Solving the puzzle of fibrosis resolution in alcohol-associated liver disease: An insight from KDM5 demethylases and LXR activation
Authors: Jing Ma, Suthat Liangpunsakul
Journal: Hepatology (2024)

Alcohol-associated liver disease (ALD) is a condition that develops due to prolonged and excessive alcohol consumption.1 It encompasses a spectrum of histopathological changes in the liver, ranging from steatosis, alcohol-associated hepatitis, and advanced fibrosis, to cirrhosis.1 The severity of hepatic fibrosis is closely linked to poor outcomes and mortality in individuals with chronic liver diseases, including ALD.2 Specifically, the stage of fibrosis has been identified as an independent predictor of 90-day mortality among patients with alcohol-associated hepatitis.3 The overall survival of patients with ALD with advanced fibrosis (F3 or F4) is significantly worse than those with early-stage fibrosis.4 Previous research in both animals and humans has revealed that hepatic fibrosis is a dynamic process involving both progression and regression.5 Alcohol-induced hepatic fibrosis can occur through various mechanisms. For instance, alcohol activates hepatic inflammation–associated profibrogenic activity and inhibits natural killer-cell–mediated interferon-gamma–induced death of activated HSCs.5 Activated HSCs produce TIMPs which downregulate the activity of extracellular matrix-degrading metalloproteinases (MMPs).5 Abstinence from alcohol is a fundamental component of managing patients with ALD, and it often leads to fibrosis regression.5,6 However, it is important to note that not all patients experience this regression.5,6 Mechanistically, alcohol cessation may induce apoptosis and inactivation of HSCs, a decrease in TIMPs, and an increase in MMP-mediated fibrolytic activity.5 The regression of fibrosis is significant as it can contribute to the restoration of liver architecture and function. Understanding the underlying mechanisms of fibrosis resolution after alcohol cessation is crucial, as it could pave the way for the identification of potential therapeutic targets. The study conducted by Schonfeld et al, in this issue of Hepatology, sheds light on the mechanism of fibrosis resolution in mice following the cessation of alcohol consumption.7 The experimental setup involved feeding mice a Western Diet high in fat (43% of calories from fat) along with alcohol in their drinking water (20% alcohol) for a period of 20 weeks. After this period, both alcohol and Western Diet were removed, and they conducted a series of assessments of hepatic histopathology over the next 8 weeks. Interestingly, the study observed that while hepatic steatosis improved rapidly after alcohol cessation, fibrosis resolution did not occur despite a decrease in the expression of profibrotic and proinflammatory genes. The investigation identified defects in fibrosis resolution that were, in part, attributed to the activity of the lysine demethylase 5 (KDM5) family of demethylases, particularly KDM5B and KDM5C, with a more significant role attributed to KDM5B. The absence of these demethylases, particularly in hepatocytes, enhanced fibrosis resolution, leading to a marked reduction in profibrotic gene expression. This finding emphasized the pivotal role of hepatocytes in orchestrating the dynamics of fibrosis. The study further revealed the involvement of liver X receptors (LXRs) and cytochrome P450 family 27 subfamily A member 1 (CYP27A1) as downstream effectors influenced by KDM5 demethylases. Notably, LXRs are known regulators of cholesterol metabolism and inflammation. The activation of LXRs emerged as a critical factor promoting a pro-resolving phenotype in liver macrophages. The interplay between hepatocyte-produced oxysterols (products of cholesterol oxidation), LXR activation, and subsequent modulation of the ratio of MMPs to TIMPs underscored the significance of hepatocyte-macrophage crosstalk in the process of fibrosis resolution.7 Epigenetics refers to changes in gene expression that do not involve alterations to the underlying DNA sequence. In the context of ALD, alcohol-induced epigenetic dysregulations play a significant role in the pathogenesis and progression of the disease. These epigenetic mechanisms have been identified in both hepatic parenchymal and nonparenchymal cells, contributing to inflammation and fibrosis.5 Cutting-edge technologies, including next-generation sequencing, DNA methylation assays, histone modification profiling, and computational methodologies