We have previously reported that constitutive expression of c-fos oncogene allows long-term proliferation of primary mouse bone marrow stromal cells favoring the granulocytic differentiation of myeloid precursors in an in vitro assay. Retrovirus-mediated gene transfer of the human c-fos gene was used here for immortalizing nine mouse bone marrow cell lines which were studied in detail. However, due to low expression of the ectopic c-fos gene, none of them showed characteristics of transformation as assayed by dependence upon serum for growth, the inability to form colonies in agar and contact inhibition. All of them displayed a fibroblastoid phenotype, as deduced from morphological observation and analysis of several differentiation markers. They mostly supported the granulocytic differentiation of bone marrow myeloid precursors in a GM-assay, as did c-fos-expressing primary stromal cells. Their potential for supporting myeloid progenitor proliferation was however significantly lower than that of the whole adherent layer of the Dexter-type long-term bone marrow culture they derived from (STNT cells). They showed significant variations with respect to their cytokine gene expression analyzed at the RNA level in keeping with the notion of stromal cell heterogeneity in the bone marrow. Interestingly, none of them secreted GM-CSF, SCF or IL-3, which are cytokines reputed for their ability to stimulate hematopoietic progenitors, and strikingly, only two of them were able to produce detectable levels of G-CSF in culture supernatants despite the propensity of all of them to favor granulocyte differentiation. Finally, in coculture assay, bone marrow cells were able to diminish the expression of several cytokine genes albeit at a much lower degree than in the original STNT cells.
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