Autodegradation of lysosomal cysteine proteinases

A Mild Inhibition of Cathepsin K Paradoxically Stimulates the Resorptive Activity of Osteoclasts in Culture

Cathepsin K (CatK) inhibition allows reducing bone resorption with specific advantages compared to the existing anti-osteoporosis drugs. Its clinical use appears even more promising with the recent development of ectosteric inhibitors. A confusing observation, however, is that a low dose of the active site CatK inhibitor odanacatib (ODN) was reported to decrease bone mineral density and increase serum levels of the bone resorption marker carboxy-terminal collagen crosslinks (CTX). The present study provides a possible explanation for this paradox. The resorptive activity of human osteoclasts seeded on bone slices was inhibited when subjected to ODN at doses of 20 nM, but about 100-fold lower doses induced a significant increase in CTX levels and in eroded surface (12 repeats). This low-dose-induced stimulation was prevented by inhibition of non-CatK cysteine proteinases, thereby indicating that the stimulation results from an interplay between CatK and other cysteine proteinases. Effective interplay between these proteinases was also shown in enzymatic assays where the CatK-mediated degradation of collagen was enhanced upon addition of cathepsins B or L. Furthermore, extracts of osteoclasts subjected to a low dose of ODN showed higher levels of cathepsin B compared with extracts of control osteoclasts. In conclusion, the low-dose-induced stimulation of resorption observed in the clinical study can be reproduced in osteoclasts cultured in the absence of any other cell. Our data support an osteoclast-intrinsic mechanism where a mild inhibition of CatK results in increased levels of other proteinases contributing to the collagen degradation process.

Chronic cathepsin inhibition by E-64 in Dahl salt-sensitive rats

Cysteine cathepsins are lysosomal enzymes expressed in the kidneys and other tissues, and are involved in the maturation and breakdown of cellular proteins. They have been shown to be integrally involved in the progression of many cardiovascular and renal diseases. The goal of this study was to determine the involvement of cysteine cathepsins in the development of salt‐sensitive hypertension and associated kidney damage. In our experiments, Dahl salt‐sensitive (SS) rats were fed an 8% high salt NaCl diet and intravenously infused with the irreversible cysteine cathepsin inhibitor E‐64 (1 mg/day) or the vehicle (control). Both the control and E‐64 infused groups developed significant hypertension and kidney damage, and no difference of the mean arterial pressure and the hypertension‐associated albuminuria was observed between the groups. We next tested basal calcium levels in the podocytes of both control and infused groups using confocal calcium imaging. Basal calcium did not differ between the groups, indicative of the lack of a protective or aggravating influence by the cathepsin inhibition. The efficacy of E‐64 was tested in Western blotting. Our findings corresponded to the previously reported, E‐64 induced increase in cathepsin B and L abundance. We conclude that the inhibition of cysteine cathepsins by E‐64 does not have any effects on the blood pressure development and kidney damage, at least under the studied conditions of this model of SS hypertension.

Differential cathepsin responses to inhibitor-induced feedback: E-64 and cystatin C elevate active cathepsin S and suppress active cathepsin L in breast cancer cells

Cathepsins are powerful proteases, once referred to as the lysosomal cysteine proteases, that have been implicated in breast cancer invasion and metastasis, but pharmaceutical inhibitors have suffered failures in clinical trials due to adverse side effects. Scientific advancement from lysosomotropic to cell impermeable cathepsin inhibitors have improved efficacy in treating disease, but off-target effects have still been problematic, motivating a need to better understand cellular feedback and responses to treatment with cathepsin inhibitors. To address this need, we investigated effects of E-64 and cystatin C, two broad spectrum cathepsin inhibitors, on cathepsin levels intra- and extracellularly in MDA-MB-231 breast cancer cells. Cathepsins S and L had opposing responses to both E-64 and cystatin C inhibitor treatments with paradoxically elevated amounts of active cathepsin S, but decreased amounts of active cathepsin L, as determined by multiplex cathepsin zymography. This indicated cellular feedback to selectively sustain the amounts of active cathepsin S even in the presence of inhibitors with subnanomolar inhibitory constant values. These differences were identified in cellular locations of cathepsins L and S, trafficking for secretion, co-localization with endocytosed inhibitors, and longer protein turnover time for cathepsin S compared to cathepsin L. Together, this work demonstrates that previously underappreciated cellular compensation and compartmentalization mechanisms may sustain elevated amounts of some active cathepsins while diminishing others after inhibitor treatment. This can confound predictions based solely on inhibitor kinetics, and must be better understood to effectively deploy therapies and dosing strategies that target cathepsins to prevent cancer progression.

