The Src Kinases Play a Crucial Role in the Growth of Hematopoietic Cells Transformed with Constitutively Activated Stat5 Mutants.

Abstract 5041 Stat5A and Stat5B transcription factors (TF) play an important role in the control of hematopoietic cell proliferation and survival and are constitutively activated in number of hematological malignancies and solid tumours. Studies conducted with constitutively active (ca) Stat5 mutants (Stat51*6 and cS5F) have shown that deregulated Stat5 activity promotes leukemogenesis. In order to evaluate the role of Src kinases in the transforming properties of Stat5 TF, caStat5 expressing Ba/F3 cells and primary murine bone marrow cells were treated with the PP1 Src kinase inhibitor and with its inactive analogue PP3 as a control. We found that PP1 but not PP3 strongly inhibited Stat5A1*6- and Stat5B1*6- expressing Ba/F3 cell growth and survival while no changes were observed in IL-3 stimulated-parental Ba/F3 cells when treated with PP1. Similarly, we were able to demonstrate specific requirement of Src kinases in cS5F-induced primary bone marrow cell growth. We then examined the contribution of Src kinases to the phosphorylation of various signaling molecules involved in cell growth and survival like Stat5, PI3-K/Akt, Ras/Mapk and NF-kB. The treatment of caStat5 (Stat51*6 or cS5F)-expressing Ba/F3 cells with PP1 resulted in a strong decrease of Erk1/2 phosphorylation, but not of Stat5, Akt and IKK species. In addition, caStat5-expressing Ba/F3 cells were found to express a constitutive molecular complex comprising Stat5, Shc, Grb2, Sos, Erk, Gab2, p85, Lyn and Hck. Finally we found that the Lyn Src kinase was overexpressed in the caStat5-expressing cells. Our data suggest a direct implication of Src kinases in the proliferation of caStat5- Ba/F3 transformed cells through the Shc/Erk1/2 signaling pathway and that Src family members are required for caStat5 (Stat51*6 and cS5F)-induced hematopoietic cell growth. Disclosures No relevant conflicts of interest to declare.