Structures at the proteolytic processing region of cathepsin D

A Lysosomal Pepstatin-Insensitive Proteinase as a Novel Biomarker for Breast Carcinoma

Lysosomal proteinases play an important role in the turnover of intracellular proteins, and acidic proteinases such as cathepsin D are known to be increased in breast carcinoma. In the present study the activity of a newly discovered acidic lysosomal pepstatin-insensitive proteinase (CLN2p) was measured in breast tissues by the most sensitive and highly specific assay that we had developed for the diagnosis of late-infantile neuronal ceroid lipofuscinosis (LINCL) (2). Samples from eight normal subjects undergoing reductive mammoplasty and 200 patients with primary breast carcinoma were analyzed. The results suggest a two- to seventeen-fold higher CLN2p activity in tumors, which was significantly and positively correlated with already known breast cancer biomarkers such as levels of cathepsin D, estrogen receptor and progesterone receptor. These results suggest a diagnostic and prognostic potential for this novel acid proteinase in breast cancer.

Development of 2-aminooxazoline 3-azaxanthene β-amyloid cleaving enzyme (BACE) inhibitors with improved selectivity against Cathepsin D

As part of an ongoing effort at Amgen to develop a disease-modifying therapy for Alzheimer's disease, we have previously used the aminooxazoline xanthene (AOX) scaffold to generate potent and orally efficacious BACE1 inhibitors. While AOX-BACE1 inhibitors demonstrated acceptable cardiovascular safety margins, a retinal pathological finding in rat toxicological studies demanded further investigation. It has been widely postulated that such retinal toxicity might be related to off-target inhibition of Cathepsin D (CatD), a closely related aspartyl protease. We report the development of AOX-BACE1 inhibitors with improved selectivity against CatD by following a structure- and property-based approach. Our efforts culminated in the discovery of a picolinamide-substituted 3-aza-AOX-BACE1 inhibitor absent of retinal effects in an early screening rat toxicology study.

Similarities and differences in the biogenesis, processing and lysosomal targeting between zebrafish and human pro-Cathepsin D: functional implications

The lysosomal protease Cathepsin D (CD) plays a role in neurodegenerative diseases, cancer, and embryo-fetus abnormalities. It is therefore of interest to know how this protein is synthesized in animal species used for modeling human diseases. Zebrafish (Danio rerio) is emerging as a valuable 'in vivo' vertebrate model for several human diseases. We have characterized the biogenetic pathways of zebrafish and human CD transgenically expressed in both human SH-SY5Y cells and zebrafish PAC2 cells. Differently from human CD, zebrafish CD was synthesized as a mono-glycosylated precursor (pro-CD) that was eventually processed into a single-chain mature polypeptide. In PAC2 cells, ammonium chloride and chloroquine impaired the N-glycosylation, and greatly stimulated the secretion, of pro-CD; still, a portion of un-glycosylated pro-CD reached the lysosomes and was processed to mature CD. The treatment with tunicamycin, which abrogates N-glycosylation, resulted in a similar effect. Zebrafish pro-CD was correctly processed when expressed in human cells, and its glycosylation, transport and maturation were not impaired by ammonium chloride. On the contrary, the transport and processing of human pro-CD expressed in zebrafish cells were profoundly altered: while the intermediate single-chain was not detectable, a small amount of double-chain mature CD still formed. This fact indicates that the enzyme machinery for single- to double-chain processing of mammal CD is present in zebrafish. Our data highlight the respective impact of the information imparted by the primary sequence and of the cellular transport and processing machineries in the biogenesis of lysosomal CD.

Cathepsins D and L reduce the toxicity of advanced glycation end products

Advanced glycation end product-modified proteins are known for accumulating during aging and in several pathological conditions such as diabetes, renal failure, and neurodegenerative disorders. There is little information about the intracellular fate of endocytosed advanced glycation end products (AGEs) and their influence on proteolytic systems. However, it is known that the lysosomal system is impaired during aging. Therefore, undegraded material may accumulate and play a considerable role in the development of diverse diseases. To investigate if AGEs can be degraded and to test whether they accumulate because of impaired lysosomal proteases we studied the effects of advanced glycation end products on the endosomal-lysosomal system. Five different types of AGEs were generated by bovine serum albumin incubation with glyoxal, methylglyoxal, glucose, fructose, and ribose. The first experiments revealed the uptake of AGEs by the macrophage cell line RAW 264.7. Further investigations demonstrated an increase in cathepsin D and L activity and an increase in mature cathepsins D and L. Increased activities were accompanied by the presence of more lysosomes, measured by staining with LysoTracker blue. To specify the roles of cathepsins D and L we used knockout cells to test the roles of both cathepsins on the toxicity of advanced glycation end products. In summary we conclude that both cathepsins are required for a reduction in advanced glycation end product-induced cytotoxicity.

