Basis for low affinity binding of a lysosomal cysteine protease to the cation-independent mannose 6-phosphate receptor

Procathepsin V Is Secreted in a TSH Regulated Manner from Human Thyroid Epithelial Cells and Is Accessible to an Activity-Based Probe

The significance of cysteine cathepsins for the liberation of thyroid hormones from the precursor thyroglobulin was previously shown by in vivo and in vitro studies. Cathepsin L is most important for thyroglobulin processing in mice. The present study aims at specifying the possible contribution of its closest relative, cysteine cathepsin L2/V, to thyroid function. Immunofluorescence analysis on normal human thyroid tissue revealed its predominant localization at the apical plasma membrane of thyrocytes and within the follicle lumen, indicating the secretion of cathepsin V and extracellular tasks rather than its acting within endo-lysosomes. To explore the trafficking pathways of cathepsin V in more detail, a chimeric protein consisting of human cathepsin V tagged with green fluorescent protein (GFP) was stably expressed in the Nthy-ori 3-1 thyroid epithelial cell line. Colocalization studies with compartment-specific markers and analyses of post-translational modifications revealed that the chimeric protein was sorted into the lumen of the endoplasmic reticulum and subsequently transported to the Golgi apparatus, while being N-glycosylated. Immunoblotting showed that the chimeric protein reached endo-lysosomes and it became secreted from the transduced cells. Astonishingly, thyroid stimulating hormone (TSH)-induced secretion of GFP-tagged cathepsin V occurred as the proform, suggesting that TSH upregulates its transport to the plasma membrane before it reaches endo-lysosomes for maturation. The proform of cathepsin V was found to be reactive with the activity-based probe DCG-04, suggesting that it possesses catalytic activity. We propose that TSH-stimulated secretion of procathepsin V is the default pathway in the thyroid to enable its contribution to thyroglobulin processing by extracellular means.

Cathepsin L Induces Proangiogenic Changes in Human Omental Microvascular Endothelial Cells via Activation of the ERK1/2 Pathway

BACKGROUND

Metastasis still remains the major cause of therapeutic failure, poor prognosis and high mortality in epithelial ovarian cancer (EOC) patients. Previously, we showed that EOC cells secrete a range of factors with potential pro-angiogenic activity, in disease-relevant human omental microvascular endothelial cells (HOMECs), including the lysosomal protease cathepsin L (CathL). Thus, the aim of this study was to examine potential pro-proliferative and pro-migratory effects of CathL in HOMECs and the activated signalling pathways, and whether these proangiogenic responses are dependent on CathL-catalytic activity.

METHODS

HOMECs proliferation was investigated using WST-1, BrdU and CyQUANT assays. Cell migration was examined using a Cultrex Cell 96 transwell migration assay. Enzyme activity was assayed at various pHs using the CathL-specific fluorogenic substrate FY-CHO. Activation of cell signalling pathways was tested using a commercially available phosphokinase array and intact cellbased ELISAs.

RESULTS

We showed for the first time that CathL has a potent pro-proliferative and pro-migratory effect on HOMECs. For instance, CathL significantly increases HOMEC proliferation (134.8±14.7% vs control 100%) and migration (146.6±17.3% vs control 100%). Our data strongly suggest that these proangiogenic effects of CathL are mediated via a non-proteolytic mechanism. Finally, we show that CathL-induced activation of the ERK1/2 pathway is involved in inducing these cellular effects in HOMECs.

CONCLUSION

These data suggest that CathL acts as an extracellular ligand and plays an important pro-angiogenic, and thus pro-metastatic, role during EOC metastasis to the omentum, by activating the omental microvasculature, and thus can potentially be targeted therapeutically in the future.

An Active 32-kDa Cathepsin L Is Secreted Directly from HT 1080 Fibrosarcoma Cells and Not via Lysosomal Exocytosis

