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Tumor Biology |
Cellular Oncology Laboratory, University of Queensland Joint Oncology Program and Queensland Institute of Medical Research, Brisbane, Queensland, 4029 [J. D. H., J. L. D., A. L. S., J. F. N., M. A. S., T. M. A.]; Department of Urology, Princess Alexandra Hospital, Woolloongabba, Queensland, 4102 [D. L. N., M. L. D.]; Centre for Medical Genetics, Department of Cytogenetics and Molecular Genetics, Womens and Childrens Hospital, Adelaide, South Australia, 5006 [H. J. E., G. R. S.]; and Institute for Reproduction and Development, Monash Medical Center, Monash University, Clayton, Victoria 3168 [K. A. L. L.], Australia
| ABSTRACT |
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| INTRODUCTION |
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Testicular germ cell maturation is dynamic, requiring cell-cell communication and localized cell-extracellular matrix interactions (4) . Regulation of such processes involves cell surface proteolysis, which is important not only for matrix remodeling but also for regulation of growth and differentiation through activation and/or release of functionally diverse effector molecules, including cytokines, growth factors, and cell surface receptors. Not only is characterization of cell surface proteolysis important for understanding germ cell maturation, but cell surface proteinases also constitute potential new targets for anticancer therapies.
The serine proteinases are a large multigene family, the members of which participate in proteolytic reactions that are essential to a diverse range of physiological and pathological processes (5) . These enzymes are generally expressed as inactive zymogens; activation results in rapid molecular responses without the requirement for de novo protein synthesis. The involvement of serine proteinases during the later stages of male germ cell maturation and in fertilization has been documented. The testis-specific serine proteinases human acrosin (6) and mouse TESP-1 and TESP-2 (7) are recognized as playing roles during the final stages of sperm development. Additionally, the prostate epithelial cell serine proteinase, PSA,4 catalyzes the liquefaction of seminal coagulum (8) . Plasminogen activators have been implicated in the degradation of tight junctions in the seminiferous tubules of rat testes (9) . Furthermore, as yet uncharacterized serine proteinases are also present on sperm cells as they pass through the epididymis and are necessary for the segregation of sperm surface proteins into distinct domains and the attainment of fertilization competence (10 , 11) .
The enzymatic properties of serine proteinases are dependent on a catalytic triad of His, Asp, and Ser amino acids (12) , which are present in motifs that are highly conserved among family members. We have exploited this property in the present study by using a "homology cloning" strategy (13, 14, 15) to identify a novel serine proteinase. Testisin is the first serine proteinase to be identified that is expressed by germ cells prior to the first meiotic division and likely functions in proteolytic reactions that are associated with male germ cell maturation. Its loss of expression by testicular tumors of germ cell origin and the localization of the testisin gene to chromosome 16 (16p13.3), a region of the genome that is subject to loss of heterozygosity and rearrangement in human testicular cancers, suggest a potential role for testisin as a tumor suppressor in testicular cancer.
| MATERIALS AND METHODS |
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450 bp were ligated into pGEM-T (Promega), cloned, and analyzed by DNA sequencing. A DNA fragment was identified that represented a partial testisin cDNA (nucleotides 267723). The 3' end of full-length testisin cDNA (Clone U, encoding nucleotides 3471073) was obtained by screening a human HeLa cell Uni-ZAP XR cDNA library (Stratagene, La Jolla, CA) with the radiolabeled partial testisin cDNA fragment obtained by reverse transcription-PCR. Clones encoding the 5' end of full-length testisin cDNA were obtained both by 5' RACE and screening of the HeLa cell library by PCR. 5' RACE (Life Technologies, Inc., Gaithersburg, MD) was performed using the nested primers 5'-TCTGTCCGGTTCTCAAA-3' and 5' -CGAAGTAACGGGTGTAGTAG-3' and the supplied anchor primer. The longest clone obtained, R1-2, spanned nucleotides 34379 of the full-length cDNA. PCR screening of the HeLa cell library was performed using two rounds of amplification with the same nested gene-specific primers and a vector-specific oligonucleotide (T3, 5'-ATTAACCCTCACTAAAGGGA-3'). Step-down cycling conditions were: 95°C for 10 min; 3 cycles at each annealing temperature of 95°C for 30 s, 7062°C in 2°C steps for 30 s, and 72°C for 3 min; followed by 18 cycles of 95°C for 30 s, 60°C for 30 s, and 72°C for 3 min; and a final extension at 72°C for 7 min. The longest clone obtained, 5-4, encoded nucleotides 1527 of the full-length cDNA.