such as machine learning, have greatly advanced our understanding of the epigenetic landscape in the pathophysiology of ALD.8 The KDMs family comprises enzymes that are involved in the removal of methyl groups from lysine residues on histone proteins, thus regulating gene expression and chromatin remodeling.9 Since the identification of the first KDM in 2004,10 ~30 KDMs have been identified and characterized.11 KDMs are divided into 2 categories based on their catalytic mechanism: lysine-specific demethylases and JmjC (Jumonji domain–containing protein KDMs).12 Studies have shown that KDMs play a role in liver fibrosis by controlling the activation of HSCs.13–15 For instance, KDM6B has been found to improve liver fibrosis induced by carbon tetrachloride (CCl4) and bile duct ligation in mice. It does so by inhibiting the TGF-β/suppressor of mothers against decapentaplegic signaling pathway and upregulating the expression of p21 and growth arrest and DNA damage–inducible 45 (Gadd45), contributing to the suppression of extracellular matrix proteins and α-smooth muscle actin.14 Another study demonstrated that overexpression of KDM4 could synergize with sterol regulatory element–binding protein 2 (SREBP2) to activate the transcription of miR-29, which in turn inhibits the activation of HSCs.15 In a CCl4-induced liver fibrosis model, the expression of KDM3A was found to be reduced in HSCs. KDM3A overexpression induced demethylation of H3K9me2 in the peroxisome proliferator-activated receptor gamma promoter, resulting in the inhibition of HSC activation.13 Specifically focusing on KDM5B and KDM5C, a study by Schonfeld et al provided insights into their roles in ALD, revealing sex differences in their effects on alcohol-associated steatosis and fibrosis.16,17 In alcohol-fed male mice, KDM5B and KDM5C were implicated in hepatocyte nuclear factor 4 alpha (Hnf4a) downregulation, hepatocyte dedifferentiation, and an increase in fatty acid synthesis.16 In contrast, in alcohol-induced liver fibrosis in female mice, KDM5B and KDM5C promoted fibrosis by increasing the transcription of fibrosis and inflammation-related genes, such as collagen type III alpha 1 chain (Col3a1), integrin subunit alpha V (Itgav), and GABA type A receptor–associated protein-like 1 (Gabarapl1) and a decrease in genes encoding aryl hydrocarbon receptor interacting proteins only in female mice.17 However, the roles of KDM5B and KDM5C in fibrosis resolution after alcohol cessation were not fully understood until the current study.7 The study unveiled the role of KDM5 demethylases, specifically KDM5B, in driving fibrosis resolution. Knockout experiments demonstrated that the absence of Kdm5b in hepatocytes significantly enhanced fibrosis resolution.7 By utilizing single-cell ATAC-seq (scATAC-seq) analysis, the study revealed intricate crosstalk between hepatocytes and nonparenchymal cells, emphasizing the complex intercellular dynamics that orchestrate fibrosis resolution.7 The liver is a highly complex organ with various cell types, and effective communication among these cells is crucial for maintaining normal physiological functions. In the context of fibrosis formation and resolution after alcohol cessation, the communication and signaling between hepatocytes and surrounding cells, such as hepatic macrophages, play a significant role. The scATAC-seq analysis conducted in the study revealed that specific hepatocytes, particularly those in zone 2 of the liver, were responsible for fibrosis resolution.7 Moreover, motif analysis highlighted the enrichment of LXRα in macrophages as a crucial mechanism underlying fibrosis resolution.7 Interestingly, the study demonstrated that the knockout of KDM5B and KDM5C demethylases increased LXRα activity, thereby contributing to fibrosis resolution. This suggests a direct link between the epigenetic regulation by KDM5 demethylases and the activation of LXRα in macrophages, emphasizing the intricate molecular mechanisms involved in the resolution of fibrosis after alcohol cessation.7 The LXR is a nuclear receptor that plays a crucial role in the regulation of cholesterol and lipid metabolism.18 It is activated by specific oxysterols, which are oxidized derivatives of cholesterol.18 The activation of LXR induces anti-inflammatory properties by inhibiting the transcriptional induction of proinflammatory genes within immune cells, achieved through the suppression of critical transcription factors such as NF-κB, activator protein 1, or signal transducer and