Replication of spleen focus-forming Friend virus in fibroblasts from C57BL mice that are genetically resistant to spleen focus formation

The envelope glycoprotein of friend spleen focus-forming virus covalently interacts with and constitutively activates a truncated form of the receptor tyrosine kinase Stk

The Friend spleen focus-forming virus (SFFV) encodes a unique envelope glycoprotein, gp55, which allows erythroid cells to proliferate and differentiate in the absence of erythropoietin (Epo). SFFV gp55 has been shown to interact with the Epo receptor complex, causing constitutive activation of various signal-transducing molecules. When injected into adult mice, SFFV induces a rapid erythroleukemia, with susceptibility being determined by the host gene Fv-2, which was recently shown to be identical to the gene encoding the receptor tyrosine kinase Stk/Ron. Susceptible, but not resistant, mice encode not only full-length Stk but also a truncated form of the kinase, sf-Stk, which may mediate the biological effects of SFFV infection. To determine whether expression of SFFV gp55 leads to the activation of sf-Stk, we expressed sf-Stk, with or without SFFV gp55, in hematopoietic cells expressing the Epo receptor. Our data indicate that sf-Stk interacts with SFFV gp55 as well as gp55(P), the biologically active form of the viral glycoprotein, forming disulfide-linked complexes. This covalent interaction, as well as noncovalent interactions with SFFV gp55, results in constitutive tyrosine phosphorylation of sf-Stk and its association with multiple tyrosine-phosphorylated signal-transducing molecules. In contrast, neither Epo stimulation in the absence of SFFV gp55 expression nor expression of a mutant of SFFV that cannot interact with sf-Stk was able to induce tyrosine phosphorylation of sf-Stk or its association with any signal-transducing molecules. Covalent interaction of sf-Stk with SFFV gp55 and constitutive tyrosine phosphorylation of sf-Stk can also be detected in an erythroleukemia cell line derived from an SFFV-infected mouse. Our results suggest that SFFV gp55 may mediate its biological effects in vivo by interacting with and activating a truncated form of the receptor tyrosine kinase Stk.

Deregulation of erythropoiesis by the Friend spleen focus-forming virus

The proliferation and differentiation of erythroid cells is a highly regulated process that is controlled primarily at the level of interaction of erythropoietin (Epo) with its specific cell surface receptor (EpoR). However, this process is deregulated in mice infected with the Friend spleen focus-forming virus (SFFV). Unlike normal erythroid cells, erythroid cells from SFFV-infected mice are able to proliferate and differentiate in the absence of Epo, resulting in erythroid hyperplasia and leukemia. Over the past 20 years, studies have been carried out to identify the viral genes responsible for the pathogenicity of SFFV and to understand how expression of these genes leads to the deregulation of erythropoiesis in infected animals. The studies have revealed that SFFV encodes a unique envelope glycoprotein which interacts specifically with the EpoR at the cell surface, resulting in activation of the receptor and subsequent activation of erythroid signal transduction pathways. This leads to the proliferation and differentiation of erythroid precursor cells in the absence of Epo. Although the precise mechanism by which the viral protein activates the EpoR is not yet known, it has been proposed that it causes dimerization of the receptor, resulting in constitutive activation of Epo signal transduction pathways. While interaction of the SFFV envelope glycoprotein with the EpoR leads to Epo-independent erythroid hyperplasia, this is not sufficient to transform these cells. Transformation requires the viral activation of the cellular gene Sfpi-1, whose product is thought to block erythroid cell differentiation. By understanding how SFFV can deregulate erythropoiesis, we may gain insights into the causes and treatment of related diseases in man.

A Friend virus mutant that overcomes Fv-2rr host resistance encodes a small glycoprotein that dimerizes, is processed to cell surfaces, and specifically activates erythropoietin receptors

The env gene of Friend spleen focus-forming virus (SFFV) encodes a membrane glycoprotein (gp55) that is inefficiently (3 to 5%) processed from the rough endoplasmic reticulum to form a larger dimeric plasma membrane derivative (gp55p). Moreover, the SFFV env glycoprotein associates with erythropoietin receptors (EpoR) to cause proliferation of infected erythroblasts [J.-P. Li, A. D. D'Andrea, H. F. Lodish, and D. Baltimore, Nature (London) 343:762-764, 1990]. Interestingly, the mitogenic effect of SFFV is blocked in mice homozygous for the Fv-2r resistance gene, but mutant SFFVs can overcome this resistance. Recent evidence suggested that these mutants contain partial env deletions that truncate the membrane-proximal extracellular domain of the encoded glycoproteins (M. H. Majumdar, C.-L. Cho, M. T. Fox, K. L. Eckner, S. Kozak, D. Kabat, and R. W. Geib, J. Virol. 66:3652-3660, 1992). Mutant BB6, which encodes a gp42 glycoprotein that has a large deletion in this domain, causes erythroblastosis in DBA/2 (Fv-2s) as well as in congenic D2.R (Fv-2r) mice. Analogous to gp55, gp42 is processed inefficiently as a disulfide-bonded dimer to form cell surface gp42p. Retroviral vectors with SFFV and BB6 env genes have no effect on interleukin 3-dependent BaF3 hematopoietic cells, but they cause growth factor independency of BaF3/EpoR cells, a derivative that contains recombinant EpoR. After binding 125I-Epo to surface EpoR on these factor-independent cells and adding the covalent cross-linking reagent disuccinimidyl suberate, complexes that had immunological properties and sizes demonstrating that they consisted of 125I-Epo-gp55p and 125I-Epo-gp42p were isolated from cell lysates. Contrary to a previous report, SFFV or BB6 env glycoproteins did not promiscuously activate other members of the EpoR superfamily. Although the related env glycoproteins encoded by dualtropic murine leukemia viruses formed detectable complexes with EpoR, strong mitogenic signalling did not ensue. Our results indicate that the SFFV and BB6 env glycoproteins specifically activate EpoR; they help to define the glycoprotein properties important for its functions; and they strongly suggest that the Fv-2 leukemia control gene encodes an EpoR-associated regulatory factor.

