Transforming growth factor beta 1 selectively regulates early murine hematopoietic progenitors and inhibits the growth of IL-3-dependent myeloid leukemia cell lines.

Transforming growth factor beta 1 (TGF-beta 1) has been shown to be associated with active centers of hematopoiesis and lymphopoiesis in the developing fetus. Therefore, the effects of TGF-beta 1 on mouse hematopoiesis were studied. TGF-beta 1 is a potent inhibitor of IL-3-induced bone marrow proliferation, but it does not inhibit the proliferation induced by granulocyte/macrophage, colony-stimulating factor (CSF), granulocyte CSF, and erythropoietin (Epo). TGF-beta 1 also inhibits IL-3-induced multipotential colony formation of bone marrow cells in soft agar, which includes early erythroid differentiation, while Epo-induced terminal differentiation is unaffected. In addition, IL-3-induced granulocyte/macrophage colonies were inhibited; however, small clusters of differentiated myeloid cells were consistently seen in cultures containing IL-3 and TGF-beta 1. Thus, TGF-beta 1 selectively inhibits early hematopoietic progenitor growth and differentiation but not more mature progenitors. TGF-beta 1 is also a potent inhibitor of IL-3-dependent and -independent myelomonocytic leukemic cell growth, while the more mature erythroid and macrophage leukemias are insensitive. Therefore, TGF-beta 1 functions as a selective regulator of differentiating normal hematopoietic cells, and suppresses myeloid leukemic cell growth.

Markedly elevated levels of an endogenous sarc protein in a hemopoietic precursor cell line.

The src gene product of Harvey murine sarcoma virus is a 21,000-dalton guanine nucleotide-binding protein. We have recently shown that a wide variety of vertebrate cell strains and cell lines express much lower levels of an endogenous p21 immunologically related to the Harvey murine sarcoma virus-coded p21. In this report, we have examined the levels of endogenous p21 in a unique hemopoietic precursor cell line, 416B, which was originally described as a continuous cell line of a hemopoietic stem cell, CFU-S. The currently available 416B cells express markedly elevated levels of endogenous p21. The level of endogenous p21 in the 416B cells is 5- to 10-fold higher than the level of p21 in Harvey murine sarcoma virus-infected cells and more than 100 times higher than the level of endogenous p21 that we have observed in a variety of other fresh or cultured cells. The results indicate that marked regulation of the levels of an endogenous sarc gene product can occur, and speculation about a possible role for endogenous p21 in normal hemopoietic stem cells is discussed.

Employment of a [3H]thymidine-incorporation assay to distinguish the effects of different Friend erythroleukemia-inducing retroviruses on erythroid cell proliferation.

Spleen cells taken from mice infected as adults with two different variants of the spleen focus-forming virus (SFFV), SFFVP and SFFVA, as well as spleen cells taken from mice infected as newborns with Friend murine leukemia virus (F-MuLV) were assayed in a proliferation assay in the presence or absence of the erythroid hormone erythropoietin (Epo). Infection of NIH Swiss mice with SFFV resulted in excessive proliferation of erythroid cells that could still differentiate, and spleen cells taken from these mice were able to incorporate high levels of tritiated thymidine ([3H]dThd) in the absence of Epo, even in the presence of antibodies to Epo. In contrast, the level of proliferation of spleen cells from SFFVA-infected mice, but not those from SFFVP-infected mice, could be greatly enhanced by the addition of Epo to the cultures. Infection of newborn mice with F-MuLV resulted in the generation of Friend mink cell focus-inducing virus, which caused excessive proliferation of erythroid cells that appeared to be blocked in differentiation, resulting in severe anemia. Spleen cells from these mice were unable to proliferate in the absence of Epo. However, when increasing doses of Epo were added to the cultures, the cells proliferated at levels equivalent to the levels seen with SFFV. These results indicate that a proliferation assay based on the incorporation of [3H]dThd into spleen cells in response to Epo can be used as a quantitative means of assessing and comparing the effects of erythroleukemia-inducing retroviruses on the proliferation of their target cells.

