Identifying pathways involved in leptin-dependent aggregation of human platelets

Leptin as predictor of cardiovascular events and high platelet reactivity in patients undergoing percutaneous coronary intervention

BACKGROUND AND AIMS

Leptin is a hormone involved in the regulation of food intake. Previous studies suggested an interplay between leptin, platelet aggregation, and cardiovascular outcome but this issue was not investigated in vivo in patients treated with percutaneous coronary intervention (PCI). We designed a study to evaluate the possible relation between leptin, cardiovascular outcome, and platelet reactivity (PR) in patients undergoing PCI.

METHODS

155 PCI patients had preprocedural measurements of PR and leptin plasma levels. The latter were assessed by ELISA. Hyperleptinemia was defined as leptin levels ≥14 ng/ml. PR was evaluated by the VerifyNowP2Y12 assay and expressed as P2Y12 reaction units (PRU). Patients were divided into three groups based on PR values and defined as low (LPR), normal (NPR), and high (HPR). Patients were followed for up 8 years. The primary endpoint was the incidence of Major Acute Cardiac Events (MACE) at long-term follow-up according to leptin groups. Secondary endpoints were the evaluation of leptin levels according to PR groups and the incidence of periprocedural myocardial infarction (PMI) according to leptin groups.

RESULTS

Long-term follow-up was completed in 140 patients. Patients with hyperleptinemia experienced a higher MACE rate than the normoleptinemic group (HR 2.3; CI 95% 1.14-4.6, P = 0.02). These results remained unchanged after adjusting for Body Mass Index, hypertension, and gender. Leptin levels were significantly different among groups of PR (P = 0.047). Leptin levels were higher in the HPR group (12.61 ± 16.58 ng/ml) compared to the LPR group (7.83 ± 8.87 ng/ml, P = 0.044) and NPR group (7.04 ± 7.03 ng/ml, P = 0.01). The rate of PMI was higher in hyperleptinemia patients (15.1% vs. 6.5%, P = 0.22).

CONCLUSIONS

This study suggests that high leptin levels are associated with a worse clinical outcome in patients undergoing PCI and with HPR. Further studies are needed to define better the pathophysiological pathways underlying this association.

Adipokines and thrombosis

1. Obesity is a major risk factor for cardiovascular disease. An increased body mass index (BMI) is associated with venous thromboembolism, myocardial infarction, stroke and stent thrombosis after percutaneous interventions. Studies in mouse models of obesity and induced arterial or venous thrombosis have provided insights into the mechanisms involved. 2. In addition to elevated circulating levels of fibrinogen, factor VII and plasminogen activator inhibitor (PAI)-1, changes in platelet biology and function may underlie the increased (athero) thrombotic risk in obesity. These include elevated platelet counts, an increase in mean platelet volume, an increased platelet aggregatory response to agonists and a reversible resistance to the anti-aggregatory effects of nitric oxide and prostacyclin I(2) . 3. Specific adipokines mediate the prothrombotic state in obesity. Of these, leptin enhances both arterial and venous thrombosis by promoting platelet adhesion, activation and aggregation. Leptin also induces tissue factor expression by human neutrophils and other cells. C-Reactive protein enhances the formation of monocyte-platelet aggregates and also promotes P-selectin expression and platelet adhesion to endothelial cells. Further, the adipose tissue is a significant source of tissue factor and PAI-1. Conversely, the circulating levels of adiponectin, a hormone that exerts vasculoprotective, anti-atherosclerotic and antithrombotic effects, are reduced in obese individuals. 4. A better understanding of the interactions of the adipose tissue with circulating and vascular cells and the dissection of the mechanisms linking adipokines to arterial and venous thrombosis may identify obese individuals at particularly high cardiovascular risk and indicate promising vasculoprotective and therapeutic targets.

The role of novel biomarkers of cardiovascular disease in chronic kidney disease: focus on adiponectin and leptin

Cardiovascular disease (CVD) remains a major cause of premature death in patients with chronic kidney disease (CKD), including renal transplant recipients. Both interplay of traditional cardiovascular and renal specific risk factors have been shown to be associated with an increased risk of cardiovascular death in patients with CKD. Recently, there has been great interest in the role of novel biomarkers, in particular adiponectin and leptin, and its association with CVD in the CKD population. Adiponectin is a multifunctional adipocyte-derived protein with anti-inflammatory, antiatherogenic and insulin sensitizing activity. Recent observational studies have shown adiponectin to be a novel risk marker of CVD in patients with stages 1 to 5 CKD. Leptin is an adipocyte-derived hormone that promotes weight loss by decreasing food intake. Similarly, there are observational studies to support an association between leptin and CVD, including patients with CKD. In the CKD population, leptin may be associated with uremic cachexia and subsequent increased mortality. This review aims to summarize the pathophysiological and potential clinical roles of these cardiovascular biomarkers in patients with CKD.

