Inhibition of class I histone deacetylases by romidepsin potently induces Epstein-Barr virus lytic cycle and mediates enhanced cell death with ganciclovir

Pan-histone deacetylase (HDAC) inhibitors, which inhibit 11 HDAC isoforms, are widely used to induce Epstein-Barr virus (EBV) lytic cycle in EBV-associated cancers in vitro and in clinical trials. Here, we hypothesized that inhibition of one or several specific HDAC isoforms by selective HDAC inhibitors could potently induce EBV lytic cycle in EBV-associated malignancies such as nasopharyngeal carcinoma (NPC) and gastric carcinoma (GC). We found that inhibition of class I HDACs, particularly HDAC-1, -2 and -3, was sufficient to induce EBV lytic cycle in NPC and GC cells in vitro and in vivo. Among a panel of selective HDAC inhibitors, the FDA-approved HDAC inhibitor romidepsin was found to be the most potent lytic inducer, which could activate EBV lytic cycle at approximate to 0.5 to 5 nM (versus approximate to 800 nM achievable concentration in patients' plasma) in more than 75% of cells. Upregulation of p21(WAF1), which is negatively regulated by class I HDACs, was observed before the induction of EBV lytic cycle. The upregulation of p21(WAF1) and induction of lytic cycle were abrogated by a specific inhibitor of PKC- but not the inhibitors of PI3K, MEK, p38 MAPK, JNK or ATM pathways. Interestingly, inhibition of HDAC-1, -2 and -3 by romidepsin or shRNA knockdown could confer susceptibility of EBV-positive epithelial cells to the treatment with ganciclovir (GCV). In conclusion, we demonstrated that inhibition of class I HDACs by romidepsin could potently induce EBV lytic cycle and mediate enhanced cell death with GCV, suggesting potential application of romidepsin for the treatment of EBV-associated cancers. What's new? In cancers associated with Epstein-Barr virus (EBV), the virus persists in latent form, becoming susceptible to drug therapy only after the lytic cycle has been reactivated. To effectively leverage lytic reactivation clinically, however, more specific and more potent pharmacologic lytic inducers are needed. A promising agent is romidepsin, a class I histone deacetylase (HDAC) inhibitor, shown in this study to potently trigger the EBV lytic cycle through HDAC-1/-2/-3 inhibition and protein kinase C- activation. Lytic induction augmented the cell-killing effects of the antiviral ganciclovir. The data warrant further investigation of romidepsin for the treatment of EBV-associated cancers.