activator of transcription 1.18 LXR has been characterized as an inducer of lipogenesis. This occurs by upregulating the expression of key lipid metabolism genes, including SREBP1C, acetyl-CoA carboxylase, stearoyl-CoA desaturase 1, and fatty acid synthase in the liver.19 However, studies on the antifibrotic effect of LXR have produced conflicting results. Some research indicated that LXR inverse agonists suppressed fibrosis in a model of NASH,20 while the deletion of LXRα exacerbated inflammation and collagen deposition in a model of liver injury induced by CCl4. Notably, inhibiting LXR activity has been found to have a hepatoprotective effect, attenuating alcohol-induced hepatic steatosis and fibrosis.21 In the current issue of Hepatology, a novel mechanism of LXR in fibrosis resolution after alcohol cessation has been identified.7 The study revealed that LXR activity in macrophages increased due to the upregulation of various oxysterols, which was secondary to an increase in the expression of CYP27A1.7 This upregulation of CYP27A1 was a result of KDM5B ablation in hepatocytes. The increased LXR activity contributed to a pro-resolving phenotype observed in macrophages. This was evidenced by the upregulation of LXR target genes such as ATP binding cassette subfamily G member 1 (Abcg1) and Mmp, along with a reduction in the Timp1 expression.7 The study provides a novel pathway illustrating the key role of hepatocyte KDM5 in regulating fibrosis resolution through a CYP27A-oxysterol-LXR-TIMP/MMP axis.7 This adds a layer of complexity to our understanding of the intricate intercellular mechanisms involved in resolving fibrosis after the cessation of alcohol consumption. This study indeed provides valuable insights into the role of KDMs and LXRs in the resolution of ALD.7 However, there are several challenges and considerations that need to be addressed for potential therapeutic applications. First, most in vitro experiments in the study were conducted in the absence of alcohol treatment.7 This may limit the direct applicability of the findings to alcohol-associated disease models. The effects of KDMs and LXRs in the presence of alcohol may differ, and future studies incorporating alcohol exposure could provide a more comprehensive understanding. Second, the study does not address the specific functions of KDM5B in HSCs during the fibrosis resolution period. Understanding the role of KDM5B in different cell types, especially in HSCs, is crucial for a comprehensive understanding of its contributions to fibrosis dynamics. Third, targeting KDMs may impact global histone methylation, potentially influencing various cellular functions. Specificity in targeting KDMs to hepatocytes is essential to minimize systemic effects and unintended consequences on other cell types. Fourth, the effectiveness of targeting KDMs might vary depending on the stage of liver fibrosis. Determining the optimal timing for KDM-targeted therapies in different stages of ALD progression is essential for developing precise and effective interventions. Lastly, the activation of LXR, while beneficial in resolving fibrosis, may have potential drawbacks, such as elevated triglycerides and cholesterol levels. This poses a risk of dyslipidemia and potentially cardiovascular complications.22 Balancing the benefits of fibrosis resolution with potential metabolic consequences is a critical consideration. In summary, the study conducted by Schonfeld et al7 significantly advances our understanding of the intricate interplay between epigenetic regulators, nuclear receptors, and cellular signaling pathways that govern fibrosis dynamics in ALD. The identification of KDM5B and KDM5C as key players in fibrosis resolution, their impact on LXR activity, and the crosstalk between hepatocytes and nonhepatic parenchymal cells provide valuable insights. Future studies should aim to further explore diverse downstream targets of KDM5B and KDM5C, especially in nonhepatic parenchymal cells such as HSCs. Understanding the broader effects of KDM5B and KDM5C on different cell types will contribute to a more comprehensive picture of their roles in ALD pathogenesis and resolution. These findings lay the groundwork for the discovery of novel therapeutic strategies targeting fibrosis resolution in ALD. Developing interventions that specifically modulate epigenetic regulators, nuclear receptors, and cellular signaling pathways involved in fibrosis dynamics holds promise for improving treatment outcomes in patients with ALD.