The impact of aging on liver health and the development of liver diseases

Abstract

The number of individuals aged 65 years and older is expected to grow significantly in the coming decades. As life expectancy improves, the likelihood of developing chronic diseases, such as liver diseases, rises sharply with age. Aging is characterized by 3 main categories of hallmarks: primary, antagonistic, and integrative hallmarks. These categories are also observed in the liver, which ages more slowly than other organs. In this review, we summarize the current understanding of the mechanisms of aging as they pertain to the liver. This includes aging-related structural and functional changes in the liver, the roles of liver parenchymal and nonparenchymal cells, oxidative stress, and the sirtuin 1 protein. We also discuss how aging may influence the development and therapeutic management of various common liver diseases, including ischemia-reperfusion injury, DILI, alcohol-associated liver disease, and metabolic dysfunction–associated liver diseases.

INTRODUCTION

The average life expectancy during the 1800s and early 1900s was projected to be around 40–50 years. During these times, lack of effective medical treatment, untreated infectious diseases, improper sanitation practices, and war were the leading causes of shortened lifespan. Since then, average expected life expectancy has drastically improved as people can live longer due to the advancement in medical practices, sanitation, and overall healthier lifestyles.^1,2 Population studies performed by both the US Census Bureau and the World Health Organization (WHO) show that the number of individuals aged 65 and older gradually increases yearly. In the United States alone, this age group is projected to comprise ~25% of the population by 2050. Though people are living longer, increasing age is associated with a decline in quality of life as basic survival-related functions are lost. ³ In its simplest terms, aging is defined as a natural biological process in which an organism experiences a decline in physiological and cellular function over time. ⁴ The loss of cellular homeostasis associated with aging results in an inability to respond to stress caused by both internal and external insults, such as increased oxidative stress or exposure to environmental toxins. ⁴

In aging, several processes occur and culminate in the aged phenotype. Collectively, these processes are termed the “hallmarks of aging” and divided into 3 subgroups. The first subgroup is categorized as primary hallmarks and includes telomere attrition, genomic instability, epigenetic alterations, loss of proteostasis, and disabled autophagy. Primary hallmarks are events that initiate cellular damage that disrupt repair and regenerative mechanisms at the molecular level, leading to the increased occurrence of the remaining hallmarks.^5–7 The second subgroup of hallmarks encompasses cellular mechanisms that function as restorative processes initially, as these processes will attempt to alleviate the damage created by the primary hallmarks and are termed as antagonistic pathways. The antagonistic subgroup hallmarks are dysregulated nutrient sensing, mitochondrial dysfunction, and cellular senescence.^4–6 Nutrient sensing involves a complex network of pathways that are responsible for detecting alterations in nutrient availability and energy imbalances. Aging can promote dysregulated nutrient sensing due to the constant upregulation of promoters of aging, such as mechanistic target of rapamycin, insulin and IGF-1 1 or the functional loss of proteins that effectively extend lifespan, such as sirtuin proteins and AMP kinase (AMP-activated protein kinase [AMPK]). ⁸ The removal of senescent cells has been of interest in targeted therapies to improve cellular and organismal longevity. ⁹ The third subgroup of hallmarks is termed the integrative hallmarks and includes altered intracellular communication, stem cell exhaustion, dysbiosis, and chronic inflammation. ⁵ Many of the hallmarks in all 3 subgroups will subsequently activate inflammatory signaling in cells, tissues, and organ systems overall. The consistent presence of elevated inflammatory signaling molecules resulting from the persistent stress associated with aging will generate a state of chronic inflammation, leading to a further decline in the organism’s health.^4–6,10 Overall, these 3 layers of hallmarks create complex interactions that may help explain certain aspects of the aging process.

Like other highly metabolic organs such as the heart and brain, the liver experiences a slower rate of aging, which increases its susceptibility to chronic liver pathologies that culminate in the degeneration of liver cells and the overall loss of hepatic function. ¹¹ This review will discuss the mechanisms of liver cell aging and the relationship between hepatic aging and the development of liver dysfunction and diseases.

