Hepatic Sinusoidal Endothelial Cells 5

CHAPTER 5 Hepatic Sinusoidal Endothelial Cells 5

Roman E. Perri, Vijay Shah

5.1 Development and Structure

Hepatic sinusoidal endothelial cells (HSEC) are a morphologically distinct population of cells that form the lining of liver sinusoids. Features that dis- tinguish HSEC from endothelial cells in other or- gans and in larger liver vessels are the presence of multiple fenestrae throughout the cells and the lack of an underlying basement membrane [3, 7, 60, 72]. The sinusoids are positioned between hepatocyte plates and initiate at the portal tract and termi- nate at the central vein. Sinusoids carry blood that converges in the liver from the portal venous supply, as well as from the hepatic artery [76] (Fig. 5.1). Sinusoids are separated from adjacent hepatocytes by the perisinusoidal space, also known as the space of Disse. Due to their position, HSEC are the first cells in contact with blood flow into the sinusoids and serve to compartmentalize the vascular sinusoidal channels from the hepatic parenchyma [60, 72]. The hepatic sinusoids range in diameter from Fig. 5.1. Vasculature architecture of the liver. Blood flow enters 4 µm near the portal triad to 5.5 µm near the cen- the liver via the portal vein (PVb) as well as the hepatic artery tral vein [30]. Because they are smaller than both (HAb). While portal blood enters directly into the sinusoids (S), red and white blood cells, there is distortion of both hepatic arterial blood perfuses into distinct anatomic locations cells and the sinusoids during passage of blood cells prior to re-entering the sinusoids. Sinusoidal blood leaves the [30, 78]. This process has been referred to as an “en- liver via the central veins (CV). Sinusoidal endothelial cells (En) dothelial massage,” which allows efficient exchange form the fenestrated sinusoidal wall. Kupffer cells (K) are lo- of compounds from the blood through sinusoidal cated within the sinusoids, whereas hepatic stellate cells, also fenestrae into the space of Disse [30, 76]. Also resid- termed fat-storing cells (FSC), lie within the space of Disse (DS), ing in the sinusoidal space are hepatic macrophages, adjacent to the single layer of hepatocytes (liver plate, LP). Bile or Kupffer cells, and hepatic natural killer cells, also canaliculi (BC) drain bile into the interlobular bile ducts (BDL) via called pit cells, each of which are covered in other the caniliculoductar junction (CD) in the opposite direction to chapters. flow in the vascular channels. (From Motta P, Muto M, Fujita T. During organogenesis, the primordial liver is The liver. An atlas of scanning electron microscopy. Igaku-Shoin vascularized by capillaries that are continuous and Medical Publishers, Inc., 1978:129) have a basement membrane. Differentiation of these capillaries occurs from precursors located in the septum transversum, a mesenchymal structure lo- septum transversum. Intrahepatic capillaries at this cated between the pericardium and the hepatic di- early stage of gestation have the phenotype of typi- verticulum [17, 28]. At 5 weeks of gestation, grow- cal fetal capillaries, containing cell–cell junctions ing cords of hepatoblasts derived from the hepatic and a basement membrane. Between 5 and 12 weeks diverticulum surround the precursor vessels of the of gestation, the vessels adjacent to hepatocytes de- 54 PART I: Cell Types and Matrix

