Molecular mass and carbohydrate structure of prostate specific antigen: studies for establishment of an international PSA standard

Reverse-Phase versus Sandwich Antibody Microarray, Technical Comparison from a Clinical Perspective

Protein microarrays are powerful tools to quantify and characterize proteins in multiplex assays. They have great potential within clinical diagnostics and prognostics, as they minimize consumption of both analyte and biological sample. Assays that do not require labeling of the biological specimen, henceforth called label-free, are vital for ease of clinical sample processing. Here, we evaluate two label-free techniques, reverse-phase and sandwich antibody assays, using microarrays on high-performance porous silicon surfaces and fluorescence detection. In view of increasing interest in reverse microarrays, this paper focuses on analytical sensitivity of the reverse assays compared to the more complex but highly sensitive sandwich assay. Sensitivity, linear range, and reproducibility of the two assays were compared using prostate-specific antigen (PSA) in buffer. The sandwich assay displayed 5 orders of magnitude lower detection limit (0.7 ng/mL) compared to the reverse assay (70 microg/mL). PSA at 50 nM (1.5 microg/mL) in cell lysates was detected by the sandwich assay but not by the reverse assay, demonstrating again a far lower detection limit for sandwich microarrays. In independent assay runs of PSA spiked in female serum, the sandwich assay had good linearity (R2 > 0.99) and reproducibility (coefficient of variation < or =15%), and the detection limit could be improved to 0.14 ng/mL. Without further signal amplification, the sandwich assay would be our choice for PSA analysis of clinical samples using a microarray technology platform.

Age related prostate-specific antigen reference range among men in south-East Caspian Sea

The purpose of this study was to describe the distribution of serum prostate specific antigen (PSA) and to determine age-specific reference range in a population of Persian men. Venous blood samples were taken from 287 men, from Gorgan located in the North of Iran, South-East of Caspian Sea, aged 15 > or = 80 year. The serum PSA levels was measured using Enzyme-linked Immunosorbant-Assay (ELISA) technique and age-specific range for PSA level was determined. The serum prostate-specific antigen level for six age group of 15-40 years, 41-50 years, 51-60 years, 61-70 years, 71-80 years and >80 years were mainly in the range of 0-2.5 ng mL(-1), for 76.6%, 2.6-4 ng mL(-1) for 9.1% and as whole 85.7% of all men in this study had < or = 4 ng mL(-1), 8.7 and 5.6% all men of six age group had PSA level of 4.1-10 ng mL(-1) and >10 ng mL(-1), respectively. The findings of present study indicated that a large proportion (76.6%) men in this region have a lower PSA level of 0-2.5 ng mL(-1) and only 9.1% of men have PSA level of 2.6-4 ng mL(-1). It is therefore concluded that acceptable reference range of 0-4 ng mL(-1) for PSA level require further reassessment.

Characterization of corpora amylacea glycoconjugates in normal and hyperplastic glands of human prostate

It has been shown that there are sugars in corpora amylacea, but little attention has been focused on the expression of glycoconjugates in corpora amylacea of normal and hyperplastic prostatic glands. The present study characterizes and compares the expression of glycoconjugates in corpora amylacea of normal and hyperplastic prostatic glands of elderly men by using alcian blue (AB) stain and lectin histochemistry. Corpora amylacea were larger and more numerous in hyperplastic glands compared to normal glands. The stain with AB revealed the presence of sulfated and carboxyl components in corpora amylacea. In hyperplastic prostatic glands the sulfur and acid contents of corpora amylacea were increased. Lectin affinities of corpora amylacea from normal prostatic glands demonstrated the presence of fucose, mannose, sialic acid, N-acetyl galactosamine and N-acetyl glucosamine residues. In the hyperplastic glands the lectin binding pattern of corpora amylacea was qualitatively similar to normal glands, but an increase in GalNAc, sialic acid, mannose and fucose residues was observed. Normal prostatic glands showed a weak to moderate content of mannose residues, and in contrast a strong GNA and Con-A staining was observed in hyperplastic glands. MAA and SNA affinities indicated that the content of sialic acid residues was higher in hyperplastic glands compared with normal prostatic glands. Also NAcGal residues were increased in hyperplastic glands. Luminal secretion, secretory cells and apical border of epithelium showed a similar although more intense Lectin-binding pattern as compared with corpora amylacea both in normal and hyperplastic prostatic glands. Lectin histochemistry shows that the glycoconjugates expressed in the glandular epithelium are similar to those found in corpora amylacea both in normal and hyperplastic glands. In addition, in hyperplastic glands, where the corpora amylacea are higher in size and more numerous, the reaction to lectins is more intense especially with mannose and sialic acid residues. The results suggest that corpora amylacea are originated at least in part from prostatic secretion.

A comparative study of serum protein-bound sialic acid in benign and malignant prostatic growth: possible role of oxidative stress in sialic acid homeostasis

Benign and malignant prostatic growths are associated with an increase in sialoconjugates (e.g. prostate-specific antigen (PSA)) in blood. Oxidative stress plays a crucial role in pathogenesis of various malignancies. The objective of this study was to evaluate oxidative stress parameters and protein-bound sialic acid level in sera of prostatic tumor cases and to asses for any association between them. Sera samples were collected and estimated for carbonylation of proteins, lipid peroxidation products, PSA and protein-bound sialic acid from 10 patients in each group with prostatic carcinoma (Ca prostate) and benign prostatic hyperplasia (BPH) along with 10 healthy male subjects of similar age group as control. In carcinoma prostate cases, lipid peroxides, protein carbonyls, protein-bound sialic acid and PSA were significantly increased compared to BPH and controls. There was significant association between oxidative stress parameters (lipid peroxide and protein carbonyl) and sialoconjugates (PSA and protein-bound sialic acid). In BPH cases, serum lipid peroxides and protein-bound sialic acid were significantly higher in comparison to controls and protein carbonyls were correlated with protein-bound sialic acid. ROC curve for sialic acid showed that it can be used as a marker to differentiate carcinoma prostate from benign growth of prostate at a cutoff level of 11.38 mug/mg protein with a sensitivity of 100% and specificity of 80%. We conclude that oxidative stress might be associated with the degree of sialylation of protein and graded changes in these parameters possibly unveil the pathogenic demarcation from benign to malignant condition of prostate.

