Functional studies of Epstein-Barr virus latent membrane protein 1 in nasopharyngeal epithelial cells
Functional studies of Epstein-Barr Virus Latent Membrane
Protein 1 in nasopharyngeal epithelial cells
for the degree of Doctor of Philosophy at The University of Hong Kong
on 31 March 2004
Nasopharyngeal carcinoma (NPC) is a common disease in Hong Kong
and Southern provinces of China. The incidence rate of NPC could reach to
25-50 per 100,000 per year. NPC is closely associated with Epstein-Barr virus
(EBV) infection. Over 95% of the world's population carries latently infected
EBV in their B cells. EBV genome could be detected in most of the NPC cells.
Role ofEBVin the pathogenesis ofNPC is largely unknown. Understanding the
mechanisms of tumorigenesis by EBV is essential for better prevention and
treatment of NPC. Previous studies have found that several of the EBV latent
genes modulate cellular phenotype and growth characteristics, in which the
oncogene Latent Membrane Protein 1 (LMP1) is believed to play important role
in malignant transformation.
The main focus of my research is on the mechanisms of LMPI In
transformation of nasopharyngeal epithelial cells. In this study, a novel function ofLMPl to induce Idl expression in nasopharyngeal epithelial cells (NP69) and HEK293 is reported. Idl is a negative transcriptional regulator of p16INK4a. Expression of Idl facilitates cellular immortalization and stimulates cell proliferation. With the combination of both specific chemical and genetic inhibitors of cell signaling, induction of Idl by LMPI was shown to be dependent on its NF-KB activation domain at the carboxy-terminal region, CTARI and CTAR2. Induction of Idl by LMPI may facilitate clonal expansion of premalignant nasopharyngeal epithelial cells infected with EBV and may promote their malignant transformation.
Previous studies have shown that LMPI activates Cdc42, which mediate some phenotypic changes such as actin reorganization. Such changes are known to relate to cell cycle progression, cytokinesis and adhesion, which are involved in carcinogenesis. LMPI was shown to be able to activate Cdc42 in 3T3 fibroblasts. Here Cdc42 was found to be activated by LMPI in HEK293 and nasopharyngeal carcinomatoid SUNEI. This supports a role of LMPI in carcinogenesis through constitutive activation of Cdc42 in nasopharyngeal epithelial cells.
Previous studies have also shown that loss or alteration of E-cadherin
occurs frequently in many different epithelial cancers. It has also been found to
be downregulated in NPC. E-cadherin is a plasma membrane protein that functions in cell-cell adhesion. In my study, E-cadherin was found to be downregulated by LMPI in nasopharyngeal carcinomatoid CNE2 and SUNEI. In NPC cells, inhibition of its expression was not related to the CTAR2 domain
Currently, most of the functional studies on the transformation ability of EBV were demonstrated in either non-nasopharyngeal epithelial or transformed cell lines, which have aberrant cellular signaling systems that complicate the experiments. Hence, in this study, a near normal human nasopharyngeal epithelial cell line (NP460 hTert) has been established with the assistance of
constitutive telomerase expression. The immortalized telomerase expressing cells have up-regulation in Idl, increased phosphorylation of IKBa, undetectable p 16INK4a expression and amplification at 17 q21-q25 on 11 p 15 chromosome. The phenotypic alterations of the immortalized nasopharyngeal epithelial cells were similar to the functional properties of EBV-encoded LMPI suggested that LMPI may partly contribute to the immortalization of nasopharyngeal epithelial cells.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:epstein barr virus membrane proteins cancer cells nasopharynx
Date of Publication:01/01/2004