Cathepsin L [EC 3.4.22.15] is secreted via lysosomal exocytosis by several types of cancer cells, including prostate and breast cancer cells. We previously reported that human cultured fibrosarcoma (HT 1080) cells secrete cathepsin L into the medium; this secreted cathepsin is 10-times more active than intracellular cathepsin. This increased activity was attributed to the presence of a 32-kDa cathepsin L in the medium. The aim of this study was to examine how this active 32-kDa cathepsin L is secreted into the medium. To this end, we compared the secreted active 32-kDa cathepsin L with lysosomal cathepsin L by using a novel gelatin zymography technique that employs leupeptin. We also examined the glycosylation and phosphorylation status of the proteins by using the enzymes endoglycosidase H [EC 3.2.1.96] and alkaline phosphatase [EC 3.1.3.1]. Strong active bands corresponding to the 32-kDa and 34-kDa cathepsin L forms were detected in the medium and lysosomes, respectively. The cell extract exhibited strong active bands for both forms. Moreover, both forms were adsorbed onto a concanavalin A-agarose column. The core protein domain of both forms had the same molecular mass of 30 kDa. The 32-kDa cathepsin L was phosphorylated, while the 34-kDa lysosomal form was dephosphorylated, perhaps because of the lysosomal marker enzyme, acid phosphatase. These results suggest that the active 32-kDa form does not enter the lysosomes. In conclusion, our results indicate that the active 32-kDa cathepsin L is secreted directly from the HT 1080 cells and not via lysosomal exocytosis.

The Potential Role of the Proteases Cathepsin D and Cathepsin L in the Progression and Metastasis of Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) is the leading cause of death from gynecologic malignancies and has a poor prognosis due to relatively unspecific early symptoms, and thus often advanced stage, metastasized cancer at presentation. Metastasis of EOC occurs primarily through the transcoelomic route whereby exfoliated tumor cells disseminate within the abdominal cavity, particularly to the omentum. Primary and metastatic tumor growth requires a pool of proangiogenic factors in the microenvironment which propagate new vasculature in the growing cancer. Recent evidence suggests that proangiogenic factors other than the widely known, potent angiogenic factor vascular endothelial growth factor may mediate growth and metastasis of ovarian cancer. In this review we examine the role of some of these alternative factors, specifically cathepsin D and cathepsin L.

Proteomics Analysis of Serum from Mutant Mice Reveals Lysosomal Proteins Selectively Transported by Each of the Two Mannose 6-Phosphate Receptors

Most mammalian cells contain two types of mannose 6-phosphate (Man-6-P) receptors (MPRs): the 300 kDa cation-independent (CI) MPR and 46 kDa cation-dependent (CD) MPR. The two MPRs have overlapping function in intracellular targeting of newly synthesized lysosomal proteins, but both are required for efficient targeting. Despite extensive investigation, the relative roles and specialized functions of each MPR in targeting of specific proteins remain questions of fundamental interest. One possibility is that most Man-6-P glycoproteins are transported by both MPRs, but there may be subsets that are preferentially transported by each. To investigate this, we have conducted a proteomics analysis of serum from mice lacking either MPR with the reasoning that lysosomal proteins that are selectively transported by a given MPR should be preferentially secreted into the bloodstream in its absence. We purified and identified Man-6-P glycoproteins and glycopeptides from wild-type, CDMPR-deficient, and CIMPR-deficient mouse serum and found both lysosomal proteins and proteins not currently thought to have lysosomal function. Different mass spectrometric approaches (spectral count analysis of nanospray LC-MS/MS experiments on unlabeled samples and LC-MALDI/TOF/TOF experiments on iTRAQ-labeled samples) revealed a number of proteins that appear specifically elevated in serum from each MPR-deficient mouse. Man-6-P glycoforms of cellular repressor of E1A-stimulated genes 1, tripeptidyl peptidase I, and heparanase were elevated in absence of the CDMPR and Man-6-P glycoforms of alpha-mannosidase B1, cathepsin D, and prosaposin were elevated in the absence of the CIMPR. Results were confirmed by Western blot analyses for select proteins. This study provides a comparison of different quantitative mass spectrometric approaches and provides the first report of proteins whose cellular targeting appears to be MPR-selective under physiological conditions.

The 46-kDa mannose 6-phosphate receptor does not depend on endosomal acidification for delivery of hydrolases to lysosomes

In mammalian cells, the mannose 6-phosphate receptor pathway accounts for the transport of most soluble acid hydrolases to lysosomes. It is believed that dissociation of mannose 6-phosphate receptors and their ligands is entirely driven by the acidic environment in endosomal compartments. Indeed, pH-perturbing substances such as ammonium chloride and monensin have been shown to inhibit lysosomal enzyme targeting in cells that express both known mannose 6-phosphate receptors. We now demonstrate that ammonium chloride and monensin exert modest effects on the intracellular retention of lysosomal hydrolases in murine cells that synthesize only the 46-kDa mannose 6-phosphate receptor. Neither ammonium chloride nor monensin induces changes to the subcellular localization of lysosomal hydrolases and the 46-kDa mannose 6-phosphate receptor in these cells. This suggests that endosomal dissociation of the receptor and its ligands still occurs in the presence of these agents. We conclude that the murine 46-kDa mannose 6-phosphate receptor has the capacity to deliver its cargo proteins to lysosomes even in the absence of endosomal acidification.

