Proteolytic Activity of Prostate-Specific Antigen (PSA) towards Protein Substrates and Effect of Peptides Stimulating PSA Activity

Prostate-specific antigen (PSA or kallikrein-related peptidase-3, KLK3) exerts chymotrypsin-like proteolytic activity. The main biological function of PSA is the liquefaction of the clot formed after ejaculation by cleavage of semenogelins I and II in seminal fluid. PSA also cleaves several other substrates, which may explain its putative functions in prostate cancer and its antiangiogenic activity. We compared the proteolytic efficiency of PSA towards several protein and peptide substrates and studied the effect of peptides stimulating the activity of PSA with these substrates. An endothelial cell tube formation model was used to analyze the effect of PSA-degraded protein fragments on angiogenesis. We showed that PSA degrades semenogelins I and II much more efficiently than other previously identified protein substrates, e.g., fibronectin, galectin-3 and IGFBP-3. We identified nidogen-1 as a new substrate for PSA. Peptides B2 and C4 that stimulate the activity of PSA towards small peptide substrates also enhanced the proteolytic activity of PSA towards protein substrates. Nidogen-1, galectin-3 or their fragments produced by PSA did not have any effect on endothelial cell tube formation. Although PSA cleaves several other protein substrates, in addition to semenogelins, the physiological importance of this activity remains speculative. The PSA levels in prostate are very high, but several other highly active proteases, such as hK2 and trypsin, are also expressed in the prostate and may cleave protein substrates that are weakly cleaved by PSA.     

Tissue kallikrein proteolytic cascade pathways in normal physiology and cancer

Human tissue kallikreins (KLKs or kallikrein-related peptidases) are a subgroup of extracellular serine proteases that act on a wide variety of physiological substrates, while they display aberrant expression patterns in certain types of cancer. Differential expression patterns lead to the exploitation of these proteins as new cancer biomarkers for hormone-dependent malignancies, in particular. The prostate-specific antigen or kallikrein-related peptidase 3 (PSA/KLK3) is an established tumor marker for the diagnosis and monitoring of prostate cancer. It is well documented that specific KLK genes are co-expressed in tissues and in various pathologies suggesting their participation in complex proteolytic cascades. Here, we review the currently established knowledge on the involvement of KLK proteolytic cascades in the regulation of physiological and pathological processes in prostate tissue and in skin. It is well established that the activity of KLKs is often regulated by auto-activation and subsequent autolytic internal cleavage leading to enzymatic inactivation, as well as by inhibitory serpins or by allosteric inhibition by zinc ions. Redistribution of zinc ions and alterations in their concentration due to physiological or pathological reasons activates specific KLKs initiating the kallikrein cascade(s). Recent studies on kallikrein substrate specificity allowed for the construction of a kallikrein interaction network involved in semen liquefaction and prostate cancer, as well as in skin pathologies, such as skin desquamation, psoriasis and cancer. Furthermore, we discuss the crosstalks between known proteolytic pathways and the kallikrein cascades, with emphasis on the activation of plasmin and its implications in prostate cancer. These findings may have clinical implications for the underlying molecular mechanism and management of cancer and other disorders in which KLK activity is elevated.     

Human kallikrein-related peptidase 14 (KLK14) is a new activator component of the KLK proteolytic cascade. Possible function in seminal plasma and skin

