engleski

The hepatic Ashwell-Morell receptor regulates thrombopoietin production.

Introduction: The highly conserved hepatic Ashwell-Morell (asialoglcyoprotein) receptor (AMR) can bind and remove blood asialoglycoproteins although the identity of endogenous ligands hasbeen elusive. We previously showed that desialylated cold-stored platelets are ingested by hepatic AMR. We now hypothesized that desialylated platelets serve as communicators between the AMR to stimulate thrombopoietin (Tpo) production and to regulate bone marrow homeostasis. Results: (i) Desialylated human platelet uptake by AMR stimulates hepatic Tpo mRNA translation in vitro. Human platelets were desialyated using a2-3, -6, -8 sialidase from Clostridium perfringens or left untreated and incubated with HepG2 cells in vitro. The relative ratio of Tpo mRNA/CycloA mRNA in the human hepatic cell line HepG2 was determined and compared. Tpo mRNA expression increased 30 min after addition of desialylated platelets, and further increased by 2.2 and 2.9-fold after 4 and 6 h, respectively. In marked contrast, Tpo mRNA translation in HepG2 cells incubated with control platelets was only slightly increased. (ii) Transfusion of desialylated (using a2-3, -6, -8 sialidase) mouse platelets into wild type (WT) mice stimulates hepatic Tpo mRNA expression in situ. Hepatic Tpo mRNA levels increased by ~50% after 12 h of platelet infusion into WT mice, and plasma Tpo levels increased by 36% (24 h) to 58% (48 h) following infusion of desialylated platelets. (iii) Endogenously desialylated platelets were isolated from St3gal4
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or AMR null mice (Asgr2
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) mice. These mice lack either the critical sialyltransferase ST3GalIV, which adds sialic acid to glycoproteins (St3gal4
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) or the ability to remove senile, desialylated platelets (Asgpr2
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). Desialylated platelets isolated from the St3gal4
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or Asgr2
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mice and infused into WT mice caused an increase in hepatic Tpo mRNA levels at 12 h posttransfusion. Plasma Tpo concentrations increased in parallel with Tpo mRNA levels, peaking by day two post-infusion. Desialylated platelets given to Asgr2
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mice had no effect on Tpo mRNA synthesis or on Tpo plasma levels. WT, but not mice receiving desialylated platelets rapidly (within 24 h) released new platelets into blood and on day 10 post-treatment, corresponding to the Tpo plasma increase. Accordingly, following desialylated platelet transfusion, megakaryocyte numbers increase in the bone marrow of WT but not of Asgr2
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mice. (iv) Desialylated platelet uptake by AMR regulates hepatic Tpo mRNA expression in situ, as evidenced by reduced hepatic Tpo mRNA levels were reduced by ~50% in Asgr2
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mice which lack the ability to take up desialylated platelets. In contrast, livers isolated from St3gal4
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mice in which platelet turnover is accelerated due to inefficient membrane glycoprotein sialyation and the uptake by the AMR receptor, exhibit increased Tpo mRNA levels relative to WT livers. The difference in the Tpo mRNA levels between the two mouse genotypes reveals that platelet uptake stimulates a 1.9-fold increase in Tpo mRNA expression. Conclusion: Our data support the hypothesis that uptake of desialylated platelets via AMR stimulates Tpo mRNA expression. We conclude, that platelets desialylate as they circulate and become the primary AMR ligand in blood, thus providing a novel physiological feedback mechanism to regulate plasma Tpo levels and platelet production.

platelets; Thrombopoetin; AMR

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