Synergistic Induction of Cell Death and Apoptosis by the Histone Deacetylase Inhibitor SAHA (Vorinostat) and the Demethylating Agent DAC in Multiple Myeloma.

Hypermethylation of CpG islands in the promoter region of genes is a well characterized epigenetic modification associated with transcriptional silencing of cancer related genes and plays a crucial role in carcinogenesis. In addition, acetylation of core histones is necessary for the maintenance of transcriptional activity of genes. DNA methylation and histone deacetylation are reversible and can be influenced by DNA methyltransferase (DNMT) inhibitors such as 5-aza-2`-deoxycytidine (DAC) or 5-azacytidine (AZA) and histone deacetylase (HDAC) inhibitors such as suberoylanilide hydroxamic acid (SAHA), respectively. Clinical trials using a strategy based on the modification of epigenetic changes with DAC or AZA in combination with HDAC inhibitors have been promising and may help to generate new strategies in treatment of hematopoietic malignancies including multiple myeloma (MM). In MM however, only limited data are published about the possible synergistic effects between DNMT inhibitors and the highly potent pan-HDAC inhibitor SAHA.

To assess the in vitro effects of SAHA on the MM cell lines U266, LP-1, RPMI8226 or OPM-2 and the possible interactions with DNMT inhibitors, cells were first incubated with DAC in a final concentration of 0.1 or 0.2 mM for 72 hours. After exposure to DAC, cells were incubated for 72 or 96 hours with SAHA in a final concentration between 0.1 and 20 mM. The toxic effect of the treatment was assessed by an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The drug concentration inducing a 50 % killing of the cells compared to control cell survival was calculated from the dose-response curve (IC50). Induction of apoptosis was analysed by flow cytometry with annexin V-binding. In addition, the expression of the epigenetically silenced tumor suppressor genes SFRP-2 and DAB2 was determined by real time RT-PCR before and after exposure to DAC and SAHA.

In LP-1 and U-266 cells, no relevant enhancement in the cytotoxic effect of SAHA was observed after previous exposure to DAC. In contrast, in OPM-2 and RPMI-8226 cells, a significant increase in cytotoxicity of SAHA was observed, when the cells were first incubated with DAC with a decrease of the IC50 from 6.5 μM to 2.43 μM and 10.37 μM to 4.5 μM, respectively. We further analysed a possible synergism between SAHA and DAC for the induction of apoptosis by flow cytometry. After sequential exposure of the cells with DAC for 72 hours and with SAHA for 72 hours, no change in the apoptotic cell fraction was observed for the cell lines OPM-2 and RPMI-8226. However, for U-266 and LP-1, a significant increase in apoptotic cells was observed after incubation with SAHA, when the cells were previously exposed to DAC with a increase in the apoptotic cell fraction of 39.5 % to 55.4 % and 2.5 % to 14.4 %, respectively. By real-time RT-PCR, corresponding transcriptional silencing for SFRP-2 and DAB2 was demonstrated in untreated cells, and exposure of cell lines to DAC and SAHA resulted in reexpression. A synergism for the induction of reexpression of these genes was observed when cells were incubated with DAC and SAHA sequentially.

After treatment with SAHA, we observed a dose-dependent induction of cell death and apoptosis as assessed by MTT and annexin V assay, respectively. In the different MM cell lines, we observed a synergism between SAHA and DAC both for cytotoxic effects and the induction of apoptosis. A synergism was also observed for the reexpression of epigenetically silenced genes after exposure to DAC and SAHA. These in vitro data can be considered as a basis for further in vitro studies and preclinical models with SAHA in combination with demethylating agents such as DAC in order to improve treatment response and survival in MM patients.

Jost:MSD: Research Funding. Bruemmendorf:Genzyme: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees.