Concentration of enzymatically active prostate-specific antigen (PSA) in the extracellular fluid of primary human prostate cancers and human prostate cancer xenograft models

KLK3 in the Regulation of Angiogenesis-Tumorigenic or Not?

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

Mipsagargin: The Beginning-Not the End-of Thapsigargin Prodrug-Based Cancer Therapeutics

Søren Brøgger Christensen isolated and characterized the cell-penetrant sesquiterpene lactone Thapsigargin (TG) from the fruit Thapsia garganica. In the late 1980s/early 1990s, TG was supplied to multiple independent and collaborative groups. Using this TG, studies documented with a large variety of mammalian cell types that TG rapidly (i.e., within seconds to a minute) penetrates cells, resulting in an essentially irreversible binding and inhibiting (IC₅₀~10 nM) of SERCA 2b calcium uptake pumps. If exposure to 50–100 nM TG is sustained for >24–48 h, prostate cancer cells undergo apoptotic death. TG-induced death requires changes in the cytoplasmic Ca²⁺, initiating a calmodulin/calcineurin/calpain-dependent signaling cascade that involves BAD-dependent opening of the mitochondrial permeability transition pore (MPTP); this releases cytochrome C into the cytoplasm, activating caspases and nucleases. Chemically unmodified TG has no therapeutic index and is poorly water soluble. A TG analog, in which the 8-acyl groups is replaced with the 12-aminododecanoyl group, afforded 12-ADT, retaining an EC₅₀ for killing of <100 nM. Conjugation of 12-ADT to a series of 5–8 amino acid peptides was engineered so that they are efficiently hydrolyzed by only one of a series of proteases [e.g., KLK3 (also known as Prostate Specific Antigen); KLK2 (also known as hK2); Fibroblast Activation Protein Protease (FAP); or Folh1 (also known as Prostate Specific Membrane Antigen)]. The obtained conjugates have increased water solubility for systemic delivery in the blood and prevent cell penetrance and, thus, killing until the TG-prodrug is hydrolyzed by the targeting protease in the vicinity of the cancer cells. We summarize the preclinical validation of each of these TG-prodrugs with special attention to the PSMA TG-prodrug, Mipsagargin, which is in phase II clinical testing.

A mechanistic insight into the anti-metastatic role of the prostate specific antigen

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Prostate specific antigen is the standard circulating biomarker for prostate cancer.

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We provide novel evidence that collagen 1 is an additional substrate for PSA.

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PSA hampers first steps of cancer invasion.

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Tissue-related PSA content/activity is inversely correlated to tumor progression.

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Tissue-related PSA levels improve prediction of prostate cancer specific mortality.

Aim

Since its discovery Prostate Specific Antigen (PSA), also referred to as kallikrein-3 (KLK3), has been used as standard circulating biomarker for prostate cancer (PCa). However, its specificity remains not adequate and its mechanism of action still elusive. Therefore, deciphering PSA role throughout PCa-pathobiology would be relevant in improving both cancer diagnosis and outcome prediction. We investigated the possible role played by PSA on/in the tumor microenvironment and over the first steps of cancer invasion.

Methods

Fresh PCa-specimens and cell lines were used for ex-vivo/in-vitro invasion assays and assessment of prostate tissue-PSA (tPSA), type 1 collagen (COL1A1) and ß1-integrin expression. Tissue Cancer Genome Atlas (TCGA) and Decipher® datasets were considered to estimate tPSA clinical relevance.

Results

A more precise, inverse, correspondence between tPSA and clinical/pathological parameters was found than for circulating PSA. KLK3 combined with Gleason grade and pathologic stage, better predicted cancer-related mortality. Consistently, we demonstrated that PSA inhibits prostate extracellular-matrix (ECM) invasion by PCa cells. As for the mechanism of action, we provided novel information that PSA is able to cleave COL1A1, a main component of the ECM. Finally, ß1-integrin, a crucial COL1A1 transducing-receptor involved in tumor adhesion/invasion, resulted to be downregulated in PCa specimens with higher levels of tPSA.

Conclusions

By interfering with type 1 collagen and its downstream targets, PSA may hamper adhesion and path of the cancer cells through ECM and their migration ability, thus explaining the inverse correlation highlighted between prostate tPSA levels and clinically significant disease.

Screening for new peptide substrates for the development of albumin binding anticancer pro-drugs that are cleaved by prostate-specific antigen (PSA) to improve the anti tumor efficacy

Several attempts have been made over the past decade to explore the concept of prodrug strategies that exploit PSA as a molecular target for the release of anticancer drugs in prostate tumors using various prostate specific antigen (PSA)-cleavable peptide linkers, but the desired antitumor and antimetastatic efficacy has not yet been fully achieved. We set out to look for new PSA-cleavable peptide substrates that could be cleaved more rapidly and efficiently than the previously used peptides. To look for the most susceptible PSA-cleavable peptide substrates, we used the so-called spot technology. With the following general formula, we designed 25 different fluorogenic heptapeptides; Cellulose-P5-P4-P3-P2-P1-P1′-P2’ (Fluorophore). The increase of the fluorescence in the supernatant of the reaction mixture was monitored using a 96-well fluorometric plate reader with excitation of λ_ex 485 nm and λ_em 535 nm. Three sequences showed a high fluorogenic liberation after incubation with PSA, i.e., Arg-Arg-Leu-His-Tyr-Ser-Leu (7), Arg-Arg-Leu-Asn-Tyr-Ser-Leu (8) and Arg-Ser-Ser-Tyr-Arg-Ser-Leu (23). Future incorporation of these optimized substrates in the PSA-cleavable prodrug formulations could further optimize the cleavage pattern and so the release characteristics of these prodrugs to rapidly and efficiently liberate the free cytotoxic agents inside the tumor tissues.

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Prostate-specific antigen (PSA) represents a molecular target for selectively releasing anticancer agents from prodrugs.

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Optimal PSA-cleavable peptide substrates are not yet identified.

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Spot technology is used to elucidate a new PSA-cleavable peptide substrates.

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We describe new three peptide sequences with a maximal PSA cleavability.

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These new peptide substrates could improve the antitumor efficacy of PSA-cleavable prodrugs.

