Biology of prostate-specific antigen

Prostate-specific antigen (PSA) is an androgen-regulated serine protease produced by both prostate epithelial cells and prostate cancer (PCa) and is the most commonly used serum marker for cancer. It is a member of the tissue kallikrein family, some of the members of which are also prostate specific. PSA is a major protein in semen, where its function is to cleave semenogelins in the seminal coagulum. PSA is secreted into prostatic ducts as an inactive 244-amino acid proenzyme (proPSA) that is activated by cleavage of seven N-terminal amino acids. PSA that enters the circulation intact is rapidly bound by protease inhibitors, primarily alpha1-antichymotrypsin, although a fraction is inactivated in the lumen by proteolysis and circulates as free PSA. This proteolytic inactivation, as well as the cleavage of proPSA to PSA, is less efficient in PCa. Serum total PSA levels are increased in PCa, and PSA screening has dramatically altered PCa presentation and management. Unfortunately, although high PSA levels are predictive of advanced PCa, a large fraction of organ-confined cancers present with much lower total PSA values that overlap those levels found in men without PCa. Measurement of free versus total PSA can increase specificity for PCa, and tests under development to measure forms of proPSA may further enhance the ability to detect early-stage PCa. PSA is also widely used to monitor responses to therapy and is under investigation as a therapeutic target. Finally, recent data indicate that there may be additional roles for PSA in the pathogenesis of PCa.

A peptide-doxorubicin 'prodrug' activated by prostate-specific antigen selectively kills prostate tumor cells positive for prostate-specific antigen in vivo

We covalently linked doxorubicin with a peptide that is hydrolyzable by prostate-specific antigen. In the presence of prostate tumor cells secreting prostate-specific antigen, the peptide moiety of this conjugate, L-377,202, was hydrolyzed, resulting in the release of leucine-doxorubicin and doxorubicin, which are both very cytotoxic to cancer cells. However, L-377,202 was much less cytotoxic than conventional doxorubicin to cells in culture that do not secrete prostate-specific antigen. L-377,202 was approximately 15 times more effective than was conventional doxorubicin at inhibiting the growth of human prostate cancer tumors in nude mice when both drugs were used at their maximally tolerated doses. Nude mice inoculated with human prostate tumor cells secreting prostate-specific antigen showed considerable reductions in tumor burden with minimal total body weight loss when treated with L-377, 202. This improvement in therapeutic index correlated with the selective localization of leucine-doxorubicin and free doxorubicin in tissues secreting prostate-specific antigen after exposure to L-377,202.

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.

Mechanisms, hypotheses and questions regarding prostate cancer micrometastases to bone

The morbidity and mortality associated with prostate cancer can almost universally be attributed to the consequences of metastases to the bone. While clinically there have been descriptive reports of these lesions and their detection by bone scan, there is an embrrassing paucity of reports as to the mechanisms of prostate cancer cell trafficking to the bone, adaptation to the bone environment, pertubation of the normal bone reformation process and the events leading to cachexia and death. In recent years, there have been numerous in vitro studies suggesting that PSA and hK2 may play a significant biological role in these events. Also, recent data generated form reverse transcription polymerase chain reaction assays reveal that metastasis to the bone may be an early event which further underscores the need to better understand this complex and critically important process. This commentary highlights several general concepts and a few specific issues related to CaP bone metastasis with the intent of revealing numerous opportunities for further investigation and inquiry.

Prostate-specific antigen expression by various tumors

There is a growing body of evidence indicating that prostate-specific antigen (PSA) may be present in many steroid hormone-stimulated epithelial tissues other than that of the prostate. In particular, breast tumor cell lines treated with steroid hormone receptor agonists, breast tumors, and normal human breast have recently been found by our group to contain PSA. To investigate whether PSA may also be present in other human tumors, we employed a highly sensitive immunofluorometric assay technique to quantify PSA immunoreactivity in tumor extracts. Using a PSA-positivity cutoff value of 0.005 ng per mg of protein, 23 of 43 diverse tumors tested positive for PSA protein. Confirmatory analyses for PSA by a commercially available method (IMx) on six samples demonstrated a high degree of concordance between the two methods. To establish the molecular weight of the immunoreactive species, the most highly positive tumor extracts of each tumor type were fractionated by high performance liquid chromatography. Whereas the majority of tumors had immunoreactivity eluting at both 100 KDa and 33 KDa, corresponding to PSA bound to alpha 1-antichymotrypsin and free PSA, respectively, the colon and parotid tumors displayed immunoreactivity only at the 33 KDa fraction. We conclude that in addition to breast tumors and normal breast, colon, ovarian, liver, kidney, adrenal, and parotid tumors can also produce PSA. The physiological role of PSA in these tumors is currently under investigation.

