Review of the pleiotropic effects of peroxisome proliferator-activated receptor gamma agonists on platelet function

Profiling Heparin-Induced Thrombocytopenia (HIT) Antibodies in Hospitalized Patients With and Without Diabetes

Heparin (H) anticoagulation in populations characterized by elevated platelet factor 4 (PF4) frequently elicits PF4/H antibodies, presenting a risk of heparin-induced thrombocytopenia. Recent studies have shown that anti-PF4/H enzyme-linked immunosorbent assays (ELISAs) detect antibodies in individuals never exposed to heparin. Platelet factor 4/H cross-reactive antibodies may result from PF4-mediated defense responses to injury or infection. This study questioned whether patients with diabetes are more likely to develop the endogenous cross-reactive antibodies. A comparison of healthy volunteers versus hospitalized patients with or without diabetes showed no significant differences in the prevalence of PF4/H ELISA-positive results. However, the group of patients who had both diabetes and an infectious condition had higher median antibody titer compared to other patients with or without diabetes regardless of reason for hospitalization. Higher PF4/H titers were also associated with patients with diabetes who were not on any medical therapy. In the future, determining whether PF4/H cross-reactive antibodies sensitize patients to respond adversely to heparin anticoagulation or predispose patients to other complications may be relevant to diabetes care.

Effects of oral hypoglycemic agents on platelet function

Platelet dysfunction plays a role in diabetic macrovascular complications. Several studies have assessed the effect of oral hypoglycemic agents (OHAs) on platelet function. Data from both in vivo and in vitro studies show a favorable effect for most of the traditional glucose-lowering therapies, while evidence is limited for the newer ones. Metformin, sulfonylureas, glitazones and acarbose exert a favorable effect on platelet function. Among incretin therapies, only sitagliptin has so far been demonstrated to have a beneficial effect on platelet aggregation. More in vivo and in vitro evidence is required to increase our knowledge on any potential beneficial effects of OHAs on platelet function. Any such effect may have implications for the reduction of cardiovascular risk in type 2 diabetes mellitus.

The fixed combination of pioglitazone and metformin improves biomarkers of platelet function and chronic inflammation in type 2 diabetes patients: results from the PIOfix study

BACKGROUND

Type 2 diabetes mellitus (T2DM) is characterized by a proinflammatory and procoagulant condition. This study investigates the impact of a pioglitazone plus metformin therapy on biomarkers of inflammation and platelet activation in comparison to a treatment with glimepiride plus metformin.

METHODS

The study was designed as a multicenter, randomized, double-blinded two-arm trial. Patients with T2DM and dyslipidemia under metformin monotherapy with hemoglobin A1c value between 6.5% and 9.0% were eligible for trial participation. Blood was drawn at baseline and after 24 weeks of treatment from patients of five centers. Markers of inflammation and thrombocyte function (soluble CD40 ligand, thromboxane, vWillebrand factor, adhesion molecules, clotting reaction) were evaluated subsequently in a central laboratory.

RESULTS

A total of 46 patients were included in the final analyses. Mean (± standard deviation) age was 58.5 ± 9.0 years (13 women, 33 men; disease duration 6.3 ± 5.0 years; body mass index 32.0 ± 4.8 kg/m(2)). A total of 25 patients were treated with pioglitazone plus metformin, and 21 patients were in the glimepiride arm. There was a significant decline of E-selectin (-3.7 ± 4.8 ng/ml, p < .001 versus baseline), vWillebrand factor (-19.5 ± 32.0%, p < .05), and high-sensitivity C-reactive protein concentrations (-1.08 ± 0.91 mg/liter, p < .05) in the metformin + pioglitazone arm only (metformin + glimepiride, -0.5 ± 3.4 ng/ml, +1.4 ± 33.2%, + 0.08 ± 0.72 mg/liter, respectively, all not significant). Also, all other surrogate markers for platelet function and inflammation showed slight improvements in the metformin + pioglitazone arm but not in the metformin + glimepiride arm.

CONCLUSIONS

The fixed metformin + pioglitazone combination treatment showed an overall improvement of laboratory surrogate markers, indicating improvement of platelet function and of chronic systemic inflammation, which was not seen with metformin + glimepiride.