velop their mature phenotype, marked by the devel- sinusoids [41]. Adhesion molecules on the surface opment of fenestrae, the lack of a basement mem- of typical endothelial cells capture leukocytes in brane, and the loss of cell–cell junctions [17, 18, 79]. the passing blood flow, permitting recruitment of Interestingly, in some hepatic disease states, HSEC inflammatory cells to areas of invasion by foreign dedifferentiate from their specialized phenotype pathogens [34]. HSEC have been shown rarely to ex- back to the more typical endothelial cell phenotype press selectins, a class of adhesion molecules com- [38, 73, 78, 79]. monly expressed in various other tissues [67]. How- ever, other adhesion molecules, such as vascular adhesion protein (VAP-1), are expressed in HSEC, and are responsible for capture of rolling leukocytes 5.2 through the sinusoids, thus recruiting them to areas Cellular Functions of HSEC of inflammation [41, 47]. In addition, other leukocyte adhesion molecules, such as vascular cell adhesion molecule 1 (VCAM-1), are induced on HSEC in 5.2.1 states of liver inflammation [1, 41, 67, 68]. VCAM-1 Fenestration/Filtration also mediates, in part, the adhesion of melanoma cells to hepatic endothelium, thereby playing a key The most prominent feature of sinusoidal en- role in the development of hepatic metastatic lesions dothelial cells is the presence of multiple fenestrae in this disease [41]. Neutrophil attraction to HSEC throughout the cell. Fenestrae have diameters of is mediated by intercellular adhesion molecule 1 about 100 nm, and while the number of fenestrae (ICAM-1), which is constitutively expressed on increases as the sinusoids reach the central vein, HSEC and is upregulated in response to liver injury their diameter decreases [3, 30, 77]. An intracellu- [1, 56, 67]. lar cytoskeleton composed of microfilaments, in- Chemokines produced by inflamed tissues bind termediate filaments, and microtubules supports to receptors on leukocytes and prompt the subse- the dynamic fenestrae [7], which can reorganize to quent firm adhesion of leukocytes to endothelium. form additional fenestrae when stimulated [9]. The This process allows migration of leukocytes into in- fenestrae of the HSEC serve as a mechanical sieve, flamed tissues. The expression of multiple chemok- which facilitates the transfer of nutrients and mole- ines is upregulated in the inflamed liver [1]. T cells cules from the sinusoidal space to the hepatic paren- that infiltrate the liver have been demonstrated to chyma [11, 77]. As HSEC lack a basement membrane, express receptors for these chemokines [67]. After the presence of fenestrae allow for steric selection of adhesion molecules expressed by tissue endothelial compounds that permeate the endothelial cell bar- cells have captured a leukocyte, the leukocyte must rier and gain access to the space of Disse and the transit through the endothelium to reach inflamed hepatic parenchymal cells [7, 77]. For example, the tissue [6]. Many tissues have inter-endothelial-cell presence of fenestrae allows the passage of chylomi- tight junctions that present a barrier to migration cron remnants into the space of Disse to be recog- of leukocytes. However, HSEC form a discontinuous nized and affected by hepatocyte metabolism [7, 30]. barrier without tight junctions, which likely allows Chylomicrons themselves, too large to pass through for unique means of leukocyte migration through the HSEC fenestrae, remain in the sinusoidal space the endothelium barrier and into the inflamed pa- eventually to pass from the liver. The effect of vari- renchyma [41]. ous disease states on HSEC fenestrae is felt to have major implications on the clinical manifestations of each disease state [10, 73]. 5.2.3 Signaling: Nitric Oxide Production

