Activation of the PI3K /Akt and beta-catenin pathways by Epstein -Barr virus latent membrane protein 2A in epithelial cells and in EBV-associated malignancies.

摘要

Epstein-Barr virus (EBV) is associated with human diseases and malignancies. The focus of this study was to investigate EBV latent membrane protein 2A (LMP2A) signaling in epithelial cells and its possible contribution to carcinogenesis.;In B cells, LMP2A maintains viral latency by blocking B cell receptor activation. In epithelial cells, the signaling functions of LMP2A are beginning to be elucidated. In a previous study, LMP2A expression in human HaCaT keratinocytes resulted in anchorage-independent growth and tumorigenicity in nude mice. The current study has extended these findings to characterize the pathways activated by LMP2A in epithelial cells, the biological consequences, and the activation status of these pathways in EBV-associated malignancies.;LMP2A expressed in normal human foreskin keratinocytes (HFK) induced activation of the phosphatidylinositol 3'OH-kinase (PI3K) target Akt kinase with inhibition of Akt targets glycogen synthase kinase 3-beta (GSK3beta) and the Forkhead family member FKHR. As GSK3beta is a negative regulator of the Wnt/beta-catenin signaling pathway, the status of the beta-catenin pathway was determined. In LMP2A-expressing cells, beta-catenin accumulated in the cytoplasm, translocated into the nucleus in a PI3K-dependent fashion, and activated transcription of a responsive reporter. The immunoreceptor tyrosine-based activation motif (ITAM) and PY motifs of LMP2A were important for these effects. Furthermore, activation of the PI3K/Akt and beta-catenin pathways was prevalent in primary nasopharyngeal carcinoma specimens. In contrast, in the EBV-associated lymphoid malignancy Hodgkin lymphoma, Akt was activated in the absence of GSK3beta inactivation and beta-catenin signaling.;LMP2A also impacted epithelial cell biology. Expression of LMP2A in HFK cells led to inhibition of differentiation. The PY motifs of LMP2A, which mediate interaction with the Nedd4 ubiquitin ligases and manipulate turnover of LMP2A-associated kinases in B lymphocytes, were critical for this effect. The ITAM motif, responsible for LMP2A-mediated activation of PI3K/Akt signaling, also contributed to the observed inhibition of differentiation.;Through activation of the PI3K/Akt and beta-catenin pathways that impact cell proliferation and survival, LMP2A could contribute to carcinogenesis. Furthermore, by preventing differentiation, LMP2A maintains infected epithelial cells in a metabolically-active state that could facilitate transformation in conjunction with other viral and cellular factors.