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Human Kallikrein 13 Protein in Ovarian Cancer Cytosols: A New Favorable Prognostic Marker

From the Department of Biochemistry and Molecular Biology, Faculty of Biology, University of Athens, Athens, Greece; Department of Pathology and Laboratory Medicine, Mount Sinai Hospital, Toronto; Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada; Clinical Research Unit, Department of Obstetrics and Gynecology, Technical University of Munich, Munich, Germany

Address reprint requests to Eleftherios P. Diamandis, MD, PhD, Department of Pathology and Laboratory Medicine, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario, Canada M5G 1X5; e-mail: ediamandis{at}mtsinai.on.ca.

	ABSTRACT

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PURPOSE: Human kallikrein 13 (hK13; encoded by the KLK13 gene) is a secreted serine protease expressed in endocrine tissues, including the prostate, testis, breast, and ovary. We have previously reported steroid hormone regulation of the KLK13 gene and its clinical value as a marker of favorable prognosis in breast cancer at the mRNA level. We hypothesized that hK13 may represent a potential biomarker for ovarian carcinomas.

PATIENTS AND METHODS: Using a newly developed enzyme-linked immunosorbent assay (ELISA), hK13 levels were quantified in 131 ovarian tumor extracts and correlated with various clinicopathological variables and outcome (progression-free survival [PFS], overall survival [OS]), over a median follow-up period of 42 months.

RESULTS: hK13 concentration in ovarian tumor cytosols ranged from 0 to 18.4 ng/mg of total protein. An optimal cutoff value of 0.13 ng/mg (67^th percentile) was selected, based on the ability of hK13 values to predict the PFS of the study population, to categorize tumors as hK13-positive or negative. Women with hK13-positive tumors most often had early stage (stage I/II) disease, no residual tumor after surgery and optimal debulking success (P < .05). Univariate and multivariate Cox regression analyses revealed that patients with hK13-positive tumors had a significantly longer PFS and OS than hK13-negative patients (P < .05). Kaplan-Meier survival curves further confirmed a reduced risk of relapse and death in women with hK13-positive tumors (P = .007 and P = .002, respectively).

CONCLUSION: These results indicate that hK13 is an independent marker of favorable prognosis in ovarian cancer.

	INTRODUCTION

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Epithelial ovarian cancer is the most lethal gynecologic malignancy, constituting approximately 90% of all ovarian cancer cases [1,2]. The high mortality rate is usually ascribed to late diagnosis, since epithelial ovarian tumors commonly lack early warning symptoms. Furthermore, ovarian carcinomas often lack definite precursor lesions, are quite heterogeneous, and the molecular pathways underlying their progression are still elusive. Thus, many attempts have been made to predict the biology of ovarian tumors in order to determine prognosis and develop individualized treatment strategies. The International Federation of Gynecology and Obstetrics (FIGO) stage at diagnosis represents the major prognostic factor in ovarian cancer. FIGO stage I patients have a 5-year survival of 80% to 90%, compared with only 15% to 20% for women with stage III and IV disease [3]. Other well established conventional prognostic markers include tumor grade, patient age, residual tumor after surgery, presence and absence of ascites, and histology [4,5].

In addition to these clinicopathologic parameters, numerous tumor markers with prognostic potential have also been identified, including DNA ploidy, oncogenes, cell cycle regulatory proteins and inhibitors, enzymes, growth factors, extracellular matrix components, and proteases [6-12]. More recently, cDNA microarray analyses and bioinformatic approaches, such as serial analysis of gene expression, have also been used to identify genes differentially expressed in ovarian cancer in order to subcategorize tumors on the basis of molecular profile, often unveiling important biologic, diagnostic, and prognostic information [13-17].

Among the newly identified prognostic factors are human tissue kallikreins, a group of serine proteases encoded by 15 structurally similar, hormonally-regulated genes, clustered in tandem on chromosome 19q13.4 [18]. Accumulating evidence indicates that at least 11 kallikrein family members are differentially expressed in ovarian cancer at both the mRNA and protein levels, and several demonstrate clinical utility as prognostic biomarkers [19,20]. Moreover, the serum levels of kallikrein proteins, hK6, hK10, and hK11, are elevated in a proportion of ovarian cancer patients and as such, they may represent putative serological screening and/or diagnostic biomarkers for ovarian cancer [21-25].

