Overexpression of Jak Kinases Promotes In Vitro Transformation.

The Jak-STAT pathway is responsible for signal transduction by a large number of cytokine receptors. While this pathway is normally tightly regulated, constitutive STAT3 and STAT5 activation is frequent in hematologic malignancies and contributes to oncogenesis. We took advantage of the IL-3-dependent pro-B Ba/F3 cell line to analyze the mechanisms of constitutive STAT activation in vitro. Ba/F3 cells transfected with a mutated hIL-9R (Ba/F3 Phe116) poorly respond to IL-9. However, after selection with this cytokine, we obtained cell lines that proliferated well in IL-9. After cytokine withdrawal, those cells, in contrast to the parental cells, gave rise to autonomous Ba/F3 cell lines showing constitutive STAT5 activation, and that were highly tumorigenic in vivo. This process mimics the multistep process of oncogenesis occurring in vivo. To dissect the mechanisms involved in this cellular transformation, we analyzed, using cDNA microarrays, genes expressed in one of these autonomous lines. Jak1, the Jak kinase associated to the IL-9 receptor, was found to be upregulated compared to parental cells. It was the key genetic event involved in this transformation since ectopic Jak1 overexpression increased the sensitivity to IL-9 and allowed, after a second selection step, for spontaneous progression towards autonomous cell lines with constitutive STAT activation. Using Jak1 mutants, we showed that the generation of autonomous lines was dependent on the tyrosine kinase activity of Jak1 and on a functional FERM domain, the domain that mediates association with the receptor. These results were extended to the other Jak family members. Therefore, we propose that Jak overexpression can be considered as one of the oncogenic events leading to the selection of cells highly responsive to growth factors and, after a second selection step, to autonomous cells with constitutive activation of the Jak-STAT pathway. This process could be extended to human malignancies and might explain, for instance, the constitutive STAT6 activation in primary mediastinal large B cell lymphomas where Jak2 overexpression was described.