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Hepatic stellate cell

Hepatic stellate cells (HSCs), also known as Ito cells or fat-storing cells, are liver-specific mesenchymal pericytes located in the perisinusoidal space of Disse between sinusoidal endothelial cells and hepatocytes, comprising about 5–15% of total liver cells and playing essential roles in retinoid storage, extracellular matrix (ECM) homeostasis, and liver regeneration in their quiescent state.^[1] In response to hepatic injury from toxins, viruses, alcohol, or metabolic stress, quiescent HSCs undergo activation, transitioning into proliferative, contractile myofibroblast-like cells that lose vitamin A droplets, express α-smooth muscle actin (α-SMA), and secrete excessive ECM components like collagen type I, thereby driving fibrosis as a central mechanism in chronic liver diseases ranging from steatosis to cirrhosis and hepatocellular carcinoma (HCC).^[2] First described as "star cells" by von Kupffer in 1876 and formally named by Ito in 1951 for their lipid-laden appearance, HSCs were isolated and characterized in the 1980s, with their nomenclature standardized as hepatic stellate cells in 1996 due to their stellate morphology and multifunctional nature.^[1] In the quiescent state, HSCs exhibit a spindle-shaped morphology with dendritic processes, an oval nucleus, and prominent cytoplasmic lipid droplets containing 80–90% of the body's retinyl esters, supported by ultrastructural features like rough endoplasmic reticulum and a Golgi complex for retinoid metabolism and ECM synthesis.^[1] They maintain liver architecture by regulating ECM turnover, supporting sinusoidal endothelial cell fenestrations via paracrine signals, and contributing to immune tolerance through interactions with regulatory T cells and myeloid-derived suppressor cells, while also secreting growth factors such as hepatocyte growth factor (HGF) and vascular endothelial growth factor (VEGF) to promote hepatocyte proliferation during regeneration.^[2] Metabolically, quiescent HSCs rely on pathways for retinoid storage and low-level autophagy, ensuring energy and nutrient homeostasis without inflammation.^[2] Activation of HSCs is a dynamic, two-phase process initiated by liver damage signals like reactive oxygen species (ROS), proinflammatory cytokines (e.g., TGF-β and PDGF), and Toll-like receptor ligands, leading to loss of lipid droplets, phenotypic plasticity, and reprogramming toward glycolysis, glutaminolysis, and fatty acid oxidation to fuel proliferation and migration.^[2] Recent single-cell transcriptomics has revealed HSC heterogeneity, with zonal subpopulations (e.g., portal vs. central vein) exhibiting distinct pro-regenerative or profibrogenic potentials, and reversible inactivation upon injury resolution through mechanisms like autophagy and retinoid signaling.^[2] In pathology, activated HSCs act as key effectors of fibrogenesis by cross-talking with macrophages and hepatocytes, exacerbating inflammation in conditions like nonalcoholic steatohepatitis (NASH) and promoting HCC progression via ECM remodeling and angiogenesis, though certain HSC subsets may exert protective effects through HGF-mediated tumor suppression.^[3] Their central role positions HSCs as promising therapeutic targets, with ongoing research exploring antifibrotic strategies like FXR agonists and PKM2 inhibitors to halt activation and reverse fibrosis.^[2]

Anatomy

Location and distribution

Hepatic stellate cells (HSCs) are primarily located in the perisinusoidal space of Disse, a narrow extracellular compartment situated between the basolateral surfaces of hepatocytes and the abluminal side of sinusoidal endothelial cells throughout the liver lobule.^[1] This positioning allows HSCs to form intimate associations with the hepatic microvasculature and parenchymal cells, facilitating their roles in maintaining liver architecture.^[4] In terms of distribution, HSCs constitute approximately 5-10% of the total resident cell population in the liver, with a heterogeneous pattern across the lobular zones. They exhibit higher density in the pericentral (zone 3) and midzonal regions compared to the periportal (zone 1) areas, particularly in rodent models where vitamin A-storing lipid droplets—a hallmark of quiescent HSCs—are more abundant in perivenular and midzonal locations.^[5]^[6] In humans, a slight predominance is observed in the pericentral zone, with single-cell analyses revealing distinct subpopulations such as central vein-associated HSCs enriched near zone 3 and portal vein-associated HSCs closer to zone 1, reflecting zonal heterogeneity in gene expression and positioning.^[1]^[7] HSCs interact closely with neighboring cells through elongated cytoplasmic extensions that extend along sinusoids, enabling direct apposition to sinusoidal endothelial cells, hepatocytes, and Kupffer cells. These processes support bidirectional communication and structural support within the sinusoidal niche, without extending into the sinusoidal lumen.^[1]^[8]

