Regulating hepatic inflammation: pathogen-associated molecular patterns take their toll

Epithelial barrier hypothesis in the context of nutrition, microbial dysbiosis, and immune dysregulation in metabolic dysfunction-associated steatotic liver

In recent years, the prevalence of chronic liver diseases, particularly Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), has increased significantly. This upward trend is largely associated with lifestyle-related factors such as unhealthy dietary habits, physical inactivity, and various environmental influences. Among the key elements contributing to the pathogenesis of MASLD, the integrity of the intestinal epithelial barrier emerges as a critical determinant, given its central role in maintaining immune homeostasis along the gut-liver axis. Disruption of this barrier, often driven by excessive consumption of saturated fats and refined carbohydrates in combination with low dietary fiber intake, can lead to microbial dysbiosis. This imbalance in the gut microbiota triggers immune dysregulation and promotes systemic inflammation, thereby exacerbating hepatic injury. This review discusses the contribution of epithelial barrier dysfunction to the development and progression of MASLD, with a particular focus on how increased intestinal permeability may initiate and sustain chronic liver inflammation. Additionally, the influence of dietary and environmental factors on epithelial integrity, immune responses, and the inflammatory cascade is addressed. A better understanding of the complex interplay between gut barrier impairment, immune modulation, and liver pathology may offer valuable insights into MASLD pathophysiology and contribute to the development of more targeted therapeutic strategies.

Function of TREM1 and TREM2 in Liver-Related Diseases

TREM1 and TREM2 are members of the triggering receptors expressed on myeloid cells (TREM) family. Both TREM1 and TREM2 are immunoglobulin superfamily receptors. Their main function is to identify foreign antigens and toxic substances, thereby adjusting the inflammatory response. In the liver, TREM1 and TREM2 are expressed on non-parenchymal cells, such as liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, and cells which infiltrate the liver in response to injury including monocyte-derived macrophages and neutrophils. The function of TREM1 and TREM2 in inflammatory response depends on Toll-like receptor 4. TREM1 mainly augments inflammation during acute inflammation, while TREM2 mainly inhibits chronic inflammation to protect the liver from pathological changes. Chronic inflammation often induces metabolic abnormalities, fibrosis, and tumorigenesis. The above physiological changes lead to liver-related diseases, such as liver injury, nonalcoholic steatohepatitis, hepatic fibrosis, and hepatocellular carcinoma. Here, we review the function of TREM1 and TREM2 in different liver diseases based on inflammation, providing a more comprehensive perspective for the treatment of liver-related diseases.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Lipopolysaccharides may aggravate apoptosis through accumulation of autophagosomes in alveolar macrophages of human silicosis

Silica dust mainly attacks alveolar macrophages (AMs) and increases the apoptosis of AMs in silicosis patients. However, it is still unclear whether autophagy is affected. Autophagy mainly has defensive functions in response to stress, contributing to cell survival in adverse conditions, and conversely it has also been implicated in cell death. Lipopolysaccharide (LPS) induces autophagy and apoptosis in macrophages. The role of LPS in autophagy and apoptosis in AMs of silicosis patients is unknown. In this study, we collected AMs from 53 male workers exposed to silica and divided them into an observer (control) group, and stage I, II and III patient groups. We found increased levels of LC3B, SQSTM1/p62 and BECN1，whereas the phosphorylation of MTOR，and levels of LAMP2, TLR4, MYD88, TICAM1, as well as the number of lysosomes decreased with the development of silicosis. LPS stimulation triggered autophagy and increased levels of SQSTM1 in AMs. The autophagy inhibitor, 3-methyladenine (3MA), inhibited LPS-induced apoptosis in the AMs of silicosis patients. Moreover, 3MA reversed the LPS-induced decrease in BCL2 and the increase in BAX and CASP3 levels in AMs. These results suggest that autophagosomes accumulate in AMs during silicosis progression. LPS can induce the formation of autophagosomes through a TLR4-dependent pathway, and LPS may exacerbate the apoptosis in AMs. Blockade of the formation of autophagosomes may inhibit LPS-induced apoptosis via the intrinsic apoptotic pathway in AMs. These findings describe novel mechanisms that may lead to new preventive and therapeutic strategies for pulmonary fibrosis.