SELDI-TOF proteomic analysis and cancer detection

BACKGROUND

In recent years proteomic approaches have been widely used to diagnose disease and the new technology of surface enhanced laser-desorption and ionization time-of-flight mass spectrometry (SELDI-TOF MS) is very promising.

METHODS

A review of English language literature was undertaken using Internet databases such as PubMed and Medline for studies using proteomic technologies for the early detection of cancer.

RESULTS

SELDI-TOF is an array based mass spectrometric method in which proteins of interest are selectively absorbed onto a chemically modified surface and the mass and amount of each protein is measured by irradiating the surface with a laser and measuring the time-of-flight. The technology has revealed a large number of previously uncharacterized biomarkers for a wide variety of cancers. Its versatility has also been demonstrated by its application as a tool for mining a wide variety of biological tissues and fluids.

CONCLUSIONS

The majority of these studies have discriminated between diseased and healthy controls with a high degree of sensitivity and specificity. The reproducibility, standardization and feasibility of this technology need to be addressed before these proteomic approaches to the discovery of novel, highly sensitive diagnostic tools can become routine clinical care.

Molecular forms of human prostate-specific antigen in urine of subjects with benign prostatic hyperplasia

In the present study, we examined mollecular forms of urinary prostate-specific antigen (PSA), focusing on its structural complexity in general and specifically on its microheterogeneity in relation to benign prostatic hyperplasia (BPH). Gel filtration, ion-exchange chromatography, and lectin-affinity chromatography were used to characterize PSA. In comparing the binding pattern of PSA isoforms, moderate changes were observed in the relative abundance of distinct molecular subpopulations separated on lectin-affinity columns. They may be related to alteration in the position and type of linkage of fucose or sialic acid, as well as to modification of the trimannosyl core by branching of the PSA oligosaccharide chain.

Prognostic value of serum markers for prostate cancer

The incidence of prostate cancer has increased dramatically during the last 10-15 years and it is now the commonest cancer in males in developed countries. The increase is mainly caused by the increasing use of opportunistic screening or case-finding based on the use of prostate-specific antigen (PSA) testing in serum. With this approach, prostate cancer is detected 5-10 years before giving rise to symptoms and on average 17 years before causing the death of the patient. While this has led to detection of prostate cancer at a potentially curable stage, it has also led to substantial overdiagnosis, i.e. detection of cancers that would not surface clinically in the absence of screening. A major challenge is thus to identify the cases that need to be treated while avoiding diagnosing patients who will not benefit from being diagnosed and who will only suffer from the stigma of being a cancer patient. It would be useful to have prognostic markers that could predict which patients need to be diagnosed and which do not. Ideally, it should be possible to measure these markers using non-invasive techniques, i.e. by means of serum or urine tests. As it is very useful for both early diagnosis and monitoring of prostate cancer, PSA is considered the most valuable marker available for any tumor. Although the prognostic value of PSA is limited, measurement of the proportion of free PSA has improved the identification of patients with aggressive disease. Furthermore, the rate of increase in serum PSA reflects tumor growth rate and prognosis but, due to substantial physiological variation in serum PSA, reliable estimation of the rate of PSA increase requires follow-up for at least 2 years. Algorithms based on the combined use of free and total PSA and prostate volume in logistic regression and neural networks can improve the diagnostic accuracy for prostate cancer, and assays for minor subfractions of PSA and other new markers may provide additional prognostic information. Markers of neuroendocrine differentiation are useful for the monitoring of androgen-independent disease and various bone markers are useful in patients with metastatic disease.

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.

Capillary electrophoresis for the investigation of prostate-specific antigen heterogeneity

Prostate-specific antigen (PSA) is a single-chain glycoprotein that is used as a biomarker for prostate-related diseases. PSA has one known posttranslational modification, a sialylated diantennary N-linked oligosaccharide attached to the asparagine residue N45. In this study capillary electrophoresis (CE) was employed to separate the isoforms of seven commercially available free PSA samples, two of which were specialized: enzymatically active PSA and noncomplexing PSA. The free PSA samples examined migrated as four to nine distinct, highly resolved peaks, indicating the presence of several isoforms differing in their oligosaccharide compositions. Overall, the use of CE provides a rapid, reproducible method for separation of PSA into its individual isoforms.

Glycosylation of urinary prostate-specific antigen in benign hyperplasia and cancer: assessment by lectin-binding patterns

OBJECTIVES

In the present study, we examined the glycosylation of urinary prostate-specific antigen (PSA) from benign prostatic hyperplasia (BPH) and prostate cancer (PCa) subjects, specifically looking at alterations in its oligosaccharide chain as a potential biomarker of these pathophysiological conditions.

DESIGN AND METHODS

First morning urine voids were collected from subjects with PCa and BPH before initiation of any treatment. Urinary PSA was characterized by ion-exchange chromatography, followed by lectin affinity chromatography on the columns using immobilized plant lectins.

RESULTS

Four isoforms of urinary PSA from both BPH and PCa samples were separated by ion-exchange chromatography. The elution profiles from lectin-affinity columns reflected molecular heterogeneity of PSA isoforms and the main differences observed were in the reactivity to Ulex europaeus agglutinin, Aleuria aurantia agglutinin, Phaseolus vulgaris erythroagglutinin and Phaseolus vulgaris leukoagglutinin.

CONCLUSIONS

The observed differences in the lectin reactivities between BPH PSA and PCa PSA may be of clinical importance in the evaluation of prostate health.

