Microbial and epigenetic factors in the pathogenesis of nasopharyngeal carcinoma
Sammanfattning: While uncommon in most of the world, nasopharyngeal carcinoma (NPC) shows an unusual geographic and ethnic distribution, being highly prevalent in Southern China and Southeast Asia. Genetic susceptibility, Epstein-Barr virus (EBV) infection and additional environmental exposures are well established risk factors for NPC in endemic areas. However, the detailed molecular mechanisms of NPC pathogenesis remain largely unknown. In this thesis, several novel pathogenic mechanisms of NPC development and progression are presented. The interaction of EBV encoded latent membrane protein 2A (LMP2A) with cellular proteins promoting invasiveness of NPC cells is described in paper I. Spleen tyrosine kinase (Syk) interacts with integrin β4 subunit (ITGβ4) in epithelial cells through an ITAM-like motif, and concurrent LMP2A expression interferes with this interaction by competitive binding to Syk. Both Syk and LMP2A affect cell surface expression of ITGβ4. Particularly, ITGβ4 concentrates at cellular protrusions in LMP2A expressing cells, which may contribute to the migration property of NPC-cells. Paper II and paper III focus on the epigenetic alteration of candidate tumor suppressor genes (TSGs) and their possible role in NPC tumorigenesis. Cadherin 4 (CDH4) and ubiquitin-conjugating enzyme 2L6 (UBE2L6) are downregulated due to promoter hypermehtylation in NPC. Both genes suppress the proliferation and colony formation of NPC-cells. CDH4 impedes cell migration and elicits cell communication; UBE2L6 induce apoptosis of NPC cells and counteracts degradation of adipocyte triglyceride lipase (ATGL) through ISG15-conjugation of valosin-containing protein (VCP). CDH4 and UBE2L6 could be involved in both initiation and progression of NPC. Suppression of UBE2L6 encoded protein UbcH8 correlated with poor outcome in NPC patients. In paper IV, we compared the response of NPC and normal nasopharyngeal epithelial (NNE) cell lines to bacteria and bacterial cell wall components. Strong nuclear translocation of NF-κB and significant induction of proinflammatory factors IL6, IL8, IL1α and CXCL2 were observed in NNE cells, but not in NPC cells upon exposure to Gram-positive bacteria streptococci and peptidoglycan (PGN). We identified three different mechanisms by which the activation of NF-κB in NPC cells could be hampered. It could be trapped by an enhanced accumulation of cytoplasmic lipids. I-κB degradation could be impaired due to downregulation of UBE2L6. We also showed that overexpression of lysine-specific demethylase-1 (LSD1) blocked the transcriptional activation of proinflammatory genes. Together these mechanisms might contribute to decreased immune reactivity in NPC and thus affect tumor progression.
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