Cysteine proteases and cell differentiation: excystment of the ciliated protist Sterkiella histriomuscorum

The process of excystment of Sterkiella histriomuscorum (Ciliophora, Oxytrichidae) leads in a few hours, through a massive influx of water and the resorption of the cyst wall, from an undifferentiated resting cyst to a highly differentiated and dividing vegetative cell. While studying the nature of the genes involved in this process, we isolated three different cysteine proteases genes, namely, a cathepsin B gene, a cathepsin L-like gene, and a calpain-like gene. Excystation was selectively inhibited at a precise differentiating stage by cysteine proteases inhibitors, suggesting that these proteins are specifically required during the excystment process. Reverse transcription-PCR experiments showed that both genes display differential expression between the cyst and the vegetative cells. A phylogenetic analysis showed for the first time that the cathepsin B tree is paraphyletic and that the diverging S. histriomuscorum cathepsin B is closely related to its Giardia homologues, which take part in the cyst wall breakdown process. The deduced cathepsin L-like protein sequence displays the structural signatures and phylogenetic relationships of cathepsin H, a protein that is known only in plants and animals and that is involved in the degradation of extracellular matrix components in cancer diseases. The deduced calpain-like protein sequence does not display the calcium-binding domain of conventional calpains; it belongs to a diverging phylogenetic cluster that includes Aspergillus palB, a protein which is involved in a signal transduction pathway that is sensitive to ambient pH.

Biosynthesis and alternate targeting of the lysosomal cysteine protease cathepsin L

Upregulation of cathepsin L expression, whether during development or cell transformation, or mediated by ectopic expression from a plasmid, alters the targeting of the protease and thus its physiological function. Upregulated procathepsin L is targeted to small dense core vesicles and to the dense cores of multivesicular bodies, as well as to lysosomes and to the plasma membrane for selective secretion. The multivesicular vesicles resemble secretory lysosomes characterized in specialized cell types in that they are endosomes that stably store an upregulated protein and they possess the tetraspanin CD63. Morphologically the multivesicular endosomes also resemble late endosomes, but they store procathepsin L, not the active protease, and they are not the major site for LAMP-1 accumulation. Distinction between the lysosomal proenzyme and active protease thus identifies two populations of multivesicular endosomes in fibroblasts, one a storage compartment and one an enzymatically active compartment. A distinctive targeting pathway using aggregation is utilized to enrich the storage endosomes with a particular lysosomal protease that can potentially activate and be secreted.

Mamestra configurata serpin-1 homologues: cloning, localization and developmental regulation

A screen of a Mamestra configurata (bertha armyworm) midgut cDNA library identified three types of cDNA clones that resemble the Manduca sexta serpin-1 gene family. Two serpins, 1b and 1c, possess a common conserved serpin amino terminal scaffold domain but bear no similarity to any members of the M. sexta gene family within the reactive centre loop. These serpins differ from one another by only two amino acids in the reactive centre loop (S(363)-->P) and serpin signature (M(369)-->T) regions. The other member, denoted serpin-1a, is closely related to the M. sexta serpin-1Z. M. configurata serpins as a group were expressed in all insect developmental stages including eggs, larvae and adult moths. Within larvae, serpin gene expression was restricted to the early to middle instar developmental phase and mainly in the fat body and hemocytes. Stress imposed by starvation strongly induced expression in fat body and to a lesser degree in alimentary organs, nervous system and Malphigian tubules. Conversely, starvation decreased expression in hemocytes. Wounding or inoculation with bacteria did not induce serpin gene transcription but did lead to the formation of higher and lower molecular weight forms, presumably serpin-protease complexes and resultant truncated serpin, respectively. Two dimensional PAGE and western blotting analysis revealed at least 12 distinct serpins consisting primarily of neutral, but also highly acidic and basic isoforms, as well as additional high and low molecular weight immuno-reactive species. Serpins-1b/1c are the more prominent serpin isoforms and are expressed predominantly in the fat body and subsequently exported to the hemolymph as revealed by western blotting and immunolocalization. The serpin-1b/1c isoform was found only as the fully glycosylated species within the hemolymph. Hemolymph protease activity was comprised mostly of serine proteases whose overall activity increased dramatically at the onset of the molt concomitant with a sharp decline in serpin gene expression.

