Stimulus-specific defect in platelet aggregation in polycythemia vera

Polycythemia vera: a comprehensive review and clinical recommendations

More than a century has elapsed since the appearance of the modern descriptions of polycythemia vera (PV). During this time, much has been learned regarding disease pathogenesis and PV-associated molecular aberrations. New information has allowed amendments to traditional diagnostic criteria. Phlebotomy remains the cornerstone treatment of PV, whereas myelosuppressive agents may augment the benefit of using phlebotomy for thrombosis prevention in high-risk patients. Excessive aspirin use is contraindicated in PV, although the use of lower-dose aspirin has been shown to be safe and effective in alleviating microvascular symptoms including erythromelalgia and headaches. Recent studies have shown the utility of selective serotonin receptor antagonists for treating PV-associated pruritus. Nevertheless, many questions remain unanswered. What is the specific genetic mutation or altered molecular pathway that is causally related to the disease? In the absence of a specific molecular marker, how is a working diagnosis of PV made? What evidence supports current practice in the management of PV? This article summarizes both old and new information on PV; proposes a modern diagnostic algorithm to formulate a working diagnosis; and provides recommendations for patient management, relying whenever possible on an evidence-based approach.

Pathogenetic mechanisms of polycythemia vera and congenital polycythemic disorders

The absolute polycythemias--those with increased red blood cell mass--can be divided into two groups: primary, caused by acquired or inherited mutations leading to a "gain-of-function" abnormalities expressed within the erythroid progenitors; and secondary, due to circulating serum factors, typically erythropoietin, stimulating erythropoiesis. This overview concentrates on the molecular biology of polycythemia vera (PV) discussed in the context of other polycythemic disorders. Recent advances in the regulation of erythropoiesis, as they may relate to polycythemic states, are discussed as a background for those well-defined polycythemic states wherein the molecular defect has not yet been elucidated. A number of cellular abnormalities associated with PV, including the hyperresponsiveness of PV progenitors to many cytokines as well as decreased expression of the thrombopoietin receptor on platelets and increased expression of Bcl-xL, suggest that the PV defect alters a number of cellular functions and is not restricted to cytokine receptor signal transduction. The increasing number of recognized instances of familial incidence of PV suggests that in these families the predisposition for PV is inherited as a dominant trait, and that PV is acquired as a new mutation that leads to a clonal hematopoiesis and may be due to loss of heterozygosity. The existence of these families provides a unique opportunity for isolation of the mutations in the gene leading to PV. Semin Hemaol 38(suppl 2):10-20.

Molecular basis for polycythemia

This overview concentrates on familial and congenital polycythemias in the context of other polycythemic disorders, with emphasis on those with established molecular lesions. Recent advances in the regulation of erythropoiesis, as they may relate to polycythemic states, are discussed as a background for those well-defined polycythemic states wherein the molecular defect has not yet been elucidated. Primary familial congenital polycythemias and congenital and familial secondary polycythemias, including hemoglobin mutants, methemoglobinemias and congenital 2,3-bisphosphoglycerate deficiency, are discussed. The most common primary polycythemia, polycythemia vera, as well as the only likely endemic congenital secondary polycythemia, known as Chuvash polycythemia, are discussed.

Impaired platelet binding of fibrinogen due to a lower number of GPIIB/IIIA receptors in polycythemia vera

