Lytic Induction Therapy against Epstein-Barr Virus-Associated Malignancies: Past, Present, and Future

Abstract

Epstein-Barr virus (EBV) lytic induction therapy is an emerging virus-targeted therapeutic approach that exploits the presence of EBV in tumor cells to confer specific killing effects against EBV-associated malignancies. Efforts have been made in the past years to uncover the mechanisms of EBV latent-lytic switch and discover different classes of chemical compounds that can reactivate the EBV lytic cycle. Despite the growing list of compounds showing potential to be used in the lytic induction therapy, only a few are being tested in clinical trials, with varying degrees of success. This review will summarize the current knowledge on EBV lytic reactivation, the major hurdles of translating the lytic induction therapy into clinical settings, and highlight some potential strategies in the future development of this therapy for EBV-related lymphoid and epithelial malignancies.

Keywords: EBV-associated gastric carcinoma; Epstein–Barr virus; Hodgkin lymphoma; T-/NK-/B-cell non-Hodgkin lymphoma; endemic Burkitt lymphoma; lytic induction therapy; nasopharyngeal carcinoma.

Figure 1

Overview of Epstein–Barr virus (EBV) lytic induction therapy. EBV lytic cycle is first reactivated by chemical inducers in which the viral protein kinase encoded by BGLF4 is produced. BGLF4 then activates the nucleoside analogue antiviral pro-drug into its cytotoxic form, and consequently results in a specific killing effect on EBV-positive cells. Moreover, the activated drugs can be transferred to adjacent cells, resulting in a “bystander killing” effect. GCV, valganciclovir.

Figure 2

Combination of currently available lytic inducers. (A) Combination of iron chelators and HDAC inhibitors. Iron chelators and HDAC inhibitors reactivate EBV lytic cycle through autophagy-dependent and independent pathways, respectively. Their combination could potentially be synergistic in reactivating EBV lytic cycle. (B) Combination of iron chelators and lenalidomide. Combination of lytic inducers with different mechanisms for EBV lytic reactivation may have synergistic effects in reactivating the EBV lytic cycle. Lenalidomide reactivates EBV lytic cycle by suppressing Ikaros, which inhibits the expression of transcription factors that inhibit EBV lytic cycle. Manipulation of Zta expression by iron chelators, together with the suppression of inhibitory factors that prevent Zta transactivation of other lytic genes by lenalidomide, may provide a feed-forward loop for lytic reactivation, thus enhancing EBV lytic induction.

Figure 3

Relationship between induction of lytic cycle of EBV and the autophagy machinery and the modes of action of compounds with lytic induction potentials. EBNA1 could be processed by the autophagy machinery for MHC-II presentation, while LMP1, LMP2A, Rta, and Zta could initiate autophagy. Rapamycin reactivates EBV lytic cycle by inhibiting mTOR. Iron chelators and C7, on the other hand, activate the ERK1/2-ATG5 axis to induce the lytic cycle of EBV. New compounds that target ATG4, ULK1, and Vps34 could potentially reactivate lytic cycle of EBV.

Figure 4

Relationship between EBV proteins, NF-κB, and STAT3 signaling pathways and the modes of action of compounds with lytic induction potentials. LMP1 could activate both canonical and non-canonical NF-κB pathways. RelA(p65) could bind and activate Qp-EBNA1 expression, while itself could, in turn, be inhibited by EBNA1 through the prevention of IκK phosphorylation. RelA(p65) interacts with Zta and abrogates its ability to transactivate other EBV genes, while Zta inhibits the activation of NF-κB-responsive gene promoters. EBV lytic protein encoded by BGLF2 was shown to interact with RelA(p65), preventing its phosphorylation and nuclear translocation. Bortezomib, PS1145, and aspirin reactivate EBV lytic cycle by preventing the degradation of NF-κB inhibitor, the phosphorylation of Iκβα, and translocation of RelA(p65), respectively. Both LMP1 and LMP2A could phosphorylate STAT and inhibit the activation of lytic cycle of EBV. STAT inhibitors such as cucurbitacin I, AZD1480, and S3I-201 reactivate lytic cycle of EBV by either inhibiting phosphorylation, homodimerization, DNA binding, or transcriptional activities of STAT3.

Figure 5

Relationship between EBV proteins, hTERT, and NOTCH signaling pathway and the modes of action of compounds with lytic induction potentials. hTERT inhibits the expression of BZLF1 through the NOTCH2/BATF pathway. EBNA2 is regarded as the functional homolog of active NOTCH intracellular domain (Notch-IC) and LMP2A can activate the NOTCH pathway. NOTCH2 inhibits the reactivation of lytic cycle of EBV through the upregulation of Zeb2 by NOTCH-IC, a transcription factor that represses BZLF1 transcription. γ-secretase inhibitors such as compound E and dibenzazepine can reactivate lytic cycle of EBV by preventing the release of Notch-IC. Other compounds that may reactivate the lytic cycle include hTERT inhibitor, BIBR1532, which selectively inhibits telomerase activity, and another γ-secretase inhibitor, DAPT.

Figure 6

Relationship between EBV proteins and c-MYC and the modes of action of compounds with lytic induction potentials. MYC represses the activation of lytic cycle of EBV by binding to OriLyt on the EBV genome and suppresses its looping to the BZLF1 promoter. A2CE’s and DRB’s inhibition of CDK2/9, a transcription factor that regulates MYC expression, can suppress the expression of EBV latent proteins. Compounds that target the DNA binding domain of the MYC-MAX complex such as KSI-3716, MYCi975, and 7594-0035 may reactivate the lytic cycle of EBV.