Molecular determinants of response of colon cancer cells to histone deacetylase (HDAC) inhibitors.

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Histone deacetylase inhibitors induce growth arrest and apoptosis in colon cancer cells, and have anti-tumor efficacy in vivo. To dissect the molecular basis of cell response to HDAC inhibitor-induced apoptosis, a panel of 30 colon cancer cell lines was treated with HDAC inhibitors, and cell lines sensitive and resistant to HDAC inhibitor-induced apoptosis identified. By correlating the basal gene expression profile of these cell lines with HDAC inhibitor-induced apoptosis, a gene expression signature predictive of response to these agents was identified. To gain insights into the transcriptional basis for this differential sensitivity the 5 most sensitive and resistant cell lines were treated with the HDAC-inhibitor, sodium butyrate, and 97 genes differentially regulated between sensitive and resistant cell lines identified by microarray analysis. Among the genes preferentially induced in sensitive cell lines were the immediate early response genes, FOS, JUN, ATF3, ARC, EGR1 and EGR3, and the stress response genes GADD45B, NDRG4, MT1B, MT1E, MT1F, MT1H and MTIX. However, while increased ROS production was observed following HDAC inhibitor treatment, this was equally evident in both sensitive and resistant cell lines, suggesting induction of these genes was unlikely to be secondary response to ROS production. A bioinformatics analysis demonstrated the promoter regions of these 97 genes to be enriched for GC-rich Sp1/Sp3 binding sites. Furthermore, Sp1/Sp3 reporter activity was preferentially induced by HDAC inhibitors in sensitive cell lines, the Sp1/Sp3 inhibitor, mithramycin, attenuated HDAC inhibitor-mediated induction of several of the genes listed above, and co-silencing of Sp1/Sp3 inhibited HDAC inhibitor-induced apoptosis. Collectively this study demonstrates the existence of a defined transcriptional response that is linked to HDAC inhibitor-induced apoptosis in colon cancer cells. Furthermore, a role for Sp1/Sp3 in mediating this transcriptional response is suggested.