Cathepsin B is a New Drug Target for Traumatic Brain Injury Therapeutics: Evidence for E64d as a Promising Lead Drug Candidate

There is currently no therapeutic drug treatment for traumatic brain injury (TBI) despite decades of experimental clinical trials. This may be because the mechanistic pathways for improving TBI outcomes have yet to be identified and exploited. As such, there remains a need to seek out new molecular targets and their drug candidates to find new treatments for TBI. This review presents supporting evidence for cathepsin B, a cysteine protease, as a potentially important drug target for TBI. Cathepsin B expression is greatly up-regulated in TBI animal models, as well as in trauma patients. Importantly, knockout of the cathepsin B gene in TBI mice results in substantial improvements of TBI-caused deficits in behavior, pathology, and biomarkers, as well as improvements in related injury models. During the process of TBI-induced injury, cathepsin B likely escapes the lysosome, its normal subcellular location, into the cytoplasm or extracellular matrix (ECM) where the unleashed proteolytic power causes destruction via necrotic, apoptotic, autophagic, and activated glia-induced cell death, together with ECM breakdown and inflammation. Significantly, chemical inhibitors of cathepsin B are effective for improving deficits in TBI and related injuries including ischemia, cerebral bleeding, cerebral aneurysm, edema, pain, infection, rheumatoid arthritis, epilepsy, Huntington's disease, multiple sclerosis, and Alzheimer's disease. The inhibitor E64d is unique among cathepsin B inhibitors in being the only compound to have demonstrated oral efficacy in a TBI model and prior safe use in man and as such it is an excellent tool compound for preclinical testing and clinical compound development. These data support the conclusion that drug development of cathepsin B inhibitors for TBI treatment should be accelerated.

Positive lysosomal modulation as a unique strategy to treat age-related protein accumulation diseases

Lysosomes are involved in degrading and recycling cellular ingredients, and their disruption with age may contribute to amyloidogenesis, paired helical filaments (PHFs), and α-synuclein and mutant huntingtin aggregation. Lysosomal cathepsins are upregulated by accumulating proteins and more so by the modulator Z-Phe-Ala-diazomethylketone (PADK). Such positive modulators of the lysosomal system have been studied in the well-characterized hippocampal slice model of protein accumulation that exhibits the pathogenic cascade of tau aggregation, tubulin breakdown, microtubule destabilization, transport failure, and synaptic decline. Active cathepsins were upregulated by PADK; Rab proteins were modified as well, indicating enhanced trafficking, whereas lysosome-associated membrane protein and proteasome markers were unchanged. Lysosomal modulation reduced the pre-existing PHF deposits, restored tubulin structure and transport, and recovered synaptic components. Further proof-of-principle studies used Alzheimer disease mouse models. It was recently reported that systemic PADK administration caused dramatic increases in cathepsin B protein and activity levels, whereas neprilysin, insulin-degrading enzyme, α-secretase, and β-secretase were unaffected by PADK. In the transgenic models, PADK treatment resulted in clearance of intracellular amyloid beta (Aβ) peptide and concomitant reduction of extracellular deposits. Production of the less pathogenic Aβ(1-38) peptide corresponded with decreased levels of Aβ(1-42), supporting the lysosome's antiamyloidogenic role through intracellular truncation. Amelioration of synaptic and behavioral deficits also indicates a neuroprotective function of the lysosomal system, identifying lysosomal modulation as an avenue for disease-modifying therapies. From the in vitro and in vivo findings, unique lysosomal modulators represent a minimally invasive, pharmacologically controlled strategy against protein accumulation disorders to enhance protein clearance, promote synaptic integrity, and slow the progression of dementia.