Is peroxisome proliferator-activated receptor gamma (PPARγ) a therapeutic target for the treatment of pulmonary hypertension?

Pulmonary hypertension (PH), a progressive disorder associated with significant morbidity and mortality, is caused by complex pathways that culminate in structural and functional alterations of the pulmonary circulation and increases in pulmonary vascular resistance and pressure. Diverse genetic, pathological, or environmental triggers stimulate PH pathogenesis culminating in vasoconstriction, cell proliferation, vascular remodeling, and thrombosis. We conducted a thorough literature review by performing MEDLINE searches via PubMed to identify articles pertaining to PPARγ as a therapeutic target for the treatment of PH. This review examines basic and preclinical studies that explore PPARγ and its ability to regulate PH pathogenesis. Despite the current therapies that target specific pathways in PH pathogenesis, including prostacyclin derivatives, endothelin-receptor antagonists, and phosphodiesterase type 5 inhibitors, morbidity and mortality related to PH remain unacceptably high, indicating the need for novel therapeutic approaches. Consequently, therapeutic targets that simultaneously regulate multiple pathways involved in PH pathogenesis have gained attention. This review focuses on peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. While the PPARγ receptor is best known as a master regulator of lipid and glucose metabolism, a growing body of literature demonstrates that activation of PPARγ exerts antiproliferative, antithrombotic, and vasodilatory effects on the vasculature, suggesting its potential efficacy as a PH therapeutic target.

The platelet as a model system for the acute actions of nuclear receptors

Platelets are circulating cell fragments which play a critical role in thrombosis, and whose activity is associated with the progress of cardiovascular diseases, diabetes, inflammation, and cancer cell metastasis. Recently, a number of nuclear receptors have been found present in human platelets, including the receptors for sex steroids, and glucocorticoids, along with peroxisome proliferator-activated receptors (PPAR)s and retinoid X receptors (RXR)s. Although the platelet contains no nucleus, selective ligands for these receptors activate their respective platelet nuclear receptors and regulate platelet aggregation and activation. The human platelet, because of its abundance and accessibility therefore represents an excellent model system to study the rapid non-genomic mechanism of nuclear receptors. Moreover, since targeting platelets is a major clinical therapeutic area, analysis of platelet nuclear receptors may provide clues for new drug targets as well as provide important information regarding the physiological roles of nuclear receptors in the circulation.

Pioglitazone and mechanisms of CV protection

BACKGROUND AND AIM

Pioglitazone has diverse multiple effects on metabolic and inflammatory processes that have the potential to influence cardiovascular disease pathophysiology at various points in the disease process, including atherogenesis, plaque inflammation, plaque rupture, haemostatic disturbances and microangiopathy.

RESULTS

Linking the many direct and indirect effects on the vasculature to the reduction in key macrovascular outcomes reported with pioglitazone in patients with type 2 diabetes presents a considerable challenge. However, recent large-scale clinical cardiovascular imaging studies are beginning to provide some mechanistic insights, including a potentially important role for improvements in high-density lipoprotein cholesterol with pioglitazone. In addition to a role in prevention, animal studies also suggest that pioglitazone may minimize damage and improve recovery during and after ischaemic cardio- and cerebrovascular events.

DESIGN AND METHODS

In this review, we consider potential cardiovascular protective mechanisms of pioglitazone by linking preclinical data and clinical cardiovascular outcomes guided by insights from recent imaging studies.

CONCLUSION

Pioglitazone may influence CVD pathophysiology at multiple points in the disease process, including atherogenesis, plaque inflammation, plaque rupture and haemostatic disturbances (i.e. thrombus/embolism formation), as well as microangiopathy.

