Molecular Microbiology and Immunology
Keck School of Medicine
USC / Norris Comprehensive Cancer Center
Research TopicsViral oncogenesis, MicroRNA, Herpesvirus latency and reactivation, Herpesvirus entry and trafficking, Angiogenesis and inflammation, KSHV-induced epigenetic reprogramming, KSHV-induced metabolic reprogramming
Research OverviewDr. Gao’s laboratory is interested in the biology of infection and oncogenesis of tumor viruses. The current focus is on Kaposi’s sarcoma-associated herpesvirus (KSHV) and its associated cancers.
Viral infection is the cause of up to 15% of human cancer. For examples, human papilloma virus (HPV) infection is associated with cervical carcinoma while hepatitis C virus (HCV) or hepatitis B virus (HBV) infection is associated with hepatocellular carcinoma. KSHV infection is causally linked to the development of Kaposi’s sarcoma (KS), primary effusion lymphoma (PEL) and a subset of multicentric Castleman’s disease (MCD). KS is a highly inflammatory and vascularized spindle cell cancer of proliferative endothelial cells commonly found in AIDS patients. In some African regions, KS has become the most common cancer in patients with and without HIV infection. As an emerging important human pathogen, KSHV is an excellent model for studying inflammation, angiogenesis, oncogenesis, and virus-host and virus-virus interactions. The lab employs comprehensive genetic, genomic, molecular, cellular, and biochemical approaches to address these complex problems.
1. Molecular basis of KSHV-cell interactions during acute infection
The lab addresses several important questions related to KSHV acute infection: How KSHV and other related viruses enter into and traffic inside the cells? What are the cellular pathways activated during viral infection? How do these pathways control viral infection? How do the viruses hijack the cellular functions to facilitate the infection?
The lab has defined the entry and trafficking pathways of KSHV and rhesus rhadinovirus (RRV). Both KSHV entry into primary human umbilical vein endothelial cells (HUVEC) and RRV entry into rhesus fibroblasts occur through the clathrin-mediated endocytosis. KSHV trafficking in HUVEC depends on actin dynamics while RRV trafficking in rhesus fibroblasts is mediated by microtubule and dynein. The works have shown that ubiquitination mediates the internalization of both KSHV and its cognate receptors, and identified c-Cbl as the E3 ligase that facilitates this process. By profiling viral and cellular transcriptomes, it is revealed that KSHV shuts off the overall host transcriptional program but selectively activates cellular pathways to facilitate its infection and replication. For examples, KSHV activates the ERK, JNK and p38 multiple mitogen-activated protein kinase (MAPK) pathways, which is necessary for virus entry into and trafficking inside the target cells. Activation of MAPK pathways also regulates viral replication by promoting the expression of the key KSHV transcription and replication activator, RTA (ORF50).
By analyzing the kinome, the lab has recently identified a number of novel cellular kinases that are activated during KSHV acute infection. The lab is determining the functions of these kinases and their related pathways in KSHV infection.
2. Mechanisms of KSHV latency and reactivation
The lab is interested in the life cycle of KSHV, and the viral and cellular factors that regulate KSHV persistent latent infection and lytic replication.
The lifecycle of KSHV has latent and lytic phases. Latency is the default KSHV replication program following acute infection and is essential for KS development. During viral latency, KSHV evades host immune surveillances by persisting as episomes in the nucleus and expresses a limited number of viral genes. In contrast, KSHV lytic replication produces large amounts of lytic proteins and infectious virions, which promote KS progression by spreading infection and modulating the intracellular and extracellular environments. Thus, it is essential to understand the mechanisms controlling KSHV latency and reactivation.
