A murine recombinant retrovirus containing the src oncogene transforms erythroid precursor cells in vitro

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

Absence of p53 allows direct immortalization of hematopoietic cells by the myc and raf oncogenes

The p53 tumor suppressor is implicated here as a crucial barrier to unlimited cell proliferation. Its role in transformation of hematopoietic cells was studied by infecting fetal liver cells from wild-type or p53-/- mice with oncogenic retroviruses. Transformed colonies arose with a raf and a myc-raf virus. Absence of p53 did not affect their frequency but proved critical for their continued propagation. Colonies of p53-/- cells bearing both myc and raf readily yielded continuous cell lines without apparent requirement for genetic alteration. The lines, mainly of erythroid or myelomonocytic origin, were diploid but highly tumorigenic from their inception. These findings imply that p53 loss contributes directly to immortalization and tumorigenesis, probably by abrogating an intrinsic senescence program.

New tools for investigating macrophage differentiation

Mouse macrophage clones were generated from liver, bone marrow, spleen, thymus and brain by in vitro immortalization of primary cultures with VN11 recombinant retroviruses, transducing an avian v-myc oncogene. Tissue macrophages from eight different strains of mice were immortalized. All macrophage clones obtained thus far constitutively expressed F4/80, Mac-1, Mac-2 and Fc receptors. Class II MHC molecules were induced with IFN gamma. Constitutive and inducible functional activities of macrophages were retained in most of the clones; phagocytosis, chemotaxis, adhesion properties, cytokine production, MHC expression, cytotoxicity and toxic reactive nitrogen intermediate synthesis were all preserved. A model for studying macrophage differentiation in vitro using cloned cells is presented.

An in vivo analysis of c-myc and c-fos expression during terminal erythroid differentiation in mouse spleen progenitors

Using thiamphenicol and scheduled bleeding, we were able to induce an adequate number of erythroid stem cells (CFU-e) in mice in order to conduct an in vivo study of the changing expression of c-myc and c-fos oncogenes during erythropoiesis. Results indicated that c-myc and c-fos are active in erythropoiesis and have a similar pattern of expression. A large decrease in expression of both c-myc and c-fos occurs when erythroid cells begin the biochemical transition into mature phenotypes, i.e., when RNA synthesis is down-regulated.

raf/myc-infected erythroid cells are restricted in their ability to terminally differentiate

A comparison was made of the in vitro erythroid colony-forming abilities of v-raf-, v-myc-, and v-raf/v-myc-containing retroviruses. In methylcellulose, v-raf efficiently produced colonies of well-differentiated hemoglobin-synthesizing erythroid cells, whereas v-raf/v-myc-infected erythroid cells were inhibited from terminally differentiating but retained the ability to replicate extensively. In contrast, v-myc was unable to stimulate the formation of erythroid colonies.

Isolation of a murine retrovirus with a temperature-sensitive src gene

A murine retrovirus containing the src gene of avian sarcoma virus ts LA31A was generated. In contrast to the avian sarcoma virus, our recombinant murine retrovirus can efficiently infect mammalian cells. Transformation of NIH3T3 cells by the recombinant murine retrovirus is temperature sensitive. At the permissive temperature of 34 degrees, cells form foci of rounded cells. At the nonpermissive temperature of 39 degrees, the infected cells remain flat and exhibit contact inhibition. No disease was observed following infection of newborn NFS/n mice with the ts mutant virus. In contrast, infection of newborn NFS/n mice with a recombinant murine retrovirus containing the wild-type src gene causes fibrosarcomas and hepatosplenomegaly.

The v-fms oncogene induces factor independence and tumorigenicity in CSF-1 dependent macrophage cell line

The McDonough strain of feline sarcoma virus (SM-FeSV) transforms fibroblast cell lines in culture and produces fibrosarcomas in domestic cats. SM-FeSV does not induce haematopoietic malignancies in spite of the fact that its viral oncogene, v-fms, codes for a glycoprotein related to the receptor for the mononuclear phagocyte colony stimulating factor, CSF-1. The v-fms-coded polypeptide includes the complete extracellular domain of the c-fms proto-oncogene product and retains the ability to bind CSF-1 specifically. The two molecules have very similar sequences except at their extreme carboxyl terminal ends where 40 amino acids of the c-fms-coded glycoprotein are replaced by 11 unrelated residues in the v-fms product. Autophosphorylation of the c-fms gene product on tyrosine is enhanced by CSF-1 addition, whereas phosphorylation of the v-fms-coded glycoprotein appears to be constitutive. We now show that introduction of the v-fms gene into simian virus-40 (SV40)-immortalized, CSF-1 dependent macrophages renders them independent of CSF-1 for growth and tumourigenic in nude mice. These factor-independent cell lines express unaltered levels of the c-fms product which is down-modulated in response to either CSF-1 or the tumour promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). The induction of factor independence by a non-autocrine mechanism suggests that the v-fms product is an unregulated kinase that provides growth stimulatory signals in the absence of ligand.