Comparison by LC-MS and MALDI-MS of prostate-specific antigen from five commercial sources with certified reference material 613

OBJECTIVE

Prostate-specific antigen (PSA) samples from five commercial sources were compared with PSA from the European Commission Community Bureau of Reference, known as CRM 613, using liquid chromatography-mass spectrometry (LC-MS) and matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS).

DESIGN AND METHODS

The effects of storage at various temperatures, multiple freeze-thaw cycles and lyophilization vs. solution storage were investigated.

RESULTS

: CRM 613 and the five commercially available samples varied in their heterogeneity and in the amount of PSA with a molecular mass of 28,430 kDa. Few changes were observed among any of the samples during the stability study or study of the effect of lyophilization and freeze-thaw cycles.

CONCLUSIONS

Use of many of the commercial PSA sources as authentic PSA standards for calibrants for in vitro diagnostic tests could lead to significantly biased PSA measurements. Although all the PSA samples appear to be relatively stable for the time periods and conditions studied, some of the commercially available samples contain limited amounts of PSA and included other unidentified components.

A comprehensive characterization of the peptide and protein constituents of human seminal fluid

BACKGROUND

Knowledge of the peptide and protein components of seminal fluid and their role in prostate diseases including benign prostatic hyperplasia and prostate carcinoma is scant. We have undertaken a proteomic analysis of semen as a forerunner to identifying sensitive and specific diagnostic markers of prostatic diseases; to aid in improved therapeutic intervention; and, to enhance our understanding of prostate health and disease.

METHODS

Peptide and protein components of pooled human seminal fluid (n = 5) were separated by gel electrophoresis (1D and 2D) and identified by either matrix-assisted laser desorption ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) or capillary liquid chromatography tandem mass spectrometry (LC-MS/MS).

RESULTS

Analysis by two-dimensional electrophoresis (2DE) established that there were multiple post-translational variants of the majority of the proteins. Hormones, growth factors and bioactive peptides were detected and identified.

CONCLUSIONS

We have identified over 100 protein and peptide components of normal human seminal fluid.

Human Tissue Kallikreins: Physiologic Roles and Applications in Cancer

Tissue kallikreins are members of the S1 family (clan SA) of trypsin-like serine proteases and are present in at least six mammalian orders. In humans, tissue kallikreins (hK) are encoded by 15 structurally similar, steroid hormone–regulated genes (KLK) that colocalize to chromosome 19q13.4, representing the largest cluster of contiguous protease genes in the entire genome. hKs are widely expressed in diverse tissues and implicated in a range of normal physiologic functions from the regulation of blood pressure and electrolyte balance to tissue remodeling, prohormone processing, neural plasticity, and skin desquamation. Several lines of evidence suggest that hKs may be involved in cascade reactions and that cross-talk may exist with proteases of other catalytic classes. The proteolytic activity of hKs is regulated in several ways including zymogen activation, endogenous inhibitors, such as serpins, and via internal (auto)cleavage leading to inactivation. Dysregulated hK expression is associated with multiple diseases, primarily cancer. As a consequence, many kallikreins, in addition to hK3/PSA, have been identified as promising diagnostic and/or prognostic biomarkers for several cancer types, including ovarian, breast, and prostate. Recent data also suggest that hKs may be causally involved in carcinogenesis, particularly in tumor metastasis and invasion, and, thus, may represent attractive drug targets to consider for therapeutic intervention.

Chemical synthesis of normal and transformed PSA glycopeptides

Chemical syntheses are reported for prostate specific antigen (PSA) N-linked glycopeptide fragments consisting of an uneicosapeptide (residues 27-47 of PSA) with di-, tri-, and tetrabranched N-acetyllactosamine-type glycans. The syntheses involve simultaneous, multiple glycosylations of the corresponding pentasaccharide acceptors prepared from a common trisaccharide precursor. Globally deprotected glycans are aminated and then aspartylated with a hexapeptide, which is then extended using native chemical ligation (NCL). The glycopeptides will be used for the generation of antibodies that may form the basis for a new prostate cancer diagnostic assay.

Kallikrein 4 is a Predominantly Nuclear Protein and Is Overexpressed in Prostate Cancer

Kallikreins (KLKs) are highly conserved serine proteases that play key roles in a variety of physiological and pathological processes. KLKs are secreted proteins that have extracellular substrates and function. For example, prostate-specific antigen (or KLK3) is a secreted protein that is widely used as a diagnostic marker for prostate cancer. KLK4 is a recently identified member of the kallikrein family that is regulated by androgens and is highly specific to prostate for expression. Here, we show that the gene product of KLK4, hK4, is the first member of the KLK family that is intracellularly localized. We provide strong evidence that the previously assigned first exon that was predicted to code for a signal peptide that would target hK4 for secretion is not part of the physiologically relevant form of KLK4 mRNA. In addition to detailed mapping of the KLK4 mRNA 5' end by RT-PCR, this conclusion is supported by predominantly nuclear localization of the hK4 protein in the cell, documented by both immunofluorescence and cell fractionation experiments. Furthermore, in addition to androgens, hK4 expression is regulated by estrogen and progesterone in prostate cancer cells. Finally, in situ hybridization on normal and hyperplastic prostate samples in tissue microarrays indicate that KLK4 is predominantly expressed in the basal cells of the normal prostate gland and overexpressed in prostate cancer. These data suggest that KLK4 has a unique structure and function compared with other members of the KLK family and may have a role in the biology and characterization of prostate cancer.

Recombinant prostate specific antigen inhibits angiogenesis in vitro and in vivo

BACKGROUND

Prostate specific antigen (PSA) is a kallikrein family member with serine protease activity commonly used as a diagnostic marker for prostate cancer. We recently described anti-angiogenic properties of PSA [Fortier et al.: JNCI 91:1635-1640].