Stability of free prostate-specific antigen in serum samples under a variety of sample collection and sample storage conditions

OBJECTIVES

Four studies were conducted to characterize the stability of free and total prostate specific antigen (PSA) under various sample collection and storage conditions.

METHODS

In the first study, fresh blood from 11 patients was drawn and allowed to clot at room temperature (RT). Serum was prepared by centrifugation 1, 3, 5, or 8 hours after the blood draw and tested to determine the free and total PSA levels. In the second study, serum specimens from 12 individuals were stored at RT or 4 degrees C and were tested on days 0, 1, 2, and 7. In the third study, four fresh serum samples were subjected to five freeze-thaw cycles and tested after each cycle. In the fourth study, 29 fresh samples were aliquoted, frozen at -20 degrees C or -70 degrees C, and monitored for long-term stability.

RESULTS

Approximately 1% of the free PSA was lost per hour of clotting time. Between 2% and 3% of the free PSA was lost per day of storage at 4 degrees C or 23 degrees C. About 0.9% of the free PSA was lost per month of storage at -20 degrees C compared with about 0.4% per month at -70 degrees C. Total PSA appeared to be stable throughout these studies.

CONCLUSIONS

Our results suggest that routine serum preparation and refrigerated storage of samples for up to 24 hours is acceptable for the measurement of both free and total PSA. Samples that are to be retained for longer than 24 hours should be frozen. Samples stored for extended periods should be kept at -70 degrees C.

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.

The prostate specific membrane antigen regulates the expression of IL-6 and CCL5 in prostate tumour cells by activating the MAPK pathways

The interleukin-6 (IL-6) and the chemokine CCL5 are implicated in the development and progression of several forms of tumours including that of the prostate. The expression of the prostate specific membrane antigen (PSMA) is augmented in high-grade and metastatic tumors. Observations of the clinical behaviour of prostate tumors suggest that the increased secretion of IL-6 and CCL5 and the higher expression of PSMA may be correlated. We hypothesized that PSMA could be endowed with signalling properties and that its stimulation might impact on the regulation of the gene expression of IL-6 and CCL5. We herein demonstrate that the cross-linking of cell surface PSMA with specific antibodies activates the small GTPases RAS and RAC1 and the MAPKs p38 and ERK1/2 in prostate carcinoma LNCaP cells. As downstream effects of the PSMA-fostered RAS-RAC1-MAPK pathway activation we observed a strong induction of NF-κB activation associated with an increased expression of IL-6 and CCL5 genes. Pharmacological blockade with specific inhibitors revealed that both p38 and ERK1/2 participate in the phenomenon, although a major role exerted by p38 was evident. Finally we demonstrate that IL-6 and CCL5 enhanced the proliferative potential of LNCaP cells synergistically and in a dose-dependent manner and that CCL5 functioned by receptor-mediated activation of the STAT5-Cyclin D1 pro-proliferative pathway. The novel functions attributable to PSMA which are described in the present report may have profound influence on the survival and proliferation of prostate tumor cells, accounting for the observation that PSMA overexpression in prostate cancer patients is related to a worse prognosis.

Semenogelins I and II bind zinc and regulate the activity of prostate-specific antigen

In semen, the gel proteins SgI and SgII (semenogelins I and II) are digested by PSA (prostate-specific antigen), resulting in liquefaction and release of motile spermatozoa. Semen contains a high concentration of Zn2+, which is known to inhibit the protease activity of PSA. We characterized the binding of Zn2+ to SgI and SgII and found evidence that these proteins are involved in regulating the activity of PSA. Intact SgI and SgII and synthetic semenogelin peptides were used in the experiments. Binding of Zn2+ was studied by radioligand blotting, titration with a zinc (II) fluorophore chelator and NMR analysis. A chromogenic substrate was used to measure the enzymatic activity of PSA. SgI and SgII bound Zn2+ with a stoichiometry of at least 10 mol (mol of protein)(-1) and with an average dissociation constant of approx. 5 microM per site. Moreover, Zn2+-inhibited PSA was activated by exposure to SgI or SgII. Since both proteins have high affinity for Zn2+ and are the dominating proteins in semen, they probably represent the major Zn2+ binders in semen, one function of which may be to regulate the activity of PSA. The system is self-regulating, and PSA is maintained in an active state by its substrate.

