Elucidating mechanisms leading to H. pylori-Induced DNA double strand breaks in host cells

Abstract

The bacterial pathogen Helicobacter pylori has colonized the human stomach for 100 000 years and represents the major etiological factor for the development of gastric cancer. The exact molecular mechanisms leading to carcinogenesis however are only poorly understood and so far were attributed to an aberrant immune reaction of the host to the pathogen.
A previous study from our lab revealed a direct potentially pro-carcinogenic mechanism of H. pylori that jeopardizes the genomic integrity of the host cell by introduction of DNA double strand breaks (DSBs). By pursuing various strategies we have identified host cell as well as bacterial factors involved in a concerted mechanism leading to the induction of DSBs. The complex cascade leading to DNA DSBs is elicited by the bacterial type IV secretion system (T4SS) and the T4SS component CagL. This protein is known to interact with host cell integrins, thus triggering activation of the NF-κB pathway. Consequently, the deletion of important proteins of the bacterial T4SS or deficiency of essential components of NF-κB significantly reduced H. pylori-linked DSBs in host cells.
Upon activation of NF-κB, the heterodimeric protein is separated from its cytoplasmic inhibitor and transported into the nucleus. NF-κB activation induces transcription of pro-inflammatory and pro-survival genes and, via a still only superficially understood mechanism, activates the nucleotide repair factors XPG, XPF and XPA. The consequence of this interaction is the activation of a mechanism described as "transcription-associated DSBs", eventually leading to the observed formation of DSBs in host cells.
We assume that induction of DNA breaks by the NER factors XPG, XPF and XPA relieves tension of the DNA and enables fast and efficient transcription of target genes. Indeed, deficiency of any one of the NER-factors not only resulted in reduction of DSBs, it also reduced transcription of NF-κB target genes. Among the differentially expressed genes in infected cells lacking XPG, XPF or XPA is an inhibitor of programmed cell death, c-IAP1. In line with these results, NF-κB and XPA were found to be required for the suppression of cell death in infected cells. In addition, only wt bacteria, and not ΔCagL mutants, are able to suppress apoptosis of host cells if an external apoptotic stimulus is applied.
Also, in an in vivo infection model, ΔCagL was found to be a critical factor. Mice infected with the ΔCagL mutant exhibited higher inflammation and a much more severe pathology as mice infected with wt bacteria, despite being colonized at a similar level. This effect is assumed to be due to the loss of anti-apoptotic NF-κB signaling in ΔCagL infected mice, leading to massive cell death of gastric epithelial cells, followed by necrosis and influx of immune cells.
In conclusion, H. pylori elicits NF-κB signaling via the T4SS followed by NER factor dependent DSB induction in host cells, and thus facilitates expression of NF-κB target genes, including inhibitors of apoptosis. By evoking this event cascade, we believe that H. pylori suppresses cell death in infected cells and manipulates its niche in the human host to its own advantage.