Analysis of the interaction of procathepsin D activation peptide with breast cancer cells

Anti-cathepsin D immunotherapy triggers both innate and adaptive anti-tumour immunity in breast cancer

BACKGROUND AND PURPOSE

Triple-negative breast cancer (TNBC) has poorer outcomes than other breast cancers (BC), including HER2+ BC. Cathepsin D (CathD) is a poor prognosis marker overproduced by BC cells, hypersecreted in the tumour microenvironment with tumour-promoting activity. Here, we characterized the immunomodulatory activity of the anti-CathD antibody F1 and its improved Fab-aglycosylated version (F1M1) in immunocompetent mouse models of TNBC (C57BL/6 mice harbouring E0771 cell grafts) and HER2-amplified BC (BALB/c mice harbouring TUBO cell grafts).

EXPERIMENTAL APPROACH

CathD expression was evaluated by western blotting and immunofluorescence, and antibody binding to CathD by ELISA. Antibody anti-tumour efficacy was investigated in mouse models. Immune cell recruitment and activation were assessed by immunohistochemistry, immunophenotyping, and RT-qPCR.

KEY RESULTS

F1 and F1M1 antibodies remodelled the tumour immune landscape. Both antibodies promoted innate antitumour immunity by preventing the recruitment of immunosuppressive M2-polarized tumour-associated macrophages (TAMs) and by activating natural killer cells in the tumour microenvironment of both models. This translated into a reduction of T-cell exhaustion markers in the tumour microenvironment that could be locally supported by enhanced activation of anti-tumour antigen-presenting cell (M1-polarized TAMs and cDC1 cells) functions. Both antibodies inhibited tumour growth in the highly-immunogenic E0771 model, but only marginally in the immune-excluded TUBO model, indicating that anti-CathD immunotherapy is more relevant for BC with a high immune cell infiltrate, as often observed in TNBC.

CONCLUSION AND IMPLICATION

Anti-CathD antibody-based therapy triggers the anti-tumour innate and adaptive immunity in preclinical models of BC and is a promising immunotherapy for immunogenic TNBC.

Cathepsin D in the Tumor Microenvironment of Breast and Ovarian Cancers

Cancer remains a major and leading health problem worldwide. Lack of early diagnosis, chemoresistance, and recurrence of cancer means vast research and development are required in this area. The complexity of the tumor microenvironment in the biological milieu poses greater challenges in having safer, selective, and targeted therapies. Existing strategies such as chemotherapy, radiotherapy, and antiangiogenic therapies moderately improve progression-free survival; however, they come with side effects that reduce quality of life. Thus, targeting potential candidates in the microenvironment, such as extracellular cathepsin D (CathD) which has been known to play major pro-tumorigenic roles in breast and ovarian cancers, could be a breakthrough in cancer treatment, specially using novel treatment modalities such as immunotherapy and nanotechnology-based therapy. This chapter discusses CathD as a pro-cancerous, more specifically a proangiogenic factor, that acts bi-functionally in the tumor microenvironment, and possible ways of targeting the protein therapeutically.

Total Syntheses of Cathepsin D Inhibitory Izenamides A, B, and C and Structural Confirmation of Izenamide B

The first total syntheses of izenamides A, B, and C, which are depsipeptides inhibitor of cathepsin D, were accomplished. In addition, the stereochemistry of izenamide B was confirmed by our syntheses. The key features of our synthetic route involve the avoidance of critical 2,5-diketopiperazine (DKP) formation and the minimization of epimerization during the coupling of amino acids for the target peptides.

Immunotherapy of triple-negative breast cancer with cathepsin D-targeting antibodies

BACKGROUND

Triple-negative breast cancer (TNBC) treatment is currently restricted to chemotherapy. Hence, tumor-specific molecular targets and/or alternative therapeutic strategies for TNBC are urgently needed. Immunotherapy is emerging as an exciting treatment option for TNBC patients. The aspartic protease cathepsin D (cath-D), a marker of poor prognosis in breast cancer (BC), is overproduced and hypersecreted by human BC cells. This study explores whether cath-D is a tumor cell-associated extracellular biomarker and a potent target for antibody-based therapy in TNBC.

METHODS

Cath-D prognostic value and localization was evaluated by transcriptomics, proteomics and immunohistochemistry in TNBC. First-in-class anti-cath-D human scFv fragments binding to both human and mouse cath-D were generated using phage display and cloned in the human IgG1 λ format (F1 and E2). Anti-cath-D antibody biodistribution, antitumor efficacy and in vivo underlying mechanisms were investigated in TNBC MDA-MB-231 tumor xenografts in nude mice. Antitumor effect was further assessed in TNBC patient-derived xenografts (PDXs).

