Biosynthesis of lysosomal cathepsins B and H in cultured rat hepatocytes

Molecular characterization of aspartylglucosaminidase, a lysosomal hydrolase upregulated during strobilation in the moon jellyfish, Aurelia aurita

The life cycle of the moon jellyfish, Aurelia aurita, alternates between a benthic asexual polyp stage and a planktonic sexual medusa (jellyfish) stage. Transition from polyp to medusa is called strobilation. To investigate the molecular mechanisms of strobilation, we screened for genes that are upregulated during strobilation using the differential display method and we identified aspartylglucosaminidase (AGA), which encodes a lysosomal hydrolase. Similar to AGAs from other species, Aurelia AGA possessed an N-terminal signal peptide and potential N-glycosylation sites. The genomic region of Aurelia AGA was approximately 9.8 kb in length and contained 12 exons and 11 introns. Quantitative RT-PCR analysis revealed that AGA expression increased during strobilation, and was then decreased in medusae. To inhibit AGA function, we administered the lysosomal acidification inhibitors, chloroquine or bafilomycin A1, to animals during strobilation. Both inhibitors disturbed medusa morphogenesis at the oral end, suggesting involvement of lysosomal hydrolases in strobilation.

The Ionic and hydrophobic interactions are required for the auto activation of cysteine proteases of Plasmodium falciparum

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 are major hemoglobinases and potential antimalarial drug targets. Our previous studies demonstrated that these enzymes are equipped with specific domains for specific functions. Structural and functional analysis of falcipains showed that they have unique domains including a refolding domain and a hemoglobin binding domain. As with many proteases, falcipain-2 and falcipain-3 are synthesized as inactive zymogens. However, it is not known how these enzymes get activated for hemoglobin hydrolysis. In this study, we are presenting the first evidence that salt bridges and hydrophobic interactions are required for the auto activation of cysteine proteases of P.falciparum. To investigate the mechanism of activation of these enzymes, we expressed the wild type protein as well as different mutants in E.coli. Refolding was assessed by circular dichroism. Both CD and trans activation data showed that the wild type enzymes and mutants are rich in secondary structures with similar folds. Our study revealed that prodomain-mature domain of falcipain-2 and falcipain-3 interacts via salt bridges and hydrophobic interactions. We mutated specific residues of falcipain-2 and falcipain-3, and evaluated their ability to undergo auto processing. Mutagenesis result showed that two salt bridges (Arg¹⁸⁵- Glu²²¹, Glu²¹⁰- Lys⁴⁰³) in falcipain-2, and one salt bridge (Arg²⁰²-Glu²³⁸) in falcipain-3, play crucial roles in the activation of these enzymes. Further study revealed that hydrophobic interactions present both in falcipain-2 (Phe²¹⁴ Trp⁴⁴⁹ Trp⁴⁵³) and falcipain-3 (Phe²³¹ Trp⁴⁵⁷ Trp⁴⁶¹) also play important roles in the activation of these enzymes. Our results revealed the interactions involved in auto processing of two major hemoglobinases of malaria parasite.

Proteinases and their inhibitors in liver cancer

Proteinases are known to be involved in many cancer-related processes, particularly in the breakdown of extracellular matrix barriers in the course of tumor invasion and metastasis. In this review we summarize the current knowledge about the role of the most important matrix-degrading proteinases (cathepsins, matrix metalloproteinases, plasmin/plasminogen activators) and their respective inhibitors in liver cancer progression and metastasis.

Oxidative stress and autophagy in the regulation of lysosome-dependent neuron death

Lysosomes critically regulate the pH-dependent catabolism of extracellular and intracellular macromolecules delivered from the endocytic/heterophagy and autophagy pathways, respectively. The importance of lysosomes to cell survival is underscored not only by their unique ability effectively to degrade metalloproteins and oxidatively damaged macromolecules, but also by the distinct potential for induction of both caspase-dependent and -independent cell death with a compromise in the integrity of lysosome function. Oxidative stress and free radical damage play a principal role in cell death induced by lysosome dysfunction and may be linked to several upstream and downstream stimuli, including alterations in the autophagy degradation pathway, inhibition of lysosome enzyme function, and lysosome membrane damage. Neurons are sensitive to lysosome dysfunction, and the contribution of oxidative stress and free radical damage to lysosome dysfunction may contribute to the etiology of neurodegenerative disease. This review provides a broad overview of lysosome function and explores the contribution of oxidative stress and autophagy to lysosome dysfunction-induced neuron death. Putative signaling pathways that either induce lysosome dysfunction or result from lysosome dysfunction or both, and the role of oxidative stress, free radical damage, and lysosome dysfunction in pediatric lysosomal storage disorders (neuronal ceroid lipofuscinoses or NCL/Batten disease) and in Alzheimer's disease are emphasized.