A novel mechanism of lysosomal acid sphingomyelinase maturation: requirement for carboxyl-terminal proteolytic processing

Acid sphingomyelinase (aSMase) catalyzes the hydrolysis of sphingomyelin (SM) to form the bioactive lipid ceramide (Cer). Notably, aSMase exists in two forms: a zinc (Zn(2+))-independent lysosomal aSMase (L-SMase) and a Zn(2+)-dependent secreted aSMase (S-SMase) that arise from alternative trafficking of a single protein precursor. Despite extensive investigation into the maturation and trafficking of aSMase, the exact identity of mature L-SMase has remained unclear. Here, we describe a novel mechanism of aSMase maturation involving C-terminal proteolytic processing within, or in close proximity to, endolysosomes. Using two different C-terminal-tagged constructs of aSMase (V5, DsRed), we demonstrate that aSMase is processed from a 75-kDa, Zn(2+)-activated proenzyme to a mature 65 kDa, Zn(2+)-independent L-SMase. L-SMase is recognized by a polyclonal Ab to aSMase, but not by anti-V5 or anti-DsRed antibodies, suggesting that the C-terminal tag is lost during maturation. Furthermore, indirect immunofluorescence staining demonstrated that mature L-SMase colocalized with the lysosomal marker LAMP1, whereas V5-aSMase localized to the Golgi secretory pathway. Moreover, V5-aSMase possessed Zn(2+)-dependent activity suggesting it may represent the common protein precursor of S-SMase and L-SMase. Importantly, the 65-kDa L-SMase, but not V5-aSMase, was sensitive to the lysosomotropic inhibitor desipramine, co-fractionated with lysosomes, and migrated at the same M(r) as partially purified human aSMase. Finally, three aSMase mutants containing C-terminal Niemann-Pick mutations (R600H, R600P, ΔR608) exhibited defective proteolytic maturation. Taken together, these results demonstrate that mature L-SMase arises from C-terminal proteolytic processing of pro-aSMase and suggest that impaired C-terminal proteolysis may lead to severe defects in L-SMase function.

Aspartic cathepsin D endopeptidase contributes to extracellular digestion in clawed lobsters Homarus americanus and Homarus gammarus

Acid digestive proteinases were studied in the gastric fluids of two species of clawed lobster (Homarus americanus and Homarus gammarus). An active protein was identified in both species as aspartic proteinase by specific inhibition with pepstatin A. It was confirmed as cathepsin D by mass mapping, N-terminal, and full-length cDNA sequencing. Both lobster species transcribed two cathepsin D mRNAs: cathepsin D1 and cathepsin D2. Cathepsin D1 mRNA was detected only in the midgut gland, suggesting its function as a digestive enzyme. Cathepsin D2 mRNA was found in the midgut gland, gonads, and muscle. The deduced amino acid sequence of cathepsin D1 and cathepsin D2 possesses two catalytic DTG active-site motifs, the hallmark of aspartic proteinases. The putatively active cathepsin D1 has a molecular mass of 36.4 kDa and a calculated pI of 4.14 and possesses three potential glycosylation sites. The sequences showed highest similarities with cathepsin D from insects but also with another crustacean cathepsin D. Cathepsin D1 transcripts were quantified during a starvation period using real-time qPCR. In H. americanus, 15 days of starvation did not cause significant changes, but subsequent feeding caused a 2.5-fold increase. In H. gammarus, starvation caused a 40% reduction in cathepsin D1 mRNA, and no effect was observed with subsequent feeding.

Isolation, biochemical characterization, and molecular modeling of American lobster digestive cathepsin D1

An aspartic proteinase was isolated from American lobster gastric fluid. The purified cathepsin D runs as a single band on native-PAGE displaying proteolytic activity on a zymogram at pH 3.0, with an isoelectric point of 4.7. Appearance of the protein in SDS-PAGE, depended on the conditions of the gel electrophoresis. SDS treatment by itself was not able to fully unfold the protein. Thus, in SDS-PAGE the protein appeared to be heterogeneous. A few minute of boiling the sample in the presence of SDS was necessary to fully denature the protein that then run in the gel as a single band of ~50 kDa. The protein sequence of lobster cathepsin D1, as deduced from its mRNA sequence, lacks a 'polyproline loop' and β-hairpin, which are characteristic of some of its structural homologues. A comparison of amino acid sequences of digestive and non-digestive cathepsin D-like enzymes from invertebrates showed that most cathepsin D enzymes involved in food digestion, lack the polyproline loop, whereas all non-digestive cathepsin Ds, including the American lobster cathepsin D2 paralog, contain the polyproline loop. We propose that the absence or presence of this loop may be characteristic of digestive and non-digestive aspartic proteinases, respectively.

Roles and regulation of secretory and lysosomal acid sphingomyelinase

Acid sphingomyelinase occupies a prominent position in sphingolipid catabolism, catalyzing the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Enzymatic dysfunction of acid sphingomyelinase results in Niemann-Pick disease, a lysosomal storage disorder characterized at the cellular level by accumulation of sphingomyelin within the endo-lysosomal compartment. Over the past decade interest in the role of acid sphingomyelinase has moved beyond its "housekeeping" function in constitutive turnover of sphingomyelin in the lysosome to include study of regulated ceramide generation. Ceramide functions as a bioactive sphingolipid with pleiotropic signaling properties, and has been implicated in diverse cellular processes of physiologic and pathophysiologic importance. Though many cellular enzymes have the capacity to generate ceramide,there is growing appreciation that "all ceramides are not created equal." Ceramides likely exert distinct effects in different cellular/subcellular compartments by virtue of access to other sphingolipid enzymes (e.g.ceramidases), effector molecules (e.g. ceramide-activated protein phosphatases), and neighboring lipids and proteins (e.g. cholesterol, ion channels). One of the unique features of acid sphingomyelinase is that it has been implicated in the hydrolysis of sphingomyelin in three different settings--the endo-lysosomal compartment,the outer leaflet of the plasma membrane, and lipoproteins. How a single gene product has the capacity to function in these diverse settings, and the subsequent impact on downstream ceramide-mediated biology is the subject of this review.

