Proteolysis of canine apolipoprotein by acid proteases in canine liver lysosomes

Leupeptin as a tool for the detection of the sites of catabolism of rat high-density lipoprotein apolipoproteins A-I and E

Leupeptin, an inhibitor of lysosomal cathepsin activity, was injected intravenously into male rats. Tissues obtained from leupeptin-treated animals showed a depressed cathepsin activity when compared with tissues from saline-treated control animals. Leupeptin treatment did not change the hepatic activities and subcellular distribution of marker enzymes for mitochondria, microsomes and plasma membranes. Hepatic lysosomal cathepsin activity was specifically inhibited, but the subcellular distribution of all lysosomal marker enzymes tested was changed, indicating the occurrence of enlarged lysosomes in the leupeptin-treated animals. No significant differences were observed in the serum concentrations of protein, cholesterol, cholesteryl esters, phospholipids and apolipoproteins A-I, A-IV and E between leupeptin-treated rats and control animals. When radioiodinated asialofetuin was injected intravenously, the radiolabel was retained for an extended period of time in the liver of leupeptin-treated animals, indicating diminished catabolism of this protein in the liver. When rat high-density lipoprotein, labelled specifically in the apolipoprotein A-I or E moiety was injected intravenously, only the kidneys and the liver showed a leupeptin-induced accumulation of radioactivity. These studies provide evidence for an important contribution of the kidneys and the liver to the in vivo catabolism of high-density lipoprotein apolipoproteins, using a method completely different from sugar-containing labelling compounds.

The sites of degradation of rat high-density-lipoprotein apolipoprotein E specifically labelled with O-(4-diazo-3-[125I]iodobenzoyl)sucrose

O-(4-Diazo-3-[125I]iodobenzoyl)sucrose ([125I]DIBS), a novel labelling compound specifically designed to study the catabolic sites of serum proteins [De Jong, Bouma, & Gruber (1981) Biochem. J. 198, 45-51], was applied to study the tissue sites of degradation of serum lipoproteins. [125I]DIBS-labelled apolipoproteins (apo) E and A-I, added in tracer amounts to rat serum, associate with high-density lipoproteins (HDL) just like conventionally iodinated apo E and A-I. No difference is observed between the serum decays of chromatographically isolated [125I]DIBS-labelled and conventionally iodinated HDL labelled specifically in either apo E or apo A-I. When these specifically labelled HDLs are injected into fasted rats, a substantial [125I]DIBS-dependent 125I accumulation occurs in the kidneys and in the liver. No [125I]DIBS-dependent accumulation is observed in the kidneys after injection of labelled asialofetuin or human low-density lipoprotein. It is concluded that the kidneys and the liver are important sites of catabolism of rat HDL apo E and A-I.

HDL3 degradation by proteases in isolated rat intestinal mucosal cells

Human 125I-labelled HDL3 is degraded by isolated rat intestinal mucosal cells. In our experimental conditions, lipoprotein degradation occurred by two different mechanisms. In one, lipoprotein was degraded within the cell, following binding and internalisation. In the other, degradation occurred in the medium, which seemed to contain protease activity released from cells during incubation. Though lipoprotein-deficient serum apparently interfered with degradation in the medium, bovine serum albumin had no such effect. The lysosomal inhibitor, chloroquine, reduced degradation by 60% without inhibiting HDL binding. Intestinal cell extracts contained at least two different proteases, with pH optima of 4.5 and 8.0, respectively. Comparing HDL and LDL degradation on a molar basis, more HDL particles were degraded by the cell-free extracts at pH 4.5. This degradation was activated by dithiothreitol and was inhibited by iodoacetic acid. From these observations we conclude that HDL3 is taken up by the rat intestinal mucosal cell through a specific binding site and subsequently degraded by a thiol-dependent protease in the lysosome.

Proteolytic degradation of low density lipoproteins by arterial smooth muscle cells: the role of individual cathepsins

Low density lipoproteins have been implicated in the pathogenesis of atherosclerosis. A study has therefore been made of their proteolytic degradation by homogenates of cultured smooth muscle cells from the pig aorta. The pH optimum of proteolysis of 125I-labelled low density lipoproteins was 4.25, thus suggesting the involvement of lysosomal cathepsins. Proteolysis at acid pH started to become saturated at low density lipoprotein concentrations of approx. 20 microgram of protein/ml, but did not obey Michaelis-Menten kinetics. After a lag period of approx. 10 min, proteolytic degradation was linear with time up to at least 4 h incubation, but showed a sigmoidal relationship with homogenate concentration. When cathepsin D was inhibited by pepstatin, the proteolysis of 125I-labeled low density lipoproteins was inhibited by more than 90%, whereas when cathepsin B was inhibited by leupeptin, the rate of proteolysis decreased by approx. 50%. Antipain, which inhibits both cathepsins A and B, did not inhibit proteolysis any more than leupeptin, thus suggesting a minor role, if any, for cathepsin A. a combination of pepstatin and either leupeptin or antipain inhibited proteolysis completely. Cathepsins B and D acted synergistically in the degradation of 125I-labelled low density lipoproteins.

