Proteolytic processing and glycosylation of cathepsin B. The role of the primary structure of the latent precursor and of the carbohydrate moiety for cell-type-specific molecular forms of the enzyme

An active zymogen: unravelling the mystery of tissue-type plasminogen activator

In contrast to almost all other proteinases, human tissue-type plasminogen activator (tPA) is also proteolytically active in its zymogen or single-chain form. The closely related plasminogen activator isolated from vampire bat saliva (vPA) acts exclusively in the single-chain form, lacking the requisite cleavage site for proteolytic activation. Recent structural studies on the proteolytic domains of vPA and human tPA in two- and single-chain forms reveal the mechanism of this anomalous activity. The PA-catalyzed proteolytic conversion of plasminogen to plasmin, responsible for the initiation of fibrinolysis, is fibrin-dependent; comparative structural analysis of the plasminogen activators provides clues as to the role of fibrin as cofactor.

Elevations in cathepsin B protein content and enzyme activity occur independently of glycosylation during colorectal tumor progression

Western blots, enzyme assays, protein glycosylation studies, and immunohistochemical staining were used to characterize cathepsin B expression at successive stages of colorectal tumor progression. In normal colon mucosa and premalignant adenomas, cathepsin B expression was predominantly due to mature two-chain protein detected on Western blots as the nonglycosylated 27-kDa form, with overexpression of this protein occurring in only 4 of 18 adenomas. Overexpression increased significantly in Dukes A and B carcinomas (26 of 37 cases), with cathepsin B protein generally detectable in carcinomas as a combination of both 27-kDa nonglycosylated and 28-kDa glycosylated mature two-chain forms. Glycosylated cathepsin B protein in carcinoma extracts was sensitive to PNGase F but resistant to Endo H, indicating a pattern consistent with complex rather than high mannose type glycosylation. When sorted by advancing tumor stage, peak expression of cathepsin B protein occurred in carcinomas involved in local invasion compared with adenomas or metastatic cancers. At all stages, cathepsin B activity correlated significantly with the levels of heavy chain mature cathepsin B protein (r = 0.6682, p < 0.0001) irrespective of glycosylation. Immunohistochemical staining of cathepsin B protein revealed fine diffuse cytoplasmic staining in both adenomas and carcinomas compared with coarse granular cytoplasmic staining (typical of lysosomes) seen in matched normal mucosa. Our results demonstrate several sequential, apparently independent changes in cathepsin B expression during colorectal tumor progression including early changes in subcellular localization, up-regulation of cathepsin B protein and activity in invasive cancers, and altered protein glycosylation detected in malignant tumors at all stages.

Major increase in endopeptidase activity of human cathepsin B upon removal of occluding loop contacts

The main feature distinguishing cathepsin B from other cysteine proteases of the papain family is the presence of a large insertion loop, termed the occluding loop, which occupies the S' subsites of the enzyme. The loop is held in place mainly by two contacts with the rest of the enzyme, involving residues His110 and Arg116 on the loop that form salt bridges with Asp22 and Asp224, respectively. The influence of this loop on the endopeptidase activity of cathepsin B has been investigated using site-directed mutagenesis and internally quenched fluorogenic (IQF) substrates. Wild-type cathepsin B displays poor activity against the substrates Abz-AFRSAAQ-EDDnp and Abz-QVVAGA-EDDnp as compared to cathepsin L and papain. Appreciable increases in kcat/KM were observed for cathepsin B containing the single mutations D22A, H110A, R116A, and D224A. The highest activity however is observed for mutants where both loop to enzyme contacts are disrupted. For the triple-mutant D22A/H110A/R116A, an optimum kcat/KM value of 12 x 10(5) M-1 s-1 was obtained for hydrolysis of Abz-AFRSAAQ-EDDnp, which corresponds to a 600-fold increase relative to wild-type cathepsin B and approaches the level of activity observed with cathepsin L or papain. By comparison, the mutations have little effect on the hydrolysis of Cbz-FR-MCA. The influence of the mutations on the pH dependency of activity also indicates that the complexity of pH activity profiles normally observed for cathepsin B is related to the presence of the occluding loop. The major increase in endopeptidase activity is attributed to an increase in loop "flexibility" and suggests that the occluding loop might move when an endopeptidase substrate binds to the enzyme. The possible contribution of these interactions in regulating endopeptidase activity and the implications for cathepsin B activity in physiological or pathological conditions are discussed.

