The emerging roles of extracellular vesicles as intercellular messengers in liver physiology and pathology

Abstract

Extracellular vesicles (EVs) are membrane-enclosed particles released from almost all cell types. EVs mediate intercellular communication by delivering their surface and luminal cargoes, including nucleic acids, proteins, and lipids, which reflect the pathophysiological conditions of their cellular origins. Hepatocytes and hepatic non-parenchymal cells utilize EVs to regulate a wide spectrum of biological events inside the liver and transfer them to distant organs through systemic circulation. The liver also receives EVs from multiple organs and integrates these extrahepatic signals that participate in pathophysiological processes. EVs have recently attracted growing attention for their crucial roles in maintaining and regulating hepatic homeostasis. This review summarizes the roles of EVs in intrahepatic and interorgan communications under different pathophysiological conditions of the liver, with a focus on chronic liver diseases including nonalcoholic steatohepatitis, alcoholic hepatitis, viral hepatitis, liver fibrosis, and hepatocellular carcinoma. This review also discusses recent progress for potential therapeutic applications of EVs by targeting or enhancing EV-mediated cellular communication for the treatment of liver diseases.

Keywords: Communication; Extracellular vesicles; Liver; Pathology; Physiology.

Figure 1.

Circulating EVs affect liver physiology and pathology. Pathophysiological conditions in both the liver and extrahepatic tissues provoke increases in circulating EVs that often accumulate in the liver and modulate pathophysiological responses. Liver-derived circulating EVs target circulating immune cells and extrahepatic tissues to transmit their pathophysiological conditions. Systemic pathophysiological conditions including aging, inflammation, metabolic dysfunction, gut dysbiosis, injury, viral infection, and cancer aggravate liver physiology and promote pathological status through EV-mediated modulation of energy homeostasis, immune responses, disease-associated phenotypes, and cancer progression. In the same way, pathophysiological liver conditions including abnormal energy homeostasis, inflammation, injury, viral infection, fibrosis, and cancer promote pathological responses in the blood and distant organs by delivering their EVs through the circulation. Schematic illustration was created with BioRender.com. DM, diabetes mellitus; DILI, drug-induced liver injury; IRI, ischemia-reperfusion injury; NK, natural killer; CVD, cardiovascular disease; EV, extracellular vesicle.

Figure 2.

Liver-derived EVs mediate intercellular communication for regulating glucose homeostasis. EV-mediated intrahepatic and liver-involved inter-organ communications regulate metabolic homeostasis. Their EVs carry cargoes directly related to insulin sensitivity and energy metabolism. Depending on pathophysiological circumstances, they induce metabolic dysregulation, insulin resistance, and ultimately metabolic syndrome. (A) Activated HSCs deliver GLUT1 and PKM2 to KCs, LSECs and quiescent HSCs by utilizing EVs and stimulate glycolysis. (B) High-glucose stimulation enriches calpain 2 in EVs released from hepatocytes. These EVs deteriorate insulin sensitivity by cleaving insulin receptors. (C) EVs derived from miR-130a-3p-overexpressed hepatocytes alleviate energy homeostasis and insulin sensitivity in adipocytes by downregulating expression of PPARG and PHLPP2 genes. Schematic illustration was created with BioRender.com. LSECs, liver sinusoidal endothelial cells; KCs, Kupffer cells; GLUT1, glucose transporter 1; PKM2, pyruvate kinase M2; HSC, hepatic stellate cell; PHLPP2, PH domain and leucine-rich repeat protein phosphatase 2; PPARG, peroxisome proliferator-activated receptor gamma; EV, extracellular vesicle.

Figure 3.

EVs derived from extrahepatic tissues regulate hepatic glucose homeostasis. Extrahepatic tissues can deliver their pathophysiological information to the liver in the form of EVs through systemic circulation. Pathophysiological statuses related to metabolic dysfunction, such as aging and obesity increase the amount of metabolic homeostasis-disrupting molecules in these EVs and transmit metabolic dysfunctions of the donor to the liver by targeting essential factors for regulating glucose homeostasis and insulin sensitivity. (A) Aged BM-MSCs target SIRT1 gene expression in hepatocytes by transferring miR-29b-3p loaded in EVs and aggravate insulin resistance. (B) Opposed to lean ATM, obese ATM send miR-155-containing EVs to hepatocytes through circulation and repress PPARG gene expression, which is related to insulin sensitivity. (C) Gut microbial EVs are eliminated from blood by phagocytic activity of CRIg+ KCs in the liver. However, obesity reduces population of CRIg+ KCs and promotes the transport of microbial EVs containing pathogenic DNA to the liver by disrupting intestinal barrier integrity. It leads to aggravation of inflammation and insulin resistance in hepatocytes by activating cGAS/STING pathway. Schematic illustration was created with BioRender.com. BM-MSC, bone marrow mesenchymal stem cell; ATM, adipose tissue macrophages; PPARG, peroxisome proliferator-activated receptor gamma; CRIg, complement receptor of the immunoglobulin superfamily; KC, Kupffer cell; mDNA, gut microbial DNA; cGAS, cyclic GMP-AMP synthase; STING, stimulator of interferon genes; EV, extracellular vesicle.

Figure 4.

Release and uptake of EVs in pathological communications of hepatic and non-hepatic cells. EV exchanges between hepatocytes, HSCs, and immune cells are reinforced under pathological conditions induced by lipotoxic stresses and alcohol consumption. EV surface cargoes are highlighted in red color. (A) Lipotoxic stresses activate caspase 8/3 in hepatocytes by activating DR5 signaling in a ligand-independent manner, which leads to ROCK1 activation. Lipotoxicity also increases IRE1A activity by inducing ER stress. In consequence, ROCK1 and IRE1A both increase EV release from hepatocytes. Alcohol also induces caspase 3-mediated activation of ROCK1 in hepatocytes and decreases autophagic activities in hepatocytes and Kupffer cells via miR-155-mediated down-regulation of LAMP1 and LAMP2. These processes result in an increase in EV release from both hepatocytes and Kupffer cells. (B) EVs released from lipid-stimulated hepatocytes harbor VNN1 on their surface. VNN1 mediates EV uptake into LSECs through a lipid raft-dependent endocytosis, which aggravates fibrosis by promoting angiogenesis, and directly induces activation of HSCs. EV release from LSECs is promoted by fibrotic stimulation. The EVs are taken up by qHSCs via dynamin2-dependent endocytosis and worsen fibrotic conditions through HSC activation. The internalization of EVs by HSCs is mediated via fibronectin-integrin interaction. (C) Apoptotic or activated T cells in hepatitis C patients release EVs with CD11a on their surface. CD11a on the EV surface interacts with ICAM1 on aHSCs, facilitating EV internalization, which in turn alleviates fibrotic conditions. In lipotoxic conditions, EVs produced by neutrophils carry APOE on their surface. These EVs can be taken up by hepatocytes through LDLR and contribute to regression of fibrosis. Schematic illustration was created with BioRender.com. DR5, death receptor 5; ROCK1, rho-associated, coiled-coil-containing protein kinase 1; ER, endoplasmic reticulum; IRE1A, endoplasmic reticulum to nucleus signaling 1; EV, extracellular vesicle; LAMP, lysosomal-associated membrane protein; VNN1, vanin-1; qHSC, quiescent hepatic stellate cell; CD11a, integrin alpha L; aHSC, activated hepatic stellate cell; LDLR, low-density lipoprotein receptor; APOE, apolipoprotein E; LSEC, liver sinusoidal endothelial cell.