Irinotecan is a natural alkaloid agent widely used in cancer therapy. High-mobility group protein B1 as a non-histone chromosomal protein plays a fundamental role in gene expression and inflammation. In this study, the effect of irinotecan on high-mobility group protein B1 and MMP9 content, gene expression, cell cycle, and cell growth in human breast cancer cells (MCF-7) was investigated. The cells were exposed to various concentrations of irinotecan and the viability determined by trypan blue exclusion and 3-(4,5-dimethylthiazal-2-yl)-2,5-diphenyltetrazolium bromide assays. Highmobility group B proteins were extracted from the control and drug-treated cells and analyzed by immunoblot. Highmobility group protein B1 and MMP9 messenger RNA expression was studied by reverse transcription polymerase chain reaction. The results demonstrated reduction of cell viability upon increasing irinotecan concentration, up-regulated highmobility group protein B1 gene expression, and down-regulated MMP9 mRNA. Although the content of high-mobility group protein B1 was decreased in chromatin extract upon drug action, no high-mobility group protein B1 release to extracellular space was detected by immunoblot analysis. Irinotecan decreased H3K9 acetylation and increased poly ADP-ribose polymerase fragmentation to 89 kDa and anion superoxide production suggesting induction of apoptosis in these cells. Propidium iodide staining of the cells 24 h after the drug treatment revealed arrest of the cells in S-phase. From the results, it is concluded that overexpression of high-mobility group protein B1 in the presence of irinotecan precedes breast cancer cells into apoptosis and in this response the binding of irinotecan to chromatin or high-mobility group protein B1 may condense/aggregate chromatin, preventing high-mobility group protein B1 release from chromatin.
