Expression of Id1 protects leukemia cells from apoptosis. (A) Induction of apoptosis in K562 cells by Trail (5 ng/mL). Cells that exhibited annexin V staining were considered to be apoptotic. Percentage of GFP positive cells were shown in cells transiently transduced with lentiviral vectors coexpressing GFP and either siRNA-GL or siRNA-Id1. Results were derived from 2 independent experiments. (B) Western blot analysis of K562 cells using PARP antibody. An increased amount of 85-kDa fragment of PARP was observed in K1 and K2, compared with parental K562 cells or cells transduced with pBabe empty vector. (C) ID1 expression in HL60 cells by Western blot analysis. Note that ID1-transfected cells (H1 and H2) showed a significant increase in Id1 expression. (D) Induction of apoptosis in HL60 cells by serum starvation. Cells that displayed annexin V staining were considered to be apoptotic. Representative experiments are shown here. (E) Western blot analysis of HL60 cells (serum-starved for 72 hours) using PARP antibody. An increased amount of 85-kDa fragment of PARP was observed in parental HL60 cells and cells transduced with MSCV empty vector, compared with H1 or H2.