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.     

Major role of human KLK14 in seminal clot liquefaction

Liquefaction of human semen involves proteolytic degradation of the seminal coagulum and release of motile spermatozoa. Several members of human kallikrein-related peptidases (KLKs) have been implicated in semen liquefaction, functioning through highly regulated proteolytic cascades. Among these, KLK3 (also known as prostate-specific antigen) is the main executor enzyme responsible for processing of the primary components of semen coagulum, semenogelins I and II. We have recently identified KLK14 as a potential activator of KLK3 and other KLKs. This study aims to elucidate the cascade-mediated role of KLK14 ex vivo. KLK14 expression was significantly lower (p = 0.0252) in individuals with clinically delayed liquefaction. Concordantly, KLK14 expression was significantly (p = 0.0478) lower in asthenospermic cases. Specific inhibition of KLK14 activity by the synthetic inhibitor ACT(G9) resulted in a significant delay in semen liquefaction, a drop in the "early" (30 min postejaculation) "chymotrypsin-like" and KLK1 activity, and an increase in the "late" (90 min postejaculation) chymotrypsin-like activity. Conversely, the addition of recombinant active KLK14 facilitated the liquefaction process, augmented the early chymotrypsin-like activity, and lowered late chymotrypsin-like activity. Given that the observed chymotrypsin-like activity was almost completely attributed to KLK3 activity, KLK3 seems to be regulated bidirectionally. Accordingly, a higher level of KLK3 fragmentation was observed in KLK14-induced coagula, suggesting an inactivation mechanism via internal cleavage. Finally, semenogelins I and II were directly cleaved by KLK14. Semenogelins were also able to reverse KLK14 inhibition by Zn2+, providing a novel regulatory mechanism for KLK14 activity. Our results show that KLK14 exerts a significant and dose-dependent effect in the process of semen liquefaction.     

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.     

LEKTI fragments specifically inhibit KLK5, KLK7, and KLK14 and control desquamation through a pH-dependent interaction

LEKTI is a 15-domain serine proteinase inhibitor whose defective expression underlies the severe autosomal recessive ichthyosiform skin disease, Netherton syndrome. Here, we show that LEKTI is produced as a precursor rapidly cleaved by furin, generating a variety of single or multidomain LEKTI fragments secreted in cultured keratinocytes and in the epidermis. The identity of these biological fragments (D1, D5, D6, D8-D11, and D9-D15) was inferred from biochemical analysis, using a panel of LEKTI antibodies. The functional inhibitory capacity of each fragment was tested on a panel of serine proteases. All LEKTI fragments, except D1, showed specific and differential inhibition of human kallikreins 5, 7, and 14. The strongest inhibition was observed with D8-D11, toward KLK5. Kinetics analysis revealed that this interaction is rapid and irreversible, reflecting an extremely tight binding complex. We demonstrated that pH variations govern this interaction, leading to the release of active KLK5 from the complex at acidic pH. These results identify KLK5, a key actor of the desquamation process, as the major target of LEKTI. They disclose a new mechanism of skin homeostasis by which the epidermal pH gradient allows precisely regulated KLK5 activity and corneodesmosomal cleavage in the most superficial layers of the stratum corneum.     

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.     

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.     

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.     

Identification of Lympho-Epithelial Kazal-Type Inhibitor 2 in Human Skin as a Kallikrein-Related Peptidase 5-Specific Protease Inhibitor

