Ferroptosis Biology And Role In Liver Disease: Implications

Ferroptosis is a novel form of iron-dependent regulated cell death, characterized by the production of toxic lipid peroxides. Increasing evidence suggests ferroptosis in liver disease, from disease initiation and progression to treatment. The liver is uniquely qualified to experience ferroptosis because it is responsible for iron storage and detoxification, and also plays a key role in lipid metabolism. Induction of ferroptosis in the liver causes a surge in inflammation, alters metabolism, and can either stop tissue damage or lead to the development of fibrosis, cirrhosis or liver cancer. Importantly, the same processes that are lethal to normal hepatocytes can also be exploited to selectively kill therapy-resistant liver tumors. Ferroptosis is therefore a promising therapeutic target in the treatment of a variety of liver diseases.

Why It Matters

1) The liver is uniquely vulnerable

The liver is particularly prone to ferroptosis because its normal function entails the management of both iron and lipids on a daily basis. Iron is an essential element that cells require for a variety of functions. However, free or reactive iron can catalyze a wide array of oxidative chemical reactions that the cell may not be able to contain. In addition to the more usual stable membrane lipids, liver cells also contain large amounts of fatty substrates. These fatty substrates are particularly vulnerable to oxidative damage, which is a major characteristic of ferroptosis. Thus, liver cells are at risk of lipid-specific death by ferroptosis when they experience oxidative stress from liver injury, from increased metabolic demands or from toxic substances.

Injury whether localised or not may become become inflammatory as a result of iron overload and lipid peroxidation. Ferroptosis, previously perceived as a silent event, can in fact cause tissue irritation through the products of lipid peroxidation either inducing an immune response or amplifying inflammation. Similar mechanisms which lead to ferroptosis in other tissues can exacerbate inflammation and tissue injury in the liver leading to ongoing swelling, immune and inflammation-mediated injury and misrepair.

2) The core biology in clear terms

Ferroptosis includes two key elements: iron and lipid per oxidation. Free iron will catalyze non-enzymatic reactions to oxidize normal membrane lipid to toxic oxidized lipids. Oxidized lipid can then propagate across the membrane leading to loss of membrane integrity and cell death. Ferroptosis can be distinguished from other forms of programmed cell death, such as apoptosis, in which ferroptosis is specifically defined by an iron chemistry, including Fenton reactions and lipid per oxidation.

Antioxidant defenses prevent ferroptosis and allow cells to remove lipid peroxide damage one lipid at a time through detoxification by the enzyme GPX4 using reduced glutathione. Decreasing reduced glutathione or inhibiting GPX4 can increase susceptibility to ferroptosis. Xc−–dependent protection therefore starts with import of cystine to support glutathione production and lipid peroxide detoxification by GPX4. As a consequence, decreased levels of cystine can lead to decreased glutathione and consequently decreased lipid protection.

In addition to these mechanisms, it is worth considering other cellular pathways that counteract lipid oxidation or membrane reconstitution stress. However, once multiple stresses (inflammation, fat accumulation, iron overload) converge, ferroptosis emerges as a main form of cell death when GPX4 is maximally challenged.

3) Why ferroptosis changes disease progression

Ferroptosis can exacerbate liver disease by promoting the progression from tissue injury to inflammation and then to fibrosis. Repeated ferroptosis in the liver leads to chronic loss of hepatocytes and subsequent tissue damage-induced inflammatory and wound-healing programs. Chronic wound healing programs then result in fibrosis, or the deposition of excess scar tissue.

Cells within the liver, whether damaged or not, react differently to injury and it is the sum of these responses that determines the final outcome for the organ as a whole. The metabolically active hepatocytes need to maintain homeostasis whereas the immune cells (whether resident or visiting) need to decide whether an inflammatory response needs to be kept under control or allowed to switch to a pathologically exaggerated form. The quiescent stellate cells are responsible for fibrosis and deposition of scar tissue. In addition, we consider here the possibility that ferroptosis of hepatocytes could provide a powerful stimulus for inflammation and how products of lipid oxidation by hepatocytes can influence other cell types in the liver. A newfound dialogue between stem cells and nerve cells may help explain how one signaling pathway can contribute to both benign development and life threatening diseases.

4) Why this matters for treatment strategy

This new antineoplastic- and tissue-protective mechanism, two-way depending on the disease under study, will certainly require a precise patient selection, because ferroptosis inhibitors could prevent further tissue damage in acute and chronic noncancer liver diseases, while inducing ferroptosis in cancer cells may become a new approach to counteract therapy resistant cancer, by selectively killing chemotherapy resistant cancer cells that do not die undergoing apoptosis.