Vigorous, but differential mononuclear cell response of cirrhotic patients to bacterial ligands

AIM: To study the role of gram-positive and gram-negative bacteria in the pathogenesis of liver injury, specifically the activation of inflammatory mediators.

METHODS: Peripheral blood mononuclear cells of 20 out-patients were studied, 10 of them with cirrhosis. Peripheral blood mononuclear cells were isolated and exposed to lipopolysaccharide or lipoteichoic acid. CD14, Toll-like receptor 2 and 4 expression was determined by flow cytometry, and tumor necrosis factor (TNF) α, interleukin (IL)-1β, IL-6, IL-12 and IL-10 secretion in supernatants was determined by ELISA.

RESULTS: Higher CD14, Toll-like receptor 2 and 4 expression was observed in peripheral blood mononuclear cells from cirrhotic patients, (P < 0.01, P < 0.006, P < 0.111) respectively. Lipopolysaccharide and lipoteichoic acid induced a further increase in CD14 expression (P < 0.111 lipopolysaccharide, P < 0.013 lipoteichoic acid), and a decrease in Toll-like receptor 2 (P < 0.008 lipopolysaccharide, P < 0.008 lipoteichoic acid) and Toll-like receptor 4 (P < 0.008 lipopolysaccharide, P < 0.028 lipoteichoic acid) expression. With the exception of TNFα, absolute cytokine secretion of peripheral blood mononuclear cells was lower in cirrhotic patients under non-exposure conditions (P < 0.070 IL-6, P < 0.009 IL-1β, P < 0.022 IL-12). Once exposed to lipopolysaccharide or lipoteichoic acid, absolute cytokine secretion of peripheral blood mononuclear cells was similar in cirrhotic and non-cirrhotic patients, determining a more vigorous response in the former (P < 0.005 TNFα, IL-1β, IL-6, IL-2 and IL-10 lipopolysaccharide; P < 0.037 TNFα; P < 0.006 IL-1β; P < 0.005 IL-6; P < 0.007 IL-12; P < 0.014 IL-10 lipoteichoic acid). Response of peripheral blood mononuclear cells was more intense after lipopolysaccharide than after lipoteichoic acid exposure.

CONCLUSION: Peripheral blood mononuclear cells of cirrhotic patients are able to respond to a sudden bacterial ligand exposure, particularly lipopolysaccharide, suggesting that immune regulation mechanisms are still present.

Soluble inflammatory markers as predictors of liver histological changes in patients with chronic hepatitis C virus infection

Host immune response seems to be mainly responsible for the progression of liver disease among patients with hepatitis C virus (HCV) infection. Immune activation involves the release of cytokines and their receptors that can be measured in plasma samples. The study aimed to evaluate the association between plasma levels of chemokines and soluble tumor necrosis factor receptors (sTNFR) and liver histological changes among patients with chronic HCV infection. Seventy-one treatment-naive patients were included. Plasma levels of CCL2, CCL3, CCL11, CCL24, CXCL9, CXCL10, sTNFR1, and sTNFR2 were measured and liver histological findings were reviewed. Plasma levels of CXCL9, sTNFR1, and sTNFR2 were significantly associated with liver fibrosis, with higher median levels found among patients with moderate/severe fibrosis (F >or= 2) if compared to those with no or mild fibrosis (p = 0.014; p = 0.012; p = 0.009, respectively). Plasma sTNFR2 levels were significantly associated with necroinflammatory activity, with higher median levels among patients with moderate/severe activity (A >or= 2) if compared to those with no or mild activity (2.34 ng/mL vs. 1.99 ng/mL; p = 0.019). In conclusion, plasma levels of CXCL9, sTNFR1, and sTNFR2 were independently associated with liver histological changes, suggesting a role of TNF activation and Th1-type cell-mediated immune response in the pathogenesis of HCV infection.