Proteolytic-antiproteolytic balance and its regulation in carcinogenesis

Cancer development is essentially a tissue remodeling process in which normal tissue is substituted with cancer tissue. A crucial role in this process is attributed to proteolytic degradation of the extracellular matrix (ECM). Degradation of ECM is initiated by proteases, secreted by different cell types, participating in tumor cell invasion and increased expression or activity of every known class of proteases (metallo-, serine-, aspartyl-, and cysteine) has been linked to malignancy and invasion of tumor cells. Proteolytic enzymes can act directly by degrading ECM or indirectly by activating other proteases, which then degrade the ECM. They act in a determined order, resulting from the order of their activation. When proteases exert their action on other proteases, the end result is a cascade leading to proteolysis. Presumable order of events in this complicated cascade is that aspartyl protease (cathepsin D) activates cysteine proteases (e.g., cathepsin B) that can activate pro-uPA. Then active uPA can convert plasminogen into plasmin. Cathepsin B as well as plasmin are capable of degrading several components of tumor stroma and may activate zymogens of matrix metalloproteinases, the main family of ECM degrading proteases. The activities of these proteases are regulated by a complex array of activators, inhibitors and cellular receptors. In physiological conditions the balance exists between proteases and their inhibitors. Proteolytic-antiproteolytic balance may be of major significance in the cancer development. One of the reasons for such a situation is enhanced generation of free radicals observed in many pathological states. Free radicals react with main cellular components like proteins and lipids and in this way modify proteolytic-antiproteolytic balance and enable penetration damaging cellular membrane. All these lead to enhancement of proteolysis and destruction of ECM proteins and in consequence to invasion and metastasis.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

Biosynthesis of lysosomal proteinases in health and disease

Proteolytic maturation of lysosomal proteinases is initiated after receptor-mediated targeting to prelysosomal compartments, while terminal processing occurs upon delivery to lysosomes. These late processing events are impaired in patients suffering from inherited lysosomal disorders, such as sialic acid storage disease and mucolipidosis II (I-cell disease). Lysosomes in the affected cells display marked changes in their physiological and morphological properties, with features reminiscent of prelysosomal compartments. This indicates that the absence of mature lysosomes interferes with the final processing steps during the biosynthesis of lysosomal proteinases. Thus, impaired proteinase maturation reflects an incompetent lysosomal apparatus and as such can be seen as a hallmark of lysosomal storage diseases.

Biosynthesis and processing of cathepsin K in cultured human osteoclasts

Cathepsin K (cat K) is the major cysteine protease expressed in osteoclasts and is thought to play a key role in matrix degradation during bone resorption. However, little is known regarding the synthesis, activation, or turnover of the endogenous enzyme in osteoclasts. In this study, we show that mature cat K protein and enzyme activity are localized within osteoclasts. Pulse-chase experiments revealed that, following the synthesis of pro cat K, intracellular conversion to the mature enzyme occurred in a time-dependent manner. Subsequently, the level of mature enzyme decreased. Little or no cat K was observed in the culture media at any timepoint. Pretreatment of osteoclasts with either chloroquine or monensin resulted in complete inhibition of the processing of newly synthesized cat K. In addition, pro cat K demonstrated susceptibility to treatment with N-glycosidase F, suggesting the presence of high-mannose-containing oligosaccharides. Treatment of osteoclasts with the PI3-kinase inhibitor, Wortmannin (WT), not only prevented the intracellular processing of cat K but also resulted in the secretion of proenzyme into the culture media. Taken together, these results suggest that the biosynthesis, processing, and turnover of cat K in human osteoclasts is constitutive and occurs in a manner similar to that of other known cysteine proteases. Furthermore, cat K is not secreted as a proenzyme, but is processed intracellularly, presumably in lysosomal compartments prior to the release of active enzyme into the resorption lacunae.

