In the process of electron transfer in aerobic respiration of mitochondria, the by-product of ROS is inevitable due to the leakage of electrons at complexes I and III [36]. However, ROS play a significant signaling role, whenever there is too much, the rate of intracellular ROS production outweighs the cell’s antioxidant capacity; they result in OS and severe damage despite the presence of antioxidant systems such as catalases, peroxiredoxins, and glutathione/glutathione peroxidase systems [37]. An increase in OS may be the cause of mitochondrial dysfunction.

MASLD is a complex multifactorial disease related to lifestyle, genetic, and environmental factors. In the “multiple-hit” hypothesis about its pathogenesis, OS is considered to be the main factor leading to liver injury and disease progression in MASLD [38] and has also been identified as an essential feature of human steatohepatitis [16]. Given the relationship between mitochondrial oxidative metabolism and the occurrence and development of MASLD, researchers have been continuously exploring over the past two decades to understand better the mechanisms of hepatic mitochondrial oxidative stress in MASLD. This endeavor aims to provide potential therapeutic targets for clinical practice.

Earlier, researchers found that the oxidative stress pathway leads to mitochondrial dysfunction in metabolic diseases. Cardiolipin (CL) is a mitochondrial phospholipid required for mitochondrial bioenergy and membrane structure [39]. Lysocardiolipin acyltransferase 1 (ALCAT1) serves as a catalyst for pathological CL remodeling in several diseases [40]. CL is highly sensitive to ROS oxidative damage due to its double bonds and unique position in mitochondria. At the same time, CL is the only phospholipid in mitochondria that undergo early oxidation during apoptosis. The experiment found that MASLD significantly up-regulated the expression of ALCAT1 protein in the liver of Wild-Type control mice, and ALCAT1 deficiency could dramatically improve insulin sensitivity and reduce liver fat production and fibrosis [41]. In short, the expression of ALCAT1 leads to CL peroxidation, leading to a vicious cycle of oxidative stress and mitochondrial dysfunction and ultimately leading to MASLD [41].

In Europe, MASLD is considered a metabolic predisposition to hepatocellular carcinoma (HCC) [42–44]. As various immunosuppressive mechanisms involving different immune cell subsets have been studied in HCC [45], the role of adaptive immune response in MASLD and HCC has been recognized [46]. Here, researchers found that the imbalance of lipid metabolism in MASLD can lead to the selective loss of CD4 T lymphocytes in the liver, but will not lead to the selective loss of CD8 T lymphocytes, and accelerate the occurrence of liver cancer [47]. It was also found that CD4 T lymphocytes had greater mitochondrial mass than CD8 T lymphocytes and could produce higher levels of ROS. Among the fatty acids accumulated in MASLD, linoleic acid can destroy the function of mitochondria, cause more oxidative damage than other free fatty acids, and mediate the selective loss of CD4 T lymphocytes in the liver. Blocking ROS in vivo can reverse the reduction of CD4 T lymphocytes in the liver induced by MASLD and delay the promotion of HCC by MASLD [47].

In 2018, Baker et al. proposed that the onset of MASLD may originate from the fetal period [48]. Previous human studies have described that infants born to obese mothers have increased intracellular lipid storage and have a higher risk of developing obesity, MASLD, cardiovascular disease, and liver cancer in later life [49–51]. Compared with adult livers, fetal livers normally have fewer mitochondria, lower activity of the fatty acid metabolic enzyme carnitine palmitoyl transferase-1 (CPT1), and little or no gluconeogenesis. However, fetal exposure to excessive maternal fuel can uniquely alter liver fatty acid oxidation, tricarboxylic acid cycle activity, de novo adipogenesis, and mitochondrial health functions. These events can promote the increase of oxidative stress and the excessive storage of this peroxidation leads to the exposure of hydrophilic phosphocholine (PC) head groups and the formation of highly reactive oxidized phospholipids diglycerides. Together with the changes in immune function and epigenetic changes, they make the fetal liver ready for the occurrence of MASLD and may increase the risk of MASLD in the next generation.