To delineate the mechanism of latency, the lab has performed mutagenesis of KSHV genes using a reverse genetics approach and identified LANA (ORF73), vFLIP (ORF71) and miRNA miR-K1 that promote KSHV latency. LANA is essential for KSHV episome persistence, represses the expression of KSHV lytic genes and promotes the proliferation of latent KSHV-infected cells. vFLIP and miR-K1 repress the expression of KSHV lytic genes and enhance cell survival by activating the NF-κB pathway. By screening cellular pathways, it is found that induction of MAPK pathways or reactive oxygen species (ROS) hydrogen peroxide is essential and sufficient for activating KSHV lytic replication. ROS, which is induced by a number of inflammatory cytokines found in KS tumors, triggers KSHV lytic replication by activating the MAPK pathways. Scavenging of ROS with antioxidants such as N-acetyl-L-cysteine (NAC) inhibits KSHV lytic replication and the growth of PEL, and prolongs animal survival in a mouse PEL model.
KSHV latent and lytic replications are tightly regulated by specific epigenetic marks. The lab has recently found that SIRT1, a NAD+-dependent class III histone deacetylase (HDAC), promotes KSHV latency by inhibiting viral lytic replication. SIRT1 binds to the promoter and silence the expression of RTA. Chemical inhibition or knock down of SIRT1 is sufficient to initiate the lytic replication program by increasing active histone mark H3K4me3 and decreasing repressive histone mark H3K27me3 in the RTA promoter. SIRT1 also interacts with RTA and inhibits RTA transactivation of its own promoter and those of downstream target genes. These findings link a metabolic sensor to KSHV life cycle.
Ongoing works have identified a number of viral and cellular miRNAs that target KSHV genes. Identification of the stages of viral replication regulated by the miRNAs and defining the mechanism of action should provide insights into the roles of these miRNAs in KSHV life cycle.
3. Mechanisms of KSHV-induced oncogenesis
The lab aims to delineate the mechanisms of KSHV-induced inflammation, angiogenesis, cellular transformation and tumorigenesis. Specifically, the lab investigates how KSHV reprograms cellular epigenome, transcriptome, metabolome and signaling pathways that might lead to these malignant manifestations.
Inflammation and angiogenesis are the hallmarks of KS tumors. The lab has found that KSHV infection promotes inflammation, angiogenesis and cell invasion. KSHV induces pro-inflammatory and pro-angiogenic cytokines including interleukin-6 (IL-6), angiopoietin-2 (Ang-2), matrix metalloproteinases (MMP) -1, -2, and -9 by regulating a number of cellular pathways.
During KSHV latency, the innate immune system is expected to be silent. However, the lab has recently found that a number of cellular innate immune pathways are activated during KSHV latency. In particular, it is found that the complement pathway is activated in KS spindle tumor cells and in latent KSHV-infected cells. KSHV activates the alternative complement pathway by downregulating complement regulatory proteins CD55 and CD59, resulting in the depositions of complement membrane attack complex (MAC) C5b-9 and the complement component C3 activated product C3b on latent KSHV-infected cells. Of interest, KSHV-infected cells are resistant to complement-mediated cytolysis. On the contrary, the activated complement activates the STAT3 pathway resulting in enhanced cell survival. The lab is investigating the role of complement activation in KSHV-induced inflammation, angiogenesis and tumorigenesis.
To understand the mechanism of KSHV-induced oncogenesis, the lab has recently developed a novel and highly efficient model of KSHV-induce cellular transformation of primary rat mesenchymal stem cells. KSHV-transformed cells grow much faster than the uninfected control cells, lost contact-inhibition, and induced KS-like tumors in nude mice. This model is used to define the viral and cellular factors required for cellular transformation. For example, recent works have shown that KSHV miRNAs are essential for cellular transformation and tumorigenesis. These miRNAs redundantly accelerate cell cycle progression and inhibit apoptosis by regulating cellular growth and survival pathways including the NF-κB pathway. It is also found that KSHV vCyclin promotes cellular transformation and tumorigenesis by relocalizing and inactivating p27.
Finally, the lab has found that KSHV extensively reprograms cellular epigenome, transcriptome, metabolome and signaling pathways in the KSHV-transformed cells. A number of cellular genes and their associated pathways are shown to be essential for KSHV-induced cellular transformation and tumorigenesis. These pathways are attractive targets for developing novel therapeutic approaches for KSHV-related malignancies.