METHODS

Two forms of PSA were cloned and expressed in Pichia pastoris: one, an intact PSA with an N-terminus of IVGGVS em leader; the second, an N-1 PSA variant. The recombinant proteins were tested for serine protease activity and for anti-angiogenic activity in vitro and in vivo.

RESULTS

The rate of substrate hydrolysis by the intact recombinant PSA was similar to that of PSA isolated and purified from human seminal plasma. In contrast, the N-1 PSA variant lacked serine protease activity. In an endothelial cell migration assay, the concentration that resulted in 50% inhibition (IC(50)) was: 0.5 microM for native PSA, 0.5 microM for intact recombinant protein, and 0.1 microM for the N-1 variant PSA. Both the intact recombinant and the N-1 recombinant PSA inhibited angiogenesis in vivo.

CONCLUSIONS

Purified recombinant PSA inhibits angiogenesis, proving the concept that PSA is an anti-angiogenic, and serine protease activity, as determined by synthetic substrate hydrolysis, is distinct from the anti-angiogenic properties of PSA.

Structural Diversity of Cancer-related and Non-Cancer-related Prostate-specific Antigen

Background: Heterogeneity among the various molecular forms of prostate-specific antigen (PSA) has not been well characterized, despite the critical importance of PSA in the detection of prostate cancer. The purpose of this study was to examine PSA heterogeneity in cancerous and noncancerous materials by extensive and systematic protein analysis.

Methods: A catalog of molecular forms of PSA was established with the PSA purified from seminal fluid. This catalog was used to analyze PSA heterogeneity in cancerous and noncancerous materials by immunoblotting with polyclonal antibodies.

Results: PSA from noncancerous materials showed a wider range of molecular mass, from 6000 to 28 000 Da. PSA from cancerous materials did not contain lower molecular mass forms.

Conclusions: The PSA protein catalog may be useful for the analysis of differences among PSA forms in men with and without prostate cancer and for analysis of antibodies used to detect PSA.

High level of alpha1-acid glycoprotein in human seminal plasma is associated with high branching and expression of Lewis(a) groups on its glycans: supporting evidence for a prostatic origin

BACKGROUND

Changes in concentration of seminal plasma alpha(1)-acid glycoprotein (AGP) have been studied in detail before. However, the source of high levels of AGP as well as the glycosylation of seminal plasma AGP has not been elucidated yet.

METHODS

The glycosylation of AGP was studied by crossed affinity immunoelectrophoresis using fucose-specific lectins and immunostaining. Glycan structure and monosaccharide analyses were performed by high pH anion exchange chromatography with pulsed amperometric detection. Fucosyltransferases were analyzed for activity and their substrate specificity was determined.

RESULTS

Two types of fucosylation were detected; Lewis(x) and Lewis(a). Lewis(a) groups were only present on AGP of individuals with a high concentration and were completely absent when the AGP concentration in seminal plasma was low. Lewis(a) expression coincides with a higher degree of branching of the glycans and a relative increased alpha4-fucosyltransferase activity. The molecular weight of all seminal plasma AGP was slightly higher than of blood plasma AGP (approx. 47 vs. 41-43 kDa).

CONCLUSIONS

The results indicate that AGP in seminal fluid most likely originates from the prostate and that it is either alpha3- or alpha4-fucosylated.

Separation methods applicable to prostate cancer diagnosis and monitoring therapy

During the last decade, significant research has been conducted using prostate-specific antigen (PSA) in the basic and clinical sciences and many advances have occurred in the clinical use of PSA for detecting and monitoring prostate cancer (PCa). Separation methods including gel-permeation chromatography, isoelectric focusing, lectin-affinity chromatography, polyacrylamide gel electrophoresis and high-performance liquid chromatography have made significant contributions to the discovery and identification of different molecular forms of PSA. Furthermore, the measurement of free and total PSA has improved the ability of PSA to detect early PCa. However, unnecessary biopsies are still needed for men with slightly elevated PSA values. On the other hand, PSA is not adequate for staging newly diagnosed PCa and prognosticating the course in individual cases. The possible application of separation methods in the basic science of prostate cancer may be associated with identification of more cancer-specific forms of PSA and discoveries of other serum proteins useful not only for detecting, but also for staging and prognosticating PCa. Such novel markers might lead to a better understanding of PCa aggressiveness and to developments in the clinical field of treatment.

The role of molecular forms of prostate-specific antigen (PSA or hK3) and of human glandular kallikrein 2 (hK2) in the diagnosis and monitoring of prostate cancer and in extra-prostatic disease

Prostate-specific antigen (PSA or hK3) is a glandular kallikrein with abundant expression in the prostate that is widely used to detect and monitor prostate cancer (PCa), although the serum level is frequently elevated also in benign and inflammatory prostatic diseases. PSA testing is useful for early detection of localized PCa and for the detection of disease recurrence after treatment. However, PSA has failed to accurately estimate cancer volume and preoperative staging. There is no PSA level in serum that definitively distinguishes men with benign conditions from those with prostate cancer, although PCa is rare in men with PSA levels in serum < 2.0 ng/ml. This prompted searches for enhancing parameters to combine with PSA testing, such as PSA density, PSA velocity, and age-specific reference ranges. Due to the protease structure, PSA occurs in different molecular forms in serum and their concentrations vary according to the type of prostatic disease. Human glandular kallikrein 2 (hK2) is very similar to PSA, but expressed at higher levels in prostate adenocarcinoma than in normal prostate epithelium. Blood testing for hK2 combined with different PSA forms improves discrimination of men with benign prostatic disease from those with prostate cancer. Many data have also been reported on the extra-prostatic expression of both PSA and hK2, and it is now believed that they may both have functions in tissues outside the prostate.

