Kaposi’s sarcoma-associated herpesvirus (KSHV) is causally linked to several acquired immune deficiency syndrome related malignancies including Kaposi’s sarcoma (KS), primary effusion lymphoma (PEL), and a subset of multicentric Castleman’s disease1. Control of viral lytic replication is essential for KSHV latency, evasion of host immune system, and induction of tumors1. Here, we show that deletion of a cluster of 14 microRNAs (miRs) from KSHV genome significantly enhances viral lytic replication as a result of reduced NF-κB activity. The miR cluster regulates NF-κB pathway by reducing the expression of IκBα protein, an inhibitor of the NF-κB complexes. Computational and miR seed mutagenesis analyses identify KSHV miR-K1 that directly mediates IκBα ?protein level by targeting the 3’UTR of its transcript. Expression of miR-K1 is sufficient to rescue the NF-κB activity and inhibit viral lytic replication while inhibition of miR-K1 in KSHV-infected PEL cells has the opposite effects. Thus, KSHV encodes a miR to control viral replication by activating NF-κB pathway. These results illustrate an important role for KSHV miRs in regulating viral latency and lytic replication by manipulating a host survival pathway.

Infection by viruses alters signaling pathways as a result of cellular response to the infection and virus modulation of its environments. Viruses have evolved to depend on these altered cellular pathways for their successful infection and replication in the host cells. For example, modulation of mitogenactivated protein kinase (MAPK) pathways is essential for infection and replication of human immunodeficiency virus, hepatitis B virus, Epstein-Barr virus, and vaccinia virus (11,23,31,54), while modulation of the NF-B pathway facilitates infection and replication of Epstein-Barr virus, herpes simplex virus type 1, and influenza virus (22, 39, 44).Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, is a gammaherpesvirus associated with Kaposi's sarcoma (KS), primary effusion lymphoma, and multicentric Castleman's disease (14). The life cycle of KSHV has two phases. The latent phase, during which the virus is maintained as episomes and has restricted expression of latent genes, is essential for the development of KSHVinduced malignancies (41, 55). The lytic phase, during which the virus produces infectious virions for dissemination, modulates cellular signaling pathways through unrestricted expression of viral genes (14).KSHV infects a variety of cell types, including B cells, epithelial cells, keratinocytes, and endothelial cells. Although KSHV establishes latency in the majority of cell types following primary infection (29), we have found that efficient infection of human umbilical vein endothelial cells (HUVEC) is productive at the early stage of infection, producing large number of infectious virions preceded by strong expression of almost all viral lytic genes (19,53).KSHV entry into the host cells relies on the interaction of its envelope glycoprotein B (gB) with cellular receptor integrin ␣3␤1 (3). This specific ligand-receptor interaction activates focal adhesion kinase and the MEK-ERK1/2 MAPK pathway but not the JNK and p38 MAPK pathways (3, 38). The activation of MEK pathway is important for KSHV infection, since specific inhibitors of MEK pathway reduce KSHV infectivity and the expression of KSHV early transcripts without affecting virus binding (38,42). Consistently, overexpression of Raf, a component of MEK pathway, enhances KSHV infectivity at the postattachment stage (1). Recently, it has been shown that binding of KSHV virions to the cells is sufficient to activate the RTA (Orf50) promoter (32). These studies indicate a role for the MEK MAPK pathway in KSHV infection.We have recently shown that besides the MEK MAPK pathway, KSHV infection also activates JNK and p38 MAPK path-

Kaposi's sarcoma is an angioproliferative disseminated tumor of endothelial cells linked to infection with

Lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV) promotes the progression of Kaposi's sarcoma (KS), a dominant malignancy in patients with AIDS. While 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced KSHV reactivation from latency is mediated by the protein kinase C delta and MEK/ERK mitogen-activated protein kinase (MAPK) pathways, we have recently shown that the MEK/ERK, JNK and p38 MAPK pathways modulate KSHV lytic replication during productive primary infection of human umbilical vein endothelial cells [Pan, H., Xie, J., Ye, F., Gao, S.J., 2006. Modulation of Kaposi's sarcoma-associated herpesvirus infection and replication by MEK/ERK, JNK, and p38 multiple mitogen-activated protein kinase pathways during primary infection. J. Virol. 80 (11), 5371-5382]. Here, we report that, besides the MEK/ERK pathway, the JNK and p38 MAPK pathways also mediate TPA-induced KSHV reactivation from latency. The MEK/ERK, JNK and p38 MAPK pathways were constitutively activated in latent KSHV-infected BCBL-1 cells. TPA treatment enhanced the levels of activated ERK and p38 but not those of activated JNK. Inhibitors of all three MAPK pathways reduced TPA-induced production of KSHV infectious virions in BCBL-1 cells in a dose-dependent fashion. The inhibitors blocked KSHV lytic replication at the early stage(s) of reactivation, and reduced the expression of viral lytic genes including RTA, a key immediate-early transactivator of viral lytic replication. Activation of MAPK pathways was necessary and sufficient for activating the promoter of RTA. Furthermore, we showed that the activation of RTA promoter by MAPK pathways was mediated by their downstream target AP-1. Together, these findings suggest that MAPK pathways might have general roles in regulating the life cycle of KSHV by mediating both viral infection and switch from viral latency to lytic replication.