Changes in cysteine protease activity and localization during midgut metamorphosis in the crucifer root maggot (Delia radicum)

We show that differential localization and/or activation of two cysteine protease activities occur at the onset of dipteran midgut metamorphosis. A 26 kDa cysteine protease activity was associated specifically with midgut tissues of late third instar larvae. Starvation of mid third instar larvae simulated the onset of prepupation and resulted in loss of the 26 kDa protease activity. A cDNA clone encoding a cysteine protease, termed DrCP1, was isolated and shown to be highly similar to those from Sarcophaga peregrina and Drosophila melanogaster (DmCP1). DrCP1 mRNA was present in all developmental stages including eggs, larvae, pupae and adults, but was highly induced at the onset of the larval-pupal transition and thereafter. The DrCP1 protein is localized to the exterior of the midgut tissues during the onset of the prepupal transition period, possibly in response to ecdysone. Analysis of transcription factor binding sites associated with the DmCP1 promoter indicated that elements exist that allow for both ecdysone-mediated as well as tissue-specific regulation. Based upon these and other studies we propose: (1) that the expression, activity and localization of the DrCP1-like cysteine proteases are highly regulated throughout development; and, (2) that cysteine protease activities are involved in aspects of tissue reconstruction at the onset of and during metamorphosis.

Different distribution patterns of the two mannose 6-phosphate receptors in rat liver

Two mannose 6-phosphate receptors, cation-dependent and -independent receptors (CDMPR and CIMPR), play an important role in the intracellular transport of lysosomal enzymes. To investigate functional differences between the two in vivo, their distribution was examined in the rat liver using immunohistochemical techniques. Positive signals corresponding to CIMPR were detected intensely in hepatocytes and weakly in sinusoidal Kupffer cells and interstitial cells in Glisson's capsule. In the liver acinus, hepatocytes in the perivenous region showed a more intense immunoreactivity than those in the periportal region. On the other hand, positive staining of CDMPR was detected at a high level in Kupffer cells, epithelial cells of interlobular bile ducts, and fibroblast-like cells, but the corresponding signal was rather weak in hepatocytes. In situ hybridization analysis also revealed a high level of expression of CIMPR mRNAs in hepatocytes and of CDMPR mRNA in Kupffer cells. By double immunostaining, OX6-positive antigen-presenting cells in Glisson's capsule were co-labeled with the CDMPR signal but were only faintly stained with anti-CIMPR. These different distribution patterns of the two MPRs suggest distinct functional properties of each receptor in liver tissue.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.

Procathepsin L self-association as a mechanism for selective secretion

The lysosomal cysteine pro-protease procathepsin L was enriched in dense vesicles detectable when microsomes prepared from wild-type or transformed mouse fibroblasts were resolved on sucrose gradients. These dense vesicles did not comigrate with proteins characteristic of the endoplasmic reticulum, Golgi, endosomes or lysosomes. When gradient fraction vesicles were lysed at acidic pH in the presence of excess mannose 6-phosphate to prevent binding to mannose phosphate receptors, the majority of the procathepsin L was associated with the membrane, not the soluble, fraction. Immunogold labeling of procathepsin L in thin sections of cells or gradient fractions, using antibodies directed against the propeptide to avoid detection of the mature enzyme in dense lysosomes, revealed that the proenzyme was concentrated in dense cores localized in small vesicles near the plasma membrane and in multivesicular bodies. Consistent with the density of the gradient fraction and the electron density of the cores, yeast two-hybrid assays indicated the proenzyme could bind itself but could not interact with the aspartic proprotease procathepsin D. The data suggest that in mouse fibroblasts procathepsin L may self-associate into aggregates, initiating the formation of dense vesicles that could mediate the selective secretion of procathepsin L independent of mannose phosphate receptors.

cDNA subtraction cloning reveals novel genes whose temporal and spatial expression indicates association with trophoblast invasion

