Early in the 19th century it was proposed that bacterial infections might be associated with
cancer development (Lax and Thomas, 2002). Indeed, chronic infections with bacteria known to perturb cell-signalling processes might contribute to cell transformation. For example, bacterial infections can lead to the promotion of anti-apoptotic proliferative states, DNA damage, or multinucleation in host cells. Any of these consequences can lead to tumour initiation.
It is now well established that H. pylori infection is the most important environmental cause for the development of gastric carcinoma. In addition, epidemiological studies indicate that the production of CagA is associated with an increased risk of developing carcinoma of the stomach (De Luca and Iaquinto, 2004). Recently it was demonstrated that CagA interacts with ASPP2, a tumour suppressor protein. In case of DNA damage, it is known that ASPP2 binds to p53 and activates it, resulting in apoptosis of the cell. Interaction between CagA and ASPP2 impedes the activation of p53, leading to its degradation. As a result, the apoptotic response of the host cell is inhibited. Promotion of p53 degradation by CagA is nowadays thought to be a major contribution to the development of stomach cancer (Buti et al., 2011).
It has been suggested that persistent Chlamydia pneumoniae infections correlate with increased risk of lung cancer. Elevated antibody titers against these bacteria are frequently observed in lung cancer samples (Laurila et al., 1997). Furthermore, it has been shown that individuals with high IgA antibody titers against Chlamydia pneumoniae have up to two-fold increased lung cancer risk (Littman et al., 2004). Epidemiological studies have shown that chronic infections of the upper genital tract caused by Chlamydia trachomatis are associated with increased risk of cervical cancer (Samoff et al., 2005; Madeleine et al., 2007). It is possible that cytokinesis failure and subsequent multinucleation caused by Chlamydia infections are involved in the development of these carcinomas.
Several findings also suggest that Mycobacterium tuberculosis can cause cancer development. An old study showed that individuals with clinical history of tuberculosis have a five-fold higher
62 risk of developing lung carcinoma than control individuals (Steinitz, 1965). In the meantime, several other studies have also established an association between tuberculosis and lung cancer risk (Brownson and Alavanja, 2000; Song et al., 2002). Furthermore, bronchogenic carcinomas have been shown to frequently localize in the infected lung lobe (Farwell et al., 1978). It has been hypothesised that the constant high levels of circulating VEFG (vascular endothelial growth factor) associated with Mycobacterium tuberculosis infections might result in carcinogenesis (Tamburini et al., 2007).
One of the most striking cases of association of a bacterial infection and cancer formation concerns Salmonella. Several studies have helped to establish that individuals having chronic infections with S. Typhi have a greater risk of developing gallbladder carcinoma (Lazcano- Ponce et al., 2001; Wistuba and Gazdar, 2004; Kumar, 2006). Epidemiological studies have shown that chronic carriers of S. Typhi have an approximately eight-fold excess risk of developing gallbladder carcinoma than non-carriers. In addition, chronic carriers also have an approximately 200-fold higher risk of developing hepatobiliary carcinoma, compared with people who have had typhoid fever and completely cleared the infection (Caygill et al., 1995; Shukla et
al., 2000). In another study, Robbins and colleagues found a strong association between S. Typhi carriers and development of cholangiocarcinoma (Robbins et al., 1988). Furthermore,
associations of typhoid carriage and cancer formation in the pancreas, lung and colorectum have also been found, but to a much less extent (Szu et al., 1994; Caygill et al., 1995). Despite the overwhelming association of S. Typhi infections with cancer formation, the pathogenic process by which this occurs is still unknown. One possible mechanistic explanation could be the activity of the toxin CDT, present in S. Typhi, although it seems unlikely that the activities of CDT could lead to transformation of host cells, since there are no associative studies between cancer formation and infections with the majority of the other CDT-producing bacteria, such as
Escherichia coli, Shigella and Campylobacter.
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4. Aims of this project
Salmonella can invade, survive and replicate within host cells. Several host cell features are
known to be targeted by this intracellular pathogen, such as the cellular cytoskeletal network, vesicular trafficking and control of apoptosis. The evidence of an association between S. Typhi infections and cancer formation, whose molecular mechanisms were not studied, suggested the hypothesis that Salmonella could interfere with the cell cycle of the host. Therefore, one objective of this study was to investigate the potential impact of the intracellular replication of
Salmonella on the cell cycle of the host and to characterize these potential interactions by
investigating the contributions of the T3SS-2 and host pathways affected. Although the association of Salmonella infections and cancer formation concerns only long-term infections of
S. Typhi in humans, the experiments presented here were carried out using S. Typhimurium
strains. Throughout the past decades, S. Typhimurium has been of invaluable use to understand S. Typhi infections, since S. Typhimurium and S. Typhi have similar phenotypes in cultured cells (Mills and Finlay, 1994), have most of their genomes in common (Sabbagh et al., 2010) and cause similar systemic diseases in mice and in humans, respectively (Santos et al., 2001).
In addition, it was hypothesised that there might be phases of the cell cycle that are more susceptible and/or refractive to Salmonella invasion. Therefore, another aim of this study was to analyse whether the different phases of the host cell cycle had an effect on bacterial invasion and to study the potential molecular mechanisms involved, for example, the contribution of the T3SS-1. These investigations led to the initial findings of this study. Therefore, the impact of the host cell cycle on Salmonella invasion became the main focus of this research and those findings are presented firstly in the Results section.
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