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Background: Histone deacetylase (HDAC) inhibitors lead to the acetylation of histones, chromatin decondensation and the sensitization of cancer cells to DNA damaging agents such as topoisomerase II inhibitors. We previously reported that treatment of breast cancer cells with the HDAC inhibitor, valproic acid (VPA), resulted in chromatin decondensation through the down-regulation of heterochromatin maintenance proteins such as heterochromatin protein 1 (HP1), structural maintenance of chromatin proteins (SMC) 1-5 and DNA methyltransferase 1 (DNMT1). In this study, we evaluated the importance of individual HDAC enzymes in the chromatin structure changes induced by VPA. Methods: HDAC enzymes were individually depleted using siRNA transfection. HDAC enzyme depletion and the downstream effects on the expression of heterochromatin maintenance proteins were evaluated by Western blot analysis. Changes in the chromatin structure were evaluated by electron microscopy. Negative controls included transfection of non-silencing siRNA. Non-transfected cells treated with VPA served as a positive control for the down-regulation of heterochromatin maintenance proteins and chromatin decondensation. Results: Transfection of MCF-7 breast cancer cells with HDAC2-specific siRNA resulted in the selective depletion of HDAC2 enzyme without affecting the expression of other HDACs. Depletion of HDAC2 led to the down-regulation of the heterochromatin maintenance proteins and sensitized cells to apoptosis induced by the topoisomerase II inhibitor, epirubicin. Transfection of cells with non-silencing siRNA did not affect heterochromatin protein expression and did not sensitize cells to epirubicin. Conclusions: Our data suggest that HDAC2 is involved in the regulation of chromatin structure and dynamics. Inhibition of HDAC2 may play an important role in HDAC inhibitor-induced sensitization of breast cancer cells to chemotherapy.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA