ABC transporters in megakaryopoiesis and platelet activity

ABCC4 impacts megakaryopoiesis and protects megakaryocytes against 6-mercaptopurine induced cytotoxicity

The role of ABCC4, an ATP-binding cassette transporter, in the process of platelet formation, megakaryopoiesis, is unknown. Here, we show that ABCC4 is highly expressed in megakaryocytes (MKs). Mining of public genomic data (ATAC-seq and genome wide chromatin interactions, Hi-C) revealed that key megakaryopoiesis transcription factors (TFs) interacted with ABCC4 regulatory elements and likely accounted for high ABCC4 expression in MKs. Importantly these genomic interactions for ABCC4 ranked higher than for genes with known roles in megakaryopoiesis suggesting a role for ABCC4 in megakaryopoiesis. We then demonstrate that ABCC4 is required for optimal platelet formation as in vitro differentiation of fetal liver derived MKs from Abcc4-/- mice exhibited impaired proplatelet formation and polyploidization, features required for optimal megakaryopoiesis. Likewise, a human megakaryoblastic cell line, MEG-01 showed that acute ABCC4 inhibition markedly suppressed key processes in megakaryopoiesis and that these effects were related to reduced cAMP export and enhanced dissociation of a negative regulator of megakaryopoiesis, protein kinase A (PKA) from ABCC4. PKA activity concomitantly increased after ABCC4 inhibition which was coupled with significantly reduced GATA-1 expression, a TF needed for optimal megakaryopoiesis. Further, ABCC4 protected MKs from 6-mercaptopurine (6-MP) as Abcc4-/- mice show a profound reduction in MKs after 6-MP treatment. In total, our studies show that ABCC4 not only protects the MKs but is also required for maximal platelet production from MKs, suggesting modulation of ABCC4 function might be a potential therapeutic strategy to regulate platelet production.

Activation of soluble guanylyl cyclase with inhibition of multidrug resistance protein inhibitor-4 (MRP4) as a new antiplatelet therapy

The intracellular levels of cyclic GMP are controlled by its rate of formation through nitric oxide-mediated stimulation of soluble guanylate cyclase (sGC) and its degradation by phosphodiesterases. Multidrug resistance protein 4 (MRP4) expressed in human platelets pumps cyclic nucleotides out of cells. In search for new antiplatelet strategies, we tested the hypothesis that sGC activation concomitant with MRP4 inhibition confers higher antiplatelet efficacy compared with monotherapy alone. This study was undertaken to investigate the pharmacological association of the sGC activator BAY 60-2770 with the MRP4 inhibitor MK571 on human washed platelets. Collagen- and thrombin-induced platelet aggregation and ATP-release reaction assays were performed. BAY 60-2770 (0.001-10 µM) produced significant inhibitions of agonist-induced platelet aggregation accompanied by reduced ATP-release. Pre-incubation with 10 µM MK571 alone had no significant effect on platelet aggregation and ATP release, but it produced a left displacement by about of 10-100-fold in the concentration-response curves to BAY 60-2770. Pre-incubation with MK571increased and decreased, respectively, the intracellular and extracellular levels of cGMP to BAY 60-2770, whereas the cAMP levels remained unchanged. The increased VASP-serine 239 phosphorylation in BAY 60-2770-treated platelets was enhanced by MK571. In Fluo-4-loaded platelets, BAY 60-2770 reduced the intracellular Ca2+ levels, an effect significantly potentiated by MK571. Flow cytometry assays showed that BAY 60-2770 reduces the αIIbβ3 integrin activation, which was further reduced by MK571 association. Blocking the MRP4-mediated efflux of cGMP may be a potential mechanism to enhance the antiplatelet efficacy of sGC activators.