Activation of tumor suppressor genes in nontumorigenic revertants of the HeLa cervical carcinoma cell line.

Two nontransformed revertants of HeLa cells, designated HA and HF, were isolated using a selection procedure based on prolonged retention of the fluorescent dye rhodamine 123 within the mitochondria of HeLa (ATCC CCL2) cells versus normal epithelial cells. Unlike the parental HeLa cells, the revertants expressed markedly reduced levels of the bone-liver-kidney, placental, and intestinal isoforms of alkaline phosphatase, exhibited a flat nonrefractile morphology, and failed to grow in suspension culture. The revertant clones had > 100-fold reduced cloning efficiencies in semisolid medium relative to HeLa cells and failed to induce s.c. tumors when injected into nude mice. Both revertant clones have retained nontransformed phenotypes after 5 years of continuous culture. Southern blot analyses performed with human papillomavirus 18-specific DNA probes indicated that the integrated viral sequences present in HeLa cells remained intact in the revertants. Furthermore, the level of the polycistronic mRNAs encoding the viral E6 and E7 oncogenes were comparable in the parental HeLa cell line and the revertants. Western blot analyses of immunoprecipitated human papillomavirus 18 E6 and E7 proteins further demonstrated that the levels of these viral oncoproteins were comparable in HeLa cells and revertants. Infection with helper-free, defective retroviruses that express E6, E7 or E6 and E7 oncogenes failed to retransform the revertants, suggesting that their nontransformed phenotype did not result from mutations in these viral oncogenes. Cell fusion experiments indicated that the revertant phenotypes of HA and HF cells resulted from mutations in cellular genes that activate one or more tumor suppressor genes.