THE C-JUN-INDUCED TRANSFORMATION PROCESS INVOLVES COMPLEX REGULATION OF TENASCIN-C EXPRESSION

In cooperation with an activated ras oncogene, the site-dependent AP-1 transcription factor c-Jun transforms primary rat embryo fibroblasts (REF). Although signal transduction pathways leading to activation of c-Jun proteins have been extensively studied, little is known about c- Jun cellular targets. We identified c-Jun-upregulated cDNA clones homo logous to the tenascin-C gene by differential screening of a cDNA libr ary from REF. This tightly regulated gene encodes a rare extracellular matrix protein involved in cell attachment and migration and in the c ontrol of cell growth. Transient overexpression of c-Jun induced tenas cin-C expression in primary REF and in FR3T3, an established fibroblas t cell line. Surprisingly, tenascin-C synthesis was repressed after st able transformation by c-Jun compared to that in the nontransformed pa rental cells. As assessed by using the tenascin-C (-220 to +79) promot er fragment cloned in a reporter construct, the c-Jun-induced transien t activation is mediated by two binding sites: one GCN4/AP-1-like site , at position -146, and one NF-kappa B site, at position -210. Further more, as demonstrated by gel shift experiments and cotransfections of the reporter plasmid and expression vectors encoding the p65 subunit o f NF-kappa B and c-Jun, the two transcription factors bind and synergi stically transactivate the tenascin-C promoter. We previously describe d two other extracellular matrix proteins, SPARC and thrombospondin-1, as c-Jun targets. Thus, our results strongly suggest that the regulat ion of the extracellular matrix composition plays a central role in c- Jun-induced transformation.