Basic mechanisms of secretion: sorting into the regulated secretory pathway

Targeting proteins to their correct cellular location is crucial for their biological function. In neuroendocrine cells, proteins can be secreted by either the constitutive or the regulated secretory pathways but the mechanism(s) whereby proteins are sorted into either pathway is unclear. In this review we discuss the possibility that sorting is either an active process occurring at the level of the trans-Golgi network, or that sorting occurs passively in the immature granules. The possible involvement of protein-lipid interactions in the sorting process is also raised.

Key words: lipid rafts, regulated secretory pathway, secretion, sorting receptors, sorting signals, trans-Golgi network.

Cell surface complex of cathepsin B/annexin II tetramer in malignant progression

The cysteine protease cathepsin B is upregulated in a variety of tumors, particularly at the invasive edges. Cathepsin B can degrade extracellular matrix proteins, such as collagen IV and laminin, and can activate the precursor form of urokinase plasminogen activator (uPA), perhaps thereby initiating an extracellular proteolytic cascade. Recently, we demonstrated that procathepsin B interacts with the annexin II heterotetramer (AIIt) on the surface of tumor cells. AIIt had previously been shown to interact with the serine proteases: plasminogen/plasmin and tissue-type plasminogen activator (tPA). The AIIt binding site for cathepsin B differs from that for either plasminogen/plasmin or tPA. AIIt also interacts with extracellular matrix proteins, e.g., collagen I and tenascin-C, forming a structural link between the tumor cell surface and the extracellular matrix. Interestingly, cathepsin B, plasminogen/plasmin, t-PA and tenascin-C have all been linked to tumor development. We speculate that colocalization through AIIt of proteases and their substrates on the tumor cell surface may facilitate: (1) activation of precursor forms of proteases and initiation of proteolytic cascades; and (2) selective degradation of extracellular matrix proteins. The recruitment of proteases to specific regions on the cell surface, regions where potential substrates are also bound, could well function as a 'proteolytic center' to enhance tumor cell detachment, invasion and motility.

Selective perturbation of early endosome and/or trans-Golgi network pH but not lysosome pH by dose-dependent expression of influenza M2 protein

Many sorting stations along the biosynthetic and endocytic pathways are acidified, suggesting a role for pH regulation in protein traffic. However, the function of acidification in individual compartments has been difficult to examine because global pH perturbants affect all acidified organelles in the cell and also have numerous side effects. To circumvent this problem, we have developed a method to selectively perturb the pH of a subset of acidified compartments. We infected HeLa cells with a recombinant adenovirus encoding influenza virus M2 protein (an acid-activated ion channel that dissipates proton gradients across membranes) and measured the effects on various steps in protein transport. At low multiplicity of infection (m.o.i.), delivery of influenza hemagglutinin from the trans-Golgi network to the cell surface was blocked, but there was almost no effect on the rate of recycling of internalized transferrin. At higher m.o.i., transferrin recycling was inhibited, suggesting increased accumulation of M2 in endosomes. Interestingly, even at the higher m.o.i., M2 expression had no effect on lysosome morphology or on EGF degradation, suggesting that lysosomal pH was not compromised by M2 expression. However, delivery of newly synthesized cathepsin D to lysosomes was slowed in cells expressing active M2, suggesting that acidification of the TGN and endosomes is important for efficient delivery of lysosomal hydrolases. Fluorescence labeling using a pH-sensitive dye confirmed the reversible effect of M2 on the pH of a subset of acidified compartments in the cell. The ability to dissect the role of acidification in individual steps of a complex pathway should be useful for numerous other studies on protein processing and transport.

Phenotypical changes of a human pancreatic adenocarcinoma cell line after selection on laminin-1/nidogen (LM/Ng) substratum

A cell line (PaTu 8902LM) exhibiting an altered phenotypic appearance was selected from a highly dedifferentiated established human pancreatic tumour cell line (PaTu 8902) by repetitive exposure to laminin-1/nidogen substratum and subsequent selection for adherent cells. Polymerase chain reaction analysis for repetitive DNA indicated that both cell lines are genetically very closely related. The original PaTu 8902 line consisted of flat cells growing in monolayers. In contrast, the obtained PaTu 8902LM cells exhibited a spherical morphology and tended to form clusters. Immunofluorescence analysis using antibodies against apical and basolateral marker enzymes indicated that the PaTu 8902LM cells were polarized, arranging their apical surfaces around central lumenal structures when growing in clusters. In addition, the selected PaTu 8902LM cell line exhibited altered levels of a number of differentiation marker enzymes like 5'-nucleotidase, transglutaminase and plasminogen activators. The different morphological characteristics of both cell lines were maintained even after injection into nude mice. In xenografts, PaTu 8902LM cells were grouped around lumenal, duct-like structures, whereas the original PaTu 8902 cell line formed solid tumours composed of undifferentiated cells. Evidence is presented that the PaTu 8902LM cells are not merely selected from preexisting cells, but that the exposure of PaTu 8902 cells to laminin-1/nidogen had induced a stable transdifferentiation towards the phenotype of the epithelial cells lining the pancreatic secretory ducts. Thus the PaTu 8902LM cells resemble more closely those cells from which tumours of the pancreas originate in vivo and therefore might be a useful cell system in future analyses of the biology of pancreatic tumours which are of increasing incidence and clinical importance.

