Functional Roles of Chemokine Receptor CCR2 and Its Ligands in Liver Disease

Abstract

Chemokines are a family of cytokines that orchestrate the migration and positioning of immune cells within tissues and are critical for the function of the immune system. CCR2 participates in liver pathology, including acute liver injury, chronic hepatitis, fibrosis/cirrhosis, and tumor progression, by mediating the recruitment of immune cells to inflammation and tumor sites. Although a variety of chemokines have been well studied in various diseases, there is no comprehensive review presenting the roles of all known chemokine ligands of CCR2 (CCL2, CCL7, CCL8, CCL12, CCL13, CCL16, and PSMP) in liver disease, and this review aims to fill this gap. The introduction of each chemokine includes its discovery, its corresponding chemotactic receptors, physiological functions and roles in inflammation and tumors, and its impact on different immune cell subgroups.

Keywords: CCL2; CCR2; PSMP; chemokine; hepatocellular carcinoma; macrophage.

Figure 1

Schematic representation of signaling pathways in the liver of CCR2 and its ligands. (A) CCL2 binds to CCR2 and activates JAK2, triggering several downstream pathways, such as STAT5 and p38MAPK, which enhance vascular permeability, promote cell migration and invasion, favor cytokine production; CCL2 binds to CCR2 and activates the Hh signaling pathway, and increases the expression of Smo and Gli-1, which induce cell invasion and EMT; CCL2 binds to CCR2 and activates the PI3K/Akt/mTORC1 pathway, promoting cell survival and inhibiting autophagosome formation and cell apoptosis. (B) CCL7 binds to CCR2 and activates the TGF-β/Smad signaling pathway, which promotes cell invasion and EMT; (C) CCL8 binds to CCR2 and activates the Ras/Raf/MEK/ERK pathway, enhancing cell growth and differentiation and promoting angiogenesis. (D) CCL16 binds to CCR2 and activates PI3K/AKT/GSK3β signaling, which results in GSK3β proteasomal degradation and enhances β-catenin stability. AKT, protein kinase B; APC, adenomatous polyposis coli; CK1, casein kinase 1; EMT, epithelial-mesenchymal transition; ERK, extracellular signal-regulated kinase; Hh, hedgehog; Gli, glioblastoma; GSK3β, glycogen synthase kinase-3 beta; JAK2, Janus kinase 2; MAPK, mitogen-activated protein kinase; mTOR, mammalian target of rapamycin; MEK, mitogenic effector kinase; PI3K, phosphatidylinositol 3-kinase; Smo, smoothened; STAT, signal transducer and activator of transcription protein. (Figure created with BioRender.com).

Figure 2

Involvement of the network of CCR2 and its ligands in regulation of immune mechanisms during liver injury, fibrosis and hepatocarcinogenesis. Sophisticated experimental mouse models of liver injury, fibrosis, and HCC revealed the complex interplay of different chemokines and liver cells. In the initial phase, upon hepatocyte injury, DAMPs activate KCs that in turn secrete inflammatory cytokines (TNF-α, IL-1β), which contribute to further hepatocyte injury and release chemokines including, CCL2, CCL7, CCL16, and PSMP. CCL2, CCL7, and PSMP promote the recruitment of CCR2⁺ circulating monocytes into the injured liver, where they develop into inflammatory, angiogenic, and fibrogenic IMs. During chronic injury, these macrophages activate HSCs by secreting TGF-β and PDGF to become collagen-producing myofibroblasts responsible for excessive extracellular matrix (ECM) synthesis and deposition, promoting liver fibrosis development. In addition, CCL16 directly inhibits the activation of HSCs, and PSMP directly promotes the activation of HSCs during liver fibrosis. In the HCC tumor microenvironment, TANs and CAFs could recruit TAMs by secreting CCL2 and CCL7. TAMs, specifically the M2 phenotype, promoted tumor growth, invasiveness, and metastasis by suppressing CD8⁺ T lymphocyte responses and producing PD-L1, IL-6, G-CSF, TNF-α, TGF-β, and VEGF. Additionally, CCL2 could directly promote HCC progression through activation of the Hh pathway. CCL7 could directly enhance the mesenchymal phenotype of HCC cells and facilitate their migration and invasion through the TGF-β signaling pathway. CAF, cancer-associated fibroblast; DAMP, danger-associated molecular pattern; ECM, extracellular matrix; G-CSF, granulocyte-colony stimulating factor; HCC, hepatocellular carcinoma; HSC, hepatic stellate cell; IM, infiltrating macrophage; KC, Kupffer cell; LSEC, liver sinusoidal endothelial cell; PDGF, platelet-derived growth factor; PD-L1, programmed death-ligand 1; PSMP, PC3-secreted microprotein; TAM, tumor-associated macrophage; TAN, tumor-associated neutrophil; TNF-α, tissue necrosis factor α; TGF-β, transforming growth factor β; VEGF, vascular endothelial growth factor. (Figure created with BioRender.com).

Figure 3

Expression and Kaplan-Meier survival curves of CCL7, CCL8, CCL13, and CCL16 in HCC based on TCGA. (A, C, E, G) The mRNA expression of CCL7, CCL8, CCL13, and CCL16 in HCC tissues was determined by RNA-seq data from the TCGA LIHC dataset. (B, D, F, H) The correlation of CCL7, CCL8, CCL13, and CCL16 mRNA expression levels with overall survival time was determined by RNA-seq in the TCGA LIHC dataset. TCGA, The Cancer Genome Atlas. (*P < 0.05; ***P < 0.001).

Figure 4

ScRNA-seq analysis of human liver. (A) t-SNE projection showing a reference map of 12 liver cell clusters with established cell-specific marker genes in human liver. (B, C) t-SNE plots showing relative distribution of CCL2, CCL7, CCL8, CCL13, CCL16 and PSMP in the 12 identified cell clusters in human healthy and cirrhotic liver and its statistical summary. scRNA-seq: Single-cell RNA sequencing; t-SNE: t-distributed stochastic neighbor embedding.