Synthesis, maturation and extracellular release of procathepsin D as influenced by cell proliferation or transformation

High Expression of the Lysosomal Protease Cathepsin D Confers Better Prognosis in Neuroblastoma Patients by Contrasting EGF-Induced Neuroblastoma Cell Growth

Neuroblastoma is a malignant extracranial solid tumor arising from the sympathoadrenal lineage of the neural crest and is often associated with N-MYC amplification. Cathepsin D has been associated with chemoresistance in N-MYC-overexpressing neuroblastomas. Increased EGFR expression also has been associated with the aggressive behavior of neuroblastomas. This work aimed to understand the mechanisms linking EGFR stimulation and cathepsin D expression with neuroblastoma progression and prognosis. Gene correlation analysis in pediatric neuroblastoma patients revealed that individuals bearing a high EGFR transcript level have a good prognosis only when CTSD (the gene coding for the lysosomal protease Cathepsin D, CD) is highly expressed. Low CTSD expression was associated with poor clinical outcome. CTSD expression was negatively correlated with CCNB2, CCNA2, CDK1 and CDK6 genes involved in cell cycle division. We investigated the biochemical pathways downstream to EGFR stimulation in human SH-SY5Y neuroblastoma cells engineered for overexpressing or silencing of CD expression. Cathepsin D overexpression decreased the proliferative potential of neuroblastoma cells through downregulation of the pro-oncogenic MAPK signaling pathway. EGFR stimulation downregulated cathepsin D expression, thus favoring cell cycle division. Our data suggest that chemotherapeutics that inhibit the EGFR pathway, along with stimulators of cathepsin D synthesis and activity, could benefit neuroblastoma prognosis.

Knock-down of cathepsin D affects the retinal pigment epithelium, impairs swim-bladder ontogenesis and causes premature death in zebrafish

The lysosomal aspartic protease Cathepsin D (CD) is ubiquitously expressed in eukaryotic organisms. CD activity is essential to accomplish the acid-dependent extensive or partial proteolysis of protein substrates within endosomal and lysosomal compartments therein delivered via endocytosis, phagocytosis or autophagocytosis. CD may also act at physiological pH on small-size substrates in the cytosol and in the extracellular milieu. Mouse and fruit fly CD knock-out models have highlighted the multi-pathophysiological roles of CD in tissue homeostasis and organ development. Here we report the first phenotypic description of the lack of CD expression during zebrafish (Danio rerio) development obtained by morpholino-mediated knock-down of CD mRNA. Since the un-fertilized eggs were shown to be supplied with maternal CD mRNA, only a morpholino targeting a sequence containing the starting ATG codon was effective. The main phenotypic alterations produced by CD knock-down in zebrafish were: 1. abnormal development of the eye and of retinal pigment epithelium; 2. absence of the swim-bladder; 3. skin hyper-pigmentation; 4. reduced growth and premature death. Rescue experiments confirmed the involvement of CD in the developmental processes leading to these phenotypic alterations. Our findings add to the list of CD functions in organ development and patho-physiology in vertebrates.

A new mechanism for prolactin processing into 16K PRL by secreted cathepsin D

Cathepsins are lysosomal enzymes that were shown to release the antiangiogenic fragments 16K prolactin (PRL), endostatin, and angiostatin by processing precursors at acidic pH in vitro. However, the physiological relevance of these findings is questionable because the neutral pH of physiological fluids is not compatible with the acidic conditions required for the proteolytic activity of these enzymes. Here we show that cathepsin D secreted from various tissues is able to process PRL into 16K PRL outside the cell. To specifically target extracellular proteolysis, we used tissues from PRL receptor-deficient mice, which are unable to internalize PRL. As assessed by the use of specific inhibitors of proton extruders, we show that the proteolytic activity of cathepsin D requires local acid secretion driven by Na(+)/H(+) exchangers and H(+)/ATPase. Although it is usually assumed that cathepsin-mediated generation of antiangiogenic peptides occurs in the moderately acidic pericellular milieu found in malignant tumors, we propose a new mechanism explaining the extracellular activity of this acidic protease under physiological pH. Our data support the concept that secreted lysosomal enzymes could be involved in the maintenance of angiogenesis dormancy via the generation of active antiangiogenic peptides in nonpathological contexts.

