Identification of renal cathepsin B as a human prorenin-processing enzyme

Cathepsin B Gene Knockout Improves Behavioral Deficits and Reduces Pathology in Models of Neurologic Disorders

Cathepsin B (CTSB) is a powerful lysosomal protease. This review evaluated CTSB gene knockout (KO) outcomes for amelioration of brain dysfunctions in neurologic diseases and aging animal models. Deletion of the CTSB gene resulted in significant improvements in behavioral deficits, neuropathology, and/or biomarkers in traumatic brain injury, ischemia, inflammatory pain, opiate tolerance, epilepsy, aging, transgenic Alzheimer's disease (AD), and periodontitis AD models as shown in 12 studies. One study found beneficial effects for double CTSB and cathepsin S KO mice in a multiple sclerosis model. Transgenic AD models using amyloid precursor protein (APP) mimicking common sporadic AD in three studies showed that CTSB KO improved memory, neuropathology, and biomarkers; two studies used APP representing rare familial AD and found no CTSB KO effect, and two studies used highly engineered APP constructs and reported slight increases in a biomarker. In clinical studies, all reports found that CTSB enzyme was upregulated in diverse neurologic disorders, including AD in which elevated CTSB was positively correlated with cognitive dysfunction. In a wide range of neurologic animal models, CTSB was also upregulated and not downregulated. Further, human genetic mutation data provided precedence for CTSB upregulation causing disease. Thus, the consilience of data is that CTSB gene KO results in improved brain dysfunction and reduced pathology through blockade of CTSB enzyme upregulation that causes human neurologic disease phenotypes. The overall findings provide strong support for CTSB as a rational drug target and for CTSB inhibitors as therapeutic candidates for a wide range of neurologic disorders. SIGNIFICANCE STATEMENT: This review provides a comprehensive compilation of the extensive data on the effects of deleting the cathepsin B (CTSB) gene in neurological and aging mouse models of brain disorders. Mice lacking the CTSB gene display improved neurobehavioral deficits, reduced neuropathology, and amelioration of neuronal cell death and inflammatory biomarkers. The significance of the compelling CTSB evidence is that the data consilience validates CTSB as a drug target for discovery of CTSB inhibitors as potential therapeutics for treating numerous neurological diseases.

Transgenic Mice Overexpressing Human Alpha-1 Antitrypsin Exhibit Low Blood Pressure and Altered Epithelial Transport Mechanisms in the Inactive and Active Cycles

Human alpha-1 antitrypsin (hAAT) is a versatile protease inhibitor, but little is known about its targets in the aldosterone-sensitive distal nephron and its role in electrolyte balance and blood pressure control. We analyzed urinary electrolytes, osmolality, and blood pressure from hAAT transgenic (hAAT-Tg) mice and C57B/6 wild-type control mice maintained on either a normal salt or high salt diet. Urinary sodium, potassium, and chloride concentrations as well as urinary osmolality were lower in hAAT-Tg mice maintained on a high salt diet during both the active and inactive cycles. hAAT-Tg mice showed a lower systolic blood pressure compared to C57B6 mice when maintained on a normal salt diet but this was not observed when they were maintained on a high salt diet. Cathepsin B protein activity was less in hAAT-Tg mice compared to wild-type controls. Protein expression of the alpha subunit of the sodium epithelial channel (ENaC) alpha was also reduced in the hAAT-Tg mice. Natriuretic peptide receptor C (NPRC) protein expression in membrane fractions of the kidney cortex was reduced while circulating levels of atrial natriuretic peptide (ANP) were greater in hAAT-Tg mice compared to wild-type controls. This study characterizes the electrolyte and blood pressure phenotype of hAAT-Tg mice during the inactive and active cycles and investigates the mechanism by which ENaC activation is inhibited in part by a mechanism involving decreased cathepsin B activity and increased ANP levels in the systemic circulation.

Transcriptome Analysis of Human Reninomas as an Approach to Understanding Juxtaglomerular Cell Biology

Renin, a key component in the regulation of blood pressure in mammals, is produced by the rare and highly specialized juxtaglomerular cells of the kidney. Chronic stimulation of renin release results in a recruitment of new juxtaglomerular cells by the apparent conversion of adjacent smooth muscle cells along the afferent arterioles. Because juxtaglomerular cells rapidly dedifferentiate when removed from the kidney, their developmental origin and the mechanism that explains their phenotypic plasticity remain unclear. To overcome this limitation, we have performed RNA expression analysis on 4 human renin-producing tumors. The most highly expressed genes that were common between the reninomas were subsequently used for in situ hybridization in kidneys of 5-day-old mice, adult mice, and adult mice treated with captopril. From the top 100 genes, 10 encoding for ligands were selected for further analysis. Medium of human embryonic kidney 293 cells transfected with the mouse cDNA encoding these ligands was applied to (pro)renin-synthesizing As4.1 cells. Among the ligands, only platelet-derived growth factor B reduced the medium and cellular (pro)renin levels, as well as As4.1 renin gene expression. In addition, platelet-derived growth factor B-exposed As4.1 cells displayed a more elongated and aligned shape with no alteration in viability. This was accompanied by a downregulated expression of α-smooth muscle actin and an upregulated expression of interleukin-6, suggesting a phenotypic shift from myoendocrine to inflammatory. Our results add 36 new genes to the list that characterize renin-producing cells and reveal a novel role for platelet-derived growth factor B as a regulator of renin-synthesizing cells.

General lysosomal hydrolysis can process prorenin accurately

Renin, an aspartyl protease that catalyzes the rate-limiting step of the renin-angiotensin system, is first synthesized as an inactive precursor, prorenin. Prorenin is activated by the proteolytic removal of an amino terminal prosegment in the dense granules of the juxtaglomerular (JG) cells of the kidney by one or more proteases whose identity is uncertain but commonly referred to as the prorenin-processing enzyme (PPE). Because several extrarenal tissues secrete only prorenin, we tested the hypothesis that the unique ability of JG cells to produce active renin might be explained by the existence of a PPE whose expression is restricted to JG cells. We found that inducing renin production by the mouse kidney by up to 20-fold was not associated with the concomitant induction of candidate PPEs. Because the renin-containing granules of JG cells also contain several lysosomal hydrolases, we engineered mouse Ren1 prorenin to be targeted to the classical vesicular lysosomes of cultured HEK-293 cells, where it was accurately processed and stored. Furthermore, we found that HEK cell lysosomes hydrolyzed any artificial extensions placed on the protein and that active renin was extraordinarily resistant to proteolytic degradation. Altogether, our results demonstrate that accurate processing of prorenin is not restricted to JG cells but can occur in classical vesicular lysosomes of heterologous cells. The implication is that renin production may not require a specific PPE but rather can be achieved by general hydrolysis in the lysosome-like granules of JG cells.