Antioxidants typically inactivate reactive oxygen species, but targeting this normal process specifically to the liver is risky because iron and oxidative stress are not just liver issues. The body requires a balance of iron homeostasis and oxidative state to maintain the function of the heart, brain, nervous system and immune system, and an attack on ferroptosis could have far-reaching effects. Indeed, use of a broad spectrum antioxidant in combination with suppression of normal immune antioxidant defenses could have severe consequences. Clearly, such a therapy must be weighed against any benefits it might offer for liver health, and it is crucial that any therapeutic strategy be directed specifically to liver tissue.

5) Why health systems should care

The diseases affecting the liver today place a heavy health care burden on the health care system in terms of hospitalization, long-term management for sequelae, and diagnosis of cancer and transplantation. However, the potential for future advances in the management of liver diseases using ferroptosis for diagnostic and therapeutic purposes could identify at risk patients early and improve management strategies for these patients leading to benefits for patients despite the increased cost to diagnosis and management using biomarkers and monitoring by specialty groups. Additionally, the use of combination therapies for ferroptosis-targeted cancer will increase expenditure.

Who It Affects

1) People with acute liver injury

Inducing acute liver injury in patients with toxins, drugs, acute ischemia or severe infections may benefit from therapies that target ferroptosis to prevent early ferroptotic loss of hepatocytes. A short-term intervention can be effective because it is timely, and suffices to prevent iron-dependent lipid peroxidation, to block depletion of antioxidant defenses, and to prevent acute liver failure.

Acetaminophen toxicity is mediated in part by the production of oxidative stress and depletion of glutathione. Ferroptosis provides a framework to consider this mechanism of toxicity since glutathione is a critical molecule for the degradation of lipid peroxides produced in this process. In cases of established toxicity, treatment is centered on established antidotes and supportive care; there is some interest in the use of ferroptosis inhibitors as adjunctive therapy to prevent additional liver injury.

2) People with metabolic-associated fatty liver disease

Metabolic-associated fatty liver disease, especially in its inflammatory or necroinflammatory form, affects a large proportion of patients with fatty liver disease. Excessive accumulation of fat in the liver predisposes to lipid peroxidation. The changed lipid composition of membranes supports the formation of more oxidizable lipids. This, in conjunction with disturbed iron metabolism, further increases ferroptosis pressure.

Ferroptosis-targeted therapies are most likely to benefit large patient populations, particularly diabetic/obese patients as well as patients with cardiovascular disease. Given the multiple medications that many of these patients are currently taking for a variety of indications, real-world evidence of safety with these emerging therapies within this complex patient population with multiple metabolic risk factors would be particularly valuable.

3) People with alcohol-related liver disease

The impact of alcohol-related liver disease may be in part due to alcohol metabolism producing oxidative stress, which can be restricted by maintaining adequate antioxidant defences to avoid excessive production of reactive oxygen species. However, repeated oxidative injury can lead to chronic liver damage resulting in sustained lipid peroxidation, self-perpetuating inflammation and liver injury. Long-term control of ferroptosis may be a treatable factor in some patients with this condition and may improve survival in addition to established measures, such as abstinence, nutritional support and management of complications.

4) People at risk of fibrosis and cirrhosis

Inhibition of ferroptosis could potentially influence liver fibrosis, either by promoting or by blocking pathways leading to liver cirrhosis, specifically during the organ’s regenerative, wound-healing attempt to hypercompensate for loss of functioning tissue. Chronic liver diseases can drive the liver into a long-lasting wound healing state leading to fibrosis, changes in the architecture of the liver sinusoids and ultimately to liver dysfunction. Once fibrosis has developed, attention might then shift from preventing wound healing to preventing further injury, i.e. to quantitating the rate of ongoing active injury.

Not all patients are at the same stage of disease and consequently treatment strategies would need to be based on a risk/benefit assessment at the specific stage of disease of the individual patient. In early disease where the patient has minimal hepatic injury, a major therapeutic objective might be the anti-ferroptotic protection of the hepatocyte. In contrast, in advanced cirrhosis, other priorities that perhaps carry greater risk for the patient might need to take precedence, particularly those related to systemic iron and redox modulation.

5) People with liver cancer

Hepatocellular carcinoma patients susceptible to ferroptosis are an important subgroup that can be cured by ferroptosis-induced tumour killing. Antioxidative defenses, iron uptake pathways and lipid metabolism have recently been shown to contribute to liver tumour resistance to therapy. By targeting these pathways, it may be possible to induce cancer-specific tumour cell death selectively supplementing or even replacing current cancer therapies to combat tumour drug resistance and improve clinical treatment outcomes.

