Written By Nick Trompeter, Ph.D., Cole Toohey, Payton Olson, Peter Lee, and Ken Dorko
Figure 1: Hepatic Stellate cells from donor lot HL230009SC, a MASH/MASLD donor, express αSMA (red), a marker of activation, and Glial Fibrillary acidic protein (green).
Functional Physiology of the Liver: Hepatic Stellate Cells, more than Vitamin A storage
Situated within the space of Dissé reside the mesenchymal hepatic stellate cells (HSCS). First discovered by von Kupffer in 1876, Kupffer noted that HSCs contain large amount of fat droplets. Indispensable to the human body, the fat droplets described by Kupffer act as the predominant (~80%) storage mechanisms of vitamin A within the human body.(54) HSCs store vitamin A as retinoids, housed within vacuoles of these morphologically star-like cells. Under normal conditions, HSCs maintain a quiescent state, exemplified by low expression of α-smooth muscle actin (αSMA) and high expression of brain-derived neurotrophic factor, Glial Fibrillary acidic protein (GFAP), Peroxisome proliferator-activated receptor gamma (PPAR-γ), and platelet-derived growth factor receptors α+β (PDGFRβ and PDGFRα).(51) HSCs integrate the paracrine signals from hepatocytes, LSECs, and KCs during injurious events, connecting signals from both sides of the space of Dissé. To sense disturbances in liver homeostasis, star-like protrusions, from which the stellate cells derive their name, extend to the abluminal space, and wrap around LSECs to promote direct cell-cell interactions.
During liver injury, either through hepatotoxic agents or damage to LSECs, HSCs undergo myofibroblastic differentiation to an active state. Activation of HSCs results in stimulation of fibroblastic markers such as αSMA, GFAP, and fibroblast activation protein (Figure 1). In response to injury, release of latent transforming growth factor-β (TGF-β) from the ECM activates HSCs. This activation induces the deposition of large quantities of fibrillary ECM proteins into the space of Dissé to create a fibrotic niche. While the deposition of collagens (type I and III) generates fibrotic lesions under chronic liver injury conditions, deposition of mitogenic factors embedded within the ECM provides the capacity of the liver to regenerate during acute injury.(52) Highlighting the essential role of HSCs to promote hepatocyte proliferation during liver regeneration, depletion of murine HSCs with gliotoxin increases hepatocyte necrosis, impairs hepatic regeneration, and decreases survival in an acetaminophen injury model.(53)
As a mesenchymal cell of the liver, HSCs mediate important cross-functional signals between other NPCs, hepatocytes, cholangiocytes, and immune cells to maintain homeostatic equilibrium and respond to liver perturbation. The synthesis of hepatocyte growth factor, epithelial growth factor, Wnt, TGF-α, and PDGF by HSCs acts as mitogens to stimulate hepatoblast proliferation (hepatocyte progenitors) during liver development and regeneration.(54) Quiescent HSCs secrete a multitude of paracrine signals, including lipoproteins, cytokines, and matrix metalloproteases (MMPs). Apolipoprotein E (ApoE) and prostaglandins produced by HSCs have physiological consequences in both the hepatic and cardiovascular systems. ApoE is a lipoprotein responsible for the transport and metabolism of lipids within plasma. Mice deficient in ApoE experience more extensive steatosis when compared to WT-controls.(55) Furthermore, HSCs secrete prostaglandins in both their quiescent and active states, which regulate hepatic metabolism and act as neural-mediated vasoactive agents.(54)
The importance of HSCs within the liver extends beyond the production of mitogens. HSCs interact directly and indirectly with the immune system. HSCs may act as antigen presenting cells, recognizing various xenobiotics through the expression of TLRs on their membranes. Similar to KCs, HSCs express MHC I and MHC II on their plasma membranes, concomitantly with T-cell co-stimulatory proteins CD40, CD80 and CD86 when activated.(56) Furthermore, the secretion of cytokines and chemokines by HSCs recruit leukocytes during hepatic insult. The production of pro-inflammatory IL-17, IL-6, CCL-2, and various CXCL chemokines by HSCs act to transduce and amplify signals from the immune system. Conversely, quiescent HSCs exert a tolerigenic effects through the interaction with Treg, while activated HSCs express programmed death ligand-1 to suppress T-cell activity.(54) Furthermore, HSCs increase production of IL-10 and follistatin, two anti-inflammatory cytokines, to promote negative feedback during inflammatory assaults on the liver.
