High density lipoprotein metabolism.

Macrophage type-I and type-II class-A scavenger receptors (MSR-A) are implicated in the pathological deposition of cholesterol during atherogenesis as a result of receptor-mediated uptake of modified low-density lipoproteins (mLDL)1–6. MSR-A can bind an extraordinarily wide range of ligands, including bacterial pathogens7, and also mediates cation-independent macrophage adhesion in vitro8. Here we show that targeted disruption of the MSR-A gene in mice results in a reduction in the size of atherosclerotic lesions in an animal deficient in apolipoprotein E. Macrophages from MSR-A-deficient mice show a marked decrease in mLDL uptake in vitro, whereas mLDL clearance from plasma occurs at a normal rate, indicating that there may be alternative mechanisms for removing mLDL from the circulation. In addition, MSR-A-knockout mice show an increased susceptibility to infection with Listeria monocytogenes or herpes simplex virus type-1, indicating that MSR-A may play a part in host defence against pathogens.

A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection

Hyperoxia increases oxygen radical production in rat lungs and lung mitochondria.

Remnant lipoprotein metabolism: key pathways involving cell-surface heparan sulfate proteoglycans and apolipoprotein E.

A recombinant beta-galactosidase gene has been expressed in a specific arterial segment in vivo by direct infection with a murine amphotropic retroviral vector or by DNA transfection with the use of liposomes. Several cell types in the vessel wall were transduced, including endothelial and vascular smooth muscle cells. After retroviral infection, a recombinant reporter gene was expressed for at least 5 months, and no helper virus was detected. Recombinant gene expression achieved by direct retroviral infection or liposome-mediated DNA transfection was limited to the site of infection and was absent from liver, lung, kidney, and spleen. These results demonstrate that site-specific gene expression can be achieved by direct gene transfer in vivo and could be applied to the treatment of such human diseases as atherosclerosis or cancer.

https://science.sciencemag.org/content/sci/249/4974/1285.full.pdf

Site-specific gene expression in vivo by direct gene transfer into the arterial wall.

The effect of a water-soluble tris-galactoside-terminated cholesterol derivative on the fate of low-density lipoproteins and liposomes

1. Modified lipoproteins have been implicated to play a significant role in the pathogenesis of atherosclerosis. In view of this we studied the fate and mechanism of uptake in vivo of acetylated human low-density lipoprotein (acetyl-LDL). Injected intravenously into rats, acetyl-LDL is rapidly cleared from the blood. At 10min after intravenous injection, 83% of the injected dose is recovered in liver. Separation of the liver into a parenchymal and non-parenchymal cell fraction indicates that the non-parenchymal cells contain a 30–50-fold higher amount of radioactivity per mg of cell protein than the parenchymal cells. 2. When incubated in vitro, freshly isolated non-parenchymal cells show a cell-association of acetyl-LDL that is 13-fold higher per mg of cell protein than with parenchymal cells, and the degradation of acetyl-LDL is 50-fold higher. The degradation of acetyl-LDL by both cell types is blocked by chloroquine (10–50μm) and NH4Cl (10mm), indicating that it occurs in the lysosomes. Competition experiments indicate the presence of a specific acetyl-LDL receptor and degradation pathway, which is different from that for native LDL. 3. Degradation of acetyl-LDL by non-parenchymal cells is completely blocked by trifluoperazine, penfluridol and chlorpromazine with a relative effectivity that corresponds to their effectivity as calmodulin inhibitors. The high-affinity degradation of human LDL is also blocked by trifluoperazine (100μm). The inhibition of the processing of acetyl-LDL occurs at a site after the binding-internalization process and before intralysosomal degradation. It is suggested that calmodulin, or a target with a similar sensitivity to calmodulin inhibitors, is involved in the transport of the endocytosed acetyl-LDL to or into the lysosomes. 4. It is concluded that the liver, and in particular non-parenchymal liver cells, are in vivo the major site for acetyl-LDL uptake. This efficient uptake and degradation mechanism for acetyl-LDL in the liver might form in vivo the major protection system against the potential pathogenic action of modified lipoproteins.

/pdf/processing-of-acetylated-human-low-density-lipoprotein-by-5gsk6t2rsj.pdf

Processing of acetylated human low-density lipoprotein by parenchymal and non-parenchymal liver cells. Involvement of calmodulin?

