The role of cholesterol in Helicobacter pylori interactions with host cells

2017-01-10T00:42:56Z (GMT) by Hutton, Melanie Lisa
Infection with Helicobacter pylori cag pathogenicity island (cagPAI)-positive strains is associated with the production of higher levels of interleukin-8 (IL-8), more destructive tissue damage and an increased risk of severe disease. The cagPAI encodes a type IV secretion system (TFSS) that is required for the secretion of the bacterial effectors cytotoxin-associated gene A (CagA) and peptidoglycan (PG) into the host cell cytoplasm. Upon delivery into epithelial cells, PG is recognised by the cytosolic pathogen recognition molecule “Nucleotide Oligomerisation Domain” (NOD1), which in turn, activates the transcription factor “Nuclear Factor kappaB” (NF κB), ultimately resulting in the upregulation of pro inflammatory gene expression. It was previously shown that interactions between the TFSS pilus protein, CagL, and host α5β1 integrin molecules were critical for CagA translocation and the induction of cytoskeletal rearrangements in epithelial cells. As α5β1 integrin is found to associate with cholesterol-rich microdomains (otherwise known as lipid rafts), we hypothesised that these domains may be utilised by H. pylori for the activation of NOD1-dependent pro-inflammatory responses mediated by host cell recognition of bacterial PG. Indeed, depletion of cholesterol from cultured epithelial cells significantly affected the levels of NF-κB and IL-8 responses induced by H. pylori bacteria and interfered with TFSS mediated PG delivery to cells. Furthermore, we demonstrated for the first time the involvement of α5β1 integrin in the induction of pro-inflammatory responses by H. pylori. Thus collectively, we propose a new model whereby α5β1 integrin, associated with host cholesterol-rich microdomains, is required for NOD1 recognition of PG and for the subsequent induction of NF κB-dependent responses to H. pylori. Asides from targeting cholesterol rich regions in host cell membranes for delivery of bacterial effector molecules, H. pylori acquires cholesterol from these microdomains for incorporation into its own cell membrane. Interestingly, it has previously been shown that H. pylori displays a specific affinity for cholesterol and is able to grow in culture media supplemented with this sterol. H. pylori has particular growth requirements that often causes difficulties when attempting to cultivate the bacterium in vitro. Growth of H. pylori depends on supplementation of a nutrient-rich complex media with serum, blood or blood derivatives. However, these supplements often require frozen storage and can show batch-to-batch variation, resulting in differences in bacterial growth. As there is a critical need for the development of a quintessential growth media for the standardised growth of H. pylori, we examined the use of a commercially available, cholesterol-enriched supplement called AlbuMAX II® as a novel nutrient replacement for serum or blood for the culture of a wide variety of H. pylori strains. Importantly, we found that the growth of H. pylori on both solid and liquid media containing AlbuMAX II® were comparable to levels obtained on blood agar or liquid media supplemented with serum. Furthermore, growth was consistently higher in media supplemented with AlbuMAX II® than in media containing β-cyclodextrin. Therefore, we propose that AlbuMAX II® can be used as a novel serum/blood replacement for the cultivation of H. pylori in either solid or liquid media. This medium could be useful for improving the current understanding of H. pylori metabolism or for antigen production. H. pylori has a unique ability to acquire cholesterol from host cells, modify it by glycosylation and incorporate the glycosylated cholesterol into its own cell membrane, thereby allowing H. pylori to escape phagocytosis, T-cell activation and bacterial clearance in vivo. However, the potential role of H. pylori membrane cholesterol in the induction of pro‑inflammatory responses has not been described. Through the use of chemically‑defined media, we have demonstrated a role for H. pylori membrane cholesterol in the induction of pro-inflammatory immune responses in epithelial cells. We have shown that the organisation of proteins into bacterial lipid raft-like structures within the H. pylori membrane may involve the presence of cholesterol. Finally, we have identified a novel protein that appears to be important for the association of cholesterol with the bacterial cell membrane as well as in facilitating bacterial interactions with host cells and mouse colonisation. Therefore, we propose that the development of treatments that target H. pylori lipid raft-like structures or that reduce interactions of H. pylori with cholesterol may offer a novel strategy to combat persistent infection.