A NOVEL MECHANISM OF HA-RAS ONCOGENE ACTION - REGULATION OF FIBRONECTIN MESSENGER-RNA LEVELS BY A NUCLEAR POSTTRANSCRIPTIONAL EVENT

Although loss of cell surface fibronectin (FN) is a hallmark of many o ncogenically transformed cells, the mechanisms responsible for this ph enomenon remain poorly understood. The present study utilized the nont umorigenic human osteosarcoma cell line TE-85 to investigate the effec ts of induced Ha-ras oncogene expression on FN biosynthesis. TE-85 cel ls were stably transfected with metallothionein-Ha-ras fusion genes, a nd the effects of metal-induced ras expression on FN biosynthesis were determined. Induction of the ras oncogene, but not proto-oncogene, wa s accompanied by a decrease in total FN mRNA and protein levels. Trans fection experiments indicated that these oncogene effects were not due to reduced FN promoter activity, suggesting that a posttranscriptiona l mechanism was involved. The most common mechanism of posttranscripti onal regulation affects cytoplasmic mRNA stability. However, in this s tudy the down-regulation of FN was identified as a nuclear event. A co mponent of the ras effect was due to a mechanism affecting accumulatio n of processed nuclear FN RNA. Mechanisms that would generate such an effect include altered RNA processing and altered stability of the pro cessed message in the nucleus. There was no effect of ras on FN mRNA p oly(A) tail length or site of polyadenylation. There was also no evide nce for altered splicing at the ED-B domain of FN mRNA. This demonstra tion of nuclear posttranscriptional down-regulation of FN by the Ha-ra s oncogene identifies a new level at which ras oncoproteins can regula te gene expression and thus contribute to development of the malignant phenotype.