Human kallikrein-related peptidases (KLKs) are a family of 15 serine proteases mainly known for their biomarker utility in various neoplastic and non-neoplastic diseases. Despite significant progress in understanding their clinical application, little is known about the activation mechanism(s) of this important family of enzymes. Emerging evidence indicates that KLKs are activated in a stepwise manner, which is a characteristic of proteolytic cascades. Thus far, KLK cascades have been implicated in semen liquefaction and skin desquamation. Many members of the KLK family have been reported to be active in seminal plasma and/or skin, suggesting their involvement in common proteolytic cascades. KLK14, in particular, is highly active and has recently been proposed as one of the key trypsin-like proteases involved in skin desquamation. This study aims to elucidate a probable cascade-mediated role of KLK14 by 1) examining KLK14-mediated cleavage of a heptapeptide library encompassing activation sites of the 15 KLKs and 2) verifying activation of certain candidate downstream targets of KLK14 (i.e. pro-KLK1, -KLK3, and -KLK11). Heptapeptides encompassing activation motifs of KLK2, -3, -5, and -11 were cleaved with a high (> or =85%) cleavage efficiency. Activation of these candidates was confirmed using full-length recombinant proteins. Pro-KLK11, -KLK3, and -KLK1 were rapidly activated in a concentration-dependent manner. Pro-KLK3 regulation was bidirectional because activation was followed by inactivation via internal cleavage of active KLK3. We are proposing a putative cascade model, operating through multiple KLKs. Identification of novel members of such proteolytic cascades will aid in further defining mechanisms involved in seminal/skin homeostasis.     

Activation profiles of human kallikrein-related peptidases by proteases of the thrombostasis axis

The human kallikrein-related peptidases (KLKs) comprise 15 members (KLK1-15) and are the single largest family of serine proteases. The KLKs are utilized, or proposed, as clinically important biomarkers and therapeutic targets of interest in cancer and neurodegenerative disease. All KLKs appear to be secreted as inactive pro-forms (pro-KLKs) that are activated extracellularly by specific proteolytic release of their N-terminal pro-peptide. This processing is a key step in the regulation of KLK function. Much recent work has been devoted to elucidating the potential for activation cascades between members of the KLK family, with physiologically relevant KLK regulatory cascades now described in skin desquamation and semen liquefaction. Despite this expanding knowledge of KLK regulation, details regarding the potential for functional intersection of KLKs with other regulatory proteases are essentially unknown. To elucidate such interaction potential, we have characterized the ability of proteases associated with thrombostasis to hydrolyze the pro-peptide sequences of the KLK family using a previously described pro-KLK fusion protein system. A subset of positive hydrolysis results were subsequently quantified with proteolytic assays using intact recombinant pro-KLK proteins. Pro-KLK6 and 14 can be activated by both plasmin and uPA, with plasmin being the best activator of pro-KLK6 identified to date. Pro-KLK11 and 12 can be activated by a broad-spectrum of thrombostasis proteases, with thrombin exhibiting a high degree of selectivity for pro-KLK12. The results show that proteases of the thrombostasis family can efficiently activate specific pro-KLKs, demonstrating the potential for important regulatory interactions between these two major protease families.     

A potential role for tissue kallikrein-related peptidases in human cervico-vaginal physiology

Human tissue kallikrein-related peptidases (KLK) are a family of 15 genes located on chromosome 19q13.4 that encode secreted serine proteases with trypsin- and/or chymotrypsin-like activity. Relatively large levels of many KLKs are present in human cervico-vaginal fluid (CVF) and in the supernatant of cultured human vaginal epithelial cells. Many KLKs are also hormonally regulated in vaginal epithelial cells, particularly by glucocorticoids and estrogens. The physiological role of KLK in the vagina is currently unknown; however, analysis of the CVF proteome has revealed clues for potential KLK functions in this environment. Here, we detail potential roles for KLKs in cervico-vaginal physiology. First, we suggest that KLKs play a role in the vagina similar to their role in skin physiology: (1) in the desquamation of vaginal epithelial cells, similar to their activity in the desquamation of skin corneocytes; and (2) in their ability to activate antimicrobial proteins in CVF as they do in sweat. Consequently, we hypothesize that dysregulated KLK expression in the vagina could lead to the development of pathological conditions such as desquamative inflammatory vaginitis. Second, we propose that KLKs may play a role in premature rupture of membranes and pre-term birth through their cleavage of fetal membrane extracellular matrix proteins.     

