Serum factors alter the extent of dephosphorylation of ligands endocytosed via the mannose 6-phosphate/insulin-like growth factor II receptor

Acid phosphatase 5 is responsible for removing the mannose 6-phosphate recognition marker from lysosomal proteins

Most newly synthesized proteins destined for the lysosome reach this location via a specific intracellular pathway. In the Golgi, a phosphotransferase specifically labels lysosomal proteins with mannose 6-phosphate (Man-6-P). This modification is recognized by receptors that target the lysosomal proteins to the lysosome where, in most cell types, the Man-6-P recognition marker is rapidly removed. Despite extensive characterization of this pathway, the enzyme responsible for the removal of the targeting modification has remained elusive. In this study, we have identified this activity. Preliminary investigations using a cell-based bioassay were used to follow a dephosphorylation activity that was associated with the lysosomal fraction. This activity was high in the liver, where endogenous lysosomal proteins are efficiently dephosphorylated, but present at a much lower level in the brain, where the modification persists. This observation, combined with an analysis of the expression of lysosomal proteins in different tissues, led us to identify acid phosphatase 5 (ACP5) as a candidate for the enzyme that removes Man-6-P. Expression of ACP5 in N1E-115 neuroblastoma cells, which do not efficiently dephosphorylate lysosomal proteins, significantly decreased the steady state levels of Man6-P glycoproteins. Analysis of ACP5-deficient mice revealed that levels of Man-6-P glycoproteins were highly elevated in tissues that normally express ACP5, and this resulted from a failure to dephosphorylate lysosomal proteins. These results indicate a central role for ACP5 in removal of the Man-6-P recognition marker and open up new avenues to investigate the importance of this process in cell biology and medicine.

Biochemical characterization and lysosomal localization of the mannose-6-phosphate protein p76 (hypothetical protein LOC196463)

Most soluble lysosomal proteins carry Man6P (mannose 6-phosphate), a specific carbohydrate marker that enables their binding to cellular MPRs (Man6P receptors) and their subsequent targeting towards the lysosome. This characteristic was exploited to identify novel soluble lysosomal proteins by proteomic analysis of Man6P proteins purified from a human cell line. Among the proteins identified during the course of the latter study [Journet, Chapel, Kieffer, Roux and Garin (2002) Proteomics, 2, 1026-1040], some had not been previously described as lysosomal proteins. We focused on a protein detected at 76 kDa by SDS/PAGE. We named this protein 'p76' and it appeared later in the NCBI protein database as the 'hypothetical protein LOC196463'. In the present paper, we describe the identification of p76 by MS and we analyse several of its biochemical characteristics. The presence of Man6P sugars was confirmed by an MPR overlay experiment, which showed the direct and Man6P-dependent interaction between p76 and the MPR. The presence of six N-glycosylation sites was validated by progressive peptide-N-glycosidase F deglycosylation. Experiments using N- and C-termini directed anti-p76 antibodies provided insights into p76 maturation. Most importantly, we were able to demonstrate the lysosomal localization of this protein, which was initially suggested by its Man6P tags, by both immunofluorescence and sub-cellular fractionation of mouse liver homogenates.

The potentials of MS-based subproteomic approaches in medical science: the case of lysosomes and breast cancer

Because of the great number of women who are diagnosed with breast cancer each year, and though this disease presents the lowest mortality rate among cancers, breast cancer remains a major public health problem. As for any cancer, the tumorigenic and metastatic processes are still hardly understood, and the biochemical markers that allow either a precise monitoring of the disease or the classification of the numerous forms of breast cancer remain too scarce. Therefore, great hopes are put on the development of high-throughput genomic and proteomic technologies. Such comprehensive techniques should help in understanding the processes and in defining steps of the disease by depicting specific genes or protein profiles. Because techniques dedicated to the current proteomic challenges are continuously improving, the probability of the discovery of new potential protein biomarkers is rapidly increasing. In addition, the identification of such markers should be eased by lowering the sample complexity; e.g., by sample fractionation, either according to specific physico-chemical properties of the proteins, or by focusing on definite subcellular compartments. In particular, proteins of the lysosomal compartment have been shown to be prone to alterations in their localization, expression, or post-translational modifications (PTMs) during the cancer process. Some of them, such as the aspartic protease cathepsin D (CatD), have even been proven as participating actively in the disease progression. The present review aims at giving an overview of the implication of the lysosome in breast cancer, and at showing how subproteomics and the constantly refining MS-based proteomic techniques may help in making breast cancer research progress, and thus, hopefully, in improving disease treatment.