Cellular morphology

Hepatic stellate cells (HSCs) in their quiescent state display a distinctive star-shaped morphology, featuring a spindle-shaped cell body with long, dendritic cytoplasmic processes that extend along the liver sinusoids in the perisinusoidal space of Disse. These processes often include thorny microprojections that facilitate interactions with neighboring cells. A hallmark of quiescent HSCs is the presence of abundant lipid droplets, which occupy 20-30% of the cell volume and primarily consist of retinyl esters, storing 80-90% of the total retinoids in the liver. These lipid droplets can be visualized through techniques such as electron microscopy, which reveals their subendothelial location, or gold chloride staining, which highlights their vitamin A content.^[1]^[9] Key immunohistochemical markers for identifying quiescent HSCs include desmin, an intermediate filament protein expressed in both rodent and human cells; glial fibrillary acidic protein (GFAP), which is strongly associated with the quiescent phenotype and early activation stages; and cytoglobin, a globin family member particularly prominent in human HSCs. Additionally, reelin, an extracellular matrix glycoprotein, is expressed in quiescent HSCs and serves as a specific marker to distinguish them from other liver mesenchymal cells and myofibroblasts, as it is absent in portal fibroblasts and periductal myofibroblasts.^[9]^[10] Upon activation, HSCs undergo a profound morphological transformation, losing their lipid droplets and adopting a flattened, myofibroblast-like appearance with an enlarged cell body and reduced or altered cytoplasmic processes. This state is characterized by the proliferation of rough endoplasmic reticulum (rER) and Golgi apparatus, supporting increased protein synthesis for extracellular matrix components, along with the formation of contractile microfilament bundles. Activated HSCs robustly express α-smooth muscle actin (α-SMA), a reliable marker of their transdifferentiation into contractile, fibrogenic cells.^[1]^[9] Ultrastructurally, quiescent HSCs possess an oval or elongated nucleus, often compressed by adjacent lipid vacuoles and containing euchromatin indicative of transcriptional activity, alongside mitochondria for energy production and peroxisomes involved in lipid metabolism. In the activated form, the nucleus may appear more ruffled, with enhanced organelle development reflecting metabolic shifts, though lipid storage structures are notably diminished.^[9]^[1]

Physiology

Quiescent state functions

In the quiescent state, hepatic stellate cells (HSCs) serve as the primary reservoir for vitamin A in the body, storing 50-80% of total retinoids primarily as retinyl palmitate esters within cytoplasmic lipid droplets.^[11] This storage function is tightly regulated by retinoid-associated proteins, including cellular retinol-binding protein type I (CRBP-I) and lecithin:retinol acyltransferase (LRAT), which facilitate retinol uptake, esterification, and mobilization to maintain systemic retinol homeostasis.^[12] These cells esterify incoming retinol from dietary sources or circulation, preventing toxicity while ensuring availability for extrahepatic needs, such as vision and immune function.^[13] Quiescent HSCs also contribute to extracellular matrix (ECM) homeostasis by producing low levels of basement membrane components, including type IV collagen and laminin, which support sinusoidal structure and endothelial integrity.^[14] This synthesis is balanced by expression of matrix metalloproteinases (MMPs), such as MMP-2 and MMP-9, and their inhibitors (TIMPs), enabling controlled turnover to prevent excessive deposition or degradation in the healthy liver.^[15] Such regulated ECM dynamics maintain vascular patency and facilitate nutrient exchange between hepatocytes and blood.^[16] Quiescent HSCs support liver regeneration by secreting growth factors such as hepatocyte growth factor (HGF) and vascular endothelial growth factor (VEGF), which promote hepatocyte proliferation and angiogenesis, respectively.^[17] Furthermore, quiescent HSCs regulate liver zonation through secretion of R-spondin 3 (RSPO3), a key activator of Wnt/β-catenin signaling that establishes metabolic gradients across hepatic lobules.^[18] RSPO3 from HSCs amplifies Wnt ligand activity, promoting periportal hepatocyte differentiation for glucose uptake and zonated expression of metabolic enzymes like glutamine synthetase in pericentral regions.^[18] This paracrine mechanism ensures patterned hepatocyte functions essential for overall liver metabolism and detoxification.^[19] In terms of immune modulation, quiescent HSCs function as liver-resident antigen-presenting cells (APCs), particularly for natural killer T (NKT) cells, by expressing MHC class I and CD1d to present glycolipids and support NKT proliferation via IL-15 production.^[20] They also secrete anti-inflammatory cytokines such as IL-10, fostering immune tolerance by suppressing pro-inflammatory T cell responses and promoting regulatory T cell activity in the hepatic microenvironment.^[21] This dual role helps maintain immune privilege in the liver, preventing unwarranted inflammation during steady-state conditions.^[22]

