Growth Properties and Tumorigenesis of MCF-7 Cells Transfected with Isogenic Mutants of ras^H

MCF-7 human breast cancer cells are estrogen dependent for maximal in vitro growth and for tumor formation in nude mice, thus providing a useful model system to study mammary tumorigenesis. A clone of MCF-7 cells transfected with the v-ras^H oncogene has been shown to form tumors in the absence of estradiol [Kasid et al., 1985, Science (Wash. DC), 228: 725–728]. To extend this observation to more clones of v-ras^H-expressing MCF-7 cells and to examine the effects of ras^H mutation, we transfected MCF-7 cells with a construct encoding the human c-ras^H protooncogene protein product and with three isogenic constructs encoding proteins containing point mutations: arg-12, thr-59, and arg-12 + thr-59 (v-ras^H). We isolated several cell lines which produced levels of c-ras^H and v-ras^H p21 at 30- to 50-fold the levels of controls. We also isolated several cell lines producing the various mutants p21s. All of the transfected cell lines were estrogen-responsive for cell growth. Transfected cells containing high levels of ras^H p21 had correspondingly high levels of growth in an anchorage-independent growth assay. Tumorigenesis studies in nude mice, however, showed that some, but not all of the cell lines expressing v-ras^H, formed tumors in the absence of estradiol. Tumor formation did not correlate with the level of ras^H p21 expression in these cell lines. No tumor formation in the absence of estradiol was observed for cell lines expressing single-mutated or unmutated forms of ras^H.