Downregulation of SnoN oncoprotein induced by antibiotics anisomycin and puromycin positively regulates transforming growth factor-β signals

Abstract

BackgroundSnoN and Ski proteins function as Smad transcriptional corepressors and are implicated in the regulation of diverse cellular processes such as proliferation, differentiation and transformation. Transforming growth factor-β (TGF-β) signaling causes SnoN and Ski protein degradation via proteasome with the participation of phosphorylated R-Smad proteins. Intriguingly, the antibiotics anisomycin (ANS) and puromycin (PURO) are also able to downregulate Ski and SnoN proteins via proteasome. MethodsWe explored the effects of ANS and PURO on SnoN protein downregulation when the activity of TGF-β signaling was inhibited by using different pharmacological and non-pharmacological approaches, either by using specific TβRI inhibitors, overexpressing the inhibitory Smad7 protein, or knocking-down TβRI receptor or Smad2 by specific shRNAs. The outcome of SnoN and Ski downregulation induced by ANS or PURO on TGF-β signaling was also studied. ResultsSnoN protein downregulation induced by ANS and PURO did not involve the induction of R-Smad phosphorylation but it was abrogated after TGF-β signaling inhibition; this effect occurred in a cell type-specific manner and independently of protein synthesis inhibition or any other ribotoxic effect. Intriguingly, antibiotics seem to require components of the TGF-β/Smad pathway to downregulate SnoN. In addition, SnoN protein downregulation induced by antibiotics favored gene transcription induced by TGF-β signaling. ConclusionsANS and PURO require TGF-β/Smad pathway to induce SnoN and Ski protein downregulation independently of inducing R-Smad2 phosphorylation, which facilitates TGF-β signaling. General significanceAntibiotic analogs lacking ribotoxic effects are useful as pharmacological tools to study TGF-β signaling by controlling Ski and SnoN protein levels.