Proteolytic activity of prostate-specific antigen (PSA) towards protein substrates and effect of peptides stimulating PSA activity

Biochemical activity induced by a germline variation in KLK3 (PSA) associates with cellular function and clinical outcome in prostate cancer

Genetic variation at the 19q13.3 KLK locus is linked with prostate cancer susceptibility. The non-synonymous KLK3 SNP, rs17632542 (c.536T>C; Ile163Thr-substitution in PSA) is associated with reduced prostate cancer risk, however, the functional relevance is unknown. Here, we identify that the SNP variant-induced change in PSA biochemical activity as a previously undescribed function mediating prostate cancer pathogenesis. The 'Thr' PSA variant led to small subcutaneous tumours, supporting reduced prostate cancer risk. However, 'Thr' PSA also displayed higher metastatic potential with pronounced osteolytic activity in an experimental metastasis in-vivo model. Biochemical characterization of this PSA variant demonstrated markedly reduced proteolytic activity that correlated with differences in in-vivo tumour burden. The SNP is associated with increased risk for aggressive disease and prostate cancer-specific mortality in three independent cohorts, highlighting its critical function in mediating metastasis. Carriers of this SNP allele had reduced serum total PSA and a higher free/total PSA ratio that could contribute to late biopsy decisions and delay in diagnosis. Our results provide a molecular explanation for the prominent 19q13.3 KLK locus, rs17632542 SNP, association with a spectrum of prostate cancer clinical outcomes.

Enzyme Responsive Delivery of Anti-Retroviral Peptide via Smart Hydrogel

In response to an urgent need for advanced formulations for the delivery of anti-retrovirals, a stimuli-sensitive hydrogel formulation that intravaginally delivers HIV-1 entry inhibitor upon being exposed to a specific protease was developed. The hydrogel formulation consists of PEG-azide and PEG-DBCO covalently linked to the entry inhibitor peptide, enfuvirtide, via substrate linker that is designed to undergo proteolysis by prostate specific antigen (PSA) present in seminal fluid and release innate enfuvirtide. Of the tested PSA substrate linkers (HSSKLQYY, GISSFYSSK, AYLMYY, and AYLMGRR), HSSKLQ was found to be an optimal candidate for PEG-based hydrogel with k_cat/K_M of 2.2 M^-1 s^-1. The PEG-based hydrogel displayed a pseudoplastic, thixotropic behavior with overall viscosity varying between 1516 and 2.2 Pa.s, within the biologically relevant shear rates of 0.01-100 s^-1. It also exhibited viscoelastic properties appropriate for uniform spreading and being retained in vagina. PEG-based hydrogels were loaded with N3-HSSKLQ-enfuvirtide (HF42) that is customarily synthesized enfuvirtide prodrug with its N-terminus connected to HSSKLQ linker. The stimuli-sensitive PEG-based hydrogel formulations upon being exposed to PSA released 36.5 ± 4.8% of enfuvirtide over 24 h in human ejaculate mimic of vaginal simulant fluid and seminal simulant fluid mixed in 1:3 ratio, which is significantly greater than its IC₅₀. The PEG-based hydrogel was non-cytotoxic to both vaginal epithelial cells (VK2/E6E7) and murine macrophages (RAW 264.7) and did not significantly induce the production of nitric oxide, an inflammatory mediator. The PEG-based hydrogel is found to have suitable physicochemical properties for an intravaginal formulation of the PSA substrate-linked anti-retrovirals and is safe towards vaginal epithelium. It is capable of delivering enfuvirtide with effective concentrations to prevent women from HIV-1 infection.

