Autostimulatory mechanisms in myeloid leukemogenesis

Induction of myeloproliferative disease in mice by tyrosine kinase fusion oncogenes does not require granulocyte-macrophage colony-stimulating factor or interleukin-3

Tyrosine kinase fusion oncogenes that occur as a result of chromosomal translocations have been shown to activate proliferative and antiapoptotic pathways in leukemic cells, but the importance of autocrine and paracrine expression of hematopoietic cytokines in leukemia pathogenesis is not understood. Evidence that leukemic transformation may be, at least in part, cytokine dependent includes data from primary human leukemia cells, cell culture experiments, and murine models of leukemia. This report demonstrates that interleukin (IL)-3 plasma levels are elevated in myeloproliferative disease (MPD) caused by the TEL/tyrosine kinase fusions TEL/platelet-derived growth factor beta receptor (PDGFbetaR), TEL/Janus kinase 2 (JAK2), and TEL/neurotrophin-3 receptor (TRKC). Plasma granulocyte-macrophage colony-stimulating factor (GM-CSF) levels were elevated by TEL/PDGFbetaR and TEL/JAK2. However, all of the fusions tested efficiently induced MPD in mice genetically deficient for both GM-CSF and IL-3, demonstrating that these cytokines are not necessary for the development of disease in this model system. Furthermore, in experiments using normal marrow transduced with TEL/PDGFbetaR retrovirus mixed with marrow transduced with an enhanced green fluorescent protein (EGFP) retrovirus, the MPD induced in these mice demonstrated minimal stimulation of normal myelopoiesis by the TEL/PDGFbetaR-expressing cells. In contrast, recipients of mixed GM-CSF-transduced and EGFP-transduced marrow exhibited significant paracrine expansion of EGFP-expressing cells. Collectively, these data demonstrate that, although cytokine levels are elevated in murine bone marrow transplant models of leukemia using tyrosine kinase fusion oncogenes, GM-CSF and IL-3 are not required for myeloproliferation by any of the oncogenes tested.

Active oxygen transforms murine myeloid progenitor cells in vitro

Active oxygen (AO) is ubiquitous in nature and its many forms can act as natural carcinogens. Their effect on the transformation of a mouse myeloid progenitor cell line was studied using anchorage-independent colony formation in methylcellulose as the primary assay. Both cytotoxic and non-toxic concentrations of t-butylhydroperoxide, hydrogen peroxide and menadione were examined. At non-cytotoxic concentrations, no AO transformation of these cells from interleukin-3 dependence to factor independence (FI) was observed, even after as many as 25 treatments. At cytotoxic concentrations, however, all 3 classes of AO transformed the cells to FI growth. The most potent agent was t-butyl hydroperoxide (43-fold induction), followed by hydrogen peroxide and then menadione. As little as one exposure to cytotoxic levels of these oxidants induced significant transformation, with relative potencies the same as those observed for multiple exposures. These inductions were not due to general cytotoxic effects, since sodium fluoride and heat-shock treatment gave minimal inductions. AO-induced colonies in methylcellulose that were removed, examined and then injected into pre-irradiated mice uniformly produced tumors. Control, non-treated cells did not form tumors. Tumorigenic cells did not form colonies in methylcellulose at lower plating densities. Furthermore, low numbers of transformed cells supplemented to high density with normal cells showed a small but insufficient increase in colony number as compared with high-density cultures of transformed cells. Our results suggest that the transformants depend upon a paracrine mechanism of growth that is mediated by the transformed cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Frequency and mechanisms of factor independence in IL-3-dependent cell lines

Interleukin-3 (IL-3) -dependent cell lines were tested for spontaneous mutation frequency with respect to two markers, growth factor-independent growth and 8-azaguanine-resistant growth. The mutation frequency for the growth factor-independent growth was approximately equal to that of 8-azaguanine-resistant growth. Investigation of the growth factor-independent phenotypes indicated that at least two different mechanisms for growth factor independence existed. One mechanisms was the activation of the IL-3 gene by mutation resulting in the constitutive expression of IL-3 and autocrine regulated growth. A second mechanisms results in IL-3-independent growth in a manner in which (1) constitutive tyrosine kinase activity was not seen and (2) c-myc transcription was constitutively activated. Interestingly, in these cells both the tyrosine phosphorylation and c-myc transcriptional pathways are still activatable following IL-3 stimulation. At present, the exact nature of the mutation that results in this phenotype is not known.

