Helicobacter pylori CagA: a new paradigm for bacterial carcinogenesis

Abstract

Infection with CagA-positive Helicobacter pylori is associated with the development of gastric adenocarcinoma. The CagA gene product CagA is injected directly from the bacterium into the bacterium-attached gastric epithelial cells via the type-IV secretion system. Upon membrane localization and subsequent tyrosine phosphorylation by Src family kinases, CagA functions as a scaffolding adaptor and interacts with a number of host proteins that regulate cell growth, cell motility and cell polarity in both CagA phosphorylation-dependent and phosphorylation-independent manners. Of special interest is the interaction of CagA with the SHP-2 tyrosine phosphatase, gain-of-function mutations that of which have recently been found in a variety of human malignancies. The CagA-SHP-2 interaction is entirely dependent on CagA tyrosine phosphorylation and, through the complex formation, SHP-2 is catalytically activated and induces morphological transformation with elevated cell motility. Intriguingly, structural diversity of the tyrosine phosphorylation sites of CagA accounts for the differential activity of individual CagA to bind and activate SHP-2. Deregulation of SHP-2 and other intracellular signaling molecules by H. pylori CagA may predispose cells to accumulate multiple genetic and epigenetic changes involved in gastric carcinogenesis. Furthermore, the differential potential of individual CagA to disturb cellular functions indicates that H. pylori strains carrying biologically more active CagA are more virulent than those with less active CagA and are more closely associated with gastric carcinoma.

Figure 1

Physical and functional interaction between H. pylori CagA and SHP‐2. CagA is translocated from H. pylori into H. pylori‐attached gastric epithelial cells via the bacterial type IV secretion system. The translocated CagA protein localizes to the inner surface of the plasma membrane and undergoes tyrosine phosphorylation by Src family kinases. Upon tyrosine phosphorylation, CagA binds specifically to the SH2 domain‐containing tyrosine phosphatase SHP‐2 and activates the phosphatase activity. The CagA‐activated SHP‐2 potentiates Erk MAP kinase activity, which regulates both cell growth and cell morphology. SHP‐2 also dephosphorylates a cellular substrate (shown as X in the figure), which is involved in induction of the hummingbird phenotype that is associated with elevated cell motility.

Figure 2

Molecular anatomy of the EPIYA‐containing region of CagA. (a) Subdivision of the EPIYA‐containing region. Prevalent CagA proteins of Western H. pylori isolates contain three EPIYA motifs, which are designated EPIYA‐A, EPIYA‐B and EPIYA‐C sites, based on the sequence surrounding each of the EPIYA motifs. Accordingly, the EPIYA‐containing region of CagA can be subdivided into the EPIYA‐A segment, the EPIYA‐B segment and the EPIYA C segment. The border of each segment was determined by comparing sequences of the EPIYA‐containing regions, which were made by extensive genetic recombination among various Western CagA isolates.^{( 34 )} (b) Comparison of the EPIYA‐containing region between Western and East Asian CagA species. Both CagA species have the conserved EPIYA‐A and EPIYA‐B segments. Following the EPIYA‐A and EPIYA‐B segments, Western CagA possesses the EPIYA‐C segment whereas East Asian CagA possesses the EPIYA‐D segment, whose sequence is unique to East Asian CagA.

Figure 3

(a) The high‐affinity binding sequence for the SH2 domains of SHP‐2. The consensus SHP‐2‐binding sequence is aligned with the EPIYA‐C site of Western CagA and the EPIYA‐D site of East Asian CagA. (b) The influence of EPIYA‐repeat polymorphism on the pathophysiological activities of CagA. EPIYA‐C and EPIYA‐D sites are major tyrosine phosphorylation sites of Western CagA and East Asian CagA, respectively. EPIYA‐D site of East Asian CagA binds SHP‐2 more strongly than does EPIYA‐C site of Western CagA. As a result, East Asian CagA exhibits greater activity to induce hummingbird cells than Western CagA. Among Western CagA species, those having larger numbers of EPIYA‐C exhibit stronger SHP‐2 binding activity and greater activity to induce the hummingbird cells than those having less numbers of EPIYA‐C do.

Figure 4

A proposed model for functional interaction between CagA and VacA. (a) H. pylori‐injected CagA deregulates SHP‐2 and other cellular target molecules that promote cell proliferation. Simultaneously, CagA activates NFAT and thereby induces NFAT‐dependent genes such as p21 ^Cip.1 . Elevated p21^Cip1 then arrests gastric epithelial cells in G₁ phase. Such G₁‐arrested cells subsequently undergo senescence, apoptosis or intestinal trans‐differentiation known as intestinal metaplasia. (b) When gastric epithelial cells simultaneously encounter VacA and CagA, VacA counteracts nuclear translocation of NFAT by CagA and thus abolishes induction of p21^Cip1 in CagA‐expressing cells. Accordingly, in the presence of adequate levels of VacA, CagA stimulates deregulated cell growth. (c) VacA inhibits production of T‐cell growth factor, interleukin‐2 (IL‐2), by suppressing NFAT activity.

Figure 5

Involvement of SHP‐2 in human malignancies. Point mutations in PTPN11, a human gene that encodes SHP‐2, have been associated with childhood leukemias and some solid tumors. The mutations abolish the inhibitory interaction between the N‐SH2 domain and the phosphatase domain of SHP‐2 and therefore produce gain‐of‐function mutations. Upon complex formation with CagA, SHP‐2 is fixed to its active state, mimicking the gain‐of‐function mutation of SHP‐2.

Figure 6

Classification of H. pylori based on the EPIYA‐repeat polymorphism of CagA. H. pylori is divided into CagA‐positive and CagA‐negative strains. CagA‐negative H. pylori is non‐pathogenic or only weakly pathogenic and is not involved in gastric carcinogenesis. CagA‐positive H. pylori is subdivided into those carrying East Asian CagA and those carrying Western CagA. Virulence of H. pylori carrying Western CagA is determined at least partly by the number of EPIYA‐C. Those having CagA with greater number of EPIYA‐C are more virulent and more closely associated with severe atrophic gastritis and gastric carcinoma than those having CagA with less EPIYA‐C. H. pylori strains carrying East Asian CagA are biologically more active than most if not all Western‐type H. pylori and individuals infected with this type of H. pylori are at the highest risk of developing gastric carcinoma.