Remodelling of the tumour microenvironment by the kallikrein-related peptidases

Kallikrein-related peptidases (KLKs) are critical regulators of the tumour microenvironment. KLKs are proteolytic enzymes regulating multiple functions of bioactive molecules including hormones and growth factors, membrane receptors and the extracellular matrix architecture involved in cancer progression and metastasis. Perturbations of the proteolytic cascade generated by these peptidases, and their downstream signalling actions, underlie tumour emergence or blockade of tumour growth. Recent studies have also revealed their role in tumour immune suppression and resistance to cancer therapy. Here, we present an overview of the complex biology of the KLK family and its context-dependent nature in cancer, and discuss the different therapeutic strategies available to potentially target these proteases.

Blocking serine protease activity prevents semenogelin degradation leading to hyperviscous semen in humans

Prostate-specific antigen (PSA) is a prostate-specific serine protease enzyme that hydrolyzes gel-forming proteins (semenogelins) and changes the semen from gel-like to watery viscosity, a process called semen liquefaction. Highly viscous semen and abnormal liquefaction reduce sperm motility and contribute to infertility. Previously, we showed that nonspecific serine protease inhibitor (AEBSF) prevented proteolytic degradation of semenogelin in mice. However, it is unclear whether similar effect could be recapitulated in fresh human ejaculates. Therefore, in this study we evaluated the effect of AEBSF on the degradation of semenogelin (SEMG1) and its subsequent impact on semen liquefaction and sperm motility in fresh semen ejaculates collected from healthy men. We found that AEBSF showed a dual contraceptive action where it effectively 1) prevented degradation of SEMG1 resulting in viscous semen and 2) decreased sperm motility in human semen samples. However, the impact of AEBSF on sperm motility and viability could be due to its inhibitory activity toward other serine proteases or simply due to its toxicity. Therefore, to determine whether inhibition of PSA activity alone could disrupt SEMG1 degradation and contribute to hyperviscous semen, a neutralizing PSA antibody was used. We found that PSA antibody effectively prevented SEMG1 degradation with a subtle impact on sperm motility. These findings suggest that the target inhibition of PSA activity can prevent proteolytic degradation of SEMG1 and block liquefaction process, resulting in hyperviscous semen. As it is currently unknown if blocking semen liquefaction alone could prevent pregnancy, it needs further extensive studies before drawing any translational conclusions.

KLK3 in the Regulation of Angiogenesis-Tumorigenic or Not?

In this focused review, we address the role of the kallikrein-related peptidase 3 (KLK3), also known as prostate-specific antigen (PSA), in the regulation of angiogenesis. Early studies suggest that KLK3 is able to inhibit angiogenic processes, which is most likely dependent on its proteolytic activity. However, more recent evidence suggests that KLK3 may also have an opposite role, mediated by the ability of KLK3 to activate the (lymph)angiogenic vascular endothelial growth factors VEGF-C and VEGF-D, further discussed in the review.

The secret identities of TMPRSS2: Fertility factor, virus trafficker, inflammation moderator, prostate protector and tumor suppressor

The human TMPRSS2 gene is pathogenetically implicated in both coronaviral lung infection and prostate cancer, suggesting its potential as a drug target in both contexts. SARS-COV-2 spike polypeptides are primed by the host transmembrane TMPRSS2 protease, triggering virus fusion with epithelial cell membranes followed by an endocytotic internalisation process that bypasses normal endosomal activation of cathepsin-mediated innate immunity; viral co-opting of TMPRSS2 thus favors microbial survivability by attenuating host inflammatory responses. In contrast, most early hormone-dependent prostate cancers express TMPRSS2:ERG fusion genes arising from deletions that eliminate the TMPRSS2 coding region while juxtaposing its androgen-inducible promoter and the open reading frame of ERG, upregulating pro-inflammatory ERG while functionally disabling TMPRSS2. Moreover, inflammatory oxidative DNA damage selects for TMPRSS2:ERG-fused cancers, whereas patients treated with antiinflammatory drugs develop fewer of these fusion-dependent tumors. These findings imply that TMPRSS2 protects the prostate by enabling endosomal bypass of pathogens which could otherwise trigger inflammation-induced DNA damage that predisposes to TMPRSS2:ERG fusions. Hence, the high oncogenic selectability of TMPRSS2:ERG fusions may reflect a unique pro-inflammatory synergy between androgenic ERG gain-of-function and fusogenic TMPRSS2 loss-of-function, cautioning against the use of TMPRSS2-inhibitory drugs to prevent or treat early prostate cancer.

