IFN-gamma transduces signals by activating the IFN-gamma receptor-associate
d Jak-1 and Jak-2 kinases and by inducing tyrosine phosphorylation and acti
vation of the Stat-1 transcriptional activator. We report that IFN-gamma ac
tivates a distinct signaling cascade involving the c-cbl protooncogene prod
uct, CrkL adapter, and small G protein Rap1, During treatment of NB-4 human
cells with IFN-gamma, c-cbl protooncogene product is rapidly phosphorylate
d on tyrosine and provides a docking site for the src homology 2 domain of
CrkL, which also undergoes IFN-gamma-dependent tyrosine phosphorylation, Cr
kL then regulates activation of the guanine exchange factor C3G, with which
it interacts constitutively via its N terminus src homology 3 domain. This
results in the IFN-gamma-dependent activation of Rap1, a protein known to
exhibit tumor suppressor activity and mediate growth inhibitory responses.
In a similar manner, Rap1 is also activated in response to treatment of cel
ls with type I IFNs (IFN-alpha, IFN-beta), which also engage CrkL in their
signaling pathways. On the other hand, IFN-gamma does not induce formation
of nuclear CrkL-Stat5 DNA-binding complexes, which are induced by IFN-alpha
and IFN-beta, indicating that pathways downstream of CrkL are differential
ly regulated by different IFN subtypes, Taken altogether, our data demonstr
ate that, in addition to activating the Stat pathway, IFN-gamma activates a
distinct signaling cascade that may play an important role in the generati
on of its growth inhibitory effects on target cells.