Characterization of PSA-RP2, a protein related to prostate-specific antigen and encoded by alternative hKLK3 transcripts

Despite the wide use of prostate-specific antigen (PSA) as a marker of prostate cancer, analysis of its gene products has not yet been completed. The structure of two alternative mRNAs (0.9 and 1.65 kb) of the hKLK3 gene that retain the third intron is reported here. These partially spliced transcripts were detected by hybridization or RT-PCR in normal prostate tissue, benign prostate hyperplasia (BPH) and cancerous prostate tissues, and also in the prostate LNCaP cell line. Insertion of the unspliced intron creates an in-frame stop codon and results in a truncated prepro PSA variant of 180 amino-acid residues. This novel variant, designated PSA-RP2, has an alternate C-terminal tail and lacks the serine residue essential for the catalytic activity of PSA. Prepro PSA-RP2 was transiently produced in COS-7 cells and detected in the spent medium using an anti-PSA serum. Secreted PSA-RP2 was glycosylated with an apparent molecular mass of 25 kDa. Our findings suggest that PSA-RP2 contributes to the molecular heterogeneity of free-PSA in the serum of patients with benign or malignant prostate tumors.

Measurement of Circulating Forms of Prostate-specific Antigen in Whole Blood Immediately after Venipuncture: Implications for Point-of-Care Testing

Background: The purpose of this study was to validate the use of whole-blood samples in the determination of circulating forms of prostate-specific antigen (PSA).

Methods: Blood samples of hospitalized prostate cancer and benign prostatic hyperplasia patients were collected and processed to generate whole-blood and serum samples. Three different rapid two-site immunoassays were developed to measure the concentrations of total PSA (PSA-T), free PSA (PSA-F), and PSA-α1-antichymotrypsin complex (PSA-ACT) to detect in vitro changes in whole-blood samples immediately after venipuncture. The possible influence of muscle movement on the release of PSA from prostate gland was studied in healthy men by measuring the rapid in vitro whole-blood kinetics of PSA forms before and after 15 min of physical exercise on a stationary bicycle.

Results: Rapid PSA-T, PSA-F, and PSA-ACT assays were designed using a 10-min sample incubation. No significant changes were detected in the concentrations of PSA-T, PSA-F, and PSA-ACT from the earliest time point of 12–16 min compared with measurements performed up to 4 h after venipuncture. Physical exercise did not influence the concentrations of the circulating forms of PSA. Hematocrit-corrected whole-blood values of PSA-T and PSA-F forms were comparable to the respective serum values. Calculation of the percentage of PSA-F (PSA F/T ratio × 100) was similar irrespective of the sample format used, i.e., whole blood or serum.

Conclusions: We found that immunodetectable PSA forms are likely at steady state immediately after venipuncture, thus enabling the use of anticoagulated whole-blood samples in near-patient settings for point-of-care testing, whereas determinations of PSA (e.g., PSA-T, PSA-F, or PSA-ACT) performed within the time frame of the office visit would provide results equivalent to conventional analyses performed in serum.

Structural characteristics of the N-glycans of two isoforms of prostate-specific antigens purified from human seminal fluid

Prostate-specific antigen (PSA) is a glycosylated chymotrypsin-like serine protease and is found mainly in prostatic tissue and seminal fluid. We purified two forms of PSA (PSA-A and PSA-B) from human seminal fluid with pI values of approx. 7.2 and approx. 6.9, respectively. To characterize the N-glycans of the two isoforms, the sugar chains were liberated by hydrazinolysis followed by N-acetylation, and derivatized with 2-aminobenzamide. Both PSA-A and PSA-B contained mono- and disialylated sugar chains, although PSA-B had a much higher content of the latter. After removal of sialic acid residues by sialidase digestion, mono- and biantennary N-glycans and three outer chain moieties (Galbeta1-4GlcNAcbeta1-, GlcNAcbeta1-, GalNAcbeta1-4GlcNAcbeta1-) were found in both samples. However, the ratios of each N-glycan were different. These results indicate that PSA-A and PSA-B differ not only in their sialic acid contents, but also in their outer chain features.

Expression, purification, and characterization of deglycosylated human pro-prostate-specific antigen

Wild-type and deglycosylated forms of human prostate-specific antigen were expressed in Chinese hamster ovary (CHO) cells as zymogens. ProPSA was collected from conditioned medium and purified using a single cation-exchange chromatographic step for the deglycosylated form and cation-exchange followed by gel filtration chromatography for the wild-type form. Recombinant wild-type proPSA produced in CHO cells has an average MW of 34.5 kDa, whereas the deglycosylated proPSA has a MW of 32.4 kDa. Both forms of proPSA were activated in vitro and the kinetic properties measured for the deglycosylated PSA are very similar to those of the wild-type recombinant PSA and the native PSA isolated from seminal fluid. These results suggest that deglycosylated PSA is likely to be very similar to native PSA with respect to its three-dimensional structure and will provide a homogeneous protein preparation necessary for X-ray crystallographic analysis.

Reference Reagents for Prostate-specific Antigen (PSA): Establishment of the First International Standards for Free PSA and PSA (90:10)

Background: Prostate-specific antigen (PSA) measurements in serum by immunoassay are widely used in the screening, diagnosis, and monitoring of patients with prostate cancer although the lack of common reference reagents has led in the past to wide differences in estimates. We report here the results of a WHO international collaborative study in which two preparations of PSA representative of the main immunoreactive components in serum, free PSA and PSA 90:10, and a preparation of recombinant DNA-derived PSA were assessed as potential standards for the calibration of diagnostic immunoassays for PSA.

Methods: Coded vials of the candidate materials and serum preparations containing PSA in the clinically important range were provided to the 10 laboratories in the study, and participants were asked to perform PSA assays currently in use in their laboratories. Data from 89 immunoassays by 26 different method-laboratory combinations were contributed to the study and analyzed centrally at the National Institute for Biological Standards and Control.

