or peroxisome proliferator-activated receptor gamma ligands, have been shown to prevent both diabetes and cardiovascular disease. This article reviews the relation between endothelial dysfunction and cardiovascular disease, assesses the endothelium in the spectrum of insulin resistance, and examines the effect of the thiazolidinediones on endothelial function.

Endothelial dysfunction in obesity and insulin resistance: a road to diabetes and heart disease

Obesity, insulin resistance, and endothelial dysfunction closely coexist throughout the natural history of type 2 diabetes. They all can be identified not only in people with type 2 diabetes, but also in various groups at risk for the disease, such as individuals with impaired glucose tolerance, family history of type 2 diabetes, hypertension, dyslipidemia, prior gestational diabetes, or polycystic ovary syndrome. Whereas their evident association cannot fully establish a cause-effect relationship, fascinating mechanisms that bring them closer together than ever before are rapidly emerging. Central or abdominal obesity leads to insulin resistance and endothelial dysfunction through fat-derived metabolic products, hormones, and cytokines. Insulin resistance leads to endothelial dysfunction through the frequent association with traditional cardiovascular risk factors and through some more direct novel mechanisms. Some specific and shared insulin signaling abnormalities in muscle, fat, and endothelial cells, as well as some new genetic and nontraditional factors, may contribute to this interesting association. Some recent clinical studies demonstrate that nonpharmacological and pharmacological strategies targeting obesity and/or insulin resistance ameliorate endothelial function and low-grade inflammation. All these findings have added a new dimension to the association of obesity, insulin resistance, and endothelial dysfunction that may become a key target in the prevention of type 2 diabetes and cardiovascular disease.

In type 2 diabetes, rosiglitazone therapy for insulin resistance ameliorates endothelial dysfunction independent of glucose control

OBJECTIVE

Insulin resistance is an independent risk factor for arteriosclerosis and cardiovascular mortality. However, the mechanism by which insulin resistance contributes to arteriosclerosis is unknown. Conceivably, endothelial dysfunction could be involved. Therefore, we asked whether therapy for insulin resistance ameliorates any endothelial dysfunction.

RESEARCH DESIGN AND METHODS

We performed a double-blind cross-over trial of 12 patients with recently diagnosed type 2 diabetes. They received rosiglitazone 4 mg b.i.d. for 12 weeks and nateglinide 60 mg b.i.d. for the same number of weeks in random order. To assess the degree of endothelial dysfunction, we used venous occlusion plethysmography. We studied vasodilation in response to acetylcholine (ACh) with and without exogenous insulin. The agents were infused into the brachial artery. Furthermore, we determined insulin resistance by euglycemic clamp.

RESULTS

Glycemic control was comparable under rosiglitazone and nateglinide. Rosiglitazone ameliorated insulin resistance by 60% compared with nateglinide. ACh response was significantly increased after rosiglitazone treatment (maximum forearm blood flow 12.8 +/- 1.3 vs. 8.8 +/- 1.3 ml/100 ml after rosiglitazone and nateglinide, respectively; P < 0.05) but did not attain the level of healthy control subjects (14.0 +/- 0.7 ml/100 ml). Coinfusion of exogenous insulin increased ACh response further in the rosiglitazone group. N-monomethyl-L-arginine-acetate (L-NMMA), an antagonist of nitric oxide synthase, largely prevented the increased vasodilation after rosiglitazone, regardless of the presence or absence of insulin. Insulin sensitivity and blood flow response were found to be correlated (P < 0.01).

CONCLUSIONS

Insulin resistance is a major contributor toward endothelial dysfunction in type 2 diabetes. Both endothelial dysfunction and insulin resistance are amenable to treatment by rosiglitazone.

Peroxisome proliferator-activated receptor gamma: implications for cardiovascular disease

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily. PPARgamma is expressed by macrophages, endothelial cells, and vascular smooth muscle cells. It regulates gene expression of key proteins involved in lipid metabolism, vascular inflammation, and proliferation contributing to atherogenesis and postangioplasty restenosis. The discovery of synthetic ligands for PPARgamma has led to significant enhancement of our understanding of the mechanism of their ligand-dependent activation and subsequent biological effects, particularly with respect to the role of PPARgamma in vascular pathophysiology. The thiazolidinedione PPARgamma agonists not only improve insulin resistance in patients with type II diabetes but also exert a broad spectrum of antiatherogenic effects in vitro and in animal models of atherosclerosis. In this review, we summarize the important role of PPARgamma as a molecular target for thiazolidinediones and its implications for the control of vascular inflammation and proliferation for the cardiovascular system.