Defining the extended substrate specificity of kallikrein 1-related peptidases

Human kallikrein 1-related peptidases (KLKs) form a subfamily of 15 extracellular (chymo)tryptic-like serine proteases. KLKs 4, 5, 13 and 14 display altered expression/activity in diverse pathological conditions, including cancer. However, their distinct (patho)physiological roles remain largely uncharacterized. As a step toward distinguishing their proteolytic functions, we attempt to define their primary and extended substrate specificities and identify candidate biological targets. Heterologously expressed KLKs 4, 5, 13 and 14 were screened against fluorogenic 7-amino-4-carbamoylmethylcoumarin positional scanning-synthetic combinatorial libraries with amino acid diversity at the P1-P4 positions. Our results indicate that these KLKs share a P1 preference for Arg. However, each KLK exhibited distinct P2-P4 specificities, attributable to structural variations in their surface loops. The preferred P4-P1 substrate recognition motifs based on optimal subsite occupancy were as follows: VI-QSAV-QL-R for KLK4; YFWGPV-RK-NSFAM-R for KLK5; VY-R-LFM-R for KLK13; and YW-KRSAM-HNSPA-R for KLK14. Protein database queries using these motifs yielded many extracellular targets, some of which represent plausible KLK substrates. For instance, cathelicidin, urokinase-type plasminogen activator, laminin and transmembrane protease serine 3 were retrieved as novel putative substrates for KLK4, 5, 13 and 14, respectively. Our findings may facilitate studies on the role of KLKs in (patho)physiology and can be used in the development of selective KLK inhibitors.     

Structures and specificity of the human kallikrein-related peptidases KLK 4, 5, 6, and 7

Human kallikrein-related peptidases (KLKs) are (chymo)-trypsin-like serine proteinases that are expressed in a variety of tissues such as prostate, ovary, breast, testis, brain, and skin. Although their physiological functions have been only partly elucidated, many of the KLKs appear to be useful prognostic cancer markers, showing distinct correlations between their expression levels and different stages of cancer. Recent advances in the purification of 'new type' recombinant KLKs allowed solution of the crystal structures of KLK4, KLK5, KLK6, and KLK7. Along with these data, enzyme kinetic studies and extended substrate specificity profiling have led to an understanding of the non-prime-side substrate preferences of KLK4, 5, 6, and 7. The shape and polarity of the specificity pockets S1-S4 explain well their substrate preferences. KLK4, 5, and 6 exhibit trypsin-like specificity, with a strong preference for Arg at the P1 position of substrates. In contrast, KLK7 displays a unique chymotrypsin-like specificity for Tyr, which is also preferred at P2. All four KLKs show little specificity for P3 residues and have a tendency to accept hydrophobic residues at P4. Interestingly, for KLK4, 5, and 7 extended charged surface regions were observed that most likely serve as exosites for physiological substrates.     

Inhibition of kallikrein-related peptidases by the serine protease inhibitor of Kazal-type 6

Kallikrein-related peptidases (KLKs) are a group of serine proteases, expressed in several tissues. Their activity is regulated by inhibitors including members of the serine protease of Kazal-type (SPINK) family. Recently, we discovered that SPINK6 is expressed in human skin and inhibits KLK5, KLK7, KLK14 but not KLK8. In this study we tested whether SPINK6 inhibits other members of the KLK family and caspase-14. Using chromogenic substrates, SPINK6 exhibited inhibitory activity against KLK12 and KLK13 with K_i around 1 nM, KLK4 with K_i = 27.3 nM, KLK6 with K_i = 140 nM, caspase-14 with a K_i approximating 1 μM and no activity against KLK1, KLK3 and KLK11. Taken together, SPINK6 is a potent inhibitor of distinct KLKs members.     

