Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis

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Abstract
Nan Wang and colleagues uncover a new function for HDL that may contribute to its anti-atherosclerotic effects. In the bone marrow, HDL removes cholesterol from megakaryocyte progenitor cells in a process requiring the cholesterol transporter ABCG4, thereby dampening megakaryocyte and platelet production. Platelets have a key role in atherogenesis and its complications. Both hypercholesterolemia and increased platelet production promote atherothrombosis; however, a potential link between altered cholesterol homeostasis and platelet production has not been explored. Here we show that transplantation of bone marrow deficient in ABCG4, a transporter of unknown function, into Ldlr−/− mice resulted in thrombocytosis, accelerated thrombosis and atherosclerosis. Although not detected in atherosclerotic lesions, Abcg4 was highly expressed in bone marrow megakaryocyte progenitors (MkPs). Abcg4−/− MkPs had defective cholesterol efflux to high-density lipoprotein (HDL), increased cell surface expression of the thrombopoietin (TPO) receptor (c-MPL) and enhanced proliferation. These consequences of ABCG4 deficiency seemed to reflect disruption of negative feedback regulation of c-MPL signaling by the E3 ligase c-CBL and the cholesterol-sensing LYN kinase. HDL infusion reduced platelet counts in Ldlr−/− mice and in a mouse model of myeloproliferative neoplasm in an ABCG4-dependent fashion. HDL infusions may offer a new approach to reducing atherothrombotic events associated with increased platelet production.