Human kallikrein gene 13 (KLK13, previously known as KLK-L4), recently discovered by the positional candidate cloning approach, is a novel androgen/progestin-regulated serine protease gene, predominantly expressed in endocrine tissues, including the prostate, testis, and breast [26]. Preliminary evidence indicates that KLK13 is implicated in hormone-dependent malignancies and has prognostic utility. For one, KLK13 is downregulated at the mRNA level in breast cancer tissues and cell lines [26]. A subsequent and extensive quantitative reverse transcriptase polymerase chain reaction study demonstrated that KLK13 expression in breast tumor tissues is an indicator of favorable prognosis, since patients with KLK13-positive tumors exhibit a longer progression-free survival (PFS) and overall survival (OS) [27]. Furthermore, five testis-specific splice variants of the KLK13 gene were identified and found to be downregulated in testicular cancer tissues compared to the matched normal counterparts [28].

In order to study hK13 expression at the protein level, we recently developed a highly specific and sensitive immunofluorometric assay [29]. Using this method, we observed high hK13 expression in esophageal, tonsil, and salivary gland tissues, as well as in endocrine-dependent tissues and their corresponding biologic fluids, namely, the prostate, testis, breast, and seminal plasma and breast milk, respectively [29]. By immuohistochemical analysis, we demonstrated that hK13 is predominantly localized in the glandular epithelia of a variety of normal tissues, including the epithelium of the prostate and surface epithelium of the breast and ovary [30]. Quantification of hK13 in normal, benign, and cancerous ovarian tissues indicated that hK13 is elevated in 50% of ovarian tumor tissues compared to the relatively low levels observed in both normal and benign tissues [29], suggesting that hK13 may have prognostic utility as an ovarian cancer biomarker. Given the above, the aim of the present study was to investigate the expression of hK13 in ovarian cancer tissues and to evaluate its prognostic significance.

	PATIENTS AND METHODS

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Ovarian Cancer Patients and Specimens
One hundred and thirty-one patients with primary epithelial ovarian cancer were examined in this study, ranging in age from 20 to 85 years, with a median age of 57 years (Table 1). Patients were monitored for survival and disease progression (no apparent progression or progression) for a median duration of 42 months. Follow-up information was available for 131 patients, among which 74 (56%) had relapsed and 54 (41%) had died.

Investigations were carried out in accordance with the ethical standards of the Helsinki Declaration of 1975, as revised in 1983, and were approved by the Institutional Review Boards of Mount Sinai Hospital and the Technical University of Munich.

Preparation of Cytosolic Extracts
Tumor specimens were snap-frozen in liquid nitrogen immediately after surgery and stored at -80°C until extraction. Frozen tissues (20 to 100 mg) were pulverized on dry ice to a fine powder and added to 10 volumes of extraction buffer (50 mmol/L Tris, pH 8.0, 150 mmol/L NaCl, 5 mmol/L EDTA, 10 g/L of NP-40 surfactant, 1 mmol/L phenylmethylsulfonyl fluoride, 1 g/L of aprotinin, 1 g/L of leupeptin). The resulting suspensions were incubated on ice for 30 minutes, with repeated shaking and vortexing every 10 minutes. The mixtures were then centrifuged at 14,000 rpm at 4°C for 30 minutes and the supernatant (cytosolic extract) was collected and stored at -80°C until further analysis. Protein concentration of the extracts was determined using the bicinchoninic acid method, with bovine serum albumin as standard (Pierce Chemical Co, Rockford, IL).

Measurement of hK13 in Ovarian Cytosolic Extracts
The concentration of hK13 in cytosolic extracts was quantified using a highly sensitive and specific noncompetitive "sandwich-type" immunoassay, previously described and evaluated [29]. Briefly, microtiter plates were coated directly with a mouse anti-hK13 monoclonal antibody (code 2–17; 500 ng/well). After a 1 hour incubation, the plates were washed six times with washing buffer (9g/L NaCl and 0.5 g/L Tween 20 in 10 mmol/L Tris buffer, pH 7.40). Then, either recombinant hK13 calibrators (50 µL/well and 50 µL of a general diluent [60 g/L BSA, 50 mmol/L Tris, pH 7.80, 0.5 g/L sodium azide]) or cytosolic extracts (50 µL/well and 50 µL of the general diluent) were applied to each well in duplicate, incubated for 2 hours with gentle shaking and washed. Rabbit anti-hK13 polyclonal antiserum (diluted 1,000-fold in assay buffer containing the components of the general diluent plus 25 mL/L normal mouse serum, 100 mL/L normal goat serum, and 10 g/L bovine IgG) was subsequently applied, incubated for 1 hour and washed. Finally, alkaline phosphatase-conjugated goat antirabbit IgG (Jackson Immunoresearch, West Grove, PA), diluted 2,000-fold in assay buffer, was added, incubated for 45 minutes and washed as before. Signal detection and data reduction were performed automatically by the CyberFluor 615 Immunoanalyzer, which uses time resolved fluorometry, as described elsewhere [34]. The detection range of this assay is 0.1 to 20 ng/mL. hK13 measurements in ng/mL were converted to ng of hK13/mg of total protein, by dividing the ng of hK13/mL of cytosolic extracts with the mg of total protein/mL of cytosolic extracts, to adjust for the amount of tumor tissue extracted.

