biomedicines Review Extra- and Intra-Cellular Mechanisms of Hepatic Stellate Cell Activation Yufei Yan †, Jiefei Zeng †, Linhao Xing and Changyong Li * Department of Physiology, School of Basic Medical Sciences, Wuhan University, Wuhan 430017, China; [email protected] (Y.Y.); [email protected] (J.Z.); [email protected] (L.X.) * Correspondence: [email protected] † These authors contributed equally to this work. Abstract: Hepatic fibrosis is characterized by the pathological accumulation of extracellular matrix (ECM) in the liver resulting from the persistent liver injury and wound-healing reaction induced by various insults. Although hepatic fibrosis is considered reversible after eliminating the cause of injury, chronic injury left unchecked can progress to cirrhosis and liver cancer. A better understanding of the cellular and molecular mechanisms controlling the fibrotic response is needed to develop novel clinical strategies. It is well documented that activated hepatic stellate cells (HSCs) is the most principal cellular players promoting synthesis and deposition of ECM components. In the current review, we discuss pathways of HSC activation, emphasizing emerging extra- and intra- cellular signals that drive this important cellular response to hepatic fibrosis. A number of cell types and external stimuli converge upon HSCs to promote their activation, including hepatocytes, liver sinusoidal endothelial cells, macrophages, cytokines, altered ECM, hepatitis viral infection, enteric dysbiosis, lipid metabolism disorder, exosomes, microRNAs, alcohol, drugs and parasites. We also discuss the emerging signaling pathways and intracellular events that individually or synergistically drive HSC activation, including TGFβ/Smad, Notch, Wnt/β-catenin, Hedgehog and Hippo signaling Citation: Yan, Y.; Zeng, J.; Xing, L.; pathways. These findings will provide novel potential therapeutic targets to arrest or reverse fibrosis Li, C. Extra- and Intra-Cellular and cirrhosis. Mechanisms of Hepatic Stellate Cell Activation. Biomedicines 2021, 9, 1014. Keywords: hepatic fibrosis; hepatic stellate cell; myofibroblast; signal pathway https://doi.org/10.3390/ biomedicines9081014 Academic Editor: Giovanni Squadrito 1. Introduction Liver fibrosis is the pathologic sequela of chronic repetitive injury and is a reversible Received: 19 July 2021 healing response in response to acute or chronic cell injury. Further development of liver Accepted: 11 August 2021 Published: 14 August 2021 fibrosis leads to cirrhosis and even liver cancer. Various factors can cause liver fibrosis, the main risk factors identified at present include viral infection, alcoholism, obesity-related Publisher’s Note: MDPI stays neutral steatohepatitis and so on [1–3]. Cirrhosis is a major cause of morbidity and mortality with regard to jurisdictional claims in globally, imposing a heavy health burden on many countries. Globally, cirrhosis currently published maps and institutional affil- causes 1.16 million deaths and is the 11th most common cause of death each year [4]. iations. Cirrhosis imposes a huge economic burden in the United States, with estimated annual direct costs of more than USD 2 billion and indirect costs of more than USD 10 billion [5]. Hepatic stellate cell (HSC) activation represents a critical event in fibrosis [6,7]. In normal liver, HSCs exist in a quiescent non-proliferative state, having a star-like shape with intracellular lipid droplet storage containing vitamin A as retinyl palmitate [8]. HSCs are a Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. type of resident non-mesenchymal cells that have features of both resident fibroblasts (em- This article is an open access article bedded in normal stroma) and pericytes (endothelial cells attached to capillaries). Locating distributed under the terms and in the space of Disse, HSCs are a major producer of extracellular matrix (ECM) [8–10], conditions of the Creative Commons which accounts for approximately 15% of total resident cells and one third of the total Attribution (CC BY) license (https:// nonparenchymal cells in the normal human liver [11]. Pathological, toxic, metabolic or creativecommons.org/licenses/by/ viral diseases lead to liver cell damage and immune cell infiltration, activating the transd- 4.0/). ifferentiation of HSCs to myofibroblasts, which is known as “activation”. It is generally Biomedicines 2021, 9, 1014. https://doi.org/10.3390/biomedicines9081014 https://www.mdpi.com/journal/biomedicines Biomedicines 2021, 9, 1014 2 of 18 Biomedicines 2021, 9, 1014 2 of 17 the transdifferentiation of HSCs to myofibroblasts, which is known as “activation”. It is believedgenerally that believed HSCs that are theHSCs main are sourcethe main of so myofibroblastsurce of myofibroblasts