Activation mechanisms

Hepatic stellate cells (HSCs) undergo activation in response to liver injury, transitioning from a quiescent state characterized by vitamin A storage to a proliferative, fibrogenic myofibroblast-like phenotype.^[23] This process is primarily triggered by paracrine signals from damaged hepatocytes and activated Kupffer cells, including transforming growth factor-β1 (TGF-β1), platelet-derived growth factor (PDGF), and reactive oxygen species (ROS).^[24] TGF-β1, secreted by injured hepatocytes, initiates early activation events, while PDGF from Kupffer cells and endothelial cells promotes migration and survival of HSCs.^[25] ROS, generated during oxidative stress from hepatocyte damage, further amplifies these signals by activating redox-sensitive pathways in HSCs.^[26] The transdifferentiation process involves several hallmark changes: HSCs lose their retinoid (vitamin A) droplets, a feature of their quiescent state, and begin to proliferate through PDGF receptor signaling, which stimulates cell cycle progression via mitogen-activated protein kinase (MAPK) pathways.^[23] Concurrently, there is upregulation of α-smooth muscle actin (α-SMA) and other contractility proteins, such as caldesmon and calponin, marking the shift to a myofibroblast phenotype capable of exerting contractile forces on the extracellular matrix.^[24] This proliferation is dose-dependent on PDGF isoforms, particularly PDGF-BB, and is essential for expanding the HSC population during chronic injury.^[25] Key signaling pathways orchestrate this activation. The canonical TGF-β/SMAD pathway is central to fibrogenic gene expression, where TGF-β1 binds to its receptors, leading to phosphorylation of SMAD2 and SMAD3, which translocate to the nucleus to induce transcription of collagen type I and other extracellular matrix components.^[23] Epigenetic modifications, including histone acetylation and methylation, promote the myofibroblast phenotype by altering chromatin accessibility; for instance, increased histone deacetylase activity sustains profibrotic gene expression during prolonged activation.^[26] These changes are reinforced by interactions with altered extracellular matrix stiffness, creating a feed-forward loop.^[24] Despite the progressive nature of activation, HSCs retain some plasticity, with potential for partial reversion to a quiescent-like state under resolving conditions. PPARγ agonists, such as rosiglitazone, can induce phenotypic reversal by enhancing lipid metabolism and downregulating fibrogenic genes, as demonstrated in experimental models of liver fibrosis.^[25] Additionally, interactions with natural killer (NK) cells provide resolution signals, where NK cell-derived interferon-γ promotes HSC apoptosis, limiting excessive activation during injury repair.^[23] This reversion capacity underscores the therapeutic potential of targeting these mechanisms to halt fibrosis progression.^[26]

Pathophysiology

Role in liver fibrosis

Activated hepatic stellate cells (HSCs) play a central role in driving liver fibrosis by undergoing transdifferentiation into myofibroblast-like cells, which excessively deposit extracellular matrix (ECM) components and disrupt normal matrix homeostasis. This process is triggered by profibrogenic signals such as transforming growth factor-β (TGF-β), leading to the accumulation of scar tissue in response to chronic liver injury.^[27] The imbalance between ECM synthesis and degradation by these cells results in progressive fibrosis, potentially advancing to cirrhosis if unresolved.^[26] A hallmark of activated HSCs is their heightened production of ECM proteins, including fibrillar collagens types I and III, fibronectin, and proteoglycans, which constitute up to 80% of the total fibrillar collagen in fibrotic livers. This overproduction creates a dense, stiff matrix in the space of Disse, favoring net deposition over physiological turnover and perpetuating fibrogenic signaling through mechanotransduction pathways.^[27] Concurrently, activated HSCs upregulate tissue inhibitors of metalloproteinases (TIMPs), particularly TIMP-1, which potently suppress the activity of matrix metalloproteinases (MMPs) such as MMP-2 and MMP-9, thereby inhibiting ECM degradation and exacerbating matrix accumulation.^[26] In addition to ECM modulation, activated HSCs acquire contractile properties through the expression of α-smooth muscle actin (α-SMA), enabling them to constrict and remodel the sinusoidal architecture. This α-SMA-mediated contraction promotes sinusoidal capillarization, where fenestrae in liver sinusoidal endothelial cells are lost, impairing blood flow and contributing to elevated portal pressure characteristic of advanced fibrosis.^[27] As a regulatory mechanism, activated HSCs can enter cellular senescence, which acts as a brake on excessive fibrosis by limiting their proliferative and fibrogenic potential. Senescent HSCs upregulate surface ligands such as MICA and MICB, which bind to the NKG2D receptor on natural killer (NK) cells, recruiting these immune cells to selectively eliminate the senescent HSCs and thereby attenuate fibrosis progression. Recent studies as of 2024 have explored inducing HSC senescence via targeted delivery of cGAS-STING activators to enhance immune clearance and reverse fibrosis.^[28]^[29]