Trophoblast invasion is a critical process in development of most mammals that shares similarities with the invasive behavior of tumor cells. In the present investigation, a cDNA subtraction library was constructed between invasive trophoblast at day 8 of murine development and mature noninvasive placenta at day 18 of gestation. One of the differentially expressed clones, Epcs26, was mapped to the X chromosome and revealed no homology to any known gene. It was predominantly expressed in parietal endoderm, undifferentiated cells of the ectoplacental cone, and a few trophoblast giant cells. Another gene, designated Epcs50, was mapped to chromosome 19. It exhibited homologies to the mouse Mps1 gene and, like Mps1, may have a distant relationship to the lytic protein perforin. High expression was detected in parietal endoderm cells and in a subset of secondary trophoblast giant cells. Two sequences, Epcs24 and Epcs68, exhibited an extensive open reading frame that shared the common features of the cysteine proteinase cathepsin L. Expression was confined to an undefined subpopulation of trophoblast giant cells. Both genes were mapped to chromosome 13 in close proximity to cathepsins L and J. The known functions of MPS1 and cathepsin L proteins indicate that the related proteins EPCS50, EPCS24, and EPCS68 participate in conferring invasive properties to the mouse trophoblast.

Lysine-based structure responsible for selective mannose phosphorylation of cathepsin D and cathepsin L defines a common structural motif for lysosomal enzyme targeting

Previous studies have shown that lysine residues on the surface of cathepsins and other lysosomal proteins are a shared component of the recognition structure involved in mannose phosphorylation. In this study, the involvement of specific lysine residues in mannose phosphorylation of cathepsin D was explored by site-directed mutagenesis. Mutation of two lysine residues in the mature portion of the protein, Lys-203 and Lys-293, cooperated to inhibit mannose phosphorylation by 70%. Other positively charged residues could not substitute for lysine at these positions, and comparison of thermal denaturation curves for the wild type and mutant proteins indicated that the inhibition could not be explained by alterations in protein folding. Structural comparisons of the two lysine residues with those required for phosphorylation of cathepsin L, using models generated from recently acquired crystal structures, revealed several relevant similarities. On both molecules, the lysine residues were positioned approximately 34 A apart (34.06 A for cathepsin D and 33.80 A for cathepsin L). When the lysine pairs were superimposed, N-linked glycosylation sites on the two proteins were found to be oriented so that oligosaccharides extending out from the sites could share a common region of space. Further similarities in the local environments of the critical lysines were also observed. These results provide details for a common lysosomal targeting structure based on a specific arrangement of lysine residues with respect to each other and to glycosylation sites on the surface of lysosomal proteins.

Effect of carbohydrate position on lysosomal transport of procathepsin L

To study the role of carbohydrate in lysosomal protein transport, we engineered two novel glycosylation signals (Asn-X-Ser/Thr) into the cDNA of human procathepsin L, a lysosomal acid protease. We constructed six mutant cDNAs encoding glycosylation signals at mutant sites Asn-138, Asn-175, or both sites together, in the presence or absence of the wild-type Asn-204 site. We stably transfected wild-type and mutant cDNAs into NIH3T3 mouse fibroblasts and then used species-specific antibodies to determine the glycosylation status, phosphorylation, localization, and transport kinetics of recombinant human procathepsin L containing one, two, or three glycosylation sites. Both novel glycosylation sites were capable of being glycosylated, although Asn-175 was utilized only 30-50% of the time. Like the wild-type glycosylation at Asn-204, carbohydrates at Asn-138 and Asn-175 were completely sensitive to endoglycosidase H, and they were phosphorylated. Mutant proteins containing two carbohydrates were capable of being delivered to lysosomes, but there was not a consistent relationship between the efficiency of lysosomal delivery and carbohydrate content of the protein. Pulse-chase labeling revealed a unique biosynthetic pattern for proteins carrying the Asn-175 glycosylation sequence. Whereas wild-type procathepsin L and mutants bearing carbohydrate at Asn-138 appeared in lysosomes by about 60 min, proteins with carbohydrate at Asn-175 were processed to a lysosome-like polypeptide within 15 min. Temperature shift, brefeldin A, and NH4Cl experiments suggested that the rapid processing did not occur in the endoplasmic reticulum and that Asn-175 mutants could interact with the mannose 6-phosphate receptor. Taken together, our results are consistent with the interpretation that Asn-175 carbohydrate confers rapid transport to lysosomes. We may have identified a recognition domain in procathepsin L that is important for its interactions with the cellular transport machinery.

