HDL, platelet progenitors, atherosclerosis and thrombosis

描述（由申请人提供）：血小板在止血和血小板相关疾病中起关键作用。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发生。血小板也能在破裂或侵蚀的动脉粥样硬化斑块上启动血栓的形成，引起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和原发性骨髓纤维化，其中血小板生成增加与明显的动脉血栓形成或动脉粥样硬化血栓形成有关。在一般人群中，异常的血小板生成也与心血管风险有更广泛的联系。即使有了目前的预防和治疗干预措施，普通人群和mpn患者中仍存在大量心血管疾病的残余风险。高密度脂蛋白可能对冠心病有保护作用。HDL的保护作用的一个主要假设是，它促进胆固醇从病变动脉粥样硬化细胞外排。最近的研究表明，atp结合盒转运蛋白ABCA1和ABCG1在促进巨噬细胞泡沫细胞向HDL的胆固醇外泄中起关键作用。ABCG4也促进胆固醇向HDL的外排，但ABCG4不在巨噬细胞中表达。我们发现Abcg4在骨髓巨核细胞祖细胞（MkPs）中选择性表达。Abcg4-/- MkPs显示胆固醇向HDL的外排缺陷和质膜胆固醇升高。Abcg4-/- BM移植到高胆固醇血症ldl -/-小鼠体内可导致血小板增多、动脉粥样硬化加速和动脉血栓形成。血小板的增加反映了MkPs和巨核细胞池的扩大，这是由于MkPs细胞表面血小板生成素（TPO）受体（MPL）的表达增加。这反映了E3泛素连接酶c-CBL对MPL负反馈调节的钝化。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域结合而受到抑制，导致c-CBL的激活受损。我们提出HDL通过ABCG4促进胆固醇从MkPs流出，激活Lyn和c-CBL，促进MPL的泛素化和降解，并限制MPL介导的TPO增殖信号。高密度脂蛋白抑制血小板产生和抑制血小板增多。我们已经提出了一系列的研究来进一步验证这一假设，并评估输注HDL或药理激活LYN作为治疗由血小板生成异常增加引起的动脉粥样硬化或动脉粥样硬化血栓的潜在方法。林恩激酶的药理激活和rHDL输注将被评估为MPL和JAK2激活突变诱导的mpn的潜在治疗方法。我们期望所提出的研究将为与血小板生成异常增加相关的动脉粥样硬化和动脉粥样硬化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.