Fertility defects in mice expressing the L68Q variant of human cystatin C: a role for amyloid in male infertility

Hereditary cystatin C amyloid angiopathy is an autosomal dominant disorder in which a variant form of cystatin C (L68Q) readily forms amyloid deposits in cerebral arteries in affected individuals resulting in early death. L68Q protein deposits in human cystatin C amyloid angiopathy patients have also been found in tissues outside of the brain including the testis, suggesting possible effects on fertility. Heterozygous transgenic mice (L68Q) that express the human L68Q variant of cystatin C under the control of the mouse cystatin C promoter were unable to generate offspring, suggesting the presence of L68Q cystatin C amyloid affected sperm function. In vitro studies showed that epididymal spermatozoa from L68Q mice were unable to fertilize oocytes and exhibited poor sperm motility. Furthermore, spermatozoa from L68Q mice exhibited reduced cell viability compared with wild type (WT) spermatozoa and often were detected in large agglutinated clumps. Examination of the epididymal fluid and spermatozoa from L68Q mice showed increased levels and distinct forms of cystatin C amyloid that were not present in WT mice. The addition of epididymal fluid from L68Q mice to WT spermatozoa resulted in a recapitulation of the L68Q phenotype in that WT spermatozoa showed reduced cell viability and motility compared with WT spermatozoa incubated in epididymal fluid from WT mice. L68Q epididymal fluid that was depleted of cystatin C amyloids, however, did not impair the motility of WT spermatozoa. Taken together these studies suggest that amyloids in the epididymal fluid can be cytotoxic to the maturing spermatozoa resulting in male infertility.

Effect of kefir and low-dose aspirin on arterial blood pressure measurements and renal apoptosis in unhypertensive rats with 4 weeks salt diet

Abstract We aim to study the effect of low-dose aspirin and kefir on arterial blood pressure measurements and renal apoptosis in unhypertensive rats with 4 weeks salt diet. Forty adult male Sprague-Dawley rats were divided into five groups: control, high-salt (HS) (8.0% NaCl), HS+aspirin (10 mg/kg), HS+kefir (10.0%w/v), HS+aspirin +kefir. We measured sistolic blood pressure (SBP), mean arterial pressure (MAP), diastolic pressure, pulse pressure in the rats. Cathepsin B, L, DNA fragmentation and caspase-3 activities were determined from rat kidney tissues and rats clearance of creatinine calculated. Although HS diet increased significantly SBP, MAP, diastolic pressure, pulse pressure parameters compared the control values. They were not as high as accepted hypertension levels. When compared to HS groups, kefir groups significantly decrease Cathepsin B and DNA fragmentation levels. Caspase levels were elevated slightly in other groups according to control group. While, we also found that creatinine clearance was higher in HS+kefir and HS+low-dose aspirin than HS group. Thus, using low-dose aspirin had been approximately decreased of renal function damage. Kefir decreased renal function damage playing as Angiotensin-converting enzyme inhibitor. But, low-dose aspirin together with kefir worsened rat renal function damage. Cathepsin B might play role both apoptosis and prorenin-processing enzyme. But not caspase pathway may be involved in the present HS diet induced apoptosis. In conclusion, kefir and low-dose aspirin used independently protect renal function and renal damage induced by HS diet in rats.

Aβ-degrading enzymes: potential for treatment of Alzheimer disease

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aβ levels and protect against cognitive impairment in mouse models of AD. The activity of several Aβ-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aβ. The age- and disease-related changes in expression of more recently recognized Aβ-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aβ-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aβ-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aβ-degrading enzymes may offer advantages.

Pharmacological and genetic evidence that cathepsin B is not the physiological activator of rodent prorenin

Renin is the first enzyme in the renin-angiotensin-aldosterone system which is the principal regulator of blood pressure and hydroelectrolyte balance. Previous studies suggest that cathepsin B is the activator of the prorenin zymogen. Here, we show no difference in plasma renin activity, or mean arterial blood pressure between wild-type and cathepsin B knockout mice. To account for potential gene compensation, a potent, selective, reversible cathepsin B inhibitor was developed to determine the role of cathepsin B on prorenin processing in rats. Pharmacological inhibition of cathepsin B in spontaneously hypertensive and double transgenic rats did not result in a reduction in renal mature renin protein levels or plasma renin activity. We conclude that cathepsin B does not play a significant role in this process in rodents.

Reduced fertility in vitro in mice lacking the cystatin CRES (cystatin-related epididymal spermatogenic): rescue by exposure of spermatozoa to dibutyryl cAMP and isobutylmethylxanthine

The cystatin CRES (cystatin-related epididymal spermatogenic; Cst8) is the defining member of a reproductive subgroup of family 2 cystatins of cysteine protease inhibitors and is present in the epididymis and spermatozoa, suggesting roles in sperm maturation and fertilization. To elucidate the role of CRES in reproduction, mice lacking the Cst8 gene were generated and their fertility examined. Although both male and female Cst8(-/-) mice generated offspring in vivo, spermatozoa from Cst8(-/-) mice exhibited a profound fertility defect in vitro. Compared to spermatozoa from Cst8(+/+) mice, spermatozoa from Cst8(-/-) mice were unable to undergo a progesterone-stimulated acrosome reaction and had decreased levels of protein tyrosine phosphorylation, suggesting a defect in the ability of Cst8(-/-) spermatozoa to capacitate. Incubation of Cst8(-/-) spermatozoa with dibutyryl cAMP and 3-isobutyl-1-methylxanthine rescued the fertility defect, including the capacity for sperm protein tyrosine phosphorylation. Both untreated Cst8(+/+) and Cst8(-/-) spermatozoa, however, exhibited similar increased total levels of cAMP and protein kinase A (PKA) activity throughout the capacitation time course compared to spermatozoa incubated under noncapacitating conditions. Taken together, these results suggest that mice lacking CRES may have altered local levels of cAMP/PKA activity, perhaps because of improper partitioning or tethering of these signaling molecules, or that the CRES defect does not directly involve cAMP/PKA but other signaling pathways that regulate protein tyrosine phosphorylation and capacitation.