Monomethylated selenium inhibits growth of LNCaP human prostate cancer xenograft accompanied by a decrease in the expression of androgen receptor and prostate-specific antigen (PSA)

OBJECTIVES

Epidemiological studies and prevention trials suggest selenium is a promising preventive agent for prostate cancer. Selenium-containing compounds inhibited the growth of prostate cancer cell lines including androgen sensitive LNCaP and androgen insensitive DU145 and PC3 cells in vitro. Previous study revealed a novel mechanism of selenium action in which selenium (methylseleninic acid (MSA)) markedly reduced androgen receptor (AR) signaling in prostate cancer cells, suggesting that selenium might act as an antiandrogen, which could serve as a therapeutic agent for prostate cancer. In this study, we tested whether selenium (methylselenocysteine (MSC)) affects tumor growth of human prostate cancer cells by targeting AR signaling in vivo.

METHODS

Prostate tumor xenografts were established in nude mice by co-inoculating LNCaP cells with Matrigel. The mice-bearing tumors were treated with or without MSC (100 microg/mouse/day) via intraperitoneal injection for 2 weeks. The effect of MSC on tumor growth, AR, and prostate-specific antigen (PSA) expression was examined.

RESULTS

Methylselenocysteine (MSC) significantly inhibited LNCaP tumor growth (P < 0.05). AR expression in tumor tissues and serum PSA levels were considerably decreased in MSC-treated mice compared to the vehicle controls.

CONCLUSIONS

Pharmacological dose of MSC inhibits the growth of LNCaP human prostate cancer in vivo accompanied by a decrease in the expression of AR and PSA. These findings suggest that selenium (MSC) can serve as a therapeutic agent aimed at disruption of AR signaling for prostate cancer.

Tissue prostate-specific antigen facilitates refractory prostate tumor progression via enhancing ARA70-regulated androgen receptor transactivation

Despite being well recognized as the best biomarker for prostate cancer, pathophysiologic roles of prostate-specific antigen (PSA) remain unclear. We report here that tissue PSA may be involved in the hormone-refractory prostate cancer progression. Histologic analyses show that the increased tissue PSA levels are correlated with lower cell apoptosis index and higher cell proliferation rate in hormone-refractory tumor specimens. By stably transfecting PSA cDNA into various prostate cancer cell lines, we found that PSA could promote the growth of androgen receptor (AR)-positive CWR22rv1 and high-passage LNCaP (hormone-refractory prostate cancer cells) but not that of AR-negative PC-3 and DU145 cells. Surprisingly, the protease activity of PSA is not crucial for PSA to stimulate growth and promote AR transactivation. We further showed that increased PSA could enhance ARA70-induced AR transactivation via modulating the p53 pathway that results in the decreased apoptosis and increased cell proliferation in prostate cancer cells. Knockdown of PSA in LNCaP and CWR22rv1 cells causes cell apoptosis and cell growth arrest at the G(1) phase. In vitro colony formation assay and in vivo xenografted tumor results showed the suppression of prostate cancer growth via targeting PSA expression. Collectively, our findings suggest that, in addition to being a biomarker, PSA may also become a new potential therapeutic target for prostate cancer. PSA small interfering RNA or smaller molecules that can degrade PSA protein may be developed as alternative approaches to treat the prostate cancer.

Structure, function, and regulation of the enzyme activity of prostate-specific antigen

Prostate-specific antigen (PSA) and human glandular kallikrein 1 (hGK-1) are structurally similar products of the human glandular kallikrein gene locus on chromosome 19 that become selectively expressed by human prostate tissue. PSA is one of the most abundant prostate-derived proteins in the seminal fluid. The mature form of PSA, a single-chain glycoprotein of 237 amino acids, is a serine protease manifesting restricted chymotrypsin-like activity. PSA is mainly responsible for gel dissolution in freshly ejaculated semen by proteolysis of the major gel-forming proteins semenogelin I and II and fibronectin. PSA complexed to alpha 1-antichymotrypsin (ACT) is the predominant molecular form of serum PSA, although complex formation is slow between the purified proteins in vitro. PSA also forms stable complexes with alpha 2-macroglobulin (alpha 2M) in vitro, but as this results in encapsulation of PSA and complete loss of the PSA epitopes, the in vivo significance of this complex formation is presently unclear. A free, noncomplexed form of PSA constitutes a minor fraction of the serum PSA, although serum ACT occurs at large molar excess.

