Autophagy acts through TRAF3 and RELB to regulate gene expression via antagonism of SMAD proteins.

Macroautophagy can regulate cell signalling and tumorigenesis via elusive molecular mechanisms. We establish a RAS mutant cancer cell model where the autophagy gene ATG5 is dispensable in A549 cells in vitro, yet promotes tumorigenesis in mice. ATG5 represses transcriptional activation by the TGFbeta-SMAD gene regulatory pathway. However, autophagy does not terminate cytosolic signal transduction by TGFbeta. Instead, we use proteomics to identify selective degradation of the signalling scaffold TRAF3. TRAF3 autophagy is driven by RAS and results in activation of the NF-kappaB family member RELB. We show that RELB represses TGFbeta target promoters independently of DNA binding at NF-kappaB recognition sequences, instead binding with SMAD family member(s) at SMAD-response elements. Thus, autophagy antagonises TGFbeta gene expression. Finally, autophagy-deficient A549 cells regain tumorigenicity upon SMAD4 knockdown. Thus, at least in this setting, a physiologic function for autophagic regulation of gene expression is tumour growth.