Effect of cathepsin k inhibitor basicity on in vivo off-target activities

Cathepsin K is a lysosomal cysteine protease that is a pharmacological target for the treatment of osteoporosis. Previous studies showed that basic, lipophilic cathepsin K inhibitors are lysosomotropic and have greater activities in cell-based assays against cathepsin K, as well as the physiologically important lysosomal cysteine cathepsins B, L, and S, than expected based on their potencies against these isolated enzymes. Long-term administration of the basic cathepsin K inhibitors N-(1-(((cyanomethyl)amino)carbonyl)cyclohexyl)-4-(2-(4-methyl-piperazin-1-yl)-1,3-thiazol-4-yl)benzamide (L-006235) and balicatib to rats at a supratherapeutic dose of 500 mg/kg/day for 4 weeks resulted in increased tissue protein levels of cathepsin B and L but had no effect on cathepsin B and L message. This is attributed to the inhibitor engagement of these off-target enzymes and their stabilization to proteolytic degradation. No such increase in these tissue cathepsins was detected at the same dose of N-(cyanomethyl)-N(2)-{(1S)-2,2,2-trifluoro-1-[4'-methylsulfonyl)biphenyl-4-yl]ethyl}-l-leucinamide (L-873724), a potent nonbasic cathepsin K inhibitor with a similar off-target profile, although all three inhibitors provided similar plasma exposures. Using an activity-based probe, (125)I-BIL-DMK, in vivo inhibition of cathepsins B, L, and S was detected in tissues of mice given a single oral dose of L-006235 and balicatib, but not in mice given L-873724. In each case, similar tissue levels were achieved by all three compounds, thereby demonstrating the in vivo cathepsin selectivity of L-873724. In conclusion, basic cathepsin K inhibitors demonstrate increased off-target cysteine cathepsin activities than their nonbasic analogs and potentially have a greater risk of adverse effects associated with inhibition of these cathepsins.

Design, Synthesis, and Evaluation of In Vivo Potency and Selectivity of Epoxysuccinyl-Based Inhibitors of Papain-Family Cysteine Proteases

The papain-family cathepsins are cysteine proteases that are emerging as promising therapeutic targets for a number of human disease conditions ranging from osteoporosis to cancer. Relatively few selective inhibitors for this family exist, and the in vivo selectivity of most existing compounds is unclear. We present here the synthesis of focused libraries of epoxysuccinyl-based inhibitors and their screening in crude tissue extracts. We identified a number of potent inhibitors that display selectivity for endogenous cathepsin targets both in vitro and in vivo. Importantly, the selectivity patterns observed in crude extracts were generally retained in vivo, as assessed by active-site labeling of tissues from treated animals. Overall, this study identifies several important compound classes and highlights the use of activity-based probes to assess pharmacodynamic properties of small-molecule inhibitors in vivo.

Calvarial osteoclasts express a higher level of tartrate-resistant acid phosphatase than long bone osteoclasts and activation does not depend on cathepsin K or L activity

