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From the Department of Biochemistry,*
Costa del Sol
Hospital, Marbella, Spain; the Department of
Pathology,
Virgen de las Nieves Hospital,
Granada, Spain; the Department of Pathology,
University of Miami School of Medicine, Miami, Florida; and the
Department of Biochemistry,§
University of
Málaga, Málaga, Spain
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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and ß) joined by a unique
five disulfide bond motif.4
The protein precursor is
encoded on a single 2 kb mRNA that is transcribed from a single copy
gene located on chromosome 8 (8p21).5
The high degree of
sequence conservation across species, the widespread tissue
distribution and the high circulating concentration suggest that
clusterin plays an important biological role. Whether clusterin is
truly a multifunctional protein or whether a common mechanism underlies
its various functions is unclear. Some functions proposed for clusterin may be relevant in the setting of tumorigenesis: these include complement defense, the initiation of apoptosis, membrane protection and the maintenance of cell-cell or cell-substratum contacts. There is no definitive biochemical evidence to support a specific function for clusterin, except for its role in the modulation of the immune system.6 Clusterin is part of the fluid phase membrane attack complex (SC5b-9) and functions as a complement inhibitor.6-8 One of the physiological roles of the complement system is the lysis of foreign cells, including virus-infected and tumor cells.9 Therefore, tumor cells that express surface components that inactivate the complement system would be able to evade destruction by the complement system and thus continue to grow in size as well as progress into more malignant tumors.
On the basis of the high level of expression in apoptotic tissues, it was originally proposed that the protein might be causally involved in apoptosis.6 Thus, clusterin mRNA has been widely used as a genetic marker of apoptotic cell death.3 In hormone-dependent tissues, such as the prostate and the mammary gland, clusterin expression is induced after hormone ablation.10,11 However, these studies did not analyze which cells account for the rise in clusterin gene expression, the cells undergoing apoptosis or those destined to survive. More recently, however, evidence has accumulated suggesting that clusterin expression is not enhanced, but rather down-regulated in the cells undergoing apoptosis, and that its expression in the apoptotic tissue is restricted to the surviving bystander cells.12-14 Moreover, the expression of the gene is induced in processes other than apoptosis, including mitosis and necrosis, as has been shown in the rat liver.15 These processes involve substantial membrane remodeling, suggesting that clusterin expression may be involved in maintaining membrane integrity and prevents complement attack during membrane remodeling.8 If clusterin is involved in preventing immune attack during membrane remodeling, aberrant expression of clusterin may be involved in the pathogenesis of inflammatory and neoplastic diseases. Previous experimental work has suggested a role for clusterin in morphological transformation.16-18
As clusterin may provide a means of eluding immune surveillance and apoptosis, a better understanding of the biology of its expression might lead to new insights into cancer progression. In this study, we have examined the expression of this molecule in a large cohort of breast tumors. This is the first report showing that clusterin is expressed in the malignant epithelium from early to late stages of carcinogenesis in the breast.
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Materials and Methods |
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One hundred and fourteen invasive breast carcinomas were resected at the Costa del Sol Hospital in Marbella (Spain), between 1994 and 1997. The age range was from 26 to 81 with an average of 54 years. Tissues were collected for evaluation from each case, fixed in formalin and embedded in paraffin. Clinical staging was performed according to the postsurgical International Union Against Cancer Tumor-Node-Metastasis Classification.19 The histological typing and grading of the tumors was performed according to the World Health Organization Classification.20 Ninety nine cases were ductal adenocarcinomas and 15 were lobular carcinomas. Local treatment consisted of either (modified) radical mastectomy or tumorectomy plus radiation therapy, in combination with axillary clearance. No patients had previously received endocrine or cytotoxic therapy.
In addition to primary carcinomas, lymph node metastases (1 to 5 per case) from 40 breast carcinomas, 35 carcinomas in situ (DCIS), 20 atypical ductal hyperplasias, 42 benign lesions (18 fibroadenomas, 6 breast fibrocystic diseases, 4 nodular adenosis, 2 papillomas and 12 ductal hyperplasias lacking atypia) and 40 normal tissues (30 of them obtained distant from the lesion and considered normal by the pathologist, and 10 obtained from breast biopsies without microscopic alterations) were analyzed. In this study DCIS was divided into high-grade and low-grade lesions. High-grade lesions were defined as having predominantly high nuclear grade usually associated with central necrosis. Low grade lesions encompassed those with low or intermediate nuclear grade and minimal or absent central necrosis.
