Down-regulation of fibronectin gene expression at cell surfaces by various oncogenes is an important phenotypic change of many transformed cells. Mechanisms effecting this loss of cell surface fibronectin remain poorly understood. An isogenic mouse fibroblast system, Balb/c 3T3, was used to investigate the effect of oncogene transformation on fibronectin gene transcription. Fibronectin mRNA levels were shown to decrease by >80% when 3T3 cells became stably transformed by EJ-Ha-ras or c-sis oncogenes. The mouse fibronectin gene promoter was subsequently cloned from a mouse genomic library and inserted into reporter gene constructs. Transient transfection assays with the 1.2 kb fibronectin promoter demonstrated that its activity was also decreased in the two transformed cell lines to a similar extent as fibronectin mRNA levels. Co-transfections into 3T3 cells with both the promoter construct and either Ha-ras or c-sis expression constructs provided further support for negative regulation of fibronectin gene transcription by oncogene expression. Potential mechanisms for such transcriptional regulation are proposed based on deletion analysis of the fibronectin promoter. These data suggest that down-regulation of fibronectin gene transcription can contribute directly to the loss of cell surface fibronectin in transformed murine fibroblasts.

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