Microparticle Encapsulation of a Prostate-targeted Biologic for the Treatment of Liver Metastases in a Preclinical Model of Castration-resistant Prostate Cancer

PRX302 is a highly potent, mutant bacterial pore-forming biologic protoxin engineered for selective activation by PSA, a serine protease expressed by benign and malignant prostate epithelial cells. Although being developed as a local therapy for benign prostatic hyperplasia and localized prostate cancer, PRX302 cannot be administered systemically as a treatment for metastatic disease due to binding to ubiquitously expressed glycosylphosphatidylinositol (GPI)-anchored proteins, which leads to poor accumulation within the tumor microenvironment. To overcome this limitation, poly-lactic-co-glycolic acid (PLGA) microparticles encapsulating the protoxin were developed, which are known to accumulate in the liver, a major site of metastasis for prostate cancer and other solid tumors. A highly sensitive and reproducible sandwich ELISA to quantify PRX302 released from microparticles was developed. Utilizing this assay, PRX302 release from different microparticle formulations was assessed over multiple days. Hemolysis assays documented PSA-dependent pore formation and lytic potential (i.e., function) of the released protoxin. MTT assays demonstrated that conditioned supernatant from PRX302-loaded, but not blank (i.e., unloaded), PLGA microparticles was highly cytotoxic to PC3 and DU145 human prostate cancer cells in the presence of exogenous PSA. Microparticle encapsulation prevented PRX302 from immediately interacting with GPI-anchored proteins as demonstrated in a competition assay, which resulted in an increased therapeutic index and significant antitumor efficacy following a single dose of PRX302-loaded microparticles in a preclinical model of prostate cancer liver metastasis with no obvious toxicity. These results document that PRX302 released from PLGA microparticles demonstrate in vivo antitumor efficacy in a clinically relevant preclinical model of metastatic prostate cancer.

Beyond the biomarker role: prostate-specific antigen (PSA) in the prostate cancer microenvironment

The prostate-specific antigen (PSA) blood test is the accepted biomarker of tumor recurrence. PSA levels in serum correlate with disease progression, though its diagnostic accuracy is questionable. As a result, significant progress has been made in developing modified PSA tests such as PSA velocity, PSA density, 4Kscore, PSA glycoprofiling, Prostate Health Index, and the STHLM3 test. PSA, a serine protease, is secreted from the epithelial cells of the prostate. PSA has been suggested as a molecular target for prostate cancer therapy due to the fact that it is not only active in prostate tissue but also has a pivotal role on prostate cancer signaling pathways including proliferation, invasion, metastasis, angiogenesis, apoptosis, immune response, and tumor microenvironment regulation. Here, we summarize the current standing of PSA in prostate cancer progression as well as its utility in prostate cancer therapeutic approaches with an emphasis on the role of PSA in the tumor microenvironment.

Enhanced anti-tumor activity of the Multi-Leu peptide PACE4 inhibitor transformed into an albumin-bound tumor-targeting prodrug

The proprotein convertase PACE4 has been validated as a potential target to develop new therapeutic interventions in prostate cancer (PCa). So far, the most effective compound blocking the activity of this enzyme has been designed based on the structure of a small peptide Ac-LLLLRVKR-NH₂ known as the Multi-Leu (ML) peptide. Optimization of this scaffold led to the synthesis of compound C23 (Ac-[DLeu]LLLRVK-amidinobenzylamide) with a potent in vivo inhibitory effect on the tumor growth. However, further developments of PACE4 inhibitors may require additional improvements to counter their rapid renal clearance and to increase their tumor targeting efficiency. Herein, we explored the transformation of the ML-peptide into an albumin-binding prodrug containing a tumor specific release mechanism based on the prostate-specific antigen. Our data confirms that intravenous treatment using the ML-peptide alone has little effect on tumor growth, whereas by using the ML-prodrug in LNCaP xenograft-bearing mice it was significantly reduced. Additionally, excellent in vivo stability and tumor-targeting efficiency was demonstrated using a radiolabelled version of this compound. Taken together, these results provide a solid foundation for further development of targeted PACE4 inhibition in PCa.

A Phase I Study to Assess the Safety and Cancer-Homing Ability of Allogeneic Bone Marrow-Derived Mesenchymal Stem Cells in Men with Localized Prostate Cancer

Animal models show that systemically administered bone marrow‐derived mesenchymal stem cells (MSCs) home to sites of primary and metastatic prostate cancer (PC)—making them candidates to selectively deliver cytotoxic agents. To further assess this potential as a cell‐based therapeutic vehicle, a phase I study testing homing of systemically infused allogeneic MSCs preprostatectomy was conducted. The primary objective was to assess safety and feasibility and to determine if MSCs accumulate within primary PC tissue. MSCs were quantified using beads, emulsion, amplification, magnetics digital polymerase chain reaction (limit of detection: ≥0.01% MSCs) to measure allogeneic MSC DNA relative to recipient DNA. MSCs were harvested from healthy donors and expanded ex vivo using standard protocols by the Johns Hopkins Cell Therapy Laboratory. PC patients planning to undergo prostatectomy were eligible for MSC infusion. Enrolled subjects received a single intravenous infusion 4–6 days prior to prostatectomy. The first three subjects received 1 x 10⁶ cells per kilogram (maximum 1 x 10⁸ cells), and subsequent four patients received 2 x 10⁶ cells per kilogram (maximum 2 x 10⁸ cells). No dose‐limiting toxicities were observed and all patients underwent prostatectomy without delay. Pathologic assessment of prostate cores revealed ≥70% tumor involvement in cores from four subjects, with benign tissue in the others. MSCs were undetectable in all subjects, and the study was stopped early for futility. MSC infusions appear safe in PC patients. Although intended for eventual use in metastatic PC patients, in this study, MSCs did not home primary tumors in sufficient levels to warrant further development as a cell‐based therapeutic delivery strategy using standard ex vivo expansion protocols. stem cells translational medicine2019;8:441–449

Albumin-linked prostate-specific antigen-activated thapsigargin- and niclosamide-based molecular grenades targeting the microenvironment in metastatic castration-resistant prostate cancer

Localized prostate cancer is curable via annihilation of the entire cancer neighborhood by surgery or local radiation. Unfortunately, once metastatic, no available therapy is curative. The vast majority will die despite aggressive systemic combinational androgen-ablation therapies. Thus, there is an urgent need for effective systemic therapeutics that sterilize the entire microenvironment in metastatic castration-resistant prostate cancer (mCRPC). To accomplish this goal, advantage can be taken of the unique biology of mCRPC cells. Like their normal cell of origin, mCRPCs retain expression of the prostate-specific differentiation protein, prostate-specific antigen (PSA), which they abundantly secrete into their extracellular fluid (ECF). This unique, and essentially universal, secretion of enzymatically active PSA into the ECF by mCRPCs creates an exploitable therapeutic index for activation of systemically delivered highly lipophilic toxins as “molecular grenades” covalently linked to cysteine-34 of human serum albumin (HSA) via a stable maleimide containing PSA cleavable peptide such that PSA-dependent hydrolysis (i.e., “detonation”) releases the grenades restrictively within the ECF of mCRPC. This approach decreases dose-limiting host toxicity while enhancing plasma half-life from minutes to days (i.e., pharmacokinetic effect) and increasing the tissue concentration of the maleimide coupled albumin delivery (MAD) in the ECF at sites of cancer due to the enhanced permeability of albumin at these sites (i.e., enhanced permeability and retention effect). This allows the MAD-PSA detonated grenades to circulate throughout the body in a non-toxic form. Only within sites of mCRPC is there a sufficiently high level of enzymatically active PSA to efficiently “pull the pin” on the grenades releasing their lipophilic cell-penetrant toxins from HSA. Thus, if a sufficient level of “detonation” occurs, this will kill mCRPC cells, and sterilize the entire PSA-rich metastatic sites via a bystander effect. In this review, two examples of such MAD-PSA detonated molecular grenades are presented—one based upon thapsigagin and the other on niclosamide.