during hepaticduring hepatic fibrosis fibro- and are independentsis and are independent of the source of ofthe damage source of [12 damage,13]. In [12,13]. chronic In liver chronic disease, liver disease, the imbalance the im- be- tweenbalance the between pro-fibrogenic the pro-fibrogenic and anti-fibrogenic and anti-f mechanismsibrogenic mechanisms leads to continuous leads to continuous activation of proliferating,activation of contractile,proliferating, and contractile, migrating and myofibroblasts, migrating myofibroblasts, resulting in excessiveresulting in production exces- ofsive ECM. production Large amounts of ECM. Large of ECM amounts deposited of ECM in deposited the liver leadin the to liver liver lead fibrosis to liver [ 14fibrosis]. Here, we[14]. review Here, extra-we review and intra-cellularextra- and intra-cellular mechanisms mechanisms of HSC activation,of HSC activation, emphasizing emphasiz- recent emerginging recent cellular emerging and cellular molecular and molecular signals that signals trigger that thistrigger important this important cellular cellular response re- to liversponse injury. to liver injury. 2.2. Extracellular Extracellular FactorsFactors of HSC Activation Activation TheThe extracellular extracellular factorsfactors thatthat promote HSC ac activationtivation have have been been identified identified as as stimu- stimula- tionlation of variousof various cell cell types, types, altered altered extracellularextracellular matrix, matrix, enteric enteric dysbiosis, dysbiosis, chronic chronic infection infection ofof hepatitis hepatitis virus, lipid lipid metabolism metabolism disorder disorder,, exosomes, exosomes, microRNA microRNA and other and otherfactors factors in- includingcluding alcohol, alcohol, drugs drugs and and parasites parasites (Figure (Figure 1).1 ). FigureFigure 1. Extracellular1. Extracellular factors factors promoting promoting HSC HSC activation.activation. Extracellu Extracellularlar factors factors including including stimulation stimulation of ofvarious various cell cell types, types, cyto-kines,cyto-kines, altered altered ECM, ECM, hepatitis hepatitis viral viral infection, infection, entericenteric dysbiosis,dysbiosis, lipid lipid metabolism metabolism disorder, disorder, exosomes, exosomes, alcohol, alcohol, drugs drugs (MTX, APAP) and schistosome promote or inhibit the activation of HSCs through production of various cytokines and (MTX, APAP) and schistosome promote or inhibit the activation of HSCs through production of various cytokines and other signaling molecules. The characteristics of A-HSCs include proliferation, contractility, fibrogenesis, inflammatory othersig-naling, signaling loss molecules. of retinoid The and characteristicsenhanced ECM of production. A-HSCs include HSC, hepatic proliferation, stellate cell; contractility, DAMPs, damage-associated fibrogenesis, inflammatory molec- sig-naling,ular patterns; loss of Hh, retinoid Hedgehog and enhancedsignaling; ECMIL-33, production.interleukin-33; HSC, M-CSF, hepatic macrophage stellate cell; colony-stimulating DAMPs, damage-associated factor; TGFβ,molecular Trans- patterns;forming Hh, growth Hedgehog factorsignaling; β; PDGFβ, IL-33, Platelet interleukin-33; derived growth M-CSF, factor macrophage β; PAMP, pathogen-associated colony-stimulating molecular factor; TGF patterns;β, Transforming TLRs, growthToll-like factor receptors;β; PDGF MMP,β, Platelet matrix derived metalloproteinases; growth factor TNF,β; PAMP, tumor pathogen-associated necrosis factor; ROS, molecular reactive oxygen patterns; species; TLRs, MCP1, Toll-like receptors;monocyte MMP, chemo-tactic matrix metalloproteinases; protein-1; NO, nitric TNF, oxide; tumor IFNγ necrosis, interferon factor; γ; NK, ROS, natural reactive killer; oxygen NKT, species; natural MCP1,killer T; monocyte MTX, methotrexate; APAP, acetaminophen; hi, high expression; low, low expression; int, intermediate expression. chemo-tactic protein-1; NO, nitric oxide; IFNγ, interferon γ; NK, natural killer; NKT, natural killer T; MTX, methotrexate; APAP, acetaminophen; hi, high expression; low, low expression; int, intermediate expression. Biomedicines 2021, 9, 1014 3 of 17 2.1. Hepatocytes In response to injury, hepatocytes change their gene expression and secretion profile, and thus affect HSC activation. Damage-associated molecular patterns (DAMPs) released by injured hepatocytes might directly or indirectly promote HSC activation. Nucleotide binding oligomerization domain-like receptors 3 (NLRP3) is one of the main components of inflammasomes and the downstream targets of DAMPs. Mice with the constitutively active mutant NLRP3 develop severe liver inflammation with pyroptotic hepatocyte death and HSC activation [15]. When the liver is damaged,