Prostate specific membrane antigen (PSMA) expression gives prostate cancer cells a growth advantage in a physiologically relevant folate environment in vitro

BACKGROUND

Prostate specific membrane antigen (PSMA) expression is correlated with stage and grade of prostate cancer suggesting that it confers a growth advantage. We studied if PSMA folate hydrolase activity provides cells a growth advantage in a low folate (LF) micro-environment by hydrolyzing extracellular poly-gamma-glutamated folate to a form that cells can import.

METHODS

Proliferation of LNCaP and DU-145 cells was assessed in media containing low (LF), physiological (PF), or high (HF) folate with or without penta-gamma-glutamated folate and a PSMA specific folate hydrolase inhibitor, 2-(phosphonomethyl)-pentanedioic acid (2-PMPA).

RESULTS

LNCaP cells, which express PSMA, and DU-145 cells, which do not, displayed decreased proliferation when grown in LF or PF compared to HF media. This reduction in proliferation was eliminated in LNCaP cells when penta-gamma-glutamated folate was added to the media. In the presence of penta-gamma-glutamated folic acid DU-145 cells displayed increased growth but this was still significantly lower than growth in HF medium. Addition of 2-PMPA attenuated the increased growth seen in LNCaP cells but had no effect on DU-145 cell growth.

CONCLUSIONS

The folate hydrolase activity of PSMA may provide a growth advantage in LF and PF environments.

Evidence that the prostate-specific antigen (PSA)/Zn2+ axis may play a role in human prostate cancer cell invasion

Prostate-specific antigen (PSA), which is used as a marker for the diagnosis and monitoring of prostate cancer, is a kallikrein protease which could potentially play a role in human prostate cancer cell invasion. Zinc ions are effective inhibitors of a number of proteases. The enzymatic activity of purified PSA was strongly inhibited by Zn(2+). The ability of LNCaP cells which express and secrete PSA to invade Matrigel was strongly suppressed by Zn(2+) at a concentration similar to that inhibiting the activity of purified PSA. Zn(2+) effectively inhibited the degradation of Matrigel by purified PSA. These results suggest that Zn(2+) in human prostate may suppress the invasion and metastasis of prostate cancer cells through the regulation of the proteolytic activity of PSA. Loss of inhibition of the proteolytic activity of PSA by Zn(2+) in prostate tumors could contribute to invasion.

Impact of prostate-specific antigen (PSA) nadir and time to PSA nadir on disease progression in prostate cancer treated with androgen-deprivation therapy

BACKGROUND

The influence of PSA kinetics on the outcome of metastatic prostate cancer after androgen deprivation therapy (ADT) is not well understood. We evaluated the prognostic significance of PSA nadir and time to PSA nadir as well as their potential interactive effect on the progression of disease after ADT.

METHODS

A total of 650 men with advanced or metastatic prostate cancer treated with ADT were studied. The prognostic significance of PSA nadir and time to PSA nadir on disease progression were analyzed using Kaplan-Meier analysis and the Cox regression model.

RESULTS

We found that both PSA nadir and time to PSA nadir were independent and significant predictors of disease progression. Patients with higher PSA nadir (≥0.2 ng/ml) and shorter time to PSA nadir (<10 months) had significant shorter time to disease progression after adjusting for other covariates. The combined analyses showed a potential synergistic effect of these two variables on disease progression. Patient with higher PSA nadir and shorter time to PSA nadir had significantly higher risk for disease progression compared to those with lower PSA nadir and longer time to PSA nadir (Hazard Ratios (HR) = 3.11, P < 0.001).

CONCLUSIONS

We concluded that both PSA nadir and time to PSA nadir are significant predictors of disease progression for prostate cancer patients receiving ADT.

Prostate-specific antigen in breast cyst fluid: possible role of prostate-specific antigen in hormone-dependent breast cancer

Prostate-specific antigen (PSA) is a 33 kDa serine protease which is produced by many different tissues in the body and has been shown to be present in low concentrations in breast milk and in about 30% of breast cancers. The presence of PSA in breast cancers is associated with the presence of steroid-hormone receptors and may be a favourable prognostic indicator. In this study, PSA immunoreactivity was measured in breast cyst fluid obtained from women with palpable breast cysts which is the most common benign breast disease. PSA was found to be present in very low concentrations in breast cyst fluid. In an attempt to understand the possible role of PSA in the breast, the effect of PSA on growth of the hormone-dependent MCF-7 and hormone-independent MDA-MB-231 human breast cancer cell lines was studied. In addition, the effect of PSA on oestrone sulphatase activity and oestrogen 17-oxidoreductase activity in these cell lines was investigated. PSA, in low concentrations, was found to inhibit MCF-7 cell growth and to stimulate the conversion of oestradiol to the less potent oestrogen oestrone in this cell line. PSA had no effect on the MDA-MB-231 cell line. Our findings suggest that PSA may act as a negative growth regulator in hormone-dependent breast cancers.

