G. R. Johnson

Transforming genes and target cells of murine spleen focus-forming viruses.

Publisher Summary This chapter outlines the factors that determine leukemogenesis induced by acutely transforming retroviruses by the interaction with either myeloid stem or progenitor cells of the mammalian system. The acutely transforming retroviruses described will be limited to the spleen focus-forming viruses, defined as those retroviruses that induce a proliferative hematopoietic disease upon the intravenous injection of adult animals. The chapter discusses the molecular features of the three known groups of acutely transforming murine viruses: (1) viruses with recombinant env genes, (2) viruses with the mos oncogene, and (3) viruses with the ras oncogene. Data pertaining to the relevant proto-oncogenes and their products are also presented. It outlines the critical factors of the viral genome or transforming gene, based on structure-function analysis, which determine the oncogenicity of the retroviruses and its target cell specificity. It also discusses retrovirus-target cell interaction and outlines the experiments that are still necessary to fully understand the oncogenesis induced by spleen focus-forming viruses in the myeloid system.

Factors altering the ability of multipotential hemopoietic colony-forming cells to self-generate or form progenitor cells

Individual in vitro colonies grown from multipotential cells have been analysed for their content of colony-forming cells including BFU-E, Mix-CFC and non-erythroid CFC. The CFC content of mixed colonies was higher in colonies grown from spleen cells than in colonies grown from bone marrow cells and was unaffected by the addition of thymocytes or postendotoxin serum. When the incubation period was extended from 7 to 14 days, CFC content of mixed colonies increased. However, plate mapping and reculturing of individual mixed colonies demonstrated that different subsets of mixed colonies were present in day 7 and day 14 cultures. Although the number of mixed colonies developing decreased when stimulated by decreasing concentrations of pokeweed mitogen-stimulated spleen cell conditioned medium (SCM) the mean number of cells per colony remained unaltered and the content of CFC in such colonies increased. With the SCM concentration range tested, as the SCM concentration was decreased, the absolute number of mixed colonies containing CFC rose from 0.6 per culture to 2.4 per culture. Over this same SCM concentration range the mean number of CFC per mixed colony increased from 1.3 to 185.2. These data suggest that amplification within mixed colonies of the number of cells capable of colony formation may occur at the expense of the production of differentiating cells.

Molecular Properties of a Factor Inducing Differentiation in Murine Myelomonocytic Leukemic Cells

The possibility of therapeutic manipulation of normal regulatory molecules in leukemia has recently gained interest with the demonstration, both in vivo and in vitro, that terminal differentiation and leukemic stem cell suppression can be induced in several mouse myeloid leukemic cell lines by normal tissue products [2–4, 8, 9].

Signaling by the Erythropoietin receptor in retrovirally transduced normal hematopoietic progenitors: A conserved proline-rich motif is required for association with JAK2

The receptor for erythropoietin is restricted to cells of mature erythroid and possibly megakaryocyte lineages. Studies in cell lines have suggested that cytokine receptors share a conserved signaling pathway for proliferation. We retrovirally transduced the erythropoietin receptor or a constitutively activated form of the EPOR into normal hematopoietic progenitors, including blast cell colonies. The EPO-R was able to support the proliferation and differentiation of early erythroid, early megakaryocytic, and macrophage progenitors, but not granulocyte progenitors. Blast cell colonies transduced with the EPO-R proliferate in response to EPO but the development of erythroid cells was not favored over other lineages. These results with normal cells suggest that some but not all cytokine receptors exhibit shared signaling pathways, and that EPO signaling alone is not sufficient to drive erythroid development. The Janus family of cytosolic tyrosine kinases mediate cytokine initiated mitogenic signals. We have determined that the EPO-R box 1 cytoplasmic motif is required for the binding and activation of JAK2. However, sequences outside the box 1 domain most likely regulate the specificity of JAK kinase association.

Proliferation In Vivo and In Vitro of Haemopoietic Progenitor Cells Induced by AF-1, a New ras-Containing Retrovirus

Many studies have been performed on the effects of murine retrovirus upon the haematopoietic systems. In some instances following a variable period after in vivo infection transplantable tumours or continuous cell lines have been developed [1, 2]. The cell lines appear to proliferate independently of haemopoietic growth factors, although the possibility remains that the transformed cells are able to produce their own growth factors. In most cases of transformation of haemopoietic cells and production of continuous cell lines by retroviruses the target cell for viral transformation remains unknown.