For many types of cancer, choosing the best option is crucial. But for liver cancer, there is an added pressure to avoid causing damage to a liver that is already dying from the cancer. Many liver cancer patients have underlying cirrhosis, making many treatments poor options. Researchers are interested in finding a liver cancer treatment that induces ferroptosis with minimal harm to healthy liver tissue.

6) Clinicians, labs, and caregivers

Management of patients with ferroptosis-inducing therapy would require a multidisciplinary team of providers and services from the hepatology and oncology teams as well as emergency medicine, intensive care unit, and pathology and laboratory services. Providers and patients would benefit from concise and practical education on the likelihood of ferroptosis-related complications and management options, as well as informed counseling of patients and families regarding anticipated benefits and risks, the need for close monitoring and management, and expected outcomes.

What Changes

1) Clinical care in noncancer liver disease

Future adjunctive therapies may target the prevention of iron induced lipid damage prior to unmasking of downstream sequelae rather than solely treating resulting symptoms. This may create an urgent unmet market need for lipid radical scavengers, compounds that induce production of glutathione (Gpx4), Gpx4 stabilizers, and therapies that stabilize and limit iron. In iron overload syndromes with acute features, a short course of iron modulating treatment administered at critical points of pathologic iron release may effectively attenuate tissue damage. In syndromes with chronic features, a longer term modulatory treatment approach may be indicated in select patients.

As the biology of ferroptosis is not equally operational in all liver diseases or patients, patient selection will become crucial. Future treatment pathways are likely to integrate clinical and biomarkers to select patients who are most likely to benefit from modulation of ferroptosis while preventing exposure of patients and their tumors to the potential harms of ferroptosis modulators and at the same time maximizing the benefits for the subset of patients who are most likely to benefit.

2) Liver cancer treatment strategy

Inducing ferroptosis may be a useful component of combination therapies aimed at overcoming resistance to currently used anti-cancer drugs. Some tumors can be primed for ferroptosis by reprogramming iron or lipid metabolism, or by depleting antioxidants. If safety and selectivity can be established, adding ferroptosis inducers to targeted therapies or immunotherapies may become a rational therapeutic strategy.

Abstract Molecular profiling may guide the use of ferroptosis-inducing agents. Tumors undergo intense selective pressure to develop defenses against such therapies. Here we identify several pathways that protect against ferroptosis, and the metabolic networks that support them, which will allow prediction of sensitivity and toxicity of these emerging agents.

3) Diagnostics and monitoring

Markers and clinical tests for ferroptosis and ferroptosis-targeting therapeutic response(s) are needed. In research studies, ferroptosis can be measured by assessing levels of lipid peroxidation products and/or iron related indices and gene/protein expression of ferroptosis-related genes. Importantly, for clinical tests, a reliable, affordable and meaningful test must correlate with one or more important patient-centered outcomes (e.g., decreased risk of disease progression, decreased number of hospitalizations per patient, increased survival time).

Monitoring iron-related gene expression, liver toxicity, oxidative stress and side effects of iron supplementation/overload will be an important aspect of the monitoring strategy. In addition, we will require protocols for off-target effects in all patients as well as disease-specific increases in risk due to co-morbid conditions (e.g. cardiovascular disease, kidney disease, immune suppression).

4) Policy, access, and health system readiness

Future health systems will need a rational, efficient and timely process to evaluate and integrate novel diagnostics and therapeutics targeting ferroptosis into practice. Decisions about coverage will likely depend on evidence demonstrating a treatment’s capacity to decrease costly complications of decompensation, cancer progression or need for transplantation for patients with liver disease. Future planning for the equitable distribution of resources to study and manage liver disease will need to consider the roles of socioeconomic status, alcohol availability, the prevalence of obesity, and access to screening, early evaluation, and treatment.

Even after ferroptosis drugs enter clinical practice, ongoing safety monitoring at multiple levels will be important. Patients with liver disease are typically “old” with multiple medical conditions and on multiple medications, so that even though rigorous clinical trials cannot possibly identify rare adverse effects, drug-drug interactions, or predictive factors of benefit, post-marketing surveillance at multiple levels will be required to identify these.

References:

https://pmc.ncbi.nlm.nih.gov/articles/PMC11362879/ https://pubmed.ncbi.nlm.nih.gov/35075250/