Identification and characterization of cathepsin B as the cellular MARCKS cleaving enzyme

The importance of regulating the cellular concentrations of the myristoylated alanine-rich C kinase substrate (MARCKS), a major cellular substrate of protein kinase C, is indicated by the fact that mice lacking MARCKS exhibit gross abnormalities of central nervous system development and die shortly after birth. We previously identified a novel means of regulating cellular MARCKS concentrations that involved a specific proteolytic cleavage of the protein and implicated a cysteine protease in this process (Spizz, G., and Blackshear, P. J. (1996) J. Biol. Chem. 271, 553-562). Here we show that p40, the carboxyl-terminal fragment resulting from this cleavage of MARCKS, was associated with the mitochondrial/lysosomal pellet fraction of human diploid fibroblasts and that its generation in cells was sensitive to treatment with NH4Cl. These data suggest the involvement of lysosomes in the generation and/or stability of p40. The MARCKS-cleaving enzyme (MCE) activity was peripherally associated with a 10,000 x g pellet fraction from bovine liver, and it co-purified with the activity and immunoreactivity of a lysosomal protease, cathepsin B. Cathepsin B catalyzed the generation of p40 from MARCKS in a cell-free system and behaved similarly to the MCE with respect to mutants of MARCKS previously shown to be poor substrates for the MCE. Treatment of fibroblasts with a cell-permeable, specific inhibitor of cathepsin B, CA074-Me, resulted in parallel time- and concentration-dependent inhibition of cathepsin B and MCE activity. Incubation of a synthetic MARCKS phosphorylation site domain peptide with purified cathepsin B resulted in cleavage of the peptide at sites consistent with preferred cathepsin B substrate sites. These data provide evidence for the identity of the MCE as cathepsin B and suggest that this cleavage most likely takes place within lysosomes, perhaps as a result of specific lysosomal targeting sequences within the MARCKS primary sequence. The data also suggest a direct interaction between MARCKS and cathepsin B in cells and leave open the possibility that MARCKS may in some way regulate the protease for which it is a substrate.

Crystal structure of the wild-type human procathepsin B at 2.5 A resolution reveals the native active site of a papain-like cysteine protease zymogen

The structure of the wild-type human procathepsin B has been refined to a crystallographic R-value of 0.18 and R-free of 0.23 exploiting the data obtained from new crystals that diffract beyond 2.5 A resolution. The structure confirms two previously presented, lower-resolution structures. The structure of the propeptide chain folds on the surface of the enzyme domains and blocks access of substrate to the already formed active site. Abundant solvent molecules fill the cavities between the propeptide and the enzyme part of the molecule. The propeptide structure is compared with a substrate model in the S2, S1, S1' and S2' binding sites. In this crystal form the cathepsin B occluding loop residues adopt yet another conformation. The structures show that the occluding loop region between the residues Cys108 and Cys119 behaves quite independently from the rest of the structure and easily adapts to changes in environment. The variety of the observed conformations of the occluding loop is in agreement with other data showing that the loop is responsible for limiting cathepsin B activity to that of a carboxydipeptidase. The region before Cys108 is essentially the same as in the mature structure, whereas the region from Cys119 to Thr125 is raised compared to the mature form by the propeptide squeezed between it and the enzyme domains, surface. The structure strongly suggests that processing of procathepsin B during its autoactivation is not unimolecular.

