The proteasome inhibitor MG132 potentiates TRAIL receptor agonist-induced apoptosis by stabilizing tBid and Bik in human head and neck squamous cell carcinoma cells

► MG132 sensitizes HNSCC cells to TRAIL-/AY4-mediated apoptotic cell death. ► MG132 potentiates TRAIL receptor-induced apoptosis by stabilizing tBid and Bik. ► Bak is constrained by Mcl-1 and Bcl-XL, but not by Bcl-2, in HNSCC cells. ► Bax did not interact with Mcl-1, Bcl-XL, or Bcl-2 in HNSCC cells. ► tBid plays a major role in Bax activation, and Bik indirectly activates Bak.