RESULTS

High CTSD mRNA levels correlated with shorter recurrence-free survival in TNBC, and extracellular cath-D was detected in the tumor microenvironment, but not in matched normal breast stroma. Anti-cath-D F1 and E2 antibodies accumulated in TNBC MDA-MB-231 tumor xenografts, inhibited tumor growth and improved mice survival without apparent toxicity. The Fc function of F1, the best antibody candidate, was essential for maximal tumor inhibition in the MDA-MB-231 model. Mechanistically, F1 antitumor response was triggered through natural killer cell activation via IL-15 upregulation, associated with granzyme B and perforin production, and the release of antitumor IFNγ cytokine. The F1 antibody also prevented the tumor recruitment of immunosuppressive tumor-associated macrophages M2 and myeloid-derived suppressor cells, a specific effect associated with a less immunosuppressive tumor microenvironment highlighted by TGFβ decrease. Finally, the antibody F1 inhibited tumor growth of two TNBC PDXs, isolated from patients resistant or not to neo-adjuvant chemotherapy.

CONCLUSION

Cath-D is a tumor-specific extracellular target in TNBC suitable for antibody-based therapy. Immunomodulatory antibody-based strategy against cath-D is a promising immunotherapy to treat patients with TNBC.

Progranulin Stimulates the In Vitro Maturation of Pro-Cathepsin D at Acidic pH

Single-copy loss-of-function mutations in the progranulin gene (PGRN) underlie the neurodegenerative disease frontotemporal lobar degeneration, while homozygous loss-of-function of PGRN results in the lysosomal storage disorder neuronal ceroid lipofuscinosis. Despite evidence that normal PGRN levels are critical for neuronal health, the function of this protein is not yet understood. Here, we show that PGRN stimulates the in vitro maturation of the lysosomal aspartyl protease cathepsin D (CTSD). CTSD is delivered to the endolysosomal system as an inactive precursor (proCTSD) and requires sequential cleavage steps via intermediate forms to achieve the mature state (matCTSD). In co-immunoprecipitation experiments, PGRN interacts predominantly with immature pro- and intermediate forms of CTSD. PGRN enhances in vitro conversion of proCTSD to matCTSD in a concentration-dependent manner. Differential scanning fluorimetry shows a destabilizing effect induced by PGRN on proCTSD folding (∆T_m = -1.7 °C at a 3:1 molar ratio). We propose a mechanism whereby PGRN binds to proCTSD, destabilizing the propeptide from the enzyme catalytic core and favoring conversion to mature forms of the enzyme. Further understanding of the role of PGRN in CTSD maturation will assist in the development of targeted therapies for neurodegenerative disease.

Functional Relationship and Gene Ontology Classification of Breast Cancer Biomarkers

Breast cancer is a complex disease that still imposes a significant healthcare burden on women worldwide. The etiology of breast cancer is not known but significant advances have been made in the area of early detection and treatment. The advent of advanced molecular biology techniques, mapping of the human genome and availability of high throughput genomic and proteomic strategies opens up new opportunities and will potentially lead to the discovery of novel biomarkers for early detection and prognostication of breast cancer. Currently, many biomarkers, particularly the hormonal and epidermal growth factor receptors, are being utilized for breast cancer prognosis. Unfortunately, none of the biomarkers in use have sufficient diagnostic, prognostic and/or predictive power across all categories and stages of breast cancer. It is recognized that more useful information can be generated if tumors are interrogated with multiple markers. But choosing the right combination of biomarkers is challenging, because 1) multiple pathways are involved, 2) up to 62 genes and their protein products are potentially involved in breast cancer-related mechanisms and 3) the more markers evaluated, the more the time and cost involved. This review summarizes the current literature on selected biomarkers for breast cancer, discusses the functional relationships, and groups the selected genes based on a Gene Ontology™ classification.

The Potential Role of the Proteases Cathepsin D and Cathepsin L in the Progression and Metastasis of Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) is the leading cause of death from gynecologic malignancies and has a poor prognosis due to relatively unspecific early symptoms, and thus often advanced stage, metastasized cancer at presentation. Metastasis of EOC occurs primarily through the transcoelomic route whereby exfoliated tumor cells disseminate within the abdominal cavity, particularly to the omentum. Primary and metastatic tumor growth requires a pool of proangiogenic factors in the microenvironment which propagate new vasculature in the growing cancer. Recent evidence suggests that proangiogenic factors other than the widely known, potent angiogenic factor vascular endothelial growth factor may mediate growth and metastasis of ovarian cancer. In this review we examine the role of some of these alternative factors, specifically cathepsin D and cathepsin L.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Procathepsin d involvement in chemoresistance of cancer cells

Background:

The role of pCD in cancer has been studied for a long time. We have focused on the hypothesis that increased expression and/or secretion of pCD in cancer cells causes increased chemoresistance to apoptosis inducing molecules.