Degradation of apolipoprotein B of low density lipoprotein by cultured bovine smooth muscle cells. Accumulation of intermediates in the presence of protease inhibitors

The incubation of bovine aortic smooth muscle cells with 125I-labeled low density lipoprotein in the presence of protease inhibitors resulted in the significant intracellular accumulation of intact apolipoprotein B, as well as a number of high molecular weight degradation intermediates. This effect was brought about both by leupeptin, a specific inhibitor of thiol proteases (40% inhibition of degradation), as well as by the more general lysosomotrophic inhibitors, chloroquine and NH4Cl. Qualitatively identical spectra of degradation intermediates were formed in the presence of chloroquine and NH4Cl as determined by autoradiography of SDS-polyacrylamide gel electrophoretic fractions, ranging from the apolipoprotein B band (Mr = 340,000) to bands with molecular weights of less than 14,000. The effect of NH4Cl was reversible and release of inhibition resulted in the sequential loss of intermediates from the cells; those species having higher molecular weights disappearing first. Inhibition by leupeptin was associated with the accumulation of degradation products in the lower molecular weight range only (Mr less than or equal to 72,000). These results provide evidence that apolipoprotein B proteolysis progresses in distinct stages via specific breakdown products and suggest that the thiol cathepsins become more active later in the degradation pathway.

Cathepsin-D-dependent initiation of the hydrolysis by lysosomal enzymes of apoprotein B from low-density lipoproteins

The degradation of 135I-apoprotein B of human low-density lipoprotein by cell extracts of cultured bovine aortic smooth muscle cells was determined by measuring the formation of acid-soluble products and by analyzing the electrophoretic patterns of digested apoprotein in gels containing sodium dodecyl sulfate. Degradation resulted in an initial rapid accumulation of a limited number of distinct smaller fragments. Two products with apparent molecular weights of 220,000 and 200,000 predominated. Pepstatin inhibited proteolysis almost completely, as measured by either assay. Leupeptin decreased hydrolysis to acid-soluble products by approximately 50%, but had no effect on the initial cleavage of intact apoprotein B. Similar results were found in the case of extracts from cultured human skin fibroblasts and from adult bovine arterial smooth muscle. Leupeptin inhibited intracellular degradation of 125I-apoprotein B in cultured cells by approximately 50%. It is concluded that the intralysosomal degradation of apoprotein B involves an initial limited endoproteolytic attack at susceptible sites by cathepsin D. This and other enzymes, including cathepsin B, then act synergistically to bring degradation to completion.

High density lipoprotein and low density lipoprotein catabolism by human liver and parenchymal and non-parenchymal cells from rat liver

The capacity of the homogenates from human liver, rat parenchymal cells, rat non-parenchymal cells and total rat liver for the breakdown of human and rat high density lipoprotein (HDL) and human low density lipoprotein (LDL) was determined. Human HDL was catabolized by human liver, in contrast to human LDL, the protein degradation of which was low or absent. Human and rat HDL were catabolized by both the rat parenchymal and non-parenchymal cell homogenates with, on protein base, a 10-times higher activity in the non-parenchymal liver cells. This implies that more than 50% of the total liver capacity for HDL protein degradation is localized in these cell types. Human LDL degradation in the rat could only be detected in the non-parenchymal cell homogenates. These findings are discussed in view of the function of HDL and LDL as carriers for cholesterol.

Chloroquine-induced interference with degradation of serum lipoproteins in rat liver, studied in vivo and in vitro

The effect of chloroquine, an inhibitor of certain lysosomal enzymes including cathepsin B (EC 3.4.22.1), on the degradation of serum lipoproteins in rat liver was studied in vivo and in liver homogenates. Chloroquine had no effect on the clearance from the circulation of 125I-labeled rat or human very low density lipoproteins or human low density lipoproteins. Pretreatment with chloroquine for 3 h, resulted in a 2-2.5 fold increase in 125i-labeled very low density lipoprotein recovered in the liver 45 min after injection of the homologous and heterologous lipoproteins. This effect was evident on both the 125I-labeled protein and 125I-labeled lipid moiety. 30 min after the injection of [3H]-cholesterol linoleate-labeled very low density lipoproteins, 70% of the injected label was recovered in the liver, both in control and chloroquine-treated rats. Since the perl and 20% in the experimental group, it was concluded that chloroquine interferes with the hydrolysis of [3H]cholesterol linoleate. Following injection of 125I-labeled human low density lipoproteins only 4% of the injected lipoprotein was recovered in the liver of control rats and not more than 10% after chloroquine treatment, when about 50% had been cleared from the circulation. Hence, while very low density lipoprotein protein and cholesterol ester are catabolized in the liver, the catabolism of low density lipoproteins occurs mainly in extra-hepatic tissues. Using post-nuclear liver suprnatant, optimal degradation of various serum lipoproteins was found at pH 4.4, and chloroquine inhibited their degradation. Degradation of very low density and low density lipoproteins was completely inhibited at 0.05 M chloroquine, while less pronounced inhibition was seen with high density lipoproteins, apolipoproteins and apolipoprotein AI. These results indicate that liver acid hydrolases in vivo participate in the degradation of serum lipoproteins. Cathepsin B is apparently responsible for the degradation of aplipoprotein B, while other cathepsins might also be active in the degradation of this and the other apolipoproteins.