Characterization and processing of prostate specific antigen (hK3) and human glandular kallikrein (hK2) secreted by LNCaP cells

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.

Study of glycosylation of prostate-specific antigen secreted by cancer tissue-originated spheroids reveals new candidates for prostate cancer detection

Prostate-specific antigen (PSA) is the most frequently used biomarker for the screening of prostate cancer. Understanding the structure of cancer-specific glycans can help us improve PSA assay. In the present study, we analysed the glycans of PSA obtained from culture medium containing cancer tissue-originated spheroids (CTOS) which have similar characteristics as that of the parent tumour to explore the new candidates for cancer-related glycoforms of PSA. The glycan profile of PSA from CTOS was determined by comparing with PSA from normal seminal plasma and cancer cell lines (LNCaP and 22Rv1) using lectin chromatography and mass spectrometry. PSA from CTOS was mostly sialylated and the content of Wisteria floribunda agglutinin reactive glycan (LacdiNAc) was similar to that of PSA derived from seminal plasma and 22Rv1. Conversely, concanavalin A (Con A)-unbound PSA was definitely detected from the three cancer origins but was almost negligible in seminal PSA. Two novel types of PSA were elucidated in the Con A-unbound fraction: one is a high molecular weight PSA with highly branched N-glycans, and the other is a low molecular weight PSA without N-glycans. Furthermore, the existence of Lewis X antigen group on PSA was indicated. These PSAs will be candidates for new cancer-related markers.

E-cadherin is downregulated in benign prostatic hyperplasia and required for tight junction formation and permeability barrier in the prostatic epithelial cell monolayer

BACKGROUND

We previously reported the presence of prostate-specific antigen (PSA) in the stromal compartment of benign prostatic hyperplasia (BPH). Since PSA is expressed exclusively by prostatic luminal epithelial cells, PSA in the BPH stroma suggests increased tissue permeability and the compromise of epithelial barrier integrity. E-cadherin, an important adherens junction component and tight junction regulator, is known to exhibit downregulation in BPH. These observations suggest that the prostate epithelial barrier is disrupted in BPH and E-cadherin downregulation may increase epithelial barrier permeability.

METHODS

The ultra-structure of cellular junctions in BPH specimens was observed using transmission electron microscopy (TEM) and E-cadherin immunostaining analysis was performed on BPH and normal adjacent specimens from BPH patients. In vitro cell line studies using benign prostatic epithelial cell lines were performed to determine the impact of small interfering RNA knockdown of E-cadherin on transepithelial electrical resistance and diffusion of fluorescein isothiocyanate (FITC)-dextran in transwell assays.

RESULTS

The number of kiss points in tight junctions was reduced in BPH epithelial cells as compared with the normal adjacent prostate. Immunostaining confirmed E-cadherin downregulation and revealed a discontinuous E-cadherin staining pattern in BPH specimens. E-cadherin knockdown increased monolayer permeability and disrupted tight junction formation without affecting cell density.

CONCLUSIONS

Our results indicate that tight junctions are compromised in BPH and loss of E-cadherin is potentially an important underlying mechanism, suggesting targeting E-cadherin loss could be a potential approach to prevent or treat BPH.

Activation profiles of human kallikrein-related peptidases by matrix metalloproteinases

The 15 human kallikrein-related peptidases (KLKs) are clinically important biomarkers and therapeutic targets of interest in inflammation, cancer, and neurodegenerative disease. KLKs are secreted as inactive pro-forms (pro-KLKs) that are activated extracellularly by specific proteolytic release of their amino-terminal pro-peptide, and this is a key step in their functional regulation. Physiologically relevant KLK regulatory cascades of activation have been described in skin desquamation and semen liquefaction, and work by a large number of investigators has elucidated pairwise and autolytic activation relationships among the KLKs with the potential for more extensive activation cascades. More recent work has asked whether functional intersection of KLKs with other types of regulatory proteases exists. Such studies show a capacity for members of the thrombostasis axis to act as broad activators of pro-KLKs. In the present report, we ask whether such functional intersection is possible between the KLKs and the members of the matrix metalloproteinase (MMP) family by evaluating the ability of the MMPs to activate pro-KLKs. The results identify MMP-20 as a broad activator of pro-KLKs, suggesting the potential for intersection of the KLK and MMP axes under pathological dysregulation of MMP-20 expression.