Accumulating evidence supports the hypothesis that HSCs actively engage in xenobiotic metabolism. HSCs express multiple isoforms of Cytochrome p450 enzymes (CYP), including CYP2C11, CYP3A2, CYP2D1, and CYP2S1, in addition to alcohol dehydrogenase. The ability to metabolize xenobiotics appears to act as a sensor for oxidative stress and pH with the liver. CYP enzymes may provide insight into whether HSCs undergo myofibroblast differentiation in the presence of the pro-fibrogenic TGF-β.(57)
Functional Physiology of the Liver: Hepatocytes, the conductor of the orchestra.
So far, we have covered the functional physiology and the integral role of nonparenchymal cells in defense of the human body from xenobiotics and other foreign particles have been discussed in depth. However, between 70% and 85% of the liver volumes derives from the hepatic parenchymal cells, hepatocytes. Hepatocytes are epithelial cells that reside at the base of the space of Dissé, forming a unique polarization of their basolateral and apical membranes. The basolateral membrane of hepatocytes faces the lumen of the sinusoid, while the apical membrane forms between neighboring hepatocytes, creating canaliculi in which hepatocytes secrete bile (Appearing as the white junctions in Figure 2,). As the predominant cell for metabolism and detoxification within the body, hepatocytes are integral for physiological homeostasis.
Figure 2: Mosaic Cell Sciences' primary human hepatocytes display the formation of bile canaliculi 12 hours post-plating, indicating polarization of basolateral and apical membranes/
The portal vein and the hepatic artery supply blood from the intestines and the heart, respectively, to provide oxygen, nutrients, and metabolites from digestion to hepatocytes. As the blood drains from the periphery to the center of the liver sinusoid, collecting in the central vein, hepatocytes interact with the blood plasma components that enter the space of Dissé. Bridging from the portal to the central vein, zonation of the liver provides functional hepatocyte heterogeneity, which results in three distinct zones of the lobule: the periportal zone (1), the mid-lobular zone (2), and the pericentral zone (3). Within each zone, hepatocytes differentially express specific enzymes and transporterss, while performing different functions due to the variation of biochemical and biophysical factors as blood moves from the periportal to the pericentral zones.(58)
At the periphery, Zone 1 contains the highest concentration of oxygen and nutrients (such as glucose), with inverse (hypoxia and nutrient depletion) established in the pericentral zone. Hepatocytes within Zone 1 have increased oxygen uptake, gluconeogenesis, oxidation, carbohydrate metabolism, sulfation, and glutathione, when compared to pericentral hepatocytes. Conversely, pericentral hepatocytes function to promote glycolysis, lipogenesis, bile acid synthesis, and glutamine synthesis by glutamine synthetase.(59) While determining the function and expression profiles of pericentral and periportal hepatocytes has been extensively studied, the recent utilization of laser-capture microdissection and spatial transcriptomic mapping has allowed for clarity into the function of mid-lobular (intermediate) hepatocytes. Using spatial transcriptomics, Sun and colleagues determined that mid-lobular hepatocytes act as a reservoir to replenish the hepatocyte pool, as highlighted by their high proliferative capacity and low metabolic function.(60,61) The heterogeneity of hepatocyte function within each zone creates an assembly line in which periportal hepatocytes metabolize nutrients and hormones, which then act as substrates for additional metabolism by pericentral hepatocytes. Furthermore, creation of a concentric ring of 10-15 hepatocyte long assembly line within the distinct zones avoids competition of resource available between hepatocytes.
