Objectives. Solid tumors are believed to be organized in a cellular hierarchy with a population of cancer stem cell (CSC) driving cancer progression and resistance to conventional treatment. Deciphering molecular mechanisms related to CSC biology will help develop CSC targeted therapeutic strategies. CSC self-renewal and differentiation programs are regulated by epigenetic modulations. Our study specifically targets breast CSCs with a novel epidrug used as a differentiating agent, a histone deacetylase inhibitor (HDACi) S78454, aka PCI-24781.

Methods. Seventeen breast cancer cell lines from several molecular subtypes (basal, luminal and mesenchymal) were treated for 72 hours with S78454 to determine their respective IC50s. Specific inhibition of histone deacetylation was measured by immunoblotting (histone and tubulin acetylation, p21). The effect of S78454 treatment on the breast CSC population was assayed by both ALDEFLUOR assay and tumorsphere formation. Cell cycle progression was evaluated along S78454 treatment. The induction of cell differentiation was monitored by tracking modifications in molecular phenotype using immunofluorescence.

Results. Two different types of sensitivity profiles to S78454 were observed (high and low), but these were not correlated to the molecular subtype of the cell lines. After six hours of S78454 treatment, specific biological effects including increased histone and tubulin acetylation and p21 protein expression were seen. After 72 hours of treatment morphological changes were induced in cells with low sensitivity, which increased in size, grew in clusters, and changed their phenotype with a lost of cytokeratins 5/6, 14, and vimentin protein expression and a gain of E-cadherin. In cell lines with low sensitivity, S78454 treatment induced a decrease in the ALDEFLUOR-positive breast CSC population as well as a decrease in tumorsphere formation whereas no specific effect was observed on CSC population from highly sensitive cell lines. Interestingly, S78454 treatment induced a G2/M cell cycle arrest in highly sensitive cell lines whereas cell lines with a low sensitivity to S78454 present a transitory G1/S cell cycle arrest after 24 hours of treatment before achieving a complete cell division.

Conclusions. In low sensitive cell lines, biological and molecular changes following S78454 might reflect the epigenetic modulation of breast cancer stem cell differentiation. Cell cycle progression from G1 to S has been defined as an important cell cycle stage controlling stem cell fate allowing equilibrium between self-renewal and committed cell fate decision. The transitory G1/S cell cycle block induced by S78454 treatment may favorized cell differentiation. In highly sensitive cell lines, S78454 treatment seems to induce an irreversible G2/M arrest inducing apoptosis in all cancer cell populations. Therefore, our findings suggest that HDACi treatment could be used as a novel therapeutic strategy through induction of differentiation of breast CSC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A57.