Mutations in the env gene of friend spleen focus-forming virus overcome Fv-2r-mediated resistance to Friend virus-induced erythroleukemia

Although Fv-2r homozygous mice are resistant to leukemias induced either by an erythropoietin-encoding virus or by wild-type Friend virus (FV) (M. E. Hoatlin, S. L. Kozak, F. Lilly, A. Chakraborti, C. A. Kozak, and D. Kabat, Proc. Natl. Acad. Sci. USA 87:9985-9989, 1990), they are susceptible to some variants of FV (R. A. Steeves, E. A. Mirand, A. Bulba, and P. J. Trudel, Int. J. Cancer 5:349-356, 1970; R. W. Geib, M. B. Seaward, M. L. Stevens, C.-L. Cho, and M. Majumdar, Virus Res. 14:161-174, 1989). To localize the virus gene involved in influencing the host range, we cloned and sequenced the env gene of the BB6 variant of FV (Steeves et al., Int. J. Cancer 5:349-356, 1970). In comparison with the wild-type env gene, the BB6 variant contains a 159-bp deletion that eliminates the membrane-proximal portion of the extracellular domain and 58 point mutations resulting in 13 amino acid changes. Substitution of the variant env gene for the wild-type env gene resulted in a recombinant virus that produced a Friend virus-like disease in Fv-2r homozygotes. Our results identify the spleen focus-forming virus env gene as the viral gene involved in this virus-host interaction. Additionally, they suggest that the product of the Fv-2r gene modifies the interaction between the spleen focus-forming virus envelope protein and the erythropoietin receptor.

Activation of erythropoietin receptors by Friend viral gp55 and by erythropoietin and down-modulation by the murine Fv-2r resistance gene

The leukemogenic membrane glycoprotein (gp55) encoded by Friend spleen focus-forming virus appears to bind to erythropoietin receptors (EpoR) sto stimulate erythroblastosis [Li, J.-P., D'Andrea, A.D., Lodish, H.F. & Baltimore, D. (1990) Nature (London) 343, 762-764]. To directly compare the effects of gp55 with erythropoietin (Epo), we produced retrovirions that encode either gp55, Epo, or EpoR. After infection with EpoR virus, interleukin 3-dependent DA-3 cells bound 125I-labeled Epo and grew without interleukin 3 in the presence of Epo. These latter cells, but not parental DA-3 cells, became factor-independent after superinfection either with Epo virus or with Friend spleen focus-forming virus. In addition, Epo virus caused a disease in mice that mimicked Friend erythroleukemia. Although Fv-2r homozygotes are susceptible to all other retroviral diseases, they are resistant to both Epo viral and Friend viral erythroleukemias. These results indicate that both gp55 and Epo stimulate EpoR and that the Fv-2 gene encodes a protein that controls response to these ligands. However, the Fv-2 protein is not EpoR because the corresponding genes map to opposite ends of mouse chromosome 9. These results have important implications for understanding signal transduction by EpoR and the role of host genetic variation in controlling susceptibility to an oncogenic protein.

RB virus: a strain of Friend virus that produces a 'Friend virus-like' disease in Fv-2rr mice

RB virus is a newly derived strain of Friend virus that was adapted to produce a 'Friend-like' disease in mice that are genetically resistant to wild-type Friend virus. RB virus was produced by passing high titers of the wild-type Friend virus (Lilly-Steeves polycythemia-producing strain) through adult Fv-2rr mice. Titration of the defective spleen focus-forming virus indicated RB virus infected similar numbers of Fv-2ss or Fv-2rr target cells. Analysis of the spleens from mice infected with RB virus indicated that RB induced the early stage of Friend disease (erythroid proliferation) in both Fv-2rr and Fv-2ss mice. Fv-2ss mice infected with RB virus developed the classical Friend disease within 3 weeks. In contrast, the percentage of Fv-2rr mice developing the 'Friend-like' disease after infection with RB virus never exceeded 60%. The latency period of RBV in Fv-2rr mice was strain dependent. D2.R16 (Fv-2rr) developed the syndrome more rapidly than C57BL/6 (Fv-2rr). RB virus retained the capacity to transform erythroprogenitor cells from both Fv-2ss and Fv-2rr animals. Cells infected with RB virus consistently produced a modified SFFV envelope protein, gp48.

