Functional interaction between macrophages and hepatocytes dictate the outcome of liver fibrosis

Macrophages Serve as Bidirectional Regulators and Potential Therapeutic Targets for Liver Fibrosis

Liver fibrosis is a dynamic pathological process in which the structure and function of the liver abnormally change due to long-term complex inflammatory reactions and chronic liver injury caused by multiple internal and external factors. Previous studies believed that the activation of hepatic stellate cells is a critical part of the occurrence and development of liver fibrosis. However, an increasing number of studies have indicated that the macrophage plays an important role as a central regulator in liver fibrosis, and it directly affects the development and recovery of liver fibrosis. Studies of macrophages and liver fibrosis in the recent 10 years will be reviewed in this paper. This review will not only clarify the molecular mechanism of liver fibrosis regulated by macrophages but also provide new strategies and methods for ameliorating and treating liver fibrosis.

Targeting GPR65 alleviates hepatic inflammation and fibrosis by suppressing the JNK and NF-κB pathways

BACKGROUND

G-protein coupled receptors (GPCRs) are recognized as attractive targets for drug therapy. However, it remains poorly understood how GPCRs, except for a few chemokine receptors, regulate the progression of liver fibrosis. Here, we aimed to reveal the role of GPR65, a proton-sensing receptor, in liver fibrosis and to elucidate the underlying mechanism.

METHODS

The expression level of GPR65 was evaluated in both human and mouse fibrotic livers. Furthermore, Gpr65-deficient mice were treated with either bile duct ligation (BDL) for 21 d or carbon tetrachloride (CCl4) for 8 weeks to investigate the role of GPR65 in liver fibrosis. A combination of experimental approaches, including Western blotting, quantitative real-time reverse transcription‑polymerase chain reaction (qRT-PCR), and enzyme-linked immunosorbent assay (ELISA), confocal microscopy and rescue studies, were used to explore the underlying mechanisms of GPR65's action in liver fibrosis. Additionally, the therapeutic potential of GPR65 inhibitor in the development of liver fibrosis was investigated.

RESULTS

We found that hepatic macrophages (HMs)-enriched GPR65 was upregulated in both human and mouse fibrotic livers. Moreover, knockout of Gpr65 significantly alleviated BDL- and CCl4-induced liver inflammation, injury and fibrosis in vivo, and mouse bone marrow transplantation (BMT) experiments further demonstrated that the protective effect of Gpr65 knockout is primarily mediated by bone marrow-derived macrophages (BMMs). Additionally, in vitro data demonstrated that Gpr65 silencing and GPR65 antagonist inhibited, while GPR65 overexpression and application of GPR65 endogenous and exogenous agonists enhanced the expression and release of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and transforming growth factor-β (TGF-β), all of which subsequently promoted the activation of hepatic stellate cells (HSCs) and the damage of hepatocytes (HCs). Mechanistically, GPR65 overexpression, the acidic pH and GPR65 exogenous agonist induced up-regulation of TNF-α and IL-6 via the Gαq-Ca2+-JNK/NF-κB pathways, while promoted the expression of TGF-β through the Gαq-Ca2+-MLK3-MKK7-JNK pathway. Notably, pharmacological GPR65 inhibition retarded the development of inflammation, HCs injury and fibrosis in vivo.

CONCLUSIONS

GPR65 is a major regulator that modulates the progression of liver fibrosis. Thus, targeting GPR65 could be an effective therapeutic strategy for the prevention of liver fibrosis.

SKIL/SnoN attenuates TGF-β1/SMAD signaling-dependent collagen synthesis in hepatic fibrosis

The ski-related novel gene (SnoN), encoded by the SKIL gene, has been shown to negatively regulated transforming growth factor-β1 (TGF-β1) signaling pathway. However, the roles of SnoN in hepatic stellate cell (HSC) activation and hepatic fibrosis (HF) are still unclear. To evaluate the role of SnoN in HF, we combined bulk RNA sequencing analysis and single-cell RNA sequencing analysis to analyse patients with HF. The role of SKIL/SnoN was verified using liver samples from rat model transfected HSC-T6 and LX-2 cell lines. Immunohistochemistry, immunofluorescence, PCR, and western blotting techniques were used to demonstrate the expression of SnoN and its regulatory effects on TGF-β1 signaling in fibrotic liver tissues and cells. Furthermore, we constructed competitive endogenous RNA regulatory network and potential drug network associated with the SnoN gene. We identified SKIL gene as a differentially expressed gene in hepatic fibrosis. SnoN protein was found to be widely expressed in the cytoplasm of normal hepatic tissues, whereas it was almost absent in HF tissues. In the rat group subjected to bile duct ligation (BDL), SnoN protein expression decreased, while TGF-β1, collagen III, tissue inhibitor of metalloproteinase 1 (TIMP-1), and fibronectin levels increased. We observed the interaction of SnoN with p-SMAD2 and p-SMAD3 in the cytoplasm. Following SnoN overexpression, apoptosis of HSCs was promoted, and the expression of HF-associated proteins, including collagen I, collagen III, and TIMP-1, was reduced. Conversely, downregulation of SnoN inhibited HSC apoptosis, increased collagen III and TIMP-1 levels, and decreased matrix metalloproteinase 13 (MMP-13) expression. In conclusion, SnoN expression is downregulated in fibrotic livers, and could attenuate TGF-β1/SMADs signaling-dependent de-repression of collagen synthesis.

Relaxin in hepatic fibrosis: What is known and where to head?

Relaxin (RLX) is a heterodimeric, polypeptide hormone that has natural anti-fibrotic activity in many organs. During the chronic liver injury, hepatic stellate cells (HSCs) are phenotypically transformed into myofibroblasts. This process is known as activation of HSCs. Activated HSCs play a central role in hepatic fibrosis. Quiescent HSCs were shown to express low levels of RLX receptors such as RXFP1 and RXFP2. Upon chronic liver injury, HSCs are activated and express high levels of the RLX receptors. ML290, an agonist of RXFP1 has been reported to have antifibrotic effect in vitro as well as in vivo. Serelaxin, a recombinant human RLX-2 treatment has reduced hepatic fibrosis and portal hypertension in experimental models due to its vasodilation properties by inducing intrahepatic nitric oxide level. Serelaxin has also produced a neutral effect when studied against human cirrhosis-related portal hypertension in clinical trials. RLX is a potent collagen synthesis inhibitor and it has extracellular matrix (ECM) remodeling properties by promoting matrix metalloproteinases and downregulating expression of metalloproteinases inhibitors. Available reports suggest that RLX could induce ECM remodeling and suppress the profibrogenic transforming growth factor-β signaling and thereby regress hepatic fibrosis. Though RLX has natural antifibrotic activity, its antifibrotic molecular mechanisms especially in hepatic fibrosis condition are not reported. This review exclusively focuses antifibrotic effect of RLX on hepatic fibrosis.