Metabolic dysfunction–associated steatohepatitis (MASH), formerly known as nonalcoholic steatohepatitis (NASH), is an advanced stage of metabolic fatty liver disease. The pathogenic mechanisms of MASH center on hepatocyte injury and the ensuing immune response within the liver microenvironment. Recent work has implicated TREM2 ⁺ macrophages in various disease conditions, and substantial induction of TREM2 ⁺ NASH-associated macrophages (NAMs) serves as a hallmark of metabolic liver disease. Despite this, the mechanisms through which NAMs contribute to MASH pathogenesis remain poorly understood. Here, we identify membrane-spanning 4-domains a7 (MS4A7) as a NAM-specific pathogenic factor that exacerbates MASH progression in mice. Hepatic MS4A7 expression was strongly induced in mouse and human MASH and associated with the severity of liver injury. Whole-body and myeloid-specific ablation of Ms4a7 alleviated diet-induced MASH pathologies in male mice. We demonstrate that exposure to lipid droplets (LDs), released upon injury of steatotic hepatocytes, triggered NAM induction and exacerbated MASH-associated liver injury in an MS4A7-dependent manner. Mechanistically, MS4A7 drove NLRP3 inflammasome activation via direct physical interaction and shaped disease-associated cell states within the liver microenvironment. This work reveals the LD-MS4A7-NLRP3 inflammasome axis as a pathogenic driver of MASH progression and provides insights into the role of TREM2 ⁺ macrophages in disease pathogenesis.

Background & Aims: SLC25A47 was initially identified as a mitochondrial HCC-downregulated carrier protein, but its physiological functions and transport substrates are unknown. We aimed to investigate the physiological role of SLC25A47 in hepatic metabolism. Approach & Results: In the treatment of hepatocytes with metformin, we found that metformin can transcriptionally activate the expression of Slc25a47, which is required for AMP-activated protein kinase α (AMPKα) phosphorylation. Slc25a47-deficient mice had increased hepatic lipid content, triglycerides, and cholesterol levels, and we found that Slc25a47 deficiency suppressed AMPKα phosphorylation and led to an increased accumulation of nuclear SREBPs, with elevated fatty acid and cholesterol biosynthetic activities. Conversely, when Slc25a47 was overexpressed in mouse liver, AMPKα was activated and resulted in the inhibition of lipogenesis. Moreover, using a diethylnitrosamine-induced mouse HCC model, we found that the deletion of Slc25a47 promoted HCC tumorigenesis and development through the activated mammalian target of rapamycin cascade. Employing homology modeling of SLC25A47 and virtual screening of the human metabolome database, we demonstrated that NAD⁺ was an endogenous substrate for SLC25A47, and the activity of NAD⁺-dependent sirtuin 3 declined in Slc25a47-deficient mice, followed by inactivation of AMPKα. Conclusions: Our findings reveal that SLC25A47, a hepatocyte-specific mitochondrial NAD⁺ transporter, is one of the pharmacological targets of metformin and regulates lipid homeostasis through AMPKα, and may serve as a potential drug target for treating NAFLD and HCC.

Nonalcoholic steatohepatitis (NASH) represents a severe stage of fatty liver disease characterized by hepatocyte injury, inflammation, and liver fibrosis. Myeloid-derived innate immune cells, such as macrophages and dendritic cells, play an important role in host defense and disease pathogenesis. Despite this, the nature of transcriptomic reprogramming of myeloid cells in NASH liver and its contribution to disease progression remain incompletely defined. In this study, we performed bulk and single-cell RNA sequencing analysis to delineate the landscape of macrophage and dendritic cell transcriptomes in healthy and NASH liver. Our analysis uncovered cell type-specific patterns of transcriptomic reprogramming upon diet-induced NASH. We identified Brain abundant membrane attached signal protein 1 (Basp1) as a myeloid-enriched gene that is markedly induced in mouse and human NASH liver. Myeloid-specific inactivation of Basp1 attenuates the severity of diet-induced NASH pathologies as shown by reduced hepatocyte injury and liver fibrosis in mice. Mechanistically, cultured macrophages lacking Basp1 exhibited diminished response to pro-inflammatory stimuli, impaired NLRP3 inflammasome activation, and reduced cytokine secretion. Together, these findings uncover Basp1 as a critical regulator of myeloid inflammatory signaling that underlies NASH pathogenesis. Export

