STAT3 confers chemoresistance to ABC-DLBCL cells by promoting a cellular antioxidant program. (A) Immunoblot analysis of PY705-STAT3 and PS727-STAT3 during a 24-hour Dox treatment course of 4 ABC-DLBCL cell lines. (B) The effect of STAT3 silencing on basal and Dox-induced ROS and cell viability change. Results from Ly3 and SUDHL2 cells were shown. ROS and cell viability measurements were made as in Figure 3. Cell viability was defined as the fraction of Annexin-neg/PI-neg cells. Forty-eight hours after transfection with either control (Ctrli) or STAT3-specific siRNA (STAT3i) oligos, changes in SOD2 mRNA (C) and protein (D) were examined by qPCR and immunoblot, respectively. (E) Luciferase reporter assay measuring the effects of inhibitors to STAT3 (10 μM STATTIC; ST) and Jak (20 μM AG490; AG) on the activity of a luciferase construct driven by the 3.6-kb human SOD2 promoter. SUDHL2 cells were used in transient transfections. (F) qChIP analysis was performed to measure in vivo binding of STAT3 to the SOD2 promoter region in Ly3 cells that were transfected with either Ctrli or STAT3i oligos. Signals enriched by STAT3 Abs were normalized to that from the control rabbit IgG. Results shown in B, C, and E are mean ± SD and representative of 2 independent experiments. *P < .05; **P < .01; ***P < .001, based on 2-tailed Student t test. (G) SOD-PEG can attenuate cell death associated with STAT3 inactivation. Ly 3 cells with or without STATTIC pretreatment were exposed to Dox. At 4 and 19 hours into the Dox treatment, aliquots of the samples were treated with SOD-PEG at 75 or 150 U/mL. Cell death was defined as the proportion of all Annexin V+ cells at 24 hours after Dox exposure.