Metallothioneins are liver-produced, metal-binding proteins rich in cysteine residues. The sulfhydryl groups provided by such residues enable chelation of several metal ions, including lead, cadmium, and zinc, creating metal-thiolate clusters. The primary metallothioneine isoforms are MT1, MT2, MT3, and MT4, each of which displays a unique biological function and tissue-specific pattern of expression. MT1 and MT2 are the key isoforms devoted to heavy metal detoxification, as well as mediating the oxidative stress response. They are expressed by nearly all cells and soft tissues, but especially by organ systems involved in detoxification processes, such as the liver, intestines, kidneys, and pancreas. MT3, in contrast, is predominantly expressed by brain neurons and glial cells and functions to promote neuronal survival and axon formation. It has been shown to provoke HIF-1a nuclear accumulation, resulting in augmented expression of vascular endothelial growth factor in the brain. Conversely, MT3 has become widely known as Growth Inhibitory Factor due to its behavior as an inhibitor of abnormal cortical neuron growth, thereby preventing neurite formation. MT3 also binds free copper and zinc ions to mitigate oxidative stress that otherwise prompts protein aggregation and generation of amyloid-beta plaques. Finally, MT4 is highly localized to stratified squamous epithelial cells where it plays a role in cell differentiation and keratinization. Keratinization is a zinc-dependent process with MT4 behaving as a zinc reservoir.
In addition to the aforementioned functions of oxidative stress abatement and heavy metal sequestration, metallothioneins have been found to influence general cell metabolism, inflammatory pathways, as well as apoptosis. Research supports a link between diminished expression of metallothioneins and the progression of various chronic diseases associated with oxidative stress, including diabetes, age-related disease, and cancer. Oxidative stress elicits lipid peroxidation, generating malondialdehyde and other toxic aldehydes. Lipid-rich membranes are subsequently heavily impacted, with both lipid peroxidation and aldehyde production compromising their structural integrity. Metallothioneins protect against oxidative stress by upregulating antioxidant defense. They can themselves directly scavenge free radicals or interact with other antioxidants like glutathione peroxidase to enhance antioxidant expression or assist in glutathione recycling. A rise in reactive oxygen nitrogen species within a cell instigates metallothioneine expression. The release of zinc by metallothioneins stimulates nuclear factor E2-related factor 2 (Nrf2) and metal-responsive transcription factor 1 (MTF-1) translocation to the nucleus. These factors bind antioxidant response elements (ARE) and metal-response elements (MRE), respectively, driving antioxidant gene transcription.
Moreover, oxidative stress provokes release of heavy metal ions from their binding proteins causing Fenton-like reactions that further amplify free radical production. Metallothioneins spring to action in these circumstances, sequestering free metal ions into metal-thiolate complexes. Metallothioneins exhibit the strongest affinity for copper, followed by cadmium and zinc. Important to note is that metallothioneins fail to adequately clear thallium due to its poor affinity for thiol groups, explaining its elevated toxicity compared to mercury, lead, cadmium, and copper. Metallothionein induction displays a positive feedback loop whereby excess oxidative stress prompts cysteine thiol group oxidation, triggering release of bound metal ions which additionally stimulate metallothionein expression. However, metal-free metallothionein is more easily degraded, resulting in eventual decline in metallothionein levels during prolonged oxidative stress.
Beyond their role in oxidative stress, metallothioneins mitigate inflammation owing to their ability to complex with zinc. Zinc-metallothionein complexes suppress the damaging effects of lipopolysaccharide on endothelial cells, as well as neutrophil release of inflammatory cytokines, consequently avoiding activation of nuclear factor kappa B (NF-κB) as well as ERK, p38, and JNK branches of MAPK signaling. The supply of zinc by MT3 also serves to moderate the TRIF-IRF3-STAT1 signaling axis and related caspase-11 activation. Furthermore, the repression of oxidative stress and inflammation enables metallothioneins to modulate apoptosis. Through the mitigation of cell damage and the curtailment of both pro-apoptotic factors and caspase activation, metallothioneins can protect against apoptosis.
Zinc is the most potent metal to promote metallothoinine transcription, and metallothioneine expression is in turn directly regulated by intracellular zinc. Displacement of zinc from the zinc-metallothioneine complex by other heavy metals, nitric oxide, or hydrogen peroxide serves to increase intracellular zinc, driving metallothioneine transcription. Heavy metals aren’t the only agents able to catalyze metallothioneine expression, however. Glucocorticoids, epigenetic regulation, as well as JAK2/STAT pathway activation by reactive oxygen and nitrogen species also spur metallothioneine transcription. Moreover, the cytokine interleukin-22, vitamin D, and vitamin C have all been found to enhance metallothioneine expression. Finally, the traditional Chinese herb Eucommia ulmoides Oliv. actuates Nrf2 nuclear translocation and subsequent metallothioneine transcription.
Given the role of metallothioneines in mitigating oxidative stress and regulating key pathways implicated in disease processes, there is considerable interest in effective modulation of metallothioneine transcription. Efforts are ongoing to develop novel compounds that act to augment metallothioneine levels, including nanomaterials and molecular interaction partners. More research is needed to develop a comprehensive understanding of the full extent of metallothioneine function. Most studies to date have relied upon animal models and cellular studies. Rigorous clinical trials are thus required to explore and assess metallothioneine-targeted interventions in humans. Metallothioneines remain a compelling therapeutic focus in chronic oxidative stress-mediated illness.
Li Y, Shao Y, Shi X. Metallothionein’s Role in Oxidative Stress: A Pathway to Innovative Treatments. Pharmacol Res. Published online June 5, 2026. doi:10.1016/j.phrs.2026.108282