Kallikreins-related peptidases (KLKs) are serine proteases and have been implicated in the desquamation process of the skin. Their activity is tightly controlled by epidermal protease inhibitors like the lympho-epithelial Kazal-type inhibitor (LEKTI). Defects of the LEKTI-encoding gene serine protease inhibitor Kazal type (Spink)5 lead to the absence of LEKTI and result in the genodermatose Netherton syndrome, which mimics the common skin disease atopic dermatitis. Since many KLKs are expressed in human skin with KLK5 being considered as one of the most important KLKs in skin desquamation, we proposed that more inhibitors are present in human skin. Herein, we purified from human stratum corneum by HPLC techniques a new KLK5-inhibiting peptide encoded by a member of the Spink family, designated as Spink9 located on chromosome 5p33.1. This peptide is highly homologous to LEKTI and was termed LEKTI-2. Recombinant LEKTI-2 inhibited KLK5 but not KLK7, 14 or other serine proteases tested including trypsin, plasmin and thrombin. Spink9 mRNA expression was detected in human skin samples and in cultured keratinocytes. LEKTI-2 immune-expression was focally localized at the stratum granulosum and stratum corneum at palmar and plantar sites in close localization to KLK5. At sites of plantar hyperkeratosis, LEKTI-2 expression was increased. We suggest that LEKTI-2 contributes to the regulation of the desquamation process in human skin by specifically inhibiting KLK5.     

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.     

Development of peptides specifically modulating the activity of KLK2 and KLK3

The prostate produces several proteases, the most abundant ones being kallikrein-related peptidase 3 (KLK3, PSA) and KLK2 (hK2), which are potential targets for tumor imaging and treatment. KLK3 expression is lower in malignant than in normal prostatic epithelium and it is further reduced in poorly differentiated tumors, in which the expression of KLK2 is increased. KLK3 has been shown to inhibit angiogenesis, whereas KLK2 may mediate tumor growth and invasion by participating in proteolytic cascades. Thus, it may be possible to control prostate cancer growth by modulating the proteolytic activity of KLK3 and KLK2. We have developed peptides that very specifically stimulate the activity of KLK3 or inhibit that of KLK2. Using these peptides we have established peptide-based methods for the determination of enzymatically active KLK3. The first-generation peptides are unstable in vivo and are rapidly cleared from the circulation. Currently we are modifying the peptides to make them suitable for in vivo applications. We have been able to considerably improve the stability of KLK2-binding peptides by cyclization. In this review we summarize the possible roles of KLK3 and KLK2 in prostate cancer and then concentrate on the development of peptides that modulate the activity of these proteases.     

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.     

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.     

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.     

Proteinase-activated receptors (PARs): differential signalling by kallikrein-related peptidases KLK8 and KLK14

We compared signalling pathways such as calcium transients, MAPK activation, β-arrestin interactions and receptor internalization triggered by kallikrein-related peptidases (KLKs) 8 and 14 in human and rat proteinase-activated receptor (PAR)2-expressing human embryonic kidney (HEK) and Kirsten transformed rat kidney (KNRK) cells. Further, we analysed processing by KLK8 vs. KLK14 of synthetic human and rat PAR2-derived sequences representing the cleavage-activation domain of PAR2. Our data show that like KLK14, KLK8 can unmask a PAR2 receptor-activating sequence from a peptide precursor. However, whilst KLK8, like KLK14, can signal in rat-PAR2-expressing KNRK cells, this enzyme cannot signal via human PAR2 in HEK or KNRK cells, but rather, disarms HEK PAR1. Thus, KLK8 and KLK14 can signal differentially via the PARs to affect tissue function.     

Non-combinatorial library screening reveals subsite cooperativity and identifies new high-efficiency substrates for kallikrein-related peptidase 14

An array of substrates link the tryptic serine protease, kallikrein-related peptidase 14 (KLK14), to physiological functions including desquamation and activation of signaling molecules associated with inflammation and cancer. Recognition of protease cleavage sequences is driven by complementarity between exposed substrate motifs and the physicochemical signature of an enzyme's active site cleft. However, conventional substrate screening methods have generated conflicting subsite profiles for KLK14. This study utilizes a recently developed screening technique, the sparse matrix library, to identify five novel high-efficiency sequences for KLK14. The optimal sequence, YASR, was cleaved with higher efficiency (k(cat)/K(m)=3.81 ± 0.4 × 10(6) M(-1) s(-1)) than favored substrates from positional scanning and phage display by 2- and 10-fold, respectively. Binding site cooperativity was prominent among preferred sequences, which enabled optimal interaction at all subsites as indicated by predictive modeling of KLK14/substrate complexes. These simulations constitute the first molecular dynamics analysis of KLK14 and offer a structural rationale for the divergent subsite preferences evident between KLK14 and closely related KLKs, KLK4 and KLK5. Collectively, these findings highlight the importance of binding site cooperativity in protease substrate recognition, which has implications for discovery of optimal substrates and engineering highly effective protease inhibitors.     

