Identification of five molecular forms of cathepsin D in bovine milk

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.

Cathepsin D non-proteolytically induces proliferation and migration in human omental microvascular endothelial cells via activation of the ERK1/2 and PI3K/AKT pathways

Epithelial ovarian cancer (EOC) frequently metastasises to the omentum, a process that requires pro-angiogenic activation of human omental microvascular endothelial cells (HOMECs) by tumour-secreted factors. We have previously shown that ovarian cancer cells secrete a range of factors that induce pro-angiogenic responses e.g. migration, in HOMECs including the lysosomal protease cathepsin D (CathD). However, the cellular mechanism by which CathD induces these cellular responses is not understood. The aim of this study was to further examine the pro-angiogenic effects of CathD in HOMECs i.e. proliferation and migration, to investigate whether these effects are dependent on CathD catalytic activity and to delineate the intracellular signalling kinases activated by CathD. We report, for the first time, that CathD significantly increases HOMEC proliferation and migration via a non-proteolytic mechanism resulting in activation of ERK1/2 and AKT. These data suggest that EOC cancer secreted CathD acts as an extracellular ligand and may play an important pro-angiogenic, and thus pro-metastatic, role by activating the omental microvasculature during EOC metastasis to the omentum.

Proteolytic Systems in Milk: Perspectives on the Evolutionary Function within the Mammary Gland and the Infant

Milk contains elements of numerous proteolytic systems (zymogens, active proteases, protease inhibitors and protease activators) produced in part from blood, in part by mammary epithelial cells and in part by immune cell secretion. Researchers have examined milk proteases for decades, as they can cause major defects in milk quality and cheese production. Most previous research has examined these proteases with the aim to eliminate or control their actions. However, our recent peptidomics research demonstrates that these milk proteases produce specific peptides in healthy milk and continue to function within the infant's gastrointestinal tract. These findings suggest that milk proteases have an evolutionary function in aiding the infant's digestion or releasing functional peptides. In other words, the mother provides the infant with not only dietary proteins but also the means to digest them. However, proteolysis in the milk is controlled by a balance of protease inhibitors and protease activators so that only a small portion of milk proteins are digested within the mammary gland. This regulation presents a question: If proteolysis is beneficial to the infant, what benefits are gained by preventing complete proteolysis through the presence of protease inhibitors? In addition to summarizing what is known about milk proteolytic systems, we explore possible evolutionary explanations for this proteolytic balance.

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.

Yeast as a tool to explore cathepsin D function

Cathepsin D has garnered increased attention in recent years, mainly since it has been associated with several human pathologies. In particular, cathepsin D is often overexpressed and hypersecreted in cancer cells, implying it may constitute a therapeutic target. However, cathepsin D can have both anti- and pro-survival functions depending on its proteolytic activity, cellular context and stress stimulus. Therefore, a more detailed understanding of cathepsin D regulation and how to modulate its apoptotic functions is clearly needed. In this review, we provide an overview of the role of cathepsin D in physiological and pathological scenarios. We then focus on the opposing functions of cathepsin D in apoptosis, particularly relevant in cancer research. Emphasis is given to the role of the yeast protease Pep4p, the vacuolar counterpart of cathepsin D, in life and death. Finally, we discuss how insights from yeast cathepsin D and its role in regulated cell death can unveil novel functions of mammalian cathepsin D in apoptosis and cancer.

Predicting the important enzymes in human breast milk digestion

Human milk is known to contain several proteases, but little is known about whether these enzymes are active, which proteins they cleave, and their relative contribution to milk protein digestion in vivo. This study analyzed the mass spectrometry-identified protein fragments found in pooled human milk by comparing their cleavage sites with the enzyme specificity patterns of an array of enzymes. The results indicate that several enzymes are actively taking part in the digestion of human milk proteins within the mammary gland, including plasmin and/or trypsin, elastase, cathepsin D, pepsin, chymotrypsin, a glutamyl endopeptidase-like enzyme, and proline endopeptidase. Two proteins were most affected by enzyme hydrolysis: β-casein and polymeric immunoglobulin receptor. In contrast, other highly abundant milk proteins such as α-lactalbumin and lactoferrin appear to have undergone no proteolytic cleavage. A peptide sequence containing a known antimicrobial peptide is released in breast milk by elastase and cathepsin D.

Role of somatic cells on dairy processes and products: a review

Somatic cells are an important component naturally present in milk, and somatic cell count is used as an indicator of udder health and milk quality. The role of somatic cells in dairy processes and products is ill-defined in most studies because the role of these cells combines also the concomitance of physicochemical modifications of milk, bacterial count, and the udder inflammation in the presence of high somatic cell count. The aim of this review is to focus on the role of somatic cells themselves and of endogenous enzymes from somatic cells in milk, in dairy transformation processes, and in characteristics of final products overcoming biases due to other factors. The immune function of somatic cells in the udder defense and their protective role in milk will be primarily considered. Different characteristics of milk induced by various somatic cell counts, types, and their endogenous enzymes influencing directly the technological properties of milk and the final quality of dairy products will be discussed as well. By comparing methods used in other studies and eliminating biases due to other factors not considered in these studies, a new approach has been suggested to evaluate the effective role of somatic cells on dairy processes and products. In addition, this new approach allows the characterization of somatic cells and their endogenous enzymes and, in future research, will allow the clarification of mechanisms involved in the release of these components from somatic cells during dairy processes, particularly in cheese technologies.

