Processing of lipoproteins in human monocyte-macrophages

Specific functions of lysosomal proteases in endocytic and autophagic pathways

Endolysosomal vesicles form a highly dynamic multifunctional cellular compartment that contains multiple highly potent proteolytic enzymes. Originally these proteases have been assigned to cooperate solely in executing the unselective ‘bulk proteolysis’ within the acidic milieu of the lysosome. Although to some degree this notion still holds true, evidence is accumulating for specific and regulatory functions of individual ‘acidic’ proteases in many cellular processes linked to the endosomal/lysosomal compartment. Here we summarize and discuss the functions of individual endolysosomal proteases in such diverse processes as the termination of growth factor signaling, lipoprotein particle degradation, infection, antigen presentation, and autophagy. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Uptake of oxidized LDL by macrophages results in partial lysosomal enzyme inactivation and relocation

The cytotoxicity of oxidized LDL (oxLDL) to several types of artery wall cells might contribute to atherosclerosis by causing cell death, presumably by both apoptosis and necrosis. After its uptake into macrophage lysosomes by receptor-mediated endocytosis, oxLDL is poorly degraded, resulting in ceroid-containing foam cells. We studied the influence ofoxLDL on lysosomal enzyme activity and, in particular, on lysosomal membrane stability and the modulation of these cellular characteristics by HDL and vitamin E (vit-E). Unexposed cells and cells exposed to acetylated LDL (AcLDL) were used as controls. The lysosomal marker enzymes cathepsin L and N-acetyl-beta-glucosaminidase (NAbetaGase) were biochemically assayed in J-774 cells after fractionation. Lysosomal integrity in living cells was assayed by the acridine orange (AO) relocation test. Cathepsin D was immunocytochemically demonstrated in J-774 cells and human monocyte-derived macrophages. We found that the total activities of NAbetaGase and cathepsin L were significantly decreased, whereas their relative cytosolic activities were enhanced, after oxLDL exposure. Labilization of the lysosomal membranes was further proven by decreased lysosomal AO uptake and relocation to the cytosol of cathepsin D, as estimated by light and electron microscopic immunocytochemistry. HDL and vit-E diminished the cytotoxicity of oxLDL by decreasing the lysosomal damage. The results indicate that endocytosed oxLDL not only partially inactivates lysosomal enzymes but also destabilizes the acidic vacuolar compartment, causing relocation of lysosomal enzymes to the cytosol. Exposure to AcLDL resulted in its uptake with enlargement of the lysosomal apparatus, but the stability of the lysosomal membranes was not changed.

LDL stimulates chemotaxis of human monocytes through a cyclooxygenase-dependent pathway

Monocyte migration into the vessel wall is an early step in atherogenesis. Even though a number of chemotactic factors have been identified, the regulation of the chemotactic response is not clearly understood. As the release of arachidonic acid has been implicated in monocyte chemotaxis, we studied the influence of LDL, which can supply this fatty acid to cells, on the chemotactic mobility of monocytes. Migration of human monocytic U937 cells was abolished by a 30-hour incubation in medium containing lipoprotein-depleted 10% fetal calf serum. Thereafter, human VLDL, LDL, acetyl LDL, methyl LDL, HDL, free cholesterol, linoleic acid, oleic acid, or arachidonic acid was added. At the end of varying incubation periods (0.5 to 8 hours), chemotaxis, viability, and cellular cholesterol content were measured. In the same experimental setting we also studied the effects of the pharmacological agents chloroquine, indomethacin, and acetylsalicylic acid on LDL-mediated chemotaxis. Chemotaxis was restored by LDL in a dose- and time-dependent manner starting at concentrations as low as 5 micrograms/mL and at incubations as brief as 30 minutes. The other lipoproteins tested (VLDL, HDL, acetyl LDL, and methyl LDL) as well as free cholesterol had no comparable effect on chemotaxis. Viability and total cholesterol content did not differ among the groups. Simultaneous incubation of cells with chloroquine, indomethacin, and acetylsalicylic acid reduced restitution of chemotaxis by LDL by 71%, 82%, and 68%, respectively. In contrast, the agents had only slight inhibitory effects on the chemotactic mobility of serum-fed control cells. Incubation with linoleic acid showed a 60% restoration of chemotaxis, whereas arachidonic acid stimulated chemotaxis by 140% compared with the positive control. Preincubation of LDL with the monoclonal antibody MB47 directed against LDL resulted in a significantly reduced migratory response. The data suggest a novel cyclooxygenase-dependent regulatory mechanism of chemotaxis by LDL.

