Functional analysis of Helicobacter pylori FrpB4 in adherence and role of the host protein IQGAP1 in apoptosis modulation

The bacterium Helicobacter pylori (H. pylori), colonizes the human stomach causing mild gastritis, peptic ulcers and gastric cancer. Adherence of H. pylori is an important factor for persistent colonization of the gastric mucosa and facilitates the delivery of virulence factors such as, the cytotoxin associated antigen (CagA) into host cells and induces different host cell signaling pathways. Although many adhesins are known, it appears that additional adherence-related proteins are involved. Moreover, CagA causes multiple changes in the host cell signaling and has been closely associated with gastric cancer. But, its molecular significance is still unknown. This study is aimed at, investigating the influence of iron regulated outer membrane protein (FrpB4) on adherence and translocation of CagA into host cells. Inactivation of frpB4 in H. pylori strains increased adherence to host cells due to higher phospholipase activity, mediated by outer membrane phospholipase A (PldA). Indeed, deletion of the pldA gene in frpB4 mutants decreased adherence, suggesting PldA could be an adhesin. Further, higher levels of phosphorylated CagA were detected in gastric epithelial cells that were infected with frpB4 mutants when compared to the wild type strains. In particular, phosphorylated CagA was observed in murine fibroblasts upon infection with the frpB4 mutant of the G27 strain in contrast to wild type strain, which by, itself is a unique feature, thereby broadening the prospects in murine models. In addition, work on the host side unravels a novel interaction between the CagA protein and IQGAP1. The host cell protein IQGAP1 (IQ motif containing GTPase activating protein 1) functions as a scaffolding protein in different signaling pathways involved in cancer and also regulates the cytoskeletal architecture. Knock-down of IQGAP1 by RNA interference made the cells strongly resistant to H. pylori, stress and receptor mediated apoptosis. The data demonstrates that IQGAP1 needs to interact with keratin18, which is essential for the proper formation of the Death Inducing Signaling Complex (DISC), during apoptosis induction. In absence of IQGAP1 the formation of DISC complex is abrogated, leading to a block in the activation of caspase-8 and consequently resulting in resistance to cell death. Finally the results add to an increasing pool of knowledge, highlighting the role of keratin in apoptosis regulation and also the new function of IQGAP1 in apoptosis may lead it to be a therapeutic target for cancer.