Harnessing metabolism of hepatic macrophages to aid liver regeneration

The multifaceted roles of macrophages in the transition from hepatitis to hepatocellular carcinoma: From mechanisms to therapeutic strategies

Macrophages are central to the progression from hepatitis to hepatocellular carcinoma (HCC), with their remarkable plasticity and ability to adapt to the changing liver microenvironment. Chronic inflammation, fibrosis, and ultimately tumorigenesis are driven by macrophage activation, making them key regulators of liver disease progression. This review explores the diverse roles of macrophages in the transition from hepatitis to HCC. In the early stages of hepatitis, macrophages are essential for pathogen clearance and tissue repair. However, chronic activation leads to prolonged inflammation, which exacerbates liver damage and promotes fibrosis. As the disease progresses to liver fibrosis, macrophages interact with hepatic stellate cells, fostering a pro-tumorigenic microenvironment that supports HCC development. In hepatocarcinogenesis, macrophages contribute to tumor initiation, growth, metastasis, immune evasion, cancer stem cell maintenance, and angiogenesis. Their functional plasticity enables them to adapt to the tumor microenvironment, thereby promoting tumor progression and resistance to therapy. Targeting macrophages represents a promising strategy for preventing and treating HCC. Therapeutic approaches, including reprogramming macrophage phenotypes to enhance anti-tumor immunity, blocking macrophage recruitment and activation, and utilizing nanoparticle-based drug delivery systems, may provide new avenues for combating HCC by modulating macrophage functions and tumor microenvironment dynamics.

MSCs Suppress Macrophage Necroptosis and Foster Liver Regeneration by Modulating SP1/SK1 Axis in Treating Acute Severe Autoimmune Hepatitis

Acute severe autoimmune hepatitis (AS-AIH) is characterized by rapid progression and poor prognosis, with a current lack of effective targeted treatments. Stem cell therapy has demonstrated significant therapeutic promise across various autoimmune diseases. However, the intricate pathogenesis of AS-AIH has hindered the widespread utilization of mesenchymal stem cells (MSCs) in this domain. Herein, it is demonstrated that necroptosis, as the primary mode of cell death in AIH, is crucial in causing AS-AIH. Inflammatory macrophages are the primary cell population involved in necroptosis. Inhibition of the specificity protein 1/sphingosine kinase 1/sphingosine-1-phosphate (SP1/SK1/S1P) axis is responsible for this phenomenon, leading to excessive activation of the intrahepatic immune system and aggravating liver damage. Furthermore, the S1P/S1PR2/YAP axis is the key pathway in initiating liver regeneration during AS-AIH. S1P synthesized by hepatocytes is the primary source, and this process is also regulated by the SP1/SK1 axis. MSCs promote S1P synthesis by macrophages through the delivery of SP1, which inhibits necroptosis and synergistically enhances liver regeneration. In addition, MSCs also promote S1P synthesis in hepatocytes through the same mechanism, further aiding liver regeneration. These findings unveil the core pathogenesis of AS-AIH and provide a theoretical foundation for using MSCs as a potential targeted therapeutic modality.

The liver's dilemma: sensing real danger in a sea of PAMPs: the (arterial) sinusoidal segment theory

The liver is susceptible to viruses and bacterial infections, tumors, and sterile tissue damage, but immunological danger recognition in the liver is highly unconventional. When analyzing innate and adaptive immunity in the organ, the valid concepts that guide danger recognition and immune response in the periphery should be put aside. In the liver, the vascular anatomy is a game changer, as about 80% of the blood that percolates the organ arrives from the hepatic portal vein, draining blood rich in molecules from the intestinal flora. This 24/7 exposure to high amounts of pathogen-associated molecular pattern (PAMPs) molecules results in hepatic immunological tolerance. In the liver, dendritic, Kupffer (KC), liver sinusoidal endothelial cells (LSECs), and even hepatocytes express PD-L1, a T lymphocyte downregulatory molecule. Most cells express Fas-L, IL-10, TGF-β, low levels of co-stimulatory molecules, lack of or have low levels of MHC-I and/or MHC-II expression. Moreover, other negative regulators such as CTLA-4, IDO-1, and prostaglandin E2 (PGE2) are regularly expressed. Then, how can real danger be discerned and recognized in this sea of PAMPs? This is an open question. Here, we hypothesize that conventional immunological danger recognition can occur in the liver but in specific and minor arterial sinusoidal segments,. Then, in the portal triad, where the hepatic artery ramificates into the stroma and carries arterial blood with no gut-derived PAMPs, there is no evolutive or environmental pressure for immunosuppressive pathways, and conventional immunological danger recognition could occur. Therefore, in arterial sinusoidal segments with no sea of PAMPs, the liver could recognize real danger and support innate and adaptive immunity.