Cellular binding and degradation of lipoprotein (a)

Lp(a) is an important contributing factor to the development of atherosclerosis, and in structure is similar to LDL. Given the central role of the LDL receptor (LDL-R) in the metabolism of LDL, we felt that a study of the binding and degradation of Lp(a) facilitated by the LDL-R of human monocyte derived macrophages (HMDM) would be of value in understanding its pathological nature. In this study we compared equimolar amounts of Lp(a) and LDL and found that nearly equal amounts of Lp(a) and LDL bound to the LDL-R of HMDM at 4 degrees C, however the affinity of both lipoproteins was much lower than has been observed for the LDL-R of fibroblasts, being 0.80 muM for Lp(a) and 0.23 muM for LDL. The binding of Lp(a) to HMDM could be competed by 63% with a 50-fold excess of LDL. Degradation of Lp(a) at 37 degree C, unlike 4 degrees C binding, was mainly nonspecific (75% of total Lp(a) degradation) and when compared on an equimolar basis, nearly 6 times more LDL than Lp(a) was processed by the LDL-R pathway in 5 hr. Lower degradation of Lp(a) appears to be the result of lower binding at 37 degree C and a lower degradation rate when compared to LDL. It was not caused by increased intracellular accumulation or retroendocytosis. Degradation of both lipoproteins was only modestly affected by up and down regulation of the LDL-R. Because the binding of LDL at 4 degrees C and degradation at 37 degree C is mainly LDL-R specific, whereas only the 4 degree C binding of Lp(a) is so, suggests that the poor LDL-R dependent degradation of Lp(a) at 37 degree C is caused by a conformational change that is inducted in Lp(a) upon lowering the temperature to 4 degree C which allows better recognition of Lp(a) by the HMDM LDL-R.

Effects of lysosomal protease inhibitors on the degradation of acetylated low density lipoprotein in cultured rat peritoneal macrophages

The effect of protease inhibitors, leupeptin and pepstatin A, on the metabolism of acetylated low density lipoprotein (acetyl-LDL) was investigated in cultured rat peritoneal macrophages and compared with that of chloroquine. While both leupeptin and pepstatin inhibited the proteolytic degradation of 125I-acetyl-LDL, a combination of both showed an additive effect. Similar to chloroquine, both protease inhibitors diminished [3H] oleate incorporation into cellular cholesteryl[3H] oleate and increased cholesterol content of macrophages. These results suggest that both thiol protease and cathepsin D participate in the physiological degradation of apolipoprotein in macrophages. The inhibition of apolipoprotein degradation seemed to have an effect on cholesterol metabolism in macrophages cultured with acetyl-LDL.

In situ immunolocalization of lipoproteins in human arteriosclerotic tissue

The concentration of serum lipoproteins, especially those of low density (LDL) and high density (HDL) lipoprotein, are related to the pathogenesis of arteriosclerosis. However, there is a lack of data concerning lipoprotein distribution in the human arteriosclerotic plaque. To detect these lipoproteins, we performed immunogold labeling on ultrathin sections of fixed and embedded human arteriosclerotic tissue. We used a panel of specific antibodies to different lipoproteins and their apolipoprotein constituents, namely LDL, formaldehyde-fixed LDL, apolipoprotein B-100, HDL, and formaldehyde-fixed apolipoprotein A-I. We also applied antibodies to alpha-actin and cathepsin D to characterize the cells and organelles involved in lipoprotein uptake and metabolism. Semiquantitative evaluation was carried out for a detailed comparison of the results obtained. Electron microscopic examination revealed that the majority of HDL and LDL in the pathological tissue was localized intracellularly in macrophage-derived foam cells and smooth muscle cells, whereas only LDL was found in the extracellular matrix. In some cases, we observed an intracellular accumulation of lipoproteins in electron-dense vesicles, which appeared to be of lysosomal origin, as shown by double labeling with an antibody to cathepsin D. These vesicles were present only in macrophage-derived foam cells, which were localized in the necrotic cores of arteriosclerotic plaques, and could not be found in healthy tissue or in the early stages of arteriosclerotic disease.