Ethanol consumption alters trafficking of lysosomal enzymes and affects the processing of procathepsin L in rat liver

In order to determine whether ethanol consumption alters the targeting of hepatic lysosomal enzymes to their organelles, we examined the sedimentation properties of lysosomal hydrolases in ethanol-fed rats and their pair-fed controls. Rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose dextrin for one to five wk. Liver extracts were fractionated by Percoll density gradient centrifugation and fractions obtained were analyzed for the distribution of lysosomal marker enzymes. Heavy lysosomes were further purified from these gradients and the activity of specific hydrolases was determined. Compared with those from controls, isolated lysosomes from ethanol-fed rats showed a 20-50% reduction in the activity of lysosomal acid phosphatase and beta-galactosidase. Decreased intralysosomal hydrolase activity in ethanol-fed rats was associated with a significant redistribution of these enzymes as well as those of cathepsins B and L to lighter fractions of Percoll density gradients. This indicated an ethanol-elicited shift of these enzymes to lower density cellular compartments. In order to determine whether ethanol administration affects the synthesis and proteolytic maturation of hepatic procathepsin L, we conducted immunoblot analyses to quantify the steady-state levels of precursor and mature forms of cathepsin L in hepatic post-nuclear fractions. Ethanol administration caused a significant elevation in the steady-state level of the 39 kDa cathepsin L precursor relative to its 30 kDa intermediate and 25 kDa mature product. These results were confirmed by pulse-chase experiments using isolated hepatocytes exposed to [35S]methionine. Hepatocytes from both control and ethanol-fed rats incorporated equal levels of radioactivity into procathepsin L. However, during the chase period, the ratios of the 39 kDa procathepsin L to its 30 kDa intermediate and 25 kDa mature product in cells from ethanol-fed rats were 1.5-3-fold higher than those in controls. These results demonstrate that ethanol consumption caused a marked impairment in the processing of procathepsin L to mature enzyme, without affecting its synthesis. Taken together, our findings suggest that chronic ethanol consumption caused a deficiency in intralysosomal enzyme content by altering the trafficking and processing of these hydrolases into lysosomes.

Activation of cathepsin B, secreted by a colorectal cancer cell line requires low pH and is mediated by cathepsin D

The aim of our study was to identify changes in secreted procathepsin B levels in a model of the human colorectal adenoma to carcinoma sequence and to determine the factors required for its extracellular activation. Conversion of the non-tumorigenic adenoma-derived cell line PC/AA to a highly tumorigenic phenotype (designated AA/CI/SB10/M) was associated with an 8-fold increase in the presence of the proform of cathepsin B in 24 hr conditioned serum-free medium (SFM). In addition, mature enzyme was only detected in the cell lines of this model with increased malignant potential. This is in agreement with the findings of a previous study, in which mature cathepsin B was only present in the 24 hr conditioned SFM of cancer-derived cell lines and not in SFM from adenoma-derived cell lines. Having demonstrated a reduction in the pH of conditioned medium from cell lines with increased malignant potential, we used a range of specific proteinase inhibitors to show that an aspartyl proteinase was involved in the initial activation of procathepsin B. Consistent with this finding, we subsequently demonstrated an increased secretion of the aspartyl proteinase cathepsin D in the medium of the AA/CI/SB10/M adenocarcinoma cells compared with the non-tumorigenic AA/Cl cell line. Therefore, the presence of mature cathpsin B in the conditioned medium of the more malignant cell lines coincided with a reduction in pH and an increase in the amount of cathepsin D secreted. Data from the human colorectal derived adenoma to carcinoma sequence indicate that an in vivo mechanism may exist that, dependent on the simultaneous presence of both a tumour-generated acidic extracellular environment and an elevated secretion of procathepsin D, could result in the activation of latent procathepsin outside the cell.

