Targeting BRD2 and BRD4 inhibit the growth of KSHV-infected immortalized endothelial cells through suppression of LANA translation.

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of several human cancers, including Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL), both of which still lack effective treatment options. Members of the bromodomain and extra-terminal domain (BET) family, especially bromodomain-containing protein 4 (BRD4), play important roles in RNA polymerase II-mediated transcriptional regulation and are required for the expression of many tumor-driving oncogenes in various cancer cells. Therefore, BET proteins have become attractive targets for anticancer drug development. Previous studies have demonstrated the high sensitivity of PEL cells to BET inhibitors, and BRD4 silencing effectively blocks tumor cell proliferation. In contrast, KSHV-infected immortalized endothelial cells display strong resistance to BET inhibitors, including (+)-JQ1. To further develop BRD-targeted therapies for KSHV-infected immortalized endothelial cells, we identified MZ-1 and SIM-1, two BRD4 PROTAC degraders, as effective inhibitors of cell growth in these cells. Mechanistically, these inhibitory effects depend on suppression of LANA translation through increased eIF2α phosphorylation in KSHV-infected cells. Similar LANA suppression was also observed following RNAi-mediated silencing of BRD2 or BRD4. Proteomic analysis identified unique protein candidates altered in MZ-1- and SIM-1-treated KSHV-infected immortalized endothelial cells compared with (+)-JQ1-treated cells. In summary, our study develops an effective strategy against KSHV-infected immortalized endothelial cells using selective BRD PROTACs, which may help improve therapeutic outcomes for KSHV-related malignancies in the future.