ReviewPathophysiological functions of cathepsin D: Targeting its catalytic activity versus its protein binding activity?
Introduction
Proteases irreversibly hydrolyze the peptide bond in proteins, which results in an important and irreversible post-translational modification. The human genome encodes over 569 proteolytic enzymes or homologs, which constitute the second largest class of human enzymes. Proteases are assigned to five classes on the basis of the active site residue that executes the nucleophilic attack on the target peptide bond: aspartic, cysteine, serine, threonine and metalloproteinases. These proteases are implicated in normal physiological processes, but deregulation of their expression and/or enzyme activity in disorders such as cancer has profound consequences.
Different families of proteases have been implicated in motility, invasion, extravasation, proliferation and metastasis: the serine proteases (uPA, uPAR and PAI1) [1], the metalloproteinase family [2], the cysteine cathepsins (cathepsin-B and cathepsin-L) [3], and the aspartic cathepsin D (cath-D) [4], respectively. Cathepsin proteases are lysosomal hydrolases that degrade proteins at acidic pH in the lysosomes, or extracellularly in the matrix. Cathepsins can be divided into three subgroups, based on their active site amino acid (i.e., cysteine (B, C, H, F, K, L, O, S, V, X, and W), aspartate (D and E), or serine (A and G) cathepsins). The possible involvement of cysteine and aspartic cathepsins in cancer has been the subject of more debate than that of metalloproteases and serine proteases. This might result from the assumption that only secreted proteases that are proteolytically active at neutral pH play an active role in cancer, whereas cathepsins, which require a more acidic pH to be proteolytically active, are thought to be likely to play only a minor role. However, it has been demonstrated that cathepsins are hyper-secreted in cancer, and cath-B and cath-D have been described as being associated with the cell surface [5], [6]. Recent studies using transgenic mouse models have stimulated fresh interest in the fundamental roles of cathepsins in cancer [7], [8], [9], [10]. Although historically studies have tended to focus on the role of lysosomal proteases within the endocytic and lysosomal compartments, recent discoveries have shown that these proteases play a critical role in other intracellular compartments, such as the cytosol [11] or the nucleus [12], and within the extracellular milieu in the tumoral stroma [13]. It has become clear that their pattern of expression and their substrate specificities are more complex than was originally envisaged. In cancer, lysosomal proteases are overexpressed and their cellular localizations are profoundly altered, leading to major changes in their targets and consequently in their biological activities.
In addition, cathepsins, metalloproteases and serine proteases act in a cascade-like manner and as part of a proteolytic pathway rather than simply functioning individually [14]. Elucidating the cascade of enzymatic activities that contribute to overall proteolysis during carcinogenesis may identify rate-limiting steps or pathways that could be targeted by anti-cancer treatments [14]. The proteolytic cascade of activation of the different classes of proteases strongly suggests that anti-cancer strategies intended to target several classes of proteases simultaneously might be more promising than those that target a single protease or class of proteases.
Recent studies have focused on extracellular proteases as primary targets for drug discovery, because of their differential expression in many pathophysiological processes, including cancer, cardiovascular conditions, and inflammatory, pulmonary, and periodontal diseases [3], [15]. Interestingly, new extracellular inhibitors of metalloproteases, serine proteases and cysteine proteases are currently under clinical investigation [15]. The aim of this review is to present the role of the cath-D protease in cancer progression and metastasis, as well as its function in apoptosis, and to discuss how it could be targeted in cancer by inhibiting its proteolytic activity and/or its protein binding activity.
Section snippets
Structure and trafficking of cath-D
Cath-D [E.C. 3.4.23.5] is a ubiquitous, lysosomal, aspartic endo-proteinase that requires an acidic pH to be proteolytically active (Fig. 1). The human cath-D gene contains 9 exons, and is located on chromosome 11p15 [16]. During its transportation to lysosomes, the 52-kDa human pro-cath-D is proteolytically processed to form a 48-kDa, single-chain, intermediate which is an active enzyme located in the endosomes. Further proteolytic processing yields the mature active lysosomal protease, which
The tight control of cath-D expression and catalytic function is fundamental in normal cells
During fetal development, the level of cath-D increases gradually in all tissues, suggesting a gradual maturation of the lysosomal system [34]. A reduction of cath-D expression or catalytic activity leads to devastating neurodegenerative disorders. Cath-D knockout mice die shortly after birth, and display a neuronal accumulation of ceroid lipofuscin, accompanied by neurodegeneration in the retina and central nervous system, and the accumulation of autophagic vacuoles [35], [36], [37], [38].
Function of cath-D in apoptosis
Cysteine and aspartic cathepsins play key roles in tumor cell death via the mediation of apoptosis [4], [11], [49], [50], [51]. The function of cath-D in apoptosis needs further investigation, since this protease has both anti-apoptotic and pro-apoptotic functions.
Cath-D is an independent marker of a poor prognosis in breast cancer
In the 1990s, several independent clinical studies showed that the cath-D level in primary breast cancer cytosols is an independent prognostic parameter correlated with the incidence of clinical metastasis and shorter survival times [76]. A meta-analysis of studies on node-negative breast cancer [77], as well as a complete study of 2810 patients in Rotterdam [78], indicate that high concentrations of cath-D are an effective marker of aggressiveness. Cath-D is now recognized as an independent
Targeting cath-D in cancer
Cathepsins have long been known to play an important role in the progression and metastasis of cancer. Cath-D stimulates cancer cell proliferation, fibroblast outgrowth, tumor angiogenesis, and metastasis. In cancer cells, overexpressed cath-D accumulates in cells where it may affect their degradation capacities, and the pro-enzyme is hyper-secreted in the tumor micro-environment (Fig. 2). Therefore, inhibiting cath-D action requires the development of inhibitors targeting extracellular cath-D,
Conclusion
Cath-D is a key protease that affects many fundamental functions in cells. The molecular mechanism by which cath-D affects cancer progression remains largely unknown. Furthermore, we still do not have any specific cath-D inhibitors that could be used to target its action in cancer. Since this protease may also act by protein–protein interaction, it will be crucial to identify its partners in order to develop inhibitors to block its protein binding function.
Acknowledgement
Grant sponsors ‘ANR Jeunes chercheurs Jeunes chercheuses’ ANR-05-JCJC-0215-01, and EU FP6; Grant number: LSHC-CT-2007-037665.
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