Plasmid Construction.
A plasmid containing full-length testisin [testi-sin(L)] cDNA was generated by ligating a BsgI-XbaI DNA fragment from clone 5-4 into BsgI-XbaI-digested clone U, generating pBluescriptHELA2(L)B65. A KasI-MscI DNA fragment from clone R1-2 was ligated into the KasI-MscI-digested pBluescriptHELA2(L)B65 to generate pBluescriptHELA2(S)B41, the construct encoding the short isoform, testisin(S).
A Sau3AI fragment of pBluescriptHELA2(S)B41 encoding amino acids 90279 was cloned into the BamHI site of pGEX-1 (Pharmacia Biotech) to generate a partial GST-testisin fusion construct for expression in Escherichia coli.
For expression in eukaryotic cells, a DNA fragment encoding nucleotides 13954 of the testisin(S) isoform from pBluescriptHELA2(S)B41 was generated by PCR using the following primers: forward, 5'-GCACAGGTACC-GAGGCCATGGGCGCGCGC-3'; and reverse, 5' -GCACATCTAGATCAG-TGGTGGTGGTGGTGGTGGACCGGCCCCAGGAGTGG-3'. The amplification product was cloned into pGEM-T (Promega). The fragment was then excised with NotI and cloned into the NotI site of pcDNA3 (Invitrogen, Carlsbad, CA), generating pcDNA3Test(S-C). An expression vector containing the long isoform, pcDNA3Test(L-C) encoding nucleotides 13960 of the testisin(L) was generated using the same procedure from pBluescriptHELA2(L)B65.
All constructs were verified by DNA sequence analysis.
Patients and Tumor Specimens.
Eight paired samples of testicular tumors and adjacent unaffected testicular tissue were obtained at the time of surgery and were snap-frozen in liquid nitrogen for RNA analyses. In addition, portions of unaffected testis, tumor tissues, and junctional specimens (at the tumor and unaffected testis tissue interface) were processed for routine histological examination by paraffin-embedding formalin-fixed tissue. Histological evaluation was based on routine pathology reports and included four seminomas, one teratoma, one yolk sac tumor, and two mixed germ cell tumors. Normal testicular tissue was also obtained from a patient undergoing bilateral orchidectomy for treatment of prostatic carcinoma.
Northern and Poly(A)+ RNA Dot Blot Analyses.
Total RNA isolated from frozen tissue specimens and the cell lines HeLa S3 and SW480 (ATCC CCL 228), was separated by denaturing gel electrophoresis and transferred to Hybond-N nylon membranes (Amersham, Aylesbury, United Kingdom) as described (16)
. Human multiple-tissue Northern blots and a human multiple-tissue dot blot (Clontech, Palo Alto, CA) were obtained commercially. The multiple-tissue Northern blot contained 2 µg of poly(A)+ RNA per lane. The dot blot contained poly(A)+ RNA from 50 normal adult and fetal tissues normalized to the mRNA expression levels of eight different housekeeping genes and ranged from 89 to 514 ng. Blots were hybridized with [32P]dCTP-labeled MscI-BamHI testisin(L) fragment (nucleotides 321861) in ExpressHyb (Clontech) solution at 65°C and washed to a final stringency of 0.2x SSC-0.1% SDS at 65°C. The dot blot was washed to a final stringency of 0.1x SSC-0.5% SDS at 60°C. Blots were reprobed with ß-actin cDNA or an oligonucleotide probe for 18S rRNA (16)
to confirm RNA loading in each lane.
Production of Affinity-purified Antipeptide Polyclonal Antibodies.
Rabbit polyclonal antibodies were generated against testisin-specific peptides derived from nonhomologous hydrophilic regions within the catalytic domain of testisin. Two peptides, each containing a cysteine residue incorporated at the COOH terminus, were synthesized (Auspep, Parkville, Australia) and conjugated to keyhole limpet hemocyanin using µ-maleimidobenzoic acid N-hydroxysuccinimide ester. The peptide sequences were as follows: T175-190, Gly-Tyr-Ile-Lys-Glu-Asp-Glu-Ala-Leu-Pro-His-Thr-Leu-Gln-Cys; and T46-63, Glu-Asp-Ala-Glu-Leu-Gly-Arg-Trp-Pro-Trp-Gln-Gly-Ser-Leu-Arg-Leu-Trp-Asp-Cys (short isoform numbering). Rabbit antisera were peptide affinity-purified using SulfoLink coupling gel (Pierce, Rockville, IL). The specificity of each antibody was tested against the immunogenic peptide by ELISA and against recombinant testisin by Western blot.
