Viral Myc oncoproteins in infected fibroblasts down-modulate thrombospondin-1, a possible tumor suppressor gene.
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.
Tikhonenko
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AT
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA. atikhone@fred.fhcrc.org
Black
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Linial
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D000595:Amino Acid Sequence; D000818:Animals; D002472:Cell Transformation, Viral; D003370:Coturnix; D015536:Down-Regulation; D005347:Fibroblasts; D015972:Gene Expression Regulation, Neoplastic; D016147:Genes, Tumor Suppressor; D016259:Genes, myc; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D016274:Oncogene Protein p55(v-myc) / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics; D012334:RNA, Neoplasm / Q000235:genetics; D051381:Rats; D016415:Sequence Alignment; D019699:Thrombospondins
D008562:Membrane Glycoproteins; D016274:Oncogene Protein p55(v-myc); D012333:RNA, Messenger; D012334:RNA, Neoplasm; D019699:Thrombospondins