BOTH THE POLYCYTHEMIA-INDUCING AND ANEMIA-INDUCING STRAINS OF FRIEND SPLEEN FOCUS-FORMING VIRUS INDUCE CONSTITUTIVE ACTIVATION OF THE RAF-1MITOGEN-ACTIVATED PROTEIN-KINASE SIGNAL-TRANSDUCTION PATHWAY/

The erythroleukemia-inducing Friend spleen focus-forming virus (SFFV) encodes a unique envelope glycoprotein which allows erythroid cells to proliferate and differentiate in the absence of erythropoietin (Epo). In an attempt to understand how the virus causes Epo independence, we have been studying signal transduction pathways activated by Epo to d etermine if SFFV exerts its biological effects by constitutively activ ating any of these pathways in the absence of Epo. We previously demon strated that Stat proteins, the downstream components of the Epo-induc ed Jak-Stat pathway, are constitutively activated in SFFV-infected cel ls. In this study, we demonstrate that SFFV also activates Raf-1, MEK and mitogen-activated protein (MAP) kinase, the downstream components of the Raf-1/MAP kinase pathway. This pathway was activated in cells i nfected with the polycythemia-inducing strain of SFFV, which induces b oth proliferation and differentiation of erythroid cells in the absenc e of Epo, as well as in cells infected,vith the anemia-inducing strain of the virus, which still require Epo for differentiation. Inhibition of Raf-l by using antisense oligonucleotides led to a partial inhibit ion of the Epo-independent proliferation of SFFV-infected cells. Expre ssion of the transcription factors c-Jun and JunB, but not c-Fos, was induced in SFFV-infected cells in the absence of Epo, suggesting that constitutive activation of the Raf-1/MAP kinase pathway by the virus m ay result in deregulation of AP-1 activity. We conclude from our studi es that infection of erythroid cells with SFFV leads to the constituti ve activation of signal transduction molecules in both the Jak-Stat an d Raf-1/MAP kinase pathways and that both of these pathways must be ac tivated to achieve maximum proliferation and differentiation of erythr oid cells in the absence of Epo.