Background/Aim: Histone deacetylase inhibitors (HDACi) are a novel class of targeted anti-cancer therapeutics currently approved for treatment of cutaneous T-cell lymphoma (CTCL). We previously demonstrated that HDACi-induced apoptosis is mediated via the induction of ’immediate early’ (IE) gene expression in cancer cells. Bioinformatics analysis indicated that proteasome inhibitors (PI) induced a similar transcriptional response to HDACi. Recently, combination HDACi and PI treatment has been shown to synergistically induce apoptosis in vitro, and led to improved progression-free survival in a Phase III trial for multiple myeloma (MM). Here, we investigate whether the molecular basis for the activity of this combination is linked to enhanced induction of IE gene expression.
Methods: Tumour cells were treated with the HDACi, Vorinostat (0.5-5 µM) alone, the Bortezomib (1-50 nM) alone, or Vorinostat + Bortezomib. Apoptosis, anti-proliferative effects and IE gene induction were determined by propidium iodide staining and FACS analysis, MTS assay, and qRT-PCR, respectively. MAPK pathway activation and histone hyperacetylation was determined using western blot.
Results: Combination Vorinostat and Bortezomib treatment synergistically induced apoptosis and inhibited cell growth in cell lines derived from multiple tumour types, including CTCL, MM and colorectal cancer. In parallel, this drug combination markedly enhanced the induction of multiple IE genes (FOS, JUN, ATF3, GADD45B) compared to single-agent treatment. IE gene induction by Bortezomib but not HDACi was mediated by activation of MAPK signalling pathways (JNK and P38), while HDACi-induced IE gene expression was associated with a global increase in histone acetylation.
Conclusions: This study establishes a link between the induction of IE gene expression and the synergistic induction of apoptosis in cancer cells treated with Vorinostat and Bortezomib. We demonstrate that HDACi and PI induce IE genes via distinct mechanisms which could explain the synergistic anti-tumour activity of this drug combination.