Western Blot Analysis.
Proteins were separated by SDS-PAGE on 1012% gels and transferred electrophoretically to Hybond-P membranes (Amersham). Membranes were blocked with 5% nonfat skim milk powder in Tris-buffered saline [10 mM Tris-HCl (pH 7.0)-150 mM NaCl], incubated with affinity-purified antipeptide antibody and then with horseradish peroxidase-conjugated sheep antirabbit immunoglobulin secondary antibody, and visualized by enhanced chemiluminescence (Amersham).
Immunohistochemistry.
Paraffin sections (5 µm) of Bouins-fixed normal human testis tissue or formalin-fixed tissues from testicular cancer patients were deparaffinized and then rehydrated before antigen retrieval in boiling 10 mM citric acid buffer (pH 6). After cooling, endogenous peroxidase activity was inhibited by a 10-min incubation in 1% hydrogen peroxide. Nonspecific antibody binding was blocked by incubating the sections in 4% nonfat skim milk powder in Tris-buffered saline for 15 min, followed by 10% normal goat serum for 20 min. Affinity-purified antitestisin T175-190 antibody was applied at 1:200 dilution and incubated overnight in a humidified chamber at room temperature. Controls included sections incubated with no primary antibody or antibody that had been preabsorbed for 2 h at room temperature with 1 µg of the antigenic peptide. Following incubation with prediluted biotinylated goat antirabbit immunoglobulins (Zymed, San Francisco, CA), streptavidin-horseradish peroxidase (Zymed) was applied, and color was developed using the chromogen 3,3'-diaminobenzidine with hydrogen peroxide as substrate. The sections were counterstained in Mayers hematoxylin.
FISH.
Plasmid DNA encoding full-length testisin cDNA was labeled with biotin-14-dATP by nick translation and hybridized in situ at a final concentration of 20 ng/ml to human metaphase chromosomes from two normal males. The method was modified from that described previously (17)
, in that chromosomes were stained before analysis with both propidium iodide (as counterstain) and 4',6-diamidino-2-phenylindole (for chromosome identification). Images of metaphase preparations were captured by a cooled charged coupled device camera using the Cyto Vision Ultra image collection and enhancement system (Applied Imaging Int., Ltd., Newcastle, United Kingdom).
| RESULTS |
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35,000 (data not shown), demonstrating that the predicted start codon was functional in vitro. A hydropathy plot (19)
of the amino acid sequence revealed a hydrophobic region located at amino acid positions 119 that conformed with the consensus for a typical NH2-terminal secretory signal peptide (20)
. Thus, this region is likely to function as a signal peptide, directing the newly synthesized protein to enter the endoplasmic reticulum. Unusually for members of the serine proteinase family, a second hydrophobic region was identified at the COOH terminus of testisin at amino acids 298314. This COOH-terminal extension likely constitutes a membrane anchor, as has been reported for the serine proteinases prostasin (21)
and CAP1 (22)
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cDNAs encoding two testisin isoforms, designated testisin(S) and testisin(L), were identified. These two isoforms differed by six nucleotides, CTATAG, at nucleotide position 276 (Fig. 1)
. This six-nucleotide insertion incorporates a consensus 3' splice site, and the position of this insertion within the cDNA sequence is consistent with the position of an intron-exon junction in other serine proteinase genes of the chymotrypsin family (23, 24, 25)
, suggesting that these isoforms may have arisen as a result of alternative mRNA splicing. The presence of these six nucleotides results in the insertion of two amino acids Tyr87-Ser in the polypeptide sequence, only four amino acids distant from the catalytic His82. The functional significance of this insertion to the catalytic activity and/or substrate specificity of testisin is not yet known.
Testisin mRNA Is Strongly Expressed Only in Testis.
Northern blot analysis showed that testisin mRNA of
1.4 kb was expressed constitutively in HeLa cells, whereas no expression was detected in the colon cancer cell line SW480 (Fig. 2A)
. The distribution of testisin mRNA in a range of normal human tissues was examined by Northern blot. Analysis of poly(A)+ RNA from 16 normal human tissues showed a single transcript of
1.4 kb detected only in human testis (Fig. 2B)
. To extend this analysis, we hybridized a commercially available dot blot containing poly(A)+ RNA from 50 normal human tissues with the testisin cDNA probe. Abundant expression of testisin mRNA was detected only in the testis (Fig. 2C)
, with prolonged exposures showing barely detectable signals in salivary gland, bone marrow, lung, and trachea.