Friend virus replication as a function of age

We have investigated the effect of age on the replication of Friend spleen focus-forming virus (SFFV). Recovery of SFFV from the spleens of four strains of mice was determined following intravenous infection with NB-tropic Friend virus (FV) complex at ages ranging from 6 to 134 weeks. In C57BL/6 mice, the virus did not replicate in adults up to 40 weeks of age, but beyond that there was a steep exponential increase with age in the amounts of SFFV recoverable. In C3H/He mice, which replicate the virus as young adults, the amount of SFFV recovered was 6-fold greater in old than in young mice. Recovery of virus was biphasic with age in SJL mice; in A strain mice no consistent change with age was noted. In C57BL/6 mice, reconstitution of lethally irradiated recipients with syngeneic marrow cells, followed by i.v. infection with FV, showed that the amounts of SFFV recovered depended on the age of the recipient. The present work shows that Friend SFFV replication is a sensitive indicator and can be used as a tool for the investigation of aging processes. The mechanisms responsible for the age-dependent change in regulation of virus replication and for the polymorphism remain to be determined.

Host control of susceptibility to erythroleukemia and to the types of leukemia induced by Friend murine leukemia virus: initial and late stages

The genetic control of the susceptibility to early erythroleukemic induction by helper-independent Friend murine leukemia virus was determined to be due to the presence of a single dominant resistant locus. This locus, (termed Fv6), however, governs only resistance to early erythroleukemia, since other types of leukemias developed late in the resistant mice: myelomonocytic leukemia (approximately 70%), T-lymphoid leukemia (approximately 25%) and late erythroleukemia (approximately 10%). Data also indicated that mice with higher DBA/2 (resistant strain) genetic background had an increased frequency of myelomonocytic leukemias. Studies of the hemopoietic modulations indicated that mice that developed early or late erythroleukemia had dramatic increases in erythroid bursts, suggesting that the leukemic blocks are at the level of BFU-E or earlier. High levels of the granulocytic progenitor colonies were also found in mice with late erythroleukemia and myelomonocytic leukemia. Analysis of these leukemias indicate that they are multistage diseases.

Expression of resistance to Friend virus-stimulated erythropoiesis in bone marrow chimeras containing Fv-2rr and Fv-2ss bone marrow

Bone marrow chimeras were formed containing mixtures of DBA/2 (Fv-2ss, Hbbdd) and B10.D2 (Fv-2rr, Hbbss) bone marrow. When these mice were infected with the polycythemia-inducing strain of Friend virus, erythropoiesis was stimulated, but the proportion of B10.D2 hemoglobin fell rapidly and newly synthesized hemoglobin was essentially all of the DBA/2 type. The treatment of infected polycythemic chimeras with phenylhydrazine lowered the hematocrit and restored the synthesis of B10.D2 hemoglobin. These results imply that B10.D2 erythroid precursors are intrinsically resistant to Friend virus-stimulated erythropoiesis. The experiments also suggest that virus-stimulated erythropoiesis is not mediated by a factor or cell-cell interactions, unless such factors or interactions do not act across strain barriers.

Inheritance of susceptibility to the myeloproliferative sarcoma virus: effect of the Fv-2 locus and evidence for a myeloproliferative sarcoma virus resistance locus

Myeloproliferative sarcoma virus (MPSV) causes a generalized stem cell leukemia with erythroid and myeloid hyperplasia in adult mice. MPSV also transforms fibroblasts. Mice congenic for the Fv-2 locus showed marked differences in susceptibility to MPSV according to the Fv-2 genotype. MPSV was injected into C57BL/6 Fvs and C57BL/6 Fv-2r mice congenic except for the Fv-2 locus. C57BL/6 mice with the Fvs genotype were much more susceptible to MPSV than were those with the Fvr genotype. Both DDD Fv-2r mice congenic with DDD Fv-2s mice except for the Fv-2 locus and DDD Fv-2s mice, however, were sensitive to spleen focus formation by MPSV. These data indicate that at least one additional resistance locus to MPSV is present in C57BL/6 mice but not in DDD mice. Both the Fv-2 locus and the putative MSPV resistance locus (loci) Mpsvr appear to be epistatic to either of the sensitivity loci. Fibroblast focus formation by MPSV was obtained well in C57BL/6 Fv-2r and C57BL/6 Fvs fibroblasts, indicating that the genes for MPSV resistance (Fv-2r and Mpsvr) were not operating in fibroblast cells. A model is proposed which may account for the differences in response of genetically different mice to MPSV and Friend spleen focus-forming virus.