Alkylator-Induced DNA Damage Response in Multiple Myeloma (MM) Cells Is Amplified by Histone Deacetylase (HDAC) Inhibition, Resulting in Significant Synergism in Cell Death

One of the main mechanisms of action of HDAC inhibitors is the transcriptional reactivation of dormant tumor-suppressor genes through acetylation of histones, thereby inducing apoptosis. Treatment with HDACI has also been shown to induce chromatin destabilization in a transcription independent way. In the current study, we sought to determine whether HDAC inhibition induces DNA damage and amplifies alkylator-induced mitotic cell death in both melphalan sensitive- and resistant-MM cell lines (RPMI8226, 8226/LR5). The IC50 values of SNDX275, a class I HDACI agent, and melphalan on the 72-hour MTT assay were 268.05 nM and 245.94 nM in the RPMI8226, and 309.91 nM and 8657.46 nM in the 8226/LR5, respectively. When combined together at clinically attainable concentrations, the combination index by the Chou-Talalay method ranged from 0.27 to 0.75 for the RPMI8226 and from 0.33 to 0.7 for the 8226/LR5, indicating a powerful synergism. For elucidation of molecular mechanisms, MM1S and RPMI8226 cell lines were investigated for apoptosis, histone acetylation, cell cycle analysis, DNA double strand break and DNA damage response serially in 48-hour culture with SNDX-275 at 500 nM and melphalan at 10 μM, alone and in combination. Cleavage of PARP was seen following treatment with each SNDX275 and melphalan, but was highest at 48 hours with the combination of both. Apoptosis was associated with cleavage of caspases of 8, 3 and 9, which was most intense on combination. Melphalan amplified SNDX275-induced acetylation of H3. In cell cycle analysis by flow cytometry, SNDX275 caused an increase in G0-G1 and a decrease in S and G2-M. Cyclin D1, E2F-1 and p53 on western blot were not affected but expression of p21 increased. Melphalan arrested the cell cycle at G2, increased expression of p53 in the RPMI8226 and of p21 in the MM1S. The combination intensified the increase in p21 in both cell lines and in p53 only in the RPMI8226. Phosphorylation of H2AX, a marker of DNA double strand break, increased in a time dependent manner following each drug, along with an increase in phosphorylation of CHK1 and CHK2, indicative of initiation of DNA damage response. The increase in γH2AX and pCHK1 & 2, however, was considerably higher on combination than each drug alone. Furthermore, morphologic assessment of dead cells by the 48 hours of culture revealed a significant increase in mitotic catastrophe on combination in the MM1S: 0% on SNDX275 alone; 10% on melphalan alone; 43.4% on combination. The current study suggests that HDAC inhibition synergizes with melphalan in MM cells and that intensification of DNA damage is one of the mechanisms. Further studies are necessary to understand the role of HDAC inhibition for induction of mitotic catastrophe.