Crystal structures of human procathepsin B at 3.2 and 3.3 Angstroms resolution reveal an interaction motif between a papain-like cysteine protease and its propeptide

A wild-type human procathepsin B was expressed, crystallized in two crystal forms and its crystal structure determined at 3.2 and 3.3 Angstroms resolution. The structure reveals that the propeptide folds on the cathepsin B surface, shielding the enzyme active site from exposure to solvent. The structure of the enzymatically active domains is virtually identical to that of the native enzyme [Musil et al. (1991) EMBO J. 10, 2321-2330]: the main difference is that the occluding loop residues are lifted above the body of the mature enzyme, supporting the propeptide structure.

Pericellular mobilization of the tissue-destructive cysteine proteinases, cathepsins B, L, and S, by human monocyte-derived macrophages

Human macrophages are believed to damage host tissues in chronic inflammatory disease states, but these cells have been reported to express only modest degradative activity in vitro. However, while examining the ability of human monocytes to degrade the extracellular matrix component elastin, we identified culture conditions under which the cells matured into a macrophage population that displayed a degradative phenotype hundreds of times more destructive than that previously ascribed to any other cell population. The monocyte-derived macrophages synthesized elastinolytic matrix metalloproteinases (i.e., gelatinase B and matrilysin) as well as cysteine proteinases (i.e., cathepsins B, L, and S), but only the cathepsins were detected in the extracellular milieu as fully processed, mature enzymes by either vital fluorescence or active-site labeling. Consistent with these observations, macrophage-mediated elastinolytic activity was not affected by matrix metalloproteinase inhibitors but could be almost completely abrogated by inhibiting cathepsins L and S. These data demonstrate that human macrophages mobilize cysteine proteinases to arm themselves with a powerful effector mechanism that can participate in the pathophysiologic remodeling of the extracellular matrix.

The preparation of catalytically active human cathepsin B from its precursor expressed in Escherichia coli in the form of inclusion bodies

A cDNA clone encoding human procathepsin B was expressed at a high level in Escherichia coli using a T7 polymerase expression system, resulting in the formation of insoluble cytoplasmic protein aggregates (inclusion bodies). The recombinant product was solubilized and renatured by refolding and reoxidation. The proenzyme was subsequently processed with pepsin to produce an enzymically active enzyme. By systematic variation of the parameters influencing the folding, formation of disulphide bonds, and processing of procathepsin B to the catalytically active mature form, a simple renaturation procedure was designed, allowing the production of about 3 mg purified active cathepsin B/l E. coli culture broth. The enzyme obtained in this way consists of a single chain and, as a consequence of pepsin treatment, possesses a three-amino-acid extension at its N-terminus. The enzyme has similar kinetic and immunological properties to native human cathepsin B.

Multiple forms of cathepsin B in human lung cancer

In this study we have examined, by means of isoelectric focusing (IEF) in native polyacrylamide gel and contact-print fluorescence zymography, whether human lung carcinomas and the lung parenchyma contain different pools of multiple charge forms of the cysteine proteinase cathepsin B. The isoelectric point (pI) patterns of cathepsin B from lung carcinoma and matched lung were similar, particularly with regard to 2 major intermediate acidic enzyme pI forms designated as I and II (pIapp of 5.10 and 4.93 in tumors, and 5.11 and 4.94 in lungs, respectively). The slightly acidic cathepsin B pI forms (pIapp 5.47-5.19) in squamous-cell lung carcinoma (SQCLC) were significantly more numerous than such enzyme pI forms in lungs. The numbers of the highly acidic cathepsin B pI forms (pIapp 4.82-4.33) were significantly higher in SQCLC and lung adenocarcinoma (ACL) than in matched lung. The activity distribution percentage in the set of highly acidic cathepsin B pI forms was significantly higher in SQCLC and ACL than in matched lung. We also observed that cathepsin B from SQCLC and matched lung was fully recoverable by IEF from inhibition by leupeptin. Using the cysteine-proteinase-specific inactivator E-64, we revealed by IEF that some cathepsin B isoforms (charge forms) from SQCLC were more resistant to inactivation by this compound than the corresponding enzyme isoforms from lungs. After IEF, the enzyme isoforms apparently lost their resistance to E-64. Our results indicate that the pool of multiple charge forms of cathepsin B in SQCLC and ACL is different from that in the lung, and also that there may be an increased level of loose complexes between cathepsin B and some proteins or polypeptides in SQCLC compared to the lung.