Lipid toxic factors produced by lipid peroxidation exist in many inflammatory environments, such as atherosclerosis [52], infectious pulmonary diseases [53], and MASLD [54,55]. Phospholipids containing PC are the main components of low-density lipoprotein, cell membranes, and lipid droplets. They usually contain polyunsaturated fatty acids at the sn-2 position, which are susceptible to free radical-induced lipid peroxidation. This peroxidation leads to the exposure of hydrophilic PC head groups and the formation of highly reactive oxidized phospholipids (OxPLs). OxPLs can induce cell stress and apoptosis by regulating the activity of intracellular signal transduction and gene expression. Studies have shown that OxPLs accumulate in MASLD in humans and mice. In the mouse model, it was found that OxPLs partially inactivate the enzyme manganese superoxide dismutase (MnSOD/SOD2) through covalent modification, which promotes an increase in ROS and induces mitochondrial dysfunction in hepatocytes. Neutralization of OxPLs can significantly reduce steatosis, inflammation, hepatocyte injury, and fibrosis. It is manifested by the reduction in the number of macrophages, the decreased expression of inflammatory genes, and lowered levels of serum cytokines, thereby alleviating liver inflammation [56]. These results suggest that targeting OxPLs may be an effective treatment strategy for MASLD.

It has been discovered that elevated levels of NAD(P)H oxidase 4 (NOX4) and nuclear factor erythroid-derived 2-like 2 (NFE2L2) in hepatocytes during the early stages of disease progression can alleviate mitochondrial oxidative stress [57]. NFE2L2 encodes nuclear factor-erythroid 2-related factor 2 (Nrf2) mediates signaling that plays a pivotal role in maintaining cellular redox homeostasis in hepatocytes [58], which can improve mitochondrial oxidative stress and membrane potential and rescue mitochondrial biogenesis. Previous studies have demonstrated that NFE2L2 drives the expression of NOX4 in muscle, subsequently enhancing NFE2L2 antioxidant defenses, thereby mitigating muscle oxidative damage and insulin resistance [59]. In the majority of patients with MASLD, progression to NASH and fibrosis does not occur, which researchers believe may be due to adaptive mechanisms that mitigate the oxidative damage that might otherwise arise during the progression of MASLD [57]. They have found that the induction and activation of NFE2L2 depend on ROS generated by mitochondria and NOX4. To a certain extent, ROS derived from NOX4 contribute to antioxidant defense, and the deletion or inhibition of NOX4 in hepatocytes decreases ROS and weakens antioxidant defense, leading to mitochondrial oxidative stress and damage to proteins and lipids. Their experimental results indicate that inducing NOX4 in the livers of obese mice triggers the ROS-dependent induction of the NFE2L2 antioxidant defense response, thereby limiting steatosis, mitochondrial oxidative stress, and tissue damage and consequently restraining the progression to NASH and fibrosis [57].

Necrotic apoptosis can be activated in both human and experimental MASLD livers as an immunogenic regulated cell death pathway [60,61], dependent on the activity of mixed lineage kinase domain-like protein (MLKL) and receptor-interacting protein kinase 3 (RIPK3) [62–64]. Research has found that RIPK3-deficient mice with NASH exhibit reduced liver infiltration of inflammatory cells, fibrosis, and precancerous nodules [65]. To investigate the role of RIPK3 in regulating mitochondrial function and the role of lipid droplet (LD) structure in MASLD, researchers conducted functional analyses assessing mitochondrial and LD biology in both NFE2L2 and RIPK3−/− mice. Their results showed that RIPK3 deficiency inhibits mitochondrial dysfunction and oxidative stress [66], indicating that RIPK3 deficiency can restore mitochondrial bioenergetics. This also suggests that RIPK3 inhibition holds promise for improving MASLD.

Relevant studies have shown that improving mitochondrial dysfunction and reducing hepatic cellular oxidative stress contribute to the amelioration of MASLD [67,68]. Acetyl CoA Carboxylase 1 (ACACA) is the key enzyme controlling the speed of fatty acid biosynthesis. It controls the generation of fatty acids via novo lipogenesis, with elevated activity enhancing adipocyte differentiation and fat deposition [69]. Adenosine monophosphate-activated protein kinase (AMPK) acts as an essential sensor of energy in cellular metabolism, which facilitates mitochondrial lipid oxidation and enhances fatty acid uptake, thus leading to reduced lipid accumulation [70]. ACACA can be inactivated by inhibiting lipid synthesis through AMPK phosphorylation. In addition, carnitine palmitoyltransferase 1α (CPT1A), as a downstream target of the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα), serves as a key regulator of β-oxidation of free fatty acids within mitochondria [71]. Previous studies have shown that the imbalance of PPARα in MASLD may be involved in the pathogenesis of the disease [72]. Recently, researchers have confirmed that suppression of ACACA leads to reduced lipid accumulation by adjusting fatty acid metabolism and influencing the AMPK/PPARα/CPT1A axis. Simultaneously, by enhancing mitochondrial function, it helps to alleviate oxidative stress [73]. These findings indicate that ACACA could be a potential therapeutic target for the treatment of MASLD.