PSA-selective activation of cytotoxic human serine proteases within the tumor microenvironment as a therapeutic strategy to target prostate cancer

Prostate cancer is the most diagnosed malignancy and the second leading cause of cancer-related death in American men. While localized therapy is highly curative, treatments for metastatic prostate cancer are largely palliative. Thus, new innovative therapies are needed to target metastatic tumors. Prostate-Specific Antigen (PSA) is a chymotrypsin-like protease with a unique substrate specificity that is secreted by both normal and malignant prostate epithelial cells. Previous studies demonstrated the presence of high levels (μM-mM) of enzymatically active PSA is present in the extracellular fluid of the prostate cancer microenvironment. Because of this, PSA is an attractive target for a protease activated pro-toxin therapeutic strategy. Because prostate cancers typically grow very slowly, a strategy employing a proliferation-independent cytotoxic payload is preferred. Recently, it was shown that the human protease Granzyme B (GZMB), at low micromolar concentrations in the extracellular space, can cleave an array of extracellular matrix (ECM) proteins thus perturbing cell growth, signaling, motility, and integrity. It is also well established that other human proteases such as trypsin can induce similar effects. Because both enzymes require N-terminal proteolytic activation, we propose to convert these proteins into PSA-activated cytotoxins. In this study, we examine the enzymatic and cell targeting parameters of these PSA-activated cytotoxic serine proteases. These pro-enzymes were activated robustly by PSA and induced ECM damage that led to the death of prostate cancer cells in vitro thus supporting the potential use of this strategy as means to target metastatic prostate cancers.

Liposomal Drug Delivery Systems and Anticancer Drugs

Cancer is a life-threatening disease contributing to ~3.4 million deaths worldwide. There are various causes of cancer, such as smoking, being overweight or obese, intake of processed meat, radiation, family history, stress, environmental factors, and chance. The first-line treatment of cancer is the surgical removal of solid tumours, radiation therapy, and chemotherapy. The systemic administration of the free drug is considered to be the main clinical failure of chemotherapy in cancer treatment, as limited drug concentration reaches the tumour site. Most of the active pharmaceutical ingredients (APIs) used in chemotherapy are highly cytotoxic to both cancer and normal cells. Accordingly, targeting the tumour vasculatures is essential for tumour treatment. In this context, encapsulation of anti-cancer drugs within the liposomal system offers secure platforms for the targeted delivery of anti-cancer drugs for the treatment of cancer. This, in turn, can be helpful for reducing the cytotoxic side effects of anti-cancer drugs on normal cells. This short-review focuses on the use of liposomes in anti-cancer drug delivery.

Development of molecules stimulating the activity of KLK3 - an update

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

Enzyme-Responsive Liposomes for the Delivery of Anticancer Drugs

Liposomes are nanocarriers that deliver the payloads at the target site, leading to therapeutic drug concentrations at the diseased site and reduced toxic effects in healthy tissues. Several approaches have been used to enhance the ability of the nanocarrier to target the specific tissues, including ligand-targeted liposomes and stimuli-responsive liposomes. Ligand-targeted liposomes exhibit higher uptake by the target tissue due to the targeting ligand attached to the surface, while the stimuli-responsive liposomes do not release their cargo unless they expose to an endogenous or exogenous stimulant at the target site. In this review, we mainly focus on the liposomes that are responsive to pathologically increased levels of enzymes at the target site. Enzyme-responsive liposomes release their cargo upon contact with the enzyme through several destabilization mechanisms: (1) structural perturbation in the lipid bilayer, (2) removal of a shielding polymer from the surface and increased cellular uptake, (3) cleavage of a lipopeptide or lipopolymer incorporated in the bilayer, and (4) activation of a prodrug in the liposomes.

A prodrug-doped cellular Trojan Horse for the potential treatment of prostate cancer

Despite considerable advances in prostate cancer research, there is a major need for a systemic delivery platform that efficiently targets anti-cancer drugs to sites of disseminated prostate cancer while minimizing host toxicity. In this proof-of-principle study, human mesenchymal stem cells (MSCs) were loaded with poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs) that encapsulate the macromolecule G114, a thapsigargin-based prostate specific antigen (PSA)-activated prodrug. G114-particles (∼950 nm in size) were internalized by MSCs, followed by the release of G114 as an intact prodrug from loaded cells. Moreover, G114 released from G114 MP-loaded MSCs selectively induced death of the PSA-secreting PCa cell line, LNCaP. Finally, G114 MP-loaded MSCs inhibited tumor growth when used in proof-of-concept co-inoculation studies with CWR22 PCa xenografts, suggesting that cell-based delivery of G114 did not compromise the potency of this pro-drug in-vitro or in-vivo. This study demonstrates a potentially promising approach to assemble a cell-based drug delivery platform, which inhibits cancer growth in-vivo without the need of genetic engineering. We envision that upon achieving efficient homing of systemically infused MSCs to cancer sites, this MSC-based platform may be developed into an effective, systemic 'Trojan Horse' therapy for targeted delivery of therapeutic agents to sites of metastatic PCa.

