Shp-2 tyrosine phosphatase functions as a negative regulator of the interferon-stimulated Jak/STAT pathway

Shp-2 is an SH2 domain-containing protein tyrosine phosphatase. Although th e mechanism remains to be defined, substantial experimental data suggest th at Shp-2 is primarily a positive regulator in cell growth and development. We present evidence here that Shp-2, while acting to promote mitogenic sign als, also functions as a negative effector in interferon (IFN)-induced grow th-inhibitory and apoptotic pathways. Treatment of mouse fibroblast cells l acking a functional Shp-2 with IFN-alpha or IFN-gamma resulted in an augmen ted suppression of cell viability compared to that of wild-type cells. To d issect the molecular mechanism, we examined IFN-induced activation of signa l transducers and activators of transcription (STATs) by electrophoretic mo bility shift assay, using a specific DNA probe (hSIE). The amounts of STAT proteins bound to hSIE upon IFN-alpha or IFN-gamma stimulation were signifi cantly increased in Shp-2(-/-) cells. Consistently, tyrosine phosphorylatio n levels of Stat1 upon IFN-gamma treatment and, to a lesser extent, upon IF N-alpha stimulation were markedly elevated in mutant cells. Furthermore, IF N-gamma induced a higher level of caspase 1 expression in Shp-2(-/-) cells than in wild-type cells. Reintroduction of wild-type Shp-2 protein reversed the hypersensitivity of Shp-2(-/-) fibroblasts to the cytotoxic effect of IFN-alpha and IFN-gamma. Excessive activation of STATs by IFNs was also dim inished in mutant cells in which Shp-2 had been reintroduced. Together, the se results establish that Shp-2 functions as a negative regulator of the Ja k/STAT pathway. We propose that Shp-2 acts to promote cell growth and survi val through two mechanisms, i.e, the stimulation of growth, factor-initiate d mitogenic pathways and the suppression of cytotoxic effect elicited by cy tokines, such as IFNs.