Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P20645 (mannose-6-phosphate receptor)
 320 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Macropinosomes formed by addition of recombinant macrophage colony-stimulating factor (rM-CSF) to mouse macrophages migrate centripetally and shrink, remaining detectable by phase microscopy for up to 15 min. This longevity allowed us to study how macropinosomes age. Macropinosomes were pulse labeled for 1 min with fixable fluorescein dextran (FDx10f), a probe for fluid phase pinocytosis, and chased for various times. To quantify changes in their antigenic profile, pulse-labeled macropinosomes of different ages were fixed and stained for immunofluorescence with a panel of antibodies specific for the transferrin receptor (TfR), the late endosome-specific, GTP-binding protein rab 7 or lysosomal glycoprotein A (lgp-A), and the percentage of antibody positive, FDx10f-labeled macropinosomes was scored. Some newly formed macropinosomes were positive for TfR, but few were rab 7 or lgp-A-positive. With intermediate chase times (2-4 min), staining for rab 7 and lgp-A increased to > 60%, while TfR staining declined. After a long chase (9-12 min), rab 7 staining returned to low levels while lgp-A staining remained at a high level. Thus, macropinosomes matured by progressive acquisition and loss of characteristic endocytic vesicle markers. However, unlike a maturation process, their merger with the tubular lysosomal compartment more nearly resembled the incorporation of a transient vesicle into a pre-existing, stable compartment. Shortly after their formation, FDx10f-labeled macropinosomes contacted and merged with Texas red dextran (TRDx10)-labeled tubular lysosomes. This occurred in two steps: macropinosomes acquired lgp-A first, and then several minutes later the cation-independent mannose-6-phosphate receptor (CI-MPR) and markers of lysosomal content (cathepsin L or pre-loaded TRDx10), all apparently derived from tubular lysosomes. Thus, macropinosome progress through macrophages showed features of both the maturation and vesicle shuttle models of endocytosis, beginning with a maturation process and ending by merger into a stable, resident lysosomal compartment.
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PMID:Macropinosome maturation and fusion with tubular lysosomes in macrophages. 809 75

Cathepsin D is overexpressed in most primary breast cancers where its concentration is correlated with increased metastatic potential. To investigate the possible role and mechanism of this lysosomal protease in metastasis, we transfected low-metastatic rat tumor cells with wild-type human cathepsin D, or mutated forms obtained by insertion of a KDEL peptide signal responsible for ER retention, or a control KDAS peptide. The overexpressed pro-cathepsin D in wild-type and KDAS clones was normally sorted and maturated in lysosomes. In KDEL clones, pro-cathepsin D was mostly retained in the ER or partially secreted by high-producer clones but was not maturated. While overexpressed cathepsin D increased experimental metastasis in athymic mice, the pro-cathepsin/D-KDEL was totally ineffective. Moreover, the effect of cathepsin D on metastasis did not seem to be due to saturation of the mannose-6-phosphate receptor since the secretion of two other rat lysosomal enzymes was unaffected by cathepsin D overexpression. We conclude that pro-cathepsin D overexpression facilitates tumor metastasis only when maturated into an active enzyme.
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PMID:Cathepsin D maturation and its stimulatory effect on metastasis are prevented by addition of KDEL retention signal. 813 16

We have studied the intracellular fate of the apolipoprotein B of copper-oxidized LDL in cultured J774 macrophages, using subcellular fractionation and immunofluorescence techniques. The oxidized apolipoprotein B, using cell fractionation, was located primarily in secondary lysosomes (identified using the lysosomal marker-enzyme aryl sulfatase). Light microscopy using antibodies to the mannose-6-phosphate receptor, the lysosomal membrane protein lgp 120, and oxidized LDL (biotinylated) confirmed that apo B of oxidized LDL did accumulate in secondary lysosomes rather than in endosomes. We conclude from these results that the oxidized apolipoprotein B of LDL reaches the secondary lysosomes, but is not efficiently degraded, leading to intracellular accumulation within this compartment. If this occurs in vivo it may influence the physiology of the macrophage and their subsequent roles in forming foam cells and the development of the fatty streaks of early atherosclerosis.
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PMID:Apolipoprotein B of oxidized LDL accumulates in the lysosomes of macrophages. 815 30