Prostate-specific kallikrein-related peptidases and their relation to prostate cancer biology and detection. Established relevance and emerging roles

Kallikreins are a family of serine proteases with a range of tissue-specific and essential proteolytic functions. Among the best studied are the prostate tissue-specific KLK2 and KLK3 genes and their secreted protease products, human kallikrein 2, hk2, and prostate-specific antigen (PSA). Members of the so-called classic kallikreins, these highly active trypsin-like serine proteases play established roles in human reproduction. Both hK2 and PSA expression is regulated by the androgen receptor which has a fundamental role in prostate tissue development and progression of disease. This feature, combined with the ability to sensitively detect different forms of these proteins in blood and biopsies, result in a crucially important biomarker for the presence and recurrence of cancer. Emerging evidence has begun to suggest a role for these kallikreins in critical vascular events. This review discusses the established and developing biological roles of hK2 and PSA, as well as the historical and advanced use of their detection to accurately and non-invasively detect and guide treatment of prostatic disease.

Microfluidic integration for automated targeted proteomic assays

A dearth of protein isoform-based clinical diagnostics currently hinders advances in personalized medicine. A well-organized protein biomarker validation process that includes facile measurement of protein isoforms would accelerate development of effective protein-based diagnostics. Toward scalable protein isoform analysis, we introduce a microfluidic "single-channel, multistage" immunoblotting strategy. The multistep assay performs all immunoblotting steps: separation, immobilization of resolved proteins, antibody probing of immobilized proteins, and all interim wash steps. Programmable, low-dispersion electrophoretic transport obviates the need for pumps and valves. A three-dimensional bulk photoreactive hydrogel eliminates manual blotting. In addition to simplified operation and interfacing, directed electrophoretic transport through our 3D nanoporous reactive hydrogel yields superior performance over the state-of-the-art in enhanced capture efficiency (on par with membrane electroblotting) and sparing consumption of reagents (ca. 1 ng antibody), as supported by empirical and by scaling analyses. We apply our fully integrated microfluidic assay to protein measurements of endogenous prostate specific antigen isoforms in (i) minimally processed human prostate cancer cell lysate (1.1 pg limit of detection) and (ii) crude sera from metastatic prostate cancer patients. The single-instrument functionality establishes a scalable microfluidic framework for high-throughput targeted proteomics, as is relevant to personalized medicine through robust protein biomarker verification, systematic characterization of new antibody probes for functional proteomics, and, more broadly, to characterization of human biospecimen repositories.

Immunoassay for the discrimination of free prostate-specific antigen (fPSA) forms with internal cleavages at Lys(₁₄₅) or Lys(₁₄₆) from fPSA without internal cleavages at Lys(₁₄₅) or Lys(₁₄₆)

Total levels of circulating prostate-specific antigen (tPSA) are strongly associated with prostate cancer (PCa) risk and outcome but benign prostate disease is the most frequent cause of a moderately elevated PSA level. Free PSA (fPSA) forms are independently associated with PCa risk and contribute modest diagnostic enhancements above and beyond tPSA alone. We developed an immunoassay for fPSA subfractions containing internal cleavages at Lys(145) or Lys(146) (fPSA-N). The assay was based on blocking intact single-chain fPSA (fPSA-I) with antibody 4D4 which does not detect PSA containing internal cleavages at Lys(145) or Lys(146). We also measured fPSA-N in blood from healthy volunteers and in anti-coagulated plasma from 76 men with or without evidence of PCa at biopsy. The analytical and functional detection limits of this assay were 0.016 ng/mL and 0.10 ng/mL, respectively. The median recovery of male fPSA-N from female plasma was 95.0%. All 12 female samples (average age 28 years) had fPSA-N concentrations at or below the analytical detection limit. The median fPSA-N concentration (0.050 ng/mL) in 9 healthy male volunteers (age<40 years) was below the functional detection limit, 0.420 ng/mL in 27 patients with benign prostate conditions and 0.239 ng/mL in 49 patients with PCa. Deming regression analysis of the patient samples showed that the measured fPSA-N concentrations were generally 23% lower than the previously calculated (fPSA minus fPSA-I) concentrations, likely due to differences in the antibody combinations used. In conclusion, we have developed a sensitive, specific and direct immunoassay for fPSA-N which can be used to study the clinical relevance of this PSA isoform.

