aPKC alters the TGFβ response in NSCLC cells through both Smad-dependent and Smad-independent pathways

Transforming growth factor beta (TGF beta) signaling controls many cellular responses including proliferation, epithelial to mesenchymal transition and apoptosis, through the activation of canonical (Smad) as well as non-canonical (e.g. Par6) pathways. Previous studies from our lab have demonstrated that aPKC inhibition regulates TGF beta receptor trafficking and signaling. Here, we report that downstream TGF beta-dependent transcriptional responses in aPKC-silenced NSCLC cells were reduced compared with those of control cells, despite a temporal extension of Smad2 phosphorylation. We assessed SARA-Smad2-Smad4 association and observed that knockdown of aPKC increased SARA (also known as ZFYVE9) levels and SARA-Smad2 complex formation, increased cytoplasmic retention of Smad2 and reduced Smad2-Smad4 complex formation, which correlated with reduced Smad2 nuclear translocation. Interestingly, we also detected an increase in p38 MAPK phosphorylation and apoptosis in aPKC-silenced cells, which were found to be TRAF6-dependent. Taken together, our results suggest that aPKC isoforms regulate Smad and non-Smad TGF beta pathways and that aPKC inhibition sensitizes NSCLC cells to undergo TGF beta-dependent apoptosis.