An immunocytochemical study for lysosomal cathepsins B and D related to the intracellular degradation of titanium at the bone-titanium interface

The morphological relationship between titanium and lysosomal proteinases, cathepsins B and D, at the bone-titanium interface using titanium-coated plastic implants placed for 28 days in the tibiae of 6-week-old rats was immunocytochemically investigated by the colloidal immunogold-silver method. Under light microscopy the titanium layer appeared to make direct contact with the bone and one or a few layers of slender cells were interposed between the bone and titanium. Ultrastructurally, the titanium came in contact with the bone or the slender cell layer through a 20 to 40 nm thin amorphous zone. The slender cells at the bone-titanium interface consisted of two types; one was an osteoblast type with glycogen granules which was found along the newly-formed bone facing titanium layer. The other was a fibroblast type which came in contact with the titanium layer and occasionally endocytosed the detached titanium fragments. In addition, some of the slender cells also showed degenerative changes. Immunocytochemically, cathepsins B and/or D were sometimes colocalized in some phagolysosomes with titanium fragments. These findings suggested that the fibroblast types at the bone-titanium interface may act as scavengers to remove both cell debris and titanium by means of some endocytotic ability, and lysosomal cathepsins also developed in response to the endocytosed titanium. The osteoblast type also appears to show a high degree of osteogenic activity around the titanium-coated plastic implants.

A fluid-phase endocytotic capacity and intracellular degradation of a foreign protein (horseradish peroxidase) by lysosomal cysteine proteinases in the rat junctional epithelium

We investigated the co-localization of lysosomal cathepsins B, H and L, and horseradish peroxidase (HRP) in junctional epithelial (JE) cells both as a fluid-phase endocytotic marker to demonstrate the fluid-phase endocytotic capacity of JE cells, and to understand the morphological relationships of the endocytosed foreign substances to lysosomal cathepsins in these cells. The diaminobenzidine (DAB) histochemical and cytochemical methods and immunohistochemical avidin-biotin-peroxidase complex and immunocytochemical post-embedding colloidal gold methods were used. Under light microscopy, DAB reaction products based on HRP were found in JE but were rare or absent in the oral sulcular epithelium and oral epithelium. Immunolabeling for cathepsins B and H was found in the granular structures of the cells, but no cathepsin L was identified. With electron microscopy, DAB reaction products, which indicated both HRP and the azurophil granules of neutrophils, were endocytosed into JE cells. Using a post-embedding technique, gold particles indicating HRP were present on the plasma membrane of JE cells, at the periphery of electronlucent vacuoles, and in the electrondense granules. Gold particles indicating cathepsin B or H were found in the electrondense granules. With different sizes of colloidal golds, the co-localization of cathepsin B or H with HRP was indicated only in the electrondense portion of the larger vacuoles consisting of electronlucent and -dense parts. This study provided the first morphological data which indicate that JE has a fluid phase endocytotic capacity, and which suggest that the lysosomal cathepsins B and H are involved in the intracellular degradation of foreign substances invading through the gingival sulcus in JE cells.