The Role of Exercise in the Treatment of Cardiovascular Disease Associated with Type 2 Diabetes Mellitus

The role of exercise training in the prevention and treatment of type 2 diabetes mellitus has been studied extensively over the past two decades. Although the primary treatment aim for patients with type 2 diabetes is metabolic control, the morbidity and mortality associated with the disease is more a function of cardiovascular disease. As exercise is associated with favourable reductions in the risk for cardiovascular disease in other high-risk populations, here we explore the role of exercise in the treatment of cardiovascular maladaptations associated with type 2 diabetes. The cardiovascular adaptation to type 2 diabetes is characterised by hypertrophy, stiffening and loss of functional reserve. Clinically, the cardiovascular adaptations to the diabetic state are associated with an increased risk for cardiovascular disease. Functionally, these adaptations have been shown to contribute to a reduced exercise capacity, which may explain the reduced cardiovascular fitness observed in this population. Exercise training is associated with improved exercise capacity in various populations, including type 2 diabetes. Several structural and functional adaptations within the cardiovascular system following exercise training could explain these findings, such as reductions in ventricular and vascular structural hypertrophy and compliance coupled with increased functional reserve. Although these cardiovascular adaptations to aerobic exercise training have been well documented in older populations with similar decrements in cardiovascular fitness and function, they have yet to be examined in patients with type 2 diabetes. For this reason, we contend that exercise training may be an excellent therapeutic adjunct in the treatment of diabetic cardiovascular disease.

Insulin impairs endothelium-dependent vasodilation independent of insulin sensitivity or lipid profile

Insulin resistance is a risk factor for atherosclerosis and is associated with hyperinsulinemia, abnormal lipid profile, and hypertension. Whether hyperinsulinemia affects vascular function independent of insulin resistance or other metabolic risk factors is unknown. This investigation aimed to assess the effects of hyperinsulinemia on endothelial function in subjects with a spectrum of insulin sensitivity and lipid profile. Endothelium-dependent (flow-mediated dilation, FMD) and -independent (nitroglycerin) responses of the brachial artery were studied by high-resolution ultrasound before and during hyperinsulinemia (euglycemic clamp) in 25 normoglycemic, normotensive subjects. Participants were divided into an insulin-sensitive and an insulin-resistant subgroup based on their sensitivity index values, with a cutoff of 8, and into a normal-cholesterol and a high-cholesterol subgroup based on their total cholesterol levels, with a cutoff of 5.2 mmol/l (200 mg/dl). In the whole population, FMD was lower during hyperinsulinemia compared with baseline (2.3 +/- 0.6% vs. 6 +/- 0.6%; P < 0.001). Resting FMD was lower in the insulin-resistant subgroup compared with the insulin-sensitive subgroup (4.2 +/- 0.9% vs. 7.4 +/- 0.8%; P = 0.014) and in the high-cholesterol subjects compared with the normal-cholesterol subjects (4.4 +/- 0.7% vs. 8 +/- 0.7%; P = 0.002). Hyperinsulinemia decreased FMD in both the insulin-sensitive (from 7.4 +/- 0.8% to 3.6 +/- 0.4%; P < 0.001) and insulin-resistant (from 4.2% to 1.22%; P = 0.012) subgroups and in both the normal-cholesterol (from 8 +/- 0.7% to 3.9 +/- 0.4%; P < 0.001) and high-cholesterol (from 4.4 +/- 0.7% to 1.1 +/- 0.8%; P = 0.01) participants. Acute hyperinsulinemia impairs conduit vessel endothelial function independent of insulin sensitivity and lipid profile. Insulin may trigger endothelial dysfunction and promote atherosclerosis.

Peroxisome proliferator-activated receptor-gamma agonists in atherosclerosis: current evidence and future directions

PURPOSE OF REVIEW

The prevalence of type 2 diabetes globally is reaching epidemic proportions. Type 2 diabetes is strongly associated with increased risk of cardiovascular disease. Atherosclerosis is thought to arise as a result of a chronic inflammatory process within the arterial wall. Insulin resistance is central to the pathogenesis of type 2 diabetes and may contribute to atherogenesis, either directly or through associated risk factors. The peroxisome proliferator-activated receptor-gamma agonists, the thiazolidinediones, pioglitazone and rosiglitazone, are insulin sensitizing agents, that are licensed for the management of hyperglycaemia. Growing evidence supports an array of additional effects of thiazolidinedione therapy, both immunomodulatory and antiinflammatory, which may attenuate atherogenesis in type 2 diabetes.