MECHANISMS OF AGING IN THE LIVER

Age-induced changes in liver structure and function

The liver is a dynamic organ responsible for performing many homeostatic functions, such as metabolic maintenance, detoxification, and bile production for fat digestion. ¹² The liver undergoes significant alterations in structure and function as a person ages. Research utilizing ultrasound technology has shown that liver volume can decline by 20%–40% with increasing age. ¹³ Furthermore, both the rate of blood flow and alterations in the hepatic vascular endothelium, including loss of porosity or fenestration, were found to correlate with a decrease in liver volume in aged livers.^14–16

The liver possesses a unique ability to regenerate after tissue damage or injury. This regeneration facilitates the restoration of liver mass through the replication of liver cells, primarily hepatocytes, in response to a signaling network initiated by cytokine release, followed by cell cycle progression stimulated by growth factors.^17,18 In aging, liver regeneration declines or becomes impaired due to disrupted signaling pathways stemming from insufficient growth ligand-to-receptor binding or an increase in cell cycle inhibitory complexes, such as the one formed by CCAAT/enhancer-binding protein alpha (CEBP/α), histone deacetylase 1, and Brm, a chromatin remodeling protein.^18,19 In addition to a decline in regenerative capacity, studies on aged rat livers have shown significant reductions in drug metabolism related to Cytochrome P450 (CYP) enzymes.^20,21 However, studies on CYP-mediated metabolism in humans have produced mixed results. While some research has indicated a substantial age-related decline in the protein levels of CYP isozymes, such as CYP3A and CYP2E1, other studies have reported no significant functional changes in other CYP isozymes.^22–24 Furthermore, as people age, there is a decrease in the protein content of albumin, which is produced by the liver. This reduction in albumin affects the drug-protein binding interactions necessary for adequate drug clearance by the liver. ²⁵

Changes in parenchymal and nonparenchymal hepatic cells during liver aging

The liver is a heterogeneous organ containing various cell populations, essential for the liver’s overall function.^12,26 The liver’s most prominent cell type, or parenchymal cell, is the hepatocyte, which comprises ~60%–80% of liver volume and performs most hepatic functions. Hepatocytes play a vital role in regulating glucose metabolism by facilitating glycolysis and gluconeogenesis. They also synthesize essential plasma proteins, such as albumin and complement proteins, which circulate in the body. In addition, hepatocytes are involved in lipid metabolism by synthesizing and secreting lipids. Furthermore, they produce bile acids, crucial for the absorption of lipids from the intestine. Moreover, hepatocytes are the primary cells responsible for detoxifying xenobiotics in the body.^12,26,27 The remaining 20%–40% of liver volume consists of nonparenchymal hepatic cells, including hepatic stellate cells (HSCs), cholangiocytes, kupffer cells (KCs), and liver sinusoidal endothelial cells (LSECs).^12,26 In the liver, HSCs are usually quiescent cells that store vitamin A and lipids and help to maintain the extracellular matrix (ECM). Upon activation due to stress or injury, HSCs promote the accumulation of scar tissue in the liver, leading to fibrosis and cirrhosis.^26,28 Cholangiocytes are cells that line the epithelium of the bile duct and are involved in the modification and secretion of bile from the liver. ²⁹ As the name implies, LSECs are specialized cells that line the hepatic sinusoids and are fenestrated to allow for blood filtration.^28,30 Lastly, the liver contains its unique population of immune cells, KCs, that regulate innate immunity and perform phagocytic activity ²⁸

The 3 general layers of aging hallmarks that apply to other tissues and organs we discussed above also pertain to the liver. In the first layer, telomere shortening, a standard marker of aging, occurs in the liver and is linked to hepatocyte senescence. This shortening is associated with a reduced regenerative capacity and an increased risk of chronic liver diseases, such as cirrhosis and hepatocellular carcinoma (HCC). ³¹ Hepatocyte polyploidy, decreased telomere length, and hypertrophy were observed in hepatocytes from aged rodent livers and human hepatocytes.^11,32 Moreover, aged hepatocytes exhibit changes in DNA methylation and histone acetylation, contributing to increased genomic instability in the aging liver.^33,34 The liver’s cellular proteostasis and organelle quality control are primarily regulated by 2 major degradation pathways: the proteasome system and the autophagy-lysosomal system. Aging causes age-dependent decreases in the liver’s 26S and 20S proteasome activity. ³⁵ The liver has 3 types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy. These types differ in the routes used to deliver autophagic cargo to the lysosome. ³⁶ Aging was connected to decreased autophagy-related proteins such as microtubule-associated protein 1B-light chain 3 (LC3), autophagosome structures, and increases in primary, or unfused, lysosomes. ³⁷ In addition, LAMP2A levels, key receptors for chaperone-mediated autophagy, decreased in the aged liver.^38–40 Intriguingly, elevated protein levels of cathepsin B were observed in the livers of aged rats, likely a compensatory response to increased expression of an endogenous cysteine peptidase inhibitor and impaired lysosomal functions. ⁴¹ Indeed, aged hepatocytes exhibit increased lysosomal leakage resulting in the progressive accumulation of lysosomal content, such as cathepsin D, in the cytosol. ⁴² One of the most common features of aging’s effects on autophagy and lysosomal function is the excessive accumulation of lipofuscin within lysosomes. ⁴³ Lipofuscin is termed the “age-pigment” as it commonly accumulates in aged tissues. ⁴⁴ Notably, hepatocyte lysosomes have increased lipofuscin accumulation in aged livers, suggesting impaired lysosomal degradation. ⁴⁵ While changes in autophagy within the liver can impair its overall function, pharmacological and genetic strategies to enhance autophagy have been shown to improve liver function in aging. ⁴⁶