Aim:

The aim was to evaluate the effects of pCD expression/release on chemoresistance.

Materials and Methods:

We tested the LC₅₀ values for various transfectants of breast cancer cell line MDA-MB-231 as well as effects of exogenous additions of pCD, its mutants, pepstatine, antibodies, and Brefeldin on the resistance.

Results:

We found that pCD levels can be correlated with chemoresistance, the pro-resistant activity seems to be localized outside the cells, proteolytic activity is not involved, and PI3-Akt signaling has an important role in antiapoptotic effects of pCD.

Conclusion:

We can conclude that overexpression of pCD has strong influence on increased resistance of tumor cells. This could, in fact, be an important contribution in the possible use of pCD level determination for prognostic and/or therapeutic purposes.

New insights into procathepsin D in pathological and physiological conditions

Procathepsin D is a major glycoprotein that is secreted from numerous types of cancer cells including breast, lung and prostrate carcinomas. It affects multiple stages of tumorigenesis that include proliferation, invasion, metastasis and apoptosis. Previous studies showed that the mitogenic effect of procathepsin D on cancer cells was mediated through its propeptide or activation peptide. Recent studies have also implicated the possible use of procathepsin D/activation peptide as a marker of cancer progression. Considering the broad range of functions of procathepsin D, the present review summarizes the three major potentials of procathepsin D-cancer progression, tumor marker and wound healing.

Cathepsin D--many functions of one aspartic protease

For years, it has been held that cathepsin D (CD) is involved in rather non-specific protein degradation in a strongly acidic milieu of lysosomes. Studies with CD knock-out mice revealed that CD is not necessary for embryonal development, but it is indispensable for postnatal tissue homeostasis. Mutation that abolishes CD enzymatic activity causes neuronal ceroid lipofuscinosis (NCL) characterized by severe neurodegeneration, developmental regression, visual loss and epilepsy in both animals and humans. In the last decade, however, an increasing number of studies demonstrated that enzymatic function of CD is not restricted solely to acidic milieu of lysosomes with important consequences in regulation of apoptosis. In addition to CD enzymatic activity, it has been shown that apoptosis is also regulated by catalytically inactive mutants of CD which suggests that CD interacts with other important molecules and influences cell signaling. Moreover, procathepsin D (pCD), secreted from cancer cells, acts as a mitogen on both cancer and stromal cells and stimulates their pro-invasive and pro-metastatic properties. Numerous studies found that pCD/CD level represents an independent prognostic factor in a variety of cancers and is therefore considered to be a potential target of anti-cancer therapy. Studies dealing with functions of cathepsin D are complicated by the fact that there are several simultaneous forms of CD in a cell-pCD, intermediate enzymatically active CD and mature heavy and light chain CD. It became evident that these forms may differently regulate the above-mentioned processes. In this article, we review the possible functions of CD and its various forms in cells and organisms during physiological and pathological conditions.

Cathepsin D propeptide: mechanism and regulation of its interaction with the catalytic core

Propeptide blocks the active site in the inactive zymogen of cathepsin D and is cleaved off during zymogen activation. We have designed a set of peptidic fragments derived from the propeptide structure and evaluated their inhibitory potency against mature cathepsin D using a kinetic assay. Our mapping of the cathepsin D propeptide indicated two domains in the propeptide involved in the inhibitory interaction with the enzyme core: the active site "anchor" domain and the N-terminus of the propeptide. The latter plays a dominant role in propeptide inhibition (nanomolar Ki), and its high-affinity binding was corroborated by fluorescence polarization measurements. In addition to the inhibitory domains of propeptide, a fragment derived from the N-terminus of mature cathepsin D displayed inhibition. This finding supports its proposed regulatory function. The interaction mechanisms of the identified inhibitory domains were characterized by determining their modes of inhibition as well as by spatial modeling of the propeptide in the zymogen molecule. The inhibitory interaction of the N-terminal propeptide domain was abolished in the presence of sulfated polysaccharides, which interact with basic propeptide residues. The inhibitory potency of the active site anchor domain was affected by the Ala38pVal substitution, a propeptide polymorphism reported to be associated with the pathology of Alzheimer's disease. We infer that propeptide is a sensitive tethered ligand that allows for complex modulation of cathepsin D zymogen activation.