Cellular heterogeneity of innate immune cells, such as macrophages, in the liver is a hallmark of metabolic dysfunction-associated steatohepatitis (MASH) pathogenesis. However, the mechanisms shaping liver macrophage heterogeneity and function during disease progression remain poorly understood. Control or myeloid-specific Tgfbr1 knockout mice (n = 9-12 per group) were fed a 12-week choline-deficient, amino acid-defined high-fat diet (CDA-HFD) or a 20-week GAN diet (40% fat, 22% fructose, 2% cholesterol). Liver tissue was analyzed using histopathology, quantitative PCR, immunoblotting, flow cytometry, and RNA sequencing (RNA-seq). Bulk RNA-seq (n = 3 per group) and single-nucleus RNA-seq were performed to investigate transcriptional reprogramming. Macrophage population dynamics were evaluated by flow cytometry and immunofluorescence. We identified TGF-β signaling as a crucial regulator of disease-associated expansion of Trem2⁺ and Fcrl5⁺ macrophages in MASH livers. Myeloid-specific inactivation of Tgfbr1 in mice exacerbated diet-induced MASH, with increased hepatocyte injury, inflammation, and liver fibrosis. Mechanistically, loss of TGF-β signaling in myeloid cells altered macrophage composition, marked by a reduction in Trem2⁺ and expansion of Fcrl5⁺ macrophages. Additionally, macrophages lacking Tgfbr1 exhibited gene signatures associated with inflammasome activation, cytokine signaling, cellular senescence, and immunosuppression. These changes in macrophage composition and function promoted effector T cell exhaustion and the development of MASH-associated hepatocellular carcinoma in Tgfbr1-deficient mice. These findings identify myeloid TGF-β signaling as a key driver of liver macrophage heterogeneity and polarization within the microenvironment during the progression of MASH and MASH-associated liver cancer. Our study reveals that myeloid TGF-β signaling plays a crucial role in shaping liver macrophage heterogeneity, which in turn influences the pathogenesis of metabolic liver disease. These findings are particularly important for researchers studying immune-metabolic interactions and for clinicians seeking new therapeutic strategies for liver disorders. By elucidating how TGF-β signaling regulates macrophage function, our work paves the way for targeted interventions that modulate immune responses to improve liver health. Future research should consider the potential translational applications of these findings while addressing limitations related to model systems and human variability.

Hepatocyte senescence is increasingly recognized as a pathogenic driver of metabolic dysfunction–associated steatohepatitis (MASH). Through single-nucleus transcriptomic profiling, we identified a discrete population of disease-associated hepatocytes (daHep) exhibiting enrichment for senescence markers in MASH livers. The emergence of senescent hepatocytes was associated with a marked induction of hepatic thymocyte selection associated (THEMIS) expression in both murine and human MASH. Genetic ablation of Themis , either globally or specifically in hepatocytes, resulted in significant expansion of daHep and senescent hepatocyte populations and exacerbated MASH pathology in mice. Single-nucleus transcriptomic analysis revealed a central role for THEMIS in shaping the cellular landscape of both parenchymal and nonparenchymal compartments within the MASH liver microenvironment. Conversely, adeno-associated virus–mediated overexpression of THEMIS suppressed hepatocyte senescence and attenuated diet-induced MASH. Mechanistic studies revealed that THEMIS deficiency promoted aberrant ERK phosphorylation and hepatocyte senescence. These findings establish THEMIS as a critical hepatoprotective factor that restrains hepatocyte senescence and mitigates metabolic liver disease progression.