Nội dung

Comparative Hepatology BioMed Central Open Access Research Liver sinusoidal endothelial cells represents an important blood clearance system in pigs Geir I Nedredal*1, Kjetil H Elvevold2, Lars M Ytrebø1, Randi Olsen3, Arthur Revhaug1 and Bård Smedsrød2 Address: 1Department of Digestive Surgery, University Hospital of Tromsø, 9038 Tromsø, Norway, 2Department of Experimental Pathology, Institute of Medical Biology, University of Tromsø, 9037 Tromsø, Norway and 3Department of Electron Microscopy, Institute of Medical Biology, University of Tromsø, 9037 Tromsø, Norway Email: Geir I Nedredal* - Geir.Ivar.Nedredal@fagmed.uit.no; Kjetil H Elvevold - Kjetilhe@fagmed.uit.no; Lars M Ytrebø - Larsmy@fagmed.uit.no; Randi Olsen - randio@fagmed.uit.no; Arthur Revhaug - arthur.revhaug@unn.no; Bård Smedsrød - baards@fagmed.uit.no * Corresponding author Published: 3 January 2003 Comparative Hepatology 2003, 2:1 Received: 24 October 2002 Accepted: 3 January 2003 This article is available from: http://www.comparative-hepatology.com/content/2/1/1 © 2003 Nedredal et al; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL. Abstract Background: Numerous studies in rats and a few other mammalian species, including man, have shown that the sinusoidal cells constitute an important part of liver function. In the pig, however, which is frequently used in studies on liver transplantation and liver failure models, our knowledge about the function of hepatic sinusoidal cells is scarce. We have explored the scavenger function of pig liver sinusoidal endothelial cells (LSEC), a cell type that in other mammals performs vital elimination of an array of waste macromolecules from the circulation. Results: 125I-macromolecules known to be cleared in the rat via the scavenger and mannose receptors were rapidly removed from the pig circulation, 50% of the injected dose being removed within the first 2–5 min following injection. Fluorescently labeled microbeads (2 µm in diameter) used to probe phagocytosis accumulated in Kupffer cells only, whereas fluorescently labeled soluble macromolecular ligands for the mannose and scavenger receptors were sequestered only by LSEC. Desmin-positive stellate cells accumulated no probes. Isolation of liver cells using collagenase perfusion through the portal vein, followed by various centrifugation protocols to separate the different liver cell populations yielded 280 × 107 (range 50–890 × 107) sinusoidal cells per liver (weight of liver 237.1 g (sd 43.6)). Use of specific anti-Kupffer cell- and anti-desmin antibodies, combined with endocytosis of fluorescently labeled macromolecular soluble ligands indicated that the LSEC fraction contained 62 × 107 (sd 12 × 107) purified LSEC. Cultured LSEC avidly endocytosed ligands for the mannose and scavenger receptors. Conclusions: We show here for the first time that pig LSEC, similar to what has been found earlier in rat LSEC, represent an effective scavenger system for removal of macromolecular waste products from the circulation. Background Pig liver is frequently used to study liver transplantation and failure, and also serves as a source of cells for bioarti- ficial livers [1]. On this background it is surprising that the knowledge about a central liver function, namely blood clearance, in the pig, has been insufficiently dealt with in Page 1 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 the literature. The concept of the reticuloendothelial system (RES) was launched by Aschoff in 1924 [2]. A fact that is often forgotten nowadays is that Aschoff included both Kupffer cells (KC) and sinusoidal endothelial cells (LSEC) as equally important members of hepatic RES. However, with time, the liver RES came to be synonymous with the liver macrophage. In fact, all major text books of pathology used today describe the RES as consisting only of macrophages. Nevertheless, very recent studies on the biology of LSEC have shown that these cells in rodents, and the few other mammals that have been studied, represent the most important site of elimination of nearly all tested soluble waste macromolecules, spanning from the unphysiological colloidal vital stains used by Aschoff and his predecessors to a number of physiological macromolecular waste products such as major matrix components [3], serum components [4], lysosomal enzymes [5], and pathophysiological substances such as oxidized low density lipoprotein (LDL) [6] and advanced glycation end products [7]. Studies carried out to compare the scavenger function of KC and LSEC have shown that these two cell types contribute to the hepatic RES function in different yet