Dual Signaling Role of the Protein Tyrosine Phosphatase SHP-2 in Regulating Expression of Acute-Phase Plasma Proteins by Interleukin-6 Cytokine Receptors in Hepatic Cells

Abstract

One of the major actions of interleukin-6 (IL-6) is the transcriptional activation of acute-phase plasma proteins (APP) genes in liver cells. Signaling by the IL-6 receptor is mediated through the signal transducing subunit gp130 and involves the activation of Janus-associated kinases (JAKs), signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein (MAP) kinase. Functional analysis of gp130 in rat hepatoma cells by using transduced chimeric G-CSFR-gp130 receptor constructs demonstrates that SHP-2, the Src homology 2 (SH2) domain-containing protein tyrosine phosphatase, acts as a negative regulator of the JAK/STAT signaling in part by downregulating JAK activity, thereby indirectly moderating the induction of STAT3-dependent APP genes. This study shows that in hepatoma cells, the recruitment and tyrosine phosphorylation of SHP-2, but not SHC, is the primary signaling event associated with the activation of MAP kinases (ERK1/2) by gp130. Overexpression of truncated SHP-2 that lacks Grb2-interacting sites, but not the full-length catalytically inactive SHP-2, reduces ERK activation by IL-6, confirming the signal-mediating role of SHP-2. Activation of ERK1/2 is correlated with induction of the immediate-early response genes. Stimulation of the c-fos, c-jun, and egr-1 genes is essentially absent in cells expressing gp130 with a Y759F mutation, which is unable to recruit SHP-2. Interestingly, both JAK/STAT and SHP-2 pathways regulate the induction of the junB gene. Moreover, disengagement of SHP-2 from gp130 signaling not only enhances APP gene induction but also further reduces cell proliferation, in part correlated with the attenuated expression of immediate-early response genes. These results suggest that IL-6 regulation of APP genes is affected by SHP-2 in two ways: SHP-2 acts as a phosphatase on the JAK/STAT pathway and serves as linker to the MAP kinase pathway, which in turn moderates APP production.

ACKNOWLEDGMENTS

We are greatly indebted to Immunex Corporation and Genetics Institute for generously providing cytokines; H. Ohnishi and G.-S. Feng for providing various SHP-2 constructs; S. Pruitt for providing probes for Egr-1, JunB, and c-Jun; A. Ullrich for providing anti-SIRP antibodies; Y. Wang and Erin Kinzie for providing assistance in experimental work; R. G. Hawley for generating recombinant G-gp130 retrovirus; C. Stewart and D. C. Sheedy for performing flow cytometric work; C.-F. Lai and O. Robledo for giving helpful advice; and L. Scere and M. Held for performing secretarial assistance.

This work was supported by NIH grant CA26122 to H.B. and Roswell Park grant CA16056.