Kallikreins on steroids: structure, function, and hormonal regulation of prostate-specific antigen and the extended kallikrein locus

The 15 members of the kallikrein-related serine peptidase (KLK) family have diverse tissue-specific expression profiles and putative proteolytic functions. The kallikrein family is also emerging as a rich source of disease biomarkers with KLK3, commonly known as prostate-specific antigen, being the current serum biomarker for prostate cancer. The kallikrein locus is also notable because it is extraordinarily responsive to steroids and other hormones. Indeed, at least 14 functional hormone response elements have been identified in the kallikrein locus. A more comprehensive understanding of the transcriptional regulation of kallikreins may help the field make more informed hypotheses about the physiological functions of kallikreins and their effectiveness as biomarkers. In this review, we describe the organization of the kallikrein locus and the structure of kallikrein genes and proteins. We also focus on the transcriptional regulation of kallikreins by androgens, progestins, glucocorticoids, mineralocorticoids, estrogens, and other hormones in animal models and human prostate, breast, and reproductive tract tissues. The interaction of the androgen receptor with androgen response elements in the promoter and enhancer of KLK2 and KLK3 is also summarized in detail. There is evidence that all kallikreins are regulated by multiple nuclear receptors. Yet, apart from KLK2 and KLK3, it is not clear whether all kallikreins are direct transcriptional targets. Therefore, we argue that gaining more detailed information about the mechanisms that regulate kallikrein expression should be a priority of future studies and that the kallikrein locus will continue to be an important model in the era of genome-wide analyses.

Prostate-specific antigen (PSA) is activated by KLK2 in prostate cancer ex vivo models and in prostate-targeted PSA/KLK2 double transgenic mice

BACKGROUND

Prostate-specific antigen (PSA) is a serine protease secreted as a zymogen. Previously, cell-free biochemical studies have identified various kallikreins (KLK) as candidate activating proteases. In this study, KLK2-mediated activation of PSA in cell-based in vitro, xenograft, and transgenic models was evaluated.

METHODS

Du145-derived PSA- or KLK2-expressing clones were coincubated in vitro and in vivo to evaluate KLK2-induced PSA activity. While mice possess orthologs of KLK4-15, they do not have functional orthologs of PSA or KLK2. Therefore, transgenic animals expressing PSA or both PSA and KLK2 were generated to assess orthotopic PSA activation.

RESULTS

PSA is activated by KLK2 when the cells are physically in contact, and through co-conditioned media. In vivo, the free (inactive PSA) to total (active + inactive PSA) ratio in the blood is decreased when PSA and KLK2-expressing cells are co-inoculated subcutaneously, suggesting increased active PSA. Additionally, double-transgenic mice expressing both genes in the prostate produce more active PSA compared to single transgenic animals. A longitudinal evaluation over a 2-year period demonstrated no morphologic changes (i.e., no PIN or prostate cancer) due to PSA or PSA/KLK2 double transgene expression relative to non-transgenic mice.

CONCLUSIONS

These data demonstrate, with biologically relevant models, that KLK2 is the protease responsible for activating PSA. While PSA is involved in the processing and release of a number of important growth factors, our results suggest that active PSA is not sufficient to induce the development of prostate cancer or prostate cancer precursors in aging PSA transgenic mice.