Targeted prodrug approaches for hormone refractory prostate cancer

Due to the propensity of relapse and resistance with prolonged androgen deprivation therapy (ADT), there is a growing interest in developing non-hormonal therapeutic approaches as alternative treatment modalities for hormone refractory prostate cancer (HRPC). Although the standard treatment for HRPC consists of a combination of ADT with taxanes and anthracyclines, the clinical use of chemotherapeutics is limited by systemic toxicity stemming from nondiscriminatory drug exposure to normal tissues. In order to improve the tumor selectivity of chemotherapeutics, various targeted prodrug approaches have been explored. Antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT) strategies leverage tumor-specific antigens and transcription factors for the specific delivery of cytotoxic anticancer agents using various prodrug-activating enzymes. In prostate cancer, overexpression of tumor-specific proteases such as prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) is being exploited for selective activation of anticancer prodrugs designed to be activated through proteolysis by these prostate cancer-specific enzymes. PSMA- and PSA-activated prodrugs typically comprise an engineered high-specificity protease peptide substrate coupled to a potent cytotoxic agent via a linker for rapid release of cytotoxic species in the vicinity of prostate cancer cells following proteolytic cleavage. Over the past two decades, various such prodrugs have been developed and they were effective at inhibiting prostate tumor growth in rodent models; several of these prodrug approaches have been advanced to clinical trials and may be developed into effective therapies for HRPC.

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

Prostate-specific antigen (PSA or kallikrein-related peptidase-3, KLK3) exerts chymotrypsin-like proteolytic activity. The main biological function of PSA is the liquefaction of the clot formed after ejaculation by cleavage of semenogelins I and II in seminal fluid. PSA also cleaves several other substrates, which may explain its putative functions in prostate cancer and its antiangiogenic activity. We compared the proteolytic efficiency of PSA towards several protein and peptide substrates and studied the effect of peptides stimulating the activity of PSA with these substrates. An endothelial cell tube formation model was used to analyze the effect of PSA-degraded protein fragments on angiogenesis. We showed that PSA degrades semenogelins I and II much more efficiently than other previously identified protein substrates, e.g., fibronectin, galectin-3 and IGFBP-3. We identified nidogen-1 as a new substrate for PSA. Peptides B2 and C4 that stimulate the activity of PSA towards small peptide substrates also enhanced the proteolytic activity of PSA towards protein substrates. Nidogen-1, galectin-3 or their fragments produced by PSA did not have any effect on endothelial cell tube formation. Although PSA cleaves several other protein substrates, in addition to semenogelins, the physiological importance of this activity remains speculative. The PSA levels in prostate are very high, but several other highly active proteases, such as hK2 and trypsin, are also expressed in the prostate and may cleave protein substrates that are weakly cleaved by PSA.

Enzyme-triggered gelation: targeting proteases with internal cleavage sites

A generalizable method for detecting protease activity via gelation is described. A recognition sequence is used to target the protease of interest while a second protease is used to remove the residual residues from the gelator scaffold. Using this approach, selective assays for both MMP-9 and PSA are demonstrated.

Characterization of the prostate-specific antigen (PSA) catalytic mechanism: a pre-steady-state and steady-state study

Prostate-specific antigen (PSA), an enzyme of 30 kDa grouped in the kallikrein family is synthesized to high levels by normal and malignant prostate epithelial cells. Therefore, it is the main biomarker currently used for early diagnosis of prostate cancer. Here, presteady-state and steady-state kinetics of the PSA-catalyzed hydrolysis of the fluorogenic substrate Mu-His-Ser-Ser-Lys-Leu-Gln-AMC (spanning from pH 6.5 to pH 9.0, at 37.0°C) are reported. Steady-state kinetics display at every pH value a peculiar feature, represented by an initial "burst" phase of the fluorescence signal before steady-state conditions are taking place. This behavior, which has been already observed in other members of the kallikrein family, suggests the occurrence of a proteolytic mechanism wherefore the acylation step is faster than the deacylation process. This feature allows to detect the acyl intermediate, where the newly formed C-terminal carboxylic acid of the cleaved substrate forms an ester bond with the -OH group of the Ser195 catalytic residue, whereas the AMC product has been already released. Therefore, the pH-dependence of the two enzymatic steps (i.e., acylation and deacylation) has been separately characterized, allowing the determination of pKa values. On this basis, possible residues are tentatively identified in PSA, which might regulate these two steps by interacting with the two portions of the substrate.

PSA enzymatic activity: a new biomarker for assessing prostate cancer aggressiveness

BACKGROUND

With the advent of widespread prostate-specific antigen (PSA) testing in recent decades, prostate cancer (PCa) has emerged as the most frequently diagnosed non-skin cancer among men in the U.S. and Europe. Greater screening rates coupled with improved detection methods have caused a controversial upsurge in the number of men undergoing prostate biopsy and subsequent treatment. However, current diagnostic techniques generally suffer from limited ability to identify which seemingly indolent cancers are biologically aggressive.

METHODS

We collected prostatic fluid from 778 post-radical prostatectomy specimens and randomly selected samples from both the clinically confirmed aggressive (n = 50) and non-aggressive (n = 50) prostate cancer populations. We measured the level of proteolytic enzyme activity of PSA (aPSA) in each sample and used receiver operating characteristic (ROC) analysis to correlate aPSA levels with prostate cancer aggressiveness.

RESULTS

We found aPSA in prostatic fluid to be inversely proportional to disease stage, such that patients with the most aggressive PCa have on average significantly reduced aPSA compared to those with less aggressive disease. Significantly, our results suggest that many (22% in our study population) of the diagnosed patients with non-aggressive PCa could have averted or delayed radical prostatectomy.

CONCLUSIONS

Given the high level of debate surrounding PSA screening effectiveness [3-5] and the recent U.S. Preventative Services Task Force recommendation to discontinue PSA screening [6], our results provide renewed hope that a clinical monitoring tool may emerge that truly refines PCa treatment decision-making.

Structural optimization, biological evaluation, and application of peptidomimetic prostate specific antigen inhibitors

Prostate-specific antigen (PSA) is a serine protease produced at high levels by normal and malignant prostate epithelial cells that is used extensively as a biomarker in the clinical management of prostate cancer. To better understand PSA's role in prostate cancer progression, we prepared a library of peptidyl boronic acid-based inhibitors. To enhance selectivity for PSA vs other serine proteases, we modified the P1 site of the inhibitors to incorporate a bromopropylglycine group. This allowed the inhibitors to participate in halogen bond formation with the serine found at the bottom of the specificity pocket. The best of these Ahx-FSQn(boro)Bpg had PSA Ki of 72 nM and chymotrypsin Ki of 580 nM. In vivo studies using PSA-producing xenografts demonstrated that candidate inhibitors had minimal effect on growth but significantly altered serum levels of PSA. Biodistribution of (125)I labeled peptides showed low levels of uptake into tumors compared to other normal tissues.

