HDL, platelet progenitors, atherosclerosis and thrombosis

描述（由申请方提供）：血小板在止血和血小板相关疾病中发挥关键作用。活化的血小板促进单核细胞和中性粒细胞向动脉壁的募集，并促进动脉粥样硬化的形成。动脉粥样硬化还可在破裂或侵蚀的动脉粥样硬化斑块上引发血栓形成，引起动脉粥样硬化的临床并发症。一个突出的例子发生在骨髓增生性肿瘤（MPN），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粥样硬化血栓形成有关。血小板生成异常也与一般人群中的心血管风险有更广泛的联系。即使在目前的预防和治疗干预措施的情况下，普通人群和MPN患者的心血管疾病的大量剩余风险仍然存在。高密度脂蛋白可以预防冠心病。HDL的保护作用的一个主要假设是它促进胆固醇从病变粥样硬化细胞流出。最近的研究表明ATP结合盒转运蛋白ABCA 1和ABCG 1在促进巨噬细胞泡沫细胞向HDL的胆固醇流出中起关键作用。ABCG 4也促进胆固醇流出到HDL，但ABCG 4在巨噬细胞中不表达。我们发现Abcg 4在骨髓巨核祖细胞（MkPs）中选择性表达。Abcg 4-/- MkPs显示胆固醇向HDL的流出缺陷和质膜胆固醇增加。Abcg 4-/- BM移植到高胆固醇血症Ldlr-/-小鼠中导致血小板增多、加速动脉粥样硬化和动脉血栓形成。血小板增加反映了MkP和巨核细胞的扩增池，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负反馈调节的钝化。进一步的研究表明，膜锚定的林恩激酶通过与Abcg 4-/-细胞中富含胆固醇的膜微区结合而被抑制，导致c-CBL的活化受损。我们认为HDL通过ABCG 4促进胆固醇从MkPs流出，激活林恩和c-CBL，促进MPL的泛素化和降解，并限制响应TPO的MPL介导的增殖信号传导。HDL抑制血小板生成和抑制血小板增多。我们已经提出了一系列的研究，以进一步验证这一假设，并评估HDL的输注或林恩的药理学活化作为动脉粥样硬化或血小板生成异常增加引起的动脉粥样硬化血栓形成的潜在治疗。将评估林恩激酶的药理学激活和rHDL输注作为MPL和JAK 2激活突变诱导的MPN的潜在治疗。我们期望这些研究将为动脉粥样硬化和血小板生成异常增加相关的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.