The UT-7 cell line was established from a patient with a megakaryoblastic leukemia (Komatsu et al, Cancer Res 51: 341, 1991). Its proliferation is strictly dependent on the presence of hematopoietic growth factors including erythropoietin (Epo), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). We investigated the differentiation capacities of this cell line under the action of several growth factors, using immunomarkers, flow cytometry, and ultrastructural techniques. In the presence of GM-CSF and IL-3, eosinophil and basophil promyelocytes were detected, as well as a few cells with erythroid and megakaryocytic (MK) differentiation features. In contrast, Epo induced a marked erythroid differentiation with an increase of glycophorin A expression, accompanied by a few hemoglobinized cells. Differentiation induced by the growth factors took 24 to 48 hours to begin, and increased with cell passages to a plateau at 2 weeks of culture. However, this was not only due to a cell selection because the differential effects of Epo and GM-CSF were observed from a single cell clone and the phenotype could be reversed by opposite growth factors, even after a long period of culture. We subsequently investigated the phenotype of UT-7 in the presence of combinations of Epo, IL-3, and GM-CSF, and showed that GM-CSF and IL-3 act predominantly over Epo. This effect was mediated by a rapid downmodulation of Epo receptors by GM-CSF at messenger RNA and binding sites levels, without a change in receptor affinities. On the other hand, Epo had no effect on number and affinity of GM-CSF receptors. This study shows that UT-7 is a growth factor-dependent pluripotent cell line in which commitment may be directed by a hierarchical action of growth factors through an early and rapid transmodulation of growth factor receptors.

The human pluripotent UT-7 cell line is growth factor-dependent for proliferation and differentiation. We have previously shown that (1) granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo) induce a myeloid and erythroid pattern of differentiation, respectively; (2) GM-CSF acts predominantly over Epo for cell differentiation; (3) GM-CSF induces a rapid downmodulation (4 hours) of Epo receptors (Epo-R) at the mRNA and binding site levels; and (4) in contrast, Epo has no effect on GM-CSF receptor (GM-CSF-R) expression. These results suggested that UT-7 cell commitment or differentiation may be directed by a hierarchical action of growth factors through an early and rapid transmodulation of growth factor receptors. To test this hypothesis, we introduced and expressed the murine Epo-R (muEpo-R) in UT-7 cells using a retroviral strategy. Two retroviral vectors were constructed: one carrying the neomycin resistance gene, and another carrying a mouse Epo-R cDNA devoid of its regulatory untranslated 3′ sequence placed under the transcriptional control of the viral long terminal repeat element (LTR) and the neomycin resistance gene. Three UT-7/Epo-R infected clones (12, 6, 10) and one UT-7/neomycin clone (Neo) were selected in medium containing G418. After growth factor deprivation (18 hours), Epo-Rs were expressed at the same level (approximately 6,000 receptors per cell) in all four clones 12, 6, 10, Neo, and in parental UT-7 cells, and exhibited similar affinity (0.1 to 0.2 nmol/L). Cross-linking experiments showed that Epo is associated with three proteins of about 66, 85, and 100 kD in cells of parental UT-7, as well as in cells of clones 10 and 12. An inhibitory antibody directed specifically against the human Epo-R (huEpo-R Ab) abolished almost completely the cross-linking on parental UT-7 cells, but not on cells of clone 12, demonstrating that more than 90% cell surface Epo-Rs were of murine origin. The presence of GM-CSF significantly reduced the number of Epo-Rs expressed on parental UT-7 cells, but not on cells of clones 12, 10, and 6. HuEpo-R Ab inhibited Epo-induced parental UT-7 cell growth, but not that of cells of clone 12, suggesting that the muEpo-R is able to induce human UT-7 cell proliferation. When cells of clone 12 were switched from a medium containing GM-CSF to one with Epo, cell surface glycophorin A (GPA) was induced, as in parental UT-7 cells without inhibition by the huEpo-R Ab, demonstrating that the muEpo-R is also able to transduce a differentiation signal in human cells. However, in cells of clones 12, 6, 10 and Neo, as well as in parental UT-7 cells, the induction of GPA by Epo was inhibited by GM-CSF. This finding demonstrates that, although GM-CSF does not downregulate muEpo-R binding sites on UT- 7/muEpo-R infected clones, it still inhibits the effects of Epo on cell differentation. Therefore, hierarchical regulation induced by growth factors for cell commitment or differntiation more likely acts downstream of cell surface receptors at either the signal transduction or transcriptional levels.

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