Proteolysis of complement factors iC3b and C5 by the serine protease prostate-specific antigen in prostatic fluid and seminal plasma

Prostate-specific Ag (PSA) is a serine protease that is expressed exclusively by normal and malignant prostate epithelial cells. The continued high-level expression of PSA by the majority of men with both high- and low-grade prostate cancer throughout the course of disease progression, even in the androgen-ablated state, suggests that PSA has a role in the pathogenesis of disease. Current experimental and clinical evidence suggests that chronic inflammation, regardless of the cause, may predispose men to prostate cancer. The responsibility of the immune system in immune surveillance and eventually tumor progression is well appreciated but not completely understood. In this study, we used a mass spectrometry-based evaluation of prostatic fluid obtained from diseased prostates after removal by radical prostatectomy to identify potential immunoregulatory proteins. This analysis revealed the presence of Igs and the complement system proteins C3, factor B, and clusterin. Verification of these findings by Western blot confirmed the high-level expression of C3 in the prostatic fluid and the presence of a previously uncharacterized C-terminal C3 cleavage product. Biochemical analysis of this C3 cleavage fragment revealed a putative PSA cleavage site after tyrosine-1348. Purified PSA was able to cleave iC3b and the related complement protein C5. These results suggest a previously uncharacterized function of PSA as an immunoregulatory protease that could help to create an environment hospitable to malignancy through proteolysis of the complement system.

Prostate-specific antigen-retargeted recombinant newcastle disease virus for prostate cancer virotherapy

Oncolytic virus (OV) therapies of cancer are based on the use of replication-competent, tumor-selective viruses with limited toxicity. Newcastle disease virus (NDV), an avian paramyxovirus, is a promising OV and is inherently tumor selective and cytotoxic only to tumor cells. Replication is restricted in normal cells. Despite encouraging phase I/II clinical trials with NDV, further refinements for tumor-specific targeting are needed to enhance its therapeutic index. Systemically delivered NDV fails to reach solid tumors in therapeutic concentrations and also spreads poorly within the tumors due to barriers including complement, innate immunity, and the extracellular matrix. Overcoming these hurdles is paramount to realizing the exceptional oncolytic efficacy of NDV. We engineered the F protein of NDV and generated a recombinant NDV (rNDV) whose F protein is cleavable exclusively by prostate-specific antigen (PSA). The rNDV replicated efficiently and specifically in prostate cancer (CaP) cells and 3-dimensional prostaspheres but failed to replicate in the absence of PSA. Induction of intracellular PSA production by a synthetic androgen analog (R1881) enhanced fusogenicity in androgen-responsive CaP cells. Further, PSA-cleavable rNDV caused specific lysis of androgen-independent and androgen-responsive/nonresponsive CaP cells and prostaspheres, with a half-maximal effective concentration (EC50) ranging from a multiplicity of infection of 0.01 to 0.1. PSA-retargeted NDV efficiently lysed prostasphere tumor mimics, suggesting efficacy in vivo. Also, PSA-cleavable NDV failed to replicate in chicken embryos, indicating no pathogenicity for chickens. Prostate-specific antigen targeting is likely to enhance the therapeutic index of rNDV owing to tumor-restricted replication and enhanced fusogenicity.

PSA forms complexes with α1-antichymotrypsin in prostate

BACKGROUND

PSA is the most useful prostate cancer marker. However, its levels are increased also in some non-malignant conditions. In circulation, the majority of PSA is complexed with protease inhibitors, including α(1) -antichymotrypsin (ACT). The proportion of the PSA-ACT complex is higher in patients with prostate cancer than in controls without cancer. The expression of ACT has been shown to be higher in prostate cancer than in benign prostatic hyperplasia. However, results regarding the extent which PSA forms complexes within the prostate and whether there are differences in complex formation between normal and malignant prostatic tissue are inconsistent and limited.

METHODS

We studied complex formation of PSA secreted by cultured human prostate tissues and in the tissue by in situ proximity ligation assay (PLA). Free, total and active PSA, and the PSA-ACT complex were determined in tissue culture media by immunoassays, immunoblotting, and chromatographic methods.

RESULTS

The majority of PSA in tissue culture medium was free and enzymatically active. However, a significant proportion (1.6 ± 0.5%) of immunoreactive PSA was found to be complexed with ACT. Complex formation was confirmed by in situ PLA, which showed more intense staining of PSA-ACT in cancers with Gleason grade 3 than in adjacent benign tissues from the same patients.

CONCLUSIONS

These results show that PSA forms complexes already within the prostate and that PSA-ACT levels are increased in moderately differentiated prostate cancer tissue. This may explain, at least partially, why the ratio of serum PSA-ACT to total PSA is increased in prostate cancer.

Enzymatically active prostate-specific antigen promotes growth of human prostate cancers

BACKGROUND

Prostate specific antigen (PSA) is the best-known member of the kallikrein-related peptidase family, with an established role as a prostatic disease biomarker. Although it is produced at high levels by all stages of prostate cancer, it is uncertain if PSA plays a role in prostate cancer initiation and progression. We decided to investigate the impact of PSA and its enzymatic activity on tumor cell growth rates.

METHODS

A gene-specific shRNA lentiviral construct reduced endogenous PSA expression in the LNCaP human prostate cancer cell line. Resulting changes in growth rates in vitro and in vivo were determined. Using a mass spectroscopy-based approach, alterations to the LNCaP proteome due to reduced PSA were measured. Finally, to evaluate the importance of PSA's proteolytic activity, the PSA-null Du145 human prostate cancer cell line was engineered to express either enzymatically inactive pro-PSA (WT) or a furin-activated variant (FR) with high enzymatic activity. The resulting clones were evaluated for PSA-induced changes in growth rates in vivo and in vitro.

RESULTS

Lowered PSA levels dramatically reduced LNCaP growth rates. Expressing active PSA (FR), but not the inactive WT variant, conferred a growth advantage on Du145 cells. Proteomics analysis revealed global changes to the LNCaP proteome as a result of reduced PSA expression.

CONCLUSIONS

These studies demonstrate the importance of PSA to prostate cancer cell growth. We also show that the enzymatic activity of PSA confers an enhanced growth rate to human prostate cancer cells, suggesting a causal role in prostate cancer progression.

PSA affects prostate cancer cell invasion in vitro and induces an osteoblastic phenotype in bone in vivo

BACKGROUND

Patients with advanced prostate cancer frequently have a poor prognosis as a result of metastasis and present with high serum PSA levels. There is evidence suggesting that the serine protease activity of PSA could be involved in the invasion and metastasis of prostate cancer. In this study, we determined the effects of PSA and its precursor, pro-PSA, on invasion and the type of bone metastasis.