Whole blood aggregation, coagulation, and markers of platelet activation in diet-induced diabetic C57BL/6J mice

AIMS

Type 2 diabetes in humans is associated with hypercoaguability; however, little is known about platelet function in mouse models of type 2 diabetes used to study this disorder. Therefore, the purpose of this study was to examine platelet aggregation, coagulation, and markers of platelet activation in a diet-induced mouse model of type 2 diabetes.

METHODS

Four week old, male, C57BL/6J mice were randomized to a standard chow or high fat (60% beef lard) diet for 4 months. To examine platelet function we measured ADP-induced whole blood aggregometry, whole blood coagulation by thromboelastography, tail bleeding times, platelet microparticle and platelet expression of p-selectin and platelet expression of CD61 by flow cytometry.

RESULTS

Diabetic mice exhibited less aggregation (p<0.05), less coagulation (p<0.01), prolonged tail bleeding times (n.s.), and lower agonist stimulated platelet CD61 expression (p<0.001) compared to non-diabetic mice. There was no difference in platelet microparticle and platelet p-selectin expression.

CONCLUSIONS

Diet-induced type 2 diabetic mouse do not demonstrate the hypercoagulability and platelet activation typically observed in humans with this disorder. More studies are warranted to further explore platelet function in this mouse model; to determine their applicability for studying these alterations in type 2 diabetes.

TRP-independent inhibition of the phospholipase C pathway by natural sensory ligands

Menthol, cinnamaldehyde, and camphor are activators for temperature-sensitive transient receptor potential ion channels (thermoTRPs). Here we found that these three compounds inhibit the phospholipase C (PLC) signaling. P2Y purinoceptor-mediated or histamine receptor-mediated cytosolic calcium mobilization through the PLC pathway was significantly suppressed by menthol, cinnamaldehyde, and camphor. Experiments using a fluorescent pleckstrin homology domain of PLCdelta1 and IP1 accumulation assays demonstrated that direct inhibition of PLC activity occurred upon the addition of the sensory compounds. P2Y receptor-mediated PLC activation is part of the mechanism of platelet aggregation. The three compounds inhibited ADP-induced platelet aggregation. Calcium influx studies showed that thermoTRPs do not function in platelets, suggesting that the anti-aggregation effect is independent of thermoTRP activity. These results suggest that menthol, cinnamaldehyde, and camphor are able to modify PLC signaling and that those effects may lead to changes in cellular functions. This study also identifies new types of compounds that could potentially modulate platelet-related pathological events.

Links Between Adipose Tissue and Thrombosis in the Mouse

Obesity has become a global epidemic and carries a considerable negative impact in regard to quality of life and life expectancy. A primary problem is that obese individuals are at increased risk of suffering from cardiovascular disease complications such as myocardial infarction and stroke. Because fat accumulation is a consistent aspect of obesity, mechanisms that may link adipose tissue to cardiovascular disease complications should be considered. Proteins expressed from adipose tissue, known as adipokines, are hypothesized to have important effects on the progression and incidence of cardiovascular disease complications. This review examines the evidence that adipokines play a direct role in vascular thrombosis, an important event in cardiovascular disease complications.

Relationship between metabolic syndrome and platelet responsiveness to leptin in overweight and obese patients

OBJECTIVE

To verify whether platelet responsiveness to leptin is associated with metabolic syndrome risk factors.

DESIGN

Cross-sectional study.

SUBJECTS

We studied 169 consecutive patients, mean age=43.6+/-9.9 years, with overweight (N=57) or obesity (N=112).

MEASUREMENTS

Cluster analysis was used to generate three clusters based on platelet responsiveness to increasing doses of leptin. Profiles of metabolic syndrome risk factors of the three clusters were compared by discriminant analysis.