Procathepsin D secreted by HaCaT keratinocyte cells - A novel regulator of keratinocyte growth

Procathepsin D (pCD), the precursor form of lysosomal aspartic protease, is overexpressed and secreted by various carcinomas. The fact that secreted pCD plays an essential role in progression of cancer has been established. In this study, we describe substantial secretion of pCD by the human keratinocyte cell line HaCaT, under serum-free conditions. Moreover, exogenous addition of purified pCD enhanced the proliferation of HaCaT cells. The proliferative effect of pCD was inhibited by a monoclonal antibody against the activation peptide (AP) of pCD. Treatment of HaCaT cells with pCD or AP led to the secretion of a set of cytokines that might promote the growth of cells in a paracrine manner. The role of secreted pCD and its mechanism of action were studied in a scratch wound model and the presence of pCD and AP enhanced regeneration, while this effect was reversed by the addition of anti-AP antibody. Expression and secretion of pCD was upregulated in HaCaT cells exposed to various stress conditions. Taken together, our results strongly suggest that the secretion of pCD is not only linked to cancer cells but also plays a role in normal physiological conditions like wound healing and tissue remodeling.

Procathepsin D expression correlates with invasive and metastatic phenotype of MDA-MB-231 derived cell lines

Procathepsin D (pCD) is a glycoprotein secreted abundantly by cancerous cells with a documented role in tumor development. The levels of pCD in primary tumors are highly correlated with an increased incidence of metastasis. Our earlier studies have shown that pCD exerts its effect on cancer cells through its activation peptide (AP) and involves both autocrine and paracrine modes of action. In this study, we analyzed the expression and role of pCD in MDA-MB-231 and its derived cell lines 1833 and 4175 possessing discrete metastatic abilities. Our results demonstrated a direct relationship between expression and secretion of pCD to the differential invasive potential of these cells. Also, the cell lines responded to AP treatment by enhancing their invasive potential, proliferation and induction of secretion of various cytokines, suggesting that pCD plays a role in metastasis through its AP region.

Secretion of Cytokines in Breast Cancer Cells: The Molecular Mechanism of Procathepsin D Proliferative Effects

Procathepsin D (pCD) is a major secreted protein in estrogen receptor-positive (ER+) breast cancer cell lines. Several independent studies have documented pronounced mitogenic effect of secreted pCD on cancer tissue-derived cell lines, including those from breast, lung, and prostate cancer. It has also been shown that the proliferative effect of pCD involves both autocrine and paracrine modes of action. Recent studies have suggested that pCD could act as a key paracrine communicator between cancer and stromal cells. We have shown earlier that the proliferative activity of pCD depends on the activation peptide sequence of pCD. The present study casts light on the mechanism by which pCD influences the proliferation of cancer cells expressing the ER. Results described in the current paper clearly show that pCD initiates secretion of cytokines interleukin-4 (IL-4), IL-8, IL-10, IL-13, macrophage inflammatory protein-1beta and (MIP-1beta) from such tumor cells. Secreted cytokines take part in the proliferation of the cancer cells, as proven by selective inhibition using antibodies. In addition, expression of cytokine receptors on tested cell lines corresponded to the effects of individual cytokines. An analogous pattern was also observed for fibroblasts, which, under physiologic conditions, are the cells in closest contact with the tumor tissue and play a role in tumor growth and invasion. Our observations were further supported by coculture experiments that are in agreement. Although very similar in response to addition of pCD, the invasive ER- cells do not secrete cytokines. Together with previous in vivo results, these data point to pCD as one of key molecules for therapeutic attack in breast cancer.

Possible role of procathepsin D in human cancer

For the past ten years, our research has been focused on elucidating the mechanism by which procathepsin D (pCD) impacts cancer development. Various studies have shown that pCD is overexpressed and secreted by numerous cancer cell lines. After secretion, it exhibits "growth hormone-like" activity on cancerous cells but the exact mechanism of this mitogenic activity is not yet understood. The activation peptide of pCD (APpCD) (which is cleaved off upon activation of the zymogen) is responsible for the mitogenic function of pCD. Various in vitro and in vivo studies support our theory that the APpCD interacts with both parent and neighborhood cancer cells and thus functions as an autocrine mitogen. We propose a model of pCD mitogenic function and also some possible approaches for treatment and prevention of certain types of cancer.

Cathepsin D: newly discovered functions of a long-standing aspartic protease in cancer and apoptosis

The lysosomal aspartic protease cathepsin D (cath-D) is over-expressed and hyper-secreted by epithelial breast cancer cells. This protease is an independent marker of poor prognosis in breast cancer being correlated with the incidence of clinical metastasis. Cath-D over-expression stimulates tumorigenicity and metastasis. Indeed it plays an essential role in the multiple steps of tumor progression, in stimulating cancer cell proliferation, fibroblast outgrowth and angiogenesis, as well as in inhibiting tumor apoptosis. A mutated cath-D devoid of catalytic activity still proved mitogenic for cancer, endothelial and fibroblastic cells, suggesting an extra-cellular mode of action of cath-D involving a triggering, either directly or indirectly, of an as yet unidentified cell surface receptor. Cath-D is also a key mediator of induced-apoptosis and its proteolytic activity has been involved generally in this event. During apoptosis, mature lysosomal cath-D is translocated to the cytosol. Since cath-D is one of the lysosomal enzymes which requires a more acidic pH to be proteolytically-active relative to the cysteine lysosomal enzymes, such as cath-B and -L, it is open to question whether cytosolic cath-D might be able to cleave substrate(s) implicated in the apoptotic cascade. This review summarises our current knowledge on cath-D action in cancer progression and metastasis, as well as its dual function in apoptosis.

