TY - JOUR T1 - Megakaryocytic Smad4 Regulates Platelet Function Through Syk and ROCK2 Expression JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.116.107417 SP - mol.116.107417 AU - Yanhua Wang AU - Lirong Jiang AU - Xi Mo AU - Yu Lan AU - Xiao Yang AU - Xinyi Liu AU - Jian Zhang AU - Li Zhu AU - Junling Liu AU - Xiaolin Wu Y1 - 2017/01/01 UR - http://molpharm.aspetjournals.org/content/early/2017/06/29/mol.116.107417.abstract N2 - Smad4, a key transcription factor in the TGF-β signaling pathway, is involved in a variety of cell physiological and pathological processes. Here, we characterized megakaryocyte/platelet-specific Smad4 deficiency in mice to elucidate its effect on platelet function. We found that megakaryocyte/platelet-specific loss of Smad4 caused mild thrombocytopenia and significantly extended first occlusion time and tail bleeding time in mice. Smad4-deficient platelets showed reduced agonist-induced platelet aggregation. Further studies showed that severe defect was seen in integrin αIIbβ3-mediated bidirectional (inside-out and outside-in) signaling in Smad4-deficient platelets, as evidenced by reduced fibrinogen (Fg) binding and α granule secretion, suppressed platelet spreading and clot retraction. Microarray analysis showed that the expression levels of multiple genes were altered in Smad4-deficient platelets. Among these genes, spleen tyrosine kinase (Syk) and Rho-associated coiled-coil containing protein kinase 2 (ROCK2) were down-regulated several times as confirmed by quantitative RT-PCR and immunoblotting. Further research showed that Smad4 directly regulates ROCK2 transcription, but indirectly regulates Syk. Megakaryocyte/platelet-specific Smad4 deficiency caused decreased expression levels of Syk and ROCK2 in platelets. These results suggest potential links among Smad4 deficiency, attenuated Syk and ROCK2 expression and defective platelet activation. ER -