Biologic effects of prolonged melphalan treatment of murine long-term bone marrow cultures and interleukin 3-dependent hematopoietic progenitor cell lines.

The mechanism of alkylating agent-induced leukemia is unknown. For the determination of whether chronic alkylating agent treatment of hematopoietic stem cells in vitro was detectably leukemogenic, murine long-term bone marrow cultures (LTBMC) and clonal interleukin 3 (IL-3)-dependent multipotential hematopoietic progenitor cell lines [B6SUtA clone (cl) 27 and Ro cl 3-1] derived from LTBMC were chronically pulse treated in vitro with the alkylating agent melphalan [L-phenylalanine mustard (L-PAM)]. Weekly treatment of C3H/HeJ or CD-1 Swiss mouse LTBMC with 3 X 10(-6)M L-PAM significantly decreased cumulative production of nonadherent granulocytes and granulocyte-macrophage progenitor cells responsive to L-cell or WEH1-3 cell colony-stimulating factor compared to the production seen in untreated control cultures; it also significantly reduced the hematopoietic longevity (13 wk compared to greater than 20 wk for untreated control cultures). Weekly, twice weekly, or daily (3 X 10(-6)M) L-PAM treatment of IL-3-dependent cell lines induced gradual L-PAM adaptation in the absence of a detectable change in the maximum binding capacity of 125I-labeled IL-3. No leukemogenic variants of line B6SUtA cl 27 were detectably induced. However, 3 stably expressed marker chromosomes were induced after 12 months of L-PAM treatment of line B6SUtA cl 27. Thus IL-3-dependent hematopoietic progenitor cells slowly adapt to L-PAM when in suspension culture in vitro. Physiologic expression of drug toxicity in LTBMC may prevent this hematopoietic cell gradual adaptation.

Transactivation of the GATA-1 promoter by a myb-ets-containing mouse retrovirus is mediated by CACCC elements.

The myb-ets-containing ME26 virus causes erythroleukemia in mice by a novel mechanism involving the inappropriate activation of erythroid-specific genes in hematopoietic precursor cells. We have previously shown that the ME26 viral protein can transactivate the GATA-1 promoter in transient transactivation assays carried out in mouse fibroblasts. The mouse GATA-1 promoter, whose activity is regulated by the GATA-1 protein itself, contains a double GATA consensus sequence at its 5' end and two CACCC elements at its 3' end, both of which are crucial for promoter activity in erythroid cells, as well as a nonconsensus GATA sequence and several putative c-myb and c-ets binding sites. In order to determine which sequences in the GATA-1 promoter are crucial for activation by the ME26 viral protein, we made deletions of the promoter, cloned them into a luciferase expression vector and tested their activity in mouse fibroblasts, which do not express GATA-1. Our results indicate that sequences in the 3' end of the GATA-1 promoter, which include two CACCC elements, are essential for transactivation by ME26 virus, while other upstream sites contribute to full activation by the virus. Mutation of the CACCC sites abolishes ME26 viral transactivation. The interaction of cell extracts containing ME26 viral protein and the GATA-1 promoter fragment containing the two CACCC elements was examined by electrophoretic mobility shift analysis (EMSA) and the results showed no direct interaction between the two. However, we could detect the ubiquitous transcription factor Sp1 bound to this sequence. These data demonstrate that the CACCC element is necessary for GATA-1 promoter transactivation by ME26 virus and that the viral protein may indirectly transactivate the promoter by binding to Sp1.

Introduction of v-abl oncogene induces monocytic differentiation of an IL-3-dependent myeloid progenitor cell line.