Profiling of heparin-induced thrombocytopenia antibody levels in patients with and without diabetes

Heparin/platelet factor 4 (H:PF4) antibodies are the causative agent in heparin-induced thrombocytopenia (HIT). The antibodies are frequently formed after exposure to heparin, most commonly without any signs of clinical HIT. Heparin-induced thrombocytopenia antibodies have been detected by enzyme-linked immunosorbent assay (ELISA) in individuals who have not been exposed to heparin. It is possible that the antibodies could be elicited by PF4 associated with endogenous, heparin-like glycosaminoglycans (GAGs). This risk would be higher in individuals with endothelial dysfunction and chronic platelet activation. In the setting of an outpatient endocrinology clinic, both diabetic and nondiabetic patients were studied and compared with healthy volunteers. Heparin/platelet factor 4 antibody titers were measured by ELISA and analyzed to determine the frequency of clinically seropositive responses, and median and interquartile ranges of baseline antibody titers. The study found no increase in frequency of ELISA-positive patients among diabetic patients. Moreover, the diabetic population had lower overall level of H:PF4 antibody titer, especially the subgroups treated with thiazolidinedione drugs or angiotensin receptor blockers. Further studies are needed to determine whether subthreshold titers of HIT antibody may be reflective of the physiological state of platelet/endothelial balance.

A multicentre, randomized, double-blind placebo-controlled trial evaluating rosiglitazone for the prevention of atherosclerosis progression after coronary artery bypass graft surgery in patients with type 2 diabetes. Design and rationale of the VeIn-Coronary aTherOsclerosis and Rosiglitazone after bypass surgerY (VICTORY) trial

BACKGROUND

The number of patients with coronary artery disease and type 2 diabetes will increase dramatically over the next decade. Diabetes has been related to accelerated atherosclerosis and many patients with diabetes will require coronary artery bypass graft (CABG) surgery utilizing saphenous vein grafts. After CABG, accelerated atherosclerosis in saphenous vein grafts leads to graft failure in approximately 50% of cases over a 10-year period. Rosiglitazone, a peroxisome proliferator-activated receptor-gamma agonist, has been shown to improve multiple metabolic parameters in patients with type 2 diabetes. However, its role in the prevention of atherosclerosis progression is uncertain.

STUDY DESIGN

VeIn-Coronary aTherOsclerosis and Rosiglitazone after bypass surgerY (VICTORY) is a cardiometabolic trial in which patients with type 2 diabetes, one to 10 years after CABG, will be randomly assigned to receive rosiglitazone (up to 8 mg/day) or a placebo after qualifying angiography and intravascular ultrasound of a segment of one vein graft with or without a native anastomosed coronary artery. A comprehensive set of athero-thrombo-inflammatory markers will be serially assessed during the 12-month follow-up period. Body fat distribution and body composition will be assessed by computed tomography and dual energy x-ray absorptiometry, respectively, at baseline, six months and 12 months follow-up. For atherosclerosis progression evaluation, repeat angiography and intravascular ultrasound will be performed after 12 months follow-up. The primary end point of the study will be the change in atherosclerotic plaque volume in a 40 mm or longer segment of one vein graft.

CONCLUSIONS

The VICTORY trial is the first cardiometabolic study to evaluate the antiatherosclerotic and metabolic effects of rosiglitazone in post-CABG patients with type 2 diabetes.

Platelet dysfunction in central obesity

Central obesity is a relevant risk factor for major cardiovascular events due to the atherosclerotic involvement of coronary, cerebral and lower limb arterial vessels. A major role in the increased cardiovascular risk is played by platelets, which show an increased activation and a reduced sensitivity to the physiological and pharmacological antiaggregating agents. This review focuses on platelet dysfunction in central obesity. The mechanisms involved are related to: i) the reduced sensitivity to insulin and other substances acting via intracellular cyclic nucleotides, such as nitrates and prostacyclin; ii) the altered intracellular ionic milieu with elevated cytosolic Ca(2+); and iii) the increased oxidative stress, which elicits isoprostane production from arachidonic acid. Therapeutic guidelines recommend a multifactorial prevention of cardiovascular disease including antiplatelet drugs in high risk patients, even though, at present, the protective effect of antiplatelet therapy in obese, insulin resistant subjects has not been evaluated by specific trials. Some reports, however, suggest a decreased sensitivity to the antiaggregating effects of both acetylsalicylic acid (aspirin) and thienopyridines in human obesity. Platelet defects may play a pivotal role in the reduced efficacy of antiplatelet therapy in obese subjects in the setting of cardiovascular prevention and acute coronary syndrome treatment. Thus, a specifically tailored antiaggregating therapy is likely necessary in obese, insulin resistant subjects, especially in the presence of type 2 diabetes mellitus.