Differentiation arrest and stromal cell-independent growth of murine erythroleukemia cells are associated with elevated expression of ets-related genes but not with mutation of p53

Downregulation of c-kit (Stem Cell Factor Receptor) in Transformed Hematopoietic Precursor Cells by Stroma Cells

We show a dramatic downregulation of the stem cell factor (SCF) receptor in different hematopoietic cell lines by murine stroma. Growth of the human erythroid/macrophage progenitor cell line TF-1 is dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3). However, TF-1 cells clone and proliferate equally well on stroma. Independent stroma-dependent TF-1 clones (TF-1S) were generated on MS-5 stroma. Growth of TF-1S and TF-1 cells on stroma still requires interaction between c-kit (SCF receptor) and its ligand SCF, because antibodies against c-kit inhibit growth to less than 2%. Surprisingly, c-kit receptor expression (RNA and protein) was downregulated by 2 to 3 orders of magnitude in TF-1S and TF-1 cells grown on stroma. This stroma-dependent regulation of the kit receptor in TF-1 was also observed on exposure to kit ligand-negative stroma, thus indicating the need for heterologous receptor ligand interaction. Removal of stroma induced upregulation by 2 to 4 orders of magnitude. Downregulation and upregulation of c-kit expression could also be shown for the megakaryocytic progenitor cell line M-07e and was comparable to that of TF-1, indicating that stroma-dependent regulation of c-kit is a general mechanism. Downregulation may be an economic way to compensate for the increased sensitivity of the c-kit/ligand interaction on stroma. The stroma-dependent c-kit regulation most likely occurs at the transcriptional level, because mechanisms, such as splicing, attenuation, differential promoter usage, or mRNA stability, could be excluded.

Downregulation of c-kit (Stem Cell Factor Receptor) in Transformed Hematopoietic Precursor Cells by Stroma Cells

We show a dramatic downregulation of the stem cell factor (SCF) receptor in different hematopoietic cell lines by murine stroma. Growth of the human erythroid/macrophage progenitor cell line TF-1 is dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3). However, TF-1 cells clone and proliferate equally well on stroma. Independent stroma-dependent TF-1 clones (TF-1S) were generated on MS-5 stroma. Growth of TF-1S and TF-1 cells on stroma still requires interaction between c-kit (SCF receptor) and its ligand SCF, because antibodies against c-kit inhibit growth to less than 2%. Surprisingly, c-kit receptor expression (RNA and protein) was downregulated by 2 to 3 orders of magnitude in TF-1S and TF-1 cells grown on stroma. This stroma-dependent regulation of the kit receptor in TF-1 was also observed on exposure to kit ligand-negative stroma, thus indicating the need for heterologous receptor ligand interaction. Removal of stroma induced upregulation by 2 to 4 orders of magnitude. Downregulation and upregulation of c-kit expression could also be shown for the megakaryocytic progenitor cell line M-07e and was comparable to that of TF-1, indicating that stroma-dependent regulation of c-kit is a general mechanism. Downregulation may be an economic way to compensate for the increased sensitivity of the c-kit/ligand interaction on stroma. The stroma-dependent c-kit regulation most likely occurs at the transcriptional level, because mechanisms, such as splicing, attenuation, differential promoter usage, or mRNA stability, could be excluded.

An Activating Mutation in the Kit Receptor Abolishes the Stroma Requirement for Growth of ELM Erythroleukemia Cells, But Does Not Prevent Their Differentiation in Response to Erythropoietin

We have previously shown that murine ELM erythroleukemia cells can only be grown in vitro in the presence of a stromal feeder layer, or alternatively stem cell factor (SCF), without which they differentiate. When grown in the presence of SCF, ELM cells can still differentiate in response to erythropoietin (Epo), but growth on stroma prevents this. We previously isolated a stroma-independent ELM variant, ELM-I-1, that is also defective in Epo-induced differentiation. We show here that this variant has an activating mutation in the Kit receptor, converting aspartic acid 814 to histidine. Expression of the mutant receptor in stroma-dependent ELM-D cells causes growth factor-independent proliferation and also gives the cells a selective advantage, in terms of proliferation rate and clonegenicity, compared with ELM-D cells grown in optimal amounts of SCF. Expression of the mutant receptor in ELM-D cells also prevents spontaneous differentiation, but not differentiation induced by Epo. Analysis of mitogenic signaling pathways in these cells shows that the mutant receptor induces constitutive activation of p42/p44 mitogen-activated protein kinases. It also selectively inhibits the expression of p66Shc but not the p46/p52 Shc isoforms (as did treatment of ELM cells with SCF), which is of interest, because p66Shc is known to play an inhibitory role in growth factor signaling.

