Platelets as a Novel Target for PPAR?? Ligands

WY-14643, a novel antiplatelet and antithrombotic agent targeting the GPIbα receptor

BACKGROUND AND PURPOSE

Hypolipidemia and platelet activation play key roles in atherosclerotic diseases. Pirinixic acid (WY-14643) was originally developed as a lipid-lowering drug. Here we focused on its antiplatelet and antithrombotic abilities and the underlying mechanism.

EXPERIMENTAL APPROACH

The effects of WY-14643 on platelet aggregation was measured using a lumi-aggregometer. Clot retraction and spreading on fibrinogen were also assayed. PPARα-/- platelets were used to identify the target of WY-14643. The interaction between WY-14643 and glycoprotein Ibα (GPIbα) was detected using cellular thermal shift assay (CETSA), surface plasmon resonance (SPR) spectroscopy and molecular docking. GPIbα downstream signaling was examined by Western blot. The antithrombotic effect was investigated using mouse mesenteric arteriole thrombosis model. Mouse tail bleeding model was used to study its effect on bleeding side effects.

KEY RESULTS

WY-14643 concentration-dependently inhibits human washed platelet aggregation, clot retraction, and spreading. Significantly, WY-14643 inhibits thrombin-induced activation of human washed platelets with an IC50 of 7.026 μM. The antiplatelet effect of WY-14643 is mainly dependent of GPIbα. CESTA, SPR and molecular docking results indicate that WY-14643 directly interacts with GPIbα and acts as a GPIbα antagonist. WY-14643 also inhibits phosphorylation of PLCγ2, Akt, p38, and Erk1/2 induced by thrombin. Noteworthily, 20 mg/kg oral administration of WY-14643 inhibits FeCl3-induced thrombosis of mesenteric arteries in mice similarly to clopidogrel without increasing bleeding.

CONCLUSION AND IMPLICATIONS

WY-14643 is not only a PPARα agonist with lipid-lowering effect, but also an antiplatelet agent as a GPIbα antagonist. It may have more significant therapeutic advantages than current antiplatelet agents for the treatment of atherosclerotic thrombosis, which have lipid-lowering effects without bleeding side effects.

β-catenin and PPAR-γ levels in bone marrow of myeloproliferative neoplasm: an immunohistochemical and ultrastructural study

In accordance with increased proliferation in myeloproliferative neoplasm (MPN), the goal is to evaluate the immunoexpression of: β-catenin, PPAR-γ and Ki67 protein, to compare them with bone marrow ultrastructural characteristics in patients with MPN. Immunoexpression and electron microscopy of bone marrow was analyzed in 30 Ph-negative MPN patients, including per 10 patients with polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). The quantity of β-catenin immunoreactive cells was significantly higher in PV then in ET (p < 0.01) or PMF group of patients (p < 0.01) and also in ET versus PMF group of patients (p < 0.01). Erythroid lineage showed absent β-catenin staining without immunoreactivity in nucleus. In contrast, immunoreactivity for PPAR-γ was localized mostly in megakaryocytes and the highest number of PPAR-γ immunopositive cells was detected in PMF group of patients. In addition, the proliferative Ki67 index was significantly increased in the PMF and PV patients compared to patients with ET. Also, the megakaryocytes showed abnormal maturation in PMF group of patients as determined by ultrastructural analysis. These results indicated that PV dominantly expressed β-catenin and proliferation marker Ki67 in bone marrow, while PMF is linked preferentially to PPAR-γ immunopositive megakaryocytes characterized by abnormal maturation.

Melatonin suppresses platelet activation and function against cardiac ischemia/reperfusion injury via PPARγ/FUNDC1/mitophagy pathways

Platelet activation is a major (patho-) physiological mechanism that underlies ischemia/reperfusion (I/R) injury. In this study, we explored the molecular signals for platelet hyperactivity and investigated the beneficial effects of melatonin on platelet reactivity in response to I/R injury. After reperfusion, peroxisome proliferator-activated receptor γ (PPARγ) was progressively downregulated in patients with acute myocardial infarction undergoing coronary artery bypass grafting (CABG) surgery and in mice with I/R injury model. Loss of PPARγ was closely associated with FUN14 domain containing 1 (FUNDC1) dephosphorylation and mitophagy activation, leading to increased mitochondrial electron transport chain complex (ETC.) activity, enhanced mitochondrial respiratory function, and elevated ATP production. The improved mitochondrial function strongly contributed to platelet aggregation, spreading, expression of P-selectin, and final formation of micro-thromboses, eventually resulting in myocardial dysfunction and microvascular structural destruction. However, melatonin powerfully suppressed platelet activation via restoration of the PPARγ content in platelets, which subsequently blocked FUNDC1-required mitophagy, mitochondrial energy production, platelet hyperactivity, and cardiac I/R injury. In contrast, genetic ablation of PPARγ in platelet abolished the beneficial effects of melatonin on mitophagy, mitochondrial ATP supply, and platelet activation. Our results lay the foundation for the molecular mechanism of platelet activation in response to I/R injury and highlight that the manipulation of the PPARγ/FUNDC1/mitophagy pathway by melatonin could be a novel strategy for cardioprotection in the setting of cardiac I/R injury.

