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 My c oncoproteins, To this end, we have fused Myc of the MC29 retrovirus with the rat glucocorticoid receptor, This chimeric protein requires d examethasone to undergo nuclear translocation and achieve an active co nformation, We employed a differential hybridization approach to ident ify mRNAs that are induced or repressed in infected avian fibroblasts in response to dexamethasone. This screen yielded one mRNA underrepres ented in the dexamethasone-treated cells. In Myc-transformed cell clon es, its level decreases B-fold as early as 4 h and more than 30-fold a fter 32 h of exposure to the hormone, This mRNA was also downregulated by recombinant Myc retroviruses in rodent fibroblasts, including thos e refractory to transformation, Sequence analysis revealed that it is homologous to the 3' untranslated regions of the mammalian thrombospon din-l genes, Using an anti-thrombospondin antibody, we confirmed that rodent cells overexpressing Myc produce very small amounts of this pro tein. Also, they do not support efficient expression of a reporter gen e driven by the thrombospondin-l promoter. Thus, thrombospondin-l is a bona fide target of Myc. Moreover, its silencing might pertain to the transforming activity of Myc, since in several systems thrombospondin -l exhibits tumor suppressor properties, presumably due to its negativ e effect on neovascularization.