Cathepsin D released by lactating rat mammary epithelial cells is involved in prolactin cleavage under physiological conditions

The 16 kDa prolactin fragment arises from partial proteolysis of the native 23 kDa prolactin pituitary hormone. The mammary gland has been involved in this processing, although it has not been clarified whether it occurs in stroma or epithelial cells or extracellularly. Also, the processing enzyme has not been defined yet. Here we show that the incubation medium of stroma-deprived mammary acini from lactating rat contains an enzymatic activity able to cleave, in a temperature- and time-dependent fashion, the 23 kDa prolactin to generate a 16 kDa prolactin detectable under reducing conditions. This cleavage was not impaired in the presence of hirudin, a thrombin inhibitor, but strongly weakened in the presence of pepstatin A, a cathepsin D inhibitor. Cathepsin D immuno-depletion abolished the capability of acini-conditioned medium to cleave the 23 kDa prolactin. Brefeldin A treatment of acini, a condition that largely abolished the apical secretion of milk proteins, did not impair the secretion of the enzymatically active single chain of cathepsin D. These results show that mature cathepsin D from endosomes or lysosomes is released, likely at the baso-lateral site of mammary epithelial cells, and that a cathepsin D-dependent activity is required to effect, under physiological conditions, the cleavage of 23 kDa prolactin in the extracellular medium. This is the first report demonstrating that cathepsin D can perform a limited proteolysis of a substrate at physiological pH outside the cell.

Destination 'lysosome': a target organelle for tumour cell killing?

Lysosomes and lysosome-related organelles constitute a system of acid compartments that interconnect the inside of the cell with the extracellular environment via endocytosis, phagocytosis and exocytosis. In recent decades it has been recognized that lysosomes are not just wastebaskets for disposal of unused cellular constituents, but that they are involved in several cellular processes such as post-translational maturation of proteins, degradation of receptors and extracellular release of active enzymes. By complementing the autophagic process, lysosomes actively contribute to the maintenance of cellular homeostasis. Proteolysis by lysosomal cathepsins has been shown to mediate the death signal of cytotoxic drugs and cytokines, as well as the activation of pro-survival factors. Secreted lysosomal cathepsins have been shown to degrade protein components of the extracellular matrix, thus contributing actively to its re-modelling in physiological and pathological processes. The malfunction of lysosomes can, therefore, impact on cell behaviour and fate. Here we review the role of lysosomal hydrolases in several aspects of the malignant phenotype including loss of cell growth control, altered regulation of cell death, acquisition of chemoresistance and of metastatic potential. Based on these observations, the lysosome is proposed as a potential target organelle for the chemotherapy of tumours. We will also present some recent data concerning the technologies for delivering chemotherapeutic drugs to the endosomal-lysosomal compartment and the strategies to improve their efficacy.

Endosomal-lysosomal proteolysis mediates death signalling by TNFalpha, not by etoposide, in L929 fibrosarcoma cells: evidence for an active role of cathepsin D

In several 'in vitro' models of apoptosis, lysosomal proteolysis has been shown to play an active role in mediating the death signal by cytokines or antiblastic drugs. Depending on the experimental cell model and the cytotoxic stimulus applied, an increased expression and the cytosolic translocation of either cathepsin D or B have been reported in apoptotic cells. We have analysed the involvement of these lysosomal proteases in a canonical apoptotic cell model, namely L929 fibroblasts, in which apoptosis was induced by cytotoxic agents acting through different mechanisms: (i) the cytokine TNFalpha, which triggers the cell suicide via interaction with its membrane receptor, and (ii) the topoisomerase II-inhibitor etoposide (VP16), which directly causes DNA damage. In both cases the activity of cathepsins B and D increased in apoptosing cultures. CA074-Me, a specific inhibitor of cathepsin B, and Leupeptin, a broad inhibitor of serine and cysteine proteases (among which is cathepsin B), did not exert any protection from TNFalpha. In contrast, pre-loading the cells with pepstatin A, a specific inhibitor of cathepsin D, protected L929 cells from TNFalpha cytotoxicity by more than 50%. However, no protection was observed if pepstatin A was added concomitantly with the cytokine. Inhibition of either cathepsin B or D did not impede apoptosis induced by etoposide. Lysosomal integrity was preserved and cathepsin D remained still confined in vesicular structures in apoptotic cells treated with either TNFalpha or etoposide. It follows that proteolysis by cathepsin D is likely to represent an early event in the death pathway triggered by TNFalpha and occurs within the endosomal-lysosomal compartment.

