Cell cycle blockade and differentiation of ovarian cancer cells by the histone deacetylase inhibitor trichostatin A are associated with changes in p21, Rb, and Id proteins.

Inhibitors of histone deacetylase activity are emerging as a potentially important new class of anticancer agents. In the current studies, exposing A2780 ovarian cancer cells to the histone deacetylase inhibitor trichostatin A (TSA) produced a marked change in cellular morphology, proliferation, and differentiation. Within 24 h of TSA treatment, there was a morphological transformation of the cells, with increased cytoplasm, a more epithelial-like columnar appearance, and the emergence of distinct cellular boundaries. Commensurate with the morphological transformation, TSA also inhibited cell proliferation; cells treated with TSA for 72 h increased to 110% of the initial cell numbers versus control cell numbers of 622%, with a corresponding reduction in mitotic activity and a flow cytometry S-phase fraction of 3.9% in TSA-treated cells versus 28.8% for control. TSA also induced epithelial-like differentiation with increased cytokeratin expression from 2% of controls to 22-25% of TSA-treated cells and the reappearance of intercellular plasma membrane junctions and primitive microvilli. Immunocytochemical analyses indicate the molecular mechanism underlying the actions of TSA on A2780 cell cycle progression and differentiation involves reexpression of the CDK inhibitor p21. Elevated levels of p21, in TSA-treated cells, were associated with a reduction in the phosphorylation of the cell cycle regulator retinoblastoma protein (Rb). TSA also caused a decrease in the helix-loop-helix inhibitor of differentiation/DNA binding protein Id1, with no change in Id2 levels. In conclusion, the observed TSA-induced changes in p21, Rb, and Id1 are consistent with cell cycle senescence and differentiation of A2780 ovarian cancer cells.