Interference-Free Duplex Detection of Total and Active Enzyme Concentrations at a Single Working Electrode

The duplex detection of both total and active enzyme concentrations without interferences at a single working electrode is challenging, especially when two different assays are combined. It is also challenging to obtain two different redox-cycling reactions without interference. Here, we present a simple but sensitive combined assay that is based on two redox-cycling reactions using two incubation periods and applied potentials at a single electrode. The assay combines an immunoassay for the determination of the total enzyme (total prostate-specific antigen, tPSA) concentration with a protease assay for the determination of the active enzyme (free PSA, fPSA) concentration. The immunoassay label and fPSA that are affinity-bound to the electrode are used for high sensitivity and specificity in the protease assay as well as the immunoassay. In the immunoassay, electrochemical-enzymatic (EN) redox cycling involving ferrocenemethanol is obtained at 0.1 V versus Ag/AgCl without incubation before the proteolytically released 4-amino-1-naphthol is generated. In the protease assay, EN redox cycling involving 4-amino-1-naphthol is obtained at 0.0 V after 30 min of incubation without ferrocenemethanol electro-oxidation. The detection procedure is almost the same as common electrochemical sandwich-type immunoassays, although the two different assays are combined. The duplex detection in buffer and serum is highly interference-free, specific, and sensitive. The detection limits for tPSA and fPSA are approximately 10 and 1 pg/mL, respectively.

Powerful Chemiluminescence Probe for Rapid Detection of Prostate Specific Antigen Proteolytic Activity: Forensic Identification of Human Semen

The prostate specific antigen (PSA), a serine protease with chymotrypsin-like activity, is predominantly expressed in the prostate and is considered as the most common marker in use to identify and follow the progress of prostate cancer. In addition, it is also now accepted as a marker for detecting semen in criminal cases. Here, we describe the design, synthesis, and evaluation of the first chemiluminescence probe for detection of PSA enzymatic activity. The probe activation mechanism is based on a catalytic cleavage of a specific peptidyl substrate, followed by a release of a phenoxy-dioxetane luminophore, that then undergoes efficient chemiexcitation to emit a green photon. The probe exhibits a significant turn-on response upon reaction with PSA and produces strong light emission signal with an extremely high signal-to-noise ratio. Comparison of the chemiluminescence probe with an analogous fluorescence probe showed superior detection capability in terms of response time and sensitivity. In addition, the probe was able to efficiently detect and image human semen traces on fabric, even after 3 days from sample preparation. The advantageous sensitivity and simplicity of a chemiluminescence assay to detect seminal fluid was effectively demonstrated by on-site measurements using a small portable luminometer. It is expected that the new chemiluminescence probe would be broadly useful for numerous applications in which PSA detection or imaging is required.

Mechanism of semen liquefaction and its potential for a novel non-hormonal contraception†

Semen liquefaction is a proteolytic process where a gel-like ejaculated semen becomes watery due to the enzymatic activity of prostate-derived serine proteases in the female reproductive tract. The liquefaction process is crucial for the sperm to gain their motility and successful transport to the fertilization site in Fallopian tubes (or oviducts in animals). Hyperviscous semen or failure in liquefaction is one of the causes of male infertility. Therefore, the biochemical inhibition of serine proteases in the female reproductive tract after ejaculation is a prime target for novel contraceptive development. Herein, we will discuss protein components in the ejaculates responsible for semen liquefaction and any developments of contraceptive methods in the past that involve the liquefaction process.