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

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

Ethanol consumption reduces the proteolytic capacity and protease activities of hepatic lysosomes

Chronic ethanol consumption causes decreased hepatic protein degradation, resulting in protein accumulation within hepatocytes. In this investigation, we sought to determine whether chronic ethanol feeding alters the degradative capacity and protease activities of isolated hepatic lysosomes. Male Sprague-Dawley-derived rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose-dextrin for 1-5 wk. Hepatic lysosomes were isolated by differential centrifugation and purified through Percoll gradients. Lysosomes obtained from livers of ethanol-fed rats degraded both endogenous protein substrates and the exogenously added radioactive substrate, 125I-RNase A, 26-42% more slowly than lysosomes from pair fed controls. The ethanol-elicited reduction in proteolytic capacity appeared to result in part, from a deficiency of the lysosomal cathepsins B, L, and H. Compared with controls, the specific activities of these enzymes were 31-45% lower in lysosomes from ethanol-fed rats. Immunoblot analyses also revealed that the intralysosomal as well as the intracellular content of cathepsin B was significantly lower in ethanol-fed rats. In contrast, ethanol consumption did not affect the cellular quantity of cathepsin L but lowered its amount in isolated lysosomes. Our findings suggest that chronic ethanol consumption causes a deficiency in lysosomal cathepsins by altering their biosynthesis and/or their trafficking into lysosomes.

Multi-step processing of procathepsin L in vitro

The proteolytic processes involved in the conversion of procathepsin L to cathepsin L on a negatively charged surface, dextran sulfate, were studied. Upon incubation for 30 min at 37 degrees C, pH 5.5 with dextran-sulfate and dithiothreitol, purified procathepsin L showed maximal activation and, correspondingly, the complete conversion to the 30 kDa, single chain mature form of enzyme was observed. In contrast, incubation under the same conditions on ice rather than at 37 degrees C for 30 or 60 min resulted in partial proteolysis to produce a 31 kDa form without a significant increase in activity. Amino terminal amino acid sequence analyses showed that the 30 kDa form obtained by incubation at 37 degrees C corresponds to the purified form of mature cathepsin L with a 2 amino acid extension at the amino terminal, and that the 31 kDa form generated by incubation on ice possesses a 6 amino acid amino terminal extension, suggesting that the activation and processing of procathepsin L are different processes, and that 4 amino acid residues (Glu-Pro-Leu-Met) at the carboxyterminal in the propeptide function to prevent the activation of processed cathepsin L.

Distinct molecular mechanisms for protein sorting within immature secretory granules of pancreatic beta-cells

In the beta-cells of pancreatic islets, insulin is stored as the predominant protein within storage granules that undergo regulated exocytosis in response to glucose. By pulse-chase analysis of radiolabeled protein condensation in beta-cells, the formation of insoluble aggregates of regulated secretory protein lags behind the conversion of proinsulin to insulin. Condensation occurs within immature granules (IGs), accounting for passive protein sorting as demonstrated by constitutive-like secretion of newly synthesized C-peptide in stoichiometric excess of insulin (Kuliawat, R., and P. Arvan. J. Cell Biol. 1992. 118:521-529). Experimental manipulation of condensation conditions in vivo reveals a direct relationship between sorting of regulated secretory protein and polymer assembly within IGs. By contrast, entry from the trans-Golgi network into IGs does not appear especially selective for regulated secretory proteins. Specifically, in normal islets, lysosomal enzyme precursors enter the stimulus-dependent secretory pathway with comparable efficiency to that of proinsulin. However, within 2 h after synthesis (the same period during which proinsulin processing occurs), newly synthesized hydrolases are fairly efficiently relocated out of the stimulus-dependent pathway. In tunicamycin-treated islets, while entry of new lysosomal enzymes into the regulated secretory pathway continues unperturbed, exit of nonglycosylated hydrolases from this pathway does not occur. Consequently, the ultimate targeting of nonglycosylated hydrolases in beta-cells is to storage granules rather than lysosomes. These results implicate a post-Golgi mechanism for the active removal of lysosomal hydrolases away from condensed granule contents during the storage process for regulated secretory proteins.

Vacuolar ATPase inactivation blocks recycling to the trans-Golgi network from the plasma membrane

TGN38/41 is an integral membrane protein which recycles between the trans-Golgi network (TGN) and the cell surface but is predominantly located in the TGN of rat (NRK) cells at steady state. As part of our studies on the mechanism and route of recycling between the TGN and the cell surface we have used chloroquine or Bafilomycin A1 to modulate the lumenal pH of endocytic organelles. The data we present demonstrate that inactivation of the proton pump which maintains the acidic environment within the lumen of endocytic organelles leads to an accumulation of TGN38/41 in early endosomes. These data confirm the observation that TGN38/41 recycles between the plasma membrane and the TGN and identifies a specific block in that recycling pathway.