Immunohistochemical Staining
A section 5 µm thick from each tumor was mounted on adhesive-coated slides, deparaffinized, and rehydrated through xylene and alcohol. After cooling for 15 minutes in jars containing 10 mmol/L citrate buffer, endogenous peroxidase was blocked with 3% H2O2 in 0.1% sodium azide for 15 minutes. The clusterin E5 monoclonal antibody was kindly provided by Dr. B. Murphy and was used at 1:1000 dilution for 1 hour at room temperature. The specificity of E5 was demonstrated in the original publication by immunoprecipitation and Western blot analysis using human serum.21 After PBS washes, the sections were incubated with biotinylated link antibody and then with peroxidase-labeled streptavidin. The staining was complete after a 10 minutes of incubation with a freshly prepared substrate-chromogen solution. The reaction products were again washed in PBS, then developed using diaminobenzidine tetrahydrochloride as chromogen. Sections were washed in running tap water and lightly counterstained with hematoxylin, followed by dehydration and coverslip mounting. Negative controls were obtained by omitting the primary antibody.
Clusterin expression was scored as follows: negative if no staining was seen or if immunoreactivity was observed in less than 10% of tumor cells; and positive if more than 10% of tumor cells showed staining. All slides were blind evaluated for immunostaining without any knowledge of the clinical outcome or of other clinical or pathological data. We also studied tumor proliferation and expression of hormone receptors, detecting the expression of Ki67, ERs and PRs (Dako, Copenhagen, Denmark). Tumors with less than 10% of the cells with nuclear staining were considered negative for the expression of hormone receptors or of low proliferation.
In Situ Localization of Apoptotic Cells
To detect apoptotic cells, in situ labeling of the 3'-ends of the DNA fragments generated by apoptosis-associated endonucleases was performed using a commercial apoptosis detection kit (Boehringer Mannheim, Germany). Briefly, deparaffinized sections were incubated with 20 µg/ml of proteinase K (Sigma Chemical Co. St. Louis, MO) for 15 minutes. Following rinsing in PBS, the slides were covered with terminal deoxynucleotidyl transferase plus nucleotide mixture at a 1:35 dilution for 60 minutes at 37°C. Then the slides were covered with an antifluorescein antibody conjugated with alkaline phosphatase. After substrate reaction, the stained cells were analyzed under a light microscope. Pretreatment of sections with DNase served as a positive control for the enzymatic procedures; omission of the enzyme served as a negative control.
Established morphological features used to identify apoptosis on H&E were also required in TUNEL-stained slides. Cells were defined as apoptotic if the whole nuclear area of the cell labeled positively. Apoptotic bodies were defined as small positively labeled globular bodies in the cytoplasm of the tumor cells which could be found either singly or in groups.
One thousand cells were counted for each specimen. The number of positively stained cells was then divided by 1000 to estimate the percentage of apoptotic cells in each specimen. As a cut off value for survival studies we used the median of the apoptosis in our series (0.76%) (range, 0.01–2.8%).
In Situ Hybridization
Human clusterin cDNA was kindly provided by Dr. B. Murphy. A digoxygenin-labeled antisense RNA probe was obtained using a HindIII-cut template and T7 RNA polymerase with a DIG RNA labeling kit (Boehringer Mannheim, Germany). Similarly, a sense probe was prepared for negative control experiments by using a NaeI-cut template and SP6RNA polymerase with the same kit. RNA stability was confirmed by hybridization using digoxygenin-labeled oligo-polyT as a probe. Deparaffinized sections were rehydrated and digested with proteinase K (15 µg/ml in 50 mmol/L Tris 20 mmol/L MgCl2 buffer) for 10 minutes. at 37°C, quenched with hydrogen peroxide and postfixed in 4% paraformaldehyde in phosphate buffered saline for 5 minutes. As an ionic block, 0.25% acetic anhydride in