Bacterial CagA protein compromises tumor suppressor mechanisms in gastric epithelial cells
Manikandan Palrasu, Elena Zaika, Wael El-Rifai, Monica Garcia-Buitrago, Maria Blanca Piazuelo, Keith T. Wilson, Richard M. Peek Jr., Alexander I. Zaika
Manikandan Palrasu, Elena Zaika, Wael El-Rifai, Monica Garcia-Buitrago, Maria Blanca Piazuelo, Keith T. Wilson, Richard M. Peek Jr., Alexander I. Zaika
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Research Article Gastroenterology

Bacterial CagA protein compromises tumor suppressor mechanisms in gastric epithelial cells

Abstract

Approximately half of the world’s population is infected with the stomach pathogen Helicobacter pylori. Infection with H. pylori is the main risk factor for distal gastric cancer. Bacterial virulence factors, such as the oncoprotein CagA, augment cancer risk. Yet despite high infection rates, only a fraction of H. pylori–infected individuals develop gastric cancer. This raises the question of defining the specific host and bacterial factors responsible for gastric tumorigenesis. To investigate the tumorigenic determinants, we analyzed gastric tissues from human subjects and animals infected with H. pylori bacteria harboring different CagA status. For laboratory studies, well-defined H. pylori strain B128 and its cancerogenic derivative strain 7.13, as well as various bacterial isogenic mutants were employed. We found that H. pylori compromises key tumor suppressor mechanisms: the host stress and apoptotic responses. Our studies showed that CagA induces phosphorylation of XIAP E3 ubiquitin ligase, which enhances ubiquitination and proteasomal degradation of the host proapoptotic factor Siva1. This process is mediated by the PI3K/Akt pathway. Inhibition of Siva1 by H. pylori increases survival of human cells with damaged DNA. It occurs in a strain-specific manner and is associated with the ability to induce gastric tumor.

Authors

Manikandan Palrasu, Elena Zaika, Wael El-Rifai, Monica Garcia-Buitrago, Maria Blanca Piazuelo, Keith T. Wilson, Richard M. Peek Jr., Alexander I. Zaika

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Figure 7

CagA protein induces phosphorylation of XIAP protein and degradation of Siva1.

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CagA protein induces phosphorylation of XIAP protein and degradation of ...
(A) Western blot analysis of Siva1 and pXIAP(S87) proteins in AGS cells cotransfected with Siva1, CagA, and GFP expression plasmids at the indicated ratios for 24 hours. GFP was used to normalize transfection efficiency. pXIAP(S87) protein levels were normalized to XIAP protein expression. (B) Western blot analysis of Siva1 and pXIAP(S87) proteins in AGS cells cotransfected with CagA expression vector (CagA-pSP65SRα) or empty vector (pSP65SRα) and Akt siRNA or scrambled siRNA as shown at the top of the panel. β-actin and GFP were used to normalize protein loading and transfection efficiency, respectively. (C) Western blot analysis of Siva1 and pXIAP(S87) proteins in AGS cells cocultured with WT H. pylori strain J166 or its cagA– or cagE– isogenic mutants. The graph panels show quantification of Siva1 and pXIAP(S87) proteins by densitometry, normalized to actin and XIAP, respectively. Expression of Siva1 and pXIAP(S87) proteins in control cells was arbitrarily set at 1. Data were analyzed using 2-tailed Student’s t test. Data are displayed as mean ± SE and are representative of 3 independent experiments. *P < 0.05; **P < 0.01; ***P < 0.001.