Increased activity of lysosomal enzymes in the peritoneal fluid of patients with gynecologic cancers and pelvic inflammatory disease

PURPOSE

To investigate whether the activity of lysosomal enzymes is increased in the peritoneal fluid of patients with gynecologic cancers compared to activity in the peritoneal fluid from normal subjects and those with pelvic inflammatory disease, and fluid from benign ovarian cysts.

PATIENTS AND METHODS

beta-glucuronidase, beta-galactosidase, and alpha-mannosidase activity was measured in the peritoneal fluid from patients with gynecologic cancer, pelvic inflammatory disease, and normal subjects, and fluid from benign ovarian cysts.

RESULTS

The mean+/-SD of beta-glucuronidase, beta-galactosidase, and alpha-mannosidase activity in the gynecologic cancers was 120+/-50 nmol, 203+/-86 nmol, and 240+/-119 nmol 4-methylumbelliferone/ml/h, respectively; in the normal control subjects it was 22+/-9 nmol, 46+/-10 nmol, and 80+/-23 nmol, respectively (P=0.00003, 0.0001, and 0.0001, respectively). The activity was increased even in cases without malignant cells in the peritoneal fluid. In pelvic inflammatory disease it was 148+/-82 nmol, 278+/-112 nmol, and 291+/-140 nmol, respectively. The activity in the fluid of the ovarian cysts was similar to that of the normal peritoneal fluid. There was a significant positive correlation between enzyme activity and stage of cancer, that was stronger for beta-glucuronidase (r=0.889, P=0.003).

CONCLUSION

The increased lysosomal enzyme activity in gynecologic cancers, without overlapping between patients and normal subjects or benign ovarian cyst fluid, indicates that such measurements might be applied for diagnostic purposes.

Cathepsin D in ovarian cancer: prognostic value and correlation with p53 expression and microvessel density

OBJECTIVE

Overexpression of ubiquitous lysosomal aspartyl protease cathepsin D (CD) is involved in the progression of cancer. This study investigates the prognostic value and the association of cathepsin D expression with clinicopathological parameters, p53 expression, and angiogenesis in ovarian cancer.

METHODS

Cathepsin D was determined immunohistochemically in 43 ovarian tumors of low malignant potential (LMP) and 80 invasive tumors FIGO stage I-IV. Results were correlated with clinicopathological characteristics, p53, and microvessel density (MVD). Survival analysis of cathepsin D expression and MVD was performed in invasive tumors.

RESULTS

Epithelial tumor cathepsin D expression was more common in LMP tumors (65.1%) compared to invasive tumors (43.7%; P = 0.02). In LMP tumors, stromal cathepsin D was associated with mucinous tumors (P = 0.01), whereas in invasive tumors, epithelial cathepsin D expression was associated with clear cell tumors (P = 0.003). Invasive tumor cathepsin D had a negative relation to p53 expression. In LMP tumors, stromal cathepsin D correlated with microvessel density (P = 0.03). Stromal cathepsin D expression was an independent prognostic factor for disease-free survival (DFS) in patients with invasive cancer (P = 0.03, Cox regression), while cathepsin D expression missed to be of prognostic value for overall survival (OS) in invasive ovarian cancer. MVD had no influence on survival in invasive ovarian cancer (P > 0.05).

CONCLUSION

Our study demonstrates a prognostic value of cathepsin D expression in invasive ovarian cancer, while cathepsin D expression in LMP tumors seems to be linked to angiogenesis. The relation among cathepsin D, p53 expression, and angiogenesis demonstrates biological differences between invasive ovarian cancer and LMP tumors.

Procathepsin D in breast cancer: what do we know? Effects of ribozymes and other inhibitors

Procathepsin D (pCD) is a major secreted glycoprotein in some human breast and other cancer cell lines. Several groups proposed that pCD served as a growth factor for these cell lines. Secreted pCD has been demonstrated in tissue section, tissue culture supernatants, carcinoma cytosols, and nipple aspirates. Moreover, several clinical studies suggested a potential role for this molecule in metastasis because its concentration in primary tumors correlated with an increased incidence of tumor metastases. In this paper, the effects of pCD were evaluated by proliferation in vitro and by mouse studies in vivo. Subsequent flow cytometry experiments showed the specificity of pCD binding to cancer cells. Cell cultivation showed that addition of either pCD or its activation peptide stimulates growth of cancer cells. These effects can be inhibited both in vitro and in vivo by anti-pCD antibodies. In addition, production of pCD can be inhibited by specifically designed ribozymes. This paper is focused on mitogenic effects of pCD, which seem to involve interaction of the activation peptide with as yet unidentified receptor. Different mechanisms by which pCD could promote development and spread of cancer cells are discussed.

