Ronald De Zanger

Structure and Function of Sinusoidal Lining Cells in the Liver

The hepatic sinusoid harbors 4 different cells: endothelial cells (100, 101), Kupffer cells (96, 102, 103), fat-storing cells (34, 51, 93), and pit cells (14, 107. 108). Each cell type has its own specific morphology and functions, and no transitional stages exist between the cells. These cells have the potential to proliferate locally, either in normal or in special conditions, that is, experiments or disease. Sinusoidal cells form a functional unit together with the parenchymal cells. Isolation protocols exist for all sinusoidal cells. Endothelial cells filter the fluids, exchanged between the sinusoid and the space of Disse through fenestrae (100), which measure 175 nm in diameter and are grouped in sieve plates. Fenestrae occupy 6-8% of the surface (106). No intact basal lamina is present under these cells (100). Various factors change the number and diameter of fenestrae [pressure, alcohol, serotonin, and nicotin; for a review, see Fraser et al (32)]. These changes mainly affect the passage of lipoproteins, which contain cholesterol and vitamin A among other components. Fat-storing cells are pericytes, located in the space of Disse, with long, contractile processes, which probably influence liver (sinusoidal) blood flow. Fat-storing cells possess characteristic fat droplets, which contain a large part of the bodys depot of vitamin A (91, 93). These cells play a major role in the synthesis of extracellular matrix (ECM) (34, 39-41). Strongly reduced levels of vitamin A occur in alcoholic livers developing fibrosis (56). Vitamin A deficiency transforms fat-storing cells into myofibroblast-like cells with enhanced ECM production (38). Kupffer cells accumulate in periportal areas. They specifically endocytose endotoxin (70), which activates these macrophages. Lipopolysaccharide, together with interferon γ, belongs to the most potent activators of Kupffer cells (28). As a result of activation, these cells secrete oxygen radicals, tumor necrosis factor, interleukin 1, interleukin 6, and a series of eicosanoids (28) and become cytotoxic against tumor cells [e.g., colon carcinoma cells (19, 22, 48)]. Toxic secretory products can cause necrosis of the liver parenchyma, which constitutes a crucial factor in liver transplantation (55). Pit cells possess characteristic azurophylic granules and display a high level of spontaneous cytolytic activity against various tumor cells, identifying themselves as natural killer cells (10). The number and cytotoxicity of pit cells can be considerably enhanced with biological response modifiers, such as Zymosan or interleukin 2 (8). Pit cell proliferation occurs within the liver, but recent evidence indicates that blood large granular lymphocytes develop into pit cells in 2 steps involving high- and low-density pit cells (88). Kupffer cells control the motility, adherence, viability, and cytotoxicity of pit cells (89), whereas cytotoxicity against tumor cells is synergistically enhanced (80, 81).

The new anti-actin agent dihydrohalichondramide reveals fenestrae-forming centers in hepatic endothelial cells

BackgroundLiver sinusoidal endothelial cells (LSECs) react to different anti-actin agents by increasing their number of fenestrae. A new structure related to fenestrae formation could be observed when LSECs were treated with misakinolide. In this study, we investigated the effects of two new actin-binding agents on fenestrae dynamics. High-resolution microscopy, including immunocytochemistry and a combination of fluorescence- and scanning electron microscopy was applied.ResultsHalichondramide and dihydrohalichondramide disrupt microfilaments within 10 minutes and double the number of fenestrae in 30 minutes. Dihydrohalichondramide induces fenestrae-forming centers, whereas halichondramide only revealed fenestrae-forming centers without attached rows of fenestrae with increasing diameter. Correlative microscopy showed the absence of actin filaments (F-actin) in sieve plates and fenestrae-forming centers. Comparable experiments on umbilical vein endothelial cells and bone marrow sinusoidal endothelial cells revealed cell contraction without the appearance of fenestrae or fenestrae-forming centers.Conclusion(I) A comparison of all anti-actin agents tested so far, revealed that the only activity that misakinolide and dihydrohalichondramide have in common is their barbed end capping activity; (II) this activity seems to slow down the process of fenestrae formation to such extent that it becomes possible to resolve fenestrae-forming centers; (III) fenestrae formation resulting from microfilament disruption is probably unique to LSECs.