Procathepsin D and cancer: From molecular biology to clinical applications

Procathepsin D (pCD) is overexpressed and secreted by cells of various tumor types including breast and lung carcinomas. pCD affects multiple features of tumor cells including proliferation, invasion, metastases and apoptosis. Several laboratories have previously shown that the mitogenic effect of pCD on cancer cells is mediated via its propeptide part (APpCD). However, the exact mechanism of how pCD affects cancer cells has not been identified. Recent observations have also revealed the possible use of pCD/APpcD as a marker of cancer progression. The purpose of this review is to summarize the three major potentials of pCD-tumor marker, potential drug, and screening agent.

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.

Proteases Involved in Mammary Tissue Damage During Endotoxin-Induced Mastitis in Dairy Cows

During and after diapedesis, milk polymorphonu-clear neutrophils (PMN) release many proteases that have the potential of degrading extracellular matrix proteins and milk proteins. However, the kinetics of milk proteolysis during inflammation and the underlying mechanisms are poorly defined. The enzymes involved in bovine mammary tissue destruction were investigated in this study using an endotoxin-induced mastitis model. Using zymography techniques, the proteolytic activity of milk and mammary tissue during mastitis was examined. Mastitic milk produced 6 caseolysis bands, 4 of which differed from the ones produced by plasmin. Peak proteolytic activity, bovine serum albumin contents, and mammary tissue damage occurred between 6 and 12 h postchallenge. Mastitic milk proteases hydrolyzed casein, gelatin, collagen, hemoglobin, mammary gland membrane proteins, and lactoferrin. These results confirm that mastitic milk proteases have a broad spectrum of activity. The hydrolytic activity of mastitic milk was partially inhibited by aprotinin, EDTA, 1,10-phenanthroline, leupeptin, and pefabloc. When cocultured with normal mammary tissue, mastitic milk, but not normal milk, caused mammary tissue degradation. In situ zymography of mammary gland showed increased proteolytic activity in mastitic tissue compared with normal tissue. The similarity of zymograms of mastitic milk, blood PMN, milk somatic cells, and PMN strongly suggests that proteases in mastitic milk mainly originate from milk PMN. These results suggest that proteases released by PMN are actively involved in udder tissue damage during mastitis.

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.

Aspartic proteinase members secreted by the ruminant placenta: specificity of three radioimmunoassay systems for the measurement of pregnancy-associated glycoproteins

Pregnancy-associated glycoproteins (PAGs) isolated from the placenta of various ruminant species are enzymatically inactive members of the aspartic proteinase family. The measurement of these proteins in the maternal blood can be a good indicator of the presence of a live embryo. As certain aspartic proteinases are present in biological fluids in physiological and pathological conditions at various concentrations, it was necessary to determine the specificity of three radioimmunoassay (RIA) systems currently used for the detection of PAG molecules. Commercially available members of the aspartic proteinase family like pepsinogen, pepsin, chymosin, rennet, cathepsin D and renin were tested in a wide concentration range (10 ng/ml - 1 mg/ml). Pepsinogen cross-reacted in RIA 1, RIA 2 and RIA 3 over 1 mg/ml, 50 microg/ml and 500 microg/ml concentrations, respectively. In the presence of pepsin, cross-reaction was observed in RIA 1, RIA 2 and RIA 3 over 1 mg/ml, 500 microg/ml and 1 mg/ml concentrations, respectively. Chymosin and rennet could cross-react in RIA 2 and RIA 3, while renin and cathepsin D did not decrease the binding of the tracer to antisera more, than that of the minimal detection limit. As the plasma/serum concentrations of the examined aspartic proteinases reported in the literature were outside the concentration range where cross-reaction was observed, it can be concluded that these RIA systems were specific for the detection of PAGs in biological fluids.

Peptide transport in the mammary gland: expression and distribution of PEPT2 mRNA and protein

The lactating mammary gland utilizes free plasma amino acids as well as those derived by hydrolysis from circulating short-chain peptides for protein synthesis. Apart from the major route of amino acid nitrogen delivery to the gland by the various transporters for free amino acids, it has been suggested that dipeptides may also be taken up in intact form to serve as a source of amino acids. The identification of peptide transporters in the mammary gland may therefore provide new insights into protein metabolism and secretion by the gland. The expression and distribution of the high-affinity type proton-coupled peptide transporter PEPT2 were investigated in rat lactating mammary gland as well as in human epithelial cells derived from breast milk. By use of RT-PCR, PEPT2 mRNA was detected in rat mammary gland extracts and human milk epithelial cells. The expression pattern of PEPT2 mRNA revealed a localization in epithelial cells of ducts and glands by nonisotopic high resolution in situ hybridization. In addition, immunohistochemistry was carried out and showed transporter immunoreactivity in the same epithelial cells of the glands and ducts. In addition, two-electrode voltage clamp recordings using PEPT2-expressing Xenopus laevis oocytes demonstrated positive inward currents induced by selected dipeptides that may play a role in aminonitrogen handling in mammalian mammary gland. Taken together, these data suggest that PEPT2 is expressed in mammary gland epithelia, in which it may contribute to the reuptake of short-chain peptides derived from hydrolysis of milk proteins secreted into the lumen. Whereas PEPT2 also transports a variety of drugs, such as selected beta-lactams, angiotensin-converting enzyme inhibitors, and antiviral and anticancer metabolites, their efficient reabsorption via PEPT2 may reduce the burden of xenobiotics in milk.