Alteration of the Tumorigenic and Metastatic Properties of Neoplastic Cells is Associated with the Process of Calcium Phosphate-Mediated DNA Transfection

During the course of studies designed to assess the effect of human Ha-ras gehe expression on the malignant behavior of transfected mouse tumor cells we noted that the process of Ca3(PO4)2-media ted DNA transfection was itself associated with profound alterations in tumorigenic or metastatic behavior. The cell line used as a recipient for these studies was a tumorigenic nonmetastatic CBA/J mouse mammary adenocarcinoma line called SP1. When cotransfected with plasmids containing the neo gene (pSV2neo) and the activated Ha-ras gene (pT24-c3), cells from the pooled (5-10 colonies) G418-resistant colonies gave rise to spontaneous lung metastases in 85% of mice after subcutaneous inoculation. However, we noted that 17% of control mice inoculated with G418-resistant pSV2neo-transfected SP1 cells also had lung metastases and that this number approached 100% as the inoculum comprised a greater pool size (50-100 colonies). When cell lines established from isolated pSV2neo-transfected colonies were examined, 3/16 were found to be metastatic. We also found that 3/16 clones grew slowly, or not at all, in CBA/J mice, whereas they grew readily in athymic (nude) mice. The increase in immunogenicity of two out of three of these latter clones was accompanied by expression of the class I H-2Dkmajor histocompatibility complex antigen that was not detectable in the parental SP1 cells. At least some of these results would appear to be due to exposure to Ca3(PO4)2alone, as we found that it resulted in 5/20 (25%) clones manifesting metastatic properties. Our results suggest that heritable changes in malignant behavior of transfected tumor cells can be observed at high frequency subsequent to the process of Ca3(PO4)2-media ted DNA transfection, and these changes may be brought about in part by inherited disturbances in expression of recipient cell genes.