Farnesoid X Receptor and Liver X Receptor Ligands Initiate Formation of Coated Platelets

OBJECTIVES

The liver X receptors (LXRs) and farnesoid X receptor (FXR) have been identified in human platelets. Ligands of these receptors have been shown to have nongenomic inhibitory effects on platelet activation by platelet agonists. This, however, seems contradictory with the platelet hyper-reactivity that is associated with several pathological conditions that are associated with increased circulating levels of molecules that are LXR and FXR ligands, such as hyperlipidemia, type 2 diabetes mellitus, and obesity.

APPROACH AND RESULTS

We, therefore, investigated whether ligands for the LXR and FXR receptors were capable of priming platelets to the activated state without stimulation by platelet agonists. Treatment of platelets with ligands for LXR and FXR converted platelets to the procoagulant state, with increases in phosphatidylserine exposure, platelet swelling, reduced membrane integrity, depolarization of the mitochondrial membrane, and microparticle release observed. Additionally, platelets also displayed features associated with coated platelets such as P-selectin exposure, fibrinogen binding, fibrin generation that is supported by increased serine protease activity, and inhibition of integrin αIIbβ3. LXR and FXR ligand-induced formation of coated platelets was found to be dependent on both reactive oxygen species and intracellular calcium mobilization, and for FXR ligands, this process was found to be dependent on cyclophilin D.

CONCLUSIONS

We conclude that treatment with LXR and FXR ligands initiates coated platelet formation, which is thought to support coagulation but results in desensitization to platelet stimuli through inhibition of αIIbβ3 consistent with their ability to inhibit platelet function and stable thrombus formation in vivo.

RXR Ligands Negatively Regulate Thrombosis and Hemostasis

Supplemental Digital Content is available in the text.

Objective—

Platelets have been found to express intracellular nuclear receptors including the retinoid X receptors (RXRα and RXRβ). Treatment of platelets with ligands of RXR has been shown to inhibit platelet responses to ADP and thromboxane A2; however, the effects on responses to other platelet agonists and the underlying mechanism have not been fully characterized.

Approach and Results—

The effect of 9-cis-retinoic acid, docosahexaenoic acid and methoprene acid on collagen receptor (glycoprotein VI [GPVI]) agonists and thrombin-stimulated platelet function; including aggregation, granule secretion, integrin activation, calcium mobilization, integrin αIIbβ3 outside-in signaling and thrombus formation in vitro and in vivo were determined. Treatment of platelets with RXR ligands resulted in attenuation of platelet functional responses after stimulation by GPVI agonists or thrombin and inhibition of integrin αIIbβ3 outside-in signaling. Treatment with 9-cis-retinoic acid caused inhibition of thrombus formation in vitro and an impairment of thrombosis and hemostasis in vivo. Both RXR ligands stimulated protein kinase A activation, measured by VASP S157 phosphorylation, that was found to be dependent on both cAMP and nuclear factor κ-light-chain-enhancer of activated B cell activity.

Conclusions—

This study identifies a widespread, negative regulatory role for RXR in the regulation of platelet functional responses and thrombus formation and describes novel events that lead to the upregulation of protein kinase A, a known negative regulator of many aspects of platelet function. This mechanism may offer a possible explanation for the cardioprotective effects described in vivo after treatment with RXR ligands.

PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signaling and platelet function through up-regulation of protein kinase A activity

Essentials peroxisome proliferator-activated receptor γ (PPARγ) agonists inhibit platelet function. PPARγ agonists negatively regulate outside-in signaling via integrin αIIbβ3. PPARγ agonists disrupt the interaction of Gα13 with integrin β3. This is attributed to an upregulation of protein kinase A activity.

