Das Tyrosin 759 des Signaltransduktors gp130 in der IL-6-abhängigen Proliferation von Prä-B-Zellen

IL-6 signaltransduction requires a cytokine-receptor complex containing the signal transducing receptor subunit glycoprotein (gp) 130. Binding of IL-6 to its specific a-receptor induces dimerization of gp130, activation of th gp130-associated Janus kinases Jak1, Jak2 and Tyk2 and phosphorylation of gp130 at cytoplasmic tyrosine residues. These phosphotyrosines serve as docking sites for SH2-domain containing proteins such as STAT1, STAT3, the protein tyrosine phosphatase SHP2 and the feedback inhibitor SOCS3. SHP2 tyrosine phosphorylation is crucial for the activation of the Ras/Raf/MAPK pathway and thus proliferation and differentiation after IL-6 stimulation. Furthermore SHP2 is involved in negative regulation of the signal transduction through the Jak/STAT pathway. Negative regulation of IL-6 signal transduction through SHP2 and SOCS3 requires the tyrosine 759 of gp130. Interleukin-6 induces B-cell proliferation. There have been contradictory results in revealing essential factors and signaling pathways for proliferation depending on IL-6. We could recently demonstrate that the presence of a single STAT-recruitement site within gp130 is sufficient for IL-6 induced proliferation of Ba/F3 cells (Schmitz et al. (2000), J. Immunol. 164: 848-854). In clear contrast, the requirement of SHP2-activation for the proliferation of pre-B Ba/F3 cells has been reported using deleted chimere receptors (Fukada et al. (1996) Immunity, 5: 449-460). To unravel this discrepancy we analysed IL-6-induced dose-dependent proliferation of Ba/F3 cells through receptor complexes lacking the SHP2/SCS3 recruitment sites. Therefore murine pre-B-cells (Ba/F3 cells) that do not express endogenous gp130 were stable transfected with gp130 (Ba/F3-gp130) or a gp130 Y759F mutant lacking the tyrosine motif essential for the recruitement of SHP2 and SOCS3 (Ba/F3-gp130(Y759F)). As expected, after IL-6 stimulation there was neither SHP2 nor ERK1/2 phosphorylation apparent in cells expressing the mutated gp130 receptor. In contrast to the published data of Fukada et al. we observed IL-6-induced proliferation of pre-B-cells in the absence of SHP2 phosphorylation. Interestingly, Ba/F3 cells expressing gp130 receptor mutants lacking Y759 proliferate even much more efficient at low concentrations of IL-6 than cells expressing wild type gp130. Our data demonstrate that the tyrosine 759 of gp130 is not essential for IL-6-induced proliferation of transfected Ba/F3 cells but rather a mediator of inhibition of cell proliferation at low doses of IL-6. Therefore, SHP2 activation appears to be relevant for IL-6-induced proliferation only after stimulation with very large (unphysiological) amounts of IL-6 as used by Fukada et al