Results: Potency estimates of the preparations relative to the in-house calibrators were in good agreement with the target value of 1 μg of total PSA/vial, the preparation of free PSA giving 1.10 μg/vial (95% confidence interval, 0.99–1.21 μg/vial) and PSA 90:10, 1.11 μg/vial (95% confidence interval, 1.04–1.18 μg/vial). No immunoreactivity was detected in ampoules containing the recombinant material. Use of a common standard of PSA 90:10 significantly reduced the between-laboratory geometric coefficients of variation for serum samples included in the study and gave a much narrower range of potency estimates.

Conclusions: The preparation of free PSA was established by WHO as the First International Standard for PSA (free) with an assigned content of 1 μg of total PSA per vial. In addition, the preparation of bound PSA was established as the First International Standard for PSA (90:10) with an assigned content of 1 μg of total PSA per vial.

Analysis of Free Prostate-specific Antigen (PSA) after Chemical Release from the Complex with α1-Antichymotrypsin (PSA-ACT)

Background: Prostate-specific antigen (PSA), a marker for prostate cancer (CaP), forms a covalent complex with α1-antichymotrypsin (ACT) in human blood. Structural analysis of the PSA-ACT complex is difficult, and complexation may be a reason for biased immunological assays when compared with the analysis of free PSA. We developed a method to cleave the PSA-ACT complex chemically. The liberated PSA was thus available for analysis as free PSA (F-PSA).

Methods: PSA was released from the PSA-ACT complex by cleaving the interprotein ester bond with ethanolamine under alkaline conditions. The release was followed by reversed-phase HPLC and an immunoassay for F-PSA. Released PSA obtained from human blood was further immunopurified and analyzed by matrix-assisted laser desorption-induced time of flight (MALDI-TOF) mass spectrometry.

Results: In vitro-prepared PSA-ACT complex was completely cleaved by treatment with nucleophilic compounds such as ethanolamine at pH 9–10. The released PSA was stable under these conditions and could be measured by reversed-phase HPLC as well as the ENZYMUN® immunoassay for F-PSA. When plasma from a CaP patient [containing 190 μg/L F-PSA and 1890 μg/L total PSA (T-PSA)] was treated under similar conditions, a concentration of ∼1600 μg/L F-PSA was measured at the end of the incubation, indicating that the PSA-ACT complex was completely cleaved. Two benign prostatic hyperplasia and CaP sera panels (12 and 13 sera, respectively) containing 4–45 μg/L T-PSA were similarly treated. The concentrations of F-PSA measured after incubation were, on average, 85% of the T-PSA values of the untreated sera. Finally, the PSA released from the complex of the CaP plasma was isolated by immunosorption, analyzed by MALDI-TOF mass spectrometry, and compared to PSA obtained from semen. The intact PSA as well as the peptides observed after digestion with endoproteinase Lys C did not reveal any structural difference between the PSA from these two sources.

Conclusions: PSA complexed to ACT in plasma of a CaP patient seems to be structurally very similar to the PSA reference material from semen. The release of PSA from the PSA-ACT complex allows F-PSA and T-PSA to be measured by the same immunological assay, thus eliminating any possible bias between two different assays.

Serial lectin affinity chromatography demonstrates altered asparagine-linked sugar-chain structures of prostate-specific antigen in human prostate carcinoma

Differences between prostate carcinoma (PCA) and benign prostatic hyperplasia (BPH) in asparagine (N)-linked sugar-chain structures of prostate-specific antigen (PSA) were investigated using serial lectin affinity chromatography. The amounts of PSA passing through columns of concanavalin A (Con A), phytohaemagglutinin E4 (PHA-E4) and PHA-L4 were significantly greater for PCA-derived PSA than BPH. We propose that the sugar moiety structure of PSA which is increased in PCA is a multiantennary complex type with branched N-acetylglucosamine beta(1->4) mannose. We suggest that N-linked sugar chains in PSA are altered during oncogenesis in the human prostate and may serve as diagnostic tools for PCA.

Epitope Analysis of a Prostate-specific Antigen (PSA) C-Terminal-specific Monoclonal Antibody and New Aspects for the Discrepancy between Equimolar and Skewed PSA Assays

Background: Immunoassays to measure prostate-specific antigen (PSA) often give different values for the same patient samples, and the calibrators among commercial immunoassays are not interchangeable. We developed three novel assays to quantify the free and complexed forms of PSA in serum.

Methods: We synthesized 46 peptides, which encompassed the entire PSA molecule, and determined the interactions between selected monoclonal antibodies (MAbs) and those peptides or the intact PSA molecule.

Results: MAb PA313 did not cross-react with human glandular kallikrein (hK2), which has 78% amino acid homology to PSA. This MAb bound with KD = 40 nmol/L to the C-terminal peptide of PSA and distinguished between a synthetic peptide derived from PSA (PSA46A: NH2-C-R226KWIKDTIVANP237-COOH) that differed from one derived from hK2 (PSA46B: NH2-C-R226KWIKDTAANP237-COOH) by a single amino acid. Only the MAb combination of PA313/PA121 showed equimolar reactivity with PSA and with PSA complexed with α1-antichymotrypsin (PSA-ACT). The free form of PSA (F-PSA) was determined by MAbs PA313/FPA503, and the amount of complexed PSA (C-PSA) in PSA-ACT was determined by αACT/PA313. The total PSA (T-PSA) measured by either of the equimolar assays (PA313/PA121 or Tandem-R) was consistent with the sum of F-PSA and C-PSA. In contrast, T-PSA by a skewed assay (IMx) was higher than F-PSA + C-PSA when the ratio of F-PSA to T-PSA (F/T) was &gt;0.15. T-PSA measured by IMx was nearly equal to F-PSA/0.55 + C-PSA. The coefficient 0.55 reflected different reactivities of the IMx assay with PSA-ACT and PSA.

Conclusion: The discrepancy between the values measured by equimolar and skewed assays depends on the ratio of free to total PSA in the sample.