5.2.2 Specific signaling pathways including those in HSEC Expression of Adhesion Molecules are covered in other chapters of this book. However, as nitric oxide (NO) is a key signaling pathway in Leukocytes are attracted and localized to sites of HSEC, some discussion of NO production in HSEC inflammation throughout the body. This occurs is provided. HSEC produce NO via endothelial nitric by attachment to adhesion molecules expressed on oxide synthase (eNOS) as well as inducible nitric ox- the surface of endothelial cells [1, 41]. The liver is ide synthase (iNOS). Production of NO via eNOS is exposed to numerous foreign antigens that traverse a characteristic that is exclusive to endothelial cells the bowel mucosa and enter the portal circulation, among hepatic cell types [41]. NO produced by the thereby gaining access to the liver via the hepatic HSEC may serve to regulate sinusoidal blood flow CHAPTER 5: Hepatic Sinusoidal Endothelial Cells 55 through a paracrine action on perisinusoidal con- heparin, albumin, lipoproteins, and hyaluronate has tractile cells [59]. Increase in NO production via been well described. For some substances such as eNOS is seen in HSEC subjected to shear stress, a heparin, the endocytotic capacity of HSEC exceeds characteristic that may serve to autoregulate blood that of Kupffer cells [3]. The ability of HSEC to take flow through the liver [62]. By this mechanism, if up hyaluronate via endocytosis has been used as a blood flow through the hepatic sinusoid is high, functional marker of sinusoidal endothelial cells eNOS-mediated production of NO may serve locally [64]. Rising serum hyaluronate levels have been to dilate the vascular bed, thus decreasing the re- demonstrated to correlate with decreased endocy- sistance of the vascular bed [62]. In cirrhotic states, totic capabilities of sinusoidal endothelial cells. production of NO via eNOS is diminished [53]. Other A typical endocytosis pathway has been de- important agonists that promote eNOS-derived NO scribed in HSEC. The mannose receptor functions generation include endothelin (via the ET-B recep- by binding and internalizing compounds or anti- tor), VEGF, estrogen and others [5, 31, 56, 59]. Every gens that contain terminal glycoproteins. The man- cell type within the liver has the capacity to gener- nose receptor exists on the cell wall, in coated pits, ate NO via iNOS upon stimulation by liver injury and there is a large intracellular pool of additional or cytokine induction, though this does not occur receptors. When the receptor binds to ligand, rapid during normal conditions [54, 63]. HSEC have been internalization occurs and the ligand is delivered to demonstrated to produce NO via induction of iNOS endosomes and lysosomes for degradation. New re- in response to cytokines such as interferon (IFN)-γ ceptors, either from intracellular stores or recycled and lipopolysaccharides (LPS), agents that do not by receptors, return to the cell surface to bind addi- themselves stimulate iNOS induction in hepatocytes tional ligand [45]. [55]. In contrast, agents such as tumor necrosis fac- The process of endocytosis through non-chath- tor (TNF-α) and interleukin (IL)-1β that stimulate rin-coated pits, such as caveolae, remains enigmatic induction of iNOS in hepatocytes do not exert a in HSEC. Some studies have demonstrated the pres- similar effect in HSEC [55]. Because HSEC are posi- ence of caveolae or similar structures termed ve- tioned adjacent to the blood supply to the liver, the siculo-vacuolar organelles, which constitute grape- majority of which is delivered from the splanchnic like clusters of caveolae in HSEC. However, protein circulation, inflammatory cytokines may act more levels of the caveolae coat protein caveolin, though quickly on HSEC to induce a rise in NO production upregulated in some disease states [61], seem very via iNOS than in other hepatic cell types. low in normal HSEC and thus it has been postulated that caveolae may not play an important role in HSEC endocytosis, perhaps because of the availabil- 5.2.4 ity of alternate endocytic pathways. Metabolism

Numerous compounds and all orally ingested medi- 5.2.6 cations pass through the liver prior to distribution Antigen Presentation throughout the body to target sites of action. It has been demonstrated that HSEC possess only very low Hepatic sinusoidal endothelial cells have been dem- levels of cytochrome P450 enzymes; however, these onstrated to be efficient presenters of bacterial anti- cells do play a role in metabolism [66]. They possess gens to T cells, allowing for immune system activa- low levels of oxidative enzymes, and high levels of tion. To this end, these cells have been demonstrated the post-oxidative enzymes epoxide hydrolase and to carry major histocompatibility complex (MHC) glutathione transferase [65]. Therefore, although class II molecules and to produce IL-1 after pro- the ability to oxidize drugs is limited in HSEC, there duction and processing of bacterial antigens, both is a relatively greater ability of the cells to conjugate features common to antigen-presenting cells [44]. or hydrolyze metabolized compounds. The ability of HSEC to function as antigen-presenting cells accounts for an additional mechanism by which they can attract T cells to the liver. In addi- 5.2.5 tion to non-specific adhesion molecules, HSEC can Endocytosis recruit antigen-specific T cells to the liver by means of antigen presentation. The MHC class II-possess- Hepatic sinusoidal endothelial cells have well-devel- ing character of HSEC is shown to lead naïve CD4+ oped apparatus for endocytotic functions, includ- cells to differentiate into IL-4 and IL-10 expressing ing lysosomes, endosomes, and pinocytotic vesicles. T cells [37, 38]. HSEC induce not only proliferation The capacity of HSEC to take up substances such as of CD4+ cells, but also the production of IFN-α [38, 56 PART I: Cell Types and Matrix