Periportal hepatocytes, in addition to oxidation, gluconeogenesis, and fatty acid metabolism, express the highest amounts or complement receptors and generate ATP for downstream hepatocytes. The expression of complement receptors allows hepatocytes to recognize foreign substances as they enter the liver lobule. Compartmentalization of triglyceride synthesis and amino acids from periportal cells regulates the energy metabolism during fasting and feeding cycles. Meanwhile, in response to fasting, pericentral hepatocytes utilize triglycerides and amino acids, produced by the tricarboxylic acid cycle in periportal cells, to allow for glycolysis to proceed in Zone 3 hepatocytes.(61)
While periportal hepatocytes initiate metabolism of nutrients, pericentral hepatocytes provide xenobiotic metabolism, as evidenced by their high expression of Cytochrome P450 enzymes (CYP)within zone 3. Furthermore, hepatotoxins (like acetaminophen) induce hepatonecrosis near the central vein, indicating metabolism of the drug by hepatocytes within this zone. When developing drugs, determining the spatial metabolism of the compound bears important consideration to avoid drug induced liver injury (DILI), bioactivity of the drug, and excretion of the drug and its metabolites. Various isoforms of CYPs exist, with the subfamilies of CYP1, CYP2, and CYP3 accounting for approximately 80% of xenobiotic metabolism in the body. Known as the Phase I reactions, CYP enzymes add reactive groups onto xenobiotics (-hydroxyl, -carboxyl, -amines, and -thiols) to either oxidize, hydrolyze, or reduce the compound/molecule. After the initial biotransformation of xenobiotics by CYP enzymes, Phase II reactions further transform the drug metabolite with the addition of hydrophilic groups (methylation, glucuronidation, acetylation, sulfation, and conjugation to glutathione or amino acids). Metabolites generated during Phase II reactions become inactivated, with enhanced water solubility to allow for excretion from the system. Further metabolism of drugs occurs during Phase III reaction. Hepatocytes utilizer transporters such as the ATP-binding cassette (ABC), organic transporting polypeptide, organic cationic transports, solute-carrier transporters, or bile salt efflux pumps uptake and efflux of drugs and/or metabolites, making these proteins important for the study of drug efficacy and pharmacotherapy.(58,62)
Beyond their role as the predominant drug metabolizer within human physiology, hepatocytes orchestrate the secretion of crucial proteins for physiologic homeostasis. The abundance of protein modifying and secretory golgi proximal to the basolateral membranes of hepatocytes indicates their importance in protein production. Hepatocytes are the only cell within the body that secrete albumin, functioning to maintain oncotic pressure within the circulatory system, producing greater than 10 grams of albumin daily.(63) In addition to the secretion of albumin, hepatocytes synthesize the majority of coagulation factors with the exception of Factors VIII, III, IV. Furthermore, hepatocytes act as cellular iron sensors within the liver via the production of hepcidin during iron loading of plasma. Hepcidin binds to ferroportin on KCs, and other macrophages that store iron, inhibiting the release of iron stores from these cells, regulating iron level within the blood.(64)
Exerting additional influence on human liver physiology and organismal wide homeostasis, hepatocytes function as innate immunity messengers and producers of acute phase proteins (APPs). Hepatocytes express PRRs, to recognize the appearance of foreign bodies from the digestive track. Like NPCs within the liver, TLRs are the most heavily studied PRRs in hepatocytes. TLR 2, -4, -5, and -9 have all been reported to stimulate a pro-inflammatory cascade when activated in hepatocytes. Activation of TLRs stimulates NF-κb nuclear translocation, eliciting production of TNF-α, IL-6, and interferons.43 Setting hepatocytes apart from other cells within the liver is the synthesis and secretion of APPs. APPs, including C-Reactive protein, alpha-2-macroglobulin, haptoglobin, alpha-1 antitrypsin, fibrinogen, and LPS binding protein, disseminate systemically to exert wide-ranging functions. These physiologic functions include bacterial scavenging, platelet inhibition, wound repair, and neutrophil elastase inhibition, but can cause severe pathological consequences. Studies suggest that C-reactive protein can potentiate the effects of osteoarthritis, enhancing IL-6 levels within synovial fluid.(65,66) Serum amyloid a, another APP, exerts pleiotropic effects by functioning as an apolipoprotein in high density lipoproteins, while also activating TLRs within the cardiovascular system to promote atherosclerosis.(67) These effects highlight how damage signals from the liver elicits system-wide outcomes that lead to comorbidities that can severely effect our health.