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35,00039,000 (Fig. 4B)
Testisin Is Associated with Primary Spermatocytes during the First Meiotic Prophase.
Maturation of male germ cells proceeds through several ordered stages, with maturation occurring from the base of the seminiferous tubules toward the tubule lumen. Committed spermatogonia undergo two rounds of meiotic division, passing in the first meiotic prophase, sequentially through preleptotene, leptotene, zygotene, pachytene, and diplotene stages, during which chromosome pairing and cross-over events occur. Following the first meiotic division, the resultant secondary spermatocytes proceed through a second meiotic division to become haploid round spermatids, which are further processed through a continuum of gross morphological changes to elongated sperm (1)
. To begin to understand the function of testisin in human testis, we examined testisin protein expression by immunohistochemical analysis of normal human adult testis. Testisin expression was first seen in zygotene spermatocytes and staining progressively increased with stage, with the most intense immune-specific staining seen in late pachytene and diplotene spermatocytes (Fig. 5A)
. Staining was diffuse within the cytoplasm of these cells with a corresponding accentuation of the plasma membrane, consistent with the identified COOH-terminal extension being involved in anchoring of testisin on the cell surface. In addition there was intense, focal, cell surface staining at some spermatocyte junctions (Fig. 5C)
. Some spermatocytes also showed evidence of dense, crescent-shaped compartmentalized staining as shown in Fig. 5D
. No detectable staining was seen in spermatogonia, spermatids, Sertoli cells, or other cells of the testicular interstitium. Control experiments using the T175-190 polyclonal antibody in the presence of competing T175-190 peptide showed absence of this specific staining pattern (Fig. 5B)
. An identical, albeit weaker, staining pattern was observed in experiments performed using a testisin-specific antibody generated against a testisin peptide, T46-63 (data not shown).
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| DISCUSSION |
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Although the sequence of testisin is unique, homology comparisons showed testisin was a member of the chymotrypsin (S1) family of serine proteinases. These serine proteinases include a signature catalytic triad of His, Asp, and Ser amino acids and are generally produced as inactive zymogens that are activated following proteolytic cleavage. Testisin contains a characteristic serine proteinase activation motif (RIVGG) with cleavage predicted to occur following Arg41. This would release a proregion linked by a disulfide bond to the catalytic domain, which is typical of many of the members of this family. On the basis of the presence of an acidic amino acid, Asp, in the putative substrate binding pocket, active testisin would be predicted to cleave its target substrate with specificity for the basic amino acids, Arg or Lys (12) .
Two isoforms of testisin, differing by two amino acids located near the catalytic histidine, result from a six-nucleotide insertion. The existence of these isoforms may be evidence of intron-exon junctional sliding (32) . The additional two amino acids occur in a loop that, in other serine proteinases, stabilizes primary interactions between surrounding structures of the proteinase and the substrate (12) . Although this insertion is unlikely to influence the primary specificity of testisin for basic amino acids, it is possible that secondary effects, such as substrate affinity, may be different between the two isoforms.
Testisin includes hydrophobic regions at the NH2 and COOH termini that are predicted to act as secretory and membrane attachment signals, respectively. Testisin is only the second human serine proteinase described with a hydrophobic COOH-terminal extension. The catalytic region of the vast majority of serine proteinases forms the COOH terminus, and most serine proteinases are either secreted or targeted to cytoplasmic storage organelles by an NH2-terminal signal sequence. Recently, however, some membrane-anchored serine proteinases have been identified in different species, including, in addition to testisin, human prostasin (20) , mouse TESP-1, and mouse TESP-2 (6) , each of which possess COOH-terminal amino acid extensions. The COOH-terminal extension of prostasin is believed to anchor prostasin to the plasma membrane of prostate epithelial cells, from which it may be proteolytically released into the semen (21) . TESP-1 and TESP-2 are thought to be anchored to the cell membrane via a glycosyl-phosphatidylinositol linkage (7) . Like these proteinases, testisin appears to be present on the plasma membrane, possibly attached via a glycosyl-phosphatidylinositol anchor.