In the second layer of aging liver hallmarks, mechanistic target of rapamycin complex 1, a key regulator of protein synthesis and cellular metabolism, is decreased in aged animal livers. ⁴⁷ AMPK is a key sensor of cellular energy and an essential regulator of cellular metabolism. In aged mice, the basal activity of AMPK α1 increases, whereas AMPK α2 does not show this increase. In addition, both isoforms of AMPK in the livers of aged mice demonstrate a lack of response to hypoxia, suggesting an overall impairment of the AMPK pathway. ⁴⁸ Aging caused increased cristae fragmentation and vacuole formation in the mitochondria of hepatocytes. ⁴⁹ The maintenance and sustainability of ATP production is highly reliant on the mitochondrial membrane potential generated by the proton pumps in the electron transport chain located on the inner mitochondrial membrane. ⁵⁰ A loss of mitochondrial membrane potential leads to mitochondrial damage and subsequent cell death. Mitochondrial membrane potential is significantly decreased in aged hepatocytes with elevated levels of oxidized mitochondrial proteins. ⁵¹ In addition, hepatocyte aging is associated with various markers of cellular senescence, such as increased senescence-associated-β-galactosidase activity, p16INK4a (p16), p21, and γ-H2AX. ⁵² Aged hepatocytes also exhibit enlarged vacuolated nuclei, which are associated with increased DNA damage and cellular senescence. ⁵³

In the third layer of liver aging hallmarks, the blood-borne systemic factors with proaging or antiaging properties for cell-to-cell communication and changes in the liver ECM are closely associated with the nonparenchymal hepatic cells. Transplanting young blood into aged rats reverses age-related hepatic dysfunction and restores liver regeneration, although the specific antiaging factors in the young blood have not yet been identified. ⁵⁴ Aged livers exhibit impaired ECM remodeling, altered composition (increased collagen and decreased growth factors), and increased stiffness, all contributing to fibrosis and liver dysfunction.^55,56 Studies evaluating the correlation between HSCs, aging, and fibrosis found that HSCs in aged livers exhibited characteristics more typical of myofibroblasts, demonstrated by increased positive staining of alpha-smooth muscle actin. Evidence also indicated elevated expression levels of the profibrogenic genes such as TGF-β and platelet-derived growth factor receptor-beta in aged livers.^32,57,58 This transition of HSCs from quiescent to activated can be partly attributable to the elevation of circulating inflammatory mediators associated with aging. ⁵⁷ Notably, the loss of efficient hepatic blood flow in the aging liver is primarily due to the thickening of the endothelium of hepatic sinusoids. Furthermore, the expression of p16, along with the reduced expression of VEGF receptor-2 and CD32b, proteins essential for LSEC development and function, is present in the aging liver.^15,16,58 In addition, the age-induced changes to LSEC impact drug clearance due to a loss of fenestration in aged livers.^59,60 Though the effect of aging on the liver microenvironment has been investigated for many years, the breadth of research surrounding changes in KC composition and function in aged livers is scarce. The number and activities of KCs increased and are associated with increased IL-6 production in the aged liver.^61,62 The phagocytic activity of KCs in aged livers has been controversial, as some studies have reported a substantial increase in KC endocytic activity, while others have noted a decline in this function of KCs in aged rat livers.^63,64 Studies on age-related effects in cholangiocytes are also limited and primarily focus on the role of cellular senescence in the progression of severe cholestatic diseases, which are more prevalent in older adults. ¹⁵ Moreover, aged mice show increased intestinal barrier impairment and higher bacterial endotoxin levels, activating Toll-like receptor 4 signaling in the liver. These changes are associated with liver degeneration, inflammation, and fibrosis. ⁶⁵ Over time and as the organism ages, the effectiveness of the immune response diminishes, leading to the occurrence of immunosenescence and a continuation of chronic inflammation, or inflammaging, where low-grade production of cytokines occurs.^66,67 As one of the molecular drivers of inflammatory signaling in macrophages, the upregulated expression of forkhead box protein O1 was correlated to a significant increase in inflammaging-associated markers such as NF-κβ and circulating cytokines like IL-1β in aged mice as well as isolated macrophages from aged mouse livers. ⁶⁸ Notably, declined liver function in aged individuals can be reversed using hepatic stem cells or liver progenitor cells, which can regenerate and repair the liver. In aged mouse livers, the activation of liver progenitor cells decreases, likely due to increased production of reactive oxygen species from infiltrating neutrophils. ⁶⁹ Accordingly, a stem cell–based approach has been proposed as a promising therapeutic strategy for enhancing liver regeneration and combating liver aging and its associated diseases. ⁷⁰ The current understanding of the 3 layers of hallmarks in liver aging is summarized in Figure 1.