Detection of procathepsin D in rat milk

The presence of procathepsin D, a zymogen of the soluble lysosomal aspartic proteinase cathepsin D, was detected in rat milk using Western blot analysis and assay of proteolytic activity in acidic buffers. No other forms of cathepsin D were found. Two different polyclonal anti-procathepsin D antibodies were used for immunochemical detection of procathepsin D. Both antibodies we found to recognize rat procathepsin D. Proteolytic activity in acidic buffers was detected using a fluorogenic substrate specific for cathepsin D and was abolished by pepstatin A, a specific inhibitor of aspartic proteinases. This study represents third demonstration of presence of procathepsin D in mammal breast milk. Potential sources and physiological functions are discussed.

Interaction of human breast fibroblasts with collagen I increases secretion of procathepsin B

Interactions of stromal and tumor cells with the extracellular matrix may regulate expression of proteases including the lysosomal proteases cathepsins B and D. In the present study, we determined whether the expression of these two proteases in human breast fibroblasts was modulated by interactions with the extracellular matrix component, collagen I. Breast fibroblasts were isolated from non-malignant breast tissue as well as from tissue surrounding malignant human breast tumors. Growth of these fibroblasts on collagen I gels affected cell morphology, but not the intracellular localization of vesicles staining for cathepsin B or D. Cathepsins B and D levels (mRNA or intracellular protein) were not affected in fibroblasts growing on collagen I gels or plastic, nor was cathepsin D secreted from these cells. In contrast, protein expression and secretion of cathepsin B, primarily procathepsin B, was induced by growth on collagen I gels. The induced secretion appeared to be mediated by integrins binding to collagen I, as inhibitory antibodies against alpha(1), alpha(2), and beta(1) integrin subunits prevented procathepsin B secretion from fibroblasts grown on collagen. In addition, procathepsin B secretion was induced when cells were plated on beta(1) integrin antibodies. To our knowledge, this is the first examination of cathepsin B and D expression and localization in human breast fibroblasts and their regulation by a matrix protein. Secretion of the cysteine protease procathepsin B from breast fibroblasts may have physiological and pathological consequences, as proteases are required for normal development and for lactation of the mammary gland, yet can also initiate and accelerate the progression of breast cancer.

Role of procathepsin D activation peptide in prostate cancer growth

BACKGROUND

Enzymatically inactive procathepsin D secreted from cancer cells has been confirmed to play a role in breast cancer development. We focused on prostate cancer and the role of activation peptide in mitogenic activity.

METHODS

Synthetic peptides and monoclonal antibodies raised against individual fragments of activation peptide were employed. Cell proliferation was measured by MTT (3-[4,5-dimethylthiatol-2-yl]-2,5-diphenyl tetrazolium bromide) assay or by in vivo growth in nude mice.

RESULTS

We demonstrated that the growth factor activity of activation peptide is localized in amino-acid region 27-44. In addition, both anti-activation peptide and anti-27-44 peptide antibodies administered in vivo inhibited the growth of human prostate tumors in mice.

CONCLUSIONS

Based on these data, we hypothesize that the interaction of procathepsin D activation peptide with an unknown receptor is mediated by amino-acid sequence 27-44. This interaction leads in certain types of tumor to a proliferation and higher motility. Blocking of this interaction by antibodies or antagonists might be a valuable tool in prostate cancer inhibition.

Cathepsin D in breast cancer: mechanisms and clinical applications, a 1999 overview

A short review of the literature first confirms the clinical value of cathepsin D as a prognostic marker in breast cancer, when using well standardized assays. We then summarize results of studies, mostly performed in our laboratory, aimed at understanding the effect of cathepsin D overexpression on metastasis and the molecular mechanisms involved. Cathepsin D-cDNA transfection increases tumor cell proliferation in vitro and the metastatic potential of 3Y1-Ad12 embryonic rat tumorigenic cells when injected in vivo into nude mice. The mechanism by which cathepsin D increases the incidence of clinical metastasis involves increased cell growth and decreased contact inhibition rather than escape of cancer cells through the basement membrane. Different mechanisms are considered to explain this mitogenic activity. Cathepsin D could act as a protease following its activation at an acidic pH, or as a ligand of different membrane receptors at a more neutral pH. In this case cathepsin D can displace IGFII from the mannose-6-phosphate/IGFII receptor to the IGFI receptor or activate another membrane receptor to be identified. The nature of the mechanisms involved in vivo may depend on the micro environment of the tumor cells. These studies should guide in the development of new therapies aimed at inhibiting the deleterious effect of overexpressed cathepsin D.