complementary ways: KC eliminate large, insoluble waste fragments by phagocytosis, whereas LSEC are geared to non-phagocytic endocytosis of soluble macromolecules [3]. In line with this notion is the curious fact that most of the colloidal vital stain that Aschoff and his predecessors used to demonstrate the existence of a RES, was recently shown to be taken up exclusively by LSEC [8]. Thus, blood clearance of soluble waste macromolecules, a major liver function, resides largely in LSEC. It should be noted that these findings have been obtained using rats and some other rodents. Furthermore, it has been shown that most vertebrates carry their so-called scavenger endothelial cells (endothelial cells endowed with the same RES-function as rat LSEC) in organs other than liver [9]. These findings justify a careful study to determine whether the liver of pig is equipped with the same type of scavenger LSEC that is present in rat liver. With the motivation to determine if pig liver contains LSEC that resemble rat LSEC, we set out to study the scavenger function of pig LSEC. Although some laboratories have reported on isolation of pig liver sinusoidal cells, those methods either yield very low purity or a very low cell number [10,11]. For this reason, we established a protocol consisting of collagenase perfusion, differential and density centrifugation, and centrifugal elutriation. This method yields both high purity and functionally intact pig liver sinusoidal cells that can be cultivated in monolayer cultures. Notably, the yield of sinusoidal cells was four orders of magnitude higher with the presently described method compared to a recently reported protocol [10]. With this method we show, for the first time, that pig http://www.comparative-hepatology.com/content/2/1/1 LSEC are as endocytically active as their rat liver counterparts. Results Rate of elimination and organ distribution of circulating formaldehyde-treated serum albumin (FSA) and α-mannosidase The circulatory survival of FSA and α-mannosidase was determined after intravenous administration of trace amounts of 125I-tyramine cellobiose-FSA (125I-TC-FSA) and 125I-α-mannosidase. Decay plots indicated efficient clearance of either probe, with 50% of injected dose being eliminated from the blood during 2–5 min (Fig. 1). The liver was the main site of uptake (Fig. 2), while a surprising finding was uptake in the lungs. Blood radioactivity after 15–20 min was 15–20% of injected dose. This equals the amount of unbound 125I after gel filtration through a PD-10 column of a sample of the intravenously administered ligands. In vivo liver cell identification Intravenuosly administered TRITC-monodisperse polymer particles (MDPP) for identification of phagocytosing KC accumulated mainly periportally in liver acini (Figs. 3A, 3B, 3C). Immunoelectron microscopy of liver sections that had been reacted with anti-TRITC-antibodies and protein A-gold revealed the presence of gold particles along the periphery of the surface of the particles, allowing a reliable identification and intracellular location of TRITC-MDPP (Figs. 4A, 4B). In contrast to these particles, FITC-FSA was taken up exclusively in LSEC-like cells lining the liver sinusoids (Fig. 3B). To distinguish LSEC from stellate cells, double immunolabeling was performed to visualize FITC-FSA and desmin in transmission electron microscopy. FITC-FSA and desmin were observed in distinct cell types along the sinusoidal lining (Fig. 5). FITCFSA was associated with organelles judged as lysosomes of LSEC. Cell separation The number of non-parenchymal cells (NPC) obtained per liver following collagenase dispersion and isopycnic density separation in iodixanol was 280 × 107 (range 50– 890 × 107) (weight of liver 237.1 g (43.6)) with a viability of 95.4% (2.5) as judged by trypan blue exclusion (Table 1). The corresponding figures for hepatocytes were 1880 × 107 (1110 × 107) and 94.1% (2.2). The cells obtained after iodixanol separation were subjected to centrifugal elutriation and collected in 4 fractions. The corresponding recoveries expressed as number of NPC and percentages of total are displayed in Table 2. Identification of cultured cells Cells, seeded on fibronectin-coated substrate, obtained from the elutriation fractions yielded LSEC cultures of var- Page 2 of 14 (page number not for citation purposes) http://www.comparative-hepatology.com/content/2/1/1 100 70 Radioactivity (% of recovered) 80 60 40 20 60 50 40 30 20 10 aorta lymphnode muscle thyroidea thymus stomach 60 duodenum 50 urine 40 heart 30 min kidney 20 blood 10 spleen 0 lung 0 0 liver Radioactivity (% remaining) Comparative Hepatology 2003, 2 organ Figure 1 Clearance kinetics. Approximately 100 × 106 cpm of 125Ityramine cellobiose-formaldehyde-treated serum albumin (TC-FSA) and 125I-α-mannosidase were injected intravenously. Radioactivity in the blood sample collected immediately after injection was taken as 100%. Blood samples were collected every minute during the first 10 minutes, then every 5 minutes for one hour. (Open boxes: 125I-TC-FSA; n = 2, closed boxes: 125I-α-mannosidase; n = 1). ying purity (Table 3). We used in vivo (Fig. 6A) or in vitro administered FITC-FSA as a specific LSEC marker, positive reaction with anti-desmin antibodies as a specific marker of stellate cells (Fig. 7A), and a specific anti-pig macrophage antibody (Fig. 7B) or phagocytosis of TRITC-MDPP (Fig. 6B) as KC specific markers. Using these criteria, cultures resulting from elutriation fraction 1 were shown to contain 63.9% stellate cells; cultures established from fraction 2 contained 80.4% LSEC, and fractions 3 and 4 contained 66.2% and 61.0% LSEC. Cells that reacted with anti-pig-macrophage antibodies or phagocytosed TRITCMDPP contained no FITC-FSA. Stellate cells were distinguished by immunolabeling with anti-desmin antibodies or by their content of characteristic autofluorescence from vitamin A droplets when irradiated with light of 328 nm of wavelength ([12]) (Fig. 6C). Specificity of endocytosis in cultured LSECs and hepatocytes The specificity of endocytosis of 125I-FSA and 125I-asialoorusomucoid protein (ASOR) in cultured LSEC and hepatocytes was studied by attempting to inhibit the uptake of trace amounts of radiolabeled ligands using excess amounts of unlabeled ligands. Incubation of LSEC cultures with 125I-FSA in the presence of excess amounts of unlabeled FSA (100 mg·mL-1) resulted in a 90% inhibition of uptake (Fig. 8). The presence of galactose (50 mmol·L-1) did not inhibit endocytosis of 125I-FSA by LSEC. Incubation of hepatocytes with 125I-ASOR in the Figure 2 Anatomical distribution. The animals used in the blood clearance studies (Fig. 1) were analyzed for anatomical distribution of radioactivity 1 h after injection. More than 90% of the injected doses were recovered in the organs listed. Results are expressed as percent total radioactivity recovered. (Grey bars: 125I-TC-FSA; n = 2, white bars: 125I-α-mannosidase; n = 1). presence of excess amounts of galactose (50 mmol·L-1) inhibited uptake by 85%. Unlabeled FSA did not inhibit endocytosis of 125I-ASOR by hepatocytes (Fig. 8). Discussion Although it is assumed that pig LSEC perform the same physiological scavenger function as it has been observed in rat LSEC [3], it has actually never been shown. Since endothelial cells of the liver of most vertebrate species are associated with clearance activity [9], we wanted to study whether pig liver clearance function resides in the scavenger activity of LSEC in the same way as it has been shown in the rat. To this end, endocytosis of both foreign and physiological waste macromolecules in pig LSEC was studied in vivo and in vitro. For the in vitro studies we also developed a method for mass isolation and culture of pig LSEC. Rate of elimination and organ distribution of FSA and αmannosidase First we studied the circulatory survival and anatomical distribution of FSA, a frequently used test ligand for the LSEC scavenger receptor in rat [13], and α-mannosidase, a physiological ligand for the mannose receptor of rat LSEC [14]. Studies in the rat and other vertebrates have shown that 125I-FSA is degraded very rapidly after uptake, resulting in rapid escape of radiotracer from the site of uptake. For this reason, FSA was labeled with 125I-TC, which is trapped in the lysosomes at the cellular site of uptake, Page 3 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Figure 3 Fluorescence micrographs of liver section. Following intravenous administration of fluorescently labeled substances, sections were prepared as described in the Methods section. A heterogeneous distribution of yellow fluorescence from TRITClabeled monodisperse polymer particles (MDPP) phagocytosed by Kupffer cells was located mainly in the periportal region of the liver acinus (arrows) (A). Green fluorescence along the lining of the liver sinusoids identifies endocytosed FITC-formaldehyde-treated serum albumin (FSA) by liver sinusoidal endothelial cells (LSEC), while the localization of phagocytosed MDPP is shown by arrows (B). Uptake of FITC-FSA (arrowheads) and MDPP (arrow) is shown more clearly at higher magnification in C. (Scale bars; A: 80 µm, B: 20 µm, C: 8 µm). Page 4 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Figure 