SUMMARY

Background Agonists for the peroxisome proliferator-activated receptor (PPARγ) have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. Objectives Profound effects on thrombus formation led us to suspect a role for PPARγ agonists in the regulation of integrin αIIbβ3 mediated signaling. Both GPVI and GPCR signaling pathways lead to αIIbβ3 activation, and signaling through αIIbβ3 plays a critical role in platelet function and normal hemostasis. Methods The effects of PPARγ agonists on the regulation of αIIbβ3 outside-in signaling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signaling components downstream of αIIbβ3 activation were also determined following adhesion to fibrinogen by Western blotting. Results Treatment of platelets with PPARγ agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbβ3 signaling, including the integrin β3 subunit, Syk, PLCγ2, focal adhesion kinase (FAK) and Akt, was also observed as a result of reduced interaction of the integrin β3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was because of an increase in PKA activity following treatment with PPARγ receptor agonists. Conclusions This study provides further evidence for antiplatelet actions of PPARγ agonists, identifies a negative regulatory role for PPARγ agonists in the control of integrin αIIbβ3 outside-in signaling, and provides a molecular basis by which the PPARγ agonists negatively regulate platelet activation and thrombus formation.

NF-κB signaling pathway as target for antiplatelet activity

In different nucleated cells, NF-κB has long been considered a prototypical proinflammatory signaling pathway with the expression of proinflammatory genes. Although platelets lack a nucleus, a number of functional transcription factors are involved in activated platelets, such as NF-κB. In platelet activation NF-κB regulation events include IKKβ phosphorylation, IκBα degradation, and p65 phosphorylation. Multiple pathways contribute to platelet activation and NF-κB is a common pathway in this activation. Therefore, in platelet activation the modulation of NF-κB pathway could be a potential new target in the treatment of inflammation-related vascular disease therapy (antiplatelet and antithrombotic activities).

Additive Renoprotection by Pioglitazone and Fenofibrate against Inflammatory, Oxidative and Apoptotic Manifestations of Cisplatin Nephrotoxicity: Modulation by PPARs

Nephrotoxicity is a major side effect for the antineoplastic drug cisplatin. Here, we employed pharmacological, biochemical, and molecular studies to investigate the role of peroxisome proliferator-activated receptors (PPARs) in cisplatin nephrotoxicity. Rats were treated with a single i.p. dose of cisplatin (5 mg/kg) alone or combined with pioglitazone (PPARγ agonist), fenofibrate (PPARα agonist), pioglitazone plus fenofibrate, or thalidomide (Tumor necrosis factor-α inhibitor; TNF-α). Cisplatin nephrotoxicity was evidenced by rises in renal indices of functional (blood urea nitrogen, BUN, and creatinine), inflammatory (TNF-α, interleukin 6, IL-6), oxidative (increased malondialdehyde, MDA, and decreased superoxide dismutase, SOD and nitric oxide metabolites, NOx), apoptotic (caspase 3), and histological (glomerular atrophy, acute tubular necrosis and vacuolation) profiles. Cisplatin effects were partly abolished upon concurrent exposure to pioglitazone, fenofibrate, or thalidomide; more renoprotection was observed in rats treated with pioglitazaone plus fenofibrate. Immunostaining showed that renal expressions of PPARα and PPARγ were reduced by cisplatin and restored to vehicle-treated values after simultaneous treatment with pioglitazone or fenofibrate. Fenofibrate or pioglitazone renoprotection remained unaltered after concurrent blockade of PPARα (GW6471) and PPARγ (GW9662), respectively. To complement the rat studies, we also report that in human embryonic kidney cells (HEK293 cells), increases caused by cisplatin in inflammatory, apoptotic, and oxidative biomarkers were (i) partly improved after exposure to pioglitazone, fenofibrate, or thalidomide, and (ii) completely disappeared in cells treated with a combination of all three drugs. These data establish that the combined use of pioglitazone and fenofibrate additively improved manifestations of cisplatin nephrotoxicity through perhaps GW6471/GW9662-insensitive mechanisms.

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.

Regulatory mechanisms of cAMP levels as a multiple target for antiplatelet activity and less bleeding risk

Platelet activation is a critical component of atherothrombosis. The multiple pathways of platelet activation limit the effect of specific receptor/pathway inhibitors, resulting in limited clinical efficacy. Recent research has confirmed that combination therapy results in enhanced antithrombotic efficacy without increasing bleeding risk. In this way, the best-known inhibitor and turn off signaling in platelet activation is cAMP. In this article we discuss the mechanisms of regulation of intraplatelet cAMP levels, a) platelet-dependent pathway: Gi/Gs protein-coupled receptors, phosphodiesterase inhibition and activation of PPARs and b) platelet-independent pathway: inhibition of adenosine uptake by erythrocytes. With respect to the association between intraplatelet cAMP levels and bleeding risk it is possible to establish that compounds/drugs with pleitropic effect for increased intraplatelet cAMP level could have an antithrombotic activity with less risk of bleeding.

