Uniformed Services University of the Health Sciences
Department of Microbiology and Immunology
4301 Jones Bridge Road
Bethesda, Maryland 20814-4799
Phone: (301) 295-9624
FAX: (301) 295-1545
Professor & Vice Chair
Ph.D., University of Connecticut, 1983
The research in my lab revolves around the molecular biology and pathogenesis of human viruses. We have a long standing interest in human T-lymphotropic virus type 1 (HTLV-1) and human immunodeficiency virus (HIV), and more recently in Kaposi sarcoma-associated herpesvirus/human herpesvirus type 8 (KSHV/HHV-8). We are particularly interested in the mechanisms of action of viral regulatory proteins and how they impact cellular mRNA transcription, cell cycle control, and signal transduction pathways to effect viral replication and pathogenesis, especially, oncogenesis. Through the study of the HTLV-1 trans-activator/oncoprotein, Tax, we have recently found that persistent and potentially oncogenic activation of I-κB kinases (IKKs)/NF-κB by Tax triggers a cellular senescence checkpoint response. This checkpoint response is induced by hyperactivated p65/RelA and is mediated by two cyclin-dependent kinase inhibitors, p21 and p27, in a p53- and pRb-independent manner. It is often impaired in cancer cells with constitutively activated NF- B. Our recent data indicate that de-regulation of G1 cyclin-dependent kinases can dampen the senescence checkpoint response to facilitate chronic NF-κB hyperactivation. We believe this represents a critical step in leukemia development. Interestingly, we have found the anti-sense protein of HTLV-1, HBZ, which down-regulates NF-κB and HTLV-1 trans-activation by Tax, can mitigate or prevent Tax-induced senescence. Most recently, we have found that similar to -herpesviruses such as EBV and KSHV, HTLV-1 infection can lead to two alternative outcomes - productive infection accompanied by senescence or latent infection followed by clonal expansion - based on the relative expression of regulatory proteins: Tax, Rex, and HBZ. When Tax/Rex expression is robust and dominant over HBZ, productive infection ensues with expression of structural proteins and NF-κB hyper-activation, which induces senescence. When Tax/Rex expression is muted and HBZ is dominant, latent infection is established with expression of regulatory (Tax/Rex/HBZ) but not structural proteins. HBZ maintains viral latency by down-regulating Tax-induced NF-κB activation and senescence, and by inhibiting Rex-mediated expression of viral structural proteins. Current efforts in the lab concentrate on understanding how chronic NF-κB activation induces cellular senescence and how the senescence response becomes impaired in cancer cells whose NF-κB signaling pathway is chronically activated. Cell-free systems are also being established to elucidate the mechanism by which Tax activates IKKs. Finally, we are actively investigating the mechanisms underlying HTLV-1 latency and reactivation with the objective of facilitating virus control in infected persons to prevent progression to disease.
A model for HTLV-1 Leukemogenesis. HTLV-1 is transmitted by cell-to-cell contact. The expression levels of Tax and HBZ modulate the outcomes of infection. Robust viral replication stimulated by Tax is accompanied by cellular senescence. HBZ moderates trans-activation by Tax, thereby down-regulates viral replication and Tax expression to allow oligoclonal expansion of infected T cells. Cytotoxic T lymphocyte (CTL) killing can control virus replication in asymptomatic carriers and select for cells that carry latent proviral DNA. HTLV-1-infected cells develop chromosomal instability. Loss of p16INK4a, p15INK4b, and other tumor suppressors and constitutive Jak/Stat activation may contribute to the inactivation of the senescence checkpoint to allow persistent Tax expression and NF-?B activation. Loss of Tax expression is favored because Tax is a primary CTL target and has a propensity to induce genomic instability and cellular senescence. Inactivation of the senescence checkpoint can facilitate potent NF-?B activation by Tax at the early stage of leukemogenesis and aid the development of Tax-independent NF-?B activation later. The mitogenic activity of HBZ mRNA may help sustain the ATL tumor phenotype.
Immunoblots of HeLa cells transduced with LV-Tax or LV-GFP. Cell lysates were prepared and immunoblotted using cyclin B1, human securin (Securin), p21CIP1/WAF1, p27KIP1, p16INK4a, Tax, and actin antibodies as described. (RIght) Expression of the senescence-associated ?-galactosidase (SA-?-Gal) in HeLa cells transduced with LV-Tax. Asynchronously growing HeLa cells (2.5 x 104 cells/well in 6-well plates) were transduced with LV-Tax or LV-GFP at an m.o.i. of 5, grown for 3 days, and stained with X-Gal overnight at 37ºC.