RESULTS

Platelet responsiveness to leptin was absent in cluster 1, whereas cluster 3 had the greatest platelet aggregation response to leptin pre-incubation. Plasma leptin levels significantly decreased from cluster 1 to cluster 3 in both gender. Patients in cluster 2 had an intermediate profile of leptin responsiveness. Highest body mass index (BMI) values were more frequent in non-responders, whereas the prevalence of high waist circumference, as well as hypertriglyceridemia and hypertension, increased with increasing responsiveness to leptin from cluster 1 to cluster 3. Pattern of metabolic syndrome risk factors qualified as group specific in 69.0% of the cluster 1, 54.9% of the cluster 2 and 55.8% of the cluster 3. Circulating leptin, waist circumference, plasma triglycerides and BMI defined distinctive patterns of metabolic syndrome risk factors in the clusters.

CONCLUSIONS

In overweight and obese outpatients, metabolic syndrome risk factors parallel to some extent platelet responsiveness to leptin. Such a correlation involves plasma leptin levels, waist circumference, plasma triglycerides and BMI, and may contribute to the excess risk of cardiovascular events in overweight and obese patients.

Perceived psychological stress and serum leptin concentrations in Japanese men

OBJECTIVE

To examine epidemiologically whether subjects with higher stress perception levels have higher leptin concentrations.

RESEARCH METHODS AND PROCEDURES

In this cross-sectional study, the study population comprised 1062 male workers at local government offices in central Japan. Self-administered questionnaires were distributed in 1997. Awareness of stress was assessed by the question: "Do you have much stress in your life?" and participants were asked to select from four possible responses: "very much," "much," "ordinary," or "little." Blood samples were also collected after fasting 12 hours overnight to determine serum leptin concentrations.

RESULTS

The mean (standard deviation) age and BMI were 50.2 (6.4) years and 23.3 (2.6) kg/m(2), respectively. Crude leptin concentrations according to stress categories were 2.86, 3.26, 3.32, and 3.54 ng/mL, respectively, and leptin concentrations adjusted for age, BMI, physical activity, drinking and smoking habits, overtime work, shift work, sleep duration, and availability of confidants were 2.96, 3.24, 3.34, and 3.43 ng/mL for "little," "ordinary," "much," and "very much," respectively (p = 0.03 by one-way analysis of covariance; p < 0.01 by test of linear trend). Significant associations were also observed among the level of perceived psychological stress and work-related stressors, variables related to sleep, and other psychological variables.

DISCUSSION

This study showed that subjects who perceived psychological stress had high leptin levels, which provides epidemiological evidence that psychological stress may have the potential effect of increasing blood levels of the pleiotropic peptide, leptin.

Analysis of leptin signalling in hematopoietic cells using an adapted MAPPIT strategy

The adipocyte-secreted hormone leptin participates in the regulation of hematopoiesis and enhances proliferation of hematopoietic cells. We used an adaptation of the MAPPIT mammalian two-hybrid method to study leptin signalling in a hematopoietic setting. We confirmed the known interactions of suppressor of cytokine signalling 3 (SOCS3) and STAT5 with the Y985 and Y1077 motifs of the leptin receptor, respectively. We also provide evidence for novel interactions at the Y1077 motif, including phospholipase C gamma and several members of the SOCS protein family, further underscoring the important role of the Y1077 motif in leptin signalling.

Leptin and atherosclerosis

Leptin, a 167-amino acid peptide hormone produced by white adipose tissue, is primarily involved in the regulation of food intake and energy expenditure. Leptin receptors are expressed in many tissues including the cardiovascular system. Plasma leptin concentration is proportional to body adiposity and is markedly increased in obese individuals. Recent studies suggest that hyperleptinemia may play an important role in obesity-associated cardiovascular diseases including atherosclerosis. Leptin exerts many potentially atherogenic effects such as induction of endothelial dysfunction, stimulation of inflammatory reaction, oxidative stress, decrease in paraoxonase activity, platelet aggregation, migration, hypertrophy and proliferation of vascular smooth muscle cells. Leptin-deficient and leptin receptor-deficient mice are protected from arterial thrombosis and neointimal hyperplasia in response to arterial wall injury. Several clinical studies have demonstrated that high leptin level predicts acute cardiovascular events, restenosis after coronary angioplasty, and cerebral stroke independently of traditional risk factors. In addition, plasma leptin correlates with markers of subclinical atherosclerosis such as carotid artery intima-media thickness and coronary artery calcifications. Inhibition of leptin signaling may be a promising strategy to slow the progression of atherosclerosis in hyperleptinemic obese subjects.

Effects of physiological leptin administration on markers of inflammation, platelet activation, and platelet aggregation during caloric deprivation

CONTEXT

Leptin is a nutritionally regulated adipocyte-derived cytokine. Previous studies in obese patients have demonstrated increased inflammatory markers and increased platelet aggregation in association with leptin. However, the effects of leptin administration on markers of inflammation and platelet aggregation in a human model of undernutrition have not previously been studied.