A guide to the interpretation of serum prostate specific antigen levels

OBJECTIVE

To review the factors that affect the concentration of prostate specific antigen (PSA) in the serum.

RESULTS

The discussion includes the structure of PSA; its distribution and metabolism; various analytical aspects of PSA measurements; the effects of clinical manipulations on PSA, including digital rectal examination, transrectal ultrasound, cystoscopy, biopsy and transurethral resection of the prostate; factors affecting PSA levels in health, in benign disease, and in prostate cancer; the effect of various treatments on PSA; and the issue of reference ranges.

CONCLUSION

Laboratory staff and physicians must take many factors into consideration when interpreting PSA results.

Inhibition of Dendropoiesis by Tumor Derived and Purified Prostate Specific Antigen

PURPOSE

Prostate specific antigen (PSA) is a serine protease produced by the prostate gland at high concentrations. Serum PSA may be significantly elevated in prostate cancer and benign prostatic diseases. It has recently become evident that, in addition to being a tissue and/or serum marker, PSA may also have biological effects. Despite the voluminous literature on this biomarker in the diagnosis of prostatic diseases relatively few reports have addressed the issue of the physiological function, biological role and immune effects of PSA in the context of prostate cancer development and progression.

MATERIALS AND METHODS

Human dendritic cell (DC) cultures were generated from CD34+ hematopoietic precursors in the presence of PSA. The DC phenotype was assessed by flow cytometry and DC ability to induce T-cell proliferation was detected by allogeneic mixed lymphocyte reaction assay. DCs were also generated in co-cultures with LNCaP cells in the presence of antiPSA antibodies. The concentrations of PSA in cultures were determined by the AXSYM System (Abbott Laboratories, Wiesbaden, Germany).

RESULTS

We noted that purified and LNCaP derived PSA inhibited the generation and maturation of DC (dendropoiesis) in vitro, which might have a crucial role in the induction and regulation of specific antitumor immune responses. The addition of active PSA to DC cultures significantly inhibited the generation and maturation of DC, as assessed by the levels of expression of CD83, CD80, CD86 and HLA DR. The ability of DC to induce T-cell proliferation, which depends on the expression of co-stimulatory and major histocompatibility complex molecules, was also suppressed in PSA treated DC cultures.

CONCLUSIONS

The antidendropoietic effect of PSA in vitro suggests a new mechanism of prostate cancer induced immunosuppression and tumor escape, and provides novel evidence of the immunoregulatory properties of PSA.

Transcriptional control: an essential component of cancer gene therapy strategies?

The therapeutic index of cancer gene therapy approaches will, at least in part, be dictated by the spatial and temporal control of expression of the therapeutic transgenes. Strategies which allow precise control of gene transcription are likely to play a crucial role in the future pre-clinical and clinical development of gene therapy. In this review, we discuss these issues as they relate to tissue and tumor specific promoters. In addition, the exciting opportunities offered by the development of regulated gene expression systems using small molecules, radiation and heat are reviewed. It is realistic to expect that the future offers the prospect of amalgamating elements of a number of these different systems in a co-ordinated gene delivery approach with the potential to increase the efficacy and reduce the toxicity of treatment.

The role of prostate specific antigen measurement in the detection and management of prostate cancer

The introduction of prostate specific antigen (PSA) testing has revolutionised the early detection, management and follow-up of patients with prostate cancer and it is considered to be one of the best biomedical markers currently available in the field of oncology. Its use with annual digital rectal examination in prostate cancer screening programmes has led to a marked change in the distribution of stage at presentation towards earlier disease and led to a significant increase in the detection of potentially curable disease. In order to improve the specificity of PSA testing and thereby reduce the number of unnecessary prostatic biopsies, a number of refinements of PSA evaluation have been proposed. These include free to total PSA ratio, PSA density, PSA density, PSA density of the transition zone, PSA velocity and age-specific PSA reference ranges. The utility of these approaches is considered in this review. The role of PSA monitoring in the detection of recurrence following radical prostatectomy and radiotherapy is discussed, as well as its role in monitoring patients treated with endocrine therapy is discussed, as well as its role in monitoring patients treated with endocrine therapy in terms of correlating PSA response with outcome, in detecting disease progression and in guiding the use of subsequent therapies. Large continuing multicentre screening and outcome studies will provide important information enabling greater refinement of the use of this important diagnostic and monitoring tool in the future detection and management of prostate cancer.

