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SPARC and thrombospondin genes are repressed by the c‐jun oncogene in rat embryo fibroblasts.

The sequence‐specific transcription factor c‐Jun displays oncogenic potential in mammalian cells either in cooperation with activated Ras in primary embryonic fibroblasts or alone in established cell lines. Although pathways for signal transduction leading to activation of c‐Jun proteins have been extensively studied, little is known about the events downstream of c‐Jun stimulation. We isolated cellular genes that are targets of c‐Jun by differential screening of a cDNA library from primary rat embryo fibroblasts. Two transcripts with sequences similar to known genes were repressed following transitory expression of a c‐Jun‐encoding vector. They correspond to the SPARC and thrombospondin 1 (TS1) genes, encoding extracellular matrix proteins. These genes are tightly regulated during embryogenesis and in adult tissues and are involved in the control of cell growth. c‐Jun transitory repression of these two genes was demonstrated both in primary cells and in FR3T3, an established fibroblast cell line. The repression was also detected in FR3T3 derivatives stably transformed by c‐Jun or Ras. Although c‐Jun regulation of the TS1 gene was found at the promoter level, preliminary results strongly suggest that repression of SPARC and TS1 gene expression are mediated by a secreted factor. In contrast, expression of these genes was unaffected by transformation with oncogenes from DNA viruses. Our results identify new, specific, probably indirect c‐Jun target genes and suggest previously unsuspected regulatory roles for SPARC and thrombospondin in the control of cell growth.