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