Prostate-specific antigen and new related markers for prostate cancer

Although prostate-specific antigen (PSA), or human kallikrein 3, is the most valuable tool available for the diagnosis and management of prostate cancer, as currently used it is insufficiently sensitive and specific for early detection or staging of the malignancy. Many new concepts have been introduced in order to optimize the clinical use of PSA measurements, but each one has its own drawbacks. The molecular forms of PSA, especially the free PSA, seem to be useful for the detection of prostate cancer in men with PSA concentrations falling in the 4-10 microg/l range. New molecular techniques, such as reverse transcriptase polymerase chain reaction for the detection of minimal amounts of PSA messenger RNA and prostate-specific membrane antigen, offer new promise for the prognosis and possibly staging of prostate cancer. On the other hand, human kallikrein 2, a serine protease closely related to PSA that is also expressed predominantly in the prostate, may be a new adjuvant marker for prostate cancer. As for its biological functions, PSA can no longer be regarded as a specific prostate molecule associated mainly with semen liquefaction when it has a possible role as a prognostic indicator in female breast cancer. The biological role of PSA in normal tissues and tumors may be much more complex than previously thought and requires further investigation.

Method for identifying prostate cells in semen using flow cytometry

BACKGROUND

Prostate-specific antigen (PSA) cannot differentiate benign prostatic hyperplasia (BPH), from prostatitis, or prostate cancer in the range of 4.0-10 ng/ml. An accurate cytologic or histologic assessment is necessary to confirm the proper diagnosis. The nature of a biopsy tends to make it a selective test not frequently repeated. We are reporting a technique employing semen as a source for the differential diagnosis of prostate epithelial cells.

METHODS

Eleven vasectomized and nonvasectomized prostate cancer patients provided semen samples (stage T1 to T2). Two patients provided repeat samples. In addition, 15 vasectomized or nonvasectomized individuals without evidence of disease provided semen samples. Three million cells fixed with 50% ethanol were stained by an antibody (7E11.C5) to prostate-specific membrane antigen (PSMA), Hybritech Antibody (399) to PSA, and cytokeratin 8 and 18. In addition to the antibodies described, a DNA stain To-Pro 3 was used to identify 2n-4n DNA containing cells. A dual laser, Becton Dickinson FACSCaliber cytometer, was used to analyze the samples.

RESULTS

All semen specimens contained diploid, cytokeratin 18-positive epithelial cells regardless of disease status. A clear difference between prostate cancer and normal prostate cell samples was observed using staining with 7E11.C5. The ratio of prostatic cells in the total epithelial cell population (PSMA:cytokeratin ratios) was calculated for each specimen. A retrospective study of sixteen semen samples from 11 prostate cancer patients had a mean PSMA:cytokeratin ratio of 0.57, whereas the samples from 15 patients without evidence of cancer had a mean PSMA:cytokeratin ratio of 0.11. This difference was significant. PSA staining was variable and inconsistent.

CONCLUSIONS

This report demonstrates that human semen contains prostate cells that can be characterized and used in the clinical diagnosis of prostate cancer.

Human Kallikrein 2 (hK2) and prostate-specific antigen (PSA): two closely related, but distinct, kallikreins in the prostate

Recent studies on human kallikrein 2 (hK2) have revealed striking similarities and significant differences with the closely related kallikrein PSA. Both PSA and hK2 are primarily localized to the prostate and share close structural similarities. Although both kallikreins are produced by the same secretory epithelial cells in the prostate, hK2 is associated more with prostate tumors than PSA and is highly expressed in poorly differentiated cancer cells. The potent trypsin-like activity of hK2 contrasts with the weak chymotrypsin-like activity of PSA. The inactive precursor form of PSA, proPSA, is converted rapidly to active PSA by hK2, suggesting an important in vivo regulatory function by hK2 on PSA activity. The high homology between hK2 and PSA results in significant cross-reactivity to hK2 by polyclonal and some monoclonal antibodies to PSA. Future studies on both PSA and hK2 need to take into account this potential for cross-reactivity. Specific monoclonal antibodies to hK2 have now demonstrated that serum levels of hK2, like PSA, are correlated with prostate cancer. The production of hK2 protein in active protease form and specific monoclonal antibodies to the hK2 antigen will allow extensive future studies delineating the physiological and clinical utility of this new prostate antigen.

Determination and analysis of antigenic epitopes of prostate specific antigen (PSA) and human glandular kallikrein 2 (hK2) using synthetic peptides and computer modeling

Prostate specific antigen (PSA) and human glandular kallikrein 2 (hK2), produced essentially by the prostate gland, are 237-amino acid monomeric proteins, with 79% identity in primary structure. Twenty-five anti-PSA monoclonal antibodies (Mabs) were studied for binding to a large array of synthetic linear peptides selected from computer models of PSA and hK2, as well as to biotinylated peptides covering the entire PSA sequence. Sixteen of the Mabs were bound to linear peptides forming four independent binding regions (I-IV). Binding region I was localized to amino acid residues 1-13 (identical sequence for PSA and hK2), II (a and b) was localized to residues 53-64, III (a and b) was localized to residues 80-91 (= kallikrein loop), and IV was localized to residues 151-164. Mabs binding to regions I and IIa were reactive with free PSA, PSA-ACT complex, and with hK2; Mabs binding to regions IIb, IIIa, and IV were reactive with free PSA and PSA-ACT complex, but unreactive with hK2; Mabs binding to region IIIb detected free PSA only. All Mabs tested (n = 7) specific for free PSA reacted with kallikrein loop (binding region IIIb). The presence of Mabs interacting with binding region I did not inhibit the catalytic activity of PSA, whereas Mabs interacting with other binding regions inhibited the catalysis. Theoretical model structures of PSA, hK2, and the PSA-ACT complex were combined with the presented data to suggest an overall orientation of PSA with regard to ACT.

