The role of LMP1 in the epithelium carcinogenesis in vivo
Epstein-Barr virus (EBV) is a human herpesvirus that has been associated over the past forty years with a number of malignancies both of lymphoid and epithelial origin. These include Burkitt's lymphoma (BL), Hodgkin's disease (HD) and nasopharyngeal carcinoma (NPC) to name only a few. One of the latent proteins of the virus, latent membrane protein 1 (LMPl) has been frequently detected in NPC biopsies and it plays a role in the initiation and development of the disease. Various strains of LMP1 have beed detected, but the LMP1CAO strain is the one most often encountered in endemic NPC cases. NPC tumours also show a deletion across chromosome 9p21which leads to loss of the tumour suppressor locus INK4a as well as deletion or hypermethylation of 3p21.3 that leads to loss of another tumour suppressor the Rassfl. The aim of the work presented in this thesis is to investigate the exact role LMPI plays both in the genesis and the development of epithelial malignancies such as NPC. In order to investigate this, transgenic mouse models expressing two different strains ofLMPl at their epithelium were used. Use of other transgenic and knock out mice was also involved, in order to investigate cooperative relationships that LMP1 may have with other oncogenes or tumour suppressor genes. Minimal skin chemical carcinogenesis was employed in order to determine the role of LMPI in initiation or progression of the tumourigenic process. LMPI CAO was found to be expressed in a wide variety of tissues in the transgenic mice, including both tissues of the epithelium as well as lymphoid tissues. LMPI CAO is a weak initiator as LMPI CAO transgenic mice develop lesions spontaneously and in some cases the ears of these mice progress from benign keratoacanthomas to malignant squamous cell carcinomas. LMPlcAO cooperates with loss ofINK4a locus to give an increased lesion load. Signalling pathways that were found to be activated by LMPI in lymphoid, epithelial cells or fibroblasts in previous studies, were investigated by Western blotting in order to determine whether they are activated by LMPI CAO in the epithelium in vivo. LMPI CAO in the epithelium in vivo, leads to activation of the p38, NF-KB, AP-I and MAPK pathways. Other proteins were shown to be upregulated or stabilised by LMPI including pS3, pI6INNK4ac,aspase-J and MMP9. Whether this is a direct effect of LMPI CAO or it is a secondary event due to the phenotype that LMPI causes is still unclear. The similarity between the LMPI transgenic mice and TGFa transgenic mice, as well as increased levels of the epidermal growth factor receptor (EGFR) in NPC biopsies and NPC cells in vitro, led us to investigate the possibility that LMPI may be acting via the TGFalEGFR pathway. Indeed, TGFa levels were found to be upregulated in transgenic affected tissues when compared to wild type sibling tissues. EGFR activates many signalling pathways including MAPK. Investigation of the MAPK pathway showed that LMPI does lead to its activation. In order to determine whether LMPI acts via upregulation of TGFa, LMPI transgenic mice were cross bred with TGFa null mice to create LMPI transgenic / TGFa null mice. The phenotype of these mice was observed and it was discovered that paradoxically, loss ofTGFa- a known oncogene- leads to a worsening of the phenotype. Further studies into the signalling pathways that may be affected by loss of TGFa showed that TGFa in this system may be acting as a tumour suppressor by upregulating Rassfl and also may be acting as a control of some of the signalling pathways activated by LMPI. The results show that LMP 1CAD is a weak initiator of proliferation but other cooperative events such as loss of tumour suppressors INK4a and/or Rassfl are needed for progression. This is consistent with previous studies performed in this laboratory as well as the facts that are currently known for NPC.