There are a variety of murine hematopoietic progenitor cell lines which are differentiation arrested, but still require growth factors such as interleukin-3 for their continued growth and survival. While oncogenes such as v-myc and v-abl have been demonstrated to abrogate the requirement for exogenous growth factors, none have been shown to have an effect on the differentiation of these cell lines. In this report, we demonstrate that the introduction and expression of Abelson murine leukemia virus into a myeloblast progenitor cell line can promote further differentiation along the monocytic lineage. There is a marked alteration in cell morphology, the acquisition of Mac-1 antigen expression, the induction of nonspecific esterase expression and the induced ability to phagocytize opsonized zymosan. Thus, the expression of Abelson murine leukemia virus protein in interleukin-3-dependent hematopoietic progenitors can provide differentiation-inducing signals in cells which are arrested in differentiation. The potential role of Abelson murine leukemia virus gene products in normal hematopoietic cell differentiation and in transformation is discussed.

Apparent Epo-independence of erythroid cells infected with the polycythemia-inducing strain of Friend spleen focus-forming virus is not due to Epo production or change in number or affinity of Epo receptors.

The polycythemia-inducing strain of the Friend spleen focus-forming virus (SFFVP) induces an acute erythroleukemia in mice. Erythroid cells from these mice differ from normal erythroid cells in that they can proliferate and differentiate in the apparent absence of the erythroid hormone erythropoietin (Epo). Although it was recently shown that the unique envelope protein encoded by SFFV is responsible for altering the hormonal requirements of erythroid cells for growth and differentiation, the mechanisms by which this occurs is not known. Since the SFFV envelope protein appears to interact with a target present only in erythroid cells and since Epo is specific for these cells, it is possible that the virus is exerting its effect through this hormone. In an effort to ascertain if this is the case, we examined cells from SFFVP-infected mice to determine (a) if they produce Epo or other erythroid growth factors that stimulate erythroid cells to grow in an autocrine-like manner and (b) if they express elevated numbers of Epo receptors that may result in a reduced requirement for the level of Epo needed for growth and differentiation. Our results indicate that SFFVP-infected cells do not secrete Epo or any other erythroid growth factors that could account for the reduced hormonal requirements of these cells. Also, our studies using iodinated Epo in cell binding assays and cross-linking studies indicate that SFFVP-infected cells are not significantly different from normal erythroid cells in the number, affinity, or size of their Epo receptors.

Constitutive activation of Stat-related DNA-binding proteins in erythroid cells by the Friend spleen focus-forming virus.

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 the erythroid hormone erythropoietin (Epo). In an attempt to understand how the virus alters the growth of erythroid cells, studies were carried out to determine if virus infection leads to the constitutive activation of the Jak-Stat pathway, one of the signal transduction pathways activated by Epo. Our data indicates that expression of SFFV in erythroid cells leads to the constitutive activation of the same Stat proteins that are transiently activated by Epo. While constitutive activation of Stat proteins by SFFV is associated with Epo-independent proliferation of splenic erythroid progenitor cells from Fv-2-sensitive mice and Epo-dependent HCD-57 cells, it is not sufficient to induce their differentiation. Although constitutive activation of the same Stat proteins is detected in erythroid cells from SFFV-infected Fv-2-resistant mice, it does not lead to their Epo-independent growth. It is also not required for transformation of erythroid cells by SFFV. Studies are in progress to identify the mechanism by which Stat proteins are phosphorylated in SFFV-infected cells in the absence of Epo. Although it has been shown that Epo activates Stat proteins through Jak2 kinase, our results suggest that the SFFV-induced Stat protein activation is Jak2-independent.

Molecular mechanism for retroviral neuropathogenesis: possible involvement of capillary endothelial cells.

A neuropathogenic variant of Friend MuLV, PVC-211, causes rapidly progressive spongiform neurodegeneration in susceptible rats and mice. Major targets of PVC-211 MuLV infection are brain capillary endothelial cells (BCEC), suggesting that virus-infected BCEC may play crucial roles in neurological disease induction. Consistent with this possibility, studies using chimeric viruses constructed between PVC-211 MuLV and non-neuropathogenic Friend MuLV have revealed that the BCEC tropism of the virus correlates with its neuropathogenicity. Possible involvement of cytokine expression by PVC-211 MuLV-infected BCEC in the induction of neuropathological changes will be discussed.

