Prostate-specific kallikreins-2 and -4 enhance the proliferation of DU-145 prostate cancer cells through protease-activated receptors-1 and -2

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

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

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

Kallikrein-related peptidases: proteolysis and signaling in cancer, the new frontier

The exact mechanism(s) by which kallikrein-related peptidases (KLKs) function, their levels of activity and their potential endogenous targets in vivo have only recently begun to be revealed. Our group and others have shown that KLKs can have hormonal properties by signaling via proteinase-activated receptors (PARs), a family of G-protein-coupled receptors. Signals by PAR(1), PAR(2), and PAR(4) can regulate calcium release or mitogen-activated protein kinase activation and lead to platelet aggregation, vascular relaxation, cell proliferation, cytokine release, and inflammation. We have further documented the presence of active KLK6 and 10 (by activity-based ELISA or proteomics) and the presence of proteinase inhibitors, such as alpha(1)-antitrypsin, in cancer-derived fluids. We suggest that tumors and inflamed tissues can release active KLKs, which are under tight regulation by proteinase inhibitors. These enzymes can potentially control cell/tissue behavior by regulating PAR activation in specific settings and disease stages.

Identification of IGFBP-3 fragments generated by KLK2 and prevention of fragmentation by KLK2-inhibiting peptides

Kallikrein-related peptidase 2 (KLK2) degrades insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) in vitro. IGFBP-3 forms complexes with IGFs, preventing them from binding to their receptors and stimulating cell proliferation and survival. IGF-independent actions have also been described for IGFBP-3. The degradation of IGFBP-3 by KLK2 or other proteases in the prostate may promote the growth of prostate cancer. We studied IGFBP-3 degradation by immunoblotting and two specific immunoassays, one recognizing only native non-fragmented IGFBP-3 and the other one recognizing both intact and proteolytically cleaved IGFBP-3. Peptides were used to inhibit the enzyme activity of KLK2 and cleavage sites in IGFBP-3 were identified by mass spectrometry. KLK2 proteolyzed IGFBP-3 into several small fragments, mostly after Arg residues, in keeping with the trypsin-like activity of KLK2. The fragmentation could be inhibited by KLK2-inhibiting peptides in a dose-dependent fashion. As degradation of IGFBP-3 could lead to a more aggressive cancer phenotype, inhibition of KLK2 activity might be useful for treatment of prostate cancer and other diseases associated with increased KLK2 activity.

Natural and synthetic inhibitors of kallikrein-related peptidases (KLKs)

Including the true tissue kallikrein KLK1, kallikrein-related peptidases (KLKs) represent a family of fifteen mammalian serine proteases. While the physiological roles of several KLKs have been at least partially elucidated, their activation and regulation remain largely unclear. This obscurity may be related to the fact that a given KLK fulfills many different tasks in diverse fetal and adult tissues, and consequently, the timescale of some of their physiological actions varies significantly. To date, a variety of endogenous inhibitors that target distinct KLKs have been identified. Among them are the attenuating Zn(2+) ions, active site-directed proteinaceous inhibitors, such as serpins and the Kazal-type inhibitors, or the huge, unspecific compartment forming α(2)-macroglobulin. Failure of these inhibitory systems can lead to certain pathophysiological conditions. One of the most prominent examples is the Netherton syndrome, which is caused by dysfunctional domains of the Kazal-type inhibitor LEKTI-1 which fail to appropriately regulate KLKs in the skin. Small synthetic inhibitory compounds and natural polypeptidic exogenous inhibitors have been widely employed to characterize the activity and substrate specificity of KLKs and to further investigate their structures and biophysical properties. Overall, this knowledge leads not only to a better understanding of the physiological tasks of KLKs, but is also a strong fundament for the synthesis of small compound drugs and engineered biomolecules for pharmaceutical approaches. In several types of cancer, KLKs have been found to be overexpressed, which makes them clinically relevant biomarkers for prognosis and monitoring. Thus, down regulation of excessive KLK activity in cancer and in skin diseases by small inhibitor compounds may represent attractive therapeutical approaches.

Expression of proteinase-activated receptor 1-4 (PAR 1-4) in human cancer

Proteinase activated receptors (PAR 1-4) are membrane receptors with a unique way of activation by proteinases like thrombin, trypsin and matrix metalloproteinases which lead to a specific cellular response. To evaluate the significance of expression and co-expression of PAR in cancer we performed a survey on published data. A Pubmed literature search on "PAR, thrombin, cancer" was performed and 46 publications were selected for systematic review based on the availability of information on tumor type, material type, detection method and specification of positive cases. PAR-1 was found in 77.3% of malignant samples (n = 678), PAR-2 in 79.5% (n = 592), PAR-3 in 12.6% (n = 87) and PAR-4 in 54.9% (n = 153). PAR-1 and -2 were present in adenocarcinomas, melanomas, osteosarcomas, glioblastomas, meningiomas, leukaemias and squamous cell carcinomas. Presence of PAR-3 was limited to kidney and liver cancer. The data on PAR-4 expression was inconclusive. Those studies analysing PAR-1 and PAR-2 reported coexpression of the two receptors. PAR-1 and -2 are widely expressed in human tumors suggesting an important role in tumorigenesis and providing potential targets for therapy. PAR-3 and PAR-4 are less frequently detectable, their expression and potential role in tumorigenesis require further investigation.

