Noncorrelative c-myc andras oncogene expression in squamous cell carcinoma cells with tumorigenic potential

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Abstract
The distribution of heterogeneous cell types within human tumors was examined, and the biological behavior of tumors and different tumor cell lines was evaluated following implantation into surrogate hosts. In situ hybridization and immunohistochemistry were used to examine the expression of oncogenes and localization of the squamous cell carcinoma cell surface‐associated antigens. Increased levels of H‐ras mRNA and p21 protein were present in six tumors, but enhanced c‐myc mRNA expression was observed in just two tumors. The distribution of oncogene mRNA and SCC antigen‐positive cells was not uniform throughout the tumor. Isolation of cells from the tumors was accomplished by cell culture, growth in soft agar, and growth in the nude mouse. One nontumorigenic immortalized cell line, SCC‐83‐01‐82, isolated by passage through soft agar, was treated with 50 μg/ml of methyl methane sulfonate (MMS). These MMS‐converted cells subsequently expressed a tumorigenic phenotype. In situ hybridization of the tumors that developed in nude mice revealed increased c‐myc and H‐ras mRNA expression. Serial passage of the MMS‐converted tumors in vivo was accompanied by consistent enhanced c‐myc expression. However, the levels of H‐ras and keratin mRNA expression decreased with passage in vitro. Northern blot analysis of c‐myc and H‐ras mRNA levels from the original SCC cell line showed no change in expression following MMS treatment. The data suggest that SCC‐83‐01‐82 is a premalignant cell line established from a mixed cell population in the tumor mass. It can be converted to a malignant phenotype by treatment with MMS, and the persistence of malignancy is under molecular control other than changes in the level of c‐myc and ras gene expression.