Aim:HMGB 1 (high-mobility group box-1) is a nuclear protein containing a con-sensus RB (retinoblastoma)-binding LXCXE motif.In this study,we studied thepotential association of HMGB 1 and RB and the in vitro and in vivo activities ofHMGB 1 in human breast cancer cells.Methods:The protein-protein interactionwas determined by immunoprecipitation-Western blotting and glutathione-S-trans-ferase capture assays;cell growth and radiosensitivity were examined by cellcounts,MTT assay,and clonogenic assay;cell cycle progression and apoptosiswere evaluated using flow cytometry;and the antitumor activity of HMGB 1 wasexamined with tumor xenografts in nude mice.Results:HMGB 1 was associatedwith RB via a LXCXE motif-dependent mechanism.HMGB 1 enhanced the abilityof RB for E2F and cyclin A transcription repression.The increased expression ofHMGB 1 conferred an altered phenotypes characterized by the suppression of cellgrowth;G1arrest and apoptosis was induced in MCF-7 cells containing the wild-type retinoblastoma (Rb) gene,but showed no activities in BT-549 cells contain-ing the Rb gene deletion.The HMGB 1-induced apoptosis accompanied by caspase3 activation and PARP (poly(ADP-ribose)polymerase) cleavage.HMGB 1 elevatedthe radiosensitivity of breast cancer cells in both the MCF-7 and BT-549 cell lines.The enhanced expression of HMGB 1 caused a suppression of growth of MCF-7tumor xenografts in nude mice,while LXCXE-defective HMGB 1 completely lostantitumor growth activity.Conclusion:HMGB 1 functions as a tumor suppressorand radiosensitizer in breast cancer.A HMGB 1-RB interaction is critical for theHMGB 1-mediated transcriptional repression,cell growth inhibition,G1cell cyclearrest,apoptosis induction,and tumor growth suppression,but is not required forradiosensitization.Therefore,it may be possible to design new therapies for thetreatment of breast cancer that exert their effects by modulating the HMGB 1 andRB regulatory pathway and HMGB 1-related gene therapy.