Cathepsin D: a cellular roadmap

Cathepsin D is a normal and major component of lysosomes, it is found in almost all cells and tissues of mammals. Present review describes different events in cellular life of cathepsin D mainly its biosynthesis, co-translational and posttranslational modifications, targeting to lysosomes and proteolytic processing and maturation within lysosomes.

Human cathepsin D

A literature survey was performed of human cathepsin D gene, cathepsin D biosynthesis, posttranslatory modifications, transport within the cell, substrate specificity and catalytic effect. Methods used to determine the activity and level of this proteinase as well as its role in the biochemistry and pathobiochemistry of cells, tissues and organs were considered.

Cathepsin D Is Secreted from M-BE Cells: Its Potential Role as a Biomarker of Lung Cancer

The early diagnosis of lung cancer is an effective approach to reduce the mortality caused by malignancy. To explore serum biomarkers of lung cancer at early stage, M-BE, a SV40T-transformed human bronchial epithelial cell line with the phenotypic features of early tumorigenesis at high passage, was cultured in the conditioned media to collect its secretory proteins. The proteins secreted from different passage M-BE cells were extracted and then separated by two-dimensional electrophoresis (2-DE). MALDI-TOF/TOF mass spectrometry was adopted to identify the passage-dependent 2-DE spots. Totally, 47 proteins were identified, including 23 that were up-regulated and 24 that were down-regulated. Of these proteins, cathepsin D was a typical secretory protein that exhibited the increased abundance either in culture media or in cells during passaging. Furthermore, the proteomic conclusions were validated in the clinical samples of lung cancer patients. When sandwich ELISA was used, the concentrations of cathepsin D in plasma showed significant differences between lung squamous cell carcinomas (SCC, 104 cases) and normal donors (36 cases, p <or= 0.015). When tissue microarray (TMA) was used, cathepsin D expression levels in SCC tissues (178 cases) were significantly higher than those in normal donors (40 cases, p < 0.001). The present study has revealed that M-BE cells at different passages could secrete or release some proteins into the living environment, which might serve as the potential resource for exploring the biomarkers of lung cancer.

Folding, activity and targeting of mutated human cathepsin D that cannot be processed into the double-chain form

The precursor of human cathepsin D (CD) is converted into the single-chain and the double-chain active polypeptides by subsequent proteolysis reactions taking place in the endosomal-lysosomal compartment and involving specific aminoacid sequences. We have mutagenized the region of aminoacids (comprising the beta-hairpin loop) involved in the latter proteolytic maturation step and generated a mutant CD that cannot be converted into the mature double-chain form. This mutant CD expressed in rodent cells reaches the lysosome and is stable as single-chain polypeptide, bears high-mannose type sugars, binds to pepstatin A and is enzymatically active, indicating that it is correctly folded. The present work provides new insights on the aminoacid region involved in the terminal processing of human CD and on the function of the processing beta-hairpin loop.

Legumain expression as a prognostic factor in breast cancer patients

Invasive tumor cells and their microenvironments are enriched with a broad spectrum of different proteases. Legumain, a novel asparaginyl endopeptidase, has been observed to be highly expressed in several types of solid tumors. However, there is no data available identifying the relationship of legumain expression and clinicopathologic or biological variables in invasive breast cancer. For the first time, the prevalence of legumain expression in invasive breast cancer (n = 432) and non-neoplastic breast tissues (n = 128) was investigated by immunohistochemistry. Three staining patterns were observed in the cytoplasm: diffuse positivity, tiny dots and vesicles. Whereas vesicular positivity in the majority of tumor cells was significantly correlated to an adverse outcome, cytoplasmic and dot-like staining showed no prognostic effect. Vesicular positivity was observed in 24% of carcinomas, but only in one case of non-neoplastic breast tissue (<1%; proliferative mastopathy). This staining pattern was found to be independent of other factors analysed as grading, nodal status or HER2 expression. Besides being of prognostic value, legumain might prove to be an important predictive factor in breast cancer, since its unique cleavage specificity is already used in prodrug activation strategies.