CC531s colon carcinoma cells induce apoptosis in rat hepatic endothelial cells by the Fas/FasL-mediated pathway

The mechanisms involved in colorectal carcinoma with liver metastasis are not well known. Metastasizing colon carcinoma cells express more FasL than primary colon carcinoma cells and cancer cells induce apoptosis in hepatocytes by the Fas/FasL pathway. Therefore, this study focused on Fas/FasL expression and functionality in rat liver sinusoidal endothelial cells (LSECs) and CC531s colon carcinoma cells in vitro and in vivo. RT‐PCR and immunochemistry revealed Fas and FasL in LSECs and CC531s, respectively. Functionality of Fas was assessed in vitro by incubation with human recombinant FasL (1–100 ng/ml) with or without enhancer. At concentrations of 10 and 100 ng/ml with enhancer, respectively 21% and 44% of endothelial cells showed signs of apoptosis using Hoechst 33342/propidium iodide staining and electron microscopy. In co‐cultures, apoptosis could be detected in endothelial cells neighboring the CC531s and could be inhibited by an antagonistic FasL antibody. Moreover, 18 h after mesenteric injection of CC531s, the sinusoidal endothelium revealed disruption. In conclusion, (i) CC531s cells induce apoptosis in LSECs in vitro by using Fas/FasL; (ii) CC531s cells damage the sinusoidal endothelial lining in vivo; and (iii) this might provide FasL‐positive tumor cells a gateway towards the hepatocytes.

Noncontact versus contact imaging: An atomic force microscopic study on hepatic endothelial cells in vitro

Liver sinusoidal endothelial cells (LEC) contain fenestrae, which control the exchange of fluids, solutes, and particles between the sinusoidal blood and the microvillous surface of the parenchymal cells. The surface of LEC can be imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM and AFM images of LEC can be used to study dynamic changes in fenestrae by comparing specimens fixed after different experimental treatments. In this article, we report the different results obtained when contact (using a constant force) or noncontact (amplitude detection) imaging on the same cells was applied. Special attention was paid on the optimalization of the image acquisition of fenestrae, because quality SEM examinations of fenestrae have already extensively been described. The following advances and conclusions are presented here: 1) High‐resolution imaging of slightly fixed LEC in fluid can be performed in noncontact AFM; 2) correct acquisition of images of fenestrae with regard to their size (Ø, ±200 nm) and shape (oval, without deformation) under liquid was possible with noncontact AFM, which was hitherto only feasible with fixed, dried, and coated LEC in contact AFM or SEM; 3) this mode of operation is more gentle to cells than contact mode; 4) images of LEC obtained in noncontact mode are of higher quality and are devoid of smearing artefacts, prominently present in contact‐mode images; 5) it is of great importance to optimize feedback and scan parameters to obtain correct surface information; and 6) LEC isolated and cultured by our method are physiologically responsive and represent an ideal object for AFM studies, because the cells are thin, smooth, and well attached to the culture substrate and show dynamic fine structural details, such as fenestrae and coated pits, which cannot be seen by light microscopy.

New observations on cytoskeleton and fenestrae in isolated rat liver sinusoidal endothelial cells

Fenestrae control the exchange of fluids, dissolved compounds and small particles between the blood and the space of Disse, and are primarily limited at one side by parenchymal cells. We recently described a simple and rapid method for the isolation, purification and cultivation of rat liver sinusoidal endothelial cells. With regard to the purity and morphology of liver endothelial cells, a detailed microscopic study was performed. Purity and viability after selective adherence was 74 and 95%, respectively. Liver endothelial cell purity was further enhanced to about 95% during adherence and spreading on collagen after 8 h of culture. Liver endothelial cells isolated by this method provide a viable cell population, enabling the study of structure and function of these cells in vitro. We investigated the cytoskeleton associated with fenestrae and sieve plates of liver endothelial cells. Cultured cells were slightly fixed and treated with cytoskeleton extraction buffer containing 0.1 % Triton. Whole mounts of extracted liver endothelial cells were prepared for scanning and transmission electron microscopy. Extracted liver endothelial cells show an integral, intricate cytoskeleton. Sieve plates and fenestrae are clearly delineated by cytoskeleton elements. Fenestrae are surrounded by a filamentous, fenestrae‐associated cytoskeleton ring with an average filament thickness of 16 nm. Additionally, sieve plates are surrounded and delineated by microtubuli, which form a network together with additional branching cytoskeletal elements. Microtubuli are sometimes found delineating linear arrangements of fenestrae. In conclusion, liver endothelial cells possess a cytoskeleton, that defines and supports sieve plates and fenestrae. Fenestrae‐associated cytoskeleton rings are involved in determining the size of fenestrae. The fenestrae‐associated cytoskeleton therefore probably controls the important hepatic function of endothelial filtration.

