Increased megakaryocytopoiesis in Lyn-deficient mice.

Previous studies in cell lines have shown Lyn kinase to be a negative regulator of thrombopoietin (TPO)-induced proliferation. To further investigate the role of Lyn during megakaryocytopoiesis, Lyn-deficient mice (lyn -/- ) were analyzed. We observed that lyn / mice have more bone marrow-derived GPIIB (CD41) and Mpl + cells when compared to their wild-type littermates. In addition, colony-forming unit-megakaryocytes (CFU-MK) are increased and TPO-induced expansion of primary marrow cells yielded a greater number of mature megakaryocytes (MKs) with increased nuclear ploidy. Histopathology of bone marrow and spleens from lyn -/- mice showed an increase in the number of MKs. Mechanistic studies revealed that TPO stimulation of MKs from lyn / mice did not affect phosphorylation of Janus kinase 2 (JAK2), signal transducer and activator of transcription (STAT) 3, STATS, or MAP kinase kinase (MEK). Lyn-deficient MKs supported greater TPO-mediated phosphorylation and kinase activity of both Erkl/2 (mitogen-activated protein kinase, MAPK) and Akt. In contrast, there was a reduction of tyrosine phosphorylation of the inositol phosphatase, SHIP. This is the first direct evidence using primary MKs from Lyn-deficient mice that confirms our prior data from cell lines that Lyn kinase is a negative regulator of TPO signaling.