Autonomous growth of lymphoid cells following IL-2 expression from retrovirus vectors containing HIV-1 trans-acting elements

The human immunodeficiency virus type I (HIV-1) possesses powerful regulatory elements that control the rate of replication of HIV-1 and subsequent processing of HIV-1 genes. We have used this regulatory mechanism to drive expression of foreign genes inserted in retrovirus vectors. This approach was used to express the human IL-2 gene in IL-2-dependent mouse CTLL-2 cells to determine the role of autonomous growth in maintaining proliferation of virus-infected T lymphocytes during HTLV-1-induced adult T-cell leukemia (ATL). Expression of IL-2 sequences in IL-2-dependent mouse CTLL-2 cells resulted in autonomous growth of IL-2-independent CTLL-2 clones. Endogenous expression of IL-2 appeared to interrupt normal constraints of growth in that these IL-2-independent clones showed reduced cell-density-dependent inhibition but not a tumorigenic phenotype. IL-2-independent CTLL-2 clones did not secrete detectable quantities of IL-2 into culture supernatant and exhibited reduced sensitivity to the inhibitory effects of both IL-2 and IL-2 receptor antibody. These results suggest that the IL-2 autocrine loop within these cells involves intracellular IL-2/IL-2 receptor binding. The apparent lack of IL-2 production and poor responsiveness to IL-2 or IL-2 antibodies displayed by cell lines from ATL patients may be explained by an intracellular IL-2/IL-2 receptor autocrine loop.

Retrovirus-mediated transfer and expression of the interleukin-3 gene in mouse hematopoietic cells result in a myeloproliferative disorder

A high-titer, recombinant retroviral vector produced in psi 2 packaging cells has been used to introduce the murine interleukin-3 (IL-3) gene into mouse hematopoietic cells. Integration and expression of the IL-3 gene was observed in spleen foci from which could be derived factor-independent, continuously proliferating cell lines. Irradiated or genetically anemic W/Wv recipients of infected hematopoietic cells developed a myeloproliferative syndrome characterized by a marked elevation in leukocyte count, bone marrow hyperplasia, and enlargement of the liver and spleen. The syndrome reflected proliferation of one or more stem cell clones, the progeny of which were capable of repopulating secondary recipients. One animal developed the syndrome primarily by a paracrine mechanism. Endogenous IL-3 production caused amplification of hematopoietic cells but did not appear to alter the maturational or self-renewal potential of these cells.

Mechanism of autocrine stimulation in hematopoietic cells producing interleukin-3 after retrovirus-mediated gene transfer

Endogenous expression of the interleukin-3 (IL3) gene introduced with a retrovirus vector renders hematopoietic cells autonomous of exogenous growth factor. To investigate the mechanism of autocrine stimulation, 25 clones were isolated after retrovirus transduction of IL3 into 32D-cl23 or FDC-P1 cells. Medium conditioned by these autonomous IL3-producing clones supported the growth of factor-dependent 32D cells. Although there was a severalfold variation in the amount of IL3 secreted (some clones secreted barely detectable levels), the doubling time of each clone in the absence of added IL3 was identical to that of the parental cell line maximally stimulated by exogenous IL3. Concentrated monoclonal and polyclonal antibodies, both highly effective in neutralizing exogenous IL3, were assayed for ability to inhibit autocrine growth. Minimal inhibitory effects were observed only on washed autocrine clones secreting low levels of IL3. To test the activity of cytoplasmically synthesized IL3, the nucleotides encoding the signal sequence of IL3 were deleted and replaced with an in-frame ATG in the context of a consensus translation initiation sequence. Ten 32D clones expressing this restructured IL3 genome were obtained. Despite the presence of biologically active IL3 in cell lysates, all clones remained dependent on exogenous IL3, with the same dose-response as that found for 32D cells. Our data are most compatible with a mechanism whereby endogenously produced IL3 interacts with its receptor prior to surface display.

