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Murine retroviruses which encode c‐myb proteins that have either complete or truncated carboxy (C) termini were used to infect haemopoietic cells from murine fetal liver. Using an agar colony assay, we could show that infection with the virus encoding the truncated protein resulted in the persistence of colony‐forming cells well beyond the short period for which such cells are present in uninfected cultures. The resultant colonies failed to give rise to cell lines; however, clonal cell lines occasionally emerged from the original infected liquid cultures. The virus which encoded a myb protein with a complete C‐terminus was virtually inactive in the colony assay; surprisingly, however, this virus could enhance proliferation in liquid cultures and has led to the generation of at least one cell line. In addition to demonstrating ‘activation’ of c‐myb by C‐terminal truncation, our results imply that an unaltered c‐myb protein can also contribute to cellular transformation and that a second event is required to establish myb‐transformed cells as a permanent cell line.

To date, cellular transformation in vitro by the myb oncogene has been described for avian haemopoietic cells only. In order to exploit the well‐characterized murine haemopoietic system to study transformation by myb, we have infected fetal liver cells with retroviral vectors carrying cDNAs that encode either complete or carboxy‐terminally truncated c‐myb proteins. We describe four cell lines which, despite our ability to efficiently infect haemopoietic target cells, were generated at low frequency. This was due, as least in part, to the requirement for a rearrangement within the vector that allowed expression of myb sequences. Three of the lines express a truncated myb protein while the fourth apparently expresses a normal c‐myb protein, and thus constitutes an exception to the general association of truncation with transformation by myb. All four cell lines resemble immature cells of the myelomonocytic lineage and are dependent on colony‐stimulating factors (CSFs) for their growth in vitro. One representative line could be converted to CSF‐independence by infection with either Abelson murine leukaemia virus or a recombinant granulocyte‐macrophage‐CSF‐encoding retrovirus; unlike the parental line, the resultant sublines were highly tumorigenic when injected into syngeneic mice.

Hexamethylenebisacetamide (HMBA)-induced differentiation of murine erythroleukemia cells (MELC) is a multistep process involving an early latent period during which a number of metabolic changes have been detected, but the cells are not yet committed irreversibly to differentiate. Commitment is defined as the capacity of MELC to go on to express the program of terminal cell division and gene expression (such as the accumulation of globin mRNA) upon removal of the HMBA from the culture. In the presence of HMBA, a small proportion of MELC are committed by 10-12 hr and greater than 90% by 48-60 hr. The present study shows that, during the initial 4 hr of culture, HMBA causes a marked decrease in c-myb and c-myc and an increase in c-fos mRNA levels. With continued culture, the decrease in c-myb and the increase in c-fos mRNA persists, while c-myc mRNA returns to control levels before the time that MELC begin to show irreversible differentiation. Dexamethasone, which blocks expression of HMBA-induced MELC differentiation, does not alter the early pattern of changes in protooncogene mRNA nor the sustained elevation of c-fos, but it does inhibit the continued suppression of c-myb allowing c-myb to return toward control levels. Hemin, which induces MELC to accumulate globins but does not initiate commitment to terminal cell division, does not alter these protooncogene mRNA levels. These studies suggest that, although the early decrease in c-myb and c-myc and increase in c-fos mRNAs may be involved in the multistep events leading to differentiation, the continued suppression of c-myb is critical for HMBA-induced MELC commitment to terminal cell division.