Bone resorption by osteoclasts depends on the activity of various proteolytic enzymes, in particular those belonging to the group of cysteine proteinases. Next to these enzymes, tartrate-resistant acid phosphatase (TRAP) is considered to participate in this process. TRAP is synthesized as an inactive proenzyme, and in vitro studies have shown its activation by cysteine proteinases. In the present study, the possible involvement of the latter enzyme class in the in vivo modulation of TRAP was investigated using mice deficient for cathepsin K and/or L and in bones that express a high (long bone) or low (calvaria) level of cysteine proteinase activity. The results demonstrated, in mice lacking cathepsin K but not in those deficient for cathepsin L, significantly higher levels of TRAP activity in long bone. This higher activity was due to a higher number of osteoclasts. Next, we found considerable differences in TRAP activity between calvarial and long bones. Calvarial bones contained a 25-fold higher level of activity than long bones. This difference was seen in all mice, irrespective of genotype. Osteoclasts isolated from the two types of bone revealed that calvarial osteoclasts expressed higher enzyme activity as well as a higher level of mRNA for the enzyme. Analysis of TRAP-deficient mice revealed higher levels of nondigested bone matrix components in and around calvarial osteoclasts than in long bone osteoclasts. Finally, inhibition of cysteine proteinase activity by specific inhibitors resulted in increased TRAP activity. Our data suggest that neither cathepsin K nor L is essential in activating TRAP. The findings also point to functional differences between osteoclasts from different bone sites in terms of participation of TRAP in degradation of bone matrix. We propose that the higher level of TRAP activity in calvarial osteoclasts compared to that in long bone cells may partially compensate for the lower cysteine proteinase activity found in calvarial osteoclasts and TRAP may contribute to the degradation of noncollagenous proteins during the digestion of this type of bone.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

The half-life of human procathepsin S

Two processes, synthesis and degradation, contribute to the intracellular concentration of a protein. As most malignant tumors or tumor cell lines show elevated levels of proteinases, we studied the half-life of a cysteine proteinase, procathepsin S, in order to determine whether tumor cells can regulate their cathepsin concentration via changing the degradation rate of the enzyme. The following procathepsin S species were examined: wild-type procathepsin S in macrophages, recombinant procathepsin S in human embryonic kidney cells (HEK 293 cells), recombinant nonglycosylated procathepsin S in HEK 293 cells, wild-type procathepsin S in the established nonsmall cell lung carcinoma cell line 97TM1. The half-lives of both wild-type procathepsins S expressed in macrophages and in HEK 293 cells were 1 h, whereas that of procathepsin S in the tumor cell line was 2 h. Nonglycosylated procathepsin S was not processed. The degradation of mature cathepsin S proceeded with a half-life of 16-18 h. All cell lines studied secreted substantial amounts of procathepsin S into the culture medium. No further maturation of secreted procathepsin S has been observed in the culture medium. We suggest a disturbed sorting mechanism in tumor cells.

Chapter 5 Role of lysosomes in cell injury

Lysosomes are acidic intracellular vacuoles of heterogeneous shape, size, and content. Lysosomes contain hydrolytic enzymes that degrade proteins, lipids, carbohydrates, and nucleic acids derived from intracellular (through autophagy) and extracellular (through heterophagy) sources. Lysosomal degradation regulates several physiological cell functions. These include turnover of cellular organelles and extracellular constituents; amino acid and glucose homeostasis; processing of proteins; lipid metabolism; cell growth, differentiation, and involution; host defenses against microorganisms and other pathogens; and removal of necrotic and foreign material from the circulation and from tissues.

Lysosomal degradation also plays an important role in the pathophysiology of acute and chronic cell injury, inflammation and repair, and tumor growth and metastasis. The participation of the lysosomes in the specific types of cell injury we have discussed is due to altered regulation of one or more of the following processes: turnover of cellular organelles by autophagic degradation; levels and activities of lysosomal hydrolases; levels of intracellular and extracellular lysosomal hydrolase inhibitors; transport of degradation products from the lysosomal matrix to the cytosol; permeability of the lysosomal membrane to hydrolases; lysosomal vacuolar acidification; transport of degradable substrates and of pathogens to the lysosomes; transport and processing of secretory proteins and lysosomal hydrolases during biogenesis; traffic and fusion of lysosomal vacuoles and vesicles; secretion of lysosomal hydrolases; and accumulation of metals, particularly iron, acidotropic agents, and undegraded and/or undegradable materials in lysosomes.