Kallikrein-related peptidase-8 (KLK8) is an active serine protease in human epidermis and sweat and is involved in a skin barrier proteolytic cascade

Kallikrein-related peptidase-8 (KLK8) is a relatively uncharacterized epidermal protease. Although proposed to regulate skin-barrier desquamation and recovery, the catalytic activity of KLK8 was never demonstrated in human epidermis, and its regulators and targets remain unknown. Herein, we elucidated for the first time KLK8 activity in human non-palmoplantar stratum corneum and sweat ex vivo. The majority of stratum corneum and sweat KLK8 was catalytically active, displaying optimal activity at pH 8.5 and considerable activity at pH 5. We also showed that KLK8 is a keratinocyte-specific protease, not secreted by human melanocytes or dermal fibroblasts. KLK8 secretion increased significantly upon calcium induction of terminal keratinocyte differentiation, suggesting an active role for this protease in upper epidermis. Potential activators, regulators, and targets of KLK8 activity were identified by in vitro kinetic assays using pro-KLK8 and mature KLK8 recombinant proteins produced in Pichia pastoris. Mature KLK8 activity was enhanced by calcium and magnesium ions and attenuated by zinc ions and by autocleavage after Arg(164). Upon screening KLK8 cleavage of a library of FRET-quenched peptides, trypsin-like specificity was observed with the highest preference for (R/K)(S/T)(A/V) at P1-P1'-P2'. We also demonstrated that KLK5 and lysyl endopeptidase activate latent pro-KLK8, whereas active KLK8 targets pro-KLK11, pro-KLK1, and LL-37 antimicrobial peptide activation in vitro. Together, our data identify KLK8 as a new active serine protease in human stratum corneum and sweat, and we propose regulators and targets that augment its involvement in a skin barrier proteolytic cascade. The implications of KLK8 elevation and hyperactivity in desquamatory and inflammatory skin disease conditions remain to be studied.     

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.     

Kallikrein-related peptidase 12 hydrolyzes matricellular proteins of the CCN family and modifies interactions of CCN1 and CCN5 with growth factors

Kallikrein-related peptidases (KLKs) are an emerging group of secreted serine proteases involved in several physiological and pathological processes. We used a degradomic approach to identify potential substrates of KLK12. MDA-MB-231 cells were treated either with KLK12 or vehicle control, and the proteome of the overlying medium was analyzed by mass spectrometry. CCN1 (cyr61, ctgf, nov) was among the proteins released by the KLK12-treated cells, suggesting that KLK12 might be responsible for the shedding of this protein from the cell surface. Fragmentation of CCN1 by KLK12 was further confirmed in vitro, and the main cleavage site was localized in the hinge region between the first and second half of the recombinant protein. KLK12 can target all six members of the CCN family at different proteolytic sites. Limited proteolysis of CCNs (cyr61, ctgf, nov) was also observed in the presence of other members of the KLK family, such as KLK1, KLK5, and KLK14, whereas KLK6, KLK11, and KLK13 were unable to fragment CCNs. Because KLK12 seems to have a role in angiogenesis, we investigated the relations between KLK12, CCNs, and several factors known to be involved in angiogenesis. Solid phase binding assays showed that fragmentation of CCN1 or CCN5 by KLK12 prevents VEGF(165) binding, whereas it also triggers the release of intact VEGF and BMP2 from the CCN complexes. The KLK12-mediated release of TGF-β1 and FGF-2, either as intact or truncated forms, was found to be concentration-dependent. These findings suggest that KLK12 may indirectly regulate the bioavailability and activity of several growth factors through processing of their CCN binding partners.     

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.