Statistical Analysis
The relationship between hK13 status and various clinicopathologic variables was analyzed with the ² test and the Fisher's exact test, as appropriate. The hK13 status of ovarian tumor extracts was categorized as either hK13-positive or hK13-negative.

For survival analysis, two different end points—cancer relapse (either local recurrence or distant metastasis) and death—were used to calculate PFS and OS, respectively. PFS was defined as the time interval between the date of surgery and the date of identification of recurrent metastatic disease. OS was defined as the time interval between the date of surgery and the date of death. The impact of hK13 on patient survival (PFS and OS) was assessed with the hazard ratio (relative risk of relapse or death in the hK13-positive group) calculated with the Cox univariate and multivariate proportional hazard regression model [35]. Only patients for whom the status of all variables was known were included in the multivariate regression models. The multivariate models were adjusted for hK13 expression in tumors and other clinical and pathologic variables that may affect survival, including stage of disease, tumor grade, cytosolic extract CA-125 values, and age. Kaplan-Meier PFS and OS curves [36] were also constructed in order to demonstrate survival differences between the hK13-positive and hK13-negative patients. The differences between the survival curves were tested for statistical significance using the log-rank test [37].

	RESULTS

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Distribution of hK13 Concentration in Ovarian Tumor Tissues
hK13 concentration in ovarian tumor cytosols from 131 patients ranged from 0 to 18.4 ng/mg of total protein, with a mean of 0.61 ng/mg total protein and a median of 0 ng/mg total protein (Table 1). An optimal cutoff value of 0.13 ng/mg total protein was identified by ² analysis, based on the ability of hK13 to predict the PFS of the study population. Based on this cutoff (67th percentile), 33% of the ovarian tumors were categorized as hK13-positive.

Relationships Between hK13 Status and Other Clinincopathologic Variables
The distributions of various clinicopathologic variables between hK13-postitive and hK13-negative patients are summarized in Table 2. The relationships between hK13 and these variables were examined with either the ² or Fisher's exact test. Patients with hK13-positive ovarian tumors were more likely to have early stage (stage I/II) disease, no residual tumor, and optimal debulking success (P < .05). Although marginally significant, hK13-positive tumors were mainly of the clear-cell and mucinous histotypes (P = .089). No relationship was observed between hK13 status and tumor grade or volume of ascites fluid.

Univariate and Multivariate Survival Analysis
The strength of association between hK13-positive tumors and survival outcome is presented in Table 3. In univariate Cox regression analysis, hK13-positive patients had a lower risk of relapse (hazard ratio [HR], 0.46; P = .009) and death (HR, 0.33; P = .004). Since the hK13-positive and negative tumors were classified according the above optimal cutoff, we also evaluated the prognostic significance of hK13 as a continuous variable. hK13 concentrations were transformed logarithmically using half of the lowest positive observation (0.015 ng/mg) for the undetectable hK13 samples. The continuous variable log (hK13) was found to have statistically significant prognostic value for overall survival (HR, 0.63; P = .028).

View larger version (20K): [in this window] [in a new window]   Fig 1. Kaplan-Meier survival curves for (A) progression-free survival and B) overall survival in patients with hK13-positive and negative ovarian tumors.