HTLV-1 Tax-expressing HeLa-FUCCI (fluorescent ubiquitin cell cycle indicator) cells bypass mitosis and become senescent. Ad-Tax-transduced cells were released from G1/S arrest and photographed every hour for 140 hours. The image of cells at the 140th hour is shown. Image processing software was used to move the Tax-expressing senescent cell (top) in close proximity to the normal growing colony (bottom). Cells with red, yellow, and green nuclei are in G1, G1/S, and S/G2 phases of the cell cycle respectively. Time-lapses movies showing the progression of Tax-expressing cells through cell cycle can be found here.
Philip S., M. A. Zahoor, H. Zhi, Y. K. Ho, and C. -Z. Giam. 2014. Regulation of Human T-Lymphotropic Virus Type I Latency and Reactivation by HBZ and Rex. PLoS.Pathog. 10:e1004040.
Zahoor M. A., S. Philip, H. Zhi, and C. Z. Giam. 2014. NF- B Inhibition Facilitates the Establishment of Cell Lines that Chronically Produce HTLV-1 Viral Particles. J. Virol. 88:3496-504
Zhi H., M. A. Zahoor, A. Shudofsky, and C. -Z. Giam. 2014. KSHV vCyclin Counters the Senescence/G1 Arrest Response Triggered by NF- B Hyper-activation. Oncogene doi: 10.1038/onc.2013.567. [Epub ahead of print]
Ho, Y. K., H. Zhi, D. DeBiaso, S. Philip, H. -M. Shih, and C. -Z. Giam. 2012. HTLV-1 Tax-Induced Rapid Senescence Is Driven by Activated IKK and p65/RelA. J Virol. 86:9474-83.
Zhi, H., L. Yang, Y. L. Kuo, Y. K. Ho, H. M. Shih, and C. -Z. Giam. 2011. NF-kappaB hyper-activation by HTLV-1 tax induces cellular senescence, but can be alleviated by the viral anti-sense protein HBZ. PLoS.Pathog. 7:e1002025.
Yang, L., N. Kotomura, Y. K. Ho, H. Zhi, S. Bixler, M. J. Schell, and C. -Z. Giam. 2011. Complex cell cycle abnormalities caused by human T-lymphotropic virus type 1 Tax. J.Virol. 85:3001-3009. (cover of March 20, 2011 issue)
Zhang, L., H. Zhi, M. Liu, Y. L. Kuo, and C. -Z. Giam. 2009. Induction of p21(CIP1/WAF1) expression by human T-lymphotropic virus type 1 Tax requires transcriptional activation and mRNA stabilization. Retrovirology. 6:35.
Liu, M., L. Yang, L. Zhang, B. Liu, R. Merling, Z. Xia, and C. -Z. Giam. 2008. Human T-cell leukemia virus type 1 infection leads to arrest in the G1 phase of the cell cycle. J.Virol. 82:8442-8455.
Merling, R., C. Chen, S. Hong, L. Zhang, M. Liu, Y. L. Kuo, and C. -Z. Giam. 2007. HTLV-1 Tax mutants that do not induce G1 arrest are disabled in activating the anaphase promoting complex. Retrovirology. 4:35.
Giam, C. Z. and K. T. Jeang. 2007. HTLV-1 Tax and adult T-cell leukemia. Front Biosci. 12:1496-1507.
Soung, N. K., Y. H. Kang, K. Kim, K. Kamijo, H. Yoon, Y. S. Seong, Y. L. Kuo, T. Miki, S. R. Kim, R. Kuriyama, C. -Z. Giam, C. H. Ahn, and K. S. Lee. 2006. Requirement of hCenexin for proper mitotic functions of polo-like kinase 1 at the centrosomes. Mol.Cell Biol. 26:8316-8335.
Zhang, L., M. Liu, R. Merling, and C. -Z. Giam. 2006. Versatile reporter systems show that transactivation by human T-cell leukemia virus type 1 Tax occurs independently of chromatin remodeling factor BRG1. J.Virol. 80:7459-7468.
Kuo, Y. L. and C. -Z. Giam. 2006. Activation of the anaphase promoting complex by HTLV-1 tax leads to senescence. EMBO J. 25:1741-1752.
Liu, B., S. Hong, Z. Tang, H. Yu, and C. -Z. Giam. 2005. HTLV-I Tax directly binds the Cdc20-associated anaphase-promoting complex and activates it ahead of schedule. Proc.Natl.Acad.Sci.U.S.A 102:63-68.