Characterization of rat cathepsin E and mutants with changed active-site residues and lacking propeptides and N-glycosylation, expressed in human embryonic kidney 293T cells

To study the roles of the catalytic activity, propeptide, and N-glycosylation of the intracellular aspartic proteinase cathepsin E in biosynthesis, processing, and intracellular trafficking, we constructed various rat cathepsin E mutants in which active-site Asp residues were changed to Ala or which lacked propeptides and N-glycosylation. Wild-type cathepsin E expressed in human embryonic kidney 293T cells was mainly found in the LAMP-1-positive endosomal organelles, as determined by immunofluorescence microscopy. Consistently, pulse-chase analysis revealed that the initially synthesized pro-cathepsin E was processed to the mature enzyme within a 24 h chase. This process was completely inhibited by brefeldin A and bafilomycin A, indicating its transport from the endoplasmic reticulum (ER) to the endosomal acidic compartment. Mutants with Asp residues in the two active-site consensus motifs changed to Ala and lacking the propeptide (Leu23-Phe58) and the putative ER-retention sequence (Ser59-Asp98) were neither processed nor transported to the endosomal compartment. The mutant lacking the ER-retention sequence was rapidly degraded in the ER, indicating the importance of this sequence in correct folding. The single (N92Q or N324D) and double (N92Q/N324D) N-glycosylation-deficient mutants were neither processed into a mature form nor transported to the endosomal compartment, but were stably retained in the ER without degradation. These data indicate that the catalytic activity, propeptides, and N-glycosylation of this protein are all essential for its processing, maturation, and trafficking.

Activation of human acid sphingomyelinase through modification or deletion of C-terminal cysteine

One form of Niemann-Pick disease is caused by a deficiency in the enzymatic activity of acid sphingomyelinase. During efforts to develop an enzyme replacement therapy based on a recombinant form of human acid sphingomyelinase (rhASM), purified preparations of the recombinant enzyme were found to have substantially increased specific activity if cell harvest media were stored for several weeks at -20 degrees C prior to purification. This increase in activity was found to correlate with the loss of the single free thiol on rhASM, suggesting the involvement of a cysteine residue. It was demonstrated that a variety of chemical modifications of the free cysteine on rhASM all result in substantial activation of the enzyme, and the modified cysteine responsible for this activation was shown to be the C-terminal residue (Cys629). Activation was also achieved by copper-promoted dimerization of rhASM (via cysteine) and by C-terminal truncation using carboxypeptidase Y. The role of the C-terminal cysteine in activation was confirmed by creating mutant forms of rhASM in which this residue was either deleted or replaced by a serine, with both forms having substantially higher specific activity than wild-type rhASM. These results indicate that purified rhASM can be activated in vitro by loss of the free thiol on the C-terminal cysteine via chemical modification, dimerization, or deletion of this amino acid residue. This method of activation is similar to the cysteine switch mechanism described previously for matrix metalloproteinases and could represent a means of posttranslational regulation of ASM activity in vivo.

Cloning of a cDNA encoding cathepsin D from salamander, Hynobius leechii, and its expression in the limb regenerates

Cathepsin D is a major lysosomal aspartic proteinase participating in the degradation or modification of intra- and extracellular matrix molecules, and its activity is known to increase in the process of tissue reorganization during the early phase of salamander limb regeneration. Here, we report the cloning of a salamander cathepsin D cDNA from Hynobius leechii and its expression profile in normal and retinoic acid (RA) treated limb regenerates. The gene expression of cathepsin D increased notably during the dedifferentiation stage and decreased gradually thereafter. Furthermore, RA that enhances dedifferentiation of regenerating salamander limb caused the elevation of cathepsin D expression both in terms of level and duration. These results suggest that cathepsin D plays important role(s) in the dedifferentiation process, and enhancement of cathepsin D expression might be closely related to RA-evoked pattern duplication.

The lysosomal protease cathepsin D is efficiently sorted to and secreted from regulated secretory compartments in the rat basophilic/mast cell line RBL

Basophils and mast cells contain a peculiar class of inflammatory granules that discharge their content upon antigen-mediated crosslinking of IgE-membrane receptors. The pathways for granule biogenesis and exocytosis in these cells are still largely obscure. In this study we employed the rat basophilic leukemia (RBL)/mast cell line to verify the hypothesis that inflammatory granules share common bioactive molecules and functional properties with lysosomes. We demonstrate that inflammatory granules, as identified by the monoclonal 5G10 antibody (which recognises an integral membrane protein) or by Toluidine Blue staining, have an intralumenal acidic pH, possess lysosomal enzymes and are accessible by fluid-phase and membrane endocytosis markers. In addition, we studied the targeting, subcellular localisation and regulated secretion of the lysosomal aspartic protease cathepsin D (CD) as affected by IgE receptor stimulation in order to obtain information on the pathways for granule biogenesis and exocytosis. Stimulation with DNP-BSA of specific IgE-primed RBL cells led to a prompt release of processed forms of CD, along with other mature lysosomal hydrolases. This release could be prevented by addition of EGTA, indicating that it was dependent on extracellular calcium influx. Antigen stimulation also induced exocytosis of immature CD forms accumulated by ammonium chloride, suggesting the existence of an intermediate station in the pathway for granule biogenesis still sensitive to regulated exocytosis. The targeting of molecules to secretory granules may occur via either a mannose-6-phosphate-dependent or mannose-6-phosphate-independent pathway. We conclude that endosomes and lysosomes in basophils/mast cells can act as regulated secretory granules or actually identify with them.