Immunocytochemical study of cathepsin L and rat salivary cystatin-3 in rat osteoclasts treated with E-64 in vivo

The localization of cathepsin L and rat salivary cystatin-3 (RSC-3) in rat osteoclasts (rat femoral and alveolar bones) treated with or without E-64 (control) was examined immunocytochemically. In osteoclasts pretreated with E-64, immunoreactivity for cathepsin L was very weak extracellularly compared to that in the control osteoclasts. However, it was strong intracellularly. The localization of RSC-3 was unclear in the control osteoclasts, while in E-64 treated osteoclasts, both the clear zone and ruffled border areas showed a very strong immunoreaction. At the electron-microscopic level, in normal osteoclasts, numerous immunoreaction products for cathepsin L were found extracellularly in the bone matrix under the ruffled border, while few intracellular products were observed. In contrast, in the E-64-treated osteoclasts, only a few immunoreaction products were found extracellularly, while intracellularly cathepsin L was found in numerous endosome-lysosomal vacuoles. In the immunoreaction for RSC-3, the cytoplasm of the ruffled border was positive, and the tips of the RSC-3-positive ruffled border appeared to enter deeply into the bone matrix. Intracellularly, the granular reaction products of RSC-3 were found in the vacuoles (probably autophagolysosomes). Thus, in E-64-treated osteoclasts, inhibition of the extracellular release of cathepsin L was demonstrated. In addition, intralysosomal accumulation of RSC-3 and deep penetration of the RSC-3-positive ruffled border into the bone matrix were found. These findings suggest that RSC-3 is associated with the inhibition of cathepsin L in both the lysosomes (in the osteoclasts) and bone matrix.

Identification of targeting proteinase for rat alpha 1-macroglobulin in vivo. Mast-cell tryptase is a major component of the alpha 1-macroglobulin-proteinase complex endocytosed into rat liver lysosomes

The alpha 1-macroglobulin-proteinase complex endocytosed into rat liver lysosomes was purified by a series of column chromatographic steps on concanavalin A-Sepharose, Sephacryl S-300, DEAE-cellulose and TSK gel DEAE-5PW columns. The complex contained no detectable alpha 2-macroglobulin. Studies on the substrate specificity indicated that the complex had tryptase-like activities towards various synthetic substrates, but no elastase, chymotrypsin, cathepsin-B and cathepsin-L activities. The proteinase activity was completely inhibited by di-isopropyl fluorophosphate, leupeptin and antipain, indicating that the proteinase bound to alpha 1-macroglobulin is a serine proteinase. Two protein bands (62 and 59 kDa) of the complex were labelled with [3H]diisopropyl fluorophosphate and both bands cross-reacted with anti-(mast-cell tryptase)antibody. These results suggest that mast-cell tryptase is a major targeting proteinase for alpha 1-macroglobulin in vivo. The main alpha-macroglobulin-proteinase complex in the adjuvant-treated rats was also the alpha 1-macroglobulin-tryptase complex, even though the plasma level of alpha 2-macroglobulin was elevated.

Chloroquine accumulation and alterations of proteolysis and pinocytosis in the rat conceptus in vitro