A completed KLK activome profile: investigation of activation profiles of KLK9, 10, and 15

We previously reported the activation profiles of the human kallikrein-related peptidases (KLKs) as determined from a KLK pro-peptide fusion-protein system. That report described the activity profiles of 12 of the 15 mature KLKs versus the 15 different pro-KLK sequences. The missing profiles in the prior report, involving KLK9, 10, and 15, are now described. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, mass spectrometry, and N-terminal sequence analyses show that KLK9 and 10 exhibit low hydrolytic activities towards all of the 15 pro-KLK sequences, while KLK15 exhibits significant activity towards both Arg- and Lys-containing KLK pro-sequences. The ability of KLK15 to activate pro-KLK8, 12, and 14 is confirmed using recombinant pro-KLK proteins, and shown to be significant for activation of pro-KLK8 and 14, but not 12. These additional data for KLK9, 10, and 15 now permit a completed KLK activome profile, using a KLK pro-peptide fusion-protein system, to be described. The results suggest that KLK15, once activated, can potentially feed back into additional pro-KLK activation pathways. Conversely, KLK9 and 10, once activated, are unlikely to participate in further pro-KLK activation pathways, although similar to KLK1 they may activate other bioactive peptides.

Quantitative Analysis of Human Kallikrein 5 (KLK5) Expression in Prostate Needle Biopsies: An Independent Cancer Biomarker

Background: Kallikrein 5 (KLK5), a recently cloned member of the kallikrein family, codes for the secreted protein KLK5. Active KLK5 protein has a trypsin activity, and the expression of KLK5 gene seems to be regulated by steroid hormones. We performed an expression analysis and clinical evaluation of the KLK5 gene, at the mRNA level, in prostate needle biopsies.

Methods: We examined KLK5 mRNA concentrations in 103 prostate tissue specimens. After testing of RNA quality, cDNA was prepared by reverse transcription. A highly sensitive quantitative real-time PCR (qRT-PCR) method for KLK5 mRNA quantification was developed using the SYBR Green chemistry. GAPDH was used as a housekeeping gene.

Results: Specimens from patients with benign prostatic hyperplasia (BPH) showed higher levels of KLK5 mRNA expression than those from patients with prostate cancer (PCa) (P = 0.024). ROC analysis demonstrated that KLK5 expression had significant discriminatory value between BPH and PCa (AUC 0.64; P = 0.016). KLK5 mRNA expression showed a statistically significant negative correlation with the total PSA serum concentration in the PCa patients (P = 0.003). Early-stage tumors showed higher KLK5 expression than late-stage ones (P = 0.014), whereas KLK5 expression was negatively correlated to Gleason score (P = 0.005).

Conclusions: KLK5 mRNA, analyzed by quantitative PCR in prostate needle biopsies, could be an independent biomarker for the differential diagnosis and prognosis in prostate cancer.

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.

New insights into the functional mechanisms and clinical applications of the kallikrein-related peptidase family

The Kallikrein-related peptidase (KLK) family consists of fifteen conserved serine proteases that form the largest contiguous cluster of proteases in the human genome. While primarily recognized for their clinical utilities as potential disease biomarkers, new compelling evidence suggests that this family plays a significant role in various physiological processes, including skin desquamation, semen liquefaction, neural plasticity, and body fluid homeostasis. KLK activation is believed to be mediated through highly organized proteolytic cascades, regulated through a series of feedback loops, inhibitors, auto-degradation and internal cleavages. Gene expression is mainly hormone-dependent, even though transcriptional epigenetic regulation has also been reported. These regulatory mechanisms are integrated with various signaling pathways to mediate multiple functions. Dysregulation of these pathways has been implicated in a large number of neoplastic and non-neoplastic pathological conditions. This review highlights our current knowledge of structural/phylogenetic features, functional role and regulatory/signaling mechanisms of this important family of enzymes.

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.

Enzymatic properties of human kallikrein-related peptidase 12 (KLK12)

Human kallikrein-related peptidase 12 (KLK12) is a new member of the human tissue kallikrein family. Preliminary studies suggest that KLK12 is differentially expressed in breast cancer and may have potential use as a cancer biomarker. It has been predicted that KLK12 is a secreted serine protease. However, the enzymatic properties of this protein have not been reported so far. Here, we report the production of recombinant KLK12 and analyses of its enzymatic characteristics, including zymogen activation, substrate specificity, and regulation of its activity. KLK12 is secreted as an inactive pro-enzyme, which is able to autoactivate to gain enzymatic activity. Through screening of a panel of fluorogenic and chromogenic peptide substrates, we establish that active KLK12 possesses trypsin-like activity, cleaving peptide bonds after both arginine and lysine. Active KLK12 quickly loses its activity due to autodegradation, and its activity can also be rapidly inhibited by zinc ions and by alpha2-antiplasmin through covalent complex formation. Furthermore, we demonstrate that KLK12 is able to activate KLK11 zymogen in vitro. Our results indicate that KLK12 may participate in enzymatic cascades involving other kallikreins.