39]. In addition, there is cross-presentation of anti- specimens in patients with primary biliary cirrho- gen to CD8+ cells by HSEC that induces CD8+ cell sis have demonstrated that as histological degree of proliferation and development of antigen tolerance. disease advances, the sinusoidal endothelium devel- This is contrasted with the development of immu- ops the capillarized phenotype [4, 73]. In addition, nity, which is typical of CD8+ cell stimulation by the functional marker of HSEC dysfunction, rising dendritic cells [37]. This function may play a role serum hyaluronate levels, is seen in the histological in avoidance of unwanted autoimmune reactions progression of primary biliary cirrhosis, and levels against food antigens. of hyaluronate appear to correlate with disease stage Bacterial antigens, as well as endotoxin derived [4]. from the intestines, are commonly present in por- Activation of hepatic stellate cells into a pheno- tal venous blood. Endotoxin has been shown to up- type that produces extracellular matrix, which leads regulate the functional activity of typical antigen- to fibrosis, is central to the development of cirrho- presenting cells, such as dendritic cells and macro- sis. HSEC produce growth factors that induce stel- phages. In contrast to these cells, HSEC activity as late cell proliferation and production of extracel- antigen-presenting cells is decreased in response to lular matrix [73]. Some studies have demonstrated endotoxin [36]. The effect of endotoxin on HSEC is that HSEC are also capable of producing extracellu- not via a decreased ability of the cells to endocytose lar matrix components such as fibronectin, laminin, bacterial antigens, but rather by affecting intracel- and type IV collagen when stimulated with trans- lular means of processing the bacterial antigens. forming growth factor (TGF)-β [52]. This suggests The pH of endosomes and lysosomes is raised in that in cirrhotic or inflammatory states, basement HSEC in response to endotoxin, therefore decreas- membranes and extracellular matrix deposition ing the ability of these cells to process antigen and may derive not only from perisinusoidal stellate to activate naïve and memory T cells [36]. The sig- cells, but also from the sinusoidal endothelial cells nificance of this effect is that bacterial antigens can themselves [52, 79]. be efficiently cleared by HSEC without inducing he- An additional change seen in HSEC in cirrhotic patic inflammation in response to the near-constant states is the expression of numerous adhesion pro- presence of endotoxin in portal venous blood. This teins not expressed in normal HSEC. Several in- allows for hepatocytes to function in a relatively im- tegrins that serve as cell-matrix adhesion proteins munologically quiescent state. are not seen in normal HSEC, but are produced in sinusoidal endothelial cells of the cirrhotic liver [18]. Integrins function to anchor the endothelial cells to the basement membrane that develops as described 5.3 in cirrhotic states. Furthermore, expression of von HSEC Pathobiology in Disease States Willebrand factor also appears to increase in HSEC in cirrhosis [4]. Platelet-endothelial cell adhesion molecule-1 is present on HSEC of cirrhotic livers, 5.3.1 but was recently found to be similarly expressed in Cirrhosis/Portal Hypertension the HSEC of normal livers as well [50].

Cirrhosis is a disease characterized by extensive scarring throughout the liver, destruction of the 5.3.2 normal hepatic architecture, and high vascular HSEC and Drug Toxicity resistance through the hepatic circulation. In cirrhotic states, NO production from HSEC is dimin- One of the earliest findings of hepatic toxicity due to ished [53]. This most likely occurs through defects acetaminophen is the development of large pores in in post-translational processing of eNOS including HSEC with separation of HSEC from the underlying enhanced expression of the NOS inhibitory protein, hepatocytes, thereby widening the space of Disse caveolin, as well as decreased eNOS phosphoryla- [25]. Collapse of the sinusoidal lumen eventually tion at serine 1179 [31, 63]. Furthermore, production occurs, likely secondary to enlargement of the space of the constrictor ET-1 is increased, in part through of Disse. Changes in HSEC due to acetaminophen HSEC production [59]. were found to be due to primary effects of the drug Hepatic sinusoidal endothelial cell fenestrae on the endothelial cells in one mouse strain, but due shrink and disappear in the cirrhotic state, a change to metabolites of acetaminophen by adjacent cells in that precedes the development of an endothelial another mouse strain and in rats [25, 42]. In addi- basement membrane and capillarization of the si- tion, depletion of cellular glutathione in both HSEC nusoids [10, 30, 49] (Fig. 5.2). Studies of liver biopsy and adjacent cells was found to be crucial to the CHAPTER 5: Hepatic Sinusoidal Endothelial Cells 57

