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Mol. Cell. Biol., Jul 1997, 3833-3840, Vol 17, No. 7
LF Stancato, M Sakatsume, M David, P Dent, F Dong, EF Petricoin, JJ Krolewski, O Silvennoinen, P Saharinen, J Pierce, CJ Marshall, T Sturgill, DS Finbloom and AC Larner
Activation of early response genes by interferons (IFNs) and other
cytokines requires tyrosine phosphorylation of a family of transcription
factors termed signal transducers and activators of transcription (Stats).
The Janus family of tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) is
required for cytokine-induced tyrosine phosphorylation and dimerization of
the Stat proteins. In order for IFNs to stimulate maximal expression of
Stat1alpha-regulated genes, phosphorylation of a serine residue in the
carboxy terminus by mitogen- activated protein kinase (MAPK) is also
required. In HeLa cells, both IFN-beta and oncostatin M (OSM) stimulated
MAPK and Raf-1 enzyme activity, in addition to Stat1 and Stat3 tyrosine
phosphorylation. OSM stimulation of Raf-1 correlated with GTP loading of
Ras, whereas IFN- beta activation of Raf-1 was Ras independent. IFN-beta-
and OSM-induced Raf-1 activity could be coimmunoprecipitated with either
Jak1 or Tyk2. Furthermore, HeLa cells lacking Jak1 displayed no activation
of STAT1alpha, STAT3, and Raf-1 by IFN-beta or OSM and also demonstrated no
increase in the relative level of GTP-bound p21ras in response to OSM. The
requirement for Jak1 for IFN-beta- and OSM-induced activation of Raf-1 was
also seen in Jak1-deficient U4A fibrosarcoma cells. Interestingly, basal
MAPK, but not Raf-1, activity was constitutively enhanced in Jak1-deficient
HeLa cells. Transient expression of Jak1 in both Jak-deficient HeLa cells
and U4A cells reconstituted the ability of IFN-beta and OSM to activate
Raf-1 and decreased the basal activity of MAPK, while expression of a
kinase-inactive form of the protein showed no effect. Moreover, U4A cells
selected for stable expression of Jak1, or COS cells transiently expressing
Jak1 or Tyk2 but not Jak3, exhibited enhanced Raf-1 activity. Therefore, it
appears that Jak1 is required for Raf-1 activation by both IFN-beta and
OSM. These results provide evidence for a link between the Jaks and the
Raf/MAPK signaling pathways.
Copyright © 1997, American Society for Microbiology
Beta interferon and oncostatin M activate Raf-1 and mitogen-activated protein kinase through a JAK1-dependent pathway
Laboratory of Cellular and Molecular Biology, National Cancer Institute, Bethesda, Maryland 20892, USA.
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