A transition metal whose hexavalent form, Cr(VI), is a potent human carcinogen. Unlike nickel (epigenetic) and arsenic (proliferative/epigenetic), chromium's primary carcinogenic mechanism is direct DNA damage through Cr-DNA adducts, crosslinks, and strand breaks generated during intracellular Cr(VI) reduction.
Chemical Forms
- Cr(VI): carcinogenic. Exists as chromate (CrO₄²⁻, 75%) and hydrochromate (HCrO₄⁻, 25%) at neutral pH.
- Cr(III): the end product of intracellular reduction. Previously considered an essential nutrient (glucose tolerance factor); this is now debated.
- Cr(VI) enters cells via sulfate channels (molecular mimicry) — rapid, massive accumulation (10-20x in 3h, 100-fold+ in 24h).
Human Exposure
- Occupational: chromate production, stainless steel welding, chrome plating, ferrochrome manufacturing.
- Environmental: contaminated drinking water. ~30% of US supplies at the time of the review had levels of concern.
- Primarily causes lung cancer (squamous cell carcinoma) in chromate workers.
- Epidemiological risks found to be substantially higher than previously thought, triggering regulatory revisions.
Carcinogenic Mechanisms [[[salnikov-2008-metal-carcinogenesis]]]
Cr(VI) Metabolism
- Cr(VI) is a pro-carcinogen — itself unreactive with DNA.
- Intracellular reduction: Cr(VI) → Cr(V) → Cr(IV) → Cr(III).
- Ascorbate is the dominant biological reductant (~90% of Cr(VI) reduction in vivo).
- Glutathione and cysteine are secondary reductants.
- The reduction process generates reactive intermediates (Cr(V), Cr(IV)) and the final Cr(III) product, all of which can damage DNA.
DNA Damage
- Cr-DNA adducts: the signature lesion. Most are ternary adducts — Cr(III) crosslinking DNA with another molecule:
- Cr-ascorbate-DNA (~50-75% of all adducts when ascorbate is the reductant)
- Cr-glutathione-DNA
- Cr-cysteine-DNA
- Only a fraction are binary (Cr directly on DNA).
- DNA-protein crosslinks: formed through attack of DNA by Cr(III) bound to proteins.
- DNA interstrand crosslinks: relatively rare but potent.
- DNA strand breaks: SSBs produced, but some reports may be assay artifacts. DSBs also occur.
- Mutagenic spectrum: primarily G→T transversions in some systems; base substitutions at sites of Cr-DNA adducts.
Genomic Instability
- Microsatellite instability (MSI): observed in lung cancers of chromate workers.
- Mismatch repair (MMR) inhibition: Cr(VI) suppresses MMR expression (hMLH1 in particular) — allowing replication errors to persist.
- This creates a selection model: low-dose Cr(VI) generates mutations while simultaneously disabling the repair system that would normally catch them.
The Ascorbate Paradox
- Ascorbate drives the reduction of Cr(VI) that creates DNA-damaging intermediates.
- But ascorbate is also needed for DNA repair and for maintaining reducing conditions in the cell.
- Net effect: ascorbate levels can paradoxically increase both Cr(VI) toxicity (more reduction, more adducts) and carcinogenic potential.
- Cellular ascorbate is typically at millimolar levels (1.3 mM in human lung), far exceeding Cr(VI) concentrations.
Cocarcinogenesis
- Cr(VI) in drinking water increases susceptibility to UV-induced skin tumors in mice.
- Dietary chromium + nickel together enhance UV carcinogenesis.
- Mechanism: Cr-generated DNA damage compounds UV damage; compromised repair allows both to persist.
Comparison with Other Metals
| Feature | Chromium | nickel | arsenic |
|---------|----------|--------|---------|
| DNA adducts | Yes (ternary Cr-DNA) | No | No |
| Strand breaks | Yes | No | No |
| Epigenetic effects | Moderate | Primary | Primary |
| Key reductant | Ascorbate | N/A | GSH/SAM |
| Repair pathway inhibited | MMR | NER | NER, BER |
| Intracellular accumulation | Massive (100x) | Moderate | Via methylation |
Open Questions
1. Role of ascorbate supplementation: would it be protective or harmful for chromate-exposed workers?
2. Drinking water standards: given revised risk estimates, are current limits adequate?
3. Is Cr(III) truly essential? The glucose tolerance factor hypothesis is weakening.
4. Interaction effects: Cr + Ni combined exposures in occupational settings.
Connections
- nickel, arsenic — co-reviewed metals
- DNA damage — primary mechanism
- oxidative stress — secondary mechanism
- metal carcinogenesis — overarching concept