Cooperative interaction between Bcl-2 and Epstein-Barr virus latent membrane protein 1 in the growth transformation of human epithelial cells

The Epstein-Barr virus latent membrane protein 1 (LMP-1) is essential for virus-induced B cell immortalization and can protect B lymphoma cell lines from apoptosis signals in vitro via induction of cellular Bcl-2 expression. However, we have reported that high-level expression of LMP-1 in epithelial cells (RHEK-1 cells) itself induces apoptosis. This apoptotic event occurs in the absence of detectable Bcl-2 expression in the LMP-1-transfected epithelial cells. In this study, we transfected the bcl-2 gene into the LMP-l-containing cells and examined the effect of Bcl-2 upon LMP-1-mediated apoptosis, and upon the growth phenotype of the transfected cells. The results show that ectopic expression of Bcl-2 specifically blocks apoptotic death induced by LMP-1. This was observed from cell culture viability and from gel electrophoresis and flow cytometric assays of the degree of DNA fragmentation in cultured cells, Furthermore, co-expression of LMP-1 and Bcl-2 in RHEK-1 cells enabled the cells to grow under low-serum conditions and to form colonies in semi-soft agar medium. These results suggest, therefore, that these two proteins play important complementary roles in the process of EBV-associated epithelial cell transformation. It appears significant, therefore, that LMP-1 and Bcl-2 are frequently co-expressed in the malignant cells of an EBV-positive epithelial tumour, nasopharyngeal carcinoma.