RECENT FINDINGS

Studies have shown that thiazolidinedione therapy may lead to risk factor modulation in type 2 diabetes. Thiazolidinediones treatment has been shown to reduce blood pressure, modify the atherogenic lipid profile associated with type 2 diabetes, reduce microalbuminuria and ameliorate the prothrombotic diathesis. Further evidence suggests that thiazolidinediones therapy inhibits the inflammatory processes which may be involved in atherosclerotic plaque initiation, propagation and destabilization.

SUMMARY

Modification of insulin resistance by thiazolidinedione therapy in type 2 diabetes and the range of pleiotropic effects may not only impact on incident type 2 diabetes, but also on associated cardiovascular disease. Numerous large clinical endpoint studies are under way to investigate these issues.

Antioxidative, antinitrative, and vasculoprotective effects of a peroxisome proliferator-activated receptor-gamma agonist in hypercholesterolemia

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR) signaling pathways have been reported to exert anti-inflammatory effects and attenuate atherosclerosis formation. However, the mechanisms responsible for their anti-inflammatory and antiatherosclerotic effects remain largely unknown. The present study tested the hypothesis that a PPARgamma agonist may exert significant endothelial protection by antioxidative and antinitrative effects.

METHODS AND RESULTS

Male New Zealand White rabbits were randomized to receive a normal (control) or a high-cholesterol diet and treated with vehicle or rosiglitazone (a PPARgamma agonist) 3 mg x kg(-1) x d(-1) for 5 weeks beginning 3 weeks after the high-cholesterol diet. At the end of 8 weeks of a high-cholesterol diet, the rabbits were killed, and the carotid arteries were isolated. Bioactive nitric oxide was determined functionally (endothelium-dependent vasodilatation) and biochemically (the phosphorylation of vasodilator-stimulated phosphoprotein, or P-VASP). Vascular superoxide production, PPARgamma, gp91phox, and inducible nitric oxide synthase (iNOS) expression, and vascular ONOO- formation were determined. Hypercholesterolemia caused severe endothelial dysfunction and reduced P-VASP, despite a marked increase in iNOS expression and total NOx production. Treatment with rosiglitazone enhanced PPARgamma expression, improved endothelium-dependent vasodilatation, preserved P-VASP, suppressed gp91phox and iNOS expression, reduced superoxide and total NOx production, and inhibited nitrotyrosine formation.

CONCLUSIONS

The PPARgamma agonist rosiglitazone exerted a significant vascular protective effect in hypercholesterolemic rabbits, most likely by attenuation of oxidative and nitrative stresses. The endothelial protective effects of PPARgamma agonists may reduce leukocyte accumulation in vascular walls and contribute to their antiatherosclerotic effect.

PPARgamma activation, by reducing oxidative stress, increases NO bioavailability in coronary arterioles of mice with Type 2 diabetes

We tested the hypothesis that short-term treatment of mice with Type 2 diabetes mellitus (DM) with rosiglitazone (ROSI), an agonist of peroxisome proliferator-activated receptor-gamma, ameliorates the impaired coronary arteriolar dilation by reducing oxidative stress via a mechanism unrelated to its effect on hyperglycemia and hyperinsulinemia. Control and Type 2 DM (db/db) mice were treated with ROSI (3 mg x kg(-1) x day(-1)) for 7 days, which did not significantly affect their serum concentration of glucose and insulin. Compared with controls, in db/db mice serum levels of 8-isoprostane and dihydroethydine-detectable superoxide production in carotid arteries were significantly elevated and were reduced by ROSI treatment. In coronary arterioles (diameter, approximately 80 microm) isolated from db/db mice, the reduced dilations to ACh, the nitric oxide (NO) donor NONOate, and increases in flow were significantly augmented either by in vitro administration of apocynin, an inhibitor of NAD(P)H-oxidase, or by in vivo ROSI treatment, responses that were then significantly reduced by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester. In aortas of db/db mice, activity of SOD and catalase was reduced, whereas NAD(P)H oxidase activity was enhanced. ROSI treatment enhanced catalase and reduced NAD(P)H oxidase activity but did not affect the activity of SOD. These findings suggest that ROSI treatment enhances NO mediation of coronary arteriolar dilations due to the reduction of vascular NAD(P)H oxidase-derived superoxide production and enhancement of catalase activity. Thus, in addition to the previously revealed beneficial metabolic effects, the antioxidant action of rosiglitazone may protect coronary arteriolar function in Type 2 DM.