Characterization of the cathepsin B gene and multiple mRNAs in human tissues: evidence for alternative splicing of cathepsin B pre-mRNA

We have cloned and characterized multiple messages for cathepsin B that differ in their 5' and 3' untranslated regions (UTRs) from human kidney and the hepatoma cell line HepG2. A comparison of these messages with the cloned human cathepsin B gene reveals that they arise by alternative splicing of a single gene. Processing at a cryptic intron donor site in exon 11 and splicing to exon 12 produces a 4.0-kb message with an alternate 3' UTR in addition to the 2.3-kb message described previously by Chan et al. (1986). Variable removal of exon 2 produces cathepsin B mRNAs which differ by 88 nucleotides in their 5'-UTRs. The ratio of the 2.3-kb to 4.0-kb transcript is about 2:1 in most of the tissues examined, but the ratio of mRNAs with variant 5' UTRs differs widely. Cathepsin B mRNAs lacking exon 2 are predominant in human tumors. In addition, human breast and colon carcinomas and a human melanoma contain a cathepsin B transcript that is also missing exon 3 encoding the signal peptide and 7 residues of the activation propeptide. An in vitro transcription/translation assay was used to demonstrate that this message could be translated from an internal methionine codon (residue 52), producing a 32-kD product lacking the signal peptide and more than half the propeptide. The transcription/translation assay also demonstrated that the variant messages differ in their rates of translation. The relative rates are about 8:2:1 for mRNA lacking exons 2 and 3 compared to mRNA lacking exon 2 and mRNA containing the full-length 5' end, respectively. These results suggest that the expression of cathepsin B in human tissues may be regulated in part at the level of mRNA processing.

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

The lysosomal cysteine proteinase cathepsin B is synthesized in cultured human hepatoma HepG2 cells as an inactive 44 kDa precursor and subsequently processed to the mature single-chain enzyme with a molecular mass of 33 kDa. Intralysosomal conversion into the two-chain form results in subunits of 27 kDa, 24 kDa (heavy chain) and 5 kDa (light chain). Enzymic deglycosylation reveals that the 27 kDa polypeptide is the glycosylated variant of the carbohydrate-free 24 kDa heavy-chain form. The intracellular transport to the lysosomes is dependent upon mannose 6-phosphate-containing N-linked oligosaccharides. Receptor-mediated endocytosis of human skin-fibroblast-derived procathepsin B by HepG2 cells resulted in processed molecular forms that are not distinguishable from endogenous cathepsin B, thus favouring rather a cell-type-specific processing than structural differences due to the source of the proenzyme. The conversion step of single-chain catehpsin B into the two-chain enzyme is inhibited in vivo by the irreversible cysteine-proteinase inhibitors Z-Phe-Ala-CHN2 and, albeit weaker, Z-Phe-Phe-CHN2. Both substances have no effect on the activation of procathepsin B to the mature enzyme. The carbohydrate moiety of cathepsin B exerts no significant influence on the stability and the enzymatic activity of the enzyme.

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.

Bafilomycin A1 inhibits the targeting of lysosomal acid hydrolases in cultured hepatocytes

Effects of bafilomycin A1, an inhibitor of vacuolar H(+)-ATPase, on the synthesis and processing of cathepsin D and cathepsin H were investigated in primary cultured rat hepatocytes. Pulse-chase experiments showed that after being synthesized as procathepsin D and procathepsin H the precursors were converted into mature forms in the control cells as the chase time elapsed. However, in the presence of 5 x 10(-7) M of bafilomycin A1, both precursors were largely secreted into the medium and no mature forms were found within the cells. Thus bafilomycin A1 mimics lysosomotropic amines with regard to perturbation of the targeting of lysosomal acid hydrolases. In contrast, bafilomycin A1 was found not to inhibit processings of proalbumin and procomplement component 3, which are thought to occur at the acidic trans-Golgi, implying that the proteolytic event of the proproteins is not sensitive to an increase of intra-Golgi pH. The results suggest that bafilomycin A1 is useful as a pH-perturbant to study the role of acidity in living cells.

