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

BACKGROUND: SnoN 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.
METHODS: We 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.
RESULTS: SnoN 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.
CONCLUSIONS: ANS 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 SIGNIFICANCE: Antibiotic analogs lacking ribotoxic effects are useful as pharmacological tools to study TGF-β signaling by controlling Ski and SnoN protein levels.