Human gastric adenocarcinoma cathepsin B: isolation and sequencing of full-length cDNAs and polymorphisms of the gene

Four full-length cDNA clones coding for preprocathepsin B were isolated from a human gastric adenocarcinoma cDNA library (AGS 1-6-30-1) and analyzed for possible sequence modifications that might be linked to altered intracellular trafficking and secretion of cathepsin B (CTSB) in malignant tumors. Comparison of AGS 1-6-30-1 cDNAs with human kidney/hepatoma cDNAs revealed: (1) three potential N-glycosylation sites instead of two, (2) a nucleotide (nt) substitution in the coding region for the propeptide from GTG to CTG which would result in a Val26-->Leu change, (3) three silent nt replacements in the coding region for the mature protein, (4) five single-nt differences in the 5'- and 3'-UTR (untranslated regions), (5) heterogeneity in the 5'-UTR, and (6) a 10-bp insertion in the 3'-UTR. The 10-bp insertion in the 3'-UTR may alter the stability of CTSB mRNA transcripts and thereby the expression of CTSB. These clones should be useful for expressing human tumor CTSB and analyzing the function of this enzyme in malignant progression. Two restriction-fragment length polymorphisms (RFLPs), EcoRI and TaqI, were detected by Southern blot analysis of genomic DNA from 36 unrelated Caucasians. Inheritance and distribution of the EcoRI alleles (13.0 and 11.0 kb) and the TaqI alleles (5.7 and 4.6 kb) indicated they were independent polymorphisms. In contrast to the EcoRI alleles of 13.0 and 11.0 kb observed in the population survey, genomic DNA from two AGS gastric adenocarcinoma subclones revealed two EcoRI alleles of 13.0 and 7.8 kb.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Expression of cathepsin B and aryl hydrocarbon hydroxylase activities, and of apolipoprotein B in human hepatoma cells maintained long-term in a serum-free medium

We have established the human hepatoma cell line, HepG2, in a defined, serum-free medium. These cells were maintained and studied over a 100-generation period (i.e. 10 serial transfers). Cells maintained in serum-free medium exhibited growth parameters (i.e. saturation density, efficiency of plating, and population doubling time) similar to those obtained with HepG2 cells maintained in serum-supplemented medium. Serum-free cells were also similar to their serum-supplemented counterparts with respect to the expression of cathepsin B activity and the induction of aryl hydrocarbon hydroxylase by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Significantly, HepG2 cells maintained in serum-free conditions also retained the ability to synthesize and secrete proteins, including the liver plasma protein, apo-lipoprotein B. These results indicate that the serum-free medium used in this study supports the long-term growth and maintenance of human hepatoma, HepG2, cells in culture. Inasmuch as these cells retain phenotypes, including differentiated markers previously reported for their serum-supplemented counterparts, they may provide a more reliable, standardized culture system to study the expression, secretion, and regulation of proteins during biological and pathologic processes.