METHODS

We stably transfected prostate adenocarcinoma cells, human DU-145 and rat MatLyLu, with either the full-length prepro-PSA sequence or pre-PSA DNA, to generate subclones of cells that secrete pro-PSA or free PSA, respectively. Secretion of PSA was measured by western blot analysis and enzyme-linked immunosorbent assay (ELISA). The invasive and migratory properties of the cells were determined using a basement membrane extract and were compared with corresponding empty vector control cells. Twelve days after injection of PSA-secreting MatLyLu cells into the femora of nude mice, bone tumor burden and histomorphometry were determined using a stereological technique.

RESULTS

The transfected cells secreted 0.15-2.23 ng PSA/10(6) cells/day. Pro-PSA-secreting subclones increased invasion and migration by 24-263%. Conversely, the PSA-secreting subclones significantly reduced both invasion and migration by 59-70%. The divergent effects on invasion and migration observed in pro-PSA- and PSA-secreting subclones indicate that different forms of PSA may have different functions. Intrafemoral injections with PSA-secreting MatLyLu cells resulted in an increase in osteoblastic parameters when compared with non-PSA-secreting subclones as measured by bone histomorphometry. Concomitantly, a decrease in osteoclasts and eroded surface was observed.

CONCLUSIONS

Our in vitro data suggest that PSA, dependent on the predominant form secreted, may decrease or increase invasive properties of prostate cancer cells. The in vivo results indicate that PSA in the bone microenvironment may contribute to the osteoblastic phenotype of bone metastasis frequently observed in prostate cancer.

The analysis of some factors that influence on serum PSA level in localized prostate cancer patients: mathematical model

INTRODUCTION

Serum prostate specific antigen (PSA) concentration in localized prostate cancer (PCa) patients depends of numerous factors related to tumor, prostate and the patient. Tumor factors include tumor volume, localization, growth rate and aggressiveness. Prostate factors include prostate volume, the activity of dihydrotestosterone (DHT) and PSA synthesis. Host factors include androgenic status and total blood volume.

OBJECTIVE

to discuss all the possible factors that influence on serum PSA levels in localized PCa patients. To present the mathematical model that predicts serum PSA concentration in localized PCa patients.

METHODS

Mathematical model is based on the hypothesis that tumor-related PSA molecules are released from the epithelial prostatic cells around the tumor, so called "Destruction Zone" around the tumor (DZ). The amount of PSA is dependent of DZ volume. Moreover, DZ volume depends on tumor volume, tumor localization in the prostate and tumor growth rate.

RESULTS

The study offers the wide spectrum of PSA values, dependent on tumor, prostate and patient characteristics. These values are comparable with the empirical results in the literature, especially if tumor-volume doubling time of three years is entered into the calculations.

CONCLUSION

Although with some limitations, this mathematical model can explain the great variety of PSA serum concentrations in PCa patients.

Development of a peptide-drug conjugate for prostate cancer therapy

TGX-221 is a highly potent phosphoinositide 3-kinase β (PI3Kβ) inhibitor that holds great promise as a novel chemotherapeutic agent to treat prostate cancer. However, poor solubility and lack of targetability limit its therapeutic applications. The objective of this present study is to develop a peptide-drug conjugate to specifically deliver TGX-221 to HER2 overexpressing prostate cancer cells. Four TGX-221 derivatives with added hydroxyl groups were synthesized for peptide conjugation. Among them, TGX-D1 exhibited a similar bioactivity to TGX-221, and it was selected for conjugation with a peptide promoiety containing a HER2-targeting ligand and a prostate specific antigen (PSA) substrate linkage. From this selection, the peptide-drug conjugate was proven to be gradually cleaved by PSA to release TGX-D1. Cellular uptake of the peptide-drug conjugate was significantly higher in prostate cancer cells compared to the parent drug. Moreover, both the peptide-drug conjugate and its cleaved products demonstrated comparable activities as the parent drug TGX-D1. Our results suggest that this peptide-drug conjugate may provide a promising chemotherapy for prostate cancer patients.

A strategy for the selective imaging of glycans using caged metabolic precursors

Glycans can be imaged by metabolic labeling with azidosugars followed by chemical reaction with imaging probes; however, tissue-specific labeling is difficult to achieve. Here we describe a strategy for the use of a caged metabolic precursor that is activated for cellular metabolism by enzymatic cleavage. An N-azidoacetylmannosamine derivative caged with a peptide substrate for the prostate-specific antigen (PSA) protease was converted to cell-surface azido sialic acids in a PSA-dependent manner. The approach has applications in tissue-selective imaging of glycans for clinical and basic research purposes.

In vivo evaluation of a novel albumin-binding prodrug of doxorubicin in an orthotopic mouse model of prostate cancer (LNCaP)

PSA, which is overexpressed in prostate carcinoma, represents a molecular target for selectively releasing an anticancer agent from a prodrug formulation. In this study, we report on the in vivo antitumor efficacy of an efficacious albumin-binding prodrug of doxorubicin (PSA9) that incorporates p-aminobenzyloxycarbonyl (PABC) as a self-immolative spacer in addition to the heptapeptide, Arg-Ser-Ser-Tyr-Tyr-Ser-Leu, which serves as a substrate for PSA. The prodrug is cleaved very efficiently by PSA releasing H-Ser-Leu-PABC-doxorubicin and subsequently doxorubicin in PSA-positive cell lysates and prostate tumor homogenates as the final cleavage product. PSA9 at 3 × 6 mg kg(-1) doxorubicin equivalents (intravenous) was compared with conventional doxorubicin at equitoxic doses (at 3 × 3 mg kg(-1); intravenous) in an orthotopic mouse model of prostate cancer using LNCaP lentiviral luciferase-neomycin cells transduced with luciferase. Whereas doxorubicin did not show any efficacy against the primary tumor or metastases, the prodrug reduced the primary tumor by 30-50% and circulating PSA levels, and in addition, showed a pronounced reduction in lung and bone metastases by ∼77% and ∼96%, respectively, and a positive trend regarding the activity against liver and lymph-node metastases compared with control and doxorubicin-treated animals. The incorporation of PABC as a self-immolative spacer together with a PSA substrate demonstrates superior antitumor effects over doxorubicin attributed to an efficient cleavage by PSA releasing doxorubicin as the final active agent in prostate tumor homogenates. Using this approach for developing effective prodrugs against prostate cancer, is worthy of further preclinical optimization.

