Cell cycle and apoptotic effects of SAHA are regulated by the cellular microenvironment in HCT116 multicellular tumour spheroids

Using multicellular tumour spheroids (MCTS) of HCT116 colon carcinoma cells, we analysed the effects of SAHA (suberoylanilide hydroxamic acid), a histone deacetylase inhibitor (HDACi). We found that, although SAHA-induced histone acetylation and ROS level upregulation occur throughout the spheroid, inhibition of cell cycle progression and induction of apoptosis are dependent on cell microenvironment. SAHA-induced growth inhibition of HCT116 MCTS results from the inhibition of cell cycle progression and induction of apoptosis. At a low concentration SAHA decreases Ki-67 and cyclin A positive cells and increases p21 positive cells in the outer layer while it induces a ROS-dependent apoptosis in the central zone of the spheroid. Coimmunostaining of p21 and apoptotic cells confirms that SAHA effects are different depending on the position of the cells within the spheroid. At a higher dose, SAHA induces mitotic defects and survivin downregulation in the outer layer of cells resulting in an additional cytotoxic effect in this part of the spheroid. Together these findings show that SAHA-induced cytostatic and cytotoxic effects occur in different cell populations, indicating that the cellular microenvironment is an important determinant in the regulation of the effects of SAHA treatment. Consequently, the MCTS model appears to be a valuable advanced tool for evaluating the effects of SAHA treatment in combination with other anticancer agents.