HMGB1-induced Autophagy Promotes Chemotherapy Resistance In Leukemia Cells

High mobility group box 1 (HMGB1), a highly conserved nuclear protein, is important in the regulation of stabilizing the structure and function of chromatin and gene transcription. Leukemia is one of the most common malignant tumors in children. Autophagy, a tightly regulated lysosome-dependent catabolic pathway, is important in the regulation of cancer development and progression and in determining the response of tumor cells to anticancer therapy. However, the role of autophagy in leukemia still remains largely unknown.Firstly, we found that HMGB1 involved in the drug resistance mechanisms in leukemia. Levels of HMGB1 expression were significantly high in cell culture supernents of human leukemia cancer cell lines (acute promyelocytic leukemia cell HL-60 and the human Jurkat T leukemia cells) following treatment with vincristine (VCR), adriamycin (ADM), cytosine arabinoside (Ara-c) and arsenic trioxide (AS2O3). Supression of HMGB1 in HL-60 and Jurkat cells by transient transfection significantly increased the susception to VCR?ADM?Ara-c?AS2O3. Pretreatment with HMGB1 increased drug resistance in HL-60 and Jurkat cells, supporting a potential prosurvival role for HMGB1 in cells exposed to chemotherapy.Secondly, we found that autophagy was required for exogenous HMGB1-mediated chemotherapy resistance. Supression of autophagy by transient transfection with Beclin 1 and pharmacological inhibiton of autophagy with bafilomycin Al resulted in a reversal of HMGBl-induced drug resistance in leukemia cells; whereas enhanced expression of HMGB1 by pretreatment with HMGB1 protein, there is a time-dependent increased autophagic flux. Moreover, exogenous HMGB1 increased autophagy as evaluated by increased expression of the autophagic marker microtubule-associated protein light chain 3-II, degradation of sequestosome 1 (p62) and autophagosome formation.Finally, we demonstrated that HMGB1 induced autophagy through the PI3K-MEK-ERK pathway. Supression of PI3K-III by gene transfection and pharmacological inhibitor 3-MA, significantly decreased HMGB1 induced LC3 conversion and P62 degradation in HL-60 and Jurkat cells. In addition, our research revealed that suppression of PI3K-III inhibited HMGB1-induced ERK1/2 phosphorylation. Supression of MEK-ERK pathway through gene transfection and pharmacological inhibitor U0126, HMGB1-induced LC3 conversion and P62 degradation were significantly decreased.In summary, we show that HMGB1 is released in leukemia cell lines in response to chemotherapy and functions as a positive regulator of autophagy, which enhances resistance to anticancer therapies. HMGB1 regulates the occurance of autophagy mainly through the PI3K-MEK-ERK pathway. In light of the therapeutic promise of autophagy inhibitors to render tumor cells more susceptible to conventional therapies, our discovery that HMGB1 is an important regulator of autophagy in leukemia cells may lead to the development of novel strategies for the treatment of human leukemia.