HDL, platelet progenitors, atherosclerosis and thrombosis

描述(申请人提供)：血小板在止血和与血小板相关的疾病中起着关键作用。活化的血小板促进单核细胞和中性粒细胞向动脉壁募集，促进动脉粥样硬化的形成。血小板也会在破裂或侵蚀的动脉粥样硬化斑块上形成血栓，导致动脉粥样硬化的临床并发症。一个显著的例子发生在骨髓增生性肿瘤(MPN)中，如原发性血小板增多症和原发性骨髓纤维化，其中血小板生成增加与显著的动脉血栓形成或动脉血栓形成相关。在普通人群中，异常的血小板产生也与心血管风险有更广泛的联系。即使有目前的预防和治疗干预措施，普通人群和MPN患者中仍有相当大的心血管疾病残留风险。高密度脂蛋白可能对预防冠心病有保护作用。关于高密度脂蛋白保护作用的一个主要假说是，它促进了病变动脉粥样瘤细胞的胆固醇外流。最近的研究表明，ATP结合盒转运体ABCA1和Abcg1在促进胆固醇从巨噬泡沫细胞外流到高密度脂蛋白中起着关键作用。ABCG4还促进胆固醇外流到高密度脂蛋白，但ABCG4在巨噬细胞中不表达。我们发现Abcg4在骨髓巨核系祖细胞(MKPs)中选择性表达。Abcg4-/-MkPs显示胆固醇外流到高密度脂蛋白的缺陷，并增加了质膜胆固醇。Abcg4-/-BM移植到高胆固醇血症的Ldlr-/-小鼠体内可导致血小板增多、动脉粥样硬化和动脉血栓形成。血小板增多反映MKP和巨核细胞池扩大，这是由于MKP细胞表面血小板生成素(TPO)受体(MPL)表达增加所致。这反映了E3泛素连接酶c-CBL对MPL的负反馈调节被钝化。进一步的研究表明，在Abcg4-/-细胞中，膜锚定的Lyn激酶通过与富含胆固醇的膜微区结合而受到抑制，从而导致c-CBL的活性受损。我们认为，高密度脂蛋白通过ABCG4促进MKPs的胆固醇外流，激活Lyn和c-CBL，促进MPL的泛素化和降解，并限制MPL介导的增殖信号对TPO的反应。高密度脂蛋白抑制血小板生成和抑制血小板增多症。我们已经提出了一系列研究来进一步验证这一假说，并评估高密度脂蛋白的输注或LYN的药理激活作为治疗动脉粥样硬化或由异常增加的血小板产生引起的动脉粥样硬化血栓的潜在治疗方法。药物激活Lyn激酶和rHDL输注将被评估为治疗因激活MPL和JAK2突变而导致的MPNS的潜在方法。我们期望拟议的研究将为动脉粥样硬化和与异常增加的血小板生成相关的动脉粥样硬化和动脉血栓形成提供新的见解和治疗策略。

DESCRIPTION (provided by applicant): Platelets play pivotal roles in hemostasis and platelet-related disorders. Activated platelets facilitate recruitment of monocytes and neutrophils to arterial wall and promote atherogenesis. Platelets also initiate the formation of thrombi on ruptured or eroded atherosclerotic plaques, causing clinical complications of atherosclerosis. A striking example occurs in myeloproliferative neoplasms (MPNs) such as essential thrombocytosis and primary myelofibrosis, in which increased platelet production is associated with prominent arterial thrombosis or athero-thrombosis. Aberrant platelet production has also been linked more broadly to cardiovascular risk in the general population. Substantial residual risks of cardiovascular disease in the general population and in patients with MPNs still remain, even with the access of current preventive and therapeutic interventions. HDL may be protective against coronary heart disease. A major hypothesis for the protective role of HDL is that it promotes cholesterol efflux from lesional atheroma cells. Recent studies indicate a key role of ATP-binding cassette transporters, ABCA1 and ABCG1 in promoting cholesterol efflux from macrophage foam cells to HDL. ABCG4 also promotes cholesterol efflux to HDL but ABCG4 is not expressed in macrophages. We found Abcg4 to be selectively expressed in bone marrow megakaryocyte progenitor cells (MkPs). Abcg4-/- MkPs showed defective cholesterol efflux to HDL and increased plasma membrane cholesterol. Abcg4-/- BM transplantation into hypercholesterolemic Ldlr-/- mice resulted in thrombocytosis, accelerated atherosclerosis and arterial thrombosis. Increased platelets reflected an expanded pool of MkPs and megakaryocytes, resulting from increased expression of the thrombopoietin (TPO) receptor (MPL) on the cell surface of MkPs. This reflected blunting of the negative feedback regulation of MPL by the E3 ubiquitin ligase, c-CBL. Further studies suggested that membrane-anchored Lyn kinase is inhibited by association with cholesterol-rich membrane microdomains in Abcg4-/- cells, resulting in impaired activation of c-CBL. We propose that HDL promotes cholesterol efflux from MkPs via ABCG4, activates Lyn and c-CBL, promotes ubiquitination and degradation of MPL and limits MPL-mediated proliferation signaling in response to TPO. HDL inhibits platelet production and suppresses thrombocytosis. We have proposed a series of studies to further test this hypothesis and assess infusion of HDL or pharmacological activation of LYN as potential treatment of atherosclerosis or atherothrombosis caused by aberrantly increased platelet production. Pharmacological activation of Lyn kinase and rHDL infusion will be assessed as potential therapy for MPNs induced by activating mutations of MPL and JAK2. We expect that the proposed studies will provide novel insights and therapeutic strategies for atherosclerosis and atherothrombosis associated with aberrantly increased platelet production.