Analysis of gene expression in prostate cancer epithelial and interstitial stromal cells using laser capture microdissection

BACKGROUND

The prostate gland represents a multifaceted system in which prostate epithelia and stroma have distinct physiological roles. To understand the interaction between stroma and glandular epithelia, it is essential to delineate the gene expression profiles of these two tissue types in prostate cancer. Most studies have compared tumor and normal samples by performing global expression analysis using a mixture of cell populations. This report presents the first study of prostate tumor tissue that examines patterns of differential expression between specific cell types using laser capture microdissection (LCM).

METHODS

LCM was used to isolate distinct cell-type populations and identify their gene expression differences using oligonucleotide microarrays. Ten differentially expressed genes were then analyzed in paired tumor and non-neoplastic prostate tissues by quantitative real-time PCR. Expression patterns of the transcription factors, WT1 and EGR1, were further compared in established prostate cell lines. WT1 protein expression was also examined in prostate tissue microarrays using immunohistochemistry.

RESULTS

The two-step method of laser capture and microarray analysis identified nearly 500 genes whose expression levels were significantly different in prostate epithelial versus stromal tissues. Several genes expressed in epithelial cells (WT1, GATA2, and FGFR-3) were more highly expressed in neoplastic than in non-neoplastic tissues; conversely several genes expressed in stromal cells (CCL5, CXCL13, IGF-1, FGF-2, and IGFBP3) were more highly expressed in non-neoplastic than in neoplastic tissues. Notably, EGR1 was also differentially expressed between epithelial and stromal tissues. Expression of WT1 and EGR1 in cell lines was consistent with these patterns of differential expression. Importantly, WT1 protein expression was demonstrated in tumor tissues and was absent in normal and benign tissues.

CONCLUSIONS

The prostate represents a complex mix of cell types and there is a need to analyze distinct cell populations to better understand their potential interactions. In the present study, LCM and microarray analysis were used to identify novel gene expression patterns in prostate cell populations, including identification of WT1 expression in epithelial cells. The relevance of WT1 expression in prostate cancer was confirmed by analysis of tumor tissue and cell lines, suggesting a potential role for WT1 in prostate tumorigenesis.

Kallikrein-related peptidase-4 initiates tumor-stroma interactions in prostate cancer through protease-activated receptor-1

In prostate cancer, the mechanism by which the stromal cells surrounding the cancer epithelium become reactive and overproduce growth factors is unclear. Furthermore, the precise process of how these stromal cells stimulate the cancer epithelium is not fully understood. We recently found that protease-activated receptor-1 (PAR-1) in these reactive stromal cells is upregulated. To investigate the role of PAR-1 in the stromal-epithelial interaction, WPMY-1 stromal myofibroblasts were stimulated with PAR-1 agonists including thrombin and PAR-1 activating peptide. We show that WPMY-1 cells have functional PAR-1 by signaling through ERK1/2. Conditioned media (CM) from PAR-1 agonists-treated WPMY-1 cells stimulate the epithelial LNCaP cells leading to ERK1/2 activation and cell proliferation. Cytokine array analysis of the CM demonstrates that PAR-1 induces stromal cells to release numerous cytokines, of which interleukin 6 (IL-6) is the major factor responsible for mitogenic signaling in LNCaP cells. CM further induces expression of prostate-specific kallikrein-related peptidase-3 (KLK3/PSA) and KLK4 in LNCaP cells via the IL-6 pathway. Moreover, KLK4 functions as a potent agonist of PAR-1 by cleaving the receptor at the proper site on cell surface. KLK4 triggers transmembrane signaling and upregulates IL-6 in WPMY-1 cells through PAR-1. Immunohistochemical analysis indicates that PAR-1 is predominantly expressed in peritumoral stroma while KLK4 is produced exclusively by the epithelial cancer cells. These data provide evidence for a novel double-paracrine mechanism whereby cancer epithelium produces KLK4 to activate PAR-1 in the surrounding stroma, which in-turn releases cytokines (IL-6) that stimulate cancer cells to proliferate and increase production of KLKs.