Differential sorting of lysosomal enzymes out of the regulated secretory pathway in pancreatic beta-cells

In cells specialized for secretory granule exocytosis, lysosomal hydrolases may enter the regulated secretory pathway. Using mouse pancreatic islets and the INS-1 beta-cell line as models, we have compared the itineraries of procathepsins L and B, two closely related members of the papain superfamily known to exhibit low and high affinity for mannose-6-phosphate receptors (MPRs), respectively. Interestingly, shortly after pulse labeling INS cells, a substantial fraction of both proenzymes exhibit regulated exocytosis. After several hours, much procathepsin L remains as precursor in a compartment that persists in its ability to undergo regulated exocytosis in parallel with insulin, while procathepsin B is efficiently converted to the mature form and can no longer be secreted. However, in islets from transgenic mice devoid of cation-dependent MPRs, the modest fraction of procathepsin B normally remaining within mature secretory granules is increased approximately fourfold. In normal mouse islets, immunoelectron microscopy established that both cathepsins are present in immature beta-granules, while immunolabeling for cathepsin L, but not B, persists in mature beta-granules. By contrast, in islets from normal male Sprague-Dawley rats, much of the proenzyme sorting appears to occur earlier, significantly diminishing the stimulus-dependent release of procathepsin B. Evidently, in the context of different systems, MPR-mediated sorting of lysosomal proenzymes occurs to a variable extent within the trans-Golgi network and is continued, as needed, within immature secretory granules. Lysosomal proenzymes that fail to be sorted at both sites remain as residents of mature secretory granules.

Differential targeting and processing of procathepsin D in normal and transformed murine 3T3 fibroblasts

The kinetics of transport and the processing of procathepsin D (proCD), the precursor of a lysosomal aspartyl protease involved in tumor-cell proliferation and metastasis, were compared in normal and SV-40- or benzo[a]pyrene-transformed 3T3 mouse fibroblasts. Sorting of newly synthesized proCD in normal cells was almost complete within 3 hr, while in transformed cells a fraction of the precursor survives a long time. In both normal and transformed 3T3 cultures, secretion of proCD started at 3 hr of chase. However, in normal cells secretion of proCD remained constant between 3 and 24 hr of chase, while in transformed cells it increased along with the chase incubation. The efficiency of formation of the mannose-6-phosphate group on proCD varied among the 3 cell types, being minimal in benzo[a]pyrene-transformed 3T3 cells. Ammonium chloride, a drug known to disrupt the segregation and to enhance the secretion of lysosomal proenzymes, was 2-fold more effective in normal than in transformed 3T3 cells. Despite vacuolar alkalinization, about one third of proCD was segregated into the endosomal-lysosomal pathway in normal and in transformed 3T3 fibroblasts, indicating the existence in these cells of alternative, mannose-6-phosphate receptor-independent mechanisms for targeting proCD. Thus, while hypersecretion of proCD and reduced sensitivity to vacuolar alkalinization are common features of both transformed cell types, the mechanisms responsible for inefficient segregation of proCD may differ between virally and chemically transformed 3T3 cells.

Re-expression of the mannose 6-phosphate receptors in receptor-deficient fibroblasts. Complementary function of the two mannose 6-phosphate receptors in lysosomal enzyme targeting

We have previously generated primary embryonic fibroblasts lacking either the cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (MPR) or the cation-dependent MPR, two trans-membrane proteins that bind the mannose 6-phosphate (Man-6-P) recognition marker on soluble lysosomal enzymes (Ludwig, T., Munier-Lehmann, H., Bauer, U., Hollinshead, M., Ovitt, C., Lobel, P., and Hoflack, B.(1994) EMBO J. 13, 3430-3437). These two cell types partially missort phosphorylated lysosomal enzymes. Using two-dimensional gel electrophoresis, we show here that they secrete, in a large part, different phosphorylated ligands. In order to better understand the sorting function of the MPRs, we have re-expressed each MPR in MPR-negative fibroblasts. We show that the MPRs have similar capacities for transporting the bulk of the newly synthesized lysosomal enzymes and that they target individual ligands with various efficiencies. However, high levels of one MPR do not fully compensate for the absence of the other, demonstrating that the two MPRs have complementary targeting functions, perhaps by recognizing different features on lysosomal enzymes. The analysis of the phosphorylated oligosaccharides shows that the ligands missorted in the absence of the cation-dependent MPR are slightly but significantly depleted in oligosaccharides with two Man-6-P residues, when compared with those missorted in the absence of the cation-independent MPR. While these results could explain some differences between the structure and the sorting function of the two MPRs, they strongly suggest that the reason why cells express two different but related MPRs is to maintain an efficient Man-6-P-dependent targeting process that could be potentially regulated by MPR expression.

