Background/Aim: Histone deacetylase inhibitors (HDACi) are approved for the treatment of cutaneous T-cell lymphoma. We have previously demonstrated that HDACi-mediated apoptosis is linked to the induction of a defined transcriptional response involving up-regulation of the immediate-early (IE) genes FOS, JUN, ATF3, EGR1 and EGR3, in cell lines derived from multiple tumour types. To identify other drugs which induce a similar transcriptional response we compared the HDACi-induced transcriptional response with that induced by >1000 bioactive molecules using the Broad Institute’s Connectivity Map database. The proteasome inhibitor, MG-262, was identified as inducing the most comparable transcriptional response to HDACi. Notably, combination treatment with HDAC and proteasome inhibitors has been shown to synergistically induce apoptosis in vitro, and a phase III trial of this combination demonstrated activity in patients multiple myeloma. The aim of this study was to determine whether these effects are mediated through the additive induction of IE genes.
Methods: The effect of the HDACi SAHAand the clinically approved proteasome inhibitor Bortezomib, alone, and in combination, on apoptosis induction was determined in colorectal, multiple myeloma and CTCL cell lines PI staining and FACS analysis. Quantitative RT-PCR was performed to determine gene expression changes upon drug treatment. MAPK pathway activation and histone hyper-acetylation was determined by western blot analysis.
Results: Single agent treatment of multiple cancer cells with bortezomib or SAHA induced expression of the IE genes FOS, JUN and ATF3. In both cases IE gene induction was sustained over 24h. Proteasome inhibitor-induced IE gene expression and apoptosis was selectively dependent on activation of the JNK and p38 stress response pathways, while HDACi-mediated effects were selectively dependent upon the Sp1 and Sp3 transcription factors, indicating these agents induce IE gene expression and apoptosis via independent mechanisms. Co-treatment with Bortezomib and SAHA resulted in synergistic induction of IE gene expression and apoptosis in cell lines derived from multiple tumour types. Importantly, apoptosis induction by the combination was inhibited by siRNA-mediated downregulation of the gene ATF3.
Conclusions: This study provides a molecular rationale for the synergistic apoptotic activity of combination treatment with HDAC and proteasome inhibitors.