Human tissue kallikreins: A road under construction

BACKGROUND

The human tissue kallikrein gene family, located at chromosome 19q13.4, is the largest contiguous family of proteases in the human genome. The locus encodes all 15 members of the family, 13 of which have been reported as potential biomarkers for several carcinomas and other non-neoplastic diseases. Kallikreins are expressed by a wide range of tissues and implicated in a number of physiological functions, including skin desquamation, semen liquefaction, neural plasticity and the regulation of blood pressure. Kallikrein function is regulated at various levels, including transcription, translation and post-translation. The proteolytic activity of kallikreins is believed to be cascade mediated and may cross-talk with other proteases. These cascades are highly regulated through a series of feedback loops, inhibitors, (auto) degradation and internal cleavage. Uncontrolled proteolytic activity of kallikreins is implicated in a large number of neoplastic and non-neoplastic pathological conditions.

CONCLUSIONS

As our understanding of their regulatory and functional mechanisms continues to expand, kallikreins are expected to become novel targets for the design of new therapeutics.

A sensitive proximity ligation assay for active PSA

Prostate-specific antigen (PSA) is a widely used marker for prostate cancer. The utility of PSA tests is limited by their inability to differentiate prostate cancer from non-malignant conditions such as benign prostatic hyperplasia and prostatitis. In circulation, PSA occurs in various complexed and free forms, and specific determination of some of these can be used to improve the diagnostic accuracy of PSA tests. We have previously identified peptides that specifically bind to enzymatically active PSA and using such a peptide we have developed an immunopeptidometric assay for this form of PSA. However, the sensitivity of that assay is too low to measure active PSA at clinically important levels. Recently a novel sensitive immunoassay for analysis of proteins, termed the proximity ligation assay, has been established. Here we describe a sensitive implementation of the proximity ligation assay, which utilizes a PSA-binding peptide and antibody as probes to detect active PSA. The assay has a sensitivity of 0.07 microg/l, which is approximately ten-fold lower than that of our previous assay. It does not cross-react with inactive proPSA or the highly similar kallikrein hK2. Our results show that a highly sensitive immunopeptidometric assay can be developed using proximity ligation. This principle should facilitate establishment of specific assays for active forms of other proteases.

Pharmacokinetics, biodistribution, and antitumor efficacy of a human glandular kallikrein 2 (hK2)-activated thapsigargin prodrug

BACKGROUND

Prostate cancer cells secrete unique proteases such as prostate-specific antigen (PSA) and human glandular kallikrein 2 (hK2) that represent targets for the activation of prodrugs as systemic treatment of metastatic prostate cancer. Previously, a combinatorial peptide library was screened to identify a highly active peptide substrate for hK2. The peptide was coupled to an analog of the potent cytotoxin thapsigargin, L12ADT, to generate an hK2-activated prodrug that was efficiently hydrolyzed by purified hK2, stable to hydrolysis in human and mouse plasma in vitro and selectively toxic to hK2 producing prostate cancer cells in vitro.

METHODS

In the current study, toxicology, pharmacokinetics, prodrug biodistribution, and antitumor efficacy studies were performed to evaluate the hK2-activated prodrug in vivo.

RESULTS

The single intravenous maximally tolerated dose of prodrug was 6 mg/kg (i.e., 3.67 micromole/kg) which produced peak serum concentration of approximately 36 microM and had a half-life of approximately 40 min. In addition, over a 24 hr period <0.5% of free L12ADT analog was observed in plasma. The prodrug demonstrated significant antitumor effect in vivo while it was being administered, but prolonged intravenous administration was not possible due to local toxicity to tail veins. Subcutaneous administration of equimolar doses produced lower plasma AUC compared to intravenous dosing but equivalent intratumoral levels of prodrug following multiple doses.

