Research Interests
Mechanisms of viral pathogenesis: virus-host interactions
Figure 1. Subcellular localization of HPIP.
Viruses often manipulate the infected cell to overcome sophisticated antiviral defenses and create an optimal environment for viral replication. It is not so surprising therefore, that detailed study of virus-host interactions has provided a unique window into the workings of the cell and uncovered attractive targets for therapeutic intervention. The major interest of our lab is how two very different herpesviruses, 1) herpes simplex virus-1 (HSV) and 2) Kaposi's sarcoma-associated herpesvirus (KSHV, HHV-8), establish a persistent (latent) infection in appropriate cell types. Besides adding to our knowledge of virus biology, we hope these studies will provide valuable insights into mechanisms governing proliferation and survival of normal cells and help answer the important question of how deregulation of these controls can lead to diseases such as cancer.
Herpes simplex virus
Regulation of the switch between lytic and latent modes of replication. Herpes simplex virus-1 (HSV) is a common human pathogen that can cause painful but benign presentations and recurrent disease (cold sores, keratitis and pharyngitis). Occasionally HSV infection of the central nervous system leads to life-threatening encephalitis. Initiation of the productive (lytic) cycle of HSV replication is in part determined by the action of VP16. This viral transcription factor is released from the virion particle and brought to its target genes through interactions with two cellular factors, Oct-1 and HCF-1.
HCF: The purification and cloning of the human HCF-1 (host cell factor-1) protein revealed a novel molecule that undergoes an unusual form of post-translational modification involving proteolytic cleavage of a large precursor and re-association of the resulting products. HCF-1's normal function is a major source of interest for us because it may explain why this protein is a target for HSV. It is known that mammalian cells require HCF-1 for progression through G1 phase of the cell cycle and the availability of functional HCF-1 protein may provides the virus with information about the proliferative capacity of the infected cell and whether it is an appropriate site lytic replication. Recent studies from our lab and others have shown that HCF-1 recruited to the activation domains of a variety of cellular transcription factors (notably LZIP and GABP) where it functions as a coactivator. Our working hypothesis is that loss of HCF-1 function leads to a cell cycle block because of reduced expression of genes required for G1 phase. We are currently trying to understand how HCF-1 promotes transcription and identify critical target genes. To understand how HCF-1 is regulated, we have isolated a number of cellular factors that interact with functionally defined domains. This collection includes a novel protein that seems to regulate HCF-1's localization within the cell. HCF-1 is predominantly nuclear in most cells but is retained in the cytoplasm of unstimulated neurons the site of HSV latency. This is a potentially important finding because HCF-1 has been shown to be cytoplasmic in sensory neurons, the primary site of HSV latency.
Maintenance of latency and cellular immortalization by Kaposi's sarcoma-associated herpes virus
Figure 2. HHV8 and AIDS-related Cancers.
A second area of investigation is the role Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8) in the development of Kaposi's sarcoma (KS), primary effusion lymphoma and other proliferative disorders. KS is the most common malignancy in AIDS patients and is a leading cancer in sub-Saharan Africa. KSHV was first identified from KS lesions in 1994 and harbors an unusual number of genes with cellular counterparts. These pirated genes are believed to perform a number of functions including blocking antiviral responses such as apoptosis and promoting angiogenesis within the highly vasculated KS tumors.
LANA: Our main focus is the Latency-associated nuclear antigen (LANA, LNA-1), a viral protein that plays a critical role in the maintenance of the viral episomal genome and in the immortalization of the host cell. LANA is a hallmark of KSHV infection and is expressed in all latently infected tumor cells implying an important role in the disease process. Recent work from other laboratories has shown that LANA disrupts the function of both the retinoblastoma protein (pRB) and the tumor suppressor protein p53. Blockade of the pRB and p53 cell cycle checkpoints is fundamental to tumorigenesis in human cells, suggesting that LANA is a key determinant of immortalization and malignancy in KSHV-associated disease. LANA also binds to the viral terminal repeat sequences and tethers the circular genome to the host chromosomes thus ensuring proper segregation during mitosis. We have shown that LANA exists as a stable dimer and functions as transcription factor capable of activating and repressing transcription. Current efforts are centered on identifying cellular and viral target genes and understanding how modulation of these activities contributes to the immortalization of latently infected cells.
Figure 3. Changes in the chromatin structure.
Figure 4. Kaposi´s sarcoma-associated herpesvirus (KSHV) latently infects human tumor cells.
Reactivation: The majority of KSHV infected cells establish a latent infection and allow the host cell to grow indefinitely. KSHV latency is remarkably stable and in most circumstances, only a tiny fraction of cells will switch to the lytic mode (reactivate). Our goal is to understand why latency is so stable and devise methods to promote reactivation. To do this we are focusing on the role of latency gene products in suppression of the lytic program and the contribution of chromatin structure to latency.