FIGURE 1.

Three sets of aging hallmarks in the liver. The scheme illustrates the 3 subsets of proposed hallmarks in liver aging. Primary hallmarks include a decline of proteasome and autophagy, an increase of hepatocyte polyploidy, telomere attrition, and epigenetic and genomic instability. Antagonistic hallmarks include decreased mTORC1 and AMPK, increased senescence, mitochondrial dysfunction, and vacuolated hepatocyte nuclei. Integrative hallmarks include altered dysbiosis, cell-cell communication, decreased LPC/stem cells, increased inflammation, and ECM. Abbreviations: AMPK, AMP-activated protein kinase; ECM, extracellular matrix; LPC, liver progenitor cell; mTOR, mechanistic target of rapamycin.

The role of oxidative stress in the aged liver

Oxidative stress increases across various tissue types in the aged organism, contributing to the increased occurrence of many of the aging hallmarks.^5–7 Oxidative stress occurs because of the imbalance between reactive oxygen and nitrogen species and the antioxidant defense systems responsible for reducing the harmful effects of reactive oxygen and nitrogen species on many intracellular biomolecules, including lipids, proteins, and DNA. Oxidative stress has been connected to the development and progression of aging as it stimulates the onset of cellular senescence through the induction of several intracellular mechanisms.^71,72 These include the onset of cell cycle arrest through the induction of cell cycle arrest mediators like p16 and p21, induction of proinflammatory mediators like NF-κB, and a persistent activation of cellular matrix remodeling proteins.^72,73 Furthermore, oxidative damage to cellular membranes induced by the elevated levels of reactive oxygen and nitrogen species can result in the release of “danger” signals from the cell, termed damage-associated molecular patterns, sparking an innate inflammatory response. ⁷⁴ In rodent models, oxidative stress was confirmed to accumulate in the aged liver as evidenced by substantial rises in lipofuscin and lipid peroxidation content in conjunction with a diminished capacity for eliminating toxic fatty acid species.^32,43,45,75 Oxidative stress and aging cooperatively impair the liver’s ability to regenerate after injury. ¹⁸

Sirtuin 1 in the aged liver: controversial topic

In aging research, several signaling pathways have been identified as playing a significant role in the progression of aging. One family of molecules extensively studied is the sirtuin family, a class of NAD⁺-dependent deacetylases. The sirtuin family comprises 7 members (1–7) that rely on NAD⁺ to function and regulate metabolism in response to cellular stress. Though all 7 sirtuins have an important biological function, mammalian Sirtuin 1 (SIRT1) is the most thoroughly studied.^76,77 SIRT1 is found in both the cytoplasm and nucleus and functions as a deacetylase to remove acetyl groups from histones, regulating gene expression. In metabolic regulation, SIRT1 activates transcriptional activators and coactivators like peroxisome proliferator–activated receptor alpha (PPARα) and PPARgamma coactivator 1 alpha (PGC1α) to induce transcription of genes involved in mitochondrial ATP production, gluconeogenesis, and fatty acid metabolism. ⁷⁸ In response to oxidative stress, SIRT1 deacetylates multiple proteins involved in DNA repair, antioxidant enzymes for mitigating reactive oxygen species in mitochondria, and genes involved in cell death versus cellular repair pathways.^78,79 Decreased SIRT1 levels in aged livers have been correlated to higher levels of oxidative stress, increased lipid accumulation, and increased expression of fibrotic factors such as alpha-smooth muscle actin and TGF-β in the presence of various hepatotoxins, as well as in metabolic dysfunction–associated steatotic liver diseases (MASLD).^80,81 Furthermore, the age-associated loss of hepatic SIRT1 or genetic ablation of SIRT1 resulted in upregulated inflammatory signaling as well as inflammasome induction, as evidenced by an increase in NLRP3 (NACHT, LRR, and PYD domains–containing protein 3) in aging rodent models. ⁸⁰ Genetic or pharmacological activation of SIRT1 decreases lipogenesis, induces fatty acid β-oxidation, decreases the presence of senescence-associated genes such as p16, and improves glucose tolerance and utilization in aged rodent livers.^81,82

