PROTEIN-TYROSINE-PHOSPHATASE-2 (SHP-2) MODERATES SIGNALING BY GP130 BUT IS NOT REQUIRED FOR THE INDUCTION OF ACUTE-PHASE PLASMA-PROTEIN GENES IN HEPATIC CELLS

Signals propagated via the gp130 subunit of the interleukin-6 (IL-6)-t ype cytokine receptors mediate, among various cellular responses, prol iferation of hematopoietic cells and induction of acute-phase plasma p rotein (APP) genes in hepatic cells, Hematopoietic growth control by g p130 is critically dependent on activation of both STAT3 and protein t yrosine phosphatase 2 (SHP-2). To investigate whether induction of APP genes has a similar requirement for SHP-2, we constructed two chimeri c receptors, G-gp130 and G-gp130(Y2F), consisting of the transmembrane and cytoplasmic domains of gp130 harboring either a wild-type or a mu tated SHP-2 binding site, respectively, fused to the extracellular dom ain of the granulocyte colony-stimulating factor (G-CSF) receptor. Rat hepatoma H-35 cells stably expressing the chimeric receptors were gen erated by retroviral transduction. Both chimeric receptors transmitted a G-CSF-induced signal characteristic of that triggered by IL-6 throu gh the endogenous gp130 receptor; i.e., both activated the appropriate JAK, induced DNA binding activity by STAT1 and STAT3, and up-regulate d expression of the target APP genes, those for alpha-fibrinogen and h aptoglobin. Notwithstanding these similarities in the patterns of sign aling responses elicited, mutation of the SHP-2 interaction site in G- gp130(Y2F) abrogated ligand-activated receptor recruitment of SHP-2 as expected, Moreover, the tyrosine phosphorylation state of the chimeri c receptor, the associated JAK activity, and the induced DNA binding a ctivity of STAT1 and STAT3 were maintained at elevated levels and for an extended period of time in G-gp130(Y2F)-expressing cells following G-CSF treatment compared to, that in cells displaying the G-gp130 rece ptor, H-35 cells ectopically expressing G-gp130(Y2F) were also found t o display an enhanced sensitivity to G-CSF and a higher level of induc tion of APP genes, Overexpression of the enzymatically inactive SHP-2 enhanced the signaling by the wild-type but not by the Y2F mutant G-gp 130 receptor. These results indicate that gp130 signaling for APP gene induction in hepatic cells differs qualitatively from that controllin g the proliferative response in hematopoietic cells in not being stric tly dependent on SHP-2. The data further suggest that SHP-2 functions normally to attenuate gp130-mediated signaling in hepatic (and, perhap s, other) cells by moderating JAK action.