Subcellular localization of mannose 6-phosphate glycoproteins in rat brain

The intracellular transport of soluble lysosomal enzymes relies on the post-translational modification of N-linked oligosaccharides to generate mannose 6-phosphate (Man 6-P) residues. In most cell types the Man 6-P signal is rapidly removed after targeting of the precursor proteins from the Golgi to lysosomes via interactions with Man 6-phosphate receptors. However, in brain, the steady state proportion of lysosomal enzymes containing Man 6-P is considerably higher than in other tissues. As a first step toward understanding the mechanism and biological significance of this observation, we analyzed the subcellular localization of the rat brain Man 6-P glycoproteins by combining biochemical and morphological approaches. The brain Man 6-P glycoproteins are predominantly localized in neuronal lysosomes with no evidence for a steady state localization in nonlysosomal or prelysosomal compartments. This contrasts with the clear endosome-like localization of the low steady state proportion of mannose-6-phosphorylated lysosomal enzymes in liver. It therefore seems likely that the observed high percentage of phosphorylated species in brain is a consequence of the accumulation of lysosomal enzymes in a neuronal lysosome that does not fully dephosphorylate the Man 6-P moieties.

Abnormal lysosomal sorting with an enhanced secretion of cathepsin D precursor molecules bearing monoester phosphate groups

It has been reported that besides defects in the phosphorylation such as in the I-cell disease, a failure in the uncovering of mannose 6-phosphate residues may result in an increase of lysosomal enzyme activities in serum [Alexander et al., Hum. Genet. 73, 53-59 (1986)]. We examined fibroblasts that were derived from the original biopsy, observed an enhanced secretion of lysosomal enzymes including cathepsin D, but found that both the phosphorylation and uncovering of mannose 6-phosphate residues were normal. The enhanced secretion of cathepsin D was characterized by an increase in the secretion of phosphorylated molecules that were sensitive to a treatment with alkaline phosphatase. The enhanced secretion of the phosphatase-sensitive form of procathepsin D was further increased in the presence of antibodies directed to cation-independent mannose 6-phosphate receptors. In contrast, antibodies specific to cation-dependent mannose 6-phosphate receptors selectively inhibited the secretion of the phosphatase-sensitive procathepsin D molecules. A chromatographic analysis of oligosaccharides from the secreted procathepsin D confirmed that the cells secrete proenzyme molecules rich in oligosaccharides with two uncovered phosphate residues. It is suggested that the enhanced secretion of procathepsin D in the variant fibroblasts results from an abnormal sorting rather than processing of phosphorylated lysosomal enzymes.

Mutant Rab7 causes the accumulation of cathepsin D and cation-independent mannose 6-phosphate receptor in an early endocytic compartment

Stable BHK cell lines inducibly expressing wild-type or dominant negative mutant forms of the rab7 GTPase were isolated and used to analyze the role of a rab7-regulated pathway in lysosome biogenesis. Expression of mutant rab7N125I protein induced a dramatic redistribution of cation-independent mannose 6-phosphate receptor (CI-MPR) from its normal perinuclear localization to large peripheral endosomes. Under these circumstances approximately 50% of the total receptor and several lysosomal hydrolases cofractionated with light membranes containing early endosome and Golgi markers. Late endosomes and lysosomes were contained exclusively in well-separated, denser gradient fractions. Newly synthesized CI-MPR and cathepsin D were shown to traverse through an early endocytic compartment, and functional rab7 was crucial for delivery to later compartments. This observation was evidenced by the fact that 2 h after synthesis, both markers were more prevalent in fractions containing light membranes. In addition, both were sensitive to HRP-DAB- mediated cross-linking of early endosomal proteins, and the late endosomal processing of cathepsin D was impaired. Using similar criteria, the lysosomal membrane glycoprotein 120 was not found accumulated in an early endocytic compartment. The data are indicative of a post-Golgi divergence in the routes followed by different lysosome-directed molecules.

Rat brain contains high levels of mannose-6-phosphorylated glycoproteins including lysosomal enzymes and palmitoyl-protein thioesterase, an enzyme implicated in infantile neuronal lipofuscinosis

Mannose 6-phosphate (Man-6-P) is a posttranslational carbohydrate modification typical of newly synthesized acid hydrolases that signals targeting from the Golgi apparatus to the lysosome via Man-6-P receptors (MPRs). Using iodinated cation independent MPR as a probe in a Western blot assay, we surveyed levels of Man-6-P glycoproteins in a number of different rat tissues. Considerable variation was observed with respect to total amounts and types of Man-6-P glycoproteins in the different tissues. Brain contained 2-8-fold more Man-6-P glycoproteins than other tissues, with relative abundance being brain >> testis approximately heart > lung approximately kidney approximately ovary approximately spleen > skeletal muscle approximately liver approximately serum. Analysis of 16 different lysosomal enzyme activities revealed that brain contains lower activities than other tissues which suggested that decreased removal of Man-6-P results in increased levels of Man-6-P glycoproteins. This was directly demonstrated by comparing activities of phosphorylated lysosomal enzymes, purified by immobilized MPR affinity chromatography, with total activities. The phosphorylated forms accounted for a considerable proportion of the MPR-targeted activities measured in brain (on average, 36.2%) but very little in lung, kidney, and liver (on average, 5.5, 2.3, and 0. 7%, respectively). Man-6-P glycoproteins were also isolated from rat brain by MPR affinity chromatography on a preparative scale. Of the 18 bands resolvable by SDS-polyacrylamide gel electrophoresis, seven bands were NH2-terminally sequenced and identified as the known lysosomal enzymes cathepsin L, cathepsin A, cathepsin D, alpha-galactosidase A, arylsulfatase A, and alpha-iduronidase. One of the major Man-6-P glycoproteins was identified as palmitoyl protein thioesterase, which was not previously thought to be lysosomal. This finding raises important questions about the cellular location and function of palmitoyl protein thioesterase, mutations in which result in the neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis.