The production and excretion of bile from the apical membranes of hepatocytes constitutes the connection from the hepatic system to the intestine. Bile is comprised of bile salts, phospholipids, cholesterol, conjugated bilirubin (bilirubin in its water-soluble form), ions, proteins, and water.(68) The amphipathic properties of bile allow from the emulsification of lipids during digestion within the small intestine. Once transported out of the apical membranes of hepatocytes into the bile canaliculi, bile flows in the opposite direction of blood, entering the bile ducts at the portal triad. Within the bile ducts, bile undergoes modifications by cholangiocytes (sharing the same common progenitor of hepatocytes, hepatoblasts) before being stored in the gallbladder for the utilization of lipid absorption during digestion. While bile and cholangiocytes have gained renewed focus in the health status of humans, specifically in the biliary system’s role in drug metabolism, an in-depth examination of this system in the gut-liver axis extends beyond the purview of this review.
References
51. Liu, X., Xu, J., Rosenthal, S., Zhang, L. juan, McCubbin, R., Meshgin, N., Shang, L., Koyama, Y., Ma, H.Y., Sharma, S., et al. (2020). Identification of Lineage-Specific Transcription Factors That Prevent Activation of Hepatic Stellate Cells and Promote Fibrosis Resolution. Gastroenterology 158, 1728-1744.e14. https://doi.org/10.1053/j.gastro.2020.01.027.
52. Dewidar, B., Meyer, C., Dooley, S., and Meindl-Beinker, N. (2019). TGF-β in Hepatic Stellate Cell Activation and Liver Fibrogenesis—Updated 2019. Cells 8. https://doi.org/10.3390/CELLS8111419.
53. Shen, K., Chang, W., Gao, X., Wang, H., Niu, W., Song, L., and Qin, X. (2011). Depletion of activated hepatic stellate cell correlates with severe liver damage and abnormal liver regeneration in acetaminophen-induced liver injury. Acta Biochim Biophys Sin (Shanghai) 43, 307–315. https://doi.org/10.1093/abbs/gmr005.
54. Sufleţel, R.T., Melincovici, C.S., Gheban, B.A., Toader, Z., and Mihu, C.M. (2020). Hepatic stellate cells - from past till present: Morphology, human markers, human cell lines, behavior in normal and liver pathology. Romanian Journal of Morphology and Embryology 61, 615–642. https://doi.org/10.47162/RJME.61.3.01.
55. Ferré, N., Martínez-Clemente, M., López-Parra, M., González-Périz, A., Horrillo, R., Planagumà, A., Camps, J., Joven, J., Tres, A., Guardiola, F., et al. (2009). Increased susceptibility to exacerbated liver injury in hypercholesterolemic ApoE-deficient mice: potential involvement of oxysterols. Am J Physiol Gastrointest Liver Physiol 296, 553–562. https://doi.org/10.1152/ajpgi.00547.2007.-The.
56. Benitez, P.C.A., Viglietti, A.I.P., Elizalde, M.M., Giambartolomei, G.H., Quarleri, J.F., and Delpino, M.V. (2020). Hepatic stellate cells and hepatocytes as liver antigen-presenting cells during B. Abortus infection. Pathogens 9, 1–14. https://doi.org/10.3390/pathogens9070527.
