Oncogene expression during progression of mouse mammary tumor cells; activity of a proviral enhancer and the resulting expression of int-2 is influenced by the state of differentiation.

The RAC cell lines are derived from a mammary tumor induced by the mouse mammary tumor virus (MMTV). Polygonal cells have retained many characteristics of epithelial cells and induce adenocarcinomas; cuboidal cells are poorly tumorigenic; and elongated cells produce highly malignant sarcoma-like tumors. MMTV proviral integrations, one of which has cis-activated the int-2 gene, demonstrated that the lines are clonal descendents from a single tumor cell. Polygonal cells have a high constitutive expression of MMTV and contain int-2 RNA. In contrast, the cuboidal and the elongated cells show no detectable expression of int-2, even in the presence of glucocorticoid hormone. Transcription of MMTV in these cells is also low but can be stimulated by dexamethasone, albeit to levels lower than in polygonal cells. Thus, expression of int-2 seems to be caused by an enhancing activity on the MMTV provirus which is not dependent on steroid hormone and is specific for mammary tumor cells with epithelial characteristics. Progression in this cell system does not require sustained expression of int-2.