Latent membrane protein 1-induced EGFR signalling is negatively regulated by TGFα prior to neoplasia

Abstract

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is an oncoprotein expressed in several EBV-associated malignancies. We have utilised mice expressing the Cao strain of LMP1 in epithelia to explore the consequences of expression in vivo, specifically the changes that occur prior to neoplasia, in the hyperplastic but degenerating tissue. Epidermal growth factor receptor (EGFR) ligands (transforming growth factor alpha (TGFalpha), heparin-binding EGF-like growth factor and epiregulin) are constitutively induced by LMP1, leading to EGFR phosphorylation but also down-regulation, degradation or turn-over, with the appearance of cleaved EGFR fragments. This is accompanied by down-regulation of Akt and activation of caspase-3 and p38 mitogen-activated protein kinase (MAPK). Surprisingly, removal of TGFalpha (using the null strain) does not ameliorate the LMP1-induced phenotype, but instead accelerates the deterioration. Consistent with this, EGFR is reduced less rapidly and MAPK/ERK kinase (MEK) and extracellular-signal-regulated kinase (ERK) are initially activated in the null background, suggesting that TGFalpha or excess of the ligands together act to divert phosphorylated EGFR into a cleavage pathway. In addition, LMP1 leads to the activation of c-Jun N-terminal kinase 2 (JNK2) followed by JNK1 in the effected tissue. Specific AP1 family members FosB, Fra-1 and JunB are constitutively induced and serum response factor, AP1 and nuclear factor kappaB (incorporating p65) are activated in the transgenic tissue compared with wild-type. This system allows the analysis of early events resulting from the expression of a viral oncogene with broad impact in the signalling milieu and the attempts at homeostasis in the responding tissue. It reveals what regulatory circuits are in place in a normal tissue, thus facilitating further prediction of causative events in carcinogenic progression.