57. Zhan, S.S., Jiang, J.X., Wu, J., Halsted, C., Friedman, S.L., Zern, M.A., and Torok, N.J. (2006). Phagocytosis of apoptotic bodies by hepatic stellate cells induces NADPH oxidase and is associated with liver fibrosis in vivo. Hepatology 43, 435–443. https://doi.org/10.1002/hep.21093.
58. Kalra, A., Ekrem, ;, Chase, Y.;, Wehrle, J., Faiz, ;, and Affiliations, T. Physiology, Liver.
59. Planas-Paz, L., Orsini, V., Boulter, L., Calabrese, D., Pikiolek, M., Nigsch, F., Xie, Y., Roma, G., Donovan, A., Mart, P., et al. (2016). The RSPO-LGR4/5-ZNRF3/RNF43 module controls liver zonation and size. Nat Cell Biol 18, 467–479. https://doi.org/10.1038/ncb3337.
60. Wei, Y., Wang, Y.G., Jia, Y., Li, L., Yoon, J., Zhang, S., Wang, Z., Zhang, Y., Zhu, M., Sharma, T., et al. (2021). Liver homeostasis is maintained by midlobular zone 2 hepatocytes. Science (1979) 371. https://doi.org/10.1126/science.abb1625.
61. Martini, T., Naef, F., and Tchorz, J.S. (2023). Spatiotemporal Metabolic Liver Zonation and Consequences on Pathophysiology. Annu. Rev. Pathol. Mech. Dis 18, 439–466. https://doi.org/10.1146/annurev-pathmechdis.
62. Zhao, M., Ma, J., Li, M., Zhang, Y., Jiang, B., Zhao, X., Huai, C., Shen, L., Zhang, N., He, L., et al. (2021). Cytochrome p450 enzymes and drug metabolism in humans. Preprint at MDPI, https://doi.org/10.3390/ijms222312808 https://doi.org/10.3390/ijms222312808.
63. Schulze, R.J., Schott, M.B., Casey, C.A., Tuma, P.L., and McNiven, M.A. (2019). Beyond the Cell: The cell biology of the hepatocyte: A membrane trafficking machine. J Cell Biol 218, 2096. https://doi.org/10.1083/JCB.201903090.
64. Nemeth, E., Tuttle, M.S., Powelson, J., Vaughn, M.D., Donovan, A., Ward, D.M.V., Ganz, T., and Kaplan, J. (2004). Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization. Science (1979) 306, 2090–2093.
65. Smith, J.W., Martins, T.B., Gopez, E., Johnson, T., Rosenberg, T.D., and Hill, H.R. (2012). Significance of C-reactive protein in osteoarthritis and total knee arthroplasty outcomes. Ther Adv Musculoskelet Dis 4, 315–325. https://doi.org/10.1177/1759720X12455959.
66. Kozijn, A.E., Tartjiono, M.T., Ravipati, S., van der Ham, F., Barrett, D.A., Mastbergen, S.C., Korthagen, N.M., Lafeber, F.P.J.G., Zuurmond, A.M., Bobeldijk, I., et al. (2019). Human C-reactive protein aggravates osteoarthritis development in mice on a high-fat diet. Osteoarthritis Cartilage 27, 118–128. https://doi.org/10.1016/j.joca.2018.09.007.
67. Hadrup, N., Zhernovkov, V., Jacobsen, N.R., Voss, C., Strunz, M., Ansari, M., Schiller, H.B., Halappanavar, S., Poulsen, S.S., Kholodenko, B., et al. (2020). Acute Phase Response as a Biological Mechanism-of-Action of (Nano)particle-Induced Cardiovascular Disease. Preprint at Wiley-VCH Verlag, https://doi.org/10.1002/smll.201907476 https://doi.org/10.1002/smll.201907476.
68. Hundt, M., Basit, H., and John, S. (2022). Physiology, Bile Secretion. StatPearls.
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