Cathepsin B is not the processing enzyme for mouse prorenin

Renin, an aspartyl protease that catalyzes the rate-limiting step in the renin-angiotensin system (RAS), is proteolytically activated by a second protease [referred to as the prorenin processing enzyme (PPE)] before its secretion from the juxtaglomerular cells of the kidney. Although several enzymes are capable of activating renin in vitro, the leading candidate for the PPE in the kidney is cathepsin B (CTSB) due to is colocalization with the renin precursor (prorenin) in juxtaglomerular cell granules and because of its site-selective activation of human prorenin both in vitro and in transfected tissue culture cell models. To verify the role of CTSB in prorenin processing in vivo, we tested the ability of CTSB-deficient (CTSB−/−) mice to generate active renin. CTSB−/− mice do not exhibit any overt symptoms (renal malformation, preweaning mortality) typical of an RAS deficiency and have normal levels of circulating active renin, which, like those in control animals, rise more than 15-fold in response to pharmacologic inhibition of the RAS. The mature renin enzyme detected in kidney lysates of CTSB−/− mice migrates at the same apparent molecular weight as that in control mice, and the processing to active renin is not affected by chloroquine treatment of the animals. Finally, the distribution and morphology of renin-producing cells in the kidney is normal in CTSB−/− mice. In conclusion, CTSB-deficient mice exhibit no differences compared with controls in their ability to generate active renin, and our results do not support CTSB as the PPE in mice.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Food vacuole plasmepsins are processed at a conserved site by an acidic convertase activity in Plasmodium falciparum

Intraerythrocytic Plasmodium falciparum digests vast amounts of hemoglobin within an acidic food vacuole (FV). Four homologous aspartic proteases participate in hemoglobin degradation within the FV. Plasmepsin (PM) I and II are thought to initiate degradation of the native hemoglobin molecule. PM IV and histo-aspartic protease (HAP) act on denatured globin further downstream in the pathway. PM I and II have been shown to be synthesized as zymogens and activated by proteolytic removal of a propiece. In this study, we have determined that the proteolytic processing of FV plasmepsins occurs immediately after a conserved Leu-Gly dipeptidyl motif with uniform kinetics and pH and inhibitor sensitivities. We have developed a cell-free in vitro processing assay that generates correctly processed plasmepsins. Our data suggest that proplasmepsin processing is not autocatalytic, but rather is mediated by a separate processing enzyme. This convertase requires acidic conditions and is blocked only by the calpain inhibitors, suggesting that it may be an atypical calpain-like protease.

A new subgroup of the family 2 cystatins

The cystatins are a superfamily of cysteine protease inhibitors. Several genes including Cres (cystatin-related epididymal spermatogenic), testatin, and cystatin T, have been identified that are related to the family 2 cystatins but lack critical consensus sites important for cysteine protease inhibition. In addition, these genes are primarily expressed in the reproductive tract suggesting they may have evolved to perform tissue-specific functions distinct from that of the typical cystatins. This review describes the CRES subgroup within the family 2 cystatins including potential new members and their putative functions in the reproductive tract.

Expression of cathepsins B, D and L in mouse corneas infected with Pseudomonas aeruginosa

C57BL/6J naïve and immunized mice were intracorneally infected with Pseudomonas aeruginosa. Semi-quantitative RT-PCR was performed to detect cathepsin gene expression and the results were further confirmed by immunoblot analysis. The enzymatic activities of cathepsins B, D and L were measured by peptidase assays. Immunohistochemical staining was carried out to localize the expression of the cathepsins. Cathepsins B, D and L were detected in the normal cornea by RT-PCR. A peptidase assay revealed activities of all three cathepsins under normal physiological conditions. In naïve mice, enzymatic activities of cathepsins B, D and L were all significantly enhanced when the corneas were infected with P. aeruginosa and the peak of the induction appeared around day 6 postinfection. Immunoblot analysis showed increased expression of cathepsins B, D and L. The infected corneal samples from immunized mice exhibited much lower induction of enzymatic activities compared to those from naïve mice. Immunohistochemistry showed that the expression of cathepsins in the normal cornea was restricted to the epithelial tissue while the induced expression of cathepsins was predominantly in the substantia propria. Our data revealed up-regulated enzymatic activities of cathepsins B, D and L in the naïve corneas infected with P. aeruginosa, which correlated well with the inflammatory response. Immunization of mice against P. aeruginosa attenuated the inducing effect on cathepsin expression caused by infection. The time sequence for induction of cathepsin proteins and enzymatic activities suggests a mechanism of host proteolytic degradation of the extracellular matrix resulting in corneal destruction after P. aeruginosa infection.

The relationship between active renin concentration and plasma renin activity in Type 1 diabetes

AIMS

Circulating activity of the renin-angiotensin-aldosterone system (RAAS) can be assessed by measuring plasma active renin concentration (ARE), as well as by measuring plasma renin activity (PRA). We aimed to assess the relationships between ARE and PRA in Type 1 diabetic compared with non-diabetic control subjects. We also assessed concentrations of the active renin precursor, prorenin.

PATIENTS AND METHODS

Thirty-five Type 1 diabetic subjects and 34 non-diabetic control subjects were assessed. Groups had similar ages, sex distributions, body mass indices, systolic and diastolic blood pressures. PRA was measured by radioimmunoassay of angiotensin I generation from endogenous substrate. ARE and total renin concentration (TRE) were measured by immunoradiometric assay (Nichols Institute Diagnostics, USA). Prorenin concentration was calculated as the difference between ARE and TRE.

RESULTS

PRA was significantly lower in Type 1 diabetic than in control subjects (0.8 (0.4-1.1) vs. 1.1 (0.9-1.9) pmol/ml per h; P < 0.005), while ARE was similar (17 (9-33) vs. 18 (15-25) mU/l; P = 0.548). PRA (loge transformed) correlated strongly with ARE in diabetic (r = 0.49; P = 0.003) and control subjects (r = 0.59; P = 0.0002), but there was significant vertical separation of the regression lines for the two groups (P < 0.0001). Prorenin concentrations were significantly higher in Type 1 diabetic subjects (249 (170-339) vs. 171 (153-219) mU/l; P = 0.005). Diabetic subjects with high prorenin concentrations (> 400 mU/l (control mean + 3 SD)) were more likely to have microalbuminuria (P = 0.027) and peripheral neuropathy (P = 0.049).

