Kallikrein-related protease in the rat ventral prostate: cDNA cloning and androgen regulation

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.

Androgen-regulated proteins in the rat ventral prostate

The rat ventral prostate synthetizes and secretes a number of relatively abundant proteins, such as Prostatic Binding Protein (PBP), the Proline-Rich Polypeptides (PRP), the 20 or 22-kDa protein and a prostatic protease, which respectively constitute approx. 50, 5, 10 and 2% of the secreted protein. The synthesis of these proteins is androgen-regulated and this is also the case for the concentration of the corresponding mRNAs. The existence of a putative androgen response element in the first intron of the PBP-C3 gene supports the possibility of a direct control of its transcription by androgen receptor complexes, but there is also evidence for the requirement of protein synthesis for mRNA stimulation. Finally, changes of tissue composition and cell-cell interactions between epithelial and stromal cells most probably play an important role in the global effect of androgens on the rat ventral prostate.

Cell-specific activation of the human skeletal alpha-actin by androgens

Although it is evident that androgens increase muscle mass and strength, little is known about the critical molecular targets of androgens in skeletal muscle. In rodents, the skeletal alpha-actin gene is a tissue-specific gene expressed only in the levator ani and other skeletal muscles but not in the prostate or preputial gland, the well-known androgen target tissue. We identified tissue-specific androgen-regulated genes in the skeletal muscle in rats after oral administration of androgens and focused on androgen-dependent up-regulation of the skeletal alpha-actin gene. To investigate the mechanism of action, an in vitro system with various cell lines and a series of deletion mutants of the alpha-actin promoter were used. The human skeletal alpha-actin promoter was activated by androgens in the muscle cell line C2C12 but not in the liver, prostate, or breast cancer cell lines in which exogenous human androgen receptor is expressed. The sequence of the promoter is sufficient for cell-specific androgen response, providing a model for the tissue specificity demonstrated in vivo. Using a series of deletion mutants, the androgen response can be maintained using just the proximal promoter region. The importance of androgen regulation of this small portion of the human skeletal alpha-actin promoter was demonstrated by the correlation between muscle and the alpha-actin promoter activity for an array of selective androgen receptor modulators (SARMs), including an orally active SARM LGD2226. Taken together, the results suggest that the regulation of skeletal alpha-actin by androgens/SARMs may represent an important model system for understanding androgen anabolic action in the muscle.

Immunocytochemical localization of seminal proteins in salivary and lacrimal glands of the rat

Antibodies against 10 different secretory proteins from the accessory sex glands of the male rat were used for immunohistochemical studies of salivary and lacrimal glands from intact and castrated rats, at the light- and electron-microscopic levels. In the parotid gland, secretory acinar cells showed immunoreactivity with antibodies against prostatic binding protein, cystatin-related peptide and acid phosphatase (isoenzyme pI 8.0; 5.6) typical of ventral prostate, and seminal vesicle secretion VI. Western blotting analysis indicated that immunoreactivity against prostatic binding protein was attributable to a subunit, presumably C3. Acid phosphatase pI 5.6 showed a molecular weight of 66 kDa, which is at variance with the prostatic form. Immunoreactivity for secretory transglutaminase, derived from the coagulating gland, was restricted to myoepithelial and stromal cells. In castrated animals, the immunoreactivity of acinar cells was reduced to the background level, whereas stromal transglutaminase immunoreactivity was unaltered. The distribution pattern of immunoreactivity for the proteins mentioned was almost identical in the lacrimal gland. Significant differences were however observed in the immunoreactivity of the inframandibular gland, where serous glandular cells were non-immunoreactive for seminal proteins, with the exception of acid phosphatase isoenzyme pI 8.0. Granules present in the convoluted granular ducts were immunoreactive particularly for acid phosphatase (isoenzyme pI 5.6) but much less for cystatin-related peptide; immunoreactivity was reduced after castration. The straight portion of the inframandibular duct system was immunoreactive for transglutaminase, but no influence of castration was visible. The distribution of immunoreactivity for seminal proteins present in the salivary and lacrimal glands and the pronounced androgen-dependence of their expression point to functional relationships of the respective proteins at both glandular sites.