Inhibition of intracellular sorting and processing of lysosomal cathepsins H and L at reduced temperature in primary cultures of rat hepatocytes

Our recent studies with pulse-chase kinetic analysis in primary cultures of rat hepatocytes suggest that newly synthesized lysosomal cathepsins H and L are initially synthesized as larger proform enzymes, and then the precursor molecules are subsequently converted to the mature enzymes by limited proteolysis during the intracellular sorting process. This proteolytic maturation of procathepsins appears to proceed within an acidic environment, and these processing events are closely connected with the activation of enzymes. To further characterize the intracellular processing site for lysosomal cathepsins H and L, the pulse-chase kinetic study was carried out at 20 degrees C in cultured rat hepatocytes, because the transport of the procathepsins was expected to be blocked at the trans-Golgi compartment at 20 degrees C. We show here that the newly synthesized procathepsins are accumulated intracellularly and the processing for lysosomal cathepsins is completely arrested at 20 degrees C along the sorting pathway. The procathepsins thus accumulated in the cell are presumed to be transported to the Golgi complex, since the oligosaccharide moieties of these polypeptides appear to be phosphorylated. When the cells were shifted to 37 degrees C after an incubation for 4 h at 20 degrees C, a gradual increase of the mature forms was found. However, the processing kinetics generating the mature enzymes were slow compared to those in control cells at 37 degrees C. When the NH4Cl was present in the cells after the temperature shift to 37 degrees C, the intracellular processing of procathepsins was considerably retarded and the release of intracellular procathepsins into the extracellular medium was observed. These results indicate that NH4Cl might exert the inhibitory effect on the mannose 6-phosphate receptor-mediated intracellular targeting mechanism for the lysosomal cathepsins. Hence, the intracellular location of procathepsins accumulated at 20 degrees C is considered to be in proximity to the trans-Golgi compartment. Taken together, the present observations suggest that the propeptide-processing step for procathepsins, which is a critical step for generating the active enzymes, proceeds within the prelysosomal compartment or the lysosomes after the enzymes leave the trans-Golgi compartment.

Molecular cloning of rat precursor cathepsin H and the expression of five lysosomal cathepsins in normal tissues and in a rat carcinosarcoma

1. A rat cathepsin H cDNA was isolated from a rat liver cDNA library with synthetic oligonucleotide probes. 2. DNA sequence analysis indicated that it codes for rat preprocathepsin H. 3. Using this clone together with the cDNA for cathepsins B, D, L and S as probes, the expression of five major lysosomal proteinases was investigated in ten different normal rat tissues and in a rat carcinoma. 4. The common feature of their expression is that the five cathepsins have relatively high mRNA levels in lung and kidney, suggesting that they all play important roles in organs engaged in active protein metabolism. 5. In other tissues, the concentrations of the five cathepsin mRNAs are significantly different. This may indicate that their expressions are differentially regulated and that they may have specialized functions in specific tissues. 6. The cathepsin B mRNA level is at least 2.5-fold higher in the rat W256-carcinoma than in any of the normal rat tissues surveyed. 7. In contrast, the mRNA levels for the other four cathepsins show no comparable elevations. 8. This finding is consistent with previous observations reporting a correlation between cathepsins B expression and malignant tumors.

Brefeldin A inhibits the targeting of cathepsin D and cathepsin H to lysosomes in rat hepatocytes

Effect of brefeldin A on the transport of lysosomal acid hydrolases (cathepsins D and H) was investigated in primary cultured rat hepatocytes. Both cathepsins were synthesized as proenzymes and progressively converted to mature enzymes in the control cells. However, BFA strongly inhibited the appearance of the mature enzymes in the cells in a dose dependent manner, suggesting that transport of newly synthesized lysosomal enzymes from the endoplasmic reticulum to lysosomes is blocked by the drug. The inhibitory effect by brefeldin A was reversible. Upon recovery from brefeldin A-intoxication, procathepsin D was effectively targeted into lysosomes, whereas a substantial amount of procathepsin H was found to be missorted, resulting in its secretion into the culture medium.

Evidence that aspartic proteinase is involved in the proteolytic processing event of procathepsin L in lysosomes

Our recent studies have shown that cathepsin L is first synthesized as an enzymatically inactive proform in endoplasmic reticulum and is successively converted into an active form during intracellular transport and we postulated that aspartic proteinases would be responsible for the intracellular propeptide-processing step of procathepsin L accompanied by the activation of enzyme (Y. Nishimura, T. Kawabata, and K. Kato (1988) Arch. Biochem. Biophys. 261, 64-71). To better understand this proposed mechanism, we investigated the effect of pepstatin, a potent inhibitor of aspartic proteinases, on the intracellular processing kinetics of cathepsin L analyzed by pulse-chase experiments in vivo with [35S]methionine in the primary cultures of rat hepatocytes. In the pepstatin-treated cells, the proteolytic conversion of cellular procathepsin L of 39 kDa to the mature enzyme was significantly inhibited and considerable amounts of proenzyme were found in the cell after 5-h chase periods. Further, the subcellular fractionation experiments demonstrated that the intracellular processing of procathepsin L in the high density lysosomal fraction was significantly inhibited and that considerable amounts of the procathepsin L form were still observed in the light density microsomal fraction after 2 h of chase. These results suggest that pepstatin treatment caused a significant inhibitory effect on the intracellular processing and also on the intracellular movement of procathepsin L from the endoplasmic reticulum to the lysosomes. These findings provide the first evidence showing that aspartic proteinase may play an important role in the intracellular proteolytic processing and activation of lysosomal cathepsin L in vivo. Therefore, we suggest that cathepsin D, a major lysosomal aspartic proteinase, is more likely to be involved in this proposed model in the lysosomes.