We know that MASLD is multifactorial, with lifestyle, genetic, and environmental factors contributing to its onset and progression. As early as ten years ago, research has shown that the widespread use of plastics and plastic products in our daily lives, their resistance to degradation, and low recycling rates can lead to significant environmental and health issues [74]. In the environment, plastics are continuously fragmented, and particles ≤5 mm and ≤1 µm are defined as microplastics (MPs) and nanoplastics (NPs), respectively. MPs and NPs can contaminate dietary products and be ingested by humans [75]. Previous studies have demonstrated that in various animal models, smaller-sized MPs and NPs preferentially accumulate in the liver [76]. Exogenous substances that induce mitochondrial abnormalities are a fundamental underlying fatty liver disease mechanism. NPs can cause mitochondrial oxidative damage, leading to excessive ROS production and calcium (Ca²⁺) overload [77]. Jie Wei and Jintao Liu et al. first proved through animal experiments that NPs inhibit proteasome-dependent degradation of the endoplasmic reticulum (ER)-resident protein inositol 1,4,5-trisphosphate receptor 1 (IP3R1), stabilizing and accumulating it at the ER-mitochondrial association membrane (MAM). This leads to the formation of the IP3R1-GRP75-VDAC1 complex, disrupting liver Ca²⁺homeostasis and redox balance in NP-exposed mice. Furthermore, the reduction of Nrf2, which is sensitive to oxidative stress in the liver of NP-exposed mice, positively regulates miR-26a by directly binding to its promoter region, resulting in downregulation of miR-26a while upregulating 10 genes involved in MAM formation, lipid uptake, inflammation, and fibrosis. Additionally, miR-26a inhibition increases MAM tether VDAC1, promotes nuclear translocation of nuclear factor kappa B p65 (NF-κB p65) and Kelch-like ECH-associated protein 1 (Keap1), and inactivates Nrf2 function, creating a vicious cycle [78]. The liver MAM-tethers/Nrf2/miR-26a feedback loop is a crucial metabolic conversion hub in the development of non-alcoholic steatohepatitis (NASH) from simple steatosis. It represents a promising therapeutic target for oxidative stress-related liver injury and NASH.

Mitochondria have their genetic material, called mitochondrial DNA (mtDNA), which codes for the 13 polypeptides of the OXPHOS system, along with the 2 rRNA (ribosomal RNA) s and 22 tRNA (transfer RNA) s playing a pivotal role in mitochondrial protein synthesis [79]. The normal mitochondrial function depends heavily on mtDNA. Still, owing to its closeness to sites where ROS are generated and the absence of histones that safeguard DNA, along with ineffective DNA repair systems, it is susceptible to oxidative damage, leading to mutations that can impair normal cellular physiological functions [80]. Mitochondrial biosynthesis processes are essential for maintaining stability during the entire mitochondrial lifecycle. In MASLD, an imbalance in redox reactions results in excessive production of ROS, producing oxidative damage to mtDNA. This mtDNA damage impairs mitochondrial function and protein synthesis, subsequently adversely affecting cellular energy metabolism and other mitochondrial functions.

The process of mitochondrial biogenesis necessitates the synchronized expression of both nuclear and mitochondrial genes, and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC1α) acts as a pivotal transcriptional activator and master regulator of this process by activating other transcription factors involved in nuclear and mitochondrial gene expression, such as transcription factor A (TFAM) and sirtuin 1 (SIRT1) [81,82]. Research has shown that PGC1α can associate the ROS defense system with mitochondrial oxidative metabolism, allowing cells to preserve their redox balance when oxidative capacity changes [83]. The expression of PGC-1α may be regulated by mitochondrial ROS, thereby establishing a tight connection with the development of MASLD.

