A thrombopoietin receptor mutant deficient in Jak-STAT activation mediatesproliferation but not differentiation in UT-7 cells

Thrombopoietin (TPO) stimulates proliferation and differentiation of cells of the megakaryocytic lineage, It exerts its function by binding and activa ting c-mpl, a member of the hematopoietic receptor superfamily. Upon bindin g of TPO to its receptor, numerous signaling events are triggered, These in clude activation of the Jak-STAT (signal transducers and activators of tran scription) pathway, mitogen-activated protein kinase (MAPK), Tee, and phosp atidylinositol (Pl) 3-kinase and phosphorylation of Shc and Vav. The contri bution of different signaling pathways to the induction of specific cellula r processes such as proliferation and differentiation is incompletely under stood. We have previously described a mutant of c-mpl that fails to activat e the Jak-STAT pathway but nevertheless retains its ability to mediate prol iferation and activation of most signaling events in the murine hematopoiet ic precursor cell lines BAF/3 and 32D, We confirm here the ability of this mutant to mediate proliferation in the absence of Jak-STAT activation in th e human cell line UT-7 and further show that this mutant fails to mediate T PO-induced megakaryocytic differentiation. Comparison of the signaling capa city of this mutant in UT-7 and BAF/3 cells shows considerable cell-type-sp ecific differences. Whereas in BAF/3 cells the mutant still mediates activa tion of Shc, MAPK, Vav, and Pl 3-kinase at levels comparable to the wild-ty pe receptor, these events are strongly diminished in UT-7 cells expressing the mutant. Furthermore, we show that the C-terminal 25 amino acid residues of the receptor mutant are crucial for the mitogenic response in UT-7 cell s. (C) 1999 by The American Society of Hematology.