Prostate-specific antigen. Its discovery and biochemical characteristics

The search for a more sensitive and specific marker for prostate cancer as well as for use in forensic investigations led to the discovery of PSA. Utilization of the purified protein resulted in the development of many immunoassays that are now in widespread use and have been applied successfully to the detection and monitoring of prostate cancer. Subsequent investigation into the biochemical characteristics of PSA revealed free and complexed forms of PSA in serum whose measurement now may provide additional clinical benefit. Further research into the molecular and biochemical properties of PSA as well as improvements in measurement techniques no doubt will lead to new applications and enhanced clinical use for this important tumor marker.

Progress in standardization of immunoassays for prostate-specific antigen

A calibrator composed of 90% prostate-specific antigen-alpha 1-antichymotrypsin (PSA-ACT) and 10% free PSA reduces the variation in control serums with widely divergent amounts of free PSA, from a mean of 28% among nine different assays to 9% after recalibration. The lyophilized form of the Stanford 90:10 calibrator is stable and therefore an excellent candidate for international standardization of PSA assays.

Human prostate-specific glandular kallikrein is expressed as an active and an inactive protein

A polymorphism in the human prostate-specific glandular kallikrein (hKLK2) gene was described by direct sequencing (by PCR) of genomic DNAs isolated from prostatic cancer tissue, benign prostatic hyperplasia tissue, and blood leukocyte specimens. Results showed two forms of human prostate-specific glandular kallikrein protein (hK2), a consequence of a change from C to T at base 792 in the hK2 coding region. Producing the two forms as recombinant proteins in insect cells demonstrated that Arg226-hK2 (CC genotype) is an active protein and Trp226-hK2 (TT genotype) is inactive. Polymorphism studies of 36 patients with prostatic diseases identified only 1 with the TT genotype. The same kind of polymorphism was not detected in the human prostate-specific antigen (hKLK3) gene. Arg226-hK2 possessed only trypsin-like enzyme activity, whereas recombinant human prostate-specific antigen (hPSA) had only chymotrypsin-like activity. Monoclonal and polyclonal antibodies raised against hPSA purified from seminal plasma detected both active and inactive hK2. Thus, because inactive as well as stable hK2 protein may be present, a lack of trypsin-like activity in hPSA standards is not enough to confirm that the materials are free of hK2 contamination.

Individual prostate-specific antigen (PSA) forms as prostate tumor markers

Prostate-specific antigen (PSA) is a kallikrein-like serine protease mainly expressed in the human prostate. It is responsible for the proteolysis of the gel-forming proteins in human semen. Two major extracellular protease inhibitors, alpha-1-antichymotrypsin (ACT) and alpha-2-macroglobulin (AMG) may inactivate PSA escaping from the prostate. The predominant immunodetected form of PSA in serum is complexed to ACT but PSA exists also in a free non-complexed form despite the large excess of inhibitors. The concentrations of PSA in serum are normally less than 4 micrograms/l. but elevated concentrations are found in a majority of patients with prostate cancer (CAP) and the analysis of PSA in serum has become invaluable in the detection and monitoring of patients with CAP. However, it is not an ideal tumor marker in the sense that there are CAP patients with normal PSA concentrations in serum and patients with benign hyperplasia of the prostate (BPH) with elevated PSA concentrations. Analysis of the various PSA forms in serum attracts much interest as there is a higher proportion of PSA in complex with ACT in patients with CAP than in those with BPH. Optimal combinations of monoclonal antibodies have been used to design sensitive noncross-reacting immunoassays for the detection of free PSA, PSA-ACT complexes and the detection of both free PSA and PSA complexes in an equimolar fashion (i.e. total PSA). Several studies have demonstrated that the analysis of the proportions of the free-to-total PSA in serum may increase the diagnostic specificity by 15-20% without significant loss in the sensitivity for detection of CAP.

In vitro stability of free prostate-specific antigen (PSA) and prostate-specific antigen (PSA) complexed to alpha 1-antichymotrypsin in blood samples

OBJECTIVES

To study the in vitro stability of free and complexed forms of prostate specific antigen (PSA) in blood samples in order to establish guidelines for specimen handling, in particular for the clinical utility of the analysis of percentage free PSA.

METHODS

Blood samples were collected and processed to generate serum, heparin plasma, and EDTA plasma. Three different two-site immunoassays were used to measure the concentrations of total PSA (PSA-T), free form of PSA (PSA-F), and PSA-alpha 1-antichymotrypsin complex (PSA-ACT) in order to determine the effect of repeated freezing and thawing, delayed separation of serum from blood cells, and stability during storage at 4 degrees C and 30 degrees C.

RESULTS

Five cycles of freezing and thawing introduced no statistically significant changes in the measured concentrations of PSA-T, PSA-F, or PSA-ACT. The effect of storing blood samples at room temperature for 1-6 h before separation of serum revealed a statistically significant decrease only for PSA-F after 5.5 h of storage (mean decrease 3.5%). PSA-T and PSA-ACT showed good stability in both serum and plasma samples, whereas PSA-F, after 1 week of storage at 4 degrees C, decreased on average by 28.8%, 7.8%, and 5.6%, respectively, in serum, heparin plasma, and EDTA plasma. The decreases of PSA-F at 4 degrees C were statistically significant (P < 0.05) relative to the controls (samples stored at -20 degrees C) after storage for 23 h in serum, 86 h in heparin plasma, and 71 h in EDTA plasma. When the same samples were stored at 30 degrees C for 24 h, only the mean decrease of PSA-F (4.8%) in serum was statistically significant.

CONCLUSIONS

PSA-F in blood samples is less stable than PSA-ACT. It is not advisable to store samples on the clot, especially if time and temperature cannot be controlled. Serum samples should be stored frozen if not analyzed during the same day. After thawing, samples can be stored up to 23 h at 4 degrees C prior to analysis. The use of plasma samples improves the stability of free PSA.