The teratogenicity of chloroquine (CQ) has been hypothesized to result from its effects on lysosomal function, specifically the ability of the visceral yolk sac (VYS) to capture and degrade external macromolecules. Using the rat whole embryo culture system, we evaluated the ability of CQ to accumulate in conceptual tissues and its effects on aspects of VYS function known to be important in the uptake and processing of nutrients. When CQ was added directly to the culture medium, it was found to accumulate rapidly in conceptual tissues, particularly the VYS. Tissue concentrations of CQ in the embryo proper reached approximately 10-fold those in the medium, whereas concentrations in the VYS exceeded by 100-fold the medium concentration within a 4-hr exposure on gestational day (GD) 10. Embryotoxic concentrations of CQ (10-30 microM) enhanced the activity of lysosomal cysteine proteinases measured in vitro under optimum pH conditions in both embryonic and VYS homogenates after a 26-hr treatment from GD 10-11. A different pattern of response in enzyme activity was observed between embryos and VYSs that could be attributed to the preferential accumulation of CQ in the VYS. Nonembryotoxic concentrations of CQ (1-7.5 microM) induced a concentration-dependent increase in VYS enzyme activity that peaked in conceptuses exposed to 20 microM CQ (an intermediate embryotoxic concentration). The enhanced cysteine proteinase activity was time dependent and appeared to increase gradually in conceptuses exposed to 10-20 microM CQ during the 26-hr culture period. This was in contrast to the rapid accumulation of CQ in conceptual tissues seen on gestational day 10. Protein content in the VYS was increased significantly after a 9-hr exposure of whole conceptuses to CQ (20 microM), indicating an inhibition of VYS proteolytic activity in situ. After 24 hr of exposure to 20 microM CQ, VYS protein content was not significantly different from control, but embryonic protein was reduced significantly by 20%. These observations are consistent with a model of reversible inhibition of VYS proteolysis by CQ followed by a compensatory increase in lysosomal proteinase activity. VYS fluid-phase pinocytosis was also assessed after CQ exposure and found to be inhibited only in the highest CQ concentration tested (30 microM). Lower concentrations of CQ that were still embryotoxic (10-20 microM) did not affect VYS fluid-phase pinocytosis, suggesting that inhibition of this activity is not primarily responsible for CQ embryotoxicity.

Increased activities of cathepsin B and other lysosomal hydrolases in fibroblasts and bone tissue cultured in the presence of cysteine proteinases inhibitors

Leupeptin is an established, reversible inhibitor of cathepsin B, a lysosomal cysteine proteinase. Yet, in rat fibroblasts as well as in foetal mouse calvaria, we observed an increase of the activity of cathepsin B in homogenates of cells and tissue harvested after culture in the presence of leupeptin. This effect was also seen for other lysosomal hydrolases, namely sphingomyelinase, N-acetyl-beta-glucosaminidase, arylsulphatase A and phospholipase A1 in fibroblasts, and beta-glucuronidase in mouse calvaria. In calvaria, antipain, another reversible cysteine proteinase inhibitor, caused a similar effect, whereas E-64, an irreversible inhibitor, was consistently inhibitory of the cathepsin B activity; yet it also caused an increase of beta-glucuronidase activity. The effect of leupeptin in fibroblasts was dose and time-dependent, required the continuous presence of the inhibitor, and was not dependent from protein synthesis. Actually, addition of cycloheximide caused a severe loss of activity of cathepsin B and of sphingomyelinase. In the presence of both cycloheximide and leupeptin, however, these two activities were retained to a value corresponding to that found in excess in cells cultivated with leupeptin alone. The data therefore suggests that leupeptin exerts the effects described in this paper by preventing the degradation of cathepsin B, sphingomyelinase and probably several other lysosomal hydrolases by cysteine proteinases. We therefore propose that cysteine proteinases play a key role in the control of the steady-state levels of these enzymes in normal conditions.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

Leupeptin causes an accumulation of lipofuscin-like substances in liver cells of young rats

Leupeptin, a thiol protease inhibitor, has previously been shown to cause a dense accumulation of substances resembling age pigment and called ceroid-lipofuscin, in brain cells of young rats. Thus far, however, attempts to produce age pigments in hepatocytes of normal young rats with protease inhibitor(s) have not been successful. The present study provides the first demonstration that leupeptin induces lipofuscin-like substances in normal young rat hepatocytes. Male Fischer-344 rats (age 4-6 weeks) were continuously infused with leupeptin or saline i.p. for 2 weeks by an osmotic minipump (dosage, 1-50 mg/100 g per day). Liver tissues were then examined by light, fluorescence and electron microscopy. Both hepatocytes and non-parenchymal cells of livers treated with leupeptin, but not saline, showed a dense accumulation of pigments which stained deeply with toluidine blue, were PAS-positive and were brightly autofluorescent. After UV excitation the pigments had an emission spectrum with a broad peak at 480-540 nm extending to 650 nm resembling the spectrum of age pigment from livers of normal aged rats. Electron microscopic examination revealed numerous lipofuscin-like deposits with heterogeneous morphology in the cytoplasm of both hepatocytes and non-parenchymal cells; lipid and myelin-like bodies were also present in hepatocytes. The results indicate that the perturbation of proteolytic activity in liver by leupeptin causes an accumulation of substances which by several criteria resemble lipofuscin. These results thus provide further support for the 'Protease Inhibitor Model of Lipofuscin Formation' as well as a potential experimental model for studying hepatocellular aging processes.

