Background & Aims Infection with the gastric mucosal pathogen H. pylori is the strongest identified risk factor for distal gastric cancer. These bacteria colonize a significant part of the world’s population. We investigated the molecular mechanisms of p53 regulation in H. pylori-infected cells. Methods Mongolian gerbils were challenged with H. pylori and their gastric tissues were analyzed by immunohistochemistry and immunoblotting with p53 antibodies. Gastric epithelial cells were co-cultured with H. pylori and the regulation of p53 was assessed by real-time PCR, immunoblotting, immunofluorescence, and cell survival assays. shRNA and dominant-negative mutants were used to inhibit activities of HDM2 and AKT. Results We found that in addition to previously reported up-regulation of p53, H. pylori can also negatively regulate p53 by increasing ubiquitination and proteasomal degradation via activation of the serine/threonine kinase AKT, which phosphorylates and activates the ubiquitin ligase HDM2. These effects were mediated by the bacterial virulence factor, CagA; ectopic expression of CagA in gastric epithelial cells increased phosphorylation of HDM2 along with the ubiquitination and proteasomal degradation of p53. The decrease in p53 levels increased survival of gastric epithelial cells that had sustained DNA damage. Conclusion H. pylori is able to inhibit the tumor suppressor p53. H. pylori activates AKT, resulting in phosphorylation and activation of HDM2 and subsequent degradation of p53 in gastric epithelial cells. H. pylori-induced dysregulation of p53 is a potential mechanism by which the microorganism increases the risk of gastric cancer in infected individuals.

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TP53, TP63, and TP73 genes comprise the p53 family. Each gene produces protein isoforms through multiple mechanisms including extensive alternative mRNA splicing. Accumulating evidence shows that these isoforms play a critical role in the regulation of many biological processes in normal cells. Their abnormal expression contributes to tumorigenesis and has a profound effect on tumor response to curative therapy. This paper is an overview of isoform diversity in the p53 family and its role in cancer.

The p53 protein plays a central role in the prevention of tumorigenesis. Cellular stresses, such as DNA damage and aberrant oncogene activation, trigger induction of p53 that halts cellular proliferation and allows cells to be repaired. If cellular damage is beyond the capability of the repair mechanisms, p53 induces apoptosis or cell cycle arrest, preventing damaged cells from becoming cancerous. However, emerging evidence suggests that the function of p53 needs to be considered as isoform-specific. Here, we report that the expression profile of p53 can be shifted toward inhibitory p53 isoforms by the pathogenic bacterium Helicobacter pylori, which is known for its strong association with gastric cancer and gastric mucosa-associated lymphoid tissue lymphoma. We found that interaction of H. pylori with gastric epithelial cells, mediated via the cag pathogenicity island, induces N-terminally truncated ∆133p53 and ∆160p53 isoforms in human cells. Induction of an orthologous p53 isoform, ∆153p53, was also found in H. pylori-infected Mongolian gerbils. The p53 isoforms inhibit p53 and p73 activities, induce NF-κB, and increase survival of infected cells. Expression of ∆133p53, in response to H. pylori infection, is regulated by phosphorylation of c-Jun and activation of activator protein-1–dependent transcription. Together, these results provide unique insights into the regulation of p53 protein and may contribute to the understanding of tumorigenesis associated with H. pylori.

Objective Infection with Helicobacter pylori is the strongest known risk factor for adenocarcinoma of the stomach. Tumorigenic transformation of gastric epithelium induced by H. pylori is a highly complex process driven by an active interplay between bacterial virulence and host factors, many aspects of which remain obscure. In this work, we investigated the degradation of p53 tumour suppressor induced by H. pylori. Design Expression of p53 protein in gastric biopsies was assessed by immunohistochemistry. Gastric cells were co-cultured with H. pylori strains isolated from high-gastric risk and low-gastric risk areas and assessed for expression of p53, p14ARF and cytotoxin-associated gene A (CagA) by immunoblotting. siRNA was used to inhibit activities of ARF-BP1 and Human Double Minute 2 (HDM2) proteins. Results Our analysis demonstrated that H. pylori strains expressing high levels of CagA virulence factor and associated with a higher gastric cancer risk more strongly suppress p53 compared with low-risk strains in vivo and in vitro. We found that degradation of p53 induced by bacterial CagA protein is mediated by host HDM2 and ARF-BP1 E3 ubiquitin ligases, while the p14ARF protein counteracts H. pylori-induced signalling. Conclusions Our results provide novel evidence that tumorigenicity associated with H. pylori infection is linked to inhibition of p53 protein by CagA. We propose a model in which CagA-induced degradation of p53 protein is determined by a relative level of p14ARF. In cells in which p14ARF levels were decreased due to hypermethylation or deletion of the p14ARF gene, H. pylori efficiently degraded p53, whereas p53 is protected in cells expressing high levels of p14ARF.