Mechanisms and kinetics of procathepsin D activation

In vitro, procathepsin D is activated to pseudocathepsin D by incubation at low pH. To investigate the mechanism of this activation, recombinant human procathepsin D and two mutants were generated in a baculovirus expression system. One mutant carried a point mutation within the catalytic domain, which resulted in a catalytically inactive enzyme form (D77A). The other carried a point mutation within the propeptide, which prevented activation by processing at the 'autoproteolysis-site' (L26P). Neither mutant is capable of processing itself to form pseudocathepsin D, and L26P is not able to process D77A. Despite the inability of L26P to cleave either its own or a wild-type prosequence, it did exhibit activity against a synthetic peptide substrate. The ability of intact precursor (zymogen) to cleave a peptide, but not a protein substrate, offers new insights into the mechanism of inhibition by the propeptide. Mature cathepsin D can process the inactive D77A mutant to the pseudoform, demonstrating that processed species are capable of cleaving zymogen molecules in an intermolecular interaction. In addition, kinetic studies provide evidence for a two-phase mechanism for the conversion of procathepsin D to pseudocathepsin D, one phase where the first molecules of pseudocathepsin D are formed at a low rate and a second phase where the process is autocatalytically accelerated by newly formed pseudocathepsin D molecules. Finally, with the help of the mutants L26P and D77A it was observed that at least two additional proteinase activities, found in conditioned media from insect cell culture, are capable of activating procathepsin D by cleaving it within the proregion. This observation suggests that there are likely to be multiple proteinases in the extracellular matrix that are capable of activating procathepsin D, thereby triggering the second autocatalytic phase. This may also be important for solid tumors, where the presence of cathepsin D has been correlated with tumor growth and invasion.

Anti-human procathepsin D activation peptide antibodies inhibit breast cancer development

Enzymatically inactive procathepsin D secreted from cancer cells has been confirmed to play a role in development of human breast cancer. In the present study, we focused on the role of activation peptide which was in our preliminary studies suggested to be most probably responsible for mitogenic activity of procathepsin D. Using synthetic fragments and antibodies raised against individual fragments, we demonstrated that the growth factor activity of activation peptide is localized in a nine amino acid stretch (AA 36-44) of activation peptide and moreover both anti-activation peptide and anti-27-44 peptide antibodies administered in vivo inhibited the growth of human breast tumors in athymic nude mice. Taking into account our previous results and presented data, we hypothesize that the interaction of procathepsin D activation peptide with an unknown surface receptor is mediated by a sequence 36-44 plus close vicinity. We also propose that this interaction leads in certain types of tumor derived cell lines to proliferation and higher motility. Blocking of the interaction of activation peptide by specific antibodies or antagonists might be a valuable tool in breast cancer inhibition.

Modulation of proliferation and chemosensitivity by procathepsin D and its peptides in ovarian cancer

Since the presence of precursors (pro-forms) of the aspartyl endoprotease, cathepsin D, appears to be linked with tumor progression, their presence was examined in sera and tumor tissues of ovarian cancer patients. The role of cathepsin D pro-forms was further assessed in the dysregulated proliferation and chemoresistance observed in advanced ovarian cancer. Cathepsin D was isolated from sera of ovarian cancer patients (n = 20) and normal volunteers (n = 11), as well as from solubilized normal ovarian epithelium (n = 8) and ovarian epithelial tumor tissue (n = 12). The specific molecular forms of cathepsin D were analyzed in these samples by Western immunoblot. Multiple circulating molecular weight forms of cathepsin D were identified in ovarian cancer patients ranging from 24 to 60 kDa, while in normal controls, a major band was observed at 34 kDa in all samples and minor bands corresponding to 27 and 48 kDa were detected in approximately half of the controls. To assess its consequences on ovarian cancer, the 52-kDa protein was immunoprecipitated from culture medium of an exponentially growing ovarian tumor cell line and was further purified by reverse-phase high-pressure liquid chromatography. Its effect on proliferation was assayed by determining cell doubling times and their chemosensitivity was measured in a standard cytotoxicity assay using cisplatin. In addition, decapeptides corresponding to the pro-portion of cathepsin D were analyzed in parallel. Procathepsin D and one decapeptide, peptide 2, as well as IGF-II (as a known positive) increased cell proliferation, with doubling times of 28.4, 28.8, and 30.3 h, respectively, versus untreated UL-1 cells (36.4 h). Procathepsin D treatment of UL-1 tumor cells significantly increased the cisplatin LD(50) (74.9 microgram/ml) over untreated (33.9 microgram/ml) as well as IGF-II-treated (38.8 microgram/ml) cells. Peptide 2 also showed a significant increase in LD(50) (69.5 microgram/ml) compared to untreated and peptide 1-treated cells (37.1 microgram/ml). There are several unique forms of cathepsin D expressed and accumulated by ovarian tumors and these forms are detectable in the sera of those with ovarian cancer. The presence of these procathepsin D can increase the proliferation of these tumor cells, while decreasing their sensitivity to chemotherapeutic agents. While procathepsin D and IGF-II both enhance proliferation, only procathepsin D (and peptide 2) appears to modulate chemosensitivity, suggesting a separate receptor or pathway for this consequence.