Saturable high affinity binding, uptake and degradation of rat plasma lipoproteins by isolated parenchymal and non-parenchymal cells from rat liver

1
Intact parenchymal and non-parenchymal cells were isolated from rat liver. The parenchymal cells were purified by differential centrifugation, while non-parenchymal cells were obtained free of parenchymal cell contamination by preferentially destroying the parenchymal cells with the aid of pronase (0.25%).

2
The ability to isolate pure intact parenchymal and non-parenchymal cells permitted the characterization and measurement of specific activities of various lysosomal enzymes, representing the main functional hydrolytic activities of the lysosomes in these distinct cell types.

3
Lysosomal enzymes catalysing the hydrolysis of the terminal carbohydrate moiety of glycoproteins and glycolipids were not particularly enriched in the non-parenchymal cells as compared to parenchymal cells. The ratio of the specific activities of non-parenchymal cells over parenchymal cells varied between 0.7 for N-acetyl-β-d-hexoseaminidase to 2.1 for α-glucosidase. This suggests no specific role of the non-parenchymal cells in the hydrolysis of terminal carbohydrate moieties of glycoproteins and glycolipids.

4
The enzymes acid phosphatase and aryl sulphatase, representing the phosphate and sulphate hydrolyzing activities, were enriched in the non-parenchymal cells as compared to the parenchymal cells by a factor of 2.5.

5
The most important peptidase cathepsin D, representing protein breakdown capacity, is enriched in the non-parenchymal cells as compared to parenchymal cells by a factor 6.0, suggesting a possible specific function of non-parenchymal cells in protein breakdown.

6
The most enriched lysosomal enzyme, representing lipid hydrolysis, is acid lipase, which is enriched in the non-parenchymal cells with a factor of 10.

7
The distribution of lysosomal enzymes between parenchymal and non-parenchymal cells suggests different functional roles of the lysosomes in these cell types. It can be concluded that the non-parenchymal cells possess a set of lysosomal enzymes which makes them extremely suitable for a phagocytic and antimicrobial function in the liver.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1975.tb02358.x

Identity and activities of lysosomal enzymes in parenchymal and non-parenchymal cells from rat liver.

Rat transferrin or asialotransferrin doubly radiolabelled with 59Fe and 125I was injected into rats. A determination of extrahepatic and hepatic uptake indicated that asialotransferrin delivers a higher fraction of the injected 59Fe to the liver than does transferrin. In order to determine in vivo the intrahepatic recognition sites for transferrin and asialotransferrin, the liver was subfractionated into parenchymal, endothelial and Kupffer cells by a low-temperature cell isolation procedure. High-affinity recognition of transferrin (competed for by an excess of unlabelled transferrin) is exerted by parenchymal cells as well as endothelial and Kupffer cells with a 10-fold higher association (expressed per mg of cell protein) to the latter cell types. In all three cell types iron delivery occurs, as concluded from the increase in cellular 59Fe/125I ratio at prolonged circulation times of transferrin. It can be calculated that parenchymal cells are responsible for 50-60% of the interaction of transferrin with the liver, 20-30% is associated with endothelial cells and about 20% with Kupffer cells. For asialotransferrin a higher fraction of the injected dose becomes associated with parenchymal cells as well as with endothelial and Kupffer cells. Competition experiments in vivo with various sugars indicated that the increased interaction of asialotransferrin with parenchymal cells is specifically inhibited by N-acetylgalactosamine whereas mannan specifically inhibits the increased interaction of asialotransferrin with endothelial and Kupffer cells. Recognition of asialotransferrin by galactose receptors from parenchymal cells or mannose receptors from endothelial and Kupffer cells is coupled to active 59Fe delivery to the cells. It is concluded that, as well as parenchymal cells, liver endothelial and Kupffer cells are also quantitatively important intrahepatic sites for transferrin and asialotransferrin metabolism, an interaction exerted by multiple recognition sites on the various cell types.

Johan K. Kruijt

Papers

The effect of a water-soluble tris-galactoside-terminated cholesterol derivative on the fate of low-density lipoproteins and liposomes

Processing of acetylated human low-density lipoprotein by parenchymal and non-parenchymal liver cells. Involvement of calmodulin?

Saturable high affinity binding, uptake and degradation of rat plasma lipoproteins by isolated parenchymal and non-parenchymal cells from rat liver

Identity and activities of lysosomal enzymes in parenchymal and non-parenchymal cells from rat liver.

The interaction in vivo of transferrin and asialotransferrin with liver cells.