As previous studies have shown that the relative content of mtDNA is affected by various conditions associated with oxidative stress [84,85], Chimienti et al. utilized a rat model of MASLD fed an obese-inducing diet. By examining the mtDNA content in control groups and obese rats without fatty liver disease, they found that the mtDNA content was decreased in obese rats [86], also indicating a significant decrease in PGC-1α. It is well known that PGC-1α regulates the biosynthesis of mtDNA, enhancing mitochondrial biogenesis and oxidative phosphorylation [87,88]. Conversely, it can be inferred that when upon downregulation of PGC-1α, the mtDNA copy count diminishes, resulting in a decreased rate of oxidative phosphorylation, resulting in a reduction in ATP levels. Recent studies showed that compared relative to the control group, the ATP levels were substantially decreased, the mtDNA copy count was markedly decreased, and the expression of PGC-1α decreased in the MASLD model group induced by free fatty acid. However, upregulation of the PGC-1α signaling pathway can enhance mitochondrial biogenesis and ATP production in HepG2 cells [89]. Therefore, mtDNA biogenesis dysfunction may lead to mitochondrial dysfunction, thereby promoting the occurrence and progression of MASLD.

The endoplasmic reticulum (ER) and mitochondria both exhibit continuous tubular membrane network morphologies within the cytoplasm. Notably, approximately 5%–20% of the mitochondrial surface is in close proximity to the ER membrane, with a minute distance of only 10–30 nm separating them. This tight adjacency facilitates the formation of a special structure known as the endoplasmic reticulum-mitochondria contact site (ERMCS). These connection points afford a stable interface for the coordinated performance of the two organelles [90]. With ongoing exploration, the ERMCS have been implicated in an increasing number of crucial physiological functions, including the interchange of calcium ions between the endoplasmic reticulum and mitochondria [91,92], the management of phospholipid production and breakdown, the adjustment of mitochondrial shape and self-eating processes, the activation of the inflammasome complex, and programmed cell death are all critical cellular processes [93,94]. These functions are indispensable for the normal operation of mitochondria. Previous research has found that reducing ER-mitochondrial interaction and calcium exchange is an early event in diet-induced obese mice, which can alter hepatic insulin sensitivity and induce hepatic steatosis. Conversely, enhancing ER-mitochondrial interaction can prevent diet-induced glucose intolerance [95]. Furthermore, they also confirmed that impaired ER-mitochondrial communication occurs in the livers of patients with MASLD [95]. Recent research has indicated that reduced ER-mitochondria contact impairs mitochondrial phospholipid metabolism and disrupts mitochondrial membrane integrity, subsequently leading to weakened mitochondrial respiration, decreased ATP production, and elevated levels of ROS. These alterations collectively contribute to mitochondrial dysfunction. In experimental mice, such dysfunction was observed to result in hepatic lipid accumulation, NASH, and liver fibrosis [96].

Mitochondrial quality control refers to the balance between mitochondrial biogenesis and the clearance of damaged mitochondria, encompassing processes such as mitochondrial fusion, fission, and mitophagy [97]. Mitochondrial fusion facilitates the exchange of metabolites and substrates within mitochondria and regulates oxidative metabolism to alleviate organelle stress. Mitochondrial fission involves the mitotic segregation of mitochondria into daughter cells and the separation of damaged or dysfunctional mitochondrial parts, which is essential for subsequent autophagic degradation. The interplay between mitochondrial membrane fusion and fission contributes to regulating mitochondrial morphology and quantity [98]. Mitophagy is the degradation and recycling of dysfunctional or damaged mitochondria, serving as a crucial mechanism for mitochondrial quality control [99,100].

The pathological changes of chronic liver injury are often associated with abnormally enhanced mitochondrial fission [101]. Research has found that excessive mitochondrial fission (such as decreased mitochondrial content, reduced mitochondrial area, and mitochondrial deformation) exists in MASLD experimental models [102]. This suggests a likelihood that alterations in mitochondrial fission play a role in the progression of MASLD. Several studies have demonstrated that mitophagy induction in hepatocytes can maintain an adequate number of functional mitochondria, thereby fulfilling cellular energy demands, preventing hepatocyte injury induced by free fatty acids, and ultimately mitigating the progression of MASLD [103,104]. Previous research findings have suggested that defects in hepatic mitophagy can exacerbate MASLD and contribute to the pathogenesis of nonalcoholic steatohepatitis [105,106]. Recent experiments have demonstrated that reduced mitophagy is an early feature of NAFLD pathogenesis. When the mitophagy process is impaired, damaged mitochondria cannot be cleared in a timely manner, leading to the accumulation of ROS and cellular damage [107]. In summary, the mechanism of mitochondria in MASLD is depicted in Fig. 3.