Mannose 6-phosphate receptors in sorting and transport of lysosomal enzymes

Mannose 6-phosphate receptors have been intensively studied with regard to their genomic organization, protein structure, ligand binding properties, intracellular trafficking and sorting functions. That their main function is sorting of newly synthesized lysosomal enzymes is commonly accepted, but much more remains to be learned about their precise recycling pathways and the mechanisms which regulate their vesicular transport. Additional functions have been reported, e.g., export of newly synthesized lysosomal enzymes from the cell by MPR 46 or a--probably indirect--participation in growth factor-mediated signal transduction by MPR 300. To understand the physiological relevance of these observations will be a challenge for future research.

Lysine-based structure in the proregion of procathepsin L is the recognition site for mannose phosphorylation

The recognition of lysosomal enzymes by UDP-GlcNAc: lysosomal-enzyme GlcNAc-1-phosphotransferase (phosphotransferase) is mediated by a protein structure on lysosomal enzymes. It has been previously demonstrated that lysine residues are required for phosphorylation of procathepsin L and are a common feature of the site on many lysosomal proteins. In this work, the procathepsin L recognition structure was further defined by identification of the region of the protein containing the structure and the critical lysine residues involved. Removal of the cathepsin L propeptide by low pH-induced autocatalytic processing abolished phosphorylation. The addition of either the purified propeptide or a glutathione S-transferase-propeptide fusion protein to the processed protein restored phosphorylation. Mutagenesis of individual lysine residues demonstrated that two propeptide lysine residues (Lys-54 and Lys-99) were required for efficient phosphorylation of procathepsin L. By comparison of the phosphorylation rates of procathepsin L, lysine-modified procathepsin L, and the procathepsin L oligosaccharide, lysine residues were shown to account for most, if not all, of the protein-dependent interaction. On this basis, it is concluded that the proregion lysine residues are the major elements of the procathepsin L recognition site. In addition, lysine residues in cathepsin D were shown to be as important for phosphorylation as those in procathepsin L, supporting a general model of the recognition site as a specific three-dimensional arrangement of lysine residues exposed on the surface of lysosomal proteins.

Biosynthesis and endocytosis of lysosomal enzymes in human colon carcinoma SW 1116 cells: impaired internalization of plasma membrane-associated cation-independent mannose 6-phosphate receptor

The human colon adenocarcinoma cell lines SW 948, SW 1116, and SW 1222 were tested for their ability to sort and internalize lysosomal enzymes. The biosynthesis of the lysosomal enzymes cathepsin B, arylsulfatase A, and beta-hexosaminidase in these cell lines exhibits no significant differences to that in human fibroblasts. The intracellular targeting of newly synthesized hydrolases to the lysosomes relies in colon carcinoma cells on the mannose 6-phosphate receptor system. Both the cation-independent mannose 6-phosphate receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor are expressed in all colon carcinoma cell lines investigated. Endocytosis of lysosomal enzymes via mannose 6-phosphate receptors is reduced in colon carcinoma cells as compared with human fibroblasts. SW 1116 cells were shown to be deficient in receptor-mediated endocytosis of mannose 6-phosphate containing ligands. Ligands of other endocytic receptors as well as the fluid-phase marker horseradish peroxidase were internalized at normal rates. While antibodies against CI-MPR bind to the surface of SW 1116 cells, these antibodies cannot be internalized. These data suggest that the cycling of CI-MPR is specifically impaired in SW 1116 cells.