Optimization of an albumin-binding prodrug of Doxorubicin that is cleaved by prostate-specific antigen

We have developed a novel albumin-binding prodrug of doxorubicin that incorporates p-aminobenzyloxycarbonyl (PABC) as a 1,6 self-immolative spacer in addition to the heptapeptide, Arg-Ser-Ser-Tyr-Tyr-Ser-Leu, as a substrate for the prostate-specific antigen (PSA) that is overexpressed in prostate carcinoma and represents a molecular target for selectively releasing an anticancer agent from a prodrug formulation. The prodrug exhibited good water solubility and was bound rapidly to the cysteine-34 position of human serum albumin. Incubation studies with PSA demonstrated that the albumin-bound form of the prodrug was cleaved rapidly at the P1-P1' scissile bond, releasing H-Ser-Leu-PABC-DOXO, which was further degraded to release doxorubicin as a final cleavage product within a few hours in prostate tumor tissue homogenates as well as in PSA-positive LNCaP LN cell lysates. Moreover, our prodrug exhibited antiproliferative activity in a low micromolar range against a PSA-expressing prostate cancer cell line (LNCaP).

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.

Complex formation between human prostate-specific antigen and protease inhibitors in mouse plasma

BACKGROUND

When secreted from the prostate, most of prostate-specific antigen (PSA) is free and enzymatically active. Upon reaching circulation, active PSA is inactivated by complex formation with protease inhibitors. To justify the use of mouse models for evaluation of the function of PSA and for studies on therapeutic modalities based on modulation of PSA activity, it is important to know whether PSA complexation is similar in mouse and man.

METHODS

To characterize the circulating forms of PSA in mouse, we used subcutaneous LNCaP and 22RV1 human prostate cancer cell xenograft tumor models. We also added PSA directly to mouse serum. Free and total PSA were measured by immunoassay, and PSA complexes were extracted by immunopurification followed by SDS-PAGE, in-gel trypsin digestion and identification of signature peptides by mass spectrometry.

RESULTS

In mice bearing xenograft tumors, 68% of the immunoreactive PSA occurred in complex, and when added to mouse serum, over 70% of PSA forms complexes that comprises alpha(2)-macroglobulin and members of the alpha(1)-antitrypsin (AAT) family.

CONCLUSION

In mouse plasma, PSA forms complexes similar to those in man, but the major immunoreactive complex contains AAT rather than alpha(1)-antichymotrypsin, which is the main complex forming serpin in man. The complex formation of PSA produced by xenograft tumor models in mice is similar to that of human prostate tumors with respect to the complexation of PSA.

A novel androgen-dependent prostate cancer xenograft model derived from skin metastasis of a Japanese patient

BACKGROUND

The incidence of, and mortality from, prostate cancer (PCa) has increased in Asian countries over the past decades, partly due to a change in dietary habits. Recent reports have revealed differences in the molecular basis of PCa among people of differing racial or ethnic backgrounds. PCa xenograft models established from Asian patients would be useful for understanding the basis of PCa in Asian populations; we therefore established and characterized a novel PCa xenograft model, JDCaP, from a metastatic skin lesion of a Japanese hormone-refractory prostate cancer (HRPC) patient.

METHODS

Skin metastatic tissue derived from poorly differentiated prostatic adenocarcinoma in a 61-year-old Japanese male was transplanted to nude mice and JDCaP was established by serial passage. Expression of androgen receptor (AR) and prostate-specific antigen (PSA) was evaluated by immunohistochemistry and the AR sequence was analyzed. Hormone sensitivity of JDCaP was investigated in vivo by orchiectomy followed by administration of steroid hormones, including testosterone, estradiol, progesterone, and hydrocortisone. Therapeutic effects of leuprorelin acetate, bicalutamide, flutamide, diethylstilbestrol (DES), and estradiol were investigated.

RESULTS

JDCaP expressed wild-type AR and PSA and showed androgen dependence. Only testosterone administration maintained tumor proliferation after orchiectomy. Administration of leuprorelin acetate, bicalutamide, and flutamide inhibited tumor growth. DES and estradiol also demonstrated significant antitumor effects.

CONCLUSIONS

JDCaP expresses wild-type ARs and exhibits androgen dependence despite its origin from a HRPC patient. The model may be useful to elucidate the molecular basis of PCa in Asian populations and to develop prevention and therapeutic strategies.

Antiangiogenic properties of prostate-specific antigen (PSA)

The prostate produces high levels of prostate-specific antigen (PSA, also known as kallikrein-related peptidase 3, KLK3), which is a potential target for tumor imaging and treatment. Although serum PSA levels are elevated in prostate cancer, PSA expression is lower in malignant than in normal prostatic epithelium and it is further reduced in poorly differentiated tumors. PSA has been shown to inhibit angiogenesis both in in vitro and in vivo models. In this review we focus on our recent studies concerning the mechanism of the antiangiogenic function of PSA. We have recently shown that the antiangiogenic activity of PSA is related to its enzymatic activity. Inactive PSA isoforms do not have antiangiogenic activity as studied by a human umbelical vein endothelial cell (HUVEC) tube formation model. Furthermore, inhibition of PSA, either by a monoclonal antibody or small molecule inhibitors abolishes the effect of PSA, while a peptide that stimulates the activity of PSA enhances the antiangiogenic effect. We have analyzed changes in gene expression associated with the PSA induced reduction of tube formation in the HUVEC model. Several small changes were observed and they were found to be opposite to those associated with tube formation. Taken together, these studies suggest that PSA exerts antiantiogenic activity related to its enzymatic activity. Thus it might be associated with the slow growth of prostate cancer.

Regulation of PSA secretion and survival signaling by calcium-independent phopholipase A(2)beta in prostate cancer cells

BACKGROUND

Serum prostate specific antigen (PSA) levels in prostate cancer patients serve as a useful biomarker for diagnosing and monitoring prostate cancer. Recently, secreted PSA has been characterized as an autocrine survival factor through activation of Akt and induction of AR. In the normal prostate, PSA is secreted in the lumen of prostatic ducts to lyse proteins in the seminal coagulum.

METHODS

However, the mechanism for constitutive PSA secretion from benign prostate and its transport across the prostate-blood barrier into serum are unknown. Regulation of peptide secretion by iPLA(2)-beta has been reported in non-prostatic tissue and in prostate tissue iPLA(2)-beta is reported to be under androgen regulation. We investigated whether iPLA(2) plays a role for in PSA secretion by comparing iPLA(2) activity and expression in normal prostate epithelial RWPE-1 cells and in LNCaP prostate cancer cells. Expression of the two active iPLA(2)-beta mRNA splice variants, LH-iPLA(2) and SH-iPLA(2), were increased and the inhibitory ankyrin-iPLA(2) isoform was markedly reduced in LNCaP cells as compared to normal prostate epithelial RWPE-1 cells.

RESULTS

These changes are consistent with a higher enzymatic activity in LNCaP cells. The iPLA(2)-beta-specific inhibitor BEL inhibited PSA secretion and induced apoptosis in LNCaP cells. iPLA(2) knockdown using SiRNA inhibited PSA secretion, downregulated AR and induced apoptosis. Exogenous PSA suppressed BEL-induced apoptosis and neutralizing anti-PSA antibody blocked the survival effect of PSA.