In vivo processing of nonanchored Yapsin 1 (Yap3p)

A C-terminally truncated form of yapsin 1 (yeast aspartic protease 3) was overexpressed in yeast and its processing through the secretory pathway was followed by pulse-labeling and immunoprecipitation studies. In the soluble cell extract, three forms of yapsin 1-87, 74, and 18 kDa-were found. Identification of these forms of yapsin 1 using different antisera suggests that the 87-kDa form is pro-yapsin 1, which is processed into two subunits, alpha (18 kDa) and beta (74 kDa), by cleavage at a loop region not found in traditional aspartic proteases. By use of a temperature-sensitive mutant strain, sec18, the generation of the two subunits was found to occur in the endoplasmic reticulum. An active site-mutated yapsin 1 was not processed into the two subunits, suggesting that this process occurs in an autocatalytic manner.

Regulation of protein sorting at the TGN by plasma membrane receptor activation

We show that in the rat basophilic leukemia cell line RBL, the physiological stimulation of the IgE receptor or direct activation of PKC leads to the missorting of proteins to the plasma membrane, diverting them from their normal intracellular destination. This is demonstrated for two classes of proteins that are normally targeted to the secretory lysosomes via completely different mechanisms, i.e. proteoglycans and the aspartic protease cathepsin D. In the latter case, normal processing of the enzyme is also affected, leading to secretion of the immature form of cathepsin. The present study shows how completely different sorting mechanisms, such as those for delivering proteoglycans and cathepsin D to secretory lysosomes, might share common regulatory signals and are similarly affected when the levels of these signals are perturbed. Finally, protein kinase C appears to be a major player in the signal transduction pathways, leading to proteoglycan and cathepsin D missorting.

Characterization of Dictyostelium discoideum cathepsin D

Previous studies using magnetic purification of Dictyostelium discoideum endocytic vesicles led us to the identification of some major vesicle proteins. Using the same purification procedure, we have now focused our interest on a 44 kDa soluble vesicle protein. Microsequencing of internal peptides and subsequent cloning of the corresponding cDNA identified this protein as the Dictyostelium homolog of mammalian cathepsins D. The only glycosylation detected on Dictyostelium cathepsin D (CatD) is common antigen 1, a cluster of mannose 6-sulfate residues on N-linked oligosaccharide chains. CatD intracellular trafficking has been studied, showing the presence of the protein throughout the entire endocytic pathway. During the differentiation process, the catD gene presents a developmental regulation, which is also observed at the protein level. catD gene disruption does not alter significantly the cell behaviour, either in the vegetative form or the differentiation stage. However, modifications in the SDS-PAGE profiles of proteins bearing common antigen 1 were detected, when comparing parental and catD(-) cells. These modifications point to a possible role of CatD in the maturation of a few Dictyostelium lysosomal proteins.

Serial Analysis of Gene Expression in Human Monocytes and Macrophages

Monocytes/macrophages serve as sentinels involved in chronic inflammation and the eradication of various pathogens. To define molecularly the differentiation of blood monocytes into macrophages, we conducted serial analysis of gene expression (SAGE) in human blood monocytes/macrophages induced by granulocyte-macrophage colony-stimulating factor (GM-CSF) or M-CSF. SAGE analysis of 57,560, 57,463, and 55,856 tags from monocytes, GM-CSF–, and M-CSF–induced macrophages, respectively, allowed identification of 35,037 different transcripts. Interestingly, the genes with the highest expression during differentiation from monocytes into macrophages were genes involved in lipid metabolism. Both CSF-induced macrophages expressed similar sets of genes except for several genes such as monocyte-derived chemokine (MDC), legumain, prostaglandin D synthetase, and lysosomal sialoglycoprotein. The identification of specific gene expression in human monocytes, GM-CSF–, or M-CSF–induced macrophages provides novel methods to define macrophage subsets and the maturation and activation stage of cells of macrophage lineage and, possibly, to diagnose diseases in which macrophages play a major role. This study represents the first extensive serial analysis of gene expression for any type of human hematopoietic cells.

Cathepsin D in cancer metastasis: a protease and a ligand

Cathepsin D (cath-D) overexpression in breast cancer cells is associated with increased risk of metastasis in patients according to several clinical studies. No alterations of pro-cath-D structure or activation have been demonstrated in cancer cells. However, overexpression and dysrouting of pro-cath-D in illegitimate compartments could have consequences on tumor progression. Transfection of a human cDNA cath-D expression vector increases the metastatic potential of 3Y1-Ad12 embryonic rat tumorigenic cells when intravenously injected into nude mice. The mechanism by which cath-D increases the incidence of clinical metastasis seems to involve increased cell growth and decreased contact inhibition rather than escape of cancer cells through the basement membrane. Different mechanisms are discussed by which cath-D could act as a protease following its activation or as a ligand of different membrane receptors at a more neutral pH.

Molecular cloning of the cDNA encoding human deoxyribonuclease II

A rapid amplification of cDNA ends method, using degenerate oligonucleotides based upon the N-terminal amino acid sequence of human hepatic deoxyribonuclease II (DNase II), allowed a novel cDNA encoding DNase II to be constructed from thyroid gland RNA. The composite nucleotide sequence (1593 bases) included an open reading frame of 1080 bases, which encoded a single polypeptide of 360 amino acids (signal peptide, 16; propeptide, 91; mature protein, 253). Although the sequence of DNase II showed no significant homology to other mammalian proteins, its cDNA structural organization resembled those of the lysosomal cathepsin families. The two parts of the cDNA corresponding to the propeptide and the mature protein were expressed in Escherichia coli, and the recombinant polypeptides thus obtained were strongly stained with an anti-DNase II antibody on Western blotting. DNase II is ubiquitously expressed in human tissues, and the DNase II gene (DNASE2) was assigned to chromosome 19.