Retrovirus-mediated transfer and expression of the interleukin-3 gene in mouse hematopoietic cells result in a myeloproliferative disorder

A high-titer, recombinant retroviral vector produced in psi 2 packaging cells has been used to introduce the murine interleukin-3 (IL-3) gene into mouse hematopoietic cells. Integration and expression of the IL-3 gene was observed in spleen foci from which could be derived factor-independent, continuously proliferating cell lines. Irradiated or genetically anemic W/Wv recipients of infected hematopoietic cells developed a myeloproliferative syndrome characterized by a marked elevation in leukocyte count, bone marrow hyperplasia, and enlargement of the liver and spleen. The syndrome reflected proliferation of one or more stem cell clones, the progeny of which were capable of repopulating secondary recipients. One animal developed the syndrome primarily by a paracrine mechanism. Endogenous IL-3 production caused amplification of hematopoietic cells but did not appear to alter the maturational or self-renewal potential of these cells.

Mechanism of autocrine stimulation in hematopoietic cells producing interleukin-3 after retrovirus-mediated gene transfer

Endogenous expression of the interleukin-3 (IL3) gene introduced with a retrovirus vector renders hematopoietic cells autonomous of exogenous growth factor. To investigate the mechanism of autocrine stimulation, 25 clones were isolated after retrovirus transduction of IL3 into 32D-cl23 or FDC-P1 cells. Medium conditioned by these autonomous IL3-producing clones supported the growth of factor-dependent 32D cells. Although there was a severalfold variation in the amount of IL3 secreted (some clones secreted barely detectable levels), the doubling time of each clone in the absence of added IL3 was identical to that of the parental cell line maximally stimulated by exogenous IL3. Concentrated monoclonal and polyclonal antibodies, both highly effective in neutralizing exogenous IL3, were assayed for ability to inhibit autocrine growth. Minimal inhibitory effects were observed only on washed autocrine clones secreting low levels of IL3. To test the activity of cytoplasmically synthesized IL3, the nucleotides encoding the signal sequence of IL3 were deleted and replaced with an in-frame ATG in the context of a consensus translation initiation sequence. Ten 32D clones expressing this restructured IL3 genome were obtained. Despite the presence of biologically active IL3 in cell lysates, all clones remained dependent on exogenous IL3, with the same dose-response as that found for 32D cells. Our data are most compatible with a mechanism whereby endogenously produced IL3 interacts with its receptor prior to surface display.

Human interleukin 3: effects on normal and leukemic cells

The effects of recombinant human interleukin 3 (IL3) on normal bone marrow cells and human leukemic cells were studied. In clonal assays, IL3 supported the growth of all colony types including megakaryocytes. Erythroid colonies were formed in the presence of IL3 and erythropoietin, but not in the absence of erythropoietin. Replating experiments using blast cell colonies derived from a cell population enriched for progenitor cells by fluorescence-activated cell sorting with the monoclonal antibody 3C5, showed that IL3 supported the continued replating of colonies. The clonal proliferation of human bone marrow cells in response to IL3 was inhibited by tumor necrosis factor and by lymphotoxin, but not by interferon-gamma. In suspension cultures, IL3 supported the proliferation of mast cells. Human IL3 had no effect on the growth responses, morphology, cytochemistry, or clonogenicity of the human leukemic cell lines HL60, U-937, KG1a, and HEL. Transcripts for IL3 mRNA were not detectable in these cells, nor in the K562 cell line, implying that autocrine secretion of IL3 was not the mechanism by which these leukemias were maintained. Although cells derived from the bone marrow or peripheral blood of twenty patients with myeloproliferative disorders, myelodysplastic syndromes or acute myeloid leukemia frequently showed proliferative responses to IL3, mRNA transcripts for IL3 were not detected in these cells.

Hemopoietic lineage switch: v-raf oncogene converts Emu-myc transgenic B cells into macrophages

Hemopoietic lineage commitment can be breached by concomitant expression of the c-myc and v-raf oncogenes. Switching to the myeloid lineage occurred frequently when B lineage cells, from either lymphomas or preleukemia bone marrow cells of Emu-myc transgenic mice, were infected with a retrovirus bearing v-raf. Cloned pre-B and B cell lines changed into either mature or immature macrophages as assessed by morphology, adherence, phagocytic activity, surface markers, and lysozyme production, but retained clonotypic immunoglobulin gene rearrangements. Although expression of the Emu-myc transgene was reduced or abolished in the more differentiated lines, the lines remained tumorigenic. The converted lines produced the myeloid growth factor GM-CSF, and most had karyotypic alterations. These results suggest that constitutive myc plus raf expression can provoke genetic reprogramming in lymphocytes.