Kallikrein-related peptidase 4: a new activator of the aberrantly expressed protease-activated receptor 1 in colon cancer cells

Certain serine proteases are considered to be signaling molecules that act through protease-activated receptors (PARs). Our recent studies have implicated PAR1 and PAR4 (thrombin receptors) and PAR2 (trypsin receptor) in human colon cancer growth. Here we analyzed the expression of KLK4, a member of the kallikrein-related peptidase (KLK) family of serine proteases and explored whether this member can activate PAR1 and PAR2 in human colon cancer cells. Immunohistochemistry showed KLK4 expression in human colon adenocarcinomas and its absence in normal epithelia. KLK4 (1 micromol/L) initiated loss of PAR1 and PAR2 from the HT29 cell surface as well as increased intracellular calcium transients in HT29 cells. This KLK4-induced Ca2+ flux was abrogated after an initial challenge of the cells with TRAP (SFLLR-NH2; 100 micromol/L), which is known to desensitize PAR1 and PAR2. Interestingly, PAR1 blocking antibody, which inhibits cleavage and activation by thrombin, dramatically reduced KLK4-induced Ca2+ influx, but blocking cleavage of PAR2 failed to attenuate the KLK4-induced Ca2+ flux. Consistently, desensitization with AP1 (TFFLR-NH2), targeting PAR1, attenuated most of the Ca2+ flux induced by KLK4. KLK4 also induced a rapid and significant ERK1/2 phosphorylation in HT29 cells. Our results demonstrate, for the first time, that KLK4 is aberrantly expressed in colon cancer and capable of inducing PAR1 signaling in cancer cells. These data suggest that KLK4 signaling via PAR1 may represent a novel pathway in colon tumorigenesis.

Protease-activated receptor-1 is upregulated in reactive stroma of primary prostate cancer and bone metastasis

BACKGROUND

Prostate cancer progression is partly facilitated by tumor-stroma interactions. We recently reported that protease-activated receptors (PAR-1 and PAR-2) are overexpressed in prostate cancer, and PAR-1 expression in peritumoral stroma is associated with biochemical recurrence. However, the nature of PAR expression in prostate tumor microenvironment is not fully understood. We therefore evaluated PAR-1 and PAR-2 expression in primary prostate cancer and bone metastasis.

METHODS

PAR-1 and PAR-2 expression in normal, primary prostate cancer and the corresponding bone metastatic tissues were examined by immunohistochemistry, and double-label immunohistochemistry with the use of additional markers.

RESULTS

PAR-1 was expressed in peritumoral stroma in the majority of primary cancer tissues (83%). Serial sections and double-label immunohistochemistry determined that these PAR-1 expressing stromal cells were predominantly myofibroblasts, the primary cell type in reactive stroma. Analysis of cancer glands revealed that PAR-1 expression was significantly increased in the reactive stroma around higher Gleason grade cancers. PAR-2 was predominantly expressed in the primary cancer cells as well as smooth muscle cells but not in reactive stroma. In bone metastasis, PAR-1 expression in cancer cells was elevated compared to the primary site from the same patient. In the bone reactive stroma, PAR-1 was present in vascular endothelial cells and fibroblasts, while both PAR-1 and PAR-2 were expressed in osteoblasts and osteoclasts.

CONCLUSIONS

In primary prostate cancer and bone metastasis, PAR-1 is upregulated in reactive stroma and PAR-2 is uniformly overexpressed in carcinoma cells, suggesting these receptors may play potentially different roles in prostate cancer development and metastasis.

Hypomaturation enamel defects in Klk4 knockout/LacZ knockin mice

Kallikrein 4 (Klk4) is believed to play an essential role in enamel biomineralization, because defects in KLK4 cause hypomaturation amelogenesis imperfecta. We used gene targeting to generate a knockin mouse that replaces the Klk4 gene sequence, starting at the translation initiation site, with a lacZ reporter gene. Correct targeting of the transgene was confirmed by Southern blot and PCR analyses. Histochemical X-gal (5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside) staining demonstrated expression of beta-galactosidase in maturation stage ameloblasts. No X-gal staining was observed in secretory stage ameloblasts or in odontoblasts. Retained enamel proteins were observed in the maturation stage enamel of the Klk4 null mouse, but not in the Klk4 heterozygous or wild-type mice. The enamel layer in the Klk4 null mouse was normal in thickness and contained decussating enamel rods but was rapidly abraded following weaning, despite the mice being maintained on soft chow. In function the enamel readily fractured within the initial rod and interrod enamel above the parallel enamel covering the dentino-enamel junction. Despite the lack of Klk4 and the retention of enamel proteins, significant levels of crystal maturation occurred (although delayed), and the enamel achieved a mineral density in some places greater than that detected in bone and dentin. An important finding was that individual enamel crystallites of erupted teeth failed to grow together, interlock, and function as a unit. Instead, individual crystallites seemed to spill out of the enamel when fractured. These results demonstrate that Klk4 is essential for the removal of enamel proteins and the proper maturation of enamel crystals.