Numerous studies have highlighted the roles of NAD and Sirtuins in extending lifespan, leading to a strong interest in finding small-molecule activators for SIRT1 to address aging.^77,78,83 However, the role of SIRT1 in regulating aging and longevity remains controversial. The Sir2 gene was first identified in yeast, which functions in transcriptional silencing. Subsequent research revealed that Sir2 and its orthologs are critical for extending the lifespan of yeast, worms, and flies.^84,85 Despite this, overexpressing Sir2 has not consistently resulted in lifespan extension in Caenorhabditis elegans and Drosophila. ³⁹ Only mice with Sirt1 overexpressed in specific brain regions, rather than those with whole body Sirt1 overexpression, exhibit lifespan extension.^82,86 This raises questions about the significance of Sir2 orthologs in aging and longevity. Furthermore, while the supplementation of NAD⁺ or its precursors has demonstrated beneficial effects on improving insulin resistance, glucose metabolism, heart health, hepatic steatosis, and chronic inflammation contributing to a “healthspan,” such supplementation does not extend lifespan in mice. ⁸⁷ It is also unclear if these benefits are primarily due to the activation of SIRT1. ⁷⁶ Regardless of its impact on lifespan, the role of SIRT1 in maintaining liver health during aging can be explored using liver-specific gain-of-function approaches (such as transgenic models or viral vector-mediated overexpression) and loss-of-function strategies (like liver-specific knockout) in various liver disease models in the future. Below, we discuss the role and mechanisms of aging in various liver diseases, including hepatic ischemia-reperfusion injury (IRI), DILI, alcohol-associated liver disease (ALD), and MASLD.

AGING IN LIVER DISEASES

Aging in hepatic IRI

Liver transplantation remains the only effective clinical method for treating end-stage liver disease. However, the shortage of donor livers has resulted in a growing number of patients on the waiting list for transplantation. There is an increasing need to expand the donor pool to include steatotic livers, older donors, and donations after cardiac death. In addition, the rising prevalence of severe liver diseases such as ALD, MASLD, and HCC in the elderly population heightens the demand for liver transplantation, which contributes to the need for more donors. ⁸⁸ Given the shortage of donor grafts, it is essential to investigate whether older patients might have poorer posttransplant survival rates and consume excessive resources postoperatively, thus affecting the procedure’s economic viability. Below, we discuss the potential impacts of aging patients receiving liver transplants and the effects of aged liver donors on transplantation outcomes.

The prospect of performing liver transplantations in elderly adults raises significant concerns. Many age-related factors exacerbate hepatic IRI and negatively impact the success and survival rates of liver transplantation. ⁸⁹ Animal studies have shown that older age worsens hepatic IRI due to changes in the hepatic microvasculature, in which LSECs respond less to signals for vasodilation, along with significant increases in capillarization. ⁹⁰ One early study indicated that patient survival rates were significantly lower among recipients of liver transplant over 60. ⁴⁰ However, this excess mortality was attributed not to liver conditions but to nonhepatic age-related issues such as infections, cardiac problems, and neurological diseases occurring within 6 months after transplant. ⁴⁰ More recent studies have indicated that the operative course, length of hospitalization, and incidence of perioperative complications for patients older than 60 years are comparable to those of their younger counterparts. As a result, advanced age is no longer deemed a contraindication to transplantation.^91,92 However, it is vital to screen for cardiopulmonary comorbidities, asymptomatic malignancies, nutritional status, and frailty to ensure optimal outcomes and prevent futile transplantation at most centers.