Differential rates of glycoprotein secretion by isolated rat hepatocytes studied in terms of concanavalin A binding

Using a concanavalin-A-based method which respects cell function, we have shown that the kinetics of glycoprotein secretion appear to depend on the nature of the oligosaccharide moiety. In 37 degrees C pulse/chase experiments using freshly isolated normal rat hepatocytes, we found that except for transferrin, whose rate of secretion was independent of its concanavalin A reactivity, the secretion of the concanavalin-A-retained forms of alpha 1 acid glycoprotein, T-kininogen, alpha 1 protease inhibitor and alpha 1 inhibitor III was slower than that of the concanavalin-A-non-retained forms. When hepatocytes were incubated at 20 degrees C, secretion was blocked with the accumulation of mainly endoglycosidase-H-sensitive forms. The secretion kinetics of the concanavalin-A-differentiated forms were still different when the temperature was shifted back to 37 degrees C. The divergence between the secretion rates of the concanavalin-A-differentiated forms would appear to be due to a late event in intracellular protein trafficking, which may depend on the sugar content and/or the number of carbohydrate chains of the glycoproteins.

Insulin-like growth factor II overexpression does not affect sorting of lysosomal enzymes in NIH-3T3 cells

The sorting of newly synthesized mannose 6-phosphate (M6P)-containing proteins and of the major excreted protein (MEP), a lysosomal thiol proteinase, was studied in NIH-3T3 cells transfected with the cDNA of human insulin-like growth factor II (IGF II) or with the vector alone. Extracts from media and cells labelled with [35S] methionine were used for chromatography on a M6P/IGF II receptor affinity matrix or for immunoprecipitation to assess the distribution of newly synthesized M6P-containing proteins and MEP, respectively. The results indicate that the overexpression of IGF II did not affect the synthesis and the sorting of M6P-containing proteins and of MEP. The binding and uptake of the lysosomal enzyme arylsulfatase A were not affected in IGF II overexpressing cells.

Cathepsin B and its endogenous inhibitors: the role in tumor malignancy

Several lysosomal proteinases including the cysteine proteinase cathepsin B have been implicated in malignant progression of tumors. Many investigators have demonstrated correlations between increased activity of cathepsin B and increased metastatic capability of animal tumors or malignancy of human tumors. Such increases in cathepsin B activity in malignant tumors may reflect alterations in synthesis, in activation and processing, and/or in intracellular trafficking and delivery as well as in the endogenous inhibitors of cathepsin B. Increases in mRNA transcripts for cathepsin B have been observed in both murine and human tumors and multiple transcripts for cathepsin B have been identified, but an association of multiple transcripts with malignancy has not been confirmed. Cathepsin B precursors found in human malignant ascites fluid do not possess mannose-rich carbohydrates suggesting that a defect in the post translational processing of carbohydrate moieties on tumor cathepsin B may be responsible for the release of cathepsin B observed in many tumor systems. However, the intracellular trafficking of cathepsin B responsible for its association with plasma membrane/endosomal systems and for its release will require further study as both latent, precursor forms of cathepsin B and native forms of cathepsin B are involved. We speculate that malignant tumor cells adherent to basement membrane are capable of forming a digestive microenvironment in which lysosomal proteinases such as cathepsin B function optimally, a microenvironment similar to that formed between adherent osteoclasts and bone. One of the endogenous cysteine proteinase inhibitors, stefin A, also is affected by malignancy. Reduced expression (mRNA and protein) of stefin A is found as well as a reduction in its inhibitory capacity against cysteine proteinases. The data to date at both the molecular and protein levels supporting a functional role(s) for cathepsin B and its endogenous inhibitors in cancer progression are only correlative. Experimental approaches utilizing well-defined model systems in conjunction with genetic manipulation of cathepsin B and its endogenous inhibitors are needed to provide convincing evidence that cathepsin B has an important role in cancer.

Estradiol increases the secretion by MCF7 cells of several lysosomal pro-enzymes

The synthesis and secretion of pro-cathepsin D is increased by estrogens in MCF7 cells. We quantified the effect of estradiol on other lysosomal enzymes in order to investigate the mechanism of this hypersecretion. Precursors of beta-hexosaminidase, cathepsin B and beta-galactosidase, which are routed to lysosomes via the mannose-6-phosphate (Man-6-P) receptor, were secreted in much lower amounts than pro-cathepsin D, but their secretion was also increased by estradiol. The activity of acid phosphatase, which is routed to lysosomes via a different transmembrane mechanism, was not altered by estradiol. While estradiol stimulated gene expression of pro-cathepsin D, it had no effect on that of pro-cathepsin B. We conclude that estradiol stimulates the secretion of several lysosomal pro-enzymes in MCF7 cells, suggesting that a general mechanism is responsible for this derouting rather than a specific alteration of cathepsin D structure.