We have previously described a stimulus-specific defect in platelet aggregation in polycythaemia vera (PV) after stimulation with surface receptor dependent agonists such as platelet activating factor (PAF). In contrast, responses to phorbol myristate acetate (PMA) were normal. We now report that after PAF stimulation, using flow cytometry, the amount of fibrinogen bound to its receptor was significantly lower in PV platelets with a median MFI of 6.0 (range 4.1-17.3) compared to controls, 12.8 (range 8-21.3; n=11; p<0.01). We found no evidence of preactivation of PV platelets. Quantitative analysis of GPIIIa gave a significantly lower number of GPIIIa on resting PV platelets, 14300 subunits of GPIIIa (range 8500-15500) vs. 19800 for controls (range 13400-26800; n=12; p<0.01). Both patients and controls increased their number of receptors on the cell surface after stimulation with PAF and PMA, but the significant difference in the number of receptors per cell remained. Indirect evaluation of PAF receptor function showed that activation of CD 62 did not differ in PV and controls after PAF stimulation. Additionally, although the basal level of serotonin in platelet-rich plasma was significantly lower in PV, there was a threefold increase of the basal level after stimulation with PAF for both PV and control platelets, also indicating a normal interaction of PAF with its receptor. Although our results indicate both an impaired PAF induced aggregation in PV and a lower number of GPIIb/IIIa complexes on single platelets, whether these phenomena are related remains uncertain.

Impaired activation of phospholipase D in polycythaemia vera-implications for the pathogenesis of the disease?

A series of studies have demonstrated a stimulus-specific defect in PMN oxidative metabolism after stimulation with surface receptor dependent stimuli such as fMLP, leukotriene B4 and platelet activating factor (PAF), whereas the response to phorbol myristate acetate was normal. Having discovered this defect, studies of the stimulus response coupling for oxidative responses were performed showing a normal interaction of fMLP with it's receptor, as well as an intact activation of phospholipase C, as measured by the generation of 1,4,5-inositoltrisphosphate, and the subsequent rise of intracellular calcium. In contrast, the formation of diacylglycerol and phosphatidylethanol was decreased in PV PMN, denoting an impaired activation of phospholipase D (PLD). It was shown by flow cytometry analyses that the hampered oxidative response was present both in single PMN and monocytes. Moreover, platelets from PV patients, whose PMN exhibit a lower oxidative response to PAF, also have a diminished aggregatory response to PAF. Thus three different cell lineages in PV have been revealed to respond abnormally to surface receptor dependent stimuli, indicating that the proposed impairment of PLD might be relevant for changes in the malignant stem cell clone. Since phosphatidic acid, produced as a result of PLD activation, may be implicated in the regulation of several oncogenes, perturbations of the PLD system could theoretically be important for the development of PV.

1-O-hexadecyl-2-metoxy-glycero-3-phosphatidylcholine--a methoxy ether lipid inhibiting platelet activating factor-induced platelet aggregation and neutrophil oxidative metabolism

Whether or not two alkylglycerols could initiate a functional response in human platelets or modify responses induced by platelet activating factor (PAF) was evaluated. It was found that 1-100 microM 1-O-hexadecyl-2-metoxy-glycero-3-phosphatidylcholine (Et-16-OCH3) induced platelet aggregation but 1-O-hexadecyl-sn-glycerol (chimyl alcohol; CA) did not. Et-16-OCH3-induced platelet aggregation was abolished by pretreatment with the PAF receptor antagonist WEB 2086. While CA had no effect on platelet aggregation induced by PAF, pretreatment with Et-16-OCH3 (0.1 microM or higher) significantly inhibited platelet aggregation induced by PAF, but had no effect on aggregation caused by ADP, thrombin or phorbol myristate acetate (PMA). A receptor binding study using radiolabelled [3H]WEB 2086 showed that Et-16-OCH3 exerts its actions through interaction with the PAF receptor. Moreover, Et-16-OCH3 inhibited neutrophil chemiluminescence responses induced by PAF, but not reactions to PMA or a formyl peptide. Finally, 1 microM Et-16-OCH3 induced a rise in the intracellular calcium concentration in platelets equal to that induced by PAF and also had an calcium ionophore-like effect at 100 microM. Thus, this study shows that Et-16-OCH3 is both a potent inducer of platelet aggregation and an inhibitor of PAF-induced platelet aggregation and neutrophil chemiluminescence, through interaction with the PAF receptor.