In experimental animal models, aged liver grafts have shown lower ATP production and increased generation of free radicals, both of which exacerbate the inflammatory response. Inflammation, driven by activated KCs and neutrophils, is increased in aged livers due to elevated NF-κB signaling, which worsens hepatocyte injury.^89,93 In addition, age-related loss of autophagy and changes in mitochondrial dynamics have been linked to increased injury in hepatic IRI. Specifically, the age-associated decline of autophagy-related protein 4b (Atg4b), impaired mitophagy, and a significant reduction in ATP production have all contributed to increased signs of IRI in aged rodent livers. ⁹⁴ Overexpression of either Atg4b or Beclin-1 enhanced autophagy, blocked the onset of mitochondrial permeability transition, and suppressed cell death following ischemia-reperfusion in old hepatocytes. ⁹⁴ Moreover, aging also exacerbates hepatic IRI by impairing age-dependent mitophagy due to insufficient PARKIN expression in mice. ⁹⁵ Consequently, strategies targeting the mechanisms involved in energy metabolism, the inflammatory response, and autophagy may be particularly effective in preventing the increased risk of IRI in aged livers following major hepatic surgery.

What about younger recipients receiving aged liver grafts? A retrospective cohort study indicated that transplanting livers from donors aged 40–59 and 60 years and older was associated with poorer graft survival in recipients younger than 40. ⁹⁶ However, due to advancements in patient care, surgical techniques, and improved donor-to-recipient matching, more recent studies have shown improved posttransplant mortality, reduced all-cause graft loss, and utilization of comparable resources in liver transplant recipients ≥ 60 years of age versus younger recipients. ⁹⁷ While there is an effort to prioritize younger organs for younger recipients, expanding the donor pool by using liver grafts from older donors may be a reasonable approach in the future.

Aging in DILI

The liver is essential for drug metabolism and clearance, which are mediated by phase I and phase II drug metabolism. Phase I is mediated by CYPs, which regulate reactions involving modifications such as oxidation, reduction, and hydrolysis. In contrast, phase II metabolism involves conjugation reactions with endogenous molecules like glucuronic acid, sulfate, or glutathione, increasing their water solubility for easier excretion. ⁹⁸ Reductions in certain CYPs occur due to advanced age, as well as decreases in hepatic blood flow and volume, all of which can alter drug metabolism in the elderly.^99,100 In this age group, hepatic drug clearance for specific medications can be reduced by up to 30%, while phase II metabolism is relatively preserved. ¹⁰⁰ These notable changes in drug metabolizing enzymes, decreased drug absorption, and clearance can lead to developing more severe adverse drug reactions and increase DILI risks in aged individuals.^25,60,101

Increased oxidative stress, the loss of function of antioxidant regulators such as superoxide dismutase and glutathione, and decreased liver regeneration all contribute to the exacerbation of acetaminophen hepatotoxicity in aged mice. ¹⁰² Clinical studies have shown that the severity of DILI is linked to a higher prevalence of severe adverse drug reactions in older patients. This is often due to the presence of comorbidities such as chronic liver disease, cardiovascular disease, and impaired renal function, along with the use of multiple prescribed medications, known as polypharmacy.^101,103,104 Age also affects the phenotypes of DILI. For instance, hepatocellular-type DILI is more common among pediatric patients, while cholestatic-type DILI and chronic DILI are more prevalent in the elderly. ¹⁰⁵ In addition, susceptibility to DILI in older adults may be influenced by certain medications and patterns of exposure (such as a higher prevalence of medication use, polypharmacy, and drug interactions). The cognitive impairments in the elderly also affect adherence, or age-related changes in pharmacokinetics and pharmacodynamics. ¹⁰⁶ While there are currently no specific therapies for DILI, understanding the risk factors associated with DILI in the aging population may help enhance diagnostic and prognostic capabilities to protect these vulnerable populations.

Aging in ALD

ALD is one of the leading causes of liver disease and alcohol-associated deaths, with reported mortality rates as high as 44% in the United States alone. ¹⁰⁷ ALD begins with lipid accumulation in hepatocytes and progresses to increased cell death. It can further develop into more severe conditions such as steatohepatitis, fibrosis, cirrhosis, and ultimately HCC.^108,109 The pathogenesis of alcohol-induced liver injury involves disrupting numerous physiological processes in the liver. In brief, alcohol is primarily metabolized in the cytoplasm by alcohol dehydrogenases 1A (ADH1A), ADH1B, ADH1C, and ADH4. ¹¹⁰ This process is followed by the breakdown of alcohol through enzymes such as mitochondrial acetaldehyde dehydrogenase 2 (ALDH2), CYP2E1 in microsomes, and catalases in peroxisomes, resulting in the production of acetaldehyde and acetate.^110,111 The metabolism of ethanol by the ADH and ALDH2 enzymes requires NAD⁺, resulting in an initial decline of hepatic NAD⁺ and a concomitant increase in the production of the reduced form of nicotinamide adenine dinucleotide (NADH). Decreased levels of hepatic NAD⁺ and an increased NADH/NAD⁺ ratio lead to impaired alcohol metabolism, mitochondrial dysfunction, megamitochondria accumulation, oxidative stress, and hepatocyte degeneration in the liver.^112,113