Relationship between Platelet PPARs, cAMP Levels, and P-Selectin Expression: Antiplatelet Activity of Natural Products

Platelets are no longer considered simply as cells participating in thrombosis. In atherosclerosis, platelets are regulators of multiple processes, with the recruitment of inflammatory cells towards the lesion sites, inflammatory mediators release, and regulation of endothelial function. The antiplatelet therapy has been used for a long time in an effort to prevent and treat cardiovascular diseases. However, limited efficacy in some patients, drug resistance, and side effects are limitations of current antiplatelet therapy. In this context, a large number of natural products (polyphenols, terpenoids, alkaloids, and fatty acids) have been reported with antiplatelet activity. In this sense, the present paper describes mechanisms of antiplatelet action of natural products on platelet P-selectin expression through cAMP levels and its role as peroxisome proliferator-activated receptors agonists.

Platelets and neurovascular inflammation

Platelets participate in haemostasis and in thrombus formation in health and disease. Moreover, they contribute to inflammation and cooperate with immune cells in a magnitude of inflammatory/immune responses. Although the inflammatory response has been recognised to be critical in neuronal diseases such as Alzheimer's disease or multiple sclerosis and its mouse counterpart, experimental autoimmune encephalomyelitis, the participation of platelets in these diseases is poorly investigated so far. Emerging studies, however, point to an interesting crosstalk between platelets and neuroinflammation. For instance, when the integrity of the blood brain barrier is compromised, platelets may be relevant for endothelial inflammation, as well as recruitment and activation of inflammatory cells, thereby potentially contributing to central nervous tissue pathogenesis. This review summarises recent insights in the role of platelets for neurovascular inflammation and addresses potential underlying mechanisms, by which platelets may affect the pathophysiology of neurovascular diseases.

Regulation of Lymphocyte Function by PPARgamma: Relevance to Thyroid Eye Disease-Related Inflammation

Thyroid eye disease (TED) is an autoimmune condition in which intense inflammation leads to orbital tissue remodeling, including the accumulation of extracellular macromolecules and fat. Disease progression depends upon interactions between lymphocytes and orbital fibroblasts. These cells engage in a cycle of reciprocal activation which produces the tissue characteristics of TED. Peroxisome proliferator-activated receptor-gamma (PPARgamma) may play divergent roles in this process, both attenuating and promoting disease progression. PPARgamma has anti-inflammatory activity, suggesting that it could interrupt intercellular communication. However, PPARgamma activation is also critical to adipogenesis, making it a potential culprit in the pathological fat accumulation associated with TED. This review explores the role of PPARgamma in TED, as it pertains to crosstalk between lymphocytes and fibroblasts and the development of therapeutics targeting cell-cell interactions mediated through this signaling pathway.

The PPAR-Platelet Connection: Modulators of Inflammation and Potential Cardiovascular Effects

Historically, platelets were viewed as simple anucleate cells responsible for initiating thrombosis and maintaining hemostasis, but clearly they are also key mediators of inflammation and immune cell activation. An emerging body of evidence links platelet function and thrombosis to vascular inflammation. peroxisome proliferator-activated receptors (PPARs) play a major role in modulating inflammation and, interestingly, PPARs (PPARβ/δ and PPARγ) were recently identified in platelets. Additionally, PPAR agonists attenuate platelet activation; an important discovery for two reasons. First, activated platelets are formidable antagonists that initiate and prolong a cascade of events that contribute to cardiovascular disease (CVD) progression. Dampening platelet release of proinflammatory mediators, including CD40 ligand (CD40L, CD154), is essential to hinder this cascade. Second, understanding the biologic importance of platelet PPARs and the mechanism(s) by which PPARs regulate platelet activation will be imperative in designing therapeutic strategies lacking the deleterious or unwanted side effects of current treatment options.

Role of peroxisome proliferator-activated receptor gamma and its ligands in the treatment of hematological malignancies

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a multifunctional transcription factor with important regulatory roles in inflammation, cellular growth, differentiation, and apoptosis. PPARgamma is expressed in a variety of immune cells as well as in numerous leukemias and lymphomas. Here, we review recent studies that provide new insights into the mechanisms by which PPARgamma ligands influence hematological malignant cell growth, differentiation, and survival. Understanding the diverse properties of PPARgamma ligands is crucial for the development of new therapeutic approaches for hematological malignancies.

