Anchorage-independent Growth-conferring Factor Production by Rat Mammary Tumor Cells

Conditioned medium from cultures of 7,12-dimethylbenz(a)anthracene-induced rat mammary tumor cells contain factors that resemble sarcoma growth factor and other transforming growth factors in biological activity but differ in their physical properties. The mammary tumor factors (MTF) are acid stable and heat and protease sensitive. They inhibit the binding of epidermal growth factor, but not insulin, to mouse embryonal carcinoma cells. MTF confers upon normal rat kidney and BALB/c-3T3 cells the ability to grow in soft agar. This effect is enhanced synergistically by high concentrations of fetal calf serum but not by epidermal growth factor. Anchorage-independent growth promotion, however, is not seen with normal mammary epithelial cells, although MTF is mitogenic for these cells as well as normal rat kidney cells, BALB/c-3T3 cells, and chick embryo fibroblasts in monolayer culture. MTF is not mitogenic for primary cultures of the tumor cells from which the factors are derived. Two major molecular weight species of MTF, eluting at M_r 6,000 and 65,000 to 70,000 on Bio-Gel P-100 columns, are present in acid-ethanol extracts of 7,12-dimethylbenz(a)anthracene- and nitrosomethylurea-induced rat mammary tumors. Transplantable tumors derived from primary 7,12-dimethylbenz(a)anthracene- or nitrosomethylurea-induced tumors have little or no MTF activity. These results demonstrate that different chemically induced rat mammary tumors contain transforming growth factor-like activities. Furthermore, it is possible that MTF is unnecessary for the maintenance of tumorigenicity, since some tumors contain no detectable MTF.