CIS3 and JAB have different regulatory roles in interleukin-6 mediated differentiation and STAT3 activation in M1 leukemia cells

We have reported JAK-signaling modulators, CIS1 (cytokine-inducible SH2 protein-1), CIS3 and JAB (JAK2 binding protein), which are structurally related. In M1 myeloid leukemia cells, CIS3 was induced by neither interleukin 6 (IL6) nor interferon gamma (IFN gamma), while JAB was induced strongly by IFN gamma and slightly by IL6 and leukemia inhibitory factor (ILF), Forced expression of CIS3 and JAB in Mi cells prevented IL6- or LIF-induced growth arrest and differentiation, even when their expression levels were comparable to endogenous ones in several cell lines such as HEL, UT-7, IFN gamma-treated Mi, and CTLL2 cells, Pretreatment of parental Mi cells with IFN gamma but not IFN beta resulted in suppression of LIF-induced STAT3 activation and differentiation, further supporting that physiological level of JAB is sufficient to inhibit LIF-signaling. However, unlike JAB, CIS3 did not inhibit IFN gamma-induced growth arrest, suggesting a difference in cytokine specificity between CIS3 and JAB. CIS3 inhibited STAT3 activation with slower kinetics than JAB and allowed rapid c-fos induction and partial Fc gamma RI expression in response to IL6, In 293 cells, CIS3 as well as JAB bound to JAK2 tyrosine kinase domain (JH1), and inhibited its kinase activity, however, the effect of CIS3 an tyrosine kinase activity was weaker than that of JAB, indicating that CIS3 possesses lower affinity to JAK kinases than JAB, These findings suggest that CIS3 is a weaker inhibitor than JAB against JAK signaling, and JAB and CIS3 possess different regulatory roles in cytokine signaling.