Inhibitory effect of α-lipoic acid on platelet aggregation is mediated by PPARs

Peroxisome proliferator-activated receptors (PPARs) isoforms (α, β/δ, and γ are present in human platelets, and activation of PPARs inhibits platelet aggregation. α-Lipoic acid (ALA), occurring naturally in human food, has been reported to exhibit an antiplatelet activity. However, the mechanisms underlying ALA-mediated inhibition of platelet aggregation remain unknown. The aim of this study was to investigate whether the antiplatelet activity of ALA is mediated by PPARs. ALA itself significantly induced PPARα/γ activation in platelets and increased intracellular amounts of PPARα/γ by blocking PPARα/γ secretion from arachidonic acid (AA)-activated platelets. Moreover, ALA significantly inhibited AA-induced platelet aggregation, Ca(2+) mobilization, and cyclooxygenase-1 (COX-1) activity, but increased cyclic AMP production in rabbit washed platelets. Importantly, ALA also enhanced interaction of PPARα/γ with protein kinase Cα (PKCα) and COX-1 accompanied by an inhibition of PKCα activity in resting and AA-activated platelets. However, the above effects of ALA on platelets were markedly reversed by simultaneous addition of selective PPARα antagonist (GW6471) or PPARγ antagonist (GW9662). Taken together, the present study provides a novel mechanism by which ALA inhibition of platelet aggregation is mediated by PPARα/γ-dependent processes, which involve interaction with PKCα and COX-1, increase of cyclic AMP formation, and inhibition of intracellular Ca(2+) mobilization.

Cytokines (interferon-γ and tumor necrosis factor-α)-induced nuclear factor-κB activation and chemokine (C-X-C motif) ligand 10 release in Graves disease and ophthalmopathy are modulated by pioglitazone

Until now, the following are not known: (1) the mechanisms underlying the induction of chemokine (C-X-C motif) ligand 10 (CXCL10) secretion by cytokines in thyrocytes; (2) if pioglitazone is able, like rosiglitazone, to inhibit the interferon (IFN)-γ-induced chemokine expression in Graves disease (GD) or ophthalmopathy (GO); and (3) the mechanisms underlying the inhibition by thiazolidinediones of the cytokines-induced CXCL10 release in thyrocytes. The aims of this study were (1) to study the mechanisms underlying the induction of CXCL10 secretion by cytokines in GD thyrocytes; (2) to test the effect of pioglitazone on IFNγ-inducible CXCL10 secretion in primary thyrocytes, orbital fibroblasts, and preadipocytes from GD and GO patients; and (3) to evaluate the mechanism of action of thiazolidinediones on nuclear factor (NF)-κB activation. The results of the study (1) demonstrate that IFNγ + TNFα enhanced the DNA binding activity of NF-κB in GD thyrocytes, in association with the release of CXCL10; (2) show that pioglitazone exerts a dose-dependent inhibition on IFNγ + TNFα-induced CXCL10 secretion in thyrocytes, orbital fibroblasts, and preadipocytes, similar to the effect observed with rosiglitazone; and (3) demonstrate that thiazolidinediones (pioglitazone and rosiglitazone) act by reducing the IFNγ + TNFα activation of NF-κB in Graves thyrocytes. To the best of our knowledge, this is the first study showing that cytokines are able to activate NF-κB in Graves thyrocytes and a parallel inhibitory effect of pioglitazone both on CXCL10 chemokine secretion and NF-κB activation. Future studies will be needed to verify if new targeted peroxisome proliferator-activated receptor-γ activators may be able to exert the anti-inflammatory effects without the risk of expanding retrobulbar fat mass.

Platelet CD40L at the interface of adaptive immunity

Initiated by the finding that platelets express functional CD40 ligand (CD40L, CD154), many new roles for platelets have been discovered in unanticipated areas, including the immune response. When current literature is considered as a whole, the picture that is emerging begins to show that platelets are able to significantly affect, for better or worse, the overall health and condition of the mammalian host. Animal models have made significant contributions to our expanding knowledge of platelet function, much of which is anticipated to be clinically relevant. While still mostly circumstantial, the evidence supports a critical role for CD40L in many normal and disease processes.

A putative role for platelet-derived PPARγ in vascular homeostasis demonstrated by anti-PPARγ induction of bleeding, thrombocytopenia and compensatory megakaryocytopoiesis

