BACKGROUND & OBJECTIVE: High mobility group box l (HMGB1), a nuclear DNA-binding protein, stabilizes the structure and function of chromatin, regulates gene transcription. Recent studies found that HMGB1 is associated with the proliferation and metastasis of many tumors, including breast cancer, colon carcinoma, and melanoma, and is rich in various solid cancer tissues and immature cells. This study was to explore the role of HMGB1 in adriamycin (ADM)-induced apoptosis in leukemia K562 cells. METHODS: K562 cells were transiently transfected with recombinant plasmid pcDNA3.1-HMGB1. The expression of HMGB1 in K562 cells were detected by Western blot and reverse transcription-polymerase chain reaction (RT-PCR). The 50% inhibition concentration (IC(50)) of ADM for K562 cells was determined by WST-8 assay. Cell apoptosis was assessed by flow cytometry. The protein level of Bcl-2 was detected by Western blot. The activities of Caspase-3 and Caspase-9 were assayed with Caspase Colorimetric Assay Kit. RESULTS: The mRNA and protein levels of HMGB1 in K562 cells transfected with pcDNA3.1-HMGB1 were increased by about 85% and 56% respectively as compared with those in K562 cells transfected with pcDNA3.1. Overexpression of HMGB1 in K562 cells by transient transfection significantly increased the resistance to ADM; the IC(50) of ADM was increased from (0.06+/-0.00) microg/mL to (3.46+/-0.06) microg/mL. When treated with 1 microg/mL ADM, the apoptosis rate was significantly lower in HMGB1-transfected K562 cells than in pcDNA3.1-transfected K562 cells [(12.00+/-1.26)% vs. (44.50+/-1.87)%, P<0.05]. Overexpression of HMGB1 in K562 cells significantly inhibited ADM-induced down-regulation of Bcl-2 protein. After treatment of ADM, the activities of Caspase-3 and Caspase-9 in HMGB1-transfected K562 cells were inhibited as compared with those in pcDNA3.1-transfected K562 cells (Caspase-3: 1.55+/-0.06 vs. 2.55+/-0.06 at 12 h, 1.86+/-0.10 vs. 2.85+/-0.06 at 24 h, P<0.05; Caspase-9: 1.40+/-0.08 vs. 2.03+/-0.05 at 12 h, 1.55+/-0.06 vs. 2.22+/-0.05 at 24 h, P<0.05). CONCLUSION: HMGB1 overexpression could inhibit ADM-induced apoptosis in leukemia K562 cells through regulating the protein level of Bcl-2 and the activities of Caspase-3 and Caspase-9.