OBJECTIVE

The objective of the study was to investigate markers of inflammation, platelet activation, and platelet aggregation in a model of caloric deprivation and increased leptin sensitivity.

DESIGN

This study was a randomized, placebo-controlled study conducted between November 2002 and November 2003.

SETTING

The study was conducted at an inpatient care setting at the General Clinical Research Center.

PARTICIPANTS

Twenty healthy, young (18-35 yr old), normal-weight (body mass index, 20-26 kg/m2) women were recruited from local advertisements. No subjects withdrew due to adverse effects.

INTERVENTION

The effects of physiological recombinant methionyl human leptin or identical placebo administration were investigated over a 4-d fast.

MAIN OUTCOME MEASURES

The primary outcome measures for this study were C-reactive protein (CRP) and indices of platelet activity.

RESULTS

Leptin administration prevented the fasting-induced decline in leptin (P < 0.05 vs. placebo at each time point). Leptin administration increased CRP (6.3 +/- 2.4 vs. 0.7 +/- 0.3 mg/liter; P = 0.04), circulating P-selectin (11.6 +/- 10.2 vs. -28.9 +/- 15.6 ng/ml; P = 0.04), and induction of platelet aggregation (5.8 +/- 2.6 vs. -2.7 +/- 2.9%, P = 0.04, percent maximum platelet aggregation) relative to placebo administration (change in leptin vs. change in placebo, respectively, for each variable). Leptin tended to increase serum amyloid A [0.1 +/- 0.2 vs. -0.3 +/- 0.1 log10 (ng/ml); P = 0.07], and the changes in serum amyloid A and CRP were highly correlated (r = 0.83; P < 0.0001). No changes in TNFalpha, IL-6, IL-10, plasminogen activator inhibitor-1, haptoglobin, intercellular adhesion molecule, or vascular cell adhesion molecule were seen between the groups.

CONCLUSIONS

Our data provide evidence that physiological leptin administration stimulates inflammatory and platelet responses in humans during caloric deprivation.

Leptin-mediated activation of human platelets: involvement of a leptin receptor and phosphodiesterase 3A-containing cellular signaling complex

An elevated circulating level of the adipocyte-derived satiety hormone leptin is an independent risk factor for cardiovascular disease. Because thrombus formation is a major cause of acute coronary events and leptin was shown previously to facilitate ADP-induced platelet aggregation, we chose to define the signaling events involved in leptin-mediated platelet activation. Using pharmacological, biochemical, and cell biological approaches, we show that leptin-induced platelet activation required activation of a signaling cascade that included the long form of the leptin receptor, three kinases [Janus kinase 2 (JAK2), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (PKB/Akt)], the insulin receptor substrate-1 (IRS-1), and the major human platelet cAMP phosphodiesterase phosphodiesterase 3A (PDE3A). Moreover, we identify a role for an intraplatelet LEPR/JAK2/IRS-1/PI3K/PKB/PDE3A molecular complex that allows for the selective leptin-mediated activation of platelets. Our data demonstrate that leptin promotes platelet activation, provides a mechanistic basis for the prothrombotic effect of this hormone, and identifies a potentially novel therapeutic avenue to limit obesity-associated cardiovascular disease.

Leptin rapidly activates PPARs in C2C12 muscle cells

Experimental evidence suggests that leptin operates on the tissues, including skeletal muscle, also by modulating gene expression. Using electrophoretic mobility shift assays, we have shown that physiological doses of leptin promptly increase the binding of C2C12 cell nuclear extracts to peroxisome proliferator-activated receptor (PPAR) response elements in oligonucleotide probes and that all three PPAR isoforms participate in DNA-binding complexes. We pre-treated C2C12 cells with AACOCF3, a specific inhibitor of cytosolic phospholipase A2 (cPLA2), an enzyme that supplies ligands to PPARs, and found that it abrogates leptin-induced PPAR DNA-binding activity. Leptin treatment significantly increased cPLA2 activity, evaluated as the release of [3H]arachidonic acid from pre-labelled C2C12 cells, as well as phosphorylation. Further, using MEK1 inhibitor PD-98059 we showed that leptin activates cPLA2 through ERK induction. These results support a direct effect of leptin on skeletal muscle cells, and suggest that the hormone may modulate muscle transcription also by precocious activation of PPARs through ERK-cPLA2 pathway.