Evolutionary history of tissue kallikreins

The gene family of human kallikrein-related peptidases (KLKs) encodes proteins with diverse and pleiotropic functions in normal physiology as well as in disease states. Currently, the most widely known KLK is KLK3 or prostate-specific antigen (PSA) that has applications in clinical diagnosis and monitoring of prostate cancer. The KLK gene family encompasses the largest contiguous cluster of serine proteases in humans which is not interrupted by non-KLK genes. This exceptional and unique characteristic of KLKs makes them ideal for evolutionary studies aiming to infer the direction and timing of gene duplication events. Previous studies on the evolution of KLKs were restricted to mammals and the emergence of KLKs was suggested about 150 million years ago (mya). In order to elucidate the evolutionary history of KLKs, we performed comprehensive phylogenetic analyses of KLK homologous proteins in multiple genomes including those that have been completed recently. Interestingly, we were able to identify novel reptilian, avian and amphibian KLK members which allowed us to trace the emergence of KLKs 330 mya. We suggest that a series of duplication and mutation events gave rise to the KLK gene family. The prominent feature of the KLK family is that it consists of tandemly and uninterruptedly arrayed genes in all species under investigation. The chromosomal co-localization in a single cluster distinguishes KLKs from trypsin and other trypsin-like proteases which are spread in different genetic loci. All the defining features of the KLKs were further found to be conserved in the novel KLK protein sequences. The study of this unique family will further assist in selecting new model organisms for functional studies of proteolytic pathways involving KLKs.     

Isolation of SPINK6 in Human Skin: SELECTIVE INHIBITOR OF KALLIKREIN-RELATED PEPTIDASES*

Kallikrein-related peptidases (KLKs) play a central role in skin desquamation. They are tightly controlled by specific inhibitors, including the lymphoepithelial Kazal-type inhibitor (LEKTI) encoded by SPINK5 and LEKTI-2 encoded by SPINK9. Herein, we identify SPINK6 as a selective inhibitor of KLKs in the skin. Unlike LEKTI but similar to LEKTI-2, SPINK6 possesses only one typical Kazal domain. Its mRNA was detected to be expressed at low levels in several tissues and was induced during keratinocyte differentiation. Natural SPINK6 was purified from human plantar stratum corneum extracts. Immunohistochemical analyses revealed SPINK6 expression in the stratum granulosum of human skin at various anatomical localizations and in the skin appendages, including sebaceous glands and sweat glands. SPINK6 expression was decreased in lesions of atopic dermatitis. Using KLK5, KLK7, KLK8, KLK14, thrombin, trypsin, plasmin, matriptase, prostasin, mast cell chymase, cathepsin G, neutrophil elastase, and chymotrypsin, inhibition with recombinant SPINK6 was detected only for KLK5, KLK7, and KLK14, with apparent K_i values of 1.33, 1070, and 0.5 nm, respectively. SPINK6 inhibited desquamation of human plantar callus in an ex vivo model. Our findings suggest that SPINK6 plays a role in modulating the activity of KLKs in human skin. A selective inhibition of KLKs by SPINK6 might have therapeutic potential when KLK activity is elevated.     

Activation profiles and regulatory cascades of the human kallikrein-related peptidases

The human kallikrein (KLK)-related peptidases are the largest family of serine peptidases, comprising 15 members (KLK1-15) and with the majority (KLK4-15) being identified only within the last decade. Members of this family are associated with important diseased states (including cancer, inflammation, and neurodegeneration) and have been utilized or proposed as clinically important biomarkers or therapeutic targets of interest. All human KLKs are synthesized as prepro-forms that are proteolytically processed to secreted pro-forms via the removal of an amino-terminal secretion signal peptide. The secreted inactive pro-KLKs are then activated extracellularly to mature peptidases by specific proteolytic release of their amino-terminal propeptide. Although a key step in the regulation of KLK function, details regarding the activation of the human pro-KLKs (i.e. the KLK "activome") are unknown, to a significant extent, but have been postulated to involve "activation cascades" with other KLKs and endopeptidases. To characterize more completely the KLK activome, we have expressed from Escherichia coli individual KLK propeptides fused to the amino terminus of a soluble carrier protein. The ability of 12 different mature KLKs to process the 15 different pro-KLK peptide sequences has been determined. Various autolytic and cross-activation relationships identified using this system have subsequently been characterized using recombinant pro-KLK proteins. The results demonstrate the potential for extensive KLK activation cascades and, when combined with available data for the tissue-specific expression of the KLK family, permit the construction of specific regulatory cascades. One such tissue-specific cascade is proposed for the central nervous system.     