4 Uptake of monodisperse polymer particles (MDPP) in Kupffer cells (KC). Following intravenous administration of fluorescently labeled substances, sections were prepared as described in the Methods section for transmission electron microscopy. MDPP are located intracellularly in Kupffer cells, as judged by their characteristic phagocytosis of the particles (A). Hepatocytes (Hep) contain numerous mitochondria. The cells that contain fat vacuoles (FV) may represent stellate cells (SC). To distinguish between vacuoles containing fat and phagocytosed MDPP, sections were immunolabeled with monoclonal antimouse TRITC-conjugate. Gold particles are located in the periphery of MDPP where the TRITC-molecules are attached (B). (Scale bars; A: 2 µm, B: 500 nm). Page 5 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Figure 5 Stellate cells (SC) and liver sinusoidal endothelial cells (LSEC). Following intravenous administration of fluorescently labeled substances, sections were prepared as described in the Methods section for transmission electron microscopy. Ultrathin sections were immunodouble labeled to visualize both FITC-labeled formaldehyde-treated serum albumin (FSA) in LSEC and desmin in SC. Figures B and C are higher magnification of segments of figure A. Cells lining the sinusoids (A) are LSEC as judged by the localization of small gold particles (5 nm, small arrow) in organelles taken as lysosomes (B). The cell containing large fatty vacuoles (FV) and large gold particles (10 nm, large arrow), was judged as a stellate cell (SC) (C). (Scale bars; A: 1 µm, B: 200 nm, C: 500 nm). Page 6 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Table 1: Parameters of liver perfusions, recovery of non-parenchymal cells (NPC), and viability (n = 10). Body wt (kg) Liver wt (g) Collagenase perfusion (min) Portal-flow (mL·min-1) Total NPC (×107) Viability NPC (%) 7.6 (0.6)* 237.1 (43.6)* 16.5 (3.2)* 304.9 (47.4)* 280 (50–890)# 95.4 (2.5)* *The values are expressed as: mean (standard deviation). #The value is expressed as: mean (range). Table 2: Yield of non-parenchymal cells (NPC) from elutriation fractions (n = 4). Fraction Flow rate (mL·min-1) Number of NPC (×107) % of total NPC 1 2 3 4 18.5 (1.0) 32.0 (0.0) 37.0 (0.0) 45.0 (0.0) 190 (68) 62 (12) 13 (8) 7 (2) 69.2 23.0 5.0 2.7 The values are expressed as: mean (standard deviation). thus preventing 125I escape from the uptake site [15]. Previous studies in the rat and other vertebrates showed that α-mannosidase, after its rapid uptake by the mannose receptor, accumulates within lysosomes and is reused for several hours before being degraded [5]. Therefore, αmannosidase was labeled with 125I in a direct, conventional manner. Both 125I-TC-FSA and 125I-α-mannosidase were rapidly eliminated from the circulation, with 50% of the ligands being removed during the first 2–5 min after intravenous administration. This rapid removal suggested a very efficient uptake mechanism. Monitoring of radioactivity in the organs showed that the liver contained 53% (FSA) and 62% (α-mannosidase) of injected dose, suggesting that a cell type(s) in liver was responsible for clearance via the scavenger and mannose receptors. Surprisingly, as much as 26% FSA and 18% α-mannosidase were recovered in lungs. This is clearly different than in the rat, where uptake in the lungs of these and other soluble macromolecular waste products have not been observed [3]. A recent report [16] showed that ligands for studies of reticuloendothelial function were taken up in both lung and liver of pig, similarly to what we found using α-mannosidase and FSA. It was concluded from that study that 198Au colloidal particles and iron oxide particles were taken up in pulmonary intravascular macrophages. The possibility that these ligands might have been taken up by scavenger endothelial cells was not mentioned in that paper. In vivo liver cell identification To determine the role of different sinusoidal cells in the clearance function of pig liver, the cellular site of uptake of FITC-FSA was compared with that of TRITC-MDPP (a functional marker of phagocytosing KC), and immunoreactive desmin (a marker of stellate cells). Since light microscopy does not allow a clear distinction between particles that are truly internalized and those that are associated with the cell surface, liver tissue was prepared for electron microscopy. To enable a distinction between vitamin A-containing lipid droplets in stellate cells and internalized MDPP in KC, sections were first incubated with anti-TRITC-antibodies, then with protein A-gold. Observations of these sections