beta-Glucuronidase undergoes proteolytic C-terminal processing during or after its transport to lysosomes or endosomes. We determined the C-terminal processing site for human placental beta-glucuronidase to be the peptide bond between Thr633-Arg634. To evaluate the role of the 18-amino acid peptide removed in C-terminal processing, we changed the codon for Arg634 to a stop codon by site-directed mutagenesis and studied expression of the truncated mutant enzyme in COS-7 cells. An increased fraction of newly synthesized enzyme from R634Stop cDNA was secreted. Pulse-chase experiments provided no evidence for increased degradation of the intracellular R634Stop enzyme. The total amount of catalytic activity expressed from the R634Stop mutant cDNA was only half that seen with the wild type cDNA, and the Kcat of the mutant enzyme was 52% that of wild type enzyme. These results indicate that the C-terminal propeptide in the precursor is important for beta-glucuronidase to achieve maximal activity. The truncated enzyme formed hybrid tetramers in cotransfection experiments with the cDNA for rat beta-glucuronidase. There appeared to be no decrease in stability of the R634Stop enzyme, since chaotropic agents, heat treatment, and pH had similar effects on the mutant and the wild type enzymes. The uptake rate of the truncated mutant (R634Stop) enzyme by beta-glucuronidase-deficient human fibroblast cells was only 55-60% that of the wild type enzyme. Binding to the immobilized cation-independent mannose-6-phosphate receptor and measurement of the 32P-labeled phosphorylated oligosaccharides revealed that the truncated mutant enzyme was 32-34% less phosphorylated and appeared to contain proportionately more covered phosphate groups than the wild type enzyme. These results suggest that the propeptide influences the accessibility to both processing enzymes that produce the mannose-6-phosphate recognition marker on beta-glucuronidase.
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PMID:C-terminal processing of human beta-glucuronidase. The propeptide is required for full expression of catalytic activity, intracellular retention, and proper phosphorylation. 822 71

The intracellular transport of [125I]tyramine cellobiose low-density lipoprotein ([125ITC]LDL) and [131ITC]beta-very-low-density lipoprotein ([131ITC]beta-VLDL) in rat liver was studied by means of centrifugation in sucrose and Nycodenz gradients. At time-points up to 45 min after intravenous injection, the two ligands were found in endosomes with distinctly different buoyant densities. In the Nycodenz gradients [131ITC]beta-VLDL appeared at 1.08 g/ml partly coinciding with the distribution of the cation independent (alpha)mannose-6-phosphate receptor, whereas [125ITC]LDL was found at 1.13 mg/ml, where the degradation of [125ITC]LDL started. [131ITC]beta-VLDL, on the other hand, was transferred to denser vesicles, banding at 1.16 g/ml, and degradation started in these organelles, similar to that observed with asialoorosomucoid (ASOR) that was used as a control ligand. Since degradation products coincided with beta-N-acetylglucosaminidase we assume that these organelles are secondary lysosomes. [125ITC]LDL was subsequently also transferred to these dense secondary lysosomes, and the distribution of degraded [125ITC]LDL was therefore bimodal until [125ITC]LDL was completely cleared from the circulation. Furthermore our results show that the different intracellular pathways observed are not due to uptake in different liver cell types, since the bimodal distribution of [125ITC]LDL was also evident in purified liver parenchymal cells. The data suggest that LDL and beta-VLDL follow different endosomal pathways in the rat hepatocytes and that both pathways meet in a common final lysosome. The data also support the notion that LDL and beta-VLDL are taken up through different endocytic receptors. However, following estradiol treatment, both ligands seem to follow a common pathway. In this case the density distributions of the two ligands coincide and resemble the pathway of LDL observed in control animals. This may be due to a pronounced up-regulation of LDL receptors following estradiol treatment, and beta-VLDL may under these conditions be taken up via the LDL receptor.
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PMID:Endocytosed LDL and beta-VLDL follow different intracellular pathways in rat liver. 825 20