Inhibitor studies indicate that active cathepsin L is probably essential to its own processing in cultured fibroblasts

The lysosomal cysteine proteinase cathepsin L is synthesized in cultured mouse NIH 3T3 cells as a 39 kDa precursor and processed intracellularly into active 29 kDa and 20 kDa + 5 kDa lysosomal forms. Addition to culture media of the peptidyl aldehyde leupeptin, a non-covalent inhibitor of cathepsin L, results in the accumulation of the 20 kDa mature form of the enzyme, resulting in increased activity of cathepsin L as measured in an in vitro assay system in the absence of leupeptin. The more potent irreversible cathepsin L inhibitors benzyloxycarbonyl-Phe-Ala-diazomethane and L-transepoxysuccinyl-L-leucylamino-(4-guanidino)butane, when added to living cells at low concentrations, result in accumulation of all partially processed forms of cathepsin L, especially the 29 kDa form, suggesting that cathepsin L is responsible for its own processing. Exogenous procathepsin L introduced into CHO cells by endocytosis via the mannose 6-phosphate receptor is processed in a manner similar to endogenous procathepsin L. We conclude that the major intracellular pathway for processing of procathepsin L, either endogenous or exogenous, probably requires active cathepsin L.

Effect of metabolic alterations on the density and the contents of cathepsins B, H and L of lysosomes in rat macrophages

Crude lysosomal preparations from non-cultured peritoneal rat macrophages were shown to separate into high-density fractions rich in cathepsin B and H and low-density fractions rich in cathepsin L when layered on Percoll density gradients. Morphologically, the heavy lysosome fractions were found to consist mainly of lysosomes labeled with gold particles for anti-(cathepsin B, H and L). The light lysosome fractions contained lysosomes labeled with anti-(cathepsin B, H and L) and many other contaminants. In addition, small vesicles labeled by anti-(cathepsin L) were detected in these fractions. Addition of calf serum to the cultured macrophages induced an increase in the density of lysosomes in both dose-dependent and time-dependent fashions. Cathepsins B, H and L all shifted to the heavy lysosome fractions following the addition of serum. Progressive increase in fluorescence-labeled calf IgG in the heavy lysosome fractions after its addition suggests that the continuous entrance of excess proteins to lysosomes causes an increase in their density. This idea is supported by the fact that the density of lysosomes increased in parallel with the accumulation of horseradish peroxidase taken up in the heavy lysosome fractions. Increase in the density of lysosomes after treatment with ethyl(2S,3S)-3[(S)-3-methyl-1-(3-methyl-butylcarbamoyl)]oxirane-2- carboxylate (E-64-d) was marked in the cells cultured with serum-containing medium but slight in serum-deprived cells. However, the level of pyruvate kinase, an autophagic sequestration marker in heavy autolysosomes from E-64-d-treated cells, was much higher in serum-deprived cells, indicating that the contribution of heterophagic sequestration towards an increase in the density of lysosomes is much greater than that of autophagy.