Activation of procathepsin B in human hepatoma cells: the conversion into the mature enzyme relies on the action of cathepsin B itself

In order to elucidate the processing mechanism of the lysosomal cysteine proteinase, cathepsin B, in mammalian cells, recombinant rat and human cathepsin B precursors were expressed in Saccharomyces cerevisiae. The active-site cysteine residue was changed to serine to prevent autoprocessing. When the purified proenzymes were incubated with the soluble fraction of postnuclear organelles obtained from human hepatoma HepG2 cells, processing to a 33 kDa form corresponding to the mature endogenous single-chain enzyme was observed. Inhibitors of metallo-, serine and aspartic proteinases exerted no significant effect on procathepsin B processing in vitro. However, the processing activity was effectively blocked by cysteine proteinase inhibitors, in particular E-64 and its cathepsin-B-selective derivative CA-074. Processing positions were identified by using anti-peptide antibodies specific for epitopes in the N- and C-terminal cleavage regions. The single-chain form produced in vitro was thus shown to contain an N-terminal extension of at least four residues relative to the mature lysosomal enzyme, as well as a C-terminal extension present in the proenzyme but usually absent in fully processed cathepsin B. On expression of the wild-type proenzyme in yeast, procathepsin B undergoes autoprocessing, yielding a single-chain form of the active enzyme, which contains similar N- and C-terminal extensions. These results indicate that maturation of procathepsin B in vivo in mammalian tissues relies on the proteolytic activity of cathepsin B itself.

Extracellular-matrix degradation at acid pH. Avian osteoclast acid collagenase isolation and characterization

Osteoclasts degrade bone matrix, which is mainly type I collagen and hydroxyapatite, in an acidic extracellular compartment. Thus we reasoned that osteoclasts must produce an acid collagenase. We purified this enzyme, a 31 kDa protein, from avian osteoclast lysates (in 100 mM acetate/1 mM CHAPS/1 mM dithiothreitol, pH 4.4), fractionated by (NH2)2SO4 precipitation, gelatin-affinity, cation exchange, and gel filtration. Fraction activity was measured using diazotized collagen or 3H-labelled cross-linked collagen (decalcified and trypsin-treated metabolically L-[4,5-3H]proline-labelled bone) as substrates. Iodoacetate, leupeptin, antipain, pepstatin and mercurials inhibited collagenolysis by the isolated proteinase; mercurial derivatives could not be re-activated by dithiothreitol. Collagen degradation was maximal at pH 4.4; purified proteinase reproduced the collagenolytic activity of cell lysates. The N-terminal amino acid sequence from the isolated protein and its CNBr degradation fragments showed sequence similarity to mammalian cathepsin Bs, and near-identity with avian liver cathepsin B. Peptide substrate specificity of the osteoclastic enzyme resembled those of mammalian cathepsin B and its avian liver counterpart, but degradation of low-molecular-mass substrates by the osteoclastic enzyme was slower, reflecting generally lower kcat. values. Further, kcat/Km varied less between arginine-containing substrates than for previously reported cathepsin Bs, indicating different substrate specificity of the osteoclast enzyme. Polyclonal antibody raised to a 25 kDa fragment of the enzyme recognized a single 31 kDa band in SDS/PAGE of osteoclast lysates blotted to poly(vinylidene difluoride), adsorbed collagenolytic activity of osteoclast lysates, and stained avian osteoclasts in tissue sections. Degenerate sense- and antisense-oligonucleotide primers, predicted from segments of primary amino acid sequence, amplified a 486 bp DNA fragment; this was cloned and sequenced. Of 162 amino acids encoded, 77% are identical with those of human cathepsin B; hybridization identified a 2.4 kb RNA in osteoclast lysates. We conclude that the major avian osteoclast collagenolytic enzyme is a cathepsin B, whose activity varies from other enzymes of its class.

Membrane proteases: roles in tissue remodeling and tumour invasion

The coordinated control of extracellular matrix degradation on the cell surface involves three crucial elements: secreted proteases and their inhibitors, surface protease receptors and integral membrane proteases. The roles that each of these elements play in cell surface proteolysis are described. The localization of proteases to the cell surface, protease activation, and regulation of cell surface proteolysis by protease inhibitors are key issues for elucidating the role of membrane proteases in tissue remodeling and tumour invasion.

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.