The human 300 kDa mannose-6-phosphate receptor (MPR 300) is phosphorylated in vivo at serine residues of its cytoplasmic domain. Two-dimensional separation can resolve tryptic phosphopeptides into four major species. To identify the kinases involved in MPR 300 phosphorylation and the phosphorylation sites the entire coding sequence of the cytoplasmic tail was expressed in Escherichia coli. The isolated cytoplasmic domain was used as a substrate for four purified serine/threonine kinases [casein kinase II (CK II), protein kinase A (PKA), protein kinase C and Ca2+/calmodulin kinase]. All kinases phosphorylate the cytoplasmic tail exclusively on serine residues. Inhibition studies using synthetic peptides, partial sequencing of isolated tryptic phosphopeptides and co-migration with tryptic phosphopeptides from MPR 300 labelled in vivo showed that (i) PKA phosphorylates the cytoplasmic MPR 300 domain at Ser20 and at a non-identified site, neither of which are phosphorylated in vivo, and that (ii) the two sites phosphorylated by CK II in vivo and in vitro are Ser82 and Ser157. The results indicate that the human MPR 300 is a physiological substrate of either CK II or a related kinase which may play a role in the transport function of MPR 300 and/or interaction with other proteins.
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PMID:Characterization of phosphorylation sites in the cytoplasmic domain of the 300 kDa mannose-6-phosphate receptor. 831 12

Osteoclasts develop from precursor cells of the monocyte series. However, specialized differentiation for efficient bone degradation separates the osteoclast from the macrophage. The physical reasons for these differences are emerging from the study of osteoclastic physiology and biochemistry. Key osteoclast specializations are multinucleation, formation of a tightly sealed extracellular compartment on bone, and high-capacity secretion of HCl and acid proteases into this extracellular site. Multinucleation increases efficiency of extracellular attachment processes. The attachment process is mediated by cell membrane integrins, and is sensitive to changes in intracellular or extracellular calcium. Acid production exploits carbonic acid as the source of acid and conjugate base equivalents, reflected in abundant osteoclastic carbonic anhydrase type II expression. Secretion of acid involves extremely high expression of vacuolar-type H(+)-ATPase and a chloride channel in the cell's specialized acid secreting organelle, the ruffled membrane, which is polarized to the osteoclast's bone attachment. Acid secretion is balanced by chloride-bicarbonate exchange in the cell's nonbone attached membranes; this functionally resembles the band 3 chloride-bicarbonate exchanger of the red cell carbon dioxide transport system. Bone collagen is degraded by acid proteases secreted into the acid degradation site via the mannose-6-phosphate receptor system, which is targeted to lysosomes in other cells. Functional deficits, as in osteopetrosis, may affect any of the elements involved in osteoclast differentiation. Furthermore, new antiosteoclastic therapeutic agents may inhibit osteoclast biochemistry intentionally, such as for the control of hypercalcemia of malignancy.
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PMID:Recent advances toward understanding osteoclast physiology. 839 72

Transforming growth factor-beta (TGF beta) is a multifunctional growth factor in bone that is secreted as a latent complex and must be activated in order to influence cellular activity. We have investigated the influence of dexamethasone (Dex; a potent glucocorticoid) on the secretion and activation of latent TGF beta by normal human osteoblast-like cells (hOBs). Dex had no significant effect on TGF beta mRNA or total protein production, but treatment with Dex resulted in a steroid dose-dependent activation of up to 90% of the TGF beta produced by the hOBs. Dex-treated hOBs activated multiple latent forms of TGF beta. Conditioned medium from Dex-treated hOBs retained the ability to activate latent TGF beta when incubated at 37 C in the absence of cells. Unlike other cell systems, Dex-induced hOB-mediated TGF beta activation did not involve binding to the mannose-6-phosphate receptor. However, the activation was prevented by treatment of hOB cells with microtubual disrupting agents, by the addition of protease inhibitors, or by weak base treatment of the medium. Dex treatment of hOBs was shown to induce a dose-dependent increase in the mRNA levels of cathepsin-B and -D and in the levels of cathepsin-B protein secreted by the cells. Taken together, these data suggest that Dex treatment of hOBs induces the production and secretion of lysosomal proteases that, when secreted, activate latent TGF beta which is secreted by the hOB cells. There is evidence for an involvement of more than one type of protease in this activation process. This activation of TGF beta may, therefore, play a role in glucocorticoid regulation of bone cell functions. Furthermore, TGF beta is most likely involved in autocrine and paracrine effects on bone cells.
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PMID:Glucocorticoid-induced activation of latent transforming growth factor-beta by normal human osteoblast-like cells. 840 70