In older adults and the elderly, alcohol use has reportedly increased in recent years. In a drug use and health survey conducted by the Substance Abuse and Mental Health Services Administration (SAMHSA), ~15% of the total participants aged 65 years and older reported that they engaged in binge drinking as well as heavy alcohol use within a 1-month timeframe in 2023. In 2019, alcohol-associated cirrhosis accounted for 21.8% of cases among the elderly, totaling 3.23 million. Furthermore, liver cancer from alcohol in this group made up 51.27% of all related cases. ¹¹⁴ Many mechanisms may explain the increased sensitivity to alcohol-induced liver injury in the elderly. First, alcohol concentrations are elevated in the blood due to diminished activity of alcohol-metabolizing enzymes such as ADH, ALDH2, and CYP2E1 in aging livers, along with a concurrent decrease in liver volume in these older livers.^13,115 Second, decreased hepatic cellular proteostasis and organelle quality control pathways involve both the proteasome and autophagy-lysosomal pathways in the aged liver.^35,45 Third, increased cellular senescence, exacerbated by alcohol consumption, may contribute to the overall decline of hepatocytes and the progression of ALD. ¹¹⁶ However, more direct studies are needed to clarify the role of senescence in alcohol-induced liver issues in aged individuals. Fourth, an elevated basal inflammatory environment in aged livers further promotes alcohol-induced inflammation and fibrosis.^117,118 Fifth, lower levels of SIRT1 in hepatocytes and HSC in aged mouse livers worsen alcohol-induced liver fibrosis. This was demonstrated by HSC-specific SIRT1 knockout mice, which were more susceptible to short-term-plus-binge ethanol-induced liver fibrosis, exhibiting upregulation of platelet-derived growth factor receptor-α expression. Restoration of SIRT1 expression in the mouse liver and HSCs improved short-term-plus-binge ethanol-induced liver injury and fibrosis in middle-aged mice. ¹¹⁸ Moreover, excessive alcohol intake can not only exacerbate the harmful effects on the aging liver but also worsen commonly occurring age-related comorbidities such as hypertension, cardiovascular disease, and diabetes mellitus, which further impair the liver’s ability to manage injury in older adults.^119,120 Therefore, alcohol treatment programs specifically designed for older adults may be more effective in meeting their unique needs and challenges by considering age-related physiological changes, comorbidities, and cognitive impairment.

Aging in MASLD

MASLD, previously known as NAFLD, affects more than 30% of the global population. MASLD is highlighted by >5% steatosis in conjunction with at least one of the cardiometabolic risk factors.^121,122 According to the consensus on the diagnostic criteria in adults, the cardiometabolic risk factors associated with MASLD include hypertriglyceridemia, hyperglycemia, hypertension, obesity, and severely decreased HDL levels.^122,123 The molecular pathogenesis driving MASLD is similar to that of ALD; however, the key difference between the 2 diseases is excessive alcohol intake.^124,125

The incidence of MASLD in the elderly population continues to grow globally. An Australian study found that the estimated rate of MASLD in patients over 70 was 33%. ¹²⁶ Elderly patients exhibited worsening conditions of fibrosis and steatohepatitis. ¹²⁷ Cellular senescence associated with aging correlates with increased hepatic fat accumulation in MASLD. ¹²⁸ Moreover, the aging liver exhibits increased lipogenesis and decreased peroxisomal fatty acid oxidation,^129,130 enhancing hepatic lipid accumulation in older individuals. In aged animals, the mitochondrial network is often more heterogeneous, fragmented, and made up of large, swollen mitochondria, which are more challenging to remove through mitophagy.^131,132 This leads to the accumulation of megamitochondria. While the formation of megamitochondria is initially an adaptive response, it may become maladaptive and contribute to increased oxidative stress and susceptibility to the pathogenesis of MASLD and ALD in the aging liver.^{112,113,132,133} Moreover, general hepatic autophagy may also decline during aging, contributing to the dysregulation of lipid metabolism, upregulated inflammatory signaling, and the development of fibrosis in advanced MASLD.^46,134 Animal studies conducted by You-Jin Choi and colleagues investigated the role of dysfunctional chaperone-mediated autophagy in isolated aged mouse hepatocytes with a knocked-down Lamp2a gene. They found that fatty acid oxidation decreased due to a significant increase in nuclear receptor corepressor 1 (NCoR1), a negative regulator of PPARα. The elevated levels of NCoR1 are likely the result of decreased autophagy in the livers of aged mice, as autophagy plays a role in selectively degrading NCoR1. ¹³⁵ This change in fatty acid metabolism was linked to increased lipid accumulation and the subsequent development of fatty liver in aged mice. Older mice experience more hepatocyte ferroptosis and liver degeneration than younger mice when fed diets that induce MASLD. By inhibiting ferroptosis, older mice’s liver gene expression profile shifts to resemble that of younger mice, effectively reversing the liver damage caused by aging. ¹³⁶ In addition to aging, sex is a significant determinant of liver gene expression and function, especially in response to a high-fat diet. Aging leads to a decline in metabolic flexibility, which affects the liver’s ability to adjust its gene expression programs. High-fat diet feeding causes higher expression levels of genes related to inflammation, fibrosis, and the ECM in aged male mice, but not in aged female mice, suggesting a sexual dimorphism in how the aged liver responds to a high-fat diet. ¹³⁷ Necroptosis is a form of nonapoptotic cell death regulated by the receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage domain kinase-like (MLKL) pathways. ¹³⁸ Increased levels of phosphorylated RIPK1, RIPK3, and MLKL were observed in the livers of aged mice, which may exacerbate diet-induced MASLD in older individuals. ¹³⁹