Fig. 5.2. Scanning electron microscopy of rat hepatic sinusoids in zone 3: A control and B after 4 weeks, C 6 weeks, and D 12 weeks of intraperitoneal injections of thioacetamide, an experimental model of cirrhosis. (From Motta P, Muto M, Fujita T. The liver. An atlas of scanning electron microscopy. Igaku-Shoin Medical Publishers, Inc., 1978:129)

pathogenesis of acetaminophen-induced toxicity, as in the experimental models. The changes induced well as the toxicity of other drugs, with abrogation in HSEC, which can be abolished by administration of toxic effects after treatment with exogenous glu- of glutathione, precede pathological changes in he- tathione [20, 24, 26]. patic parenchymal cells [26]. Thus, the disruption of Veno-occlusive disease (VOD) is a well-de- the hepatic circulation by HSEC damage is the cause scribed hepatotoxin-induced condition character- of hepatic parenchymal damage, rather than paren- ized by hepatomegaly, jaundice, and ascites within chymal damage being the cause of hepatic circula- 10–20 days after starting a chemotherapeutic regi- tory dysfunction [26]. men [58]. VOD has been most commonly associated with chemotherapy regimens used for marrow ablation in bone marrow transplants. Histological 5.3.3 features include fibrosis of the liver sinusoids and Cellular Rejection necrosis of zone 3 hepatocytes [22, 58]. It has been demonstrated that the earliest microscopic changes Rejection of the transplanted organ is a major con- of VOD are seen in HSEC [23]. The earliest histo- cern after orthotopic liver transplantation. One of logic changes seen in the liver in VOD are dilatation the earliest signs of acute liver rejection is the pres- and engorgement of the hepatic sinusoids with ex- ence of infiltrating immune cells within the sinu- travasation of red blood cells into the space of Disse soids. There is also evidence of functional abnor- [22, 58]. Electron microscopy demonstrates closure malities of HSEC during episodes of acute rejection, of fenestrae in HSEC and accumulation of collagen such as increases of serum hyaluronate levels [80], in the pores of sinusoids [23]. Animal models of reflecting diminished endocytotic capabilities and VOD have demonstrated early morphologic changes damage of HSEC. in the HSEC such as loss of fenestration and the ap- Hepatic sinusoidal endothelial cells also play an pearance of gaps in the junctions between HSEC [22, important role in the inflammation of non-acute 23]. It has also been seen that HSEC undergo further graft rejection. Leukocytes are arrested in sinusoids morphologic changes such as cellular rounding and by binding to adhesion molecules that are induced sloughing from the lining of the hepatic sinusoid. by inflammatory cytokines such as TNF-α and IL- In experimental models of VOD, embolization of 1β [68]. HSEC display a different pattern of adhesion sloughed HSEC, as well as Kupffer cells, can cause molecules during graft rejection than do portal vein downstream occlusion of the hepatic sinusoids and endothelial cells [68]. The lack of adhesion molecules resultant portal hypertension [21]. Activation of such as selectins on HSEC may be necessary to pre- matrix metalloproteinases and diminished NO gen- vent the firm adherence and microvascular throm- eration have also been implicated in this syndrome bosis of the sinusoids by leukocytes in conditions 58 PART I: Cell Types and Matrix