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

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

Effect of carbohydrate position on lysosomal transport of procathepsin L

To study the role of carbohydrate in lysosomal protein transport, we engineered two novel glycosylation signals (Asn-X-Ser/Thr) into the cDNA of human procathepsin L, a lysosomal acid protease. We constructed six mutant cDNAs encoding glycosylation signals at mutant sites Asn-138, Asn-175, or both sites together, in the presence or absence of the wild-type Asn-204 site. We stably transfected wild-type and mutant cDNAs into NIH3T3 mouse fibroblasts and then used species-specific antibodies to determine the glycosylation status, phosphorylation, localization, and transport kinetics of recombinant human procathepsin L containing one, two, or three glycosylation sites. Both novel glycosylation sites were capable of being glycosylated, although Asn-175 was utilized only 30-50% of the time. Like the wild-type glycosylation at Asn-204, carbohydrates at Asn-138 and Asn-175 were completely sensitive to endoglycosidase H, and they were phosphorylated. Mutant proteins containing two carbohydrates were capable of being delivered to lysosomes, but there was not a consistent relationship between the efficiency of lysosomal delivery and carbohydrate content of the protein. Pulse-chase labeling revealed a unique biosynthetic pattern for proteins carrying the Asn-175 glycosylation sequence. Whereas wild-type procathepsin L and mutants bearing carbohydrate at Asn-138 appeared in lysosomes by about 60 min, proteins with carbohydrate at Asn-175 were processed to a lysosome-like polypeptide within 15 min. Temperature shift, brefeldin A, and NH4Cl experiments suggested that the rapid processing did not occur in the endoplasmic reticulum and that Asn-175 mutants could interact with the mannose 6-phosphate receptor. Taken together, our results are consistent with the interpretation that Asn-175 carbohydrate confers rapid transport to lysosomes. We may have identified a recognition domain in procathepsin L that is important for its interactions with the cellular transport machinery.

Differential targeting and processing of procathepsin D in normal and transformed murine 3T3 fibroblasts

The kinetics of transport and the processing of procathepsin D (proCD), the precursor of a lysosomal aspartyl protease involved in tumor-cell proliferation and metastasis, were compared in normal and SV-40- or benzo[a]pyrene-transformed 3T3 mouse fibroblasts. Sorting of newly synthesized proCD in normal cells was almost complete within 3 hr, while in transformed cells a fraction of the precursor survives a long time. In both normal and transformed 3T3 cultures, secretion of proCD started at 3 hr of chase. However, in normal cells secretion of proCD remained constant between 3 and 24 hr of chase, while in transformed cells it increased along with the chase incubation. The efficiency of formation of the mannose-6-phosphate group on proCD varied among the 3 cell types, being minimal in benzo[a]pyrene-transformed 3T3 cells. Ammonium chloride, a drug known to disrupt the segregation and to enhance the secretion of lysosomal proenzymes, was 2-fold more effective in normal than in transformed 3T3 cells. Despite vacuolar alkalinization, about one third of proCD was segregated into the endosomal-lysosomal pathway in normal and in transformed 3T3 fibroblasts, indicating the existence in these cells of alternative, mannose-6-phosphate receptor-independent mechanisms for targeting proCD. Thus, while hypersecretion of proCD and reduced sensitivity to vacuolar alkalinization are common features of both transformed cell types, the mechanisms responsible for inefficient segregation of proCD may differ between virally and chemically transformed 3T3 cells.

Exposed thiols confer localization in the endoplasmic reticulum by retention rather than retrieval

The cysteine present in the Ig micro chain tailpiece (microtp) prevents the secretion of unpolymerized IgM intermediates and causes their accumulation in the endoplasmic reticulum (ER). In principle, this can be the consequence of actual retention in this organelle or of retrieval from the Golgi. To determine which of the two mechanisms underlies the cysteine-dependent ER localization, we analyze here the post-translational modifications of suitably engineered cathepsin D (CD) molecules. The glycans of this protease are phosphorylated by post-ER phosphotransferases and further modified in the trans-Golgi to generate a mannose 6-phosphate lysosome targeting signal. Only trace amounts of the mutp-tagged CD (CDM&mutpCys) are phosphorylated, unless retention is reversed by exogenous reducing agents or the critical cysteine mutated (CDMmutpSer). In contrast, a KDEL-tagged CD, that is retrieved from the Golgi into the ER, acquires phosphates, though mainly resistant to alkaline phosphatase. Similarly to CDMmutpSer, the few CDMmutpCys molecules that escape retention and acquire phosphates in the cis-Golgi are transported beyond the KDEL retrieval compartment, as indicated by their sensitivity to alkaline phosphatase. These results demonstrate that the thiol-dependent ER localization arises primarily from true retention, without recycling through the Golgi.