Novel small molecule inhibitors for prostate-specific antigen

BACKGROUND

Prostate-specific antigen (PSA or KLK3) has been shown to inhibit angiogenesis, but it might also have tumor promoting activities. Thus, it may be possible to modulate prostate cancer growth by stimulating or inhibiting the activity of PSA. To this end we have previously identified peptides that stimulate the activity of PSA. As peptides have several limitations as drug molecules, we screened a chemical library to find drug-like compounds that could be used to modulate the function(s) of PSA.

METHODS

Almost 50,000 compounds were analyzed for their ability to modulate PSA activity towards a fluorescent PSA-substrate. The ability of the most active compounds to affect the anti-angiogenic activity of PSA was analyzed by human umbilical vein endothelial cell (HUVEC) tube formation assay.

RESULTS

In the initial screening we identified two compounds that inhibited PSA activity. Based on these, similar compounds were selected and tested for activity to define structure-activity relationships. Several compounds with micromolar IC50-values were found, but they were not entirely specific towards PSA, e.g., they inhibited chymotrypsin, which has similar substrate specificity as PSA. However, it was possibly to improve the selectivity of the compounds towards PSA by small structural changes. These compounds inhibited the anti-angiogenic activity of PSA in the HUVEC model, proving that the proteolytic activity of PSA is essential for inhibition of angiogenesis.

CONCLUSIONS

We found several PSA inhibitors that could be useful tools for studying the role of PSA in cancer models and in normal physiology as showed in angiogenesis model.

Expression and characterization of trypsinogen produced in the human male genital tract

Trypsinogen is a serine proteinase produced mainly by the pancreas, but it has recently been found to be expressed also in several cancers such as ovarian and colon cancer and in vascular endothelial cells. In this study, we found that trypsinogen-1 and -2 are present at high concentrations (median levels, 0.4 and 0.5 mg/L, respectively) in human seminal fluid and purified them to homogeneity by immunoaffinity and anion exchange chromatography. Purified trypsinogen isoenzymes displayed a M(r) of 25 to 28 kd in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Most of the trypsinogen-1 purified from seminal fluid was enzymatically active whereas trypsinogen-2 occurred as the proform, which could be activated by enteropeptidase in vitro. Immunohistochemically, trypsinogen protein was detected in the human prostate, urethra, utriculus, ejaculatory duct, seminal vesicles, deferent duct, epididymal glands, and testis. Expression of trypsinogen mRNA in the same organs was demonstrated by in situ hybridization. Trypsinogen mRNA was also detected in the prostate and seminal vesicles by reverse transcriptase-polymerase chain reaction and Northern blotting. Isolated trypsin was shown to activate the proenzyme form of prostate-specific antigen. These results suggest that trypsinogen isoenzymes found in seminal fluid are produced locally in the male genital tract and that they may play a physiological role in the semen.

Prostate-specific antigen and human glandular kallikrein: two kallikreins of the human prostate

Prostate-specific antigen (PSA) is a 33 kD protein synthesized in the epithelial cells of the prostate gland. It is a serine protease that belongs to the subgroup of kallikreins, among which it is very similar to a putative enzyme called human glandular kallikrein (hGK-1). Although the hGK-1 enzyme remains to be characterized in vivo, the hGK-1 gene is expressed in the same prostatic epithelial cells as the PSA gene. Expression of the PSA gene is under complex control and the steady-state level of PSA mRNA is increased by androgens, and decreased by epidermal growth factor and activation of protein kinase C. This suggests the existence of several regulatory elements within the cis-acting control elements of the PSA gene. As a seminal serine protease, PSA has been shown to digest the high molecular weight seminal vesicle protein, seminogelin. However, it is likely that this does not constitute the only natural substrate of PSA, as PSA has been shown to degrade insulin-like growth factor-binding protein-3. Serum PSA concentrations are frequently increased in patients with prostatic cancer, but this is also the case in patients with benign prostatic hyperplasia. Thus, PSA measurements alone are not useful as a screening tool for undiagnosed prostatic cancer. However, serum PSA concentrations can be successfully used together with other methods in diagnosing prostatic diseases and in monitoring the successfulness of treatments for prostatic cancer.