such as rejection [68]. The difference in patterns of been shown to be reversible upon abstinence from adhesion molecule expression by different types of alcohol [30]. In addition, the scavenging activity of endothelial cells also may explain the recruitment HSEC has been shown to be dysfunctional after only of different cell types to different tissues. At sites of short periods of alcohol ingestion. These changes in portal vein acute rejection, eosinophils are typically the morphology and functional characteristics of seen, while in the liver sinusoid, natural killer cells HSEC precede the effects of alcohol on hepatic pa- are more typically seen [68]. renchymal cells. The effect of alcohol on HSEC has been demonstrated not to be caused primarily by the alcohol itself, but rather via intermediates pro- 5.3.4 duced by neighboring Kupffer cells. The effects of Ischemia-Reperfusion Injury alcohol on sinusoidal endothelial cells can be elimi- nated by administration of compounds that inacti- Hepatic sinusoidal endothelial cells are the most sen- vate Kupffer cells prior to treatment of experimental sitive cells in the liver to cold preservation damage. animals with alcohol [8, 19, 71]. Studies of rat livers perfused with University of Wis- Chronic alcohol ingestion has also been dem- consin solution for varying times demonstrate that onstrated to affect the endocytotic ability of HSEC morphologic changes occurred after as few as 8 h of [71]. Receptor-mediated uptake of ligands is slower perfusion [51], earlier than changes seen in hepato- in HSEC from animals fed ethanol chronically. In cytes. The upper limit of time that graft tissue can addition, the ability of HSEC to degrade ligands that be kept in the state of cold ischemia is dictated by had been imported by endocytosis is also dimin- the ability of HSEC to survive [13, 69]. Morphologic ished after alcohol ingestion [70]. The implications changes noted during periods of cold ischemia are of these findings are that bacterial antigens may small blebs and changes of surface texture including not be as efficiently removed by HSEC-mediated pits [46, 51]. Fenestrae are widened during periods endocytosis in states of chronic ethanol ingestion. of cold ischemia, prior to reperfusion. However, it This may lead to prolonged antigen presence and is at the time of reperfusion that the most marked immune stimulation, resulting in increased inflam- destructive changes in sinusoidal endothelial cells mation in the liver [70]. are seen [13, 15]. Upon reperfusion, lethal injury, Endotoxin-induced hepatitis is also character- rounding of the cells, denudation of sinusoids and ized by distinctive changes to HSEC, such as a de- condensation of HSEC nuclei herald loss of viability crease in the number of fenestrae, as well as a distur- of the tissue graft [12, 13]. The changes that occur bance of HSEC functional markers such as uptake in HSEC upon reperfusion are probably apoptotic of hyaluronate. Much like alcohol-induced changes rather than necrotic [15, 32, 40, 69]. The changes of in HSEC, these effects are caused by intermediates reperfusion injury to HSEC are most marked in per- produced by Kupffer cells [27]. Only Kupffer cells iportal areas compared to pericentral areas [40]. In- appear directly affected by endotoxin, as studies in terestingly, apoptosis of rat HSEC upon reperfusion which Kupffer cells have been inactivated by gado- after prolonged cold storage times is inhibited by linium chloride demonstrate that HSEC are not af- fasting the donor animal prior to organ harvest, al- fected directly by endotoxin [57]. though the mechanism of this is unclear [69].

5.3.6 5.3.5 HSEC and Liver Cancer Endotoxemia/Alcoholemia Hepatocellular carcinoma (HCC) is a primary liver The role of alcohol as a cause of chronic liver dis- tumor that is characterized by blood supply de- ease has been well described. It has recently been rived exclusively from the hepatic artery. During described that the earliest changes in the liver due development of small HCC, the blood supply to the to alcohol occur in the sinusoidal endothelium. Al- tumor transitions from a dual blood supply to the cohol intake has been demonstrated to dilate the exclusively arterial supply. During this transition, HSEC fenestrae [29], though chronic, excessive in- the hepatic sinusoids undergo alterations of capil- take of alcohol has been shown paradoxically to lead larization, with loss of fenestrae and development of to capillarization of the endothelium with closure of a basement membrane [35, 73]. These changes may the fenestrae and the development of an endothe- occur to preserve sinusoidal structure in the setting lial basement membrane. Both size and number of of increased intrasinusoidal pressures resulting fenestrae are decreased with chronic alcohol inges- from the arterial blood flow. tion [33]. These changes of HSEC fenestrae have CHAPTER 5: Hepatic Sinusoidal Endothelial Cells 59