	DISCUSSION

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Typically, the expression of proteases in cancer tissues correlates with poor patient prognosis in different malignancies [39-44], including ovarian cancer [45-49], likely as a result of the well established roles of proteolytic enzymes in extracellular matrix degradation, which facilitates invasion and metastasis [50,51]. However, in recent years, with the identification of non-extracellular matrix substrates for secreted proteases, new roles for proteolytic enzymes have emerged in the regulation of cellular functions during tumor development, including cell proliferation, differentiation, survival, genomic (in)stability, and angiogenesis [52-54]. In fact, it has been documented that certain proteases, including matrix metalloproteinase-19 [55], and serine proteases such as testisin [56], prostasin [57,58], and human kallikreins 3, 4, 5, 10, 12, 13, and 14 [18], are downregulated in hormone-dependent cancers, and several may function as tumor suppressors [59-61]. These findings may help to explain why certain proteases, including hK13, are associated with a favorable prognosis in cancer patients [62-65]. The human kallikreins 6, 8, 10, and 11 were also analyzed in a fraction of the same samples. hK13 was not found to have statistically significant correlation with the other examined kallikreins (data not shown).

Accumulating evidence suggests that at least 10 of the 15 human kallikrein family members have prognostic value in ovarian cancer (listed in Table 4), exclusive of hK13 [19]. Among these, kallikreins 8, 9, 11, and 14 are comparable to hK13 since they are most often expressed in early stage ovarian tumors and correlate with a favorable patient prognosis [66-69]. The remainder, kallikreins 4, 5, 6, 7, 10, and 15, are expressed in advanced ovarian tumors and are markers of poor prognosis [70-76]. Since the vast majority of kallikreins are coexpressed, and likely coordinately regulated in ovarian cancer, we speculate that they may represent an enzymatic pathway involved in ovarian carcinogenesis by, as yet, unknown mechanisms [19].

Previously, we have shown that KLK13, at the mRNA level, is an independent marker of favorable prognosis in women with breast carcinomas [27], in agreement with our present findings in ovarian cancer. Although the underlying biologic mechanism of hK13 involvement in the progression of breast and ovarian cancers is currently unknown, it is plausible that this effect is related to steroid hormones. First, we have shown that hK13 is encoded by an androgen-regulated gene [26]. Second, epidemiologic and experimental evidence suggests that steroid hormones, such as androgens, are implicated in the etiology of both breast and ovarian carcinomas [77-79]. Third, it has been documented that the androgen receptor (AR) is present in over 80% of breast [80] and in 84% of ovarian tumors [81] and that androgens, acting through the AR, can stimulate and inhibit breast [78] and increase ovarian [79] cancer cell proliferation. As such, it is likely that AR complexes regulate KLK13 gene expression during breast and ovarian carcinogenesis. We further speculate that hK13, under androgenic stimulation, may inhibit breast and ovarian cancer metastasis in early stage carcinomas, by initiating or terminating events through the activation of favorable proteins (ie, inhibitors) or cleavage of unfavorable ones (ie, growth factors). Identification of downstream AR-regulated genes, such as KLK13, is also important in our understanding of the mechanism by which androgens are implicated in hormone-related malignancies. These findings may have therapeutic applications.

Quantification of hK13 in normal, benign, and cancerous ovarian tissues indicated that hK13 is elevated in 50% of ovarian tumor tissues compared to the relatively low levels observed in both normal and benign tissues. It seems that hK13 is not expressed much in the normal ovarian tissue, but its expression increases in the first stages of tumorigenesis, and then it is progressively suppressed as the malignancy advances. Because there is no information available on the pathologic role of hK13 in ovarian tissue, it would be difficult to formulate a hypothesis that could explain the way of regulation of gene expression and the mechanism by which KLK13 confers a favorable prognostic outcome in ovarian cancer.

CA-125, a traditional serologic ovarian cancer biomarker, has clinical value for disease diagnosis and it is used as an aid for the early detection of relapse and for assessing response to treatment. CA-125, quantified in ovarian cancer cytosols, was used as a continuous variable in the present study and it was not found to have statistically significant prognostic value. The prognostic significance of CA-125 in cytosolic extracts, in combination with other biomarkers and clinocopathologic features, is still under investigation.

In conclusion, this is the first report to describe the prognostic utility of hK13 as an independent indicator of favorable prognosis in ovarian cancer patients. By virtue of its predominance in nonserous ovarian tumors, hK13 may also be applied clinically in the corresponding subgroup of patients. The biologic basis and significance of our findings are unclear and warrant further basic and clinical studies.

	Authors' Disclosures of Potential Conflicts of Interest

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The authors indicated no potential conflicts of interest.

	Acknowledgment

 
Supported by a grant to E.P. Diamandis from the National Sciences and Engineering Research Council of Canada and IBEX Technologies, under a University-Industry Program.

	NOTES

 
Authors' disclosures of potential conflicts of interest are found at the end of this article.

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Submitted May 21, 2003; accepted December 2, 2003.

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