From LAL-D to MASLD: Insights into the role of LAL and Kupffer cells in liver inflammation and lipid metabolism

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a prevalent liver pathology worldwide, closely associated with obesity and metabolic disorders. Increasing evidence suggests that macrophages play a crucial role in the development of MASLD. Several human studies have shown an inverse correlation between circulating lysosomal acid lipase (LAL) activity and MASLD. LAL is the sole enzyme known to degrade cholesteryl esters (CE) and triacylglycerols in lysosomes. Consequently, these substrates accumulate when their enzymatic degradation is impaired due to LAL deficiency (LALD). This study aimed to investigate the role of hepatic LAL activity and liver-resident macrophages (i.e., Kupffer cells (KC)) in MASLD. To this end, we analyzed lipid metabolism in hepatocyte-specific (hep)Lal-/- mice and depleted KC with clodronate treatment. When fed a high-fat/high-cholesterol diet (HF/HCD), hepLal-/- mice exhibited CE accumulation and an increased number of macrophages in the liver and significant hepatic inflammation. KC were depleted upon clodronate administration, whereas infiltrating/proliferating CD68-expressing macrophages were less affected. This led to exacerbated hepatic CE accumulation and dyslipidemia, as evidenced by increased LDL-cholesterol concentrations. Along with proteomic analysis of liver tissue, these findings indicate that hepatic LAL-D in HF/HCD-fed mice leads to macrophage infiltration into the liver and that KC depletion further exacerbates hepatic CE concentrations and dyslipidemia.

Mitochondria-Regulated Information Processing Nanosystem Promoting Immune Cell Communication for Liver Fibrosis Regression

Liver fibrosis is a coordinated response to tissue injury that is mediated by immune cell interactions. A mitochondria-regulated information-processing (MIP) nanosystem that promotes immune cell communication and interactions to inhibit liver fibrosis is designed. The MIP nanosystem mimics the alkaline amino acid domain of mitochondrial precursor proteins, providing precise targeting of the mitochondria. The MIP nanosystem is driven by light to modulate the mitochondria of hepatic stellate cells, resulting in the release of mitochondrial DNA into the fibrotic microenvironment, as detected by macrophages. By activating the STING signaling pathway, the developed nanosystem-induced macrophage phenotype switches to a reparative subtype (Ly6Clow) and downstream immunostimulatory transcriptional activity, fully restoring the fibrotic liver to its normal tissue state. The MIP nanosystem serves as an advanced information transfer system, allowing precise regulation of trained immunity, and offers a promising approach for effective liver fibrosis immunotherapy with the potential for clinical translation.

Macrophage plasticity: signaling pathways, tissue repair, and regeneration

Macrophages are versatile immune cells with remarkable plasticity, enabling them to adapt to diverse tissue microenvironments and perform various functions. Traditionally categorized into classically activated (M1) and alternatively activated (M2) phenotypes, recent advances have revealed a spectrum of macrophage activation states that extend beyond this dichotomy. The complex interplay of signaling pathways, transcriptional regulators, and epigenetic modifications orchestrates macrophage polarization, allowing them to respond to various stimuli dynamically. Here, we provide a comprehensive overview of the signaling cascades governing macrophage plasticity, focusing on the roles of Toll-like receptors, signal transducer and activator of transcription proteins, nuclear receptors, and microRNAs. We also discuss the emerging concepts of macrophage metabolic reprogramming and trained immunity, contributing to their functional adaptability. Macrophage plasticity plays a pivotal role in tissue repair and regeneration, with macrophages coordinating inflammation, angiogenesis, and matrix remodeling to restore tissue homeostasis. By harnessing the potential of macrophage plasticity, novel therapeutic strategies targeting macrophage polarization could be developed for various diseases, including chronic wounds, fibrotic disorders, and inflammatory conditions. Ultimately, a deeper understanding of the molecular mechanisms underpinning macrophage plasticity will pave the way for innovative regenerative medicine and tissue engineering approaches.

Treatment with β-Adrenoceptor Agonist Isoproterenol Reduces Non-parenchymal Cell Responses in LPS/D-GalN-Induced Liver Injury

There is an increasing evidence indicating the involvement of the sympathetic nervous system (SNS) in liver disease development. To achieve an extensive comprehension of the obscure process by which the SNS alleviates inflammatory damage in non-parenchymal liver cells (NPCs) during acute liver failure (ALF), we employ isoproterenol (ISO), a beta-adrenoceptor agonist, to mimic SNS signaling. ISO was administered to C57BL/6J mice to establish an acute liver failure (ALF) model using LPS/D-GalN, which was defined as ISO + ALF. Non-parenchymal cells (NPCs) were isolated from liver tissues and digested for tandem mass tag (TMT) labeled proteomics to identify differentially expressed proteins (DEPs). The administration of ISO resulted in a decreased serum levels of pro-inflammatory cytokines, e.g., TNF-α, IL-1β, and IL-6 in ALF mice, which alleviated liver damage. By using TMT analysis, it was possible to identify 1587 differentially expressed proteins (DEPs) in isolated NPCs. Notably, over 60% of the DEPs in the ISO + ALF vs. ALF comparison were shared in the Con vs. ALF comparison. According to enrichment analysis, the DEPs influenced by ISO in ALF mice were linked to biological functions of heme and fatty acid metabolism, interferon gamma response, TNFA signaling pathway, and mitochondrial oxidation function. Protein-protein interaction network analysis indicated Mapk14 and Caspase3 may serve as potentially valuable indicators of ISO intervention. In addition, the markers on activated macrophages, such as Mapk14, Casp1, Casp8, and Mrc1, were identified downregulated after ISO initiation. ISO treatment increased the abundance of anti-inflammatory markers in mouse macrophages, as evidenced by the immunohistochemistry (IHC) slides showing an increase in Arg + staining and a reduction in iNOS + staining. Furthermore, pretreatment with ISO also resulted in a reduction of LPS-stimulated inflammation signaling markers, Mapk14 and NF-κB, in human THP-1 cells. Prior treatment with ISO may have the potential to modify the biological functions of NPCs and could serve as an innovative pharmacotherapy for delaying the pathogenesis and progression of ALF.