Background: Combination treatment with histone deacetylase inhibitors (HDACi) and proteasome inhibitors synergistically induce apoptosis in a number of cancer cell line model systems and is approved for the treatment of patients with refractory multiple myeloma (MM). The mechanistic basis for the enhanced activity of this combination is yet to be clearly defined. We have previously demonstrated that HDACi-induced apoptosis in multiple tumour types is linked to the induction of a specific transcriptional response involving up-regulation of multiple immediate-early (IE) response genes, of which ATF3 plays a key role driving apoptosis. Several studies have also demonstrated ATF3 induction in response to proteasome inhibitor treatment. The aim of this study was to determine whether the enhanced activity of this combination was mediated through enhanced induction of ATF3.
Results: Treatment of multiple colorectal cancer and MM cell lines with HDACi or proteasome inhibitors induced ATF3 mRNA and protein expression. In both cases ATF3 gene induction was sustained over 8-48 hours. Proteasome inhibitor induced ATF3 gene expression and apoptosis was selectively dependent on activation of the p38 stress response pathways, while HDACi-mediated effects were selectively dependent upon the Sp1 and Sp3 transcription factors, indicating these agents induce ATF3 gene expression and apoptosis via independent mechanisms. Co-treatment with Bortezomib and Vorinostat resulted in enhanced induction of ATF3 and synergistically induced apoptosis in colorectal cancer and MM cell lines. The synergistic induction of apoptosis by the combination was attenuated by siRNA-mediated downregulation of ATF3.
Conclusion: This study provides a molecular rationale for the synergistic apoptotic activity of combination treatment with HDACi and proteasome inhibitors and demonstrates that assessment of ATF3 induction may serve as a biomarker of response to this combination.