Widely known for its role in adipogenesis and energy metabolism, PPARγ also plays a role in platelet function. To further understand functions of platelet-derived PPARγ, we produced rabbit polyclonal (PoAbs) and mouse monoclonal (MoAbs) antibodies against PPARγ 14mer/19mer peptide-immunogens. Unexpectedly, our work produced two key findings. First, MoAbs but not PoAbs produced against PPARγ peptide-immunogens displayed antigenic crossreactivity with highly conserved PPARα and PPARβ/δ. Similarly, Santa Cruz PoAb sc-7196 was monospecific for PPARγ while MoAb sc-7273 crossreacted with PPARα and PPARβ/δ. Second, immunized rabbits and mice exhibited unusual pathology including cachexia, excessive bleeding, and low platelet counts leading to thrombocytopenia. Spleens from immunized mice were fatty, hemorrhagic and friable. Although passive administration of anti-PPARγ PoAbs failed to induce experimental thrombocytopenia, megakaryocytopoiesis was induced 4-8-fold in mouse spleens. Similarly, marrow megakaryocytopoiesis was enhanced 1.8-4-fold in immunized rabbits. These peptide-immunogens are 100% conserved in human, rabbit and mouse; thus, immune-mediated platelet destruction via crossreactivity with platelet-derived PPARγ likely caused bleeding, thrombocytopenia, and compensatory megakaryocytopoiesis. Such overt pathology would cause significant problems for large-scale production of anti-PPARγ PoAbs. Furthermore, a major pitfall associated with MoAb production against closely related molecules is that monoclonicity does not guarantee monospecificity, an issue worth further scientific scrutiny.

Novel anti-adipogenic activity produced by human fibroblasts

Fatty tissue is generally found in distinct "depots" distributed throughout the human body. Adipocytes from each of the various depots differ in their metabolic capacities and their responses to environmental stimuli. Although a general understanding of the factors responsible for adipogenic transformation has been achieved, much is not understood about the mechanisms of adipose tissue deposition and the phenotypes of the adipocytes found within each depot. A clue to the factors regulating fat deposition may come from studies of adipogenesis using primary human orbital fibroblasts from patients with thyroid eye disease, a condition in which intense inflammation leads to expansion of orbital adipose tissue via differentiation of fibroblasts to adipocytes. We have previously demonstrated that adipogenesis of orbital fibroblasts is negatively correlated with cellular expression of the Thy-1 surface marker. In this study, we developed a novel imaging flow cytometric approach for the assessment of adipogenesis to test the hypothetical dependence of adipogenic potential on lack of Thy-1 expression. Using this technique, we learned that Thy-1-positive fibroblasts are, in fact, capable of differentiating into adipocytes but are less likely to do so because they secrete a paracrine anti-adipogenic factor. It is possible that such a factor plays an important role in the prevention of excess fat deposition in the normal orbit and may even be exploited as a therapy for the treatment of obesity, a major worldwide health concern.

Anticancer Role of PPARgamma Agonists in Hematological Malignancies Found in the Vasculature, Marrow, and Eyes

The use of targeted cancer therapies in combination with conventional chemotherapeutic agents and/or radiation treatment has increased overall survival of cancer patients. However, longer survival is accompanied by increased incidence of comorbidities due, in part, to drug side effects and toxicities. It is well accepted that inflammation and tumorigenesis are linked. Because peroxisome proliferator-activated receptor (PPAR)-gamma agonists are potent mediators of anti-inflammatory responses, it was a logical extension to examine the role of PPARgamma agonists in the treatment and prevention of cancer. This paper has two objectives: first to highlight the potential uses for PPARgamma agonists in anticancer therapy with special emphasis on their role when used as adjuvant or combined therapy in the treatment of hematological malignancies found in the vasculature, marrow, and eyes, and second, to review the potential role PPARgamma and/or its ligands may have in modulating cancer-associated angiogenesis and tumor-stromal microenvironment crosstalk in bone marrow.

Roles of lysophosphatidic acid in cardiovascular physiology and disease

The bioactive lipid mediator lysophosphatidic acid (LPA) exerts a range of effects on the cardiovasculature that suggest a role in a variety of critical cardiovascular functions and clinically important cardiovascular diseases. LPA is an activator of platelets from a majority of human donors identifying a possible role as a regulator of acute thrombosis and platelet function in atherogenesis and vascular injury responses. Of particular interest in this context, LPA is an effective phenotypic modulator of vascular smooth muscle cells promoting the de-differentiation, proliferation and migration of these cells that are required for the development of intimal hyperplasia. Exogenous administration of LPA results in acute and systemic changes in blood pressure in different animal species, suggesting a role for LPA in both normal blood pressure regulation and hypertension. Advances in our understanding of the molecular machinery responsible for the synthesis, actions and inactivation of LPA now promise to provide the tools required to define the role of LPA in cardiovascular physiology and disease. In this review we discuss aspects of LPA signaling in the cardiovasculature focusing on recent advances and attempting to highlight presently unresolved issues and promising avenues for further investigation.