CONCLUSIONS

Type 1 diabetes is associated with an altered relationship between ARE and PRA, such that ARE is higher for a given PRA compared with non-diabetic control subjects. Both ARE and PRA are used to assess circulating RAAS activity. The altered relationship between the two in Type 1 diabetic subjects suggests that neither parameter alone is necessarily an adequate and reliable index of such activity. Higher prorenin concentrations, particularly in association with microvascular complications, were confirmed in the Type 1 diabetic subjects. Diabet. Med. 18, 451-458 (2001)

Prorenin accumulation and activation in human endothelial cells: importance of mannose 6-phosphate receptors

ACE inhibitors improve endothelial dysfunction, possibly by blocking endothelial angiotensin production. Prorenin, through its binding and activation by endothelial mannose 6-phosphate (M6P) receptors, may contribute to this production. Here, we investigated this possibility as well as prorenin activation kinetics, the nature of the prorenin-activating enzyme, and M6P receptor-independent prorenin binding. Human umbilical vein endothelial cells (HUVECs) were incubated with wild-type prorenin, K/A-2 prorenin (in which Lys42 is mutated to Ala, thereby preventing cleavage by known proteases), M6P-free prorenin, and nonglycosylated prorenin, with or without M6P, protease inhibitors, or angiotensinogen. HUVECs bound only M6P-containing prorenin (K(d) 0.9+/-0.1 nmol/L, maximum number of binding sites [B(max)] 1010+/-50 receptors/cell). At 37 degrees C, because of M6P receptor recycling, the amount of prorenin internalized via M6P receptors was >25 times B(max). Inside the cells, wild-type and K/A-2 prorenin were proteolytically activated to renin. Renin was subsequently degraded. Protease inhibitors interfered with the latter but not with prorenin activation, thereby indicating that the activating enzyme is different from any of the known prorenin-activating enzymes. Incubation with angiotensinogen did not lead to endothelial angiotensin generation, inasmuch as HUVECs were unable to internalize angiotensinogen. Most likely, therefore, in the absence of angiotensinogen synthesis or endocytosis, M6P receptor-mediated prorenin internalization by endothelial cells represents prorenin clearance.

A furin-like convertase mediates propeptide cleavage of BACE, the Alzheimer's beta -secretase

The novel transmembrane aspartic protease BACE (for Beta-site APP Cleaving Enzyme) is the beta-secretase that cleaves amyloid precursor protein to initiate beta-amyloid formation. As such, BACE is a prime therapeutic target for the treatment of Alzheimer's disease. BACE, like other aspartic proteases, has a propeptide domain that is removed to form the mature enzyme. BACE propeptide cleavage occurs at the sequence RLPR downward arrowE, a potential furin recognition motif. Here, we explore the role of furin in BACE propeptide domain processing. BACE propeptide cleavage in cells does not appear to be autocatalytic, since an inactive D93A mutant of BACE is still cleaved appropriately. BACE and furin co-localize within the Golgi apparatus, and propeptide cleavage is inhibited by brefeldin A and monensin, drugs that disrupt trafficking through the Golgi. Treatment of cells with the calcium ionophore, leading to inhibition of calcium-dependent proteases including furin, or transfection with the alpha(1)-antitrypsin variant alpha(1)-PDX, a potent furin inhibitor, dramatically reduces cleavage of the BACE propeptide. Moreover, the BACE propeptide is not processed in the furin-deficient LoVo cell line; however, processing is restored upon furin transfection. Finally, in vitro digestion of recombinant soluble BACE with recombinant furin results in complete cleavage only at the established E46 site. Taken together, our results strongly suggest that furin, or a furin-like proprotein convertase, is responsible for cleaving the BACE propeptide domain to form the mature enzyme.

Biochemical characterization of human cathepsin X revealed that the enzyme is an exopeptidase, acting as carboxymonopeptidase or carboxydipeptidase

Cathepsin X, purified to homogeneity from human liver, is a single chain glycoprotein with a molecular mass of approximately 33 kDa and pI 5.1-5.3. Cathepsin X was inhibited by stefin A, cystatin C and chicken cystatin (Ki = 1.7-15.0 nM), but poorly or not at all by stefin B (Ki > 250 nM) and L-kininogen, respectively. The enzyme was also inhibited by two specific synthetic cathepsin B inhibitors, CA-074 and GFG-semicarbazone. Cathepsin X was similar to cathepsin B and found to be a carboxypeptidase with preference for a positively charged Arg in P1 position. Contrary to the preference of cathepsin B, cathepsin X normally acts as a carboxymonopeptidase. However, the preference for Arg in the P1 position is so strong that cathepsin X cleaves substrates with Arg in antepenultimate position, acting also as a carboxydipeptidase. A large hydrophobic residue such as Trp is preferred in the P1' position, although the enzyme cleaved all P1' residues investigated (Trp, Phe, Ala, Arg, Pro). Cathepsin X also cleaved substrates with amide-blocked C-terminal carboxyl group with rates similar to those of the unblocked substrates. In contrast, no endopeptidase activity of cathepsin X could be detected on a series of o-aminobenzoic acid-peptidyl-N-[2,-dinitrophenyl]ethylenediamine substrates. Furthermore, the standard cysteine protease methylcoumarine amide substrates (kcat/Km approximately 5.0 x 103 M-1.s-1) were degraded approximately 25-fold less efficiently than the carboxypeptidase substrates (kcat/Km approximately 120.0 x 103 M-1.s-1).

Pathologic gene expression in Gaucher disease: up-regulation of cysteine proteinases including osteoclastic cathepsin K

Deficiency of lysosomal acid β-glucosidase induces glycolipid storage in the macrophages of Gaucher disease but the pathways of multisystem tissue injury and destruction are unknown. To investigate the cognate molecular pathology of this inflammatory disorder, genes that were differentially expressed in spleen samples from a patient with Gaucher disease (Gaucher spleen) were isolated. Of 64 complementary DNA (cDNA) fragments sequenced from an enriched Gaucher cDNA library, 5 encode lysosomal proteins (cathepsins B, K, and S, α-fucosidase, and acid lipase), 10 encode other known proteins, and 2 represent novel sequences from human macrophage cell lines. Transcript abundance of the cathepsins, novel genes, pulmonary and activation-regulated chemokine (PARC), and NMB, a putative tumor suppressor gene, was greatly increased. Immunoblotting showed increased mature forms of all 3 cathepsins found in samples of Gaucher spleens. Immunofluorescence microscopy showed strong cathepsin B and K reactions in sinusoidal endothelium and Gaucher cells. The respective means, plus or minus SD, of cathepsin B, K, and S activities were 183 ± 35, 97 ± 39, and 91 ± 45 nmol/min/mg protein in 4 Gaucher spleens, and 26 ± 4, 10.5 ± 2, and 4.0 ± 2.1 nmol/min/mg protein in 3 control spleens. Plasma cathepsin B, K, and S activities were also elevated in Gaucher disease plasma (P < .001), but compared with control plasma samples, neither cathepsin B nor K activities were significantly elevated in 8 patients with nonglycosphingolipid lysosomal storage diseases or in 9 patients with other glycosphingolipidoses, which suggests disease specificity. All 3 cathepsin activities were increased 2-fold to 3-fold in Gaucher sera compared with control sera. In all 6 patients treated by enzyme replacement for 16-22 months, serum cathepsin activities decreased significantly (P < .01). Longitudinal studies confirmed the progressive reduction of proteinase activities during imiglucerase therapy but in 3 Gaucher patients with mild disease not so treated, serum cathepsin activities remained constant or increased during follow-up. Enhanced expression of cysteine proteinases may promote tissue destruction. Moreover, the first identification of aberrant cathepsin K expression in hematopoietic tissue other than osteoclasts implicates this protease in the breakdown of the matrix that characterizes lytic bone lesions in Gaucher disease.