Peptides released by physiological cleavage of semen coagulum proteins form amyloids that enhance HIV infection

Semen serves as a vehicle for HIV and promotes sexual transmission of the virus, which accounts for the majority of new HIV cases. The major component of semen is the coagulum, a viscous structure composed predominantly of spermatozoa and semenogelin proteins. Due to the activity of the semen protease PSA, the coagulum is liquefied and semenogelins are cleaved into smaller fragments. Here, we report that a subset of these semenogelin fragments form amyloid fibrils that greatly enhance HIV infection. Like SEVI, another amyloid fibril previously identified in semen, the semenogelin fibrils exhibit a cationic surface and enhance HIV virion attachment and entry. Whereas semen samples from healthy individuals greatly enhance HIV infection, semenogelin-deficient semen samples from patients with ejaculatory duct obstruction are completely deficient in enhancing activity. Semen thus harbors distinct amyloidogenic peptides derived from different precursor proteins that commonly enhance HIV infection and likely contribute to HIV transmission.     

Role of tissue kallikrein-related peptidases in cervical mucus remodeling and host defense

Human tissue kallikrein-related peptidases (KLKs) are 15 hormonally regulated genes on chromosome 19q13.4 encoding secreted serine proteases. Many KLKs are expressed throughout the female reproductive system and found in cervico-vaginal fluid (CVF). Immunohistochemistry was performed to determine KLK localization in the female reproductive system (fallopian tube, endometrium, cervix and vagina tissues). KLK levels were measured in CVF and saliva over the menstrual cycle to study whether KLKs are regulated by hormonal changes during the cycle. In vitro cleavage analysis was performed to establish whether KLKs may play a role in vaginal epithelial desquamation, mucus remodeling or processing of antimicrobial proteins. KLKs were localized in the glandular epithelium of the fallopian tubes and endometrium, the cervical mucus-secreting epithelium and vaginal stratified squamous epithelium. KLK levels peaked in CVF and saliva after ovulation. In vitro cleavage analysis confirmed KLKs 5 and 12 as capable of digesting desmoglein and desmocollin adhesion proteins and cervical mucin proteins 4 and 5B. KLK5 can digest defensin-1alpha, suggesting it may aid in cervico-vaginal host defense. We provide evidence of potential physiological roles for KLKs in cervico-vaginal physiology: in desquamation of vaginal epithelial cells, remodeling of cervical mucus and processing of antimicrobial proteins.     

Novel Biological Substrates of Human Kallikrein 7 Identified through Degradomics

Kallikrein-related peptidases (KLKs) are a group of serine proteases widely expressed in various tissues and involved in a wide range of physiological and pathological processes. Although our understanding of the pathophysiological roles of most KLKs has blossomed in recent years, identification of the direct endogenous substrates of human KLKs remains an unmet objective. In this study we employed a degradomics approach to systemically investigate the endogenous substrates of KLK7 in an effort to understand the molecular pathways underlying KLK7 action in skin. We identified several previously known as well as novel protein substrates. Our most promising candidates were further validated with the use of targeted quantitative proteomics (selected reaction monitoring methods) and in vitro recombinant protein digestion assays. Our study revealed midkine, CYR61, and tenascin-C as endogenous substrates for KLK7. Interestingly, some of these substrates (e.g. midkine) were prone to proteolytic cleavage only by KLK7 (and not by other skin-associated KLKs), whereas others (e.g. CYR61 and tenascin-C) could be digested by several KLKs. Furthermore, using melanoma cell line, we show that KLK7-mediated cleavage of midkine results in an overall reduction in the pro-proliferative and pro-migratory effect of midkine. An inverse relation between KLK7 and midkine is also observed in human melanoma tissues. In summary, our degradomics approach revealed three novel endogenous substrates for KLK7, which may shed more light on the pathobiological roles of KLK7 in human skin. Similar substrate screening approaches could be applied for the discovery of biological substrates of other protease.     

