EGCG neutralizes the cytotoxic effect of MG-262 and proteasome inhibitor IX but not NFV, MG-132, and proteasome inhibitor I. (A) Chemical structures of the proteasome inhibitors NFV, proteasome inhibitor I (PS-I), MG-132, MG-262, and proteasome inhibitor IX (PS-IX), and their grouping according to the presence of a boronic acid. (B-F) LN229 cells were treated with increasing concentrations of the individual proteasome inhibitors in the presence or absence of 20 μM EGCG for 48 hours; thereafter, the number of long-term surviving cells that were able to spawn colonies during the following 2 weeks (in the absence of drug treatment) was determined by colony-formation assays. Shown is the mean plus or minus SE (n ≥ 3). (G) Typical result of MTT survival assay of LN229 cells treated for 48 hours with increasing concentrations of BZM in the presence of increasing concentrations of either MG-262 or MG-132. Shown are wells from a 96-well plate. Deep purple color represents fully viable cells; pink(ish) color indicates reduced viability; yellow indicates absence of viable cells.