Growth factor-independent proliferation of erythroid cells infected with friend spleen focus-forming virus is protein kinase C dependent but does notrequire Ras-GTP

Interaction of erythropoietin (Epo) with its cell surface receptor activate s signal transduction pathways which result in the proliferation and differ entiation of erythroid cells. Infection of erythroid cells with the Friend spleen focus-forming virus (SFFV) leads to the interaction of the viral env elope glycoprotein with the Epo receptor and renders these cells Epo indepe ndent. We previously reported that SFFV induces Epo independence by constit utively activating components of several Epo signal transduction pathways, including the Jak-Stat and the Raf-1/mitogen-activated protein kinase (MAPK ) pathways. To Further evaluate the mechanism by which SFFV activates the R af-1/MAPK pathway, me investigated the effects of SFFV on upstream componen ts of this pathway, and our results indicate that SFEV activates Shc and Gr b2 and that this leads to Ras activation. While studies with a dominant-neg ative Ras indicated that Ras was required for Epo-induced proliferation of normal erythroid cells, the Epo-independent growth of SFFV-infected cells c an still occur in the absence of Ras, although at reduced levels. In contra st, protein kinase C (PKC) was shown to be required for the Epo-independent proliferation of SFFV-infected cells. Further studies indicated that PKC, which is thought to be involved in the activation of both Raf-1 and MAPK, w as required only for the activation of MAPK, not Raf-1, in SFFV-infected ce lls. Our results indicate that Ras and PKC define two distinct signals conv erging on MAPK in both Epo-stimulated and SFFV-infected erythroid cells and that activation of only PKC is sufficient for the Epo-independent prolifer ation of SFFV-infected cells.