Persistence of Ha-ras-induced metastatic potential of SP1 mouse mammary tumors despite loss of the Ha-ras shuttle vector

Abstract

Previous studies have shown that the SP1 mouse mammary adenocarcinoma cell line, which is tumorigenic but nonmetastatic, acquires metastatic potential when transfected with the activated human Ha-ras gene. In addition, the process of calcium phosphate-mediated DNA transfection, as well as treatment with the calcium ionophore A23187 or with phorbol 12-myristate 13-acetate, can also result in heritable changes in the malignant behavior of SP1 cells. It was of interest, therefore, to determine whether the metastatic consequences of Ha-ras oncogene expression in SP1 cells are a primary effect of the transfected gene or whether heritable secondary changes are induced by Ha-ras oncogene expression. In the latter case, continued expression of the Ha-ras oncogene would not be required to maintain the metastatic phenotype. To test this hypothesis we introduced the Ha-ras oncogene into SP1 cells on a shuttle vector in which maintenance of the vector was dependent on selection for resistance to the antibiotic G418. Subclones which had lost the transfected Ha-ras gene were subsequently isolated following growth in nonselective medium. The Ha-ras-transfected clones and the revertant subclones were found to be equally metastatic, indicating that transfection with the Ha-ras gene does induce stable secondary changes in the metastatic phenotype of SP1 cells.