Viral Myc oncoproteins in infected fibroblasts down-modulate thrombospondin-1, a possible tumor suppressor gene
A T Tikhonenko1, D J Black, M L Linial
1Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA. atikhone@fred.fhcrc.org
Abstract:
We are interested in identifying the transcriptional targets of the Myc oncoproteins. To this end, we have fused Myc of the MC29 retrovirus with the rat glucocorticoid receptor. This chimeric protein requires dexamethasone to undergo nuclear translocation and achieve an active conformation. We employed a differential hybridization approach to identify mRNAs that are induced or repressed in infected avian fibroblasts in response to dexamethasone. This screen yielded one mRNA underrepresented in the dexamethasone-treated cells. In Myc-transformed cell clones, its level decreases 6-fold as early as 4 h and more than 30-fold after 32 h of exposure to the hormone. This mRNA was also down-regulated by recombinant Myc retroviruses in rodent fibroblasts, including those refractory to transformation. Sequence analysis revealed that it is homologous to the 3' untranslated regions of the mammalian thrombospondin-1 genes. Using an anti-thrombospondin antibody, we confirmed that rodent cells overexpressing Myc produce very small amounts of this protein. Also, they do not support efficient expression of a reporter gene driven by the thrombospondin-1 promoter. Thus, thrombospondin-1 is a bona fide target of Myc. Moreover, its silencing might pertain to the transforming activity of Myc, since in several systems thrombospondin-1 exhibits tumor suppressor properties, presumably due to its negative effect on neovascularization.
Insights
Myc oncoproteins down-regulate thrombospondin-1 expression. This finding identifies a novel Myc target and suggests its role in cancer by potentially inhibiting tumor suppressor functions like anti-neovascularization.
Area of Science:
- Oncology
- Molecular Biology
- Gene Regulation
Background:
- Myc oncoproteins are crucial regulators of cell proliferation and transformation.
- Identifying Myc's transcriptional targets is key to understanding its oncogenic mechanisms.
- Thrombospondin-1 (TSP-1) has known tumor suppressor functions, including anti-angiogenesis.
Purpose of the Study:
- To identify novel transcriptional targets of Myc oncoproteins.
- To investigate the relationship between Myc activity and thrombospondin-1 expression.
- To explore the potential role of TSP-1 regulation in Myc-mediated transformation.
Main Methods:
- Construction of a chimeric Myc-glucocorticoid receptor fusion protein.
- Differential hybridization to screen for gene expression changes.
- Quantitative analysis of mRNA and protein levels.
- Reporter gene assays to assess promoter activity.
Main Results:
- A specific mRNA, homologous to thrombospondin-1 (TSP-1) 3' UTR, was identified as significantly downregulated by Myc.
- Myc overexpression led to a substantial decrease in TSP-1 mRNA and protein levels in avian and rodent cells.
- Myc significantly repressed the thrombospondin-1 promoter activity.
Conclusions:
- Thrombospondin-1 is a direct transcriptional target of Myc oncoproteins.
- Myc-mediated repression of TSP-1 may contribute to Myc's transforming activity by diminishing its tumor suppressor functions.
- The downregulation of TSP-1 by Myc could impact neovascularization, a process relevant to tumor growth.
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