Involvement in other liver diseases

Hepatic stellate cells (HSCs) play a multifaceted role in hepatocellular carcinoma (HCC), primarily as the main source of cancer-associated fibroblasts within the tumor microenvironment, where they foster a desmoplastic stroma that supports tumor progression.^[30] Activated HSCs promote angiogenesis by secreting vascular endothelial growth factor A (VEGFA), facilitating tumor vascularization and nutrient supply essential for HCC growth.^[30] They also contribute to immune evasion through expression of programmed death-ligand 1 (PD-L1), which suppresses antitumor immune responses by inhibiting T-cell activity.^[30] Furthermore, HSCs aid metastasis by remodeling the extracellular matrix and secreting factors like stromal cell-derived factor 1 (SDF-1) via CXCR4 signaling, creating a permissive niche for tumor cell invasion and dissemination.^[30] In liver regeneration, activated HSCs transiently aid repair by releasing HGF and cytokines like platelet-derived growth factor (PDGF) and transforming growth factor-β (TGF-β), promoting hepatocyte mitosis and vascular remodeling within the first week of recovery. However, prolonged activation risks fibrotic scarring through excessive ECM deposition, potentially distorting liver architecture if resolution via apoptosis or senescence fails; recent findings as of 2024 highlight PFKFB3-mediated glycolysis in HSCs as a key driver of regenerative proliferation with fibrogenic potential.^[17]^[31] In alcoholic and metabolic dysfunction-associated steatohepatitis (MASH), HSCs exacerbate disease progression by responding to lipotoxic signals, including lipid peroxidation products like oxidized low-density lipoproteins, which upregulate Toll-like receptor 4 (TLR4) and sensitize cells to TGF-β via suppression of bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI).^[32] Activated HSCs release proinflammatory cytokines such as CCL2, CCL5, IL-8, and CXCL12, recruiting macrophages and amplifying inflammation through p38 MAPK signaling and stellakine production.^[32] This cytokine milieu, combined with Hedgehog and PDGF pathways, drives sustained myofibroblast differentiation and collagen I/III deposition, accelerating transition from steatohepatitis to advanced fibrosis and cirrhosis.^[32] Autophagy in HSCs further sustains activation by facilitating lipid release and profibrogenic signaling, underscoring their central role in metabolic liver injury outcomes.^[32] During viral hepatitis caused by hepatitis B virus (HBV) or hepatitis C virus (HCV), HSCs contribute to chronic inflammation by directly responding to viral signals, such as HBV surface antigen (HBsAg) and X protein (HBx), which activate TLR/MyD88/NF-κB pathways leading to increased expression of α-smooth muscle actin (α-SMA), collagen I (COL1A1), and PDGF-B.^[33] Paracrine factors from infected hepatocytes, including TGF-β (induced by HBx/Egr-1) and high-mobility group box 1 (HMGB1) via NF-κB, further perpetuate HSC activation and ECM production.^[33] In HBV, HBx inhibits ferroptosis by binding PPARγ and promotes Th17 cell recruitment via IL-17/IL-22, fostering a profibrotic inflammatory environment that transitions to fibrosis.^[33] For HCV, core protein and exosomes containing miR-222 enhance HSC migration and contraction, while co-infection with HBV amplifies TGF-β1-mediated OCT4/Nanog signaling, driving chronic inflammation and fibrogenesis.^[33]

