LOSS OF ONCOGENIC RAS EXPRESSION DOES NOT CORRELATE WITH LOSS OF TUMORIGENICITY IN HUMAN-CELLS

ras oncogenes are mutated in a variety of human tumors, which suggests that they play an important role in human carcinogenesis. To determin e whether continued oncogenic ras expression is necessary to maintain the malignant phenotype, we studied the human fibrosarcoma cell line, HT1080, which contains one mutated and one wild-type N-ras allele, We isolated a variant of this cell line that no longer contained the muta ted copy of the N-ras gene. Loss of mutant N-ras resulted in cells tha t displayed a less transformed phenotype characterized by a flat morph ology, decreased growth rate, organized actin stress fibers, and loss of anchorage-independent growth. The transformed phenotype was restore d following reintroduction of mutant N-ras. Although loss of the oncog enic N-ras drastically affected in vitro growth parameters, the varian t remained tumorigenic in nude mice indicating that mutated N-ras expr ession is not necessary for maintenance of the tumorigenic phenotype. We confirmed this latter observation in colon carcinoma cell lines tha t have lost activated K-ras expression via targeted knockout of the mu tant K-ras gene.