Human follicular fluid proteome reveals association between overweight status and oocyte maturation abnormality

Background

Human follicular fluid (HFF), which is composed by essential proteins required for the follicle development, provides an important microenvironment for oocyte maturation. Recently, overweight status has been considered as a detrimental impact factor on oocyte maturation, but whether HFF proteome could provide protein markers for assessing overweight-based oocyte maturation deficiency is still unknown.

Methods

To reveal the HFF-based molecular characteristics associated with abnormal oocyte maturation, an iTRAQ-based comparative proteomic analysis was performed to investigate different HFF protein expression profiles from normal weight women and overweight status women.

Results

Two hundred HFF proteins were quantified in our data, of which 43% have not been overlapped by two previous publications. Compared with the HFF proteins of normal weight women, 22 up-regulated HFF proteins and 21 down-regulated HFF proteins were found in the overweight status women. PANTHER database showed these altered HFF proteins participated in development, metabolism, immunity, and coagulation, and STRING database demonstrated their complicated interaction networks. The confidence of proteomic outcome was verified by Western blot analysis of WAP four-disulfide core domain protein 2 (WFDC2), lactotransferrin (LTF), prostate-specific antigen (KLK3), fibronectin (FN1), and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Further, ELISA assay indicated WFDC2 might be a potentially useful candidate HFF marker for the diagnosis of oocyte maturation arrest caused by overweight status.

Conclusions

Our work provided a new complementary high-confidence HFF dataset involved in oocyte maturation, and these altered HFF proteins might have clinical relevance and diagnostic and prognostic value for abnormal oocyte maturation in overweight status women.

Association study between common variations in some candidate genes and prostate adenocarcinoma predisposition through multi-stage approach in Iranian population

BACKGROUND

Prostate cancer is one of the five common cancers and has the second incidence rate and the third mortality rate in Iranian population. The purpose of this study was to evaluate the association of rs16901979, rs4242382 and rs1447295 on 8q24 locus, rs2735839 (KLK3 gene) and rs721048 (EHBP1 gene) with prostate adenocarcinoma through multi-stage approach to identify the polymorphisms associated with prostate cancer and use them as screening factors. Screening tests can identify people who may have a chance of developing the disease before detection and any symptoms.

METHODS

The case-control study included 103 cases (prostate adenocarcinoma) and 100 controls (benign prostatic hyperplasia). Tetra-primer ARMS-PCR was used to genotyping of each participant. A Multi-stage approach was used for efficient genomic study. In this method, a smaller number of people can be used. Chi-squared, Fisher's exact test and logistic regression were used to investigate the SNPs associated with prostate cancer and Gleason score.

RESULTS

In the first stage (59 men), the frequency of polymorphisms rs16901979, rs4242382, rs1447295, rs2735839 and rs721048 in the prostate adenocarcinoma group was evaluated compared to the control group (P-value < 0.3) in order to select meaningful polymorphisms. There was not any significant difference between genotype frequency rs16901979 (P = 0.671) and rs721048 (P = 0.474) in the case group compared to BPH. Therefore, these polymorphisms were eliminated, and in the second step (144 men), rs4242382, rs2735839 and rs1447295 were evaluated (P-value < 0.05). According to the total population (203 men), there was significant difference between genotype frequency rs4242382 (P = 0.001), rs2735839 (P = 0.000) and rs1447295 (P = 0.005) even after using Bonferroni correction (p = 0.016). The effect of these three polymorphisms on prostate cancer was not modified by age and PSA. There was a significant difference between the allelic frequency of A vs G (rs4242382, rs2735839) at all classes of Gleason score and A vs C (rs1447295) at Gleason score ≥ 8.

CONCLUSIONS

The results of this study for rs2735839, rs4242382 and rs1447295 indicate the association of these polymorphisms with prostate adenocarcinoma predisposition in Iranian population. Exposure effect is homogeneous between different ages and PSA level categories. These three polymorphisms should be studied in a larger population to confirm these results.