Individual heterogeneity in platelet response to lysophosphatidic acid: evidence for a novel inhibitory pathway

OBJECTIVE

The bioactive lipid lysophosphatidic acid (LPA) stimulates platelet actin reorganization, adhesion, shape change, and aggregation. LPA is present in blood and exposure or release of LPA after atherosclerotic plaque rupture has been proposed to trigger platelet thrombus formation.

METHODS AND RESULTS

In this report, we characterize heterogeneity in LPA responses among individuals. Platelets isolated from approximately 20% of healthy donors consistently failed to aggregate in response to LPA. Our studies indicate that, rather than lacking stimulatory pathways, platelets from "nonresponsive" donors respond to LPA by triggering inhibitory pathway(s) to block platelet aggregation. Consistent with this observation, LPA-induced aggregation could be partially restored to "nonresponsive" platelets by pharmacological inhibition of cAMP generation. LPA "nonresponsiveness" may be related to heightened platelet expression of LPA receptor 4 and PPARgamma. Among 70 patients with stable coronary artery disease (CAD), only 1 (1.4%) was identified as an LPA nonresponder. Moreover, in 33 patients presenting for diagnostic catheterization, CAD was identified as having a bivariate association with platelet LPA responder/nonresponder status.

CONCLUSIONS

Platelet LPA signaling may involve stimulatory and inhibitory pathways, with the inhibitory pathway predominating in approximately 20% of individuals. Diseases such as CAD that affect platelet reactivity may attenuate this inhibitory pathway in platelets.

Thiazolidinediones as anti-inflammatory and anti-atherogenic agents

In the last few years, there has been increasing focus on the impact of interventions on cardiovascular outcomes in patients with type 2 diabetes. Insulin resistance and hyperglycaemia often co-exist with a cluster of risk factors for coronary artery disease, but the underlying mechanisms leading to the development of such vascular complications are complex. The over-production of free radicals in patients suffering from diabetes results in a state of oxidative stress, which leads to endothelial dysfunction and a greater risk of atherosclerosis. Moreover, inflammatory factors which play a critical role in atherothrombosis and plaque rupture are often found to be at elevated levels in this patient population. Thiazolidinediones (TZDs) are now routinely used to manage glucose levels, and have been suggested to influence other cardiovascular risk factors and therefore the pathways leading to macrovascular events. Consequently, recent studies have investigated the anti-inflammatory and anti-atherogenic properties of TZDs. The data available up to the present time, in the context of the emerging cardiovascular outcome profiles of rosiglitazone and pioglitazone, will be discussed here.

The relationship of the peroxisome proliferator-activated receptor-gamma 2 exon 2 and exon 6 gene polymorphism in Turkish type 2 diabetic patients with and without nephropathy

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPAR gamma) has recently been shown to be associated with type 2 diabetes. We aim to investigate Turkish type 2 diabetic patients with/without diabetic nephropathy and healthy group and examined the contribution of the G/C exon 2 and T/C exon 6 of the PPAR gamma gene polymorphism to the development of diabetic nephropathy.

METHODS

The PPAR gamma genotypes were determined retrospectively in 43 patients with nephropathy and 48 without nephropathy and a control group of 50 healthy individuals. Genotyping of the G/C exon 2 and T/C exon 6 of the PPAR-gamma gene polymorphism for all individuals was performed by melting curve analysis of the generated amplicons after real-time online PCR.

RESULTS

This genotype (exon 2 and exon 6) distribution did not differ between control subjects and type 2 diabetic patients. The genotype frequencies and allele exon 2 were CC, 100%; GC, 0%; GG, 0% and C, 100%; G, 0% in diabetic patients with nephropathy versus CC, 97.9%; GC, 2.1%; GG, 0% and C 98.9%, G 1.1% in those without nephropathy. Genotype exon 6 frequencies in diabetic patients with nephropathy were (T/T) 0%, (T/C) 14%; (C/C) 86% versus (G/G) 0%; (G/C) 2.1%; (C/C) 97.9% in those without nephropathy. The PPAR gamma exon 2 and exon 6 genotype and allele frequencies were not different between diabetic patients with and without nephropathy.

CONCLUSIONS

PPAR gamma exon 2 and exon 6 gene polymorphism is not associated with the development of diabetic nephropathy in Turkish type 2 diabetic patients.

