(A) RV4 lymphoma cells which lack GSTP1-1 expression are sensitive to ATO/EA induced apoptosis at low EA concentration. RV4 cells and RG19 (high GSTP1-1 expression) cells were treated with 1 μM ATO/15 μM EA for 24 h and the apoptotic cells were detected by FACS after staining with annexin V-FITC. (B) ATO/EA induced mitochondrial apoptosis pathway is muted in RG19 cells. RV4 and RG19 cells were treated with 1 μM ATO/15 μM EA for 24 h and the protein levels of the mitochondrial apoptotic pathways were analyzed by Western blot analysis. (C) Lymphoma cell lines differ widely in GSTP1-1protein and activity. The protein levels of GSTP1-1 were determined by Western blotting and the activity was determined biochemically. (D) Low GSTP1-1 activity lymphoma cells, Raji, Namalwaand Daudi, are highly sensitive to apoptosis induction by low concentrations of ATO/EA; Jurkat and Su-DHL-4 cells with high GSTP1-1activity are resistant. Raji, Namalwa, Daudi, Su-DHL-4 and Jurkat cells were treated with 1 μM ATO together with EA at the indicated concentrations for 24 h. Apoptotic cells were detected by FACS after staining with annexin V-FITC.