Proteolytic Regulation of Parathyroid Hormone-Related Protein: Functional Implications for Skeletal Malignancy

Parathyroid hormone-related protein (PTHrP), with isoforms ranging from 139 to 173 amino acids, has long been implicated in the development and regulation of multiple tissues, including that of the skeleton, via paracrine and autocrine signaling. PTHrP is also known as a potent mediator of cancer-induced bone disease, contributing to a vicious cycle between tumor cells and the bone microenvironment that drives the formation and progression of metastatic lesions. The abundance of roles ascribed to PTHrP have largely been attributed to the N-terminal 1-36 amino acid region, however, activities for mid-region and C-terminal products as well as additional shorter N-terminal species have also been described. Studies of the protein sequence have indicated that PTHrP is susceptible to post-translational proteolytic cleavage by multiple classes of proteases with emerging evidence pointing to novel functional roles for these PTHrP products in regulating cell behavior in homeostatic and pathological contexts. As a consequence, PTHrP products are also being explored as potential biomarkers of disease. Taken together, our enhanced understanding of the post-translational regulation of PTHrP bioactivity could assist in developing new therapeutic approaches that can effectively treat skeletal malignancies.

Characterization of Monoclonal and Polyclonal Antibodies Recognizing Prostate Specific Antigen: Implication for Design of a Sandwich ELISA

BACKGROUND

Prostate cancer is the second most common cancer in men. Prostate-Specific Antigen (PSA) is a tumor-associated glycoprotein with enzymatic activity which is secreted by the prostate gland. Following entry to the blood, 70-90% of PSA forms complexes with protease inhibitors and its enzymatic activity is inhibited. The serum level of PSA is increased and the rate of free PSA (fPSA) to total PSA is decreased in prostate cancer patients. Therefore, measurement of PSA and fPSA in serum is very valuable for diagnosis and prognosis of prostate cancer.

METHODS

In the present study, five anti PSA monoclonal Antibodies (mAb) were characterized by Enzyme-Linked Immunosorbent Assay (ELISA) and immunoblotting. For designing a sandwich ELISA, epitope specificity of these antibodies was studied by a competition ELISA. Free PSA was purified by electroelution technique from seminal plasma and used to produce polyclonal anti-fPSA antibody in rabbit. Purified polyclonal antibody (pAb) and mAbs were conjugated with HRP enzyme and Biotin (Bio) to set up the sandwich ELISA.

RESULTS

Three of the mAbs were found to recognize PSA similarly. One of these mAbs (2G3) was paired with anti-fPSA pAb to detect fPSA in serum. Eventually, serum fPSA concentration of 356 subjects was measured and compared by our designed ELISA and a commercial ELISA kit. Our results demonstrated a significant correlation (r=0.68; p<0.001) between the two assays. Sensitivity and specificity of our designed ELISA was 72.4 and 82.8%, respectively.

CONCLUSION

These results imply suitability of our designed ELISA for detection of fPSA in patients with prostate cancer.

Prostate Cancer Risk-Associated Single-Nucleotide Polymorphism Affects Prostate-Specific Antigen Glycosylation and Its Function

BACKGROUND

Genetic association studies have reported single-nucleotide polymorphisms (SNPs) at chromosome 19q13.3 to be associated with prostate cancer (PCa) risk. Recently, the rs61752561 SNP (Asp84Asn substitution) in exon 3 of the kallikrein-related peptidase 3 (KLK3) gene encoding prostate-specific antigen (PSA) was reported to be strongly associated with PCa risk (P = 2.3 × 10^-8). However, the biological contribution of the rs61752561 SNP to PCa risk has not been elucidated.

METHODS

Recombinant PSA protein variants were generated to assess the SNP-mediated biochemical changes by stability and substrate activity assays. PC3 cell-PSA overexpression models were established to evaluate the effect of the SNP on PCa pathogenesis. Genotype-specific correlation of the SNP with total PSA (tPSA) concentrations and free/total (F/T) PSA ratio were determined from serum samples.