Thiazolidinediones in type 2 diabetes mellitus: current clinical evidence

Type 2 diabetes mellitus is characterised by insulin resistance as well as progressive pancreatic beta cell dysfunction. The cornerstone of current oral blood-glucose lowering therapy consists of metformin, which primarily lowers hepatic glucose production, and the sulphonylureas that act by stimulating pancreatic beta-cells to secrete insulin. Recently, a novel class of agents, the thiazolidinediones, has been introduced that favourably influence insulin sensitivity and possibly also pancreatic beta-cell function. The thiazolidinediones are synthetic ligands that bind to the nuclear peroxisome proliferator-activated receptor-gamma and exert their action by activating transcription of genes that, among others, regulate adipocyte differentiation and adipogenesis as well as glucose and lipid metabolism. To date, the precise mechanisms underlying the actions of thiazolidinediones are largely unknown. When given as monotherapy or in combination with sulphonylureas, metformin or insulin in patients with type 2 diabetes, the currently available thiazolidinediones (rosiglitazone and pioglitazone) ameliorate glycaemic control, by lowering fasting and postprandial blood glucose levels, and improve insulin sensitivity in placebo-controlled trials. They seem to have differential effects on dyslipidaemia in patients with type 2 diabetes; rosiglitazone increases total cholesterol as well as high-density lipoprotein (HDL) and low-density lipoprotein cholesterol levels and affects plasma triglyceride levels depending on the baseline values, whereas pioglitazone lowers triglycerides and increases HDL cholesterol levels. The adverse events of both agents that occur with greater frequency than in patients treated with placebo are fluid retention and oedema. As demonstrated, mainly in preclinical studies to date, rosiglitazone and pioglitazone possess beneficial effects on other cardiovascular risk factors associated with the insulin resistance syndrome. Thus, these agents were shown to decrease blood pressure, enhance myocardial function and fibrinolysis, as well as possess anti-inflammatory and other beneficial vascular effects. Long-term efficacy and surveillance of this promising class of drugs in patients, however, still need to be demonstrated in outcome trials.

The dysmetabolic syndrome, insulin resistance and increased cardiovascular (CV) morbidity and mortality in type 2 diabetes: aetiological factors in the development of CV complications

Insulin resistance often clusters with other cardiovascular risk factors, such as obesity, impaired glucose tolerance (IGT), hypertension, dyslipidaemia and impaired fibrinolysis. Collectively, these endocrine and metabolic disturbances are described as the dysmetabolic syndrome, which is also commonly called the "insulin resistance syndrome", the "metabolic syndrome", or "syndrome X". Insulin resistance, working in concert with the other components of the dysmetabolic syndrome, induces deleterious changes to the vascular endothelium and lipid profiles that directly and indirectly promote the progression of atherosclerosis. Insulin resistance in adipocytes, leading to decreased suppression of lipolysis by insulin, may be especially important in this regard. Reduced suppression of lipolysis by insulin in obese subjects is associated with increased levels of fatty acids that damage the arterial wall and promote atherosclerosis. The lipid profiles of insulin-resistant subjects are often characterised by the appearance of hypertriglyceridaemia and small, dense LDL-cholesterol, together with low HDL-cholesterol. In addition, adipocytes are highly active endocrine organs and secrete a range of substances that reduce insulin sensitivity further. The net result of these derangements is a vicious circle, wherein the development of insulin resistance is strongly associated with atherogenic lipid profiles and endothelial dysfunction which, in turn, exacerbates insulin resistance. The consequences for the individual with dysmetabolic syndrome include an increased risk of cardiovascular disease of up to 4-fold compared with subjects without the dysmetabolic syndrome.

Insulin resistance and endothelial dysfunction

Insulin has multiple metabolic actions, including effects on blood vessels. Insulin normally increases blood flow by a mechanism which involves generation of nitric oxide (NO) via the arginine-NO pathway. Although insulin itself is a weak and physiologically unimportant vasodilatator, it appears to markedly potentiate endothelium-dependent vasodilatation. Therefore, anything that impairs insulin action in endothelial cells can be expected to be associated with endothelial dysfunction, i.e. loss of NO bioactivity in the vessel wall. Consistent with the idea that insulin resistance and endothelial dysfunction frequently coexist, all insulin-resistant conditions examined to date have been associated with endothelial dysfunction. However, the latter can also be caused by factors other than insulin resistance-such as a high concentration of low-density lipoprotein (LDL) cholesterol. Therapies which reverse insulin resistance-such as exercise, insulin and inhibitors of the renin-angiotensin-aldosterone (RAA) axis-also reverse endothelial dysfunction, which may thus be an inherent feature of insulin resistance.