To improve and treat MASLD in older patients, lifestyle modifications such as weight loss through diet and exercise, as well as the management of related conditions like diabetes, high cholesterol, and high blood pressure, can be beneficial. Medications like GLP-1 receptor agonists and resmetirom (brand name Rezdiffra), the first FDA-approved treatment for MASH, may also aid in managing this condition in elderly patients. Research has indicated that in liver biopsies from patients with MASH, the expression of thyroid hormone receptor beta is inversely correlated with the MASH score and tends to decline with age. ¹⁴⁰ Therefore, it is crucial to consider these age-related changes when designing management and treatment plans for older patients with MASLD.

CONCLUSIONS AND FUTURE PERSPECTIVES

As the global population grows, the number of older adults is increasing rapidly. While the liver can maintain some of its homeostatic functions throughout aging, it becomes highly susceptible to liver injury and chronic liver diseases. This susceptibility is mainly due to decreased hepatic volume, reduced blood flow, altered microvasculature, defective metabolizing enzymes, impaired proteostasis, mitochondrial dysfunction, and reduced expression of hormone receptors. Furthermore, chronic exposure to external factors such as polypharmacy, excessive alcohol consumption, and nutritional imbalances may exacerbate harmful inflammatory signaling, cellular senescence, and elevated oxidative stress. These changes ultimately hinder the aged liver’s ability to manage cell death and disease progression effectively. These age-related anatomical and molecular changes in the aged liver exacerbate IRI, DILI, ALD, and MASLD (Figure 2). The aging-related changes also significantly influence how liver diseases are managed and treated in elderly patients. In addition, there is limited information regarding sexual differences in aging liver disease, which warrants further studies using aged animal models to explore how sex hormones and menopause affect liver diseases in older females. Given the rapid advancements in artificial intelligence within biological research, it will be exciting to see how artificial intelligence can predict liver disease progression in aging populations in future studies.

FIGURE 2.

The proposed role of aging in liver diseases. (A) IRI: Aging livers experience reduced levels of ATG4B and PARKIN, leading to decreased autophagy/mitophagy and increased ROS. Enhanced NF-κB activation in macrophages causes greater inflammation, worsening IRI. (B) DILI: Older adults have decreased liver drug metabolism due to reduced activity of CYP enzymes and lower clearance, along with diminished GSH and SOD levels. These factors, combined with polypharmacy, raise susceptibility to DILI. (C) ALD: Aging diminishes alcohol-metabolizing enzymes and proteostasis mechanisms like autophagy/proteasome. Decreased SIRT1 levels and increased inflammation further exacerbate ALD. (D) MASLD: The aging liver shows elevated necroptosis and ferroptosis proteins, increased lipogenesis, and inflammation, while fatty acid beta-oxidation and autophagy decrease, leading to more severe MASLD. Abbreviations: ADH, alcohol dehydrogenase; ALD, alcohol-associated liver disease; ALDH2, acetaldehyde dehydrogenase 2; CYP, cytochrome; GSH, glutathione; IRI, ischemia-reperfusion injury; MASLD, metabolic dysfunction–associated steatotic liver disease; ROS, reactive oxygen species; SIRT1, sirtuin 1; SOD, superoxide dismutase.