A new feature of Mpl receptor: ligand-induced transforming activity in FRErat fibroblasts

Mpl is the receptor for thrombopoietin, the primary regulator of platelet p roduction by megakaryocytes. Upon stimulation by its ligand, Mpl receptor i nduces proliferation and differentiation of hematopoietic cell lines of var ious origins. In this paper, we show that Mpl is also able to transform FRE rat fibroblasts in the presence of MGDF (pegylated Megakaryocyte Growth an d Development Factor), a modified form of its ligand. We also demonstrate t hat upon MGDF stimulation Mpl receptor activates the classical transduction pathways described for hematopoietic cell lines in FRE cells. Introduction of Mpl deletion mutants in FRE cells allowed us to demonstrate that the C- terminal region of the Mpl intracytoplasmic domain, which is involved in he matopoietic differentiation, is necessary for the transformation process. W ithin that region, site-directed mutagenesis showed that the Y112 residue, which is required for She phosphorylation, is essential for rat fibroblast transformation by Mpl/MGDF, suggesting the involvement of She in Mpl-mediat ed transformation. Interestingly, we showed that transformation correlated with strong and sustained MAPK activation. Neither Jak2 Stat3 nor Stat5 pho sphorylation was sufficient to induce the transformation process. Taken alt ogether, our results suggest the oncogenicity of Mpl in fibroblastic cells in the presence of its ligand.