RESULTS

Functional analysis showed that the rs61752561 SNP affects PSA stability and structural conformation and creates an extra glycosylation site. This PSA variant had reduced enzymatic activity and the ability to stimulate proliferation and migration of PCa cells. Interestingly, the minor allele is associated with lower tPSA concentrations and high F/T PSA ratio in serum samples, indicating that the amino acid substitution may affect PSA immunoreactivity to the antibodies used in the clinical immunoassays.

CONCLUSIONS

The rs61752561 SNP appears to have a potential role in PCa pathogenesis by changing the glycosylation, protein stability, and PSA activity and may also affect the clinically measured F/T PSA ratio. Accounting for these effects on tPSA concentration and F/T PSA ratio may help to improve the accuracy of the current PSA test.

MiRNA-27b Regulates Angiogenesis by Targeting AMPK in Mouse Ischemic Stroke Model

Stroke is a leading cause of mortality and serious disability worldwide with limited treatment options. Angiogenesis has been reported to be involved in post-stroke recovery. Although the molecular mechanisms that regulate angiogenesis remain ambiguous, microRNAs have emerged as effective regulators of angiogenesis, involved in neurological function outcome. The present study aims to investigate the regulatory effects of miRNA-27b on post-stroke angiogenesis. In primary cultured brain microvascular endothelial cells (BMECs), the inhibition of miRNA-27b induced the activation of adenosine monophosphate-activated protein kinase (AMPK), which increased tube formation and migration. This action was attenuated when AMPKα2 was knocked down. Mice were subjected to middle cerebral artery occlusion (MCAo) surgery and administrated with Lentivirus miR-27b inhibitor. Enhanced angiogenesis in ischemic boundary zone (IBZ) was observed, and the neurological outcome during the entire study period was improved. The number of phosphate-AMPKα2⁺ cells that co-expressed endothelial cell marker CD31 was significantly increased. Taken together, the present study demonstrated that downregulated miRNA-27b promoted recovery after ischemic stroke via AMPK stimulus.

Kallikrein-related peptidases represent attractive therapeutic targets for ovarian cancer

INTRODUCTION

Aberrant levels of kallikrein-related peptidases (KLK1-15) have been linked to cancer cell proliferation, invasion and metastasis. In ovarian cancer, the KLK proteolytic network has a crucial role in the tissue and tumor microenvironment. Publically available ovarian cancer genome and expression data from multiple patient cohorts show an upregulation of most KLKs. Areas covered: Here, we review the expression levels of all 15 members of this family in normal and ovarian cancer tissues, categorizing them into highly and moderately or weakly expressed KLKs, and their association with patient prognosis and survival. We summarize their tumor-biological functions determined in cell-based assays and xenograft models, further highlighting their suitability as cancer biomarkers and attractive candidates for drug development. Finally, we discuss some different pharmaceutical approaches, including peptide-based and small molecule inhibitors, cyclic peptides, depsipeptides, engineered natural inhibitors, antibodies, RNA/DNA-based aptamers, prodrugs, miRNA and siRNA. Expert opinion: In light of the results from clinical and tumor-biological studies, together with the available pharmaceutical tools, we suggest KLK4, KLK5, KLK6 and possibly KLK7 as preferred targets for inhibition in ovarian cancer.

Targeting strategies of adenovirus‑mediated gene therapy and virotherapy for prostate cancer (Review)

Prostate cancer (PCa) poses a high risk to older men and it is the second most common type of male malignant tumor in western developed countries. Additionally, there is a lack of effective therapies for PCa at advanced stages. Novel treatment strategies such as adenovirus-mediated gene therapy and virotherapy involve the expression of a specific therapeutic gene to induce death in cancer cells, however, wild-type adenoviruses are also able to infect normal human cells, which leads to undesirable toxicity. Various PCa-targeting strategies in adenovirus-mediated therapy have been developed to improve tumor-targeting effects and human safety. The present review summarizes the relevant knowledge regarding available adenoviruses and PCa-targeting strategies. In addition, future directions in this area are also discussed. In conclusion, although they remain in the early stages of basic research, adenovirus-mediated gene therapy and virotherapy are expected to become important therapies for tumors in the future due to their potential targeting strategies.

