HDAC inhibition synergistically enhances alkylator-induced DNA damage responses and apoptosis in multiple myeloma cells

Histone deacetylase (HDAC) inhibitors induce chromatin destabilization. We sought to determine whether HDAC inhibition may amplify alkylator-induced mitotic cell death in multiple myeloma (MM) cells. The combination of SNDX-275, a class I HDAC inhibitor, with melphalan, showed a powerful synergism on growth inhibition with the combination index ranged from 0.27 to 0.75 in MM1.S and RPMI8226 cells. Their combinations as compared with either agent alone promoted much more caspase-dependent apoptosis. Flow cytometry analysis showed that SNDX-275 had minimal effects on cell cycle progression of MM1.S cells, but clearly increased the percentage of S phase in RPMI8226 cells associated with an upregulation in p21waf1 and a reduction in cyclin D1 and E2F1. Melphalan alone significantly arrested both MM1.S and RPMI8226 cells at S phase and enhanced expression of p53 and p21waf1. Furthermore, studies on DNA damage response revealed that phospho-histone H2A.X (γH2A.X), a hall marker of DNA double strand break, along with phosphorylated CHK1 (P-CHK1) and CHK2 (P-CHK2) was dramatically induced by SNDX-275 or melphalan. The increase in γH2A.X and P-CHK1 was considerably higher on combination than either agent alone. These molecular changes correlated well with the significant increase in mitotic catastrophe. Our data indicate that SNDX-275 synergistically enhances melphalan-induced apoptosis in MM cells via intensification of DNA damage, suggesting that SNDX-275 in combination with melphalan may be a novel therapeutic strategy for MM.