Altered cell metabolism and migration in nasopharyngeal carcinoma

Abstract

Nasopharyngeal carcinoma (NPC) is a subtype of head and neck cancer, originating from the nasopharynx. The etiology of NPC is considered a complex interaction between Epstein–Barr virus (EBV) infection, genetic susceptibility, environmental factors, and lifestyle factors. NPC has the highest metastasis rate among all head and neck cancers. Patients with NPC managed with radical radiotherapy plus chemotherapy can still develop local recurrence and distant metastases after the initial treatment. Recently, reprogrammed metabolism has been reported as a hallmark of NPC and the altered metabolism also affects the metastatic process in NPC. In this thesis, I studied these two hallmarks, activating metastasis and deregulating cellular metabolism, as well as the association between the two. The thesis focuses on the role of EBV encoded latent membrane protein 2A (LMP2A), microflora components, and metastasis suppressor 1 (MTSS1) in the altered metabolism and migration of NPC cells.

In paper I, we show that forced expression of LMP2A resulted in increased epidermal growth factor receptor (EGFR) activation, elevated levels of intracellular Ca2+, calpain activation, and accelerated cleavage of integrin β4 subunit (ITGβ4). Activation of EGFR and increased calpain activity was responsible for the ITGβ4 redistribution from the basal layer to the peripheral membrane in NPC cells, which correlated with enhanced migratory potential of NPC cells. Moreover, we demonstrated that the expression of calpastatin, the calpain inhibitor, was decreased in NPC tissues, compared to the control nasopharyngeal epithelium. In conclusion, our results pointed to that LMP2A enhances cancer cell migration via the EGFR/Ca2+/calpain/ITGβ4 signaling pathway in NPC cells. In paper II, we show that LMP2A could block lipid degradation by downregulating the lipolytic gene adipose triglycerol lipase (ATGL), consistent with the finding that LMP2A increases lipid accumulation in NPC cells. Suppression of ATGL not only promoted cell migration but also enhanced lipid accumulation in NPC cells, which mimicked the function of LMP2A in NPC cells. We found that ATGL is downregulated in NPC biopsies and the reduced expression level of ATGL associates with poor overall survival in NPC patients. Our findings revealed a novel effect on lipid metabolism in NPC and a link between lipid metabolism and cell migration in LMP2A positive NPC cells. In paper III, we show that microbes and their cell wall components induce an inflammatory response in normal nasopharyngeal epithelium (NNE) cells in vitro. NNE cells showed strong induction of nuclear nuclear factor-κB (NF-κB) shuttling and inflammatory gene expression when treated with components of the gram-positive bacterial cell wall: gram-positive bacteria (streptococci) or peptidoglycan (PGN). However, this response was abrogated in NPC cells. Furthermore, we found that the induction of inflammatory response by NF-κB signaling is blocked at two levels in NPC cells: NF-κB trapped in lipid droplets (LDs) in the cytoplasm and the reduction of the inflammatory response by increased expression of lysine-specific histone demethylase 1 which is a repressive nuclear factor. The refractory response of NPC cells might be a consequence of long-term exposure to microbes in vivo, which contributes to the decreased anti-tumor immune responses in NPC. In paper IV, we show that MTSS1 is downregulated in NPC tissues and the lower expression of MTSS1 predicts a worse patient prognosis. This could be explained by the ability of MTSS1 to suppress NPC cell migration and invasion through cytoskeletal remodeling at cell cell borders and assembly of epithelial cadherin/β-catenin in adhesion complexes, as we show in vitro. Moreover, we found that the inverse Bin–Amphiphysin–Rvs (I-BAR) domain of MTSS1 is both necessary and sufficient to promote the formation of adherens junctions. These findings suggest that MTSS1 suppresses metastasis by controlling the integrity of the adherens junctions.

In summary, in this thesis, I investigate the mechanism of altered metabolism and migration of NPC cells and revealed an association between metabolic reprogramming and activation of migration in NPC cells. These findings may have useful clinical and therapeutic implications in NPC but can also be extended to other EBV-associated epithelial cancers as well as other cancer types with decreased expression of MTSS1.