Novel uses for anti-platelet agents as anti-inflammatory drugs

An alteration in the character and function of platelets is manifested in patients with inflammatory diseases, and these alterations have been dissociated from the well-characterized involvement of platelets in thrombosis and haemostasis. Recent evidence reveals platelet activation is sometimes critical in the development of inflammation. The mechanisms by which platelets participate in inflammation are diverse, and offer numerous opportunities for future drug intervention. There is now acceptance that platelets act as innate inflammatory cells in immune responses, with roles as sentinel cells undergoing surveillance, responding to microbial invasion, orchestrating leukocyte recruitment, and migrating through tissue, causing damage and influencing repair processes in chronic disease. Some of these processes are targeted by drugs that are being developed to target platelet participation in atherosclerosis. The actions of platelets therefore influence the pathogenesis of diverse inflammatory diseases in various body compartments, encompassing parasitic and bacterial infection, allergic inflammation (especially asthma and rhinitis), and non-atopic inflammatory conditions, for example, chronic obstructive pulmonary disease (COPD), rheumatoid arthritis (RA), inflammatory bowel disease (IBD) and atherosclerosis. This review will first discuss the evidence for platelet activation in these various inflammatory diseases, and secondly discuss the mechanisms by which this pathogenesis occurs and the various anti-platelet agents which have been developed to combat platelet activation in atherosclerosis and their potential future use for the treatment of other inflammatory diseases.

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

The primary target receptor for thiazolidinediones (TZDs) or peroxisome proliferator-activated receptor gamma (PPARgamma) agonists is a transcription factor in the nucleus of adipocytes and other metabolically active cells, where they improve insulin sensitivity and glucose utilization. TZDs are also able to modify gene expression in macrophages, smooth muscle cells, and endothelial cells. Although PPARgamma is considered to be a nuclear receptor, enucleate platelets also highly express this receptor. The aim of this review is to present the current understanding of a direct or indirect effect of TZDs on platelet function. By means of a comprehensive literature search (January 1990-June 2006), publications were obtained that contained specific information about in vitro and in vivo effects of TZDs on platelet function. The effects were studied for different risk biochemical markers, i.e., proteins found to be elevated in the state of procoagulant inflammation and endothelial dysfunction. Improvement of platelet function was reported for all TZDs-troglitazone, pioglitazone, and rosiglitazone. The described effects included reduction of platelet aggregation, suppression of thrombin-induced protein kinase C-alpha and -beta activation, decrease in plasma P-selectin and platelet P-selectin expression, increase in nitric oxide production, inhibition of the Rho/Rho kinase pathway, and inhibition of tissue factor- and platelet-activating factor-induced morphological changes in macrophages. These findings appeared in parallel with reduction of the plasma concentrations of pro-inflammatory risk markers. TZDs seem to have a direct pleiotropic positive influence on platelet function and coagulation and may be helpful in treating the prothrombotic state observed in patients with type 2 diabetes and metabolic syndrome.

Pioglitazone: update on an oral antidiabetic drug with antiatherosclerotic effects

Pioglitazone, a member of the PPAR-gamma agonist drug family, has been demonstrated to improve both metabolic and vascular insulin resistance when applied to patients with Type 2 diabetes mellitus. The drug is well tolerated with fluid retention and weight gain being the most frequently described side effects. The observed effects (e.g., improvements in glucose and lipid metabolism, improvements of endothelial function and microcirculation, reduction of surrogate markers of atherosclerosis and inflammation and an improvement in hypertension) have made pioglitazone one of the frequently prescribed antidiabetic drugs in the US and Europe. Several trials have shown its potency to reduce carotid intima-media thickness, and outcome studies with pioglitazone have shown its potential to delay the progression of Type 2 diabetes and atherosclerosis and even reduce cardiovascular mortality. The purpose of this review is to provide an overview about recently published clinical results with pioglitazone. They underline the value of this drug when used alone or in combination with other antidiabetic drugs for a successful management of Type 2 diabetes mellitus.

Vascular effects of PPARgamma activators - from bench to bedside

Activation of the nuclear transcription factor peroxisome proliferator-activated receptor-gamma (PPARgamma) plays an important role in adipogenesis, insulin resistance, and glucose homeostasis. Activators of PPARgamma include the anti-diabetic thiazolidinediones (TZDs), drugs that are in clinical use to treat patients with type 2 diabetes mellitus. Experimental as well as clinical data gathered over the last decade suggest that PPARgamma activators may exert direct modulatory function in the vasculature in addition to their metabolic effects. PPARgamma is expressed in all vascular cells, where its activators exhibit anti-inflammatory and anti-atherogenic properties, suggesting that PPARgamma ligands could influence important processes in all phases of atherogenesis. Results from clinical trials demonstrated that TZDs reduce blood levels of inflammatory biomarkers of arteriosclerosis, improve endothelial function, and directly influence lesion morphology and plaque stability, underscoring that PPAR activators may have direct effects in the vasculature in humans. This review will focus on the vascular effects of PPARgamma activators and summarize the current knowledge of their modulatory function on atherogenesis and vascular disease.