Proteolytic enzymes in yolk-sac membrane of quail egg. Purification and enzymatic characterisation

Degradation of yolk protein is essential for the early development of the avian embryo. In Japanese quail (Coturnix coturnix japonica), proteolysis in the surrounding tissue of the yolk, the yolk-sac membrane, can be inhibited by class-specific inhibitors of cysteine proteinases as well as of aspartic proteinases. Purification of the enzymes leads to one cysteine proteinase and one aspartic proteinase with an apparent molecular mass of 29 kD and 44 kD, respectively. Both enzymes were purified in a two-chain form, although a single-chain form is also present in the homogenate of yolk-sac membrane. The cysteine proteinase was identified by NH2-terminal sequence analysis as well as by kinetic studies as a new cathepsin B from quail. Like mammalian cathepsin B, this avian cathepsin B exhibits two different kinds of proteolytic activity, an endopeptidase activity and a dipeptidyl carboxypeptidase activity. Chicken egg white cystatin, a protein-aceous cysteine proteinase inhibitor, inhibits quail cathepsin B with an equilibrium dissociation constant (Ki) of 3.3 nM. Likewise the aspartic proteinase was identified as a new cathepsin D from quail. This avian cathepsin D has a different processing site to all known mammalian cathepsins D. In quail cathepsin D one NH2-termini is homologous to amino acids 211-230 in mammalian cathepsin D. This is more than 100 amino acids downstream of the mammalian processing site. Comparison of the enzymatic properties of quail and bovine cathepsin D indicate that the different processing site has no influence on the enzymatic properties.

Cloning, isolation, and characterization of mammalian legumain, an asparaginyl endopeptidase

Legumain is a cysteine endopeptidase that shows strict specificity for hydrolysis of asparaginyl bonds. The enzyme belongs to peptidase family C13, and is thus unrelated to the better known cysteine peptidases of the papain family, C1 (Rawlings, N. D., and Barrett, A. J. (1994) Methods Enzymol. 244, 461-486). To date, legumain has been described only from plants and a blood fluke, Schistosoma mansoni. We now show that legumain is present in mammals. We have cloned and sequenced human legumain and part of pig legumain. We have also purified legumain to homogeneity (2200-fold, 8% yield) from pig kidney. The mammalian sequences are clearly homologous with legumains from non-mammalian species. Pig legumain is a glycoprotein of about 34 kDa, decreasing to 31 kDa on deglycosylation. It is an asparaginyl endopeptidase, hydrolyzing Z-Ala-Ala-Asn-7-(4-methyl)coumarylamide and benzoyl-Asn-p-nitroanilide. Maximal activity is seen at pH 5.8 under normal assay conditions, and the enzyme is irreversibly denatured at pH 7 and above. Mammalian legumain is a cysteine endopeptidase, inhibited by iodoacetamide and maleimides, but unaffected by compound E64 (trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane). It is inhibited by ovocystatin (cystatin from chicken egg white) and human cystatin C with Ki values < 5 nM. We discuss the significance of the discovery of a cysteine endopeptidase of a new family and distinctive specificity in man and other mammals.

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.

Vitellogenesis-related ovary cathepsin D from Xenopus laevis: purification and properties in comparison with liver cathepsin D

Cathepsin D was purified from ovaries of Xenopus laevis by both QAE-cellulose and pepstatin-Sepharose chromatography and then characterized and compared with Xenopus liver cathepsin D. Ovary cathepsin D appeared predominantly as a 43-kilodalton (kDa) molecular mass, as revealed by SDS-polyacrylamide gel electrophoresis, whereas the liver enzyme was obtained exclusively as a 36-kDa protein. The purified 43-kDa ovary enzyme cleaved vitellogenin limitedly to produce yolk proteins at pH 5.6. The specific activity of ovary cathepsin D was five to six times lower than that of the liver enzyme, as measured by hemoglobin-hydrolysis at pH 3, but the ovary enzyme was shown to be superior to the liver enzyme in terms of vitellogenin-cleaving activity, as examined at pH 5.6. Ovarian enzyme preparations contained variable amounts of 36-kDa species; this form was considered to be an autolytic product of the 43-kDa form arising during purification, because it was not detected in oocyte extracts but was generated by incubation of the purified 43-kDa enzyme alone in an acid solution. The conversion of the 43-kDa form by hepatic factors was accompanied by a marked increase in hemoglobin-hydrolytic activity.

