TGF-β/Smad signaling is important for v-Rel mediated transformation
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The v-rel oncogene is the most efficiently transforming member of the Rel/NF-κB family of transcription factors. Identification of genes or signal transduction pathways that contribute to v-Rel transformation provide insight into the mechanisms of tumorigenesis by Rel/NF-κB proteins. In these studies, the contribution of TGF-β/Smad signaling to v-Rel transformation was assessed. TGF-β/Smad signaling regulates several cellular processes, including growth, differentiation, and apoptosis and has been implicated in a number of different cancers. Using microarray technology and Northern blot analysis, key components of the TGF-β/Smad pathway (tgf-β2 and tgf-β3 ligands, TGF-β type II receptor, and receptor-activated smad3) were identified with upregulated mRNA expression in v-Rel-transformed fibroblasts and lymphoid cells relative to control cells. A corresponding change in their protein levels was also observed. Further analysis revealed elevated levels of the phosphorylated, active form of Smad3, which correlated with its increased DNA-binding activity in v-Rel transformed cells. In contrast, the overexpression of c-Rel resulted in little to no alteration in the RNA and protein expression of members of the TGF-β/Smad pathway. Further studies demonstrated that elevated TGF-β/Smad signaling is required for the transforming ability of v-Rel. Blocking TGF-β signaling with a kinase inhibitor of TGF-β type I receptor inhibited the activation of Smad3 and dramatically reduced the ability of v-Rel transformed cells to form colonies in soft agar. Overexpression of a constitutively active form of Smad3 in the inhibitor-treated cells restored their ability to form colonies in soft agar close to the levels seen in untreated cells. Additional experiments with dominant negative Smad3 also revealed its ability to hinder the oncogenic potential of v-Rel. In complementary experiments, a stimulatory effect on v-Rel transformation was observed with cells treated with recombinant TGF-β2 ligand or overexpressed with wild-type Smad3. Taken together, these studies demonstrate that TGF-β signaling is crucial for the transformation potential of v-Rel and is primarily mediated by Smad3 activity.