Pathologic gene expression in Gaucher disease: up-regulation of cysteine proteinases including osteoclastic cathepsin K

Deficiency of lysosomal acid β-glucosidase induces glycolipid storage in the macrophages of Gaucher disease but the pathways of multisystem tissue injury and destruction are unknown. To investigate the cognate molecular pathology of this inflammatory disorder, genes that were differentially expressed in spleen samples from a patient with Gaucher disease (Gaucher spleen) were isolated. Of 64 complementary DNA (cDNA) fragments sequenced from an enriched Gaucher cDNA library, 5 encode lysosomal proteins (cathepsins B, K, and S, α-fucosidase, and acid lipase), 10 encode other known proteins, and 2 represent novel sequences from human macrophage cell lines. Transcript abundance of the cathepsins, novel genes, pulmonary and activation-regulated chemokine (PARC), and NMB, a putative tumor suppressor gene, was greatly increased. Immunoblotting showed increased mature forms of all 3 cathepsins found in samples of Gaucher spleens. Immunofluorescence microscopy showed strong cathepsin B and K reactions in sinusoidal endothelium and Gaucher cells. The respective means, plus or minus SD, of cathepsin B, K, and S activities were 183 ± 35, 97 ± 39, and 91 ± 45 nmol/min/mg protein in 4 Gaucher spleens, and 26 ± 4, 10.5 ± 2, and 4.0 ± 2.1 nmol/min/mg protein in 3 control spleens. Plasma cathepsin B, K, and S activities were also elevated in Gaucher disease plasma (P &lt; .001), but compared with control plasma samples, neither cathepsin B nor K activities were significantly elevated in 8 patients with nonglycosphingolipid lysosomal storage diseases or in 9 patients with other glycosphingolipidoses, which suggests disease specificity. All 3 cathepsin activities were increased 2-fold to 3-fold in Gaucher sera compared with control sera. In all 6 patients treated by enzyme replacement for 16-22 months, serum cathepsin activities decreased significantly (P &lt; .01). Longitudinal studies confirmed the progressive reduction of proteinase activities during imiglucerase therapy but in 3 Gaucher patients with mild disease not so treated, serum cathepsin activities remained constant or increased during follow-up. Enhanced expression of cysteine proteinases may promote tissue destruction. Moreover, the first identification of aberrant cathepsin K expression in hematopoietic tissue other than osteoclasts implicates this protease in the breakdown of the matrix that characterizes lytic bone lesions in Gaucher disease.

Hydrolysis by plasma kallikrein of fluorogenic peptides derived from prorenin processing site

Human plasma kallikrein (HPK) activates plasma prorenin to renin, and the physiological significance of this activation is still unknown. In this paper we investigated the efficiency and the cleavage pattern of the hydrolysis by HPK of the internally quenched fluorescent peptides (qf-peptides) derived from the amino acid sequence of human prorenin cleavage site. The peptide Abz-F-S-Q-P-M-K-R-L-T-L-G-N-T-T-Q-EDDnp (Abz=ortho-aminobenzoic acid, and EDDnp=N-[2,4-dinitrophenyl]-ethylene diamine), that corresponds to the amino acid sequence P(7) to P(7)' of human prorenin cleavage site, is hydrolyzed at the correct processing site (R-L bond) with k(cat)/K(m)=85 mM(-1) s(-1). Alanine was scanned in all positions from P(5) to P(5)' in order to investigate the substrate specificity requirements of HPK. The qf-peptides derived from the equivalent segment of rat prorenin, that has Lys-Lys as basic amino acid pair, and the peptide Abz-NVTSPVQ-EDDnp that contains the proposed cleavage site of rat prorenin have very low susceptibility to hydrolysis by rat plasma kallikrein. These data are according to the previously reported absence of rat plasma prorenin activation by rat plasma kallikrein (RPK), and with the view that prorenin activation in rat requires alternative enzymes and/or mechanism. All the obtained peptides described in this paper were also assayed with bovine trypsin that was taken as a reference protease because it is commonly used to activate prorenin.

Hydrolysis by cathepsin B of fluorescent peptides derived from human prorenin

Cathepsin B is a lysosomal thiolprotease that, because of its colocalization with renin and its ability to activate prorenin, has been proposed as a prorenin processing enzyme. To characterize the biochemical aspect of this potential cathepsin B activity in more detail, we synthesized and assayed with human cathepsin B the internally quenched fluorescent peptide Abz-FSQPMKRLTLGNTTQ-EDDnp (Abz, ortho-aminobenzoic acid fluorescent group and EDDnp, N-¿2, 4-dinitrophenyl-ethylenediamine quencher group) that contains 7 amino acids for each side of the R-L bond that is the processing site of human prorenin. Human cathepsin B hydrolyzed this peptide at the correct site (R-L bond), with k(cat)/K(m)=75 mmol/L(-1) s(-1). Analogues of this peptide obtained by Ala scanning at positions P(5) to P(5)' were also synthesized and assayed as substrates for human cathepsin B. The obtained specificity constant (k(cat)/K(m)) values have a significant parallel with the previous data of prorenin activation by AtT-20 cells and in vitro by cathepsin B. In addition, we demonstrated the presence of cathepsin B-like activity in rat mesangial cells and the ability of its whole soluble fraction lysates, as well as that of purified cloned rat cathepsin B, to hydrolyze Abz-IKKSSF-EDDnp at the K-S bond, which contains 6 amino acids of rat prorenin processing site. The specificity data of cathepsin B toward peptides derived from prorenin processing site support the view that human or rodent cathepsin B could be involved in the intracellular processing of prorenin that is locally synthesized or taken up from the extracellular compartment.