Alterations in lipolytic activity at hepatic subcellular sites of fed and fasted rats

This study investigates the relationship between the nutritional state of rats and lipid metabolism in distinct hepatic intracellular sites. Hepatic uptake of both protein and triacylglycerol (TG) moieties of injected very low-density lipoprotein (VLDL) is increased in fasted rats compared with fed controls. The VLDL-TG hydrolysis rate is increased in the plasma of fasted rats. This is shown by a higher ratio of labeled free fatty acid (FFA) to TG (FFA/TG). In both fed and fasted rats, a much greater increase of the labeled FFA/TG ratio in endosomes, compared with that in plasma, shows that further TG hydrolysis occurs in prelysosomal compartments. However, in fasted rats, this increase (18-fold) is much less than that in fed rats (69-fold). This observation is supported by the finding of significantly lower TG-lipase activity at pH 5, 7, and 8.6 in the endosomes of fasted rats. In contrast, during fasting, TG-lipase activity in whole liver homogenate and in isolated lysosomes is increased at pH 5. These observations suggest that after feeding there is a shift in intracellular lipolytic activity from lysosomes to prelysosomal organelles.

Characterization of inhibition of Mycobacterium avium replication in macrophages by normal human serum

Serum from some AIDS patients permits the rapid multiplication of Mycobacterium avium in cultured human macrophages. Serum from human immunodeficiency virus-negative individuals inhibits replication. The characteristics of the serum-induced inhibition were examined here. M. avium 7497 serovar 4 grew exponentially in macrophages when they were cultured in serumless medium. Growth was measured by determining the CFU after infected macrophages were lysed at 0 to 7 days after infection. Normal AB serum (5 to 10%) added to infected macrophages resulted in an initial 4-day lag of bacterial growth followed by rapid replication from 4 to 7 days. Serum also inhibited bacterial replication in medium without macrophages. This inhibition was not biphasic but was sustained over 7 days. Macrophage-associated M. avium became less responsive to serum inhibitor within 24 h after infection of macrophages. Within 2 days of culture, M. avium no longer responded to inhibitor. Replication of macrophage-derived M. avium (Vi) was in some instances serum inhibitable and at other times was enhanced by serum, when it was used to infect fresh macrophages. The Vi phenotype remained serum inhibitable without macrophages. Preinfection of macrophages with heat-killed M. avium did not alter serum-induced bacterial inhibition or escape from inhibition.

Antigen processing by endosomal proteases determines which sites of sperm-whale myoglobin are eventually recognized by T cells

This study reports an identification of the major processing products of an exogenous protein antigen, viz, sperm-whale myoglobin, as obtained after cell-free processing with partially purified macrophage endosomes. It is demonstrated that such a system yields fragments that are indistinguishable by high performance liquid chromatography analysis from those generated after uptake of myoglobin inside live macrophages. The concerted action of the endosomal proteases cathepsin D and cathepsin B can account for nearly all cleavages observed. Cathepsin D appears to be mainly responsible for the initial cleavage of myoglobin, while cathepsin B catalyzes the C-terminal trimming of initially released fragments. The fragments released by cathepsin D contain most, if not all, major epitopes for murine myoglobin-specific helper T cells. Interestingly, each known T cell epitope of myoglobin is located at the very N terminus of a different myoglobin fragment released upon processing. In order to explain this correspondence, noted also in several other protein antigens, a structural relationship is proposed between antigen processing by cathepsin D and antigen recognition by major histocompatibility complex (MHC) class II products. As is demonstrated here, this relationship may be used as a predictive tool for the identification of MHC-binding sequences as well as of T cell epitopes in their naturally occurring form.