The liver is a common site to find evidence of tumor metastasis. The sequence of events required to 5.4. establish metastatic tumor is: tumor cell arrest by Summary binding to endothelial adhesion molecules, migration through the endothelial barrier, migration into In summary, there are a number of unique signaling the sub-endothelial space, and proliferation [81]. pathways and anatomic functions in HSEC, some Melanoma cells have been found, in experimental of which have been outlined in this chapter; others conditions, to use mannose receptors and the adhe- will be discussed in other chapters of this textbook. sion molecule VCAM-1 to effect adherence to sinu- A number of additional biologic and pathobiologic soidal endothelium [48, 75]. The binding of these pathways remain under active investigation. melanoma cells to HSEC was found to be dependent on IL-1, which induces expression of VCAM-1 [2, 48, 74, 75]. This is consistent with other studies that have demonstrated enhanced binding of tumor cells Selected Reading to endothelium that has been "primed" by inflammation. Wisse E, Braet F, Luo D et al. Structure and function of sinusoidal With increasing evidence that anti-angiogenic lining cells in the liver. Toxicol Pathol 1996;24:100–111. (This approaches may have efficacy in tumor growth, it review summarizes the biology and function of HSEC and its will be interesting to determine the influence of in- neighboring cell types.) hibition of HSEC migration and proliferation on he- Selzner N, Rudiger H, Graf R, Clavien PA. Protective strate- patic tumor growth. gies against ischemic injury of the liver. Gastroenterology 2003;125:917–936. (This review outlines approaches to pro- tect the hepatic microcirculation from ischemic injury.) 5.3.7 Shah V. Cellular and molecular basis of portal hypertension. Clin HSEC in the Aging Process Liver Dis 2001;5:629–644. (This review outlines mechanisms of endothelial cell pathobiology relevant to portal hyper- The fenestrations seen in HSEC allow transfer of tension.) macromolecules from the hepatic blood supply to Shah V, Kamath PS. Nitric oxide in liver transplantation: pathobi- the hepatic parenchyma. Due to the lack of a charged ology and clinical implications. Liver Transplant 2003;9:1–11. basement membrane, size is essentially the only fac- (This review article outlines NO signaling as it relates to bio- tor that dictates which compounds will traverse the logic processes relevant to liver transplantation.) sieve-like fenestrae [77]. With aging, changes in the number of fenestrae in the HSEC are seen [43]. This aging process resembles the capillarization seen in other pathologic states of the liver because, with the References loss of fenestrae, the sinusoidal endothelium more closely resembles the capillary beds in other vascu- 1. Adams D. Leucocyte adhesion molecules and alcoholic liver lar tissues. The etiology of these age-related changes disease. Alcohol Alcohol 1994;29:249–260. is uncertain, though chronic exposure to alcohol 2. Anasagasti M, Alvarez A, Martin J et al. Sinusoidal endothe- and oxidants has been shown to induce changes in lium release of hydrogen peroxide enhances very late anti- HSEC [16, 30]. The implications of hepatic sinusoi- gen-4-mediated melanoma cell adherence and tumor cyto- dal capillarization are uncertain, though it has been toxicity during interleukin-1 promotion of hepatic melano- postulated that this process may lead to disordered ma metastasis in mice. Hepatology 1997;25:840–846. lipid metabolism [43]. Chylomicron remnants, rich 3. Arii S, Imamura M. Physiological role of sinusoidal en- in triglycerides, are unable to traverse the fenestrae dothelial cells and Kupffer cells and their implication in the of HSEC that have undergone capillarization due pathogenesis of liver injury. J Hepatobiliary Pancreat Surg to aging, a process that may lead to post-prandial 2000;7:40–48. hypertriglyceridemia [14]. It has been proposed that 4. Babbs C, Haboubi N, Mellor J et al. Endothelial cell transfor- the ability of the liver to remove dietary cholesterol mation in primary biliary cirrhosis: a morphological and bio- via the sieve function of the sinusoidal fenestrae chemical study. Hepatology 1990;11:723–729. plays a key role in the pathogenesis of atherosclero- 5. Bauer M, Bauer I, Sonin N et al. Functional significance of sis [10]. endothelin B receptors in mediating sinusoidal and extrasi- nusoidal effects of endothelins in the intact rat liver. Hepa- tology 2000;31:937–947. 60 PART I: Cell Types and Matrix

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