Liver Fibrosis: From Basic Science towards Clinical Progress, Focusing on the Central Role of Hepatic Stellate Cells

Abstract

The burden of chronic liver disease is globally increasing at an alarming rate. Chronic liver injury leads to liver inflammation and fibrosis (LF) as critical determinants of long-term outcomes such as cirrhosis, liver cancer, and mortality. LF is a wound-healing process characterized by excessive deposition of extracellular matrix (ECM) proteins due to the activation of hepatic stellate cells (HSCs). In the healthy liver, quiescent HSCs metabolize and store retinoids. Upon fibrogenic activation, quiescent HSCs transdifferentiate into myofibroblasts; lose their vitamin A; upregulate α-smooth muscle actin; and produce proinflammatory soluble mediators, collagens, and inhibitors of ECM degradation. Activated HSCs are the main effector cells during hepatic fibrogenesis. In addition, the accumulation and activation of profibrogenic macrophages in response to hepatocyte death play a critical role in the initiation of HSC activation and survival. The main source of myofibroblasts is resident HSCs. Activated HSCs migrate to the site of active fibrogenesis to initiate the formation of a fibrous scar. Single-cell technologies revealed that quiescent HSCs are highly homogenous, while activated HSCs/myofibroblasts are much more heterogeneous. The complex process of inflammation results from the response of various hepatic cells to hepatocellular death and inflammatory signals related to intrahepatic injury pathways or extrahepatic mediators. Inflammatory processes modulate fibrogenesis by activating HSCs and, in turn, drive immune mechanisms via cytokines and chemokines. Increasing evidence also suggests that cellular stress responses contribute to fibrogenesis. Recent data demonstrated that LF can revert even at advanced stages of cirrhosis if the underlying cause is eliminated, which inhibits the inflammatory and profibrogenic cells. However, despite numerous clinical studies on plausible drug candidates, an approved antifibrotic therapy still remains elusive. This state-of-the-art review presents cellular and molecular mechanisms involved in hepatic fibrogenesis and its resolution, as well as comprehensively discusses the drivers linking liver injury to chronic liver inflammation and LF.

Keywords: hepatic stellate cells; hepatocytes; liver fibrosis; liver fibrosis resolution; liver sinusoidal endothelial cells; macrophages; myofibroblasts.

Figure 1

Overview of the cellular and molecular mechanisms of liver fibrogenesis. During chronic liver injury, hepatocytes activate signaling via Janus kinase (JNK), Notch, osteopontin, and hedgehog and produce exosomes harboring microRNAs (miRNAs) to initiate HSC activation. Inflammation triggers KCs and recruits monocyte-derived macrophages through C-C motif chemokine receptor (CCR)9 and C-C motif chemokine ligand (CCL)2, CCl₄, and CCL25. The crosstalk between C-X3-C motif chemokine ligand 1 (CX3CL1) and C-X3-C motif chemokine receptor 1 (CX3CR1) orchestrates macrophage survival, differentiation, and polarization. KCs trigger the HSC activation by TGF-β, PDGF, and IL-1-β. Activated HSCs produce ECM proteins and secrete inflammatory chemokines CCL2, CCL3, and CX3CL1, whereby accumulating proinflammatory monocytes. HSC-originated matrix metalloproteinase (MMP) and tissue inhibitor of metalloproteinase (TIMP) contribute to ECM perpetuation, remodeling, and fibrosis. Activated HSCs lead to portal hypertension by enhancing the hepatic sinusoids’ contractility. Some molecules and pathways, including endothelin 1, TGF-β, Jak2, and the Wnt/β/catenin pathway, affect sinusoidal contractility.

Figure 2

Molecular pathways and cellular interactions involved in HSC activation and deactivation. Activated HSCs are the main effector cells during hepatic fibrosis. In the healthy liver, they metabolize and store retinoids. Upon activation by fibrogenic stimuli, quiescent HSCs transdifferentiate into myofibroblasts, lose their vitamin A, upregulate α-smooth muscle actin (αSMA), and produce collagen I. Various factors, including immune cell-derived fibrogenic molecules, growth factors, and lipopolysaccharide, as well as profibrotic lipid mediators such as lysophosphatidylinositol and lysophosphatidic acid, induce HSC activation in the course of chronic liver disease. TGF-β is the most HSC potent activator, which is produced by infiltrating lymphocytes and monocytes, Kupffer cells (KCs), and damaged hepatocytes. IL-17, produced by neutrophils and Th17 cells, sensitizes HSCs to TGF-β by upregulating TGF-β receptor II (TGF-βRII). In addition, platelet-derived growth factor (PDGF), which is produced by endothelial cells and macrophages, further promotes HSC activation. During fibrosis resolution, HSCs either die or revert to an inactive state by upregulating transcription factors such as peroxisome proliferator-activated receptor-γ (PPARγ), GATA-binding factor 4 (GATA4), GATA6, and transcription factor 21 (TCF21). NK and CD8⁺ T cells can eliminate activated HSCs by inducing apoptosis (Further abbreviations: GM-CSF, granulocyte/macrophage colony-stimulating factor; HH, hedgehog ligands; IHH, Indian Hedgehog; LPA, lysophosphatidic acid; LPI, lysophosphatidylinositol; LPS, lipopolysaccharide; miRNA, microRNA; MSR1, macrophage scavenger receptor 1; NF-κB, nuclear factor κ-light chain-enhancer of activated B cells; OPN, osteopontin; oxLDL, oxidized low-density lipoprotein; ROS, reactive oxygen species; S1P, sphingosine-1-phosphate; SHH, sonic hedgehog; TLR4, Toll-like receptor 4). Modified from reference [5].

Figure 3

The path towards liver fibrosis: Kupffer cell activation and macrophage recruitment in the chronic inflammatory microenvironment of the diseased liver. (A) Ingestion of fat-laden apoptotic hepatocytes and free cholesterol activates KCs by promoting the production of proinflammatory mediators. (B) The liver’s chronic inflammatory microenvironment recruits monocytes from the circulation, which, due to local proinflammatory signaling, differentiate into monocyte-derived KC-like inflammatory, as well as lipid-associated, macrophages. (C) Macrophage populations are the major contributors in shaping both profibrotic and antifibrotic drivers within the fibrotic niche. Relevant phenotypic markers of the macrophage populations detected in mouse models are indicated in the figure (Abbreviations: CEACAM1, carcinoembryonic antigen-related cell adhesion molecule 1; CLEC4F, C-type lectin domain family 4 member F; LAM, lipid-associated macrophage; Mac1, macrophage-1 antigen; Mar1, macrophage scavenger receptor1; MMP, matrix metalloproteinase; SAM, scar-associated macrophages; SatM, segregated nucleus-containing atypical monocytes; TGF-β, transforming growth factor-β; TNF-α, tumor necrosis factor-α; VSIG4, V-set and immunoglobulin domain containing 4).