Gastric Cancer

The fifth most common cancer worldwide and third leading cause of cancer death, with over 1 million new cases annually. Gastric cancer stands unique in this wiki as the disease where a single microorganism — helicobacter pylori — provides the dominant causal pathway, and where that pathogen's virulence depends critically on nickel-dependent metalloenzymes. The chain from nickel → hydrogenase → CagA translocation → gastric carcinogenesis is one of the most direct metal-to-cancer pathways in human disease.

The H. pylori-Nickel-Cancer Chain

Step 1: Nickel Enables Colonization

H. pylori cannot survive gastric acid without two nickel-dependent enzymes:

  • Urease — a Ni-containing enzyme that hydrolyzes urea to ammonia + CO2, locally neutralizing gastric acid. Contains 24 nickel ions per holoenzyme. Without nickel, urease is inactive and H. pylori cannot colonize.
  • [Ni-Fe] Hydrogenase — oxidizes molecular H2 (produced by other gut bacteria) to generate energy for H. pylori survival in the microaerobic gastric niche. The hydrogenase is essential for full colonization density.

Step 2: Nickel Powers Virulence

The CagA oncoprotein translocation depends on the energy derived from nickel metalloenzymes:

  • H. pylori injects CagA into gastric epithelial cells via the type IV secretion system (T4SS)
  • T4SS assembly and function require ATP generated in part by hydrogenase-dependent metabolism
  • CagA is phosphorylated by host kinases, then hijacks SHP-2, Grb2, and other signaling molecules
  • CagA disrupts cell polarity, tight junctions, and proliferation control — the "oncoprotein"
  • Higher nickel availability → more active urease/hydrogenase → denser colonization → more CagA delivery → higher cancer risk

Step 3: The Carcinogenic Cascade

H. pylori infection progresses through the Correa cascade:

  1. Normal mucosa → chronic active gastritis
  2. Chronic gastritis → atrophic gastritis (loss of acid-secreting parietal cells)
  3. Atrophic gastritis → intestinal metaplasia
  4. Intestinal metaplasia → dysplasia
  5. Dysplasia → adenocarcinoma

Each step is driven by chronic inflammation (nf kappa b, oxidative stress, DNA damage), immune responses (Th1/Th17 — see immune balance), and epithelial damage. The process typically spans decades.

Beyond H. pylori: Other Metal Contributions

Cadmium

  • IARC Group 1 carcinogen with stomach as a target organ
  • Cd accumulates in gastric mucosa, generating oxidative stress and inhibiting DNA repair
  • Cd exposure correlates with gastric cancer incidence in occupational and environmental studies
  • Cd may synergize with H. pylori: metal-induced inflammation + bacterial virulence = compounding carcinogenesis. See metal carcinogenesis.

Lead

  • Pb exposure associated with gastric cancer risk in occupational cohorts
  • Pb inhibits DNA repair enzymes (PARP, OGG1) and promotes epigenetic silencing of tumor suppressors
  • May compound H. pylori-driven DNA damage

Iron

  • Iron deficiency from chronic H. pylori gastritis may paradoxically promote cancer by inducing compensatory proliferation
  • Conversely, excess luminal iron in atrophic gastritis (achlorhydria reduces iron absorption, but bleeding adds luminal Fe) feeds pathobionts
  • H. pylori actively sequesters host iron for its own use

Nickel (Beyond H. pylori)

  • Dietary nickel itself may contribute: high-nickel diets provide substrate for H. pylori's metalloenzymes
  • Nickel compounds are IARC Group 1 carcinogens (occupational inhalation → nasal/lung cancer; gastric route less studied)
  • Dietary nickel exposure in H. pylori-infected individuals could accelerate the carcinogenic cascade

The Gastric Microbiome Beyond H. pylori

H. pylori dominates the gastric microbiome in infected individuals but is not alone:

  • Atrophic gastritis → loss of acid barrier → colonization by oral and intestinal bacteria (Streptococcus, Prevotella, Neisseria, Rothia)
  • This "opened niche" microbiome may contribute to carcinogenesis through nitrosamine production, bile acid modification, and additional inflammation
  • Lactobacillus species may be protective: competition with H. pylori, acid production, immunomodulation
  • Post-gastrectomy microbiome shifts associate with nutritional deficiencies and altered metal absorption

Dietary Risk Factors

  • Salt — high salt intake damages gastric mucosa, enhances CagA expression, and synergizes with H. pylori
  • Nitrates/nitrites — converted to N-nitroso compounds by bacterial nitrate reductases; potent mutagens
  • Smoked/processed foods — polycyclic aromatic hydrocarbons + nitrosamines + metals (Cd in smoked foods)
  • Low fruit/vegetable intake — reduced antioxidants (vitamin C, Se) to counter oxidative stress
  • Nickel-rich foods — hypothetically, high dietary nickel fuels H. pylori metalloenzymes in infected individuals

Prevention and Therapeutic Angles

  • H. pylori eradication — the single most effective gastric cancer prevention strategy; triple/quadruple antibiotic therapy. But eradication alters the gastric and intestinal microbiome (a pharmacomicrobiomics concern).
  • Nickel restriction — untested but theoretically could reduce H. pylori virulence by starving metalloenzymes
  • Selenium supplementation — Se deficiency associates with gastric cancer risk; Se supports antioxidant defense via glutathione peroxidase
  • Probiotics — Lactobacillus supplementation during H. pylori eradication reduces antibiotic side effects and may improve eradication rates
  • Cadmium avoidance — smoking cessation (tobacco is a major Cd source), reducing dietary Cd

Connections

  • helicobacter pylori — the causative organism; nickel-dependent urease and hydrogenase power colonization and CagA delivery
  • nickel — essential cofactor for H. pylori urease and [Ni-Fe] hydrogenase; dietary nickel fuels virulence
  • hydrogenase — nickel-dependent energy enzyme enabling full colonization density
  • cadmium — IARC Group 1 carcinogen targeting gastric mucosa; synergizes with H. pylori inflammation
  • iron — H. pylori sequesters host iron; iron deficiency from chronic gastritis paradoxically promotes proliferation
  • lead — associated with gastric cancer risk in occupational cohorts; inhibits DNA repair
  • selenium — deficiency associates with gastric cancer risk; antioxidant defense via glutathione peroxidase
  • inflammation — chronic NF-kB-driven inflammation powers the Correa cascade from gastritis to adenocarcinoma
  • oxidative stress — metal-induced and infection-driven ROS as central mutagenic mechanism
  • DNA damage — the molecular basis of carcinogenic transformation from H. pylori and metal exposure
  • metal carcinogenesis — gastric cancer exemplifies the metal-infection-cancer triad
  • dietary nickel exposure — dietary nickel as substrate for H. pylori metalloenzymes in infected individuals
  • probiotics — Lactobacillus supplementation during H. pylori eradication improves outcomes
  • pharmacomicrobiomics — H. pylori eradication therapy reshapes the gastric and intestinal microbiome
  • colorectal cancer — shared metal-carcinogenesis pathways and microbiome-driven inflammation

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