An Activating Mutation in the Kit Receptor Abolishes the Stroma Requirement for Growth of ELM Erythroleukemia Cells, But Does Not Prevent Their Differentiation in Response to Erythropoietin

We have previously shown that murine ELM erythroleukemia cells can only be grown in vitro in the presence of a stromal feeder layer, or alternatively stem cell factor (SCF), without which they differentiate. When grown in the presence of SCF, ELM cells can still differentiate in response to erythropoietin (Epo), but growth on stroma prevents this. We previously isolated a stroma-independent ELM variant, ELM-I-1, that is also defective in Epo-induced differentiation. We show here that this variant has an activating mutation in the Kit receptor, converting aspartic acid 814 to histidine. Expression of the mutant receptor in stroma-dependent ELM-D cells causes growth factor-independent proliferation and also gives the cells a selective advantage, in terms of proliferation rate and clonegenicity, compared with ELM-D cells grown in optimal amounts of SCF. Expression of the mutant receptor in ELM-D cells also prevents spontaneous differentiation, but not differentiation induced by Epo. Analysis of mitogenic signaling pathways in these cells shows that the mutant receptor induces constitutive activation of p42/p44 mitogen-activated protein kinases. It also selectively inhibits the expression of p66Shc but not the p46/p52 Shc isoforms (as did treatment of ELM cells with SCF), which is of interest, because p66Shc is known to play an inhibitory role in growth factor signaling.

Both Stroma and Stem Cell Factor Maintain Long-Term Growth of ELM Erythroleukemia Cells, but Only Stroma Prevents Erythroid Differentiation in Response to Erythropoietin and Interleukin-3

Defining how the stromal requirements of hematopoietic progenitors change during leukemia progression is an important topic that is not well understood at present. The murine ELM erythroleukemia is an interesting model because the erythroid progenitors retain dependence on bone marrow-derived stromal cells for long-term growth in vitro, and they also undergo erythroid differentiation in the presence of erythropoietin (EPO) and interleukin-3 (IL-3). In this report, we have shown using neutralizing antibodies that stem cell factor (SCF), insulin-like growth factor (IGF)-1, and integrin signaling pathways are all involved. We then determined whether ELM cells can be maintained long-term without stroma in various combinations of growth factors produced by stroma cells or growth factors for which ELM cells have receptors. This showed that ELM cells could be maintained with high efficiency in SCF alone; furthermore, the cells remained absolutely SCF-dependent and did not become more tumorigenic than cells maintained on stroma. In contrast, ELM cells underwent clonal extinction when serially cloned in IGF1; any cells that survived long-term growth in IGF-1 were found to be IGF1-independent. One important difference between maintaining ELM cells on stroma and growth in SCF is that stroma reversibly inhibits their differentiation in response to EPO and IL-3, whereas SCF does not.

Both Stroma and Stem Cell Factor Maintain Long-Term Growth of ELM Erythroleukemia Cells, but Only Stroma Prevents Erythroid Differentiation in Response to Erythropoietin and Interleukin-3

Defining how the stromal requirements of hematopoietic progenitors change during leukemia progression is an important topic that is not well understood at present. The murine ELM erythroleukemia is an interesting model because the erythroid progenitors retain dependence on bone marrow-derived stromal cells for long-term growth in vitro, and they also undergo erythroid differentiation in the presence of erythropoietin (EPO) and interleukin-3 (IL-3). In this report, we have shown using neutralizing antibodies that stem cell factor (SCF), insulin-like growth factor (IGF)-1, and integrin signaling pathways are all involved. We then determined whether ELM cells can be maintained long-term without stroma in various combinations of growth factors produced by stroma cells or growth factors for which ELM cells have receptors. This showed that ELM cells could be maintained with high efficiency in SCF alone; furthermore, the cells remained absolutely SCF-dependent and did not become more tumorigenic than cells maintained on stroma. In contrast, ELM cells underwent clonal extinction when serially cloned in IGF1; any cells that survived long-term growth in IGF-1 were found to be IGF1-independent. One important difference between maintaining ELM cells on stroma and growth in SCF is that stroma reversibly inhibits their differentiation in response to EPO and IL-3, whereas SCF does not.

Cloning and characterization of a novel murine beta chemokine receptor, D6. Comparison to three other related macrophage inflammatory protein-1alpha receptors, CCR-1, CCR-3, and CCR-5

The beta-chemokine macrophage inflammatory protein-1alpha (MIP-1alpha) is chemotactic for many hemopoietic cell types and can inhibit hemopoietic stem cell (HSC) proliferation, effects mediated through G-protein coupled heptahelical receptors. We have isolated cDNAs for seven chemokine receptors, CCR-1 to -5, MIP-1alphaRL1, and a novel cDNA, D6. Chinese hamster ovary cells expressing CCR-1, -3, -5, and D6 bound 125I-murine MIP-1alpha: the order of affinity was D6 > CCR-5 > CCR-1 > CCR-3. Each bound a distinct subset of other beta-chemokines: the order of competition for 125I-murine MIP-1alpha on D6 was murine MIP-1alpha > human and murine MIP-1beta > human RANTES approximately JE > human MCP-3 > human MCP-1. Human MIP-1alpha and the alpha-chemokines did not compete. Like other chemokine receptors, D6 induced transient increases in [Ca2+] in HEK 293 cells upon ligand binding. D6 mRNA was abundant in lung and detectable in many other tissues. Bone marrow cell fractionation demonstrated T-cell and macrophage/monocyte expression of D6, and CCR-1, -3, and -5. Moreover, we could detect expression of CCR-3, CCR-5, and to a greater extent D6 in a cell population enriched for HSCs. Thus, we have characterized four murine beta chemokine receptors that are likely involved in mediating the pro-inflammatory functions of MIP-1alpha and other chemokines, and we present D6, CCR-3, and CCR-5 as candidate receptors in MIP-1alpha-induced HSC inhibition.