LMP1 of Epstein-Barr Virus Induces Proliferation of Primary Mouse Embryonic Fibroblasts and Cooperatively Transforms the Cells with a p16-Insensitive CDK4 Oncogene

Abstract

The latent membrane protein LMP1 of Epstein-Barr virus (EBV) is often present in EBV-Epstein-Barr virus (EBV) is a human gammaherpesvirus commonly carried in the majority of the human population. It is the causative agent for proliferative diseases such as infectious mononucleosis and oral hairy leukoplakia in AIDS patients. It is also implicated in a variety of human malignancies that include Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's lymphoma, nasal T-cell lymphoma, and immunoblastic lymphomas in posttransplant and AIDS patients (39). The virus often adopts latent forms in EBV-associated cancers. In nasopharyngeal carcinoma and Hodgkin's lymphoma, type II latency gene expression was observed. During this type II latency, only three viral proteins are routinely detected: EBV nuclear antigen 1 (EBNA1) and two latent membrane proteins, LMP1 and LMP2. Among the three EBV genes, the LMP1 gene is the only one implicated in cell immortalization and transformation. It is one of the viral genes required for B-cell immortalization, together with the EBNA2, EBNA3a, and EBNA3c genes (8,15,22,23,48). In addition, LMP1 has been shown to induce the transformation of certain established rodent fibroblast cell lines, including Rat-1 and BALB/c 3T3 (2, 33, 49). Furthermore, LMP1 induces the morphological transformation of the RHEK-1 cell line (12) and the tumorigenicity of epithelial cell lines in severe combined immunodeficient mice (18, 36).Although EBV is an important oncogenic virus in human, the LMP1 genes appeared to act differently from oncogenic genes of other DNA oncogenic viruses, including simian virus 40 (SV40) large T antigen, adenovirus E1A and E1B, and human papillomavirus (HPV) E6 and E7. There is little evidence suggesting that the LMP1 gene can function similarly to these viral oncogenes or even a cellular oncogene such as ras or myc. Though LMP1 transforms several immortalized rodent fibroblast cell lines and participates in B-cell immortalization, a single LMP1 gene has not been shown to induce continuous proliferation of any primary cell culture. Similarly, although it cooperates with a number of EBV genes in inducing B-cell proliferation, there is no report on the cooperativity of the LMP1 gene with another cellular oncogene in transforming primary cells.Protein sequence analysis of LMP1 revealed at least three domains: an amino-terminal cytoplasmic domain of 20 amino acid residues, a six-transmembrane domain of 185 amino acid residues, and a carboxy-terminal cytoplasmic domain of 200 amino acid residues (24). The six-transmembrane domain was shown to be required to form cytoplasmic membrane patches. The oligomerization of LMP1 in the cytoplasmic membrane may mimic that of other membrane receptor molecules induced by ligand-receptor interactions, resulting in constitutive activation of the LMP1 signaling pathway (14). The C-terminal cytoplasmic region was shown to interact with two families of proteins, TRAF (tumor necrosis factor [TNF] receptor-associated factor) and TRADD (TNF receptor-assoc...

Epstein–Barr virus latent membrane protein 1 (LMP1) upregulates Id1 expression in nasopharyngeal epithelial cells

“…We have previously reported that Id1 expression in NPC cells downregulated the Rb/p16 INK4a pathway . This is in agreement to our previous study in rodent fibroblasts, which showed that inactivation of the CDK4/p16 INK4a regulatory pathway was a downstream event of LMP1 (Yang et al, 2000). The p16 INK4a protein regulates cell cycle G1/S entry and is largely responsible for the onset of replicative senescence in human epithelial cells (Ohtani et al, 2001).…”

“…Constitutive expression of LMP1 in NP69 cells was achieved by retroviral expression of an NPC-derived LMP1. The construction of the LMP1 expression vectors had been previously described (Yang et al, 2000). NP69 cells infected with the empty vector, pLNSX, were also established as a negative control.…”

Molecular and cytogenetic changes involved in the immortalization of nasopharyngeal epithelial cells by telomerase

“…17,19 Our previous studies in rodent fibroblasts and human nasopharyngeal epithelial cells showed that the Cdk4/p16 regulatory pathway is repressed by the EBV-encoded LMP1 and conferred growth advantage to cells. 20,21 Hypermethylation of the p16 gene has been shown to be a common event in NPC. 18 Loss of heterozygosity of the chromosome 9p at the p16 INK4A locus could be detected in premalignant nasopharyngeal lesions.…”

LMP1 of Epstein–Barr virus suppresses cellular senescence associated with the inhibition of p16INK4a expression

“…We recently observed for the ®rst time that LMP1, as a single EBV gene, is sucient to induce the proliferation of primary mouse ®broblasts (Yang et al, 2000). In this study, we examine the roles and mechanisms of LMP1 in inducing the proliferation of primary cells.…”