revealed gold staining along the surface of the MDPP particles, corresponding to the surface localization of TRITC. Double immunolabeling showed that FITC-FSA (5 nm gold) was always associated with endothelial like lining cells that neither took up MDPP nor contained desmin (10 nm gold), indicating that the hepatic uptake in vivo of FSA was exclusively in LSEC, similar to what has been found in the rat [13]. Separation, cultivation, and characterization of cells in vitro To allow a more detailed study of the tentative scavenger function of pig LSEC, we developed a protocol for isolation of sinusoidal cells. The protocol was modified as compared to rat [17] and mouse liver. According to the literature, rat and mouse liver sinusoidal cells can be isolated in high yield and purity using isopycnic separation. We found that this method was insufficient to isolate such cells from pig due to the high number of desmin-positive cells; therefore, we included centrifugal elutriation to separate the cells according to size. Using collagenase perfusion through the portal vein, followed by differential centrifugation, isopycnic centrifugation, and centrifugal elutriation we obtained 4 fractions, of which fraction 2, Page 7 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Figure 6 Fluorescence micrographs of cultured liver sinusoidal endothelial cells (LSEC). Cultures were prepared as described in the Methods section. The cultures were fixed in 4% paraformaldehyde, after 6 h of incubation. FITC-labeled formaldehyde-treated serum albumin (FSA) and TRITC-labeled monodisperse polymer particles (MDPP) were administered intravenously prior to isolation of liver cells. Fluorescent microscopy reveals a homogeneous LSEC culture contaminated by a few cells with TRITC-MDPP and lipid containing vacuoles. The green fluorescence from endocytosed FITC-FSA demonstrates that most cells are LSEC, and that the probe is localized in cytoplasmic vacuoles (A), whereas the yellow fluorescence from phagocytosed TRITC-MDPP identifies Kupffer cells (arrows) (B). Autofluorescence from vitamin A identifies stellate cells (arrows) (C). (Scale bars; 20 µm). Page 8 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Figure 7 Fluorescent micrographs of cultured stellate cells and Kupffer cells. Cultures were prepared as described in Methods. The cultures were fixed in 4% paraformaldehyde, after 1 h of incubation. Micrographs of cultured stellate cells stained with monoclonal anti-desmin antibody (A) and cultured Kupffer cells stained with monoclonal anti-pig macrophage antibody (B). (Scale bars; 20 µm). Page 9 of 14 (page number not for citation purposes) Comparative Hepatology 2003, 2 http://www.comparative-hepatology.com/content/2/1/1 Table 3: Identification of cells after cultivation of elutriation fractions. Fraction FITC – FSA Desmin KC Hepatocytes 1 2 3 4 32.1 (12.6) 80.4 (6.4) 66.2 (11.1) 61.0 (17.5) 63.9 (15.4) 10.9 (9.2) 7.1 (3.8) 2.1 (1.7) 4.0 (6.9) 7.0 (1.4) 15.2 (13.5) 10.9 (10.9) 0.0 (0.0) 1.7 (1.6) 11.5 (17.5) 26.0 (19.4) Radioactivity (% of control) Prior to isolation of cells, pigs received FITC-labeled formaldehyde-treated serum albumin (FSA) intravenously. Stellate cells stained with monoclonal mouse anti-human desmin antibody and Kupffer cells (KC) stained with anti-pig macrophage antibodies. Hepatocytes were identified by simple morphology. Values are percent of total number of cells per culture (n = 3). The values are expressed as: mean (standard deviation). 100 80 60 40 20 0 Control Galactose FSA Figure 8 Specificity of endocytosis of 125I-formaldehyde-treated serum albumin (FSA) in cultured liver sinusoidal endothelial cells (LSEC) (grey and white bars), and 125I-asialo-orusomucoid protein (ASOR) in cultured hepatocytes (black and hatched bars). Monolayer cultures were incubated for 2 hrs, at 37°C, with trace amounts of labeled ligand alone (control) or together with excess amounts of unlabeled FSA (100 µg·mL-1) or galactose (50 mmol·L-1). The presence of unlabeled FSA inhibited effectively the endocytosis of 125I-FSA in LSEC, while galactose showed no such inhibitory effect. Galactose had an inhibitory effect on endocytosis of 125I-ASOR in hepatocytes, whereas unlabeled FSA showed no such inhibitory effect. Results, given as percent of control, are the means of triplicate experiments. Grey and white bars: 100% corresponds to 12.7% of added cpm, black and hatched bars: 100% corresponds to 14.6% of added cpm. White and hatched areas of bars represent % degraded ligand. Grey and black areas of bars represent % cell-associated ligand. Page 10 of 14 (page number not for citation purposes)