Purification of enzymes of the kallikrein gene family (rK8 and rK9) from the rat prostate

The rat kallikrein family consists of multiple closely related proteins. A method for demonstration and identification of kallikrein-like proteins has been developed based on their differences in isoelectric point and their immunological similarity. The method, which involved separation in flat-bed isoelectro-focusing gels (pH range 3-9) and detection by immunoblotting using polyclonal antiserum against one of the family members, has been used in the present study to detect kallikrein-like proteins in the rat prostate. Nine immunoreactive kallikrein-like protein bands were detected with pI ranging from 5.30 to 8.35. Of these, six were completely purified and three were partially purified. Two proteins (pI 5.30 and 6.75-6.90) corresponded to protein bands in gels of rat submandibular-gland extracts, and were identified by partial amino acid sequence analysis as rK8 and rK9 respectively. In addition, sequence analysis revealed complete sequence similarity between rK9 and the immunoreactive prostate proteins with pI 7.15, 7.25, 7.50 and 8.27. On the basis of this finding and immunological and biochemical characterization, we concluded that all the kallikrein-like proteins detected, except for rK8, represented isoenzymes of rK9. The molecular masses of the prostate rK9 isoenzymes (24,600-29,300 Da) were close to that of submandibular-gland rK9 (24,600 Da), although differences were observed after reduction with mercaptoethanol. The prostate rK9 isoenzymes were, like submandibular-gland rK9, inhibited by soya-bean trypsin inhibitor but not by aprotinin, and were classified as serine proteases as they were inhibited by phenylmethanesulphonyl fluoride. rK8 (28,700 Da) showed no activity with any of the substrates tested, and its inhibitory profile could therefore not be studied. No other enzymes of the kallikrein family were found in the rat prostate.

Androgen-dependent expression of cystatin-related protein (CRP) in the exorbital lacrimal gland of the rat

Cystatin-related protein (CRP), also known as 20 (22)-kDa glycoprotein is expressed not only in the ventral prostate, but also in the lacrimal gland of adult male rats. In this study the expression of CRP in androgen-treated female animals is studied. CRP mRNA is absent in the lacrimal gland of untreated adult female rats, but can be induced by androgens, although this induction is slower than in castrated male rats. Estradiol, progesterone or glucocorticoids have no effect. In testicular feminized rats, however, CRP mRNA is not induced in the lacrimal gland by androgens. At the protein level, the presence of CRP in tears of adult male rats is demonstrated. In female animals or castrated male animals CRP can be induced by androgens in a dose-dependent way. Here also the induction is slower in female rats, even during secondary induction after previous full stimulation by androgens. These results indicate that androgens and a functionally normal androgen receptor are essential for the expression of CRP in the lacrimal gland. The time course of induction depends on the dose of androgens, the previous contact with androgens, the duration of the androgen-free interval and the sex of the animals.

Structure of rat genes encoding androgen-regulated cystatin-related proteins (CRPs): a new member of the cystatin superfamily

Cystatin-related proteins (CRPs) are abundant androgen-regulated secretory glycoproteins that are specifically synthesized in the ventral prostate and lachrymal gland of the rat. Two complete 6-kb genes, Crp1 and Crp2, have been cloned and characterized. They are differentially expressed and encode slightly different proteins. The genes each contain four exons which are interrupted by large introns. An alignment of their sequences demonstrates an overall homology of 90%. The 3' end of a third gene, Crp3, from which only a 1.5-kb fragment was isolated, displays a sequence identity of 84%. These data indicate the existence of a Crp multigene family. The 5' flanking regions of Crp1 and Crp2 are highly homologous and contain a GATAAA sequence 29 nt upstream from the transcription start point. This TATA-box-like element is also found in the promoters of the genes encoding cystatin type-2 proteins. No other recognizable transcription control elements can be detected. Potential binding sites (ARE) for the androgen receptor are scattered throughout the entire genes. The exon/intron organization of the genes encoding CRPs, the size of the exons and their encoding amino acid sequences exhibiting a characteristic spacing of the Cys residues are structural elements displaying a remarkable similarity with the corresponding elements in the genes encoding cystatin type-2 proteins. CRPs must therefore belong to the cystatin superfamily. However, due to their additional domain encoded in an extra exon 2, CRPs must be classified as a new family, type 5.