Regulation of proteinase levels in the nematode Caenorhabditis elegans. Preferential depression by acute or chronic starvation

Acute starvation of the wild-type of the nematode Caenorhabditis elegans depresses the level of cathepsin D by 65% within 4-8 h and the level of the thiol cathepsins Ce1 and Ce2 to about the same extent after 24 h. There is no parallel loss of lysosomal beta-glucosidase or beta-hexosaminidase activities. In strains which are chronically starved as a result of mutations which compromise feeding behaviour (unc-52) or nutrient uptake into the intestinal cells (daf-4), cathepsin D levels are decreased to about 15% of the level in fully fed wild-type animals. We suggest that the decline in the cathepsin D level results from autodigestion when alternative protein substrates are depleted in the lysosomes.

Action of the major protease from Entamoeba histolytica on proteins of the extracellular matrix

The action of the major protease from the parasitic protozoon Entamoeba histolytica, a cysteine protease of Mr 27,000-29,000, on some important proteins of the extracellular matrix has been studied. The isolated protease degraded the extracellular matrix proteins from human tissue collagen type IV and V as well as laminin and fibronectin with different velocities and specificities under native conditions. Whereas the degradation of fibronectin and laminin proceeded rapidly, yielding distinct fragment patterns, the breakdown of the collagen types happened more slowly and incompletely. The digestion of the denatured isolated alpha 2-chain of bovine collagen type I was very fast and unspecific requiring only 1/10 of the enzyme activities as compared with the other substrates mentioned above. Nearly 85% of the overall proteolytic activity of a soluble fraction of E. histolytica was strongly inhibited by antibodies against the purified histolytic protease as well as by cystatin from chicken egg white, a specific protein inhibitor of cysteine proteases. We conclude that the histolytic protease represents by far the highest portion of soluble proteolytic activity in E. histolytica which is sufficient to destroy the extracellular matrix of the host.

The turnover of lysosomal glycosylasparaginase in rat liver

Rat liver glycosylasparaginase (N4-(beta-N-acetyl-D-glucosaminyl)-L-asparagine amidohydrolase (EC 3.5.1.26) was irreversibly inhibited in vitro by the asparagine analogue 5-diazo-4-oxo-L-norvaline (DONV). Following the abolition of this lysosomal hydrolase by DONV in rats, the enzyme activity in liver recovered exponentially. Assuming a zero order rate of protein synthesis, the half-life of the glycosylasparaginase in rat liver was measured to be 2 days.

Effect of proteinase inhibitors on intracellular processing of cathepsin B, H and L in rat macrophages

The effects of various proteinase inhibitors on the processing of lysosomal cathepsins B, H and L were investigated in cultured rat peritoneal macrophages. The processing of newly synthesized pro-cathepsins B, H and L to the mature single-chain enzymes was sensitive to a metal chelator,1,10-phenanthroline, and a synthetic metalloendopeptidase substrate, Z-Gly-Leu-NH2, and insensitive to inhibitors of serine proteinases, aspartic proteinases and cysteine proteinases. Inhibitors of cysteine proteinases, E-64-d and leupeptin, inhibited the processing of the single-chain forms of cathepsins B, H and L to the two-chain forms. These results suggest that (a) metal endopeptidase(s) is (are) involved in the propeptide processing of cathepsin B, H and L, and that proteolytic cleavages of the mature single-chain cathepsins are accomplished by cysteine proteinases in lysosomes.

Active center differences between cathepsins L and B: the S1 binding region

The substrate peptide bond cleaved by cathepsins B and L is determined not by the amino acid contributing the carboxyl group to this bond as in the case of serine proteases but rather by the presence of a neighboring amino acid with a large hydrophobic side chain. From a study of the inhibitory potency in a series, Cbz-Phe-X-CHN2, in which Phe promotes binding at S2 (terminology of [(1968) Biochem. Biophys. Res. Commun. 32, 898-902]) while the amino acid X probes S1, it is shown that this region of cathepsin L also has the ability to accommodate large hydrophobic side chains. In this respect cathepsin L differs from cathepsin B. Thus Cbz-Phe-Tyr(O-t-Bu)CHN2 inactivates cathepsin L with a rate 2.5 x 10(4) greater than that for cathepsin B.