Biosynthesis of lysosomal endopeptidases

Despite the clear differences between the amino acid sequence and enzymatic specificity of aspartic and cysteine endopeptidases, the biosynthetic processing of lysosomal members of these two families is very similar. With in vitro translation and pulse-chase analysis in tissue culture cells, the biosynthesis of cathepsin D, a aspartic protease, and cathepsins B, H and L, cysteine proteases, are compared. Both aspartic and cysteine endopeptidases undergo cotranslational cleavage of an amino-terminal signal peptide that mediates transport across the endoplasmic reticulum (ER) membrane. Addition of high-mannose carbohydrate also occurs cotranslationally in the lumen of the ER. Proteases of both enzyme classes are initially synthesized as inactive proenzymes possessing amino-terminal activation peptides. Removal of the propeptide generates an active single-chain enzyme. Whether the single-chain enzyme undergoes asymmetric cleavage into a light and a heavy chain appears to be cell type specific. Finally, late during their biosynthesis both classes of enzymes undergo amino acid trimming, losing a few amino acid residues at the cleavage site between the light and heavy chains and/or at their carboxyltermini. During biosynthesis these enzymes are also secreted to some extent. In most cells the secreted enzyme is the proenzyme bearing some complex carbohydrate. Under certain physiological conditions the inactive secreted enzymes may become activated as a result of a conformational change that may or may not result in autolysis. Analysis of the biochemical nature of the various processing steps helps define the cellular pathway followed by newly synthesized proteases targeted to the lysosome.

Biosynthesis and processing of lysosomal cathepsin L in primary cultures of rat hepatocytes

The biosynthesis and proteolytic processing of lysosomal cathepsin L was studied using in vitro translation system and in vivo pulse-chase analysis with [35S]methionine and [32P]phosphate in primary cultures of rat hepatocytes. Messenger RNA prepared from membrane-bound but not free polysomes directed the synthesis of a primary translation product of an immunoprecipitable 37.5-kDa cathepsin L in vitro. The 37.5-kDa form was converted to the 39-kDa form when translated in the presence of dog pancreas microsomes. During pulse-chase experiments with [35S]methionine in cultured rat hepatocytes, cathepsin L was first synthesized as a 39-kDa protein, presumably the proform, after a short time of labeling, and was subsequently processed into the mature forms of 30 and 25 kDa in the cell. On the other hand, considerable amounts of the proenzyme were found to be secreted into the culture medium without further proteolytic processing during the chase. The precursor and mature enzymes were N-glycosylated with high-mannose-type oligosaccharides, and the proenzyme molecule contained phosphorylated oligosaccharides. The effects of tunicamycin and chloroquine were also investigated. In the presence of tunicamycin, a 36-kDa unglycosylated polypeptide appeared in the cell and this protein was exclusively secreted from the cells without undergoing proteolytic processing. These results suggest that cathepsin L is initially synthesized on membrane-bound polysomes as a 37.5-kDa prepropeptide and that the cotranslational cleavage of the 1.5-kDa signal peptide and the core glycosylation convert the precursor to the 39-kDa proform, which is subsequently processed to the mature form during biosynthesis. Thus, the biosynthesis and secretion of lysosomal cathepsin L in rat hepatocytes seem to be analogous to those of the major excreted protein of transformed mouse fibroblasts [S. Gal, M. C. Willingham, and M. M. Gottesman (1985) J. Cell Biol. 100, 535-544] and the mouse cysteine proteinase of activated macrophages [D.A. Portnoy, A. H. Erickson, J. Kochan, J. V. Ravetch, and J. C. Unkeless (1986) J. Biol. Chem. 261, 14697-14703].