Cytotoxic reactions of CC531s towards liver sinusoidal endothelial cells: a microscopical study

Colorectal cancer cells can induce apoptosis in cells of various tissues [1]. Apoptosis can be induced by a number of factors such as Fas inducing apoptosis through the Fas/FasL pathway; other factors involve the TRAIL pathway and TNF. It is known that metastasizing colon cancer cells express more FasL then primary carcinoma cells [2]. We investigated whether a rat colon carcinoma cell line CC531s could induce apoptosis in liver sinusoidal endothelial cells (LSECs). LSECs and CC531s were co-cultured for 18 hrs and cells were visualized by SEM and TEM. Apoptosis was visualized by markers such as Hoechst and Propidium iodide. Furthermore, cells were recorded by time lapse video microscopy with and without an antagonistic antibody for FasL.

The effect of cytochalasin B – Loaded liposomes on the ultrastructure of the liver sieve

Liver sinusoidal endothelial cells (LSECs) possess fenestrae whose number can be increased both in vitro and in situ by depolymerizing the actin cytoskeleton [1]. Specially designed liposomes can be targeted with a high efficiency to LSECs. These liposomes, which were surface grafted with poly-anionized albumin (Aco-HSA) [2], can be filled with various substances, such as microfilament-disrupting drugs. This technique opens up attractive possibilities to modulate the liver sieve of LSECs with liposome-encapsulated microfilament-disrupting drugs in vivo. The goal of this study was to alter the sieves porosity by using cytochalasin B-loaded Aco-HSA liposomes. The increase in the liver sieve porosity induced by cytochalasin B (CB) may be exploited therapeutically to improve the extraction of atherogenic lipoproteins from the circulation; or to enhance the efficiency of liposome-mediated gene or drug delivery to hepatocytes.

Confocal laser scanning microscopic study of the killing of metastatic colon carcinoma cells by Kupffer cells in the early onset of hepatic metastasis

During hepatic metastasis, tumor cells are exposed to the sinusoidal environment, involving endothelial cells, Kupffer cells, pit cells (NK cells) and fat-storing cells [1]. It is known that Kupffer cells and pit cells play a direct role in killing metastasizing colon carcinoma cells [1,2]. Only few studies describe the interactions of Kupffer cells and tumor cells within the first 24 hrs of metastasis [3-6]. The importance of this early phase lies in the substantial but not complete killing of tumor cells. Also, at later stages, when different elements of host defense are involved, it is apparent that the local immune system is not capable of preventing metastasis. Investigating rare cellular events is facilitated by studying the full depth of thick sections with confocal laser scanning microscopy. Complicated preparation and histochemical procedures and a low sampling-volume [7] characterize classical microscopic methods. In contrast, confocal laser scanning microscopy (CLSM) has the ability to study thick sections (100 micrometers), gathering 3D information and allowing the use of different fluorescent probes for different variables [8]. Localization of cells can be done by immunohistochemistry, by labeling cells in vitro prior to injection or by labeling them in vivo. The anti-metastatic function of Kupffer cells was studied by labeling tumor cells with the lipophilic probe DiO in vitro and Kupffer cells with fluorescent latex particles in vivo.

Interaction of colon carcinoma cells with rat hepatic sinusoidal cells during early stages of metastasis

Interaction of colon carcinoma cells with rat hepatic sinusoidal cells during early stages of metastasis

The Structure of Different Types of Liver Cells in Relation to Uptake and Exchange Processes

The structural unit of the liver is classically named the liver lobule and is defined as a unit of parenchymal tissue characterized by peripheral branches of the portal vein and hepatic artery, and by a centrilobular branch of the hepatic vein, i.e. the central vein. The blood enters the lobule via the portal tracts through sinusoidal inlets and, after interaction with the parenchymal tissue during passage through the hepatic sinusoids, leaves the lobule through the central veins [Wisse and De Leeuw, 1984].