NUCLEOLAR STRESS SUPPORTS LYTIC REACTIVATION OF THE KAPOSI`S SARCOMA-ASSOCIATED HERPESVIRUS

Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), is a gamma-2 herpesvirus. KSHV is causally linked to Kaposi’s sarcoma, primary effusion lymphoma, and plasmablastic multicentric Castleman’s disease. Primary infection with KSHV precedes lifelong latent infection which may reactivate towards the lytic cycle. Initiation of the lytic cycle proceeds mainly through the activities of the viral replication and transcription activator (RTA) protein, which activates the expression of selected viral genes, eventually leading to upregulation of the entire lytic cascade. The lytic cycle is a critical step in the maintenance of lifelong infection and for the progression and pathogenesis of KSHV-related neoplasm. Accordingly, compromised immunological surveillance, coupled with increased loads of KSHV, is strongly associated with the development of KSHV-related diseases.

We identified nucleolar stress as a regulator of KSHV reactivation. Specifically, we interfered with ribosomal DNA transcription to induce nucleolar stress by treatment with low dose Actinomycin D, CX-5461 or BMH-21. In accordance with the reported activities of these drugs alterations in the nucleolar morphology, coupled with increased levels of p53 and p21, were documented. Treatment with these compounds alone, at different concentrations, did not stimulate lytic cycle reactivation. However, a dose dependent lytic cycle induction of KSHV by these compounds was evident when combined with induction of RTA protein expression. Further experiments employing isogenic p53+/+ and p53-/- cells indicated that the induction of lytic reactivation by nucleolar stress is p53-independent. Taken together, our study identifies nucleolar stress as a new regulator of KSHV infection which synergizes with RTA to increase lytic reactivation. Nucleolar stress can be induced in patients receiving chemotherapeutic drugs and thus our findings may have clinical implications.