Human tissue kallikreins as promiscuous modulators of homeostatic skin barrier functions

Human tissue kallikreins (KLKs) are the largest family of secreted serine protease endopeptidases encoded by 15 genes clustered on chromosome 19q13.4. Multiple KLK enzymes are co-localized in the upper stratum granulosum and stratum corneum of human epidermis, and in associated appendages such as hair follicle epithelia and sweat glands. Until recently, kallikrein proteolytic activity in the skin was exclusively attributed to KLK5 and KLK7. However, wider cutaneous roles of kallikreins became evident in recent years as the proposal of KLK proteolytic activation cascades emerged. We postulate that these proteolytic enzymes may serve as promiscuous mediators of different skin barrier functions, since they are capable of proteolysing different substrates that govern skin desquamation, antimicrobial defense, and lipid permeability. Growing evidence now attests to potential kallikrein involvement in skin inflammation, pigmentation, and tumor suppression via their ability to target proteinase-activated receptor signaling pathways. Current knowledge on kallikrein roles in skin physiology and pathobiology is described in this review.     

Crystal structure of a ternary complex between human prostate-specific antigen, its substrate acyl intermediate and an activating antibody

Human prostate-specific antigen (PSA or KLK3) is an important marker for the diagnosis and management of prostate cancer. This is an androgen-regulated glycoprotein of the kallikrein-related protease family secreted by prostatic epithelial cells. Its physiological function is to cleave semenogelins in the seminal coagulum and its enzymatic activity is strongly modulated by zinc ions.Here we present the first crystal structure of human PSA in complex with monoclonal antibody (mAb) 8G8F5 that enhances its enzymatic activity. The mAb recognizes an epitope composed of five discontinuous segments including residues from the kallikrein loop and stabilizes PSA in an “open and active conformation” that accelerates catalysis.We also present the crystal structure of PSA in complex with both the mAb 8G8F5 and a fluorogenic substrate Mu-KGISSQY-AFC, derived from semenogelin I. By exploiting the inhibition of PSA by zinc ions, we were able to obtain a substrate acyl intermediate covalently linked to the catalytic serine, at pH 7.3 but not at pH 5.5.Moreover, the inhibition of PSA activity by zinc was found to be modulated by pH variations but not by the antibody binding. The correlation of the different data with the physiological conditions under which PSA can cleave semenogelins is discussed.     

The kallikrein 14 gene is down-regulated by androgen receptor signalling and harbours genetic variation that is associated with prostate tumour aggressiveness

Kallikrein 14 (KLK14) has been proposed as a useful prognostic marker in prostate cancer, with expression reported to be associated with tumour characteristics such as higher stage and Gleason score. KLK14 tumour expression has also shown the potential to predict prostate cancer patients at risk of disease recurrence after radical prostatectomy. The KLKs are a remarkably hormone-responsive family of genes, although detailed studies of androgen regulation of KLK14 in prostate cancer have not been undertaken to date. Using in vitro studies, we have demonstrated that unlike many other prostatic KLK genes that are strictly androgen responsive, KLK14 is more broadly expressed and inversely androgen regulated in prostate cancer cells. Given these results and evidence that KLK14 may play a role in prostate cancer prognosis, we also investigated whether common genetic variants in the KLK14 locus are associated with risk and/or aggressiveness of prostate cancer in approximately 1200 prostate cancer cases and 1300 male controls. Of 41 single nucleotide polymorphisms assessed, three were associated with higher Gleason score (≥7): rs17728459 and rs4802765, both located upstream of KLK14, and rs35287116, which encodes a p.Gln33Arg substitution in the KLK14 signal peptide region. Our findings provide further support for KLK14 as a marker of prognosis in prostate cancer.     