Fibrosis and cirrhosis reversibility - molecular mechanisms

The concept that liver fibrosis is a dynamic process with potential for regression as well as progression has emerged in parallel with clinical evidence for remodeling of fibrotic extracellular matrix in patients who can be effectively treated for their underlying cause of liver disease. This article reviews recent discoveries relating to the cellular and molecular mechanisms that regulate fibrosis regression, with emphasis on studies that have used experimental in vivo models of liver disease. Apoptosis of hepatic myofibroblasts is discussed. The functions played by transcription factors, receptor-ligand interactions, and cell-matrix interactions as regulators of the lifespan of hepatic myofibroblasts are considered, as are the therapeutic opportunities for modulating these functions. Growth factors, proteolytic enzymes, and their inhibitors are discussed in detail.

Regulation of TREM expression in hepatic macrophages and endothelial cells during acute endotoxemia

Triggering receptor expressed on myeloid cells (TREM) regulates inflammatory responses to lipopolysaccharide (LPS). In these studies, we analyzed the expression of TREM in hepatic macrophages and endothelial cells which play a central role in LPS clearance. LPS administration to C3H/HeOuJ mice resulted in a rapid induction of TREM-1 and TREM-3, but a decrease in TREM-2 in liver macrophages and endothelial cells. The observation that TREM family members are detectable in endothelial cells is novel and demonstrates that their expression is not limited to myeloid cells. LPS-induced alterations in TREM expression were not evident in cells from C3H/HeJ TLR-4 mutant mice, indicating that the response is dependent on TLR-4. IL-1beta and TNFalpha upregulated TREM-1 and TREM-3 expression and suppressed TREM-2 expression in macrophages and endothelial cells. This activity involved PI3-kinase and p38 MAP kinase signaling. Interestingly, no significant differences were noted in TREM expression between wild-type and TNFR1-/- mice treated with LPS. Treatment of macrophages and endothelial cells with LPS upregulated expression of nitric oxide synthase-2 (NOS-2). This was blocked by TREM-1 Fc/fusion protein, indicating that TREM-1 mediates LPS-induced NOS-2 expression. These results suggest that TREM proteins are important in the inflammatory response of hepatic macrophages and endothelial cells to acute endotoxemia.

Role of TLR-4 in liver macrophage and endothelial cell responsiveness during acute endotoxemia

Liver macrophages and endothelial cells have been implicated in hepatotoxicity induced by endotoxin (ETX). In these studies, we analyzed the role of toll-like receptor 4 (TLR-4) in the response of these cells to acute endotoxemia. Treatment of control C3H/HeOuJ mice with ETX (3 mg/kg, i.p.) resulted in increased numbers of activated macrophages in the liver. This was associated with morphological changes in the cells and a rapid (within 3 h) induction of nitric oxide synthase-2, cyclooxygenase-2, microsomal PGE synthase-1, interleukin-1 beta and tumor necrosis factor alpha gene expression. In endothelial cells, acute endotoxemia led to increased expression of these genes, as well as 5-lipoxygenase. In contrast, liver sinusoidal cells from C3H/HeJ TLR-4 mutant mice were relatively unresponsive to ETX. Treatment of C3H/HeOuJ, but not C3H/HeJ mice with ETX, resulted in activation of transcription factors AP-1 and NF-kappaB in liver sinusoidal cells, which was evident within 3 h. Whereas in macrophages, transcription factor activation was transient, in endothelial cells, it persisted for 24 h. In C3H/HeOuJ mice treated with ETX, activation of p38 MAP kinase was also evident in macrophages and endothelial cells, and JNK kinase in macrophages. In contrast, reduced protein kinase B (AKT) was noted in macrophages. In C3H/HeJ mice, ETX administration also led to activation of p38 MAP kinase in macrophages with no effects on JNK, p44/42 MAP kinase or AKT. These studies demonstrate that liver macrophages and endothelial cells are highly responsive to acute endotoxemia. Moreover, this activity is largely dependent on TLR-4.

Clinical significance of Toll-like receptor 4 and 2 expression on the surface of peripheral blood mononuclear cells in patients with liver cirrhosis

AIM: To explore the correlation of Toll-like receptor 4, Toll-like receptors 2 expression on the surface of peripheral blood mononuclear cells with bacterial infection in patients with liver cirrhosis, and observe the effect of antibiotics on PBMC expression of TLR4 and TLR2 in cirrhosis.