Biochemical characterization and substrate specificity of rat prostate kallikrein (S3): comparison with tissue kallikrein, tonin and T-kininogenase

A tissue kallikrein-like enzyme encoded by S3 mRNA was purified to homogeneity from rat prostate gland. The apparent molecular mass of the prostate enzyme is 32 kDa as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The intact 32 kDa enzyme is split into two bands of lower molecular mass, 18 and 14 kDa, under reducing conditions on SDS-PAGE. NH2-terminal amino acid sequence analyses of the intact enzyme and heavy and light chains revealed the identity to the translated sequence of a prostate kallikrein cDNA (S3). Isoelectric focusing indicated that the prostate enzyme is a basic protein with pI of 7.30-7.45. Specific activities of the prostate kallikrein toward angiotensin I, angiotensinogen and rat low M(r) kininogen as well as tripeptide chromogenic substrates were compared with those of tissue kallikrein, tonin and T-kininogenase. The kinin-releasing activity is inhibited by leupeptin, antipain, benzamidine and soybean trypsin inhibitor. A sensitive and specific radioimmunoassay for the rat prostate kallikrein shows that the immunoreactive kallikrein levels in prostate and submandibular gland were 23.78 +/- 2.62 micrograms/mg protein (n = 5) and 12.29 +/- 2.25 micrograms/mg protein (n = 5), respectively. The results indicate that the prostate kallikrein S3 is expressed at high levels in both prostate and submandibular glands.

Functional diversity of proteinases encoded by genes of the rat tissue kallikrein family

A group of proteinases closely related to tissue kallikrein was purified from the rat submandibular gland. Physicochemical characterization of these proteinases, including amino terminal sequencing, allowed correlation with the genes of the rat kallikrein family. In spite of their similar structure, these proteinases have different substrate specificities and different susceptibilities to inhibitors which suggest that they do not share the same biological function. Kallikrein-like proteinases also have restricted specificities that are probably related to their extended substrate binding site. This makes them good candidates for processing inactive protein or peptide precursors into biologically active peptides. A general approach to identifying the putative biological substrates of individual proteinases based on analysis of the specific cleavage of synthetic and natural peptide substrates by kallikrein-related proteinases is described.

Identification and androgen-regulated expression of two major human glandular kallikrein-1 (hGK-1) mRNA species

The screening of an oligo(dT)-primed prostate cDNA library with a human glandular kallikrein-1 (hGK-1) genomic DNA fragment resulted in the isolation of two different hGK-1 cDNAs. A 1.2 kb cDNA (pGK-1) contains an open reading frame of 510 bp, encoding the major part of the previously predicted hGK-1 protein (Schedlich et al. (1987) DNA 6, 429-437). This cDNA contains a 3'-untranslated region of 677 nucleotides and terminates in a poly(A) stretch, preceded by the canonical AATAAA polyadenylation signal. A second cDNA (pGK-10A), with a size of 1.5 kb, contains an open reading frame of 669 nucleotides preceded by 16 nucleotides of the 5'-untranslated region. pGK-10A differs from pGK-1 by the presence of an additional 37 bp fragment, interrupting the protein coding region of hGK-1, which results from the use of an alternative splice donor site of intron IV of the hGK-1 gene. The mature protein (excluding presumed pre- and propeptides) as deduced from the pGK-10A cDNA sequence, has a size of 199 amino acids and differs at the COOH-terminus from the 237 amino acid hGK-1 protein. The alternatively spliced mRNA comprises approximately 20% of the hGK-1 transcripts, as deduced from analysis of mRNA from prostate cells by PCR amplification of specific fragments. The regulation of hGK-1 mRNA expression was studied in different human prostate tumors and cell lines by Northern blotting, using a hGK-1-specific oligonucleotide probe. A high level of hGK-1 expression was found in the androgen-dependent tumors PC 82 and PC EW. hGK-1 mRNA was also present in the androgen-sensitive LNCaP cell line, but undetectable in the androgen-insensitive prostate tumors PC 133, PC 135 and the PC 3 cell line. In LNCaP cells, the expression of hGK-1 mRNA was strongly induced by androgens. Regulation of expression of the closely related prostate-specific antigen (PA) gene showed a similar pattern.