Cloning and characterization of the Schistosoma japonicum aspartic proteinase involved in hemoglobin degradation

A cDNA encoding a Schistosoma japonicum aspartic proteinase was cloned, sequenced, and found to encode a zymogen of 380 amino acid residues, and its gene was shown to be present as a single copy in the S. japonicum genome. Identity comparisons showed that the enzyme (Sjpasp) was most closely related to the cathepsin Ds. The deduced amino acid sequence has four potential glycosylation sites, two of which are in identical positions to the two glycosylation sites of human kidney lysosomal cathepsin D. Furthermore, all four disulfide bonds found in mammalian cathepsin D sequences are present in Sjpasp, although the beta-hairpin (loop 3), which is cleaved during maturation of vertebrate cathepsin Ds to yield light and heavy chain subunits, is absent from Sjpasp. While most residues involved in substrate specificity and catalysis of aspartic proteinases are preserved in Sjpasp, several residues in these regions exhibit changes that may result in a novel substrate specificity. Aspartic proteinase activity is present in extracts of adult S. japonicum and Schistosoma mansoni and in culture media in which schistosomes were maintained and was capable of digesting hemoglobin. The schistosome aspartic proteinase may play a pivotal role in the catabolism of hemoglobin obtained from host erythrocytes.

Lysine residues in the C-terminal lobe and lysosomal targeting of procathepsin D

A major pathway to the lysosome for soluble hydrolases involves the 6-phosphorylation of mannose residues. The initial step in this reaction is catalyzed by a phosphotransferase which recognizes lysosomal precursors. We constructed mutants of human procathepsin D whose targeting to the lysosome could be assayed directly in intact cells. Eight lysine residues were individually converted to glutamic acid on the surface of the carboxyl terminal lobe of the protein. Mutants with as many as four Lys to Glu mutations were normally targeted to the lysosome and processed to the mature form of the enzyme in transfected cells. We conclude that the C-terminal lobe of procathepsin D may not carry a determinant essential for lysosomal targeting in intact fibroblasts.

Significance of estrogen receptors and cathepsin D tissue detection in gastric adenocarcinoma

Estrogen receptors (ERs) have recently been reported to be present in carcinomas of stomach, an organ that has so far been considered as nontarget for sex hormones. Cathepsin D is an estrogen-regulated lysosomal protease that has been overexpressed in breast cancer. ER and cathepsin D immunohistochemical expression were studied in this research in order to estimate their association to known histopathological and clinical parameters and their possible prognostic significance as well. Sixty-two patients with gastric adenocarcinomas were included in this study. The cancers were studied immunohistochemically concerning ER positivity in tumor cell nuclei and cathepsin D cytoplasmic expression. Nuclear ER staining was detected in tumor cells of 25% of male and 27% of female patients. ER positivity was demonstrated mainly in the well and moderately differentiated carcinomas; 87.5% of ER(+) tumors were also characterized as cathepsin D positive and a significant correlation between ER and cathepsin D positive expression was demonstrated (P < 0.05). Cytoplasmic cathepsin D expression was observed in carcinomatous cells of 70.9% of gastric tumors. Early tumor stage and good differentiation were significantly associated with increased cathepsin D expression (P < 0.05, P < 0.001). Histologic type, degree of differentiation and tumor stage were significantly correlated to survival (P < 0.05, P < 0.001 and P < 0.001). The patients who were cathepsin D(+) had a significant prognostic advantage over the cathepsin D(-) patients (P < 0.001). The presence of ER and estrogen-regulated cathepsin D indicates the involvement of sex hormonal factors in these tumors and cathepsin D positive expression in tumor cells seems to be related to better prognosis. Their biological, clinical, and prognostic roles remain to be further elucidated.

Human procathepsin D: three-dimensional model and isolation

Human procathepsin D was isolated from medium of human breast cancer cell line ZR-75-1 potentiated with estrogen. The isolation involved both immunoaffinity chromatography and ion-exchange chromatography. The affinity chromatography employed polyclonal antibodies raised against a synthetic activation peptide of human cathepsin D. We have started preliminary crystallization trials using the isolated material. A model of human procathepsin D was also built using coordinates of human cathepsin D and pig pepsinogen. The model aids understanding of multiple roles played by activation peptides of aspartic proteinases and will be used as a starting model for molecular replacement.

Cathepsin-D and tumor associated antigen DF3 in salivary gland neoplasia. Differential diagnostic and prognostic applications

Salivary gland tumors pose considerable difficulty in diagnostic and prognostic assessment based on the histopathologic features alone. Cathepsin-D is overexpressed in cancer cells where its concentration in the primary tumor is correlated with increased risk of metastasis. DF3 antigen is a tumor associated glycoprotein that is specific for malignant cells of glandular origin. We examined the distribution patterns of cathepsin-D and DF3 antigens in benign (n = 11) and malignant (n = 44) salivary gland tumors of various histologic types. The frequency of cathepsin-D expression is significantly increased (p < 0.001) in salivary gland carcinomas compared to benign mixed tumors (BMT). High levels of cathepsin-D expression was frequent in carcinomas ex BMT, mucoepidermoid carcinomas, poorly differentiated adenocarcinomas-NOS and adenoid cystic carcinomas (ACC). Acinic cell carcinomas and polymorphous low-grade adenocarcinomas were mostly negative. Intense cytoplasmic staining for DF3 antigen was noted in the tumor cells of mucoepidermoid carcinomas, carcinomas ex BMT and poorly differentiated adenocarcinomas-NOS whereas other types of salivary gland carcinomas exhibited either negative or only focal membrane staining. The noted differences in the reactive patterns of cathepsin-D and DF3 antigen among various histologic types of salivary gland carcinomas may have differential diagnostic and prognostic applications.

pH-dependent processing of yeast procarboxypeptidase Y by proteinase A in vivo and in vitro