Crystal structure of cathepsin X: a flip-flop of the ring of His23 allows carboxy-monopeptidase and carboxy-dipeptidase activity of the protease

BACKGROUND

Cathepsin X is a widespread, abundantly expressed papain-like mammalian lysosomal cysteine protease. It exhibits carboxy-monopeptidase as well as carboxy-dipeptidase activity and shares a similar activity profile with cathepsin B. The latter has been implicated in normal physiological events as well as in various pathological states such as rheumatoid arthritis, Alzheimer's disease and cancer progression. Thus the question is raised as to which of the two enzyme activities has actually been monitored.

RESULTS

The crystal structure of human cathepsin X has been determined at 2.67 A resolution. The structure shares the common features of a papain-like enzyme fold, but with a unique active site. The most pronounced feature of the cathepsin X structure is the mini-loop that includes a short three-residue insertion protruding into the active site of the protease. The residue Tyr27 on one side of the loop forms the surface of the S1 substrate-binding site, and His23 on the other side modulates both carboxy-monopeptidase as well as carboxy-dipeptidase activity of the enzyme by binding the C-terminal carboxyl group of a substrate in two different sidechain conformations.

CONCLUSIONS

The structure of cathepsin X exhibits a binding surface that will assist in the design of specific inhibitors of cathepsin X as well as of cathepsin B and thereby help to clarify the physiological roles of both proteases.

Lysosomal cysteine proteases: more than scavengers

Lysosomal cysteine proteases were believed to be mainly involved in intracellular protein degradation. Under special conditions they have been found outside lysosomes resulting in pathological conditions. With the discovery of a series of new cathepsins with restricted tissue distributions, it has become evident that these enzymes must be involved in a range of specific cellular tasks much broader than as simple housekeeping enzymes. It is therefore timely to review and discuss the various physiological roles of mammalian lysosomal papain-like cysteine proteases as well as their mechanisms of action and the regulation of their activity.

Exocytosis of active cathepsin B enzyme activity at pH 7.0, inhibition and molecular mass

Lysosomal cathepsin B has been implicated in parasitic, inflammatory and neoplastic diseases. Most of these pathologies suggest a role for cathepsin B outside the cells, although the origin of extracellular active enzyme is not well defined. The activity of extracellular cathepsin B is difficult to assess because of the presence of inhibitors and inactivation of the enzyme by oxidizing agents. Therefore, we have developed a continuous assay for measurement of cathepsin B activity produced pericellularly by living cells. The kinetic rate of Z-Arg-Arg-NHMec conversion was monitored and the assay optimized for enzyme stability, cell viability and sensitivity. To validate the assay, we determined that human liver cathepsin B was stable and active under the conditions of the assay and its activity could be inhibited by the selective epoxide derivative CA-074. Via this assay, we were able to demonstrate that active cathepsin B was secreted pericellularly by viable cells. Both preneoplastic and malignant cells secreted active cathepsin B. Pretreatment of cells with the membrane-permeant proinhibitor CA-074Me completely abolished pericellular and total cathepsin B activity whereas pretreatment with the active drug CA-074 had no effect. Immunoprecipitation and immunoblotting experiments suggested that the active enzyme species was 31-kDa single-chain cathepsin B. Exocytosis of cathepsin B was not related to secretion of proenzyme or secretion from mature lysosomes. Our results suggest an alternative pathway for exocytosis of active cathepsin B.

Transmural pressure inhibits prorenin processing in juxtaglomerular cell

Pressure control of renin secretion involves a complex integration of shear stress, stretch, and transmural pressure (TP). This study was designed to delineate TP control of renin secretion with minimal influence of shear stress or stretch and to determine its mechanism. Rat juxtaglomerular (JG) cells were applied to a TP-loading apparatus for 12 h. In cells conditioned with atmospheric pressure or atmospheric pressure + 40 mmHg, renin secretion rate (RSR) averaged 29.6 +/- 3.7 and 14.5 +/- 3.3% (P < 0.05, n = 8 cultures), respectively, and active renin content (ARC) averaged 47.3 +/- 4.6 and 38.4 +/- 3.4 ng of ANG I. h(-1). million cells(-1) (P < 0.05, n = 10 cultures), respectively. Total renin content and renin mRNA levels were unaffected by TP. The TP-induced decrease in RSR was prevented by Ca(2+)-free medium and the Ca(2+) channel blocker verapamil and was attenuated by thapsigargin and caffeine, which deplete intracellular Ca(2+) stores. Thapsigargin and caffeine, but not Ca(2+)-free medium or verapamil, prevented TP-induced decreases in ARC. The adenylate cyclase activator forskolin did not modulate TP-induced decreases in RSR or ARC. These findings suggest that TP not only stimulates Ca(2+) influx but also inhibits prorenin processing through an intracellular Ca(2+) store-dependent mechanism and thus inhibits active renin secretion by JG cells.

Prorenin processing by cathepsin B in vitro and in transfected cells

Renin, which catalyzes the initial proteolytic cleavage reaction in the production of angiotensins, is first synthesized as a zymogen, prorenin, and requires the proteolytic removal of an amino-terminal prosegment for activation in vivo. The lysosomal hydrolase cathepsin B has been proposed as a prorenin processing enzyme based on reports of its co-localization with renin in the secretory granules of certain tissues and its ability to activate prorenin in vitro. In the current study, scanning mutagenesis was used to identify the amino acids which determine the site selectivity of prorenin cleavage by human cathepsin B in vitro. Co-expression assays in AtT-20 cells were also used to test for the ability of cathepsin B to cleave human prorenin within cells. Our results suggest that a basic lysine residue at the -2 position from the cleavage site is required for cathepsin B cleavage of prorenin in vitro and that the structure of prorenin itself may account for the selection of the proper cleavage site. In addition, although cathepsin B appears to be correctly sorted to lysosomes, the enzyme exhibits prorenin processing activity in transfected AtT-20 cells, raising the question of the cellular localization in which the processing event occurs.