CONCLUSIONS

These data indicate that iPLA(2)-beta participates in regulating PSA secretion and supports the concept that secreted PSA provides an autocrine survival function in LNCaP cells.

Optimization of peptide-based inhibitors of prostate-specific antigen (PSA) as targeted imaging agents for prostate cancer

Prostate-specific antigen (PSA) is a serine protease biomarker that may play a role in prostate cancer development and progression. The inhibition of PSA's enzymatic activity with small molecule inhibitors is an attractive and, as of yet, unexploited target. Previously, we reported a series of peptidyl aldehyde and boronic acid based inhibitors of PSA. In this study, the structural requirements in the P2 and P3 positions of peptide-based PSA inhibitors are explored through the substitution of a series of natural and unnatural amino acids in these positions. This analysis demonstrated a preference for hydrophobic residues in the P2 position and amino acids with the potential to hydrogen bond in the P3 position. Using this information, a peptide boronic acid inhibitor with the sequence Cbz-Ser-Ser-Gln-Nle-(boro)-Leu was identified with a K(i) for PSA of 25nM. The attachment of a bulky metal chelating group to the amino terminal of this peptide did not adversely affect PSA inhibition. This result suggests that a platform of PSA inhibitor chelates could be developed as SPECT or PET-based imaging agents for prostate cancer.

Development of peptides specifically modulating the activity of KLK2 and KLK3

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

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.

Structural characterization and anti-angiogenic properties of prostate-specific antigen isoforms in seminal fluid

BACKGROUND

The prostate produces high levels of prostate-specific antigen (PSA), which has been shown to exert anti-angiogenic properties and thus might slow down prostate tumor growth. It has been suggested that the protease activity of PSA is not needed for its anti-angiogenic function. We have previously shown that seminal fluid contains both active and inactive, internally cleaved forms of PSA. The precise structural differences between these isoforms and their function are not known.

METHODS

To elucidate the structures, we purified PSA from seminal fluid and separated it by anion-exchange chromatography into six different isoforms, which were characterized by mass spectrometry. The anti-angiogenic activity of these PSA-isoforms was analyzed by human umbilical vein endothelial cell (HUVEC) tube formation assay.

RESULTS

The enzymatically active PSA-isoforms had an intact peptide moiety but could be separated into three isoforms based on differences in glycosylation. The major isoform contained PSA with a biantennary carbohydrate with terminal sialic acids on both antennae. The other active isoforms showed significant carbohydrate heterogeneity, containing one or no sialic acid. The inactive isoforms were internally cleaved at several different positions, but the fragments were held together by disulphide bonds. The enzymatic activity of PSA correlated with its inhibitory effect on the endothelial cell tube formation and the inhibition was dose-dependent at physiological concentrations, whereas enzymatically inactive internally cleaved PSA-isoforms had no effect.

CONCLUSIONS

Our results show that the anti-angiogenic effect of PSA is based on its proteolytic activity.

Gene expression changes associated with the anti-angiogenic activity of kallikrein-related peptidase 3 (KLK3) on human umbilical vein endothelial cells

Kallikrein-related peptidase 3 (KLK3, also known as prostate-specific antigen, PSA) is a chymotrypsin-like kallikrein that has anti-angiogenic properties. We have previously shown in a human umbilical vein endothelial cell (HUVEC) model that the anti-angiogenic effect of KLK3 is related to its enzyme activity. However, the mechanism of this effect remains to be clarified. To this end, we used a DNA microarray to study KLK3-induced changes in gene expression associated with reduction of HUVEC tube formation. Among the 41 000 genes studied, 311 were differentially expressed between control and KLK3-treated cells. These changes were enriched in several pathways, including those associated with proteasome, ubiquitin-mediated proteolysis, focal adhesion and regulation of the actin cytoskeleton. Furthermore, the changes were opposite to those previously described to occur during tubulogenesis. In conclusion, our results show that KLK3 induces gene expression changes in HUVECs. Although these changes might be relevant for the mechanism by which KLK3 exerts its anti-angiogenic activity, it cannot be judged from the present results whether they reflect the primary mechanism mediating the effect of KLK3 or are secondary to morphogenic differentiation.

Tumour endoproteases: the cutting edge of cancer drug delivery?

Despite progression in anticancer drug development and improvements in the clinical utilization of therapies, current treatment regimes are still dependent upon the use of systemic antiproliferative cytotoxic agents. Although these agents are unquestionably potent, their efficacy is limited by toxicity towards 'normal' cells and a lack of tumour selective targeting, resulting in a therapeutic index which is modest at best. Consequently, the development of more tumour selective cancer treatments, with better discrimination between tumour and normal cells is unequivocally an important goal for cancer drug discovery. One such strategy is to exploit the tumour phenotype as a mechanism for tumour-selective delivery of potent therapeutics. An exciting approach in this area is to develop anticancer therapeutics as prodrugs, which are non-toxic until activated by enzymes localized specifically in the tumour. Enzymes suitable for tumour-activated prodrug development must have increased activity in the tumour relative to non-diseased tissue and an ability to activate the prodrug to its active form. One class of enzyme satisfying these criteria are the tumour endoproteases, particularly the serine- and metallo-proteases. These proteolytic enzymes are essential for tumour angiogenesis, invasion and metastasis, the major defining features of malignancy. This review describes the concept behind development of tumour-endoprotease activated prodrugs and discusses the various studies to date that have demonstrated the huge potential of this approach for improvement of cancer therapy.

Targeted inhibition of hedgehog signaling by cyclopamine prodrugs for advanced prostate cancer

A promising agent for use in prostate cancer therapy is the Hedgehog (Hh) signaling pathway inhibitor, cyclopamine. This compound, however, has the potential for causing serious side effects in non-tumor tissues. To minimize these bystander toxicities, we have designed and synthesized two novel peptide-cyclopamine conjugates as prostate-specific antigen (PSA)-activated prodrugs for use against prostate cancer. These prodrugs were composed of cyclopamine coupled to one of two peptides (either HSSKLQ or SSKYQ) that can be selectively cleaved by PSA, converting the mature prodrug into an active Hedgehog inhibitor within the malignant cells. Of the two prodrugs, Mu-SSKYQ-Cyclopamine was rapidly hydrolyzed, with a half-life of 3.2h, upon incubation with the PSA enzyme. Thus, modulating cyclopamine at the secondary amine with PSA-cleavable peptides is a promising strategy for developing prodrugs to target prostate cancer.