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

Abstract

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.

The Wilms' tumor gene product represses the transcription of thrombospondin 1 in response to overexpression of c-Jun

“…In a previous study we have shown that the transcription factor c-Jun, negatively regulates the transcription of TSP1 (Mettouchi et al, 1994). In this study, we show that this repression occurs by an indirect mechanism involving a factor secreted (or activated) by c-Jun over-expressing cells.…”

“…TSP1 repression is also observed in FR3T3 cells transformed by constitutive overexpression of c-Jun (Mettouchi et al, 1994). In one of these c-Jun-transformed cell lines, cJ4, the TSP1 mRNA level was reduced to 15% of that in FR3T3 cells (Figure 1a).…”

“…In previous experiments we had shown that the high level of repression of the TSP1 gene observed in c-Jun transfection experiments was, at least in part, due to a factor secreted in the medium by cells overexpressing cJun. Consequently, TSP1 expression was repressed even in non-transfected cells (Mettouchi et al, 1994). To con®rm that this repression was transcriptional, conditioned media from cells transformed by c-Jun, cJ4 cells (CM-cJ4), were tested on the TSP1 ± SEAP reporter and showed a marked (90%) inhibition of SEAP activity as compared to control levels ( Figure 1c).…”

“…In order to better understand the molecular mechanisms involved in cellular transformation by c-Jun, we constructed a di erential cDNA library to identify new target genes for this transcription factor. Among the genes whose expression was regulated by c-Jun, we found three genes coding for extracellular matrix proteins: SPARC, thrombospondin 1 (TSP1) (Mettouchi et al, 1994) and tenascin-C (Mettouchi et al, 1997).…”

“…Due to these multiple interactions, TSP1 has been shown to be implicated in development, in¯ammation, wound healing, platelet aggregation, coagulation, tumor growth and metastasis. Many, but not all, transformed cell lines express very low levels of TSP1 (Mettouchi et al, 1994;Salnikow et al, 1994;Slack and Bornstein, 1994;Sheibani and Frazier, 1996;Tikhonenko et al, 1996). In some cases, re-expression of TSP1 in transformed cells suppresses their tumorigenicity (Weinstat et al, 1994;Sheibani and Frazier, 1995).…”