Activity of suberoylanilide hydroxamic acid against human breast cancer cells with amplification of Her-2

Purpose: We determined the effects of suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, on hsp90 and its client proteins Her-2, AKT, and c-Raf, as well as evaluated the cytotoxic effects of cotreatment of SAHA with trastuzumab or docetaxel in human breast cancer BT-474 and SKBR-3 cells containing amplification of Her-2. Experimental Design: The cells were treated with SAHA (1.0-5.0 μmol/L) and/or trastuzumab (5-40 μg/mL) or docetaxel (5-20 nmol/L). Following this, apoptosis and the levels of p21 ^WAF1, p27^KIP1, AKT, c-Raf, and Her-2, as well as of the key regulators of apoptosis were determined. Synergistic interaction between drugs was evaluated by median dose-effect analysis. Results: Treatment with SAHA up-regulated p21^WAF1 and p27^KIP1 levels, increased the percentage of cells in 6²-M phase of the cell cycle, as well as induced apoptosis in a dose-dependent manner. This was associated with up-regulation of the pro-death Bak and Bim, as well as with attenuation of the levels of Her-2 and XIAP, survivin, Bcl-2, and Bcl-x^L proteins. SAHA treatment induced acetylation of hsp90. This reduced the chaperone association of Her-2 with hsp90, promoting polyubiquitylation and degradation of Her-2. SAHA also attenuated the levels of c-Raf and AKT. Cotreatment with SAHA significantly increased trastuzumab or docetaxel-induced apoptosis of BT-474 and SKBR-3 cells. Additionally, median dose-effect analysis revealed that cotreatment with SAHA and trastuzumab or docetaxel induced synergistic cytotoxic effects against the breast cancer cells. Conclusions: These preclinical findings support the development of SAHA in combination with docetaxel and/or trastuzumab against Her-2-amplified breast cancer.