Dephosphorylation of the mannose-6-phosphate recognition marker is localized in later compartments of the endocytic route. Identification of purple acid phosphatase (uteroferrin) as the candidate phosphatase

The mannose-6-phosphate (Man6P) recognition marker in lysosomal proteins is known to be dephosphorylated after the delivery of lysosomal proteins to the endosome/lysosome compartment. The rate of Man6P recognition marker inactivation depends on the cell type and lysosomal protein. In the present study we show that in BHK 21 cells, which rapidly dephosphorylate lysosomal proteins, the recognition marker is stable in the endosomal compartment, to which lysosomal enzymes such as arylsulfatase A are delivered during endocytosis at 20 degrees C. Dephosphorylation depends on the transfer of internalized lysosomal enzymes from the 20 degrees C compartment to later compartments, most likely lysosomes. This transfer is sensitive to NH4C1 and nocodazole. In vitro experiments identified purple acid phosphatase (uteroferrin) as a candidate for the lysosomal phosphatase catalyzing in vivo the dephosphorylation of Man6P recognition marker.

IGF receptor function and regulation in autocrine human neuroblastoma cell growth

Insulin-like growth factor-II (IGF-II) and its receptors (type I and II IGF receptors) are expressed in the nervous system in a tissue and developmentally specific manner. We have previously shown that SH-SY5Y human neuroblastoma cells synthesize and secrete high levels of IGF-II, and respond to it with increased neuritic outgrowth, DNA synthesis, and cell proliferation. SH-SY5Y cells also produce type I IGF and IGF-II/M6P receptors; however, it is not known whether these receptors mediate the observed growth promoting effects of IGF-II. In this study, we assayed the role of type I IGF receptor and IGF-II/M6P receptor expression in mediating autocrine IGF-II induced growth. Using anti-receptor antibodies, we found that IGF-II stimulates cell proliferation via the type I IGF receptor but not via the IGF-II/M6P receptor. By Northern analysis, we detected increased mRNA expression of both receptors, with more dramatic changes in type I IGF receptor expression. Collectively, our results indicate a role for the type I IGF receptor in mediating IGF-II induced autocrine neuroblastoma cell growth.

Cell- and ligand-specific dephosphorylation of acid hydrolases: evidence that the mannose 6-phosphatase is controlled by compartmentalization

Mouse L cells that possess the cation-independent mannose 6-phosphate (Man 6-P)/insulin-like growth factor (IGF) II receptor change the extent to which they dephosphorylate endocytosed acid hydrolases in response to serum (Einstein, R., and C. A. Gabel. 1989. J. Cell Biol. 109:1037-1046). To investigate the mechanism by which dephosphorylation competence is regulated, the dephosphorylation of individual acid hydrolases was studied in Man 6-P/IGF II receptor-positive and -deficient cell lines. 125I-labeled Man 6-P-containing acid hydrolases were proteolytically processed but remained phosphorylated when endocytosed by receptor-positive L cells maintained in the absence of serum; after the addition of serum, however, the cell-associated hydrolases were dephosphorylated. Individual hydrolases were dephosphorylated at distinct rates and to different extents. In contrast, the same hydrolases were dephosphorylated equally and completely after entry into Man 6-P/IGF II receptor-positive Chinese hamster ovary (CHO) cells. The dephosphorylation competence of Man 6-P/IGF II receptor-deficient mouse J774 cells was more limited. beta-Glucuronidase produced by these cells underwent a limited dephosphorylation in transit to lysosomes such that diphosphorylated oligosaccharides were converted to monophosphorylated species. The overall quantity of phosphorylated oligosaccharides associated with the enzyme, however, did not decrease within the lysosomal compartment. Likewise, beta-glucuronidase was not dephosphorylated when introduced into J774 cells via Fc receptor-mediated endocytosis. The CHO and J774 cell lysosomes, therefore, display opposite extremes with respect to their capacity to dephosphorylate acid hydrolases; within CHO cell lysosomes acid hydrolases are rapidly and efficiently dephosphorylated, but within J774 cell lysosomes the same acid hydrolases remain phosphorylated. This difference in processing indicates that lysosomes themselves exist in a dephosphorylation-competent and -incompetent state. Man 6-P-bearing acid hydrolases endocytosed by the L+ cells in the absence of serum were not distributed uniformly throughout the lysosomal compartment. The change in the dephosphorylation competence of L cells in response to serum suggests, therefore, that these cells contain multiple populations of lysosomes that differ with respect to their content of a mannose 6-phosphatase, and that serum factors affect the distribution of hydrolases between the different compartments.