We have followed the transfer of EGF-EGF receptor (EGFR) complexes from endosomal vacuoles that contain transferrin receptors (TfR) to lysosome vacuoles identified by their content of HRP loaded as a 15-min pulse 4 h previously. We show that the HRP-loaded lysosomes are lysosomal-associated membrane protein-1 (LAMP-1) positive, mannose-6-phosphate receptor (M6PR) negative. and contain active acid hydrolase. EGF-EGFR complexes are delivered to these lysosomes intact and are then rapidly degraded. Preactivating the HRP contained within the preloaded lysosomes inhibits the delivery of EGFR and degradation of EGF, and results in the accumulation of EGFR-containing multivesicular bodies (MVB). With time these accumulating MVB undergo a series of maturation changes that include the loss of TfR, the continued recruitment of EGFR, and the accumulation of internal vesicles, but they remain LAMP-1 and M6PR negative. The mature MVB are often seen to make direct contact with lysosomes containing preactivated HRP, but their perimeter membranes remain intact. Together our observations suggest that the transfer of EGF-EGFR complexes from the TfR-containing endosome compartment to the lysosomes that degrade them employs a single vacuolar intermediate, the maturing MVB, and can be achieved by a single heterotypic fusion step.
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PMID:Multivesicular endosomes containing internalized EGF-EGF receptor complexes mature and then fuse directly with lysosomes. 860 81

The translocation of a unique facilitative glucose transporter isoform (GLUT4) from an intracellular site to the plasma membrane accounts for the large insulin-dependent increase in glucose transport observed in muscle and adipose tissue. The intracellular location of GLUT4 in the basal state and the pathway by which it reaches the cell surface upon insulin stimulation are unclear. Here, we have examined the colocalization of GLUT4 with the transferrin receptor, a protein which is known to recycle through the endosomal system. Using an anti-GLUT4 monoclonal antibody we immunoisolated a vesicular fraction from an intracellular membrane fraction of 3T3-L1 adipocytes that contained > 90% of the immunoreactive GLUT4 found in this fraction, but only 40% of the transferrin receptor (TfR). These results suggest only a limited degree of colocalization of these proteins. Using a technique to cross-link and render insoluble ("ablate') intracellular compartments containing the TfR by means of a transferrin-horseradish peroxidase conjugate (Tf-HRP), we further examined the relationship between the endosomal recycling pathway and the intracellular compartment containing GLUT4 in these cells. Incubation of non-stimulated cells with Tf-HRP for 3 h at 37 degrees C resulted in quantitative ablation of the intracellular TfR, GLUT1 and mannose-6-phosphate receptor and a shift in the density of Rab5-positive membranes. In contrast, only 40% of intracellular GLUT4 was ablated under the same conditions. Ablation was specific for the endosomal system as there was no significant ablation of either TGN38 or lgp120, which are markers for the trans Golgi reticulum and lysosomes respectively. Subcellular fractionation analysis revealed that most of the ablated pools of GLUT4 and TfR were found in the intracellular membrane fraction. The extent of ablation of GLUT4 from the intracellular fraction was unchanged in cells which were insulin-stimulated prior to ablation, whereas GLUT1 exhibited increased ablation in insulin-stimulated cells. Pretreatment of adipocytes with okadaic acid, an inhibitor of Type-I and -IIa phosphatases, increased GLUT4 ablation in the presence of insulin, consistent with okadaic acid increasing the internalization of GLUT4 from the plasma membrane under these conditions. Using a combination of subcellular fractionation, vesicle immunoadsorption and compartment ablation using the Tf-HRP conjugate we have been able to resolve overlapping but distinct intracellular distributions of the TfR and GLUT4 in adipocytes. At least three separate compartments were identified: TfR-positive/GLUT4-negative. TfR-negative/GLUT4-positive, and TfR-positive/GLUT4-positive, as defined by the relative abundance of these two markers. We propose that the TfR-negative/GLUT4-positive compartment, which contains approximately 60% of the intracellular GLUT4, represents a specialized intracellular compartment that is withdrawn from the endosomal system. The biosynthesis and characteristics of this compartment may be fundamental to the unique insulin regulation of GLUT4.
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PMID:Compartment ablation analysis of the insulin-responsive glucose transporter (GLUT4) in 3T3-L1 adipocytes. 861 19


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