Carboxypeptidase Y is a vacuolar enzyme from Saccharomyces cerevisiae. It enters the vacuole as a zymogen, procarboxypeptidase Y, which is immediately processed in a reaction involving two endoproteases, proteinase A and proteinase B. We have investigated the in vitro activation of purified procarboxypeptidase Y by purified proteinase A. This has identified two different processing intermediates; one active and one inactive. The intermediates define a 33 amino acid segment of the 91 amino acid propeptide as sufficient for maintaining the enzyme in an inactive state. The inactive intermediate was isolated from a processing reaction at neutral pH. In order to investigate the influence of vacuolar pH on processing in vivo, the autoactivation of proteinase A and its processing of procarboxypeptidase Y were studied in a vma2 prb1 mutant, which is deficient in vacuolar acidification and proteinase B activity. Efficient processing of procarboxypeptidase Y in the absence of proteinase B is dependent on acidic vacuolar pH, and the processing at neutral pH is slow and takes place in two steps similar to those identified in vitro.

Prognostic significance of immunohistochemical analysis of cathepsin D in low-stage breast cancer

BACKGROUND

Cathepsin D is an estrogen-regulated lysosomal protease that may be overexpressed in breast cancer. Several studies based on biochemical analysis of tumor cytosol have shown that high levels of cathepsin D are associated with poor outcome in patients with breast cancer. In contrast, a few immunohistochemical studies have shown that cathepsin D positivity in breast cancer cells indicates a favorable prognostic outcome or is of no prognostic significance.

METHODS

Because of the current controversy, the authors studied, using immunohistochemistry, the distribution (0 to 3) and intensity (0 to 3) of cathepsin D in Stage I and II infiltrating ductal carcinoma of the breast from 245 patients treated at the City of Hope National Medical Center, Duarte, California, from 1970 to 1986. A commercially available rabbit antiserum to purified human cathepsin D and the standard avidin-biotin complex method were used. Statistical analysis was based on a combined low (0 or 2) or high (3 to 6) score.

RESULTS

A high cathepsin D score was associated with Stage II disease (P = 0.007), positive lymph nodes (P = 0.019), high nuclear grade (P = 0.005), and c-erbB-2 oncogene amplification (P = 0.013). Cathepsin D positivity was not an independent prognostic indicator of disease-free survival (DFS) or overall survival (OS). In a subgroup analysis, the presence of cathepsin D had only a modest association with predicting a shorter DFS in patients with negative lymph nodes (P = 0.072) or positive progesterone receptors (PR) (P = 0.086).

CONCLUSIONS

Immunohistochemical analysis of cathepsin D, with the antiserum used in this study, is not an independent predictor of outcome in patients with breast cancer because of its strong associations with several well-established prognostic indicators.

Procathepsin D cannot autoactivate to cathepsin D at acid pH

The amino acid sequence of the propart of bovine procathepsin D was determined at the protein level. Incubation of the isolated procathepsin D at pH 3.5-5.0 for 30-120 min leads to a 2 kDa reduction in its molecular mass, as seen by SDS-PAGE. The activation product is pseudocathepsin D and is the result of a proteolytic cleavage between LeuP26 and IleP27 in the propart. Incubation at pH 5.0 for 20 h of either procathepsin D or pseudocathepsin D results in both cases in approximately equal amounts of pseudocathepsin D and a further processed intermediate, nine amino acids shorter than pseudocathepsin D. No reaction products corresponding to cathepsin D with a mature amino terminus were observed, showing that autoproteolysis alone cannot generate the mature form found in the lysosomes.

Molecular cloning and functional characterization of chicken cathepsin D, a key enzyme for yolk formation

Upon receptor-mediated endocytosis of very-low-density lipoprotein (VLDL) and vitellogenin into growing chicken oocytes, the protein moieties of these lipoproteins are proteolytically cleaved. Unlike the complete lysosomal degradation in somatic cells, enzymatic ligand breakdown in oocytes generates a characteristic set of polypeptides, which enter yolk storage compartments for subsequent utilization by the embryo. Here, we demonstrate directly that the catalyst for the intraoocytic processing of both apolipoprotein B and vitellogenin is cathepsin D. The enzyme was purified from oocytic yolk, preovulatory follicle homogenates, and liver by affinity chromatography. When plasma VLDL and vitellogenin were incubated with the purified enzyme, fragments indistinguishable from those found in yolk were generated from both precursors under identical, mildly acidic conditions. Amino-terminal sequencing of the pure enzyme demonstrated 88% identity with mammalian cathepsin Ds over 34 residues. On the basis of this information, a full-length clone specifying chicken preprocathepsin D was isolated from a chicken follicle cDNA library by screening with a human cathepsin D probe. Whereas previous studies have demonstrated that the receptors for lipoproteins in somatic cells and oocytes, respectively, of the chicken are the products of different genes, Southern and Northern blot hybridization experiments showed that the enzymes expressed in oocytes and liver are the product of a single gene, giving rise to a 3.3-kb transcript. The primary structure of the 335-residue mature protein suggests a high degree of conservation of known crucial features of aspartyl proteases; however, the absence of the so-called processing region and of an aromatic residue in a region thought to partake in catalysis raise questions with possible evolutionary implications.