Indeed, IFN-α did not adversely affect the total pY-STAT6 levels induced by IL-4, as compared to IFN-γ, which significantly suppressed the IL-4-induced pY-STAT6 levels (Fig S1-B). Such differential actions of IFN-α and IFN-γ on STAT6 phosphorylation were previously observed in human primary B cells 21. This may be due to the different capacity of IFN-γ and IFN-α for the induction of SOCS proteins in B cells. While IFN-γ is a potent inducer of SOCS proteins in various cell types, the induction of SOCS by IFN-α
seems to be limited to certain cells. In fact we failed to observe a significant induction of SOCS1 or SOCS3 by IFN-α in Ramos B cells by 8 h (data not shown), which correlates with no effects of IFN-α on the IL-4-induced STAT6 phosphorylation up to 8 h (Supporting Information Fig. S2). Considering the potential inhibitory function of SOCS1 or SOCS3 on Jak activation and the selleck inhibitor lack of SOCS induction by IFN-α, it is reasonable to see no changes in Jak1/Jak3 phosphorylation levels in B cells pretreated with IFN-α (Fig. 2A). In support of this notion, a modest inhibitory effect of IFN-α on the IL-4-induced pY-STAT6 levels was observed in PBMCs containing diverse cell types (Fig. S4). With a small decrease in total pY-STAT6 levels, both cytoplasmic and nuclear pY-STAT6 levels were reduced
without cytoplasmic retention of pY-STAT6 in PBMCs and isolated primary B cells (Supporting Information Fig. S4 and data not shown). These observations suggest that the cytosolic retention of pY-STAT6 through a complex Selleck Ridaforolimus formation with pY-STAT2, resulting in the inhibition of nuclear translocation of activated STAT6 by IFN-α seen in Ramos cells, may be a characteristic of transformed B-cell lines representing a specific stage of B-cell differentiation. IFN-α is capable of inducing STAT6 activation in the early phase of signal transduction, which is implicated in the enhancement of the biological response of IL-4, or in the induction of antiproliferative effect of IFN-α 11, 24. In line with this finding, a STAT6:STAT2 complex induced by IFN-α treatment alone has been
described in B cells, which binds to both IRF1 GAS and CD23b GAS in EMSA, representing the Dipeptidyl peptidase IFN-α-responsive and the IL-4-responsive element, respectively. However, the role of such STAT complex in the transcriptional activation or target gene expression was not examined. In these studies, the complex was found physically associated with the IFN-α receptor upon ligand stimulation, suggesting a direct activation of STAT6 by IFN-α 11, 24. On the other hand, we have identified the complex containing pY-STAT6 and pY-STAT2 during the inhibition of IL-4 signaling by IFN-α and vice versa. Moreover, it is noted that pY-STAT6 dissociates from the activated IL-4R upon the treatment with IFN-α in a time-dependent manner by 4 h (Supporting Information Fig. S5).