CONCLUSIONS

The hK2-activated prodrug was stable in vivo. The prodrug, however, was rapidly cleared and difficult to administer over prolonged dosing interval. Additional studies are underway to assess antitumor efficacy with prolonged administration of higher subcutaneous doses of prodrug. Second-generation hK2-activated thapsigargin prodrugs with increased half-lives and improved formulations are also under development.

Different glycan structures in prostate-specific antigen from prostate cancer sera in relation to seminal plasma PSA

Prostate-specific antigen (PSA), the tumor marker currently used for prostate cancer (PCa), is not specific enough to distinguish between PCa and benign prostate hyperplasia (BPH). Glycan processing is normally perturbed in tumors, therefore we investigated whether changes in glycosylation of PSA could be useful diagnostic indicators. Previously we determined that the glycosylation of PSA secreted by the tumor prostate cell line LNCaP differs significantly from that of PSA from seminal plasma (normal control). We therefore undertook a detailed glycan analysis of PSA derived from sera from PCa patients and, importantly, established that the glycosylation of the PCa serum PSA was significantly different from the PSA from the LNCaP cell line. In comparison with seminal plasma PSA, the fucose content of PSA from the PCa patient serum was significantly lower and there was a decrease in alpha2,3-linked sialic acid. Differences in the glycosylation of PSA derived from PCa patients' sera, seminal plasma, and LNCaP cells were further established by lectin detection, glycosylation immunosorbent assay, and two-dimensional electrophoresis. We also investigated whether the impact of glycosylation changes initiated by the tumor was reflected in the serum glycome. By comparing the glycans released from the total glycoproteins in PCa patient serum with those of normal serum we found an increase in the proportion of sialyl-Lewis x structures. Further analysis of the glycosylation of PSA from PCa and BPH sera will be required in order to determine the utility of these glycan differences to discriminate specifically between benign and malignant prostate states.

Biology of prostate-specific antigen

The human kallikrein (hk) family, located on chromosome 19, encodes prostate-specific antigen (PSA [or hK3]), hK2, hK4, and hK15 (prostin), as well as other serine proteases. Although PSA has been used in the detection of prostate cancer for several years, much remains unknown about its function and forms. The regulatory mechanisms of PSA are vital to its understanding. A particular mechanism by which PSA forms complexes with either alpha1-antichymotrypsin or alpha2-macroglobulin may provide important information for disease detection and progression. Data are emerging that show that active hK2, hK4, and hK15 may be important to convert pro-PSA to the active PSA enzyme. This information, along with insights into the precise mechanisms of PSA expression, may be used to suggest that PSA and, perhaps, other members of the hK family contribute critical control mechanisms to tumor invasion or progression. Although much remains to be revealed on the role of these gene products in the detection and progression of prostate cancer, findings from studies that show sensitive signaling of the disease > or =20 years before the diagnosis of clinically significant prostate cancer may alter screening procedures and improve treatment options.

Discrimination of Prostate Cancer from Benign Disease by Plasma Measurement of Intact, Free Prostate-specific Antigen Lacking an Internal Cleavage Site at Lys145-Lys146

Background: The proportion of free prostate-specific antigen (PSA) is higher in the sera of patients with benign prostatic hyperplasia compared with patients with prostate cancer (PCa). We developed an immunoassay that measures intact, free PSA forms (fPSA-I), but does not detect free PSA that has been internally cleaved at Lys145-Lys146 (fPSA-N), and investigated whether this form could discriminate patients with PCa from those without PCa.

Methods: The assay for fPSA-I uses a novel monoclonal antibody (MAb) that does not detect PSA that has been internally cleaved at Lys145-Lys146. A MAb specific for free PSA was used as a capture antibody, and purified recombinant proPSA was used as a calibrator. The concentrations of fPSA-I, free PSA (PSA-F), and total PSA (PSA-T) were analyzed in EDTA-plasma samples (n = 276) from patients who participated in a screening program for PCa (PSA-T, 0.83–76.3 μg/L).