Human cathepsin X: a novel cysteine protease of the papain family with a very short proregion and unique insertions

A novel cDNA encoding a cysteine protease of the papain family named cathepsin X was obtained by PCR amplification from a human ovary cDNA library. The cathepsin X cDNA is ubiquitously expressed in human tissues and contains an open reading frame of 912 nucleotides encoding a predicted protein of 303 amino acids. All highly conserved regions in papain-like cysteine proteases including the catalytic residues are present in cathepsin X. The mature part of cathepsin X is 26-32% identical to human cathepsins B, C, H, K, L, O, S and W. The cathepsin X sequence contains several unique features: (i) a very short proregion; (ii) a three amino acid residue insertion in a highly conserved region between the glutamine of the putative oxyanion hole and the active site cysteine; and (iii) a second insertion of 15 amino acid residues that can be aligned with the occluding loop region in cathepsin B.

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.

Revised definition of substrate binding sites of papain-like cysteine proteases

A review of kinetic and structural data has enabled us to reconsider the definition of substrate binding sites in papain-like cysteine proteases. Only three substrate binding sites, S2, S1 and S1', involve main as well as side chain contacts between substrate and enzyme residues. Interactions between the enzymes and the substrate P3 and P2' residues are based on side chains (an exception is cathepsin B which is a carboxydipeptidase), so their interaction surface spreads over a relatively wide area. The location and definition of substrate binding sites beyond S3 and S2' is even more questionable.

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.

Maturation and degradation of beta-galactosidase in the post-Golgi compartment are regulated by cathepsin B and a non-cysteine protease

Lysosomal beta-galactosidase precursor is processed to a mature form and associated with protective protein in lysosomes. In this study we used two cysteine protease proinhibitors, E64-d for cathepsins B, S, H, and L, and CA074Me for cathepsin B. They are converted intracellularly to active forms, E-64c and CA074, respectively. Both active compounds inhibited maturation of the exogenous beta-galactosidase precursor, but E-64c did not inhibit further degradation to an inactive 50-kDa product. We concluded that cathepsin B participated exclusively in maturation of beta-galactosidase, and a non-cysteine protease was involved in further degradation and inactivation of the enzyme molecule.

Prorenin activation and prohormone convertases in the mouse As4.1 cell line

The precise identification of prorenin-processing enzymes has been hampered by the very low abundance of juxtaglomerular cells in the kidney. Recently, an immortalized renin-producing renal tumor cell line (As4.1) has been proposed as a model to carry out such studies. Despite the fact that they contain secretory granules, we found no evidence (on the basis of enzymatic assays of renin activity in the supernatant of the cells and of immunoprecipitations experiments) that the As4.1 cells can secrete active renin through the regulated pathway. As4.1 cells produce only renin-1, as they derive from a strain of mice expressing only one renin gene. However, stable transfection of these cells with a renin-2 expression plasmid increased the capacity of this cell line to secrete active renin in the regulated pathway. Northern blot and reverse transcriptase-polymerase chain reaction amplification (RT-PCR) assays revealed that furin, PACE4 and PC5 were the only members of the proprotein convertase (PC) family to be present in these cells. As PC5 is the only such enzyme with the demonstrated ability to process mouse prorenin 2, it may constitute a candidate enzyme for the processing of prorenin-2 in mouse juxtaglomerular cells. However, it is not likely to be involved in the processing of mouse prorenin 1.

Cathepsin B and D expression in squamous cell carcinoma

To elucidate involvement of proteinases in malignancy of keratinocytes, expression of cathepsin B, a cysteine proteinase, and cathepsin D, an aspartic proteinase, was ascertained in formalin-fixed paraffin-embedded specimens of normal skin, squamous cell carcinoma (SCC). Bowen's disease, seborrhoeic keratosis and basal cell carcinoma (BCC). Presence of procathepsin B and an intermediate form of cathepsin D was confirmed by Western blotting and enzyme activity analysis. Cathepsin B stained more intensely in SCC tumour cells than in normal epidermis; staining patterns were diffuse, granular or both. Diffuse and granular patterns (procathepsin B and mature enzyme, respectively) appeared in inner and outer parts of tumour islands, respectively. Five of 20 cases of Bowen's disease showed diffuse enhanced cathepsin B expression; 20 cases of seborrhoeic keratosis or BCC did not. Cathepsin D stained intensely in tumour cells of half the SCC cases. The staining manner and distribution of cathepsins B and D was similar in the cytoplasm of cancer cells. No enhanced staining of cathepsin D was seen in any cases of Bowen's disease, seborrhoeic keratosis, or BCC. Coexistence and localization of active mature forms of cathepsins B and D suggests that cooperation between the two enzymes may play an important part in invasion of SCC.

Prohormone convertase PC5 is a candidate processing enzyme for prorenin in the human adrenal cortex

We isolated a cDNA clone encoding the human prohormone convertase PC5 from human adrenal gland mRNA. The deduced protein sequence would encode a 915 amino acid preproPC5 that shares a very high degree of homology with previously cloned rat and mouse homologues. PC5 mRNA was detected in multiple human tissues, including the brain, adrenal and thyroid glands, heart, placenta, lung, and testes. PC5 mRNA was undetectable in the liver and was present at lower levels in skeletal muscle, kidney, pancreas, small intestine, and stomach. Co-transfection of human PC5 and human prorenin expression vectors in cultured GH4C1 cells led to secretion of active renin. The activation of human prorenin by PC5 depended on a pair of basic amino acids at positions 42 and 43 of the prorenin prosegment and occurred only in cells containing dense core secretory granules. Human PC5 was colocalized with renin by immunohistochemistry in the zona glomerulosa of the adrenal gland, suggesting that it could participate in the activation of a local renin-angiotensin system in the human adrenal cortex.

Serum total renin is increased before microalbuminuria in diabetes

Serum prorenin is increased in patients with insulin dependent diabetes mellitus (IDDM) with microvascular complications. The present longitudinal study investigated whether increases in serum total renin concentration (TRC, active+prorenin) can predict the development of microalbuminuria in IDDM patients over a 10 year period. TRC and albumin excretion rates (AER) were determined in 78 IDDM patients who were followed longitudinally for 10.4 +/- 0.2 (mean +/- SE) years. Twelve patients had progressively increasing albuminuria (progressors), and these were compared to 66 patients in whom albuminuria did not rise (non-progressors). The two groups had similar duration of diabetes, age, follow-up, glycemic control and blood pressure at the start of the study. Serum TRC was increased in progressors [350 (1.1) mIU/liter, geometric mean (tolerance factor)] compared to non-progressors [189 (1.2)] after 5 to 10 years duration of diabetes, and continued to rise in this group, reaching a mean of 923 mIU/liter (normal range 131 to 170) after 20 years of diabetes. When serial measurements of TRC and AER were compared in individual progressors, a significant increase in TRC was apparent up to five years before the onset of microalbuminuria. Microalbuminuria in patients with IDDM is preceded by a substantial increase in serum TRC, suggesting that serum TRC may predict the subsequent development of incipient nephropathy.