Cathepsin D in cancer metastasis: a protease and a ligand

Cathepsin D (cath-D) overexpression in breast cancer cells is associated with increased risk of metastasis in patients according to several clinical studies. No alterations of pro-cath-D structure or activation have been demonstrated in cancer cells. However, overexpression and dysrouting of pro-cath-D in illegitimate compartments could have consequences on tumor progression. Transfection of a human cDNA cath-D expression vector increases the metastatic potential of 3Y1-Ad12 embryonic rat tumorigenic cells when intravenously injected into nude mice. The mechanism by which cath-D increases the incidence of clinical metastasis seems to involve increased cell growth and decreased contact inhibition rather than escape of cancer cells through the basement membrane. Different mechanisms are discussed by which cath-D could act as a protease following its activation or as a ligand of different membrane receptors at a more neutral pH.

Endocytosis of pro-cathepsin D into breast cancer cells is mostly independent of mannose-6-phosphate receptors

Cathepsin D trafficking is altered in cancer cells, leading to increased secretion of the pro-enzyme, which can be reinternalized by the same cancer cells and by stromal cells. We studied pro-cathepsin D endocytosis in two human breast cancer cell lines (MDA-MB231, MCF-7) and in human normal fibroblasts. Pro-enzyme uptake was studied indirectly through immunofluorescence analysis of anti-pro-cathepsin D monoclonal antibodies internalized in living cells. Both cancer cell lines internalized the pro-cathepsin D-antibody complex into endosomal compartments in the presence of 10 mM mannose-6-phosphate. Non-malignant fibroblasts, which do not secrete pro-cathepsin D, only internalized anti-cathepsin D antibody when purified pro-cathepsin D was added and this endocytosis was totally inhibited by mannose-6-phosphate. Cathepsin D endocytosis in cancer cells was not mediated by lectins or another receptor binding the cathepsin profragment. It was not due to fluid endocytosis, since another protein pS2 secreted by MCF-7 was not endocytosed with its antibody in the same conditions. Double-immunofluorescence and confocal microscopy analyses revealed that antibodies specific to pro-cathepsin D (M2E8) and to the mannose-6-phosphate/IGFII receptor were co-internalized independently in non-permeabilized MDA-MB231 cells and MCF-7 cells, but not in fibroblasts. Moreover, when metabolically labelled pro-cathepsin D secreted by MCF-7 or MDA-MB231 cells was incubated with homologous or heterologous non-radioactive cells, the time-dependent uptake and maturation of the pro-enzyme into fibroblasts were totally inhibited by mannose-6-phosphate, whereas they were not in the two breast cancer cell lines. The percentage of mannose-6-phosphate-independent binding of radioactively labelled pro-cathepsin D to MDA-MB231 cells at 16 degrees C was higher (7–8%) at low pro-cathepsin D concentration than at high concentration (1.5%), indicating the presence of saturable binding site(s) at the cell surface that are different from the mannose-6-phosphate receptors. We conclude that, in contrast to fibroblasts, breast cancer cells can endocytose the secreted pro-cathepsin D by a cell surface receptor that is different from the mannose-6-phosphate receptors or other lectins. The nature of this alternative receptor and its significance in the action of secreted pro-cathepsin D remain to be elucidated.

Effect of procathepsin D and its activation peptide on prostate cancer cells

Cathepsin D, a lysosomal aspartic proteinase, is secreted in the form of enzymatically inactive precursor in some cancer cells. This precursor, called procathepsin D, was found to exhibit growth factor activity toward breast cancer cell lines and this activity was later shown to be mediated by its activation peptide. In the present investigation we have used human procathepsin D and a synthetic 44 amino acid peptide corresponding to the activation peptide of procathepsin D to test its growth factor activity for human prostate cancer-derived cell lines PC3, DU145 and LNCaP. We have tested the level of proliferation of these cell lines depending on the presence of either procathepsin or activation peptide in the medium. In parallel, we have also measured the time dependency of this growth and established the optimal dose of activation peptide. These findings represent the first experimental data showing the direct effects of procathepsin D on prostate cancer cells.