Development of molecules stimulating the activity of KLK3 - an update

Kallikrein-related peptidase-3 (KLK3, known also as prostate-specific antigen, PSA) is highly expressed in the prostate. KLK3 possess antiangiogenic activity, which we have found to be related to its proteolytic activity. Thus, it may be possible to slow down the growth of prostatic tumors by enhancing this activity. We have developed peptides that enhance the proteolytic activity of KLK3. As these peptides are degraded in circulation and rapidly excreted, we have started to modify them and have succeeded in creating bioactive and more stable pseudopeptides. We have also identified small molecules stimulating the activity of KLK3, especially in synergy with peptides.

Absence of TGF-β Receptor Activation by Highly Purified hCG Preparations

Recently, several LH/human chorionic gonadotropin (hCG) receptor-independent activities for hCG have been described, including activation of the TGF-β receptor (TGFβR) by hyperglycosylated hCG and stimulation of trophoblast invasion. Because the hCG concentrations used in these studies have been rather high, reflecting physiological hCG levels in pregnancy, even a minor contamination with growth factors, which act at very low concentrations, may be significant. Several commercial hCG preparations have been found to contain significant amounts of epidermal growth factor (EGF), which we also confirmed here. Furthermore, we found that some hCG preparations also contain significant amounts of TGF-β1. These hCG preparations were able to activate ERK1/2 in JEG-3 choriocarcinoma cells or TGFβR in mink lung epithelial cells transfected with a reporter gene for TGFβR activation. No such activation was found with highly purified hCG or its free β-subunit (hCGβ), irrespective of whether they were hyperglycosylated or not. Taken together, our results suggest that the growth factor contaminations in the hCG preparations can cause activation of TGFβR and, at least in JEG-3 cells, MAPK signaling. This highlights the importance to carefully control for potential contaminations and that highly purified hCG preparations have to be used for biological studies.

Kallikreins - The melting pot of activity and function

The human tissue kallikrein and kallikrein-related peptidases (KLKs), encoded by the largest contiguous cluster of protease genes in the human genome, are secreted serine proteases with diverse expression patterns and physiological roles. Because of the broad spectrum of processes that are modulated by kallikreins, these proteases are the subject of extensive investigations. This review brings together basic information about the biochemical properties affecting enzymatic activity, with highlights on post-translational modifications, especially glycosylation. Additionally, we present the current state of knowledge regarding the physiological functions of KLKs in major human organs and outline recent discoveries pertinent to the involvement of kallikreins in cell signaling and in viral infections. Despite the current depth of knowledge of these enzymes, many questions regarding the roles of kallikreins in health and disease remain unanswered.

Selection of High-Affinity Peptidic Serine Protease Inhibitors with Increased Binding Entropy from a Back-Flip Library of Peptide-Protease Fusions

We have developed a new concept for designing peptidic protein modulators, by recombinantly fusing the peptidic modulator, with randomized residues, directly to the target protein via a linker and screening for internal modulation of the activity of the protein. We tested the feasibility of the concept by fusing a 10-residue-long, disulfide-bond-constrained inhibitory peptide, randomized in selected positions, to the catalytic domain of the serine protease murine urokinase-type plasminogen activator. High-affinity inhibitory peptide variants were identified as those that conferred to the fusion protease the lowest activity for substrate hydrolysis. The usefulness of the strategy was demonstrated by the selection of peptidic inhibitors of murine urokinase-type plasminogen activator with a low nanomolar affinity. The high affinity could not have been predicted by rational considerations, as the high affinity was associated with a loss of polar interactions and an increased binding entropy.