Results: The detection limit of the fPSA-I assay was 0.035 μg/L. Both the measured concentrations of fPSA-I and the concentrations of fPSA-N (calculated as PSA-F − fPSA-I) provided statistically significant discrimination of the two clinical groups. By contrast, PSA-F did not discriminate between these groups. Each of the ratios fPSA-I/PSA-F, fPSA-N/PSA-T, and PSA-F/PSA-T separated cancer samples from noncancer samples in a statistically significant manner (P &lt;0.0001). The ratio fPSA-I/PSA-F was significantly higher in cancer (median, 59%) compared with noncancer samples (47%).

Conclusions: The ratio fPSA-I/PSA-F is significantly higher in cancer compared with noncancer. The percentages of both fPSA-N/PSA-T and fPSA-I/PSA-F may provide interesting diagnostic enhancements alone or in combination with other markers and require further studies.

Activation of latent protease function of pro-hK2, but not pro-PSA, involves autoprocessing

BACKGROUND

Human glandular kallikrein 2 (hK2) and prostate-specific antigen (PSA) are members of an extensive kallikrein family of proteases. Both proteases are secreted as zymogens or proenzymes containing a seven amino acid propeptide that must be proteolytically removed for enzymatic activation. The physiological proteases that activate pro-hK2 and pro-PSA are not known.

METHODS

The pro-hK2 peptide sequence is Val-Pro-Leu-Ile-Gln-Ser-Arg (VPLIQSR). For PSA, the amino acid sequence of the propeptide is Ala-Pro-Leu-Ile-Leu-Ser-Arg (APLILSR). Fluorescent substrates were made by coupling these peptide sequences to 7-amino-4-methylcoumarin (AMC). The hydrolysis of the VPLIQSR-AMC and APLILSR-AMC substrates by hK2, PSA, and a panel of purified proteases was determined.

RESULTS

HK2 readily cleaved the pro-hK2 peptide substrate VPLIQSR-AMC with a rate of hydrolysis that was approximately 8-fold higher than an equimolar amount of purified trypsin. HK2 also had the highest hydrolysis rate from among a group of other trypsin-like proteases. In contrast, neither hK2 nor PSA was able to appreciably cleave the pro-PSA substrate APLILSR-AMC. The pro-PSA substrate was most readily hydrolyzed by urokinase and trypsin.

CONCLUSIONS

HK2 can hydrolyze the pro-hK2 substrate suggesting that maturation of pro-hK2 to enzymatically active hK2 involves autoprocessing. As expected, PSA, a chymotrypsin-like protease, was unable to hydrolyze either of the propeptide substrates. Therefore, it is unlikely that PSA can auto-process its own enzymatic function. HK2 has trypsin-like specificity but was unable to hydrolyze the pro-PSA substrate. These results raise the possibility that an additional processing protease may be required to fully process PSA to an enzymatically active form.

Production and Characterization of Novel Anti-Prostate-specific Antigen (PSA) Monoclonal Antibodies That Do Not Detect Internally Cleaved Lys145-Lys146 Inactive PSA

Background: The nature of free, uncomplexed prostate-specific antigen (PSA) in the circulation is still unknown. In this study, we developed novel anti-PSA antibodies using PSA produced by a metastasized cancer cell line, LNCaP, as an immunogen.

Methods: Hybridoma cell lines were screened with different methods that aimed at finding antibodies specific for the forms of free PSA produced by LNCaP cell line. Obtained antibodies were further studied for their characteristics related to previously characterized monoclonal antibodies.

Results: Numerous anti-PSA antibodies were obtained, of which four represented unique epitopes previously unrecognized by us. One free-PSA-specific antibody was bound to PSA on two distinct epitopes, and one antibody was bound to the carboxyl-terminal peptide of PSA. Two antibodies were found to bind to the peptide sequence adjacent to the internal cleavage site Lys145-Lys146. These antibodies failed to recognize internally cleaved PSA at Lys145-Lys146. We could not find anti-proPSA antibodies despite the fact that LNCaP PSA contained more than one-half of the zymogen form of PSA.

Conclusions: We report, for the first time, novel anti-PSA antibodies that do not recognize internally cleaved PSA at Lys145-Lys146 and thus are specific for intact, unclipped PSA.