History and Research

Discovery and nomenclature

The discovery of hepatic stellate cells (HSCs) dates back to the late 19th century, when German anatomist Karl Wilhelm von Kupffer first observed star-shaped perisinusoidal cells in the liver using gold chloride staining techniques originally intended to visualize neural elements.^[34] These cells, termed "Sternzellen" (star cells), were initially described as residing in the space of Disse and were thought to be part of the liver's connective tissue framework, though von Kupffer later contributed to the confusion by associating similar structures with the phagocytic cells now known as Kupffer cells (liver macrophages).^[4] Over the subsequent decades, these observations were largely overlooked or misinterpreted, with the cells occasionally referenced in histological studies but not distinctly characterized.^[35] A pivotal advancement occurred in 1951, when Japanese anatomist Toshio Ito refined the gold chloride staining method to clearly delineate lipid-laden stellate cells in the livers of vitamin A-supplemented rats, highlighting their perisinusoidal location and cytoplasmic extensions that ensheath sinusoidal endothelial cells. Ito's work emphasized the cells' prominent lipid droplets, which became more evident under conditions of high vitamin A intake, distinguishing them from other liver cell types and reviving interest in their identity. This staining approach allowed for the first reliable visualization of these cells in mammalian livers, marking a key milestone in their identification as a unique population.^[9] The nomenclature of these cells evolved through the mid-20th century, reflecting their morphological and functional attributes: Ito's descriptions led to the term "Ito cells," while their lipid content prompted names like "fat-storing cells," "lipocytes," or "perisinusoidal cells," and some early works likened them to pericytes due to their encircling processes. In 1971, Keith Wake unified these concepts by demonstrating through electron microscopy that von Kupffer's Sternzellen, Ito's fat-storing cells, and other perisinusoidal descriptions referred to the same cell type, further establishing their role in vitamin A storage via fluorescence microscopy that revealed autofluorescent retinoid droplets comprising up to 80% of the body's total reserves. By the 1980s, as isolation techniques advanced and their broader physiological significance emerged in scientific literature, the term "hepatic stellate cells" (HSCs) gained prominence to encapsulate their star-like morphology and hepatic specificity. The nomenclature was formally standardized as "hepatic stellate cells" in 1996.^[36]^[37]

Recent advances

Recent advances in hepatic stellate cell (HSC) research, particularly through single-cell RNA sequencing (scRNA-seq), have unveiled significant heterogeneity among these cells, identifying distinct subpopulations such as lipid-rich quiescent HSCs and matrix-producing activated myofibroblasts.^[38] These studies reveal spatiotemporal diversity, with peri-central vein HSCs emerging as primary contributors to myofibroblast formation during injury.^[39] Furthermore, a 2024 single-cell atlas demonstrates conserved activation trajectories across various liver injuries, including chemical and metabolic insults, highlighting shared transcriptional programs that drive fibrogenesis regardless of etiology.^[40] Emerging functions of HSCs extend beyond fibrosis to include regulation of liver size, zonation, and metabolism via the RSPO3-Wnt/β-catenin signaling axis, as elucidated in a 2025 study showing HSC-derived RSPO3 modulates hepatocyte proliferation and metabolic gradients.^[18] This pathway maintains liver homeostasis by enhancing Wnt receptor expression on hepatocytes, with disruptions linked to impaired regeneration and fibrosis progression.^[41] Additionally, HSCs engage in exosome-mediated intercellular communication, transferring miRNAs and proteins to neighboring cells like macrophages and hepatocytes, which influences both homeostatic immune tolerance and disease states such as inflammation in non-alcoholic steatohepatitis (NASH). For instance, HSC-derived exosomes promote pro-fibrotic cytokine release from macrophages, amplifying extracellular matrix deposition.^[42] Therapeutic strategies targeting HSCs have advanced with HSC-specific inhibitors and agonists showing promise in reversing fibrosis. FXR agonists, such as obeticholic acid (OCA) and the non-steroidal INT-787, inhibit HSC activation by upregulating matrix-degrading enzymes like RECK, reducing steatosis and inflammation in NASH models.^[43] Senolytics, including dasatinib and quercetin combinations, selectively clear senescent activated HSCs in preclinical NASH and hepatocellular carcinoma (HCC) models, alleviating fibrosis and tumor progression by disrupting senescence-associated secretory phenotypes.^[44] Gliotoxin, a fungal metabolite, targets activated HSCs via selective apoptosis induction, demonstrating fibrosis reversal in rodent models without broad hepatotoxicity.^[45] HSCs exhibit sophisticated mechanical sensing through integrins, which detect extracellular matrix (ECM) stiffness and transduce signals promoting activation and fibrogenesis. Increased ECM rigidity, as seen in fibrotic livers, engages integrin β1 and αv pathways to activate YAP/TAZ transcription factors, driving HSC contractility and collagen production.^[46] Conceptual biomechanical models illustrate how stiffness gradients (from ~0.5 kPa in healthy liver to >10 kPa in fibrosis) reinforce a feed-forward loop, where HSC-derived matrix stiffens the microenvironment further, perpetuating disease.^[47] This mechanotransduction is a promising target, with integrin inhibitors like cilengitide showing potential to attenuate HSC responses in advanced fibrosis.^[48]

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