Cytokine receptor common β subunit-mediated STAT5 activation confers NF-κB activation in murine proB cell line Ba/F3 cells

The cytokine receptor common 0 subunit (P.) transmits intracellular signals upon binding ligand such as granulocyte-macrophage colony-stimulating factor or interleukin-3 (IL-3); however, transcriptional regulation under the control of signaling events downstream of the P,, is not fully understood. Using murine Ba/F3 cells, here we demonstrate that the beta(c)-mediated signals stimulate NF-kappaB-driven gene expression of not only the reporter construct but also endogenous target genes such as IL-6. Analyzing the effects of several inhibitors or mutant receptors revealed that this NF-kappaB activation is mediated neither by MEK/ERK/MAPK nor by the phosphatidylinositol 3-kinase pathway but by STAT5. Overexpression experiments of the wild-type or constitutive active form of STAT5 further confirmed this notion. In addition, STAT5-dependent NF-kappaB activation is mediated not through an inducible nuclear translocation but via up-regulation of both DNA binding activity and transactivation potential of NF-kappaB. Furthermore, we also show that as yet undefined humoral factor(s) may be involved in this NF-kappaB activation process. Taken together, we may propose that cytokine receptor-mediated STAT5 activation and expression of its target genes culminates in a unique mode of NF-kappaB activation and gene expression.