A potential role for multiple tissue kallikrein serine proteases in epidermal desquamation

Desquamation of the stratum corneum is a serine protease-dependent process. Two members of the human tissue kallikrein (KLK) family of (chymo)tryptic-like serine proteases, KLK5 and KLK7, are implicated in desquamation by digestion of (corneo)desmosomes and inhibition by desquamation-related serine protease inhibitors (SPIs). However, the epidermal localization and specificity of additional KLKs also supports a role for these enzymes in desquamation. This study aims to delineate the probable contribution of KLK1, KLK5, KLK6, KLK13, and KLK14 to desquamation by examining their interactions, in vitro, with: 1) colocalized SPI, lympho-epithelial Kazal-type-related inhibitor (LEKTI, four recombinant fragments containing inhibitory domains 1-6 (rLEKTI(1-6)), domains 6-8 and partial domain 9 (rLEKTI(6-9')), domains 9-12 (rLEKTI(9-12)), and domains 12-15 (rLEKTI(12-15)), secretory leukocyte protease inhibitor, and elafin and 2) their ability to digest the (corneo)desmosomal cadherin, desmoglein 1. KLK1 was not inhibited by any SPI tested. KLK5, KLK6, KLK13, and KLK14 were potently inhibited by rLEKTI(1-6), rLEKTI(6-9'), and rLEKTI(9-12) with Ki values in the range of 2.3-28.4 nm, 6.1-221 nm, and 2.7-416 nm for each respective fragment. Only KLK5 was inhibited by rLEKTI(12-15) (Ki = 21.8 nm). No KLK was inhibited by secretory leukocyte protease inhibitor or elafin. Apart from KLK13, all KLKs digested the ectodomain of desmoglein 1 within cadherin repeats, Ca2+ binding sites, or in the juxtamembrane region. Our study indicates that multiple KLKs may participate in desquamation through cleavage of desmoglein 1 and regulation by LEKTI. These findings may have clinical implications for the treatment of skin disorders in which KLK activity is elevated.     

Functional Roles of Human Kallikrein-related Peptidases

Kallikrein-related peptidases constitute a single family of 15 (chymo)trypsin-like proteases (KLK1–15) with pleiotropic physiological roles. Aberrant regulation of KLKs has been associated with diverse diseases such as hypertension, renal dysfunction, skin disorders, inflammation, neurodegeneration, and cancer. Recent studies suggested that coordinated activation and regulation of KLK activity are achieved via a complex network of interactions referred to as the “KLK activome.” However, it remains to be validated whether these hypothetical KLK activation cascade pathways are operative in vivo. In addition, KLKs have emerged as versatile signaling molecules. In summary, KLKs represent attractive biomarkers for clinical applications and potential therapeutic targets for common human pathologies.     

KLKs促进肿瘤增殖、迁移与侵袭的分子机制

组织激肽释放酶家族（kallikrein-related peptidases,KLKs）是一类具有胰蛋白酶或胰凝乳蛋白酶活性的分泌型丝氨酸蛋白酶,由15个成员（KLK1～15）组成。虽然发现较早,但关于其在肿瘤的发生与发展中所扮演的角色研究相对较少。KLKs作为肿瘤标志物在临床上的应用也有相应报道,如KLK3(PSA)作为前列腺特异性抗原用于前列腺癌的筛查。近年研究发现,KLKs家族在肿瘤进程中发挥重要作用,被认为是调节肿瘤细胞增殖、迁移和侵袭的关键因素。本文概述了在肿瘤进展过程中KLKs作用机制的最新进展,为其作为抗肿瘤治疗靶点的研究提供理论基础。