The restricted expression pattern of testisin is consistent with a specialized role during male germ cell development. In the testes, testisin is expressed exclusively by primary spermatocytes, with intense staining in germ cells prior to the first meiotic division. This is the first described serine proteinase strongly expressed at such an early stage in germ cell development. Other described germ cell serine proteinases, acrosin (27) , TESP1, and TESP2 (7) , are synthesized at a much later stage and are present in mature sperm. Thus, testisin may represent a component of an as yet unrecognized proteolytic cascade involved in germ cell maturation, analogous to the fibrinolytic and coagulation cascades.
The physiological function of testisin is not yet known. Proteolysis is important for proliferation, apoptosis, differentiation, and cell migration; all processes that are integral to normal germ cell development. In testis, diploid spermatogonia differentiate into haploid spermatozoa following successive rounds of mitotic and meiotic cell divisions and extensive morphological restructuring. Biochemical events occurring during meiosis are poorly defined, although it is known that processes such as chromatin condensation, formation of synaptonemal complexes, and genetic recombination are proceeding (33) . We hypothesize that loss of testisin expression may alter differentiation of immature germ cells and/or lead to arrest of testicular germ cell maturation and unregulated proliferation. Testisin could participate in proteolytic events required for migration of maturing germ cells in the adluminal space of the seminiferous tubule or in matrix remodeling. Alternatively, because exchange of soluble factors and coordinated cell surface interactions between developing germ cells and Sertoli cells are essential for spermatogenesis (34) , testisin may participate in proteolytic cleavage and release of specific factors and/or activation of bioactive molecules. Such events may be essential for normal meiotic cell division in spermatogenesis and, clearly, have implications for abnormalities in germ cell maturation, such as those that occur in sterility, fertility, and testicular cancer.
The testisin gene has been localized near the telomere of human chromosome 16, at 16p13.3. This region of human chromosome 16 is associated with high genetic instability: documented rearrangements underlie a variety of common human genetic disorders, including
-thalassemia, polycystic kidney disease, tuberous sclerosis, familial Mediterranean fever, and Rubenstein-Taybi syndrome (35)
. Loss of heterozygosity studies have identified 16p13.3 as a potential locus for a tumor suppressor gene associated with male germ cell tumors (2)
. It remains to be determined whether testisin functions as a tumor suppressor gene. However, the demonstrated loss of testisin mRNA and protein expression in testicular germ cell tumors implies its absence may contribute directly or indirectly to testicular tumor development or progression. Such a role parallels that proposed for the recently identified serine proteinase, normal epithelial cell specific-1 (NES1). NES1 is expressed in normal mammary epithelial cells and is down-regulated in most breast cancer cell lines (36)
. Expression of NES1 has recently been shown to inhibit anchorage-independent growth and suppress oncogenicity in nude mice (37)
, indicating a tumor suppressor role for this serine proteinase.
Taken together, our data support a role for testisin in testicular germ cell maturation and, possibly, in the initiation and/or progression of testicular cancers. The restricted expression of testisin and the similarities between testisin and other known serine proteinases suggest that testisin may have a unique and critical biological function in germ cell growth and/or differentiation in the testis.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by grants from the Queensland Cancer Fund, Brisbane, Australia, and AMRAD Operations Pty. Ltd, Melbourne, Australia. J. D. H. was supported by a John Earnshaw Scholarship from the Queensland Cancer Fund and by the Bancroft Scholarship, Queensland Institute of Medical Research. J. L. D. was supported in part by a Dora Lush Post-Graduate Biomedical Scholarship from the National Health and Medical Research Council of Australia. ![]()
2 Present address: The Eye Clinic, University of Tasmania, Hobart, Tasmania, 7000, Australia. ![]()
3 To whom requests for reprints should be addressed, at Queensland Institute of Medical Research, Post Office Royal Brisbane Hospital, Brisbane, 4029, Queensland, Australia. Phone: 61 7 3362 0312; Fax: 61 7 3362 0107; E-mail: toniA{at}qimr.edu.au ![]()
4 The abbreviations used are: PSA, prostate-specific antigen; RACE, rapid amplification of cDNA ends; GST, glutathione S-transferase; FISH, fluorescence in situ hybridization. ![]()
5 The nucleotide sequence reported in this paper has been deposited in the DDBJ/GenBank/EMBL database (accession no. AF058300; deposited April 8, 1998). ![]()
Received 2/18/99. Accepted 4/29/99.
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