METHODS: Blood cells from a total of 42 patients (30 with ascites, 12 without ascites) with liver cirrhosis and 15 normal controls were stained with fluorescent labeling anti-TLR2/anti-TLR4/anti-CD14 monoclonal antibodies. Samples were collected and analyzed for three-color immunofluorescence by flow cytometry.

RESULTS: The expression of TLR4 and TLR2 were significantly higher in patients with ascites than those in the controls (TLR4: 22.28 ± 0.80 vs 14.45 ± 3.23, P < 0.05; TLR2: 47.65 ± 0.75 vs 24.40 ± 2.77, P < 0.05), and marked difference also existed before and after treatment (TLR4: 28.58 ± 0.79 vs 12.37 ± 0.35, P < 0.05; TLR2: 47.79 ± 0.76 vs 17.22 ± 2.48, P < 0.05). TLR4 and TLR2 expression were notably increased in patients with ascites as compared with those in ones without ascites (TLR2: 25.37 ± 1.62; TLR4: 14.81 ± 0.29) (P < 0.05), and they were not significantly different between patients without ascites and normal controls (P > 0.05).

CONCLUSION: PBMC expression of TLR4 and TLR2 are up-regulated in patients with liver cirrhosis, and they can be down-regulated by antibiotics treatment.

Modulation of non-alcoholic steatohepatitis by pattern recognition receptors in mice: the role of toll-like receptors 2 and 4

Toll-like receptors (TLR) recognize pathogen-derived molecules and induce downstream activation of inflammatory pathways. Fatty liver has been shown to result in increased sensitivity to lipopolysaccharide (LPS), a TLR4 ligand. In this study, we investigated the roles of TLR2 and TLR4 in liver damage and on cytokine induction in a methionine-choline deficient (MCD) diet-induced model of nonalcoholic steatohepatitis. We found that mice with nonalcoholic fatty liver had increased liver injury and inflammatory cytokine induction after challenge with a TLR4 but not with a TLR2 ligand. TLR2 deficient mice were not protected against the development of steatohepatitis after MCD diet feeding. On the contrary, TLR2 mice had significantly higher levels of serum ALT and greater TNF-alpha levels after LPS challenge suggesting increased liver injury. This was associated with reduced production of IL-6, a cytokine with hepatoprotective effects in fatty liver. Increased liver injury in the MCD diet-fed TLR2 mice was associated with reduced baseline and LPS-induced NF-kB and PPRE binding compared to MCS controls. These results demonstrate that TLR2 deficiency results in increased liver injury in association with nonalcoholic steatohepatitis and may suggest a protective role for TLR2-mediated signals in liver injury.

Tissue Localization of Toll-Like Receptors in Biopsy Specimens of Liver from Children Infected with Hepatitis C Virus

Toll-like receptors (TLR) are important tools of innate immunity, localized mainly on cells of the immune system, but also have been shown on cells of other origin. In the current study, they have been searched in biopsy specimens of liver from children bearing chronic viral hepatitis of C type (HCV). TLR2, TLR3 and TLR4 were traced by means of polyclonal antibodies and avidin-biotin complex (ABC) immunohistochemistry. Besides, mRNA for TLR was looked for using specific primers and polymerase chain reaction. Several controls, including neutralization of primary antibody with respective blocking peptide, confirmed the specificity of the immunohistochemical reaction. All TLR tested could be visualized in a focal distribution in single hepatocytes and some cells of inflammatory infiltrates. There was no reaction whatsoever in liver samples not infected with hepatotropic virus. In molecular studies, mRNA for TLR2 and TLR4 was detected in both noninfected and hepatitis B virus-infected established cell lines of human hepatoma as well as in HCV(+) biopsy samples. These data indicate that TLR can be traced in liver cells, both at the protein and at the mRNA level. Their irregular and focal distribution in HCV(+), but not in HCV(-), liver suggests some role of TLR in the pathogenesis of chronic viral hepatitis, at least in children.