Previous studies reported that LMP1 is involved in several signaling pathways including NF-κB, AP-1, JAK/STAT, PI3K/AKT and ERK-MAPK and regulate their downstream effects [6
]. LMP1 activate the PI3K/AKT/mTOR signaling pathway in B lymphocytes [21
], and the mTOR signaling pathway has been identified as a downstream component of the PI3K/AKT pathway in the LMP2A-transfected NPC cell lines HONE1 and AD/AH [22
]. The mTOR signaling pathway might positively regulate cyclin D1 expression in NPC [23
]. In this study, microarray analysis of the NPC HONE1 cell line stably transfected with LMP1 identified several differentially expressed genes of mTOR signaling pathways. This is the first report that LMP1 can regulate the mTOR signaling pathway in NPC. Furthermore, LMP1 overexpression and knockdown studies confirmed that LMP1-regulated genes are involved in the mTOR signaling pathway, and LMP1 expression was essential for the activation of p-mTOR and p-4EBP1 in NPC cell lines. In addition, our in vitro
studies found that LMP1 expression positively correlated with overexpression of p-mTOR, p-P70S6K and p-4EBP1 in NPC tumors.
As a well-known oncogene, one of the functions of LMP1 is to promote cell proliferation in NPC [24
]. The mTOR signaling pathway is also a major effector in cell growth, cell proliferation and cell survival, through regulation of protein synthesis, while P70S6K and 4EBP1 play particularly important roles in the mTOR signaling pathway growth acceleration function [10
]. In this study, our findings suggest that activation of P70S6K and 4EBP1 requires LMP1, and that when these genes are phosphorylated by LMP1, activated P70S6K and 4EBP1 initiate a sequence of events that promotes protein synthesis, cell growth and proliferation. Further studies need to be done to investigate the mechanism by which LMP1 regulates mTOR signaling in NPC tumorigenesis.
Deregulation of the mTOR signaling pathway is reported in many malignancies, and some of the signaling molecules in this pathway are predictors of prognosis in different types of cancers. Cytoplasmic p-mTOR expression correlates with poorer survival in gastric cancer and cervix adenocarcinoma [26
]. High expression of p-mTOR, p-P70S6K and p-4EBP1 correlate with poor outcome in glioblastoma [28
], and p-4EBP1 was demonstrated to be a potential prognostic factor in breast cancer and an independent prognostic marker in ovarian cancer [29
]. Our results revealed that NPC patients with high p-P70S6K and p-4EBP1 expression had a significantly shorter overall survival than those with low p-P70S6K (p
= 0.049) and p-4EBP1 (p
= 0.010) expression. These results are in accordance with previous studies on malignancies. p-P70S6K is required for 5'-TOP mRNA translation, especially translation of all ribosomal proteins, elongation factors, and poly (A)-binding protein. 4EBP1 forms a complex with eIF4E by closely interaction, and once 4EBP1 is phosphorylated, 4EBP1 loses its high affinity for eIF4E. When eIF4E dissociates, activated 4EBP1 enhances protein synthesis [11
]. High expression of p-P70S6K and p-4EBP1 in NPC tissues might result in a high level of protein synthesis and cell proliferation, and the poor prognosis of the NPC patients.
In this study, a large sample size of NPC cases were used for IHC staining of LMP1, and LMP1 overexpression was detected in 62.9% (141/224) of NPC tumors, in accordance with previous studies [31
]. Interestingly, we found that LMP1 overexpression in NPC patients was significantly associated with poorer overall survival (p
= 0.020). This result differed from previous reports, which found that LMP1 overexpression suggested a better prognosis of NPC patients [34
], and LMP1 was not an effective indicator of NPC outcomes [35
]. The possible reasons for the differences might be different sample sizes, regional distribution, or different LMP1 variants. Compared to previous studies, our study had a larger sample size for LMP1 expression and NPC prognosis.
Although high-expression of LMP1, p-P70S6K and p-4EBP1 was associated with poor survival of NPC patients, multivariate analysis revealed that only LMP1 expression (p = 0.013), as well as gender (p = 0.014) and metastasis (p = 0.003), were independent prognostic factors. We found that the mTOR signaling pathway was triggered by LMP1, suggesting that LMP1 may have more important roles than mTOR signaling molecules in the carcinogenesis and development of NPC.