The expression of Top2 isoforms in differentiated cardiomyocytes and their responses to DRZ. (A) Differentiation of H9C2 cells into myotubes after 3 and 10 days of culturing assessed by light microscopy. Magnification: 10 fold. (B) Western blot analysis of Top2a and Top2b expression in H9C2 cells during the first 14 days of culturing. (C) The effect of 24 h incubation with DRZ and its analogues ICRF-161 and ICRF-193 (100 μM each) on Top2 expression in H9C2 cells. B and C are Western blots using antibody reacting with both Top2a and Top2b. In B, band densities of either isoform were divided through those of the time point-matching internal standard β-tubulin and plotted with day 2 Top2/β-tubulin ratio taken as 100%. (D) The effect of co-treatment with 100 μM DRZ or with its analogue ICRF-161 on DSB γ-H2AX protein levels induced by 1 μM DOX (24 h).