The anticancer potency of green tea and its individual components is being 
intensely investigated, and some cancer patients already self-medicate with this 
"miracle herb" in hopes of augmenting the anticancer outcome of their 
chemotherapy. Bortezomib (BZM) is a proteasome inhibitor in clinical use for 
multiple myeloma. Here, we investigated whether the combination of these 
compounds would yield increased antitumor efficacy in multiple myeloma and 
glioblastoma cell lines in vitro and in vivo. Unexpectedly, we discovered that 
various green tea constituents, in particular (-)-epigallocatechin gallate 
(EGCG) and other polyphenols with 1,2-benzenediol moieties, effectively 
prevented tumor cell death induced by BZM in vitro and in vivo. This pronounced 
antagonistic function of EGCG was evident only with boronic acid-based 
proteasome inhibitors (BZM, MG-262, PS-IX), but not with several non-boronic 
acid proteasome inhibitors (MG-132, PS-I, nelfinavir). EGCG directly reacted 
with BZM and blocked its proteasome inhibitory function; as a consequence, BZM 
could not trigger endoplasmic reticulum stress or caspase-7 activation, and did 
not induce tumor cell death. Taken together, our results indicate that green tea 
polyphenols may have the potential to negate the therapeutic efficacy of BZM and 
suggest that consumption of green tea products may be contraindicated during 
cancer therapy with BZM.