Simultaneous isolation of human kidney cathepsins B, H, L and C and their characterisation

A procedure for the simultaneous isolation of four cysteine proteinases, cathepsins B, H, L and C, from human kidney is described. The method includes concentration of the acidified homogenate by ammonium sulphate precipitation. The resuspended and dialysed precipitate was chromatographed on DEAE-cellulose DE-32, to allow separation of cathepsins H and C from cathepsins B and L. The main isoform of cathepsin H was separated from cathepsin C by cation-exchange chromatography on CM-Sephadex C-50. These two enzymes were further purified by covalent chromatography on thiopropyl Sepharose and gel permeation on Sephacryl S-200. The last step allowed separation of cathepsin C and the minor isoform of cathepsin H. Purification of the other two enzymes, cathepsins B and L, was carried out on thiol Sepharose, followed by chromatography on CM-Sepharose C-50. In this step, pure cathepsin L was obtained, while two isoforms of cathepsin B had to be finally purified on Sephacryl S-200 columns. The purity of each enzyme was analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis, isoelectric focusing on polyacrylamide gels and N-terminal sequencing. The activities of the purified cathepsins B, H and L were determined in terms of kcat/KM for three substrates, Z-Phe-Arg-MCA, Z-Arg-Arg-MCA and Arg-MCA. The method produced 25 mg of cathepsin B, 6.5 mg of cathepsin H, 1.5 mg of cathepsin L and 3.8 mg of cathepsin C from 3.5 kg of human kidney.

Cathepsin B is a prorenin processing enzyme

Conversion of prorenin to renin results from proteolytic cleavage of a 43-amino-acid prorenin prosegment in renal juxtaglomerular cells. The enzyme that performs this processing is not known. Of several enzymes proposed, cathepsin B is a candidate because it colocalizes with renin in juxtaglomerular cell secretory granules and accurately cleaves the prosegment of human prorenin in vitro. It is not known whether cathepsin B can perform this function in the cell. We examined this using secretory granule-containing rat GH4C1 cells transfected with a human preprorenin expression vector. When treated with secretagogue (KCl 50 mmol/L + forskolin 10 micromol/L), these cells secrete 95% prorenin and 5% active renin into the medium, indicating little prorenin processing activity. In contrast, when the cells are cotransfected with a vector that expresses human preprocathepsin B or mouse prohormone convertase 1, secretagogue-induced secretion of active renin increased to 12% and 16.5%, respectively. With antisera that recognize the prosegment and renin, prorenin and renin were identified as proteins of 47 and 43 kD, respectively, and an antibody specific to the prosegment precipitated only the 47-kD species. These results do not address whether cathepsin B is the authentic renal prorenin processing enzyme. However, the results do demonstrate that cathepsin B can localize to the appropriate subcellular compartment and process prorenin to renin in GH4C1 cells and are consistent with a role for this enzyme in prorenin processing.

Proteolytic processing of human prorenin in renal and non-renal tissues

Previous studies have demonstrated that the mouse proprotein convertase PC1 (mPC1) accurately cleaves human prorenin to generate active renin and that this processing event appears to require co-packaging in secretory granules. In the current study, we have tested human PC1 (hPC1; also called PC3) for its ability to activate human prorenin. Our results suggest that while hPC1 is capable of carrying out the specific cleavage of human prorenin, it does so at a reduced efficiency as compared to mPC1. This difference is due to sequences in the carboxy-terminus of PC1 as demonstrated by the activity of hybrid hPC1/mPC1 molecules. These studies demonstrate that PC1 cleavage of prorenin can occur in humans and identify a functionally important region in the hPC1 protein for this interaction. Moreover, the localization of PC1 in human tissues suggests that it may participate in the generation of active renin in the adrenal medulla and possibly in certain adrenal tumors.

Latency of cathepsin B secreted by human colon carcinoma cells is not linked to secretion of cystatin C and is relieved by neutrophil elastase

The lysosomal cysteine proteinase cathepsin B is shown to be secreted by ten human colon carcinoma cell lines and to accumulate in culture media as a latent enzyme. The cell lines also secrete a physiological inhibitor of cathepsin B, cystatin C. A significant correlation was found between secretion of the latent enzyme and the inhibitor (r = 0.755, P < 0.01). The aim of the present study was to modulate the respective secretion of the two antagonists to test whether or not latency of cathepsin B was due to the concomitant secretion of the inhibitor. SW480 colon carcinoma cells were treated with the acidotropic agent ammonium chloride, phorbol 12-myristate 13-acetate, and the inflammatory cytokines TGF-beta, TNF-alpha, and IL-1 beta. Ammonium chloride significantly increased latent cathepsin B levels without affecting the constitutive secretion of cystatin C. Phorbol 12-myristate 13-acetate induced a 4- to 5-fold increase in secreted latent cathepsin B, but did not alter significantly the accumulation of cystatin C in media. The cytokines, TGF-beta, TNF-alpha, and IL-1 beta, had no major effect on the expression of these two antagonists. Latent cathepsin B released from human carcinoma cells could be efficiently activated by neutrophil elastase at neutral pH. It is concluded that latent cathepsin B is a true proenzyme rather than an enzyme-inhibitor complex. In addition, our data from neutrophil elastase activation experiments indicate that a proteolytic system for activation of the tumor cell-secreted latent enzyme may exist in vivo.

Analysis of latent forms of renin using antibodies raised against the propart segment of human prorenin: validation with representative samples of ovarian cyst and follicular fluids

Antisera were raised against synthetic peptides from the prosegment of human prorenin. The use of each of these for detection of the appropriate prosegment region of prorenin was validated by development of an ELISA protocol standardised with recombinant prorenin present in culture medium conditioned by myeloma cells transfected with a prorenin expression plasmid. Detection of the respective epitopes in the prosegment required prior exposure of the prorenin in the medium to acid pH in order to partially unfold the prorenin molecule by dislodging the prosegment from the main body of the protein. By these ELISA protocols, the form of latent renin present in representative samples from ovarian cyst and follicular fluids was analysed; one follicular cyst fluid was found to contain full-length prorenin whereas the fluid from a benign cyst and ovarian follicular fluid samples contained the precursor in truncated form.