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.

Pathogenesis of paralysis and lymphoma associated with a wild mouse retrovirus infection. Part 1. Age- and dose-related effects in susceptible laboratory mice.

Wild mouse ecotropic retrovirus (Cas-Br-M) induced paralysis and non-thymic lymphomas in susceptible NIH Swiss and NFS/N mice. The incidence of paralysis was highest and latency shortest in mice receiving high doses of virus. Lower dose inoculation and inoculation of older mice produced less paralysis with longer latency, but resulted in more lymphomas. However, 10-day-old mice did not develop paralysis and had fewer lymphomas. Anti-Cas-Br-M antibody was detectable in sera from 10-day-old infected mice but not from paralyzed mice. These data suggest that while paralysis and lymphoma may result from different virus-host interactions, the development of immunocompetence may play a role in the age-dependent resistance to Cas-Br-M-associated paralysis and lymphoma in these mice.

Generation of mink cell focus-inducing retroviruses: a model for understanding how viral-viral and viral-cellular interactions can result in biological consequences.

Using neoplastic cell lines as substrates for vaccine development could inadvertently result in viral-viral or viral-cellular interactions whose biological consequences are unclear. In this review, the generation of mink cell focus-inducing (MCF) retroviruses in the mouse is discussed as a model for understanding how viral-viral and viral-cellular interactions can result in the generation of new retroviruses with pathological consequences.

Erythroleukaemia induction by the Friend spleen focus-forming virus.

The Friend spleen focus-forming virus has been a valuable tool for understanding the molecular events involved in the multiple stages of leukaemia. As summarized in Figure 3, the primary effect of SFFV, which occurs within days, is to cause a polyclonal proliferation of erythroid precursor cells that can proliferate in the absence of their normal regulator erythropoietin. This is the direct result of the unique envelope glycoprotein encoded by SFFV, which is transported to the cell surface and apparently interacts with the EpoR or another component of the multimeric EpoR complex, resulting in the constitutive activation of the Epo signal transduction pathway. Within this proliferating population of erythroid cells is a rare cell that has undergone several genetic changes due to the integration of the viral genome in specific sites in the mouse DNA. This leads to the activation of a gene encoding the PU.1 transcription factor, whose high expression in erythroid cells may be the cause of the block in differentiation that is characteristic of SFFV-transformed erythroid cells. SFFV integration can also lead to the inactivation of the p53 tumour supressor gene, giving these cells a growth advantage in the mouse. The disease induced by SFFV in mice is very similar to polycythaemia vera in humans (Golde et al, 1981). The major clinical feature of polycythaemia vera is the continuous expansion of the number of mature red blood cells in the presence of low serum Epo levels. Also, BFU-E and CFU-E from these patients can form in the absence of Epo like the analogous cells from SFFV-infected mice (Casadevall et al, 1982). It is possible that haematopoietic cells from individuals suffering from this disease express a protein similar to the envelope glycoprotein of SFFV that can interact with the EpoR and lead to its constitutive activation. Alternatively, these patients may contain a mutant EpoR gene that is constitutively activated like the mutant EpoR described earlier. As we understand more fully how the SFFV envelope protein constitutively activates te EpoR complex, we can begin to design therapies to counteract its action that can then be applied to treating patients with polycythaemia vera or other human diseases associated with uncontrolled erythropoiesis.

Expression of a transformation-related protein (p53) in the malignant stage of Friend virus-induced diseases.

Stage 1 (pre-malignant) and stage 2 (malignant) cells derived from mice infected with Friend murine leukemia virus or polycythemia-inducing Friend virus complex were examined and compared for the expression of a transformation-related cellular protein, p53. Stage 2 cells were found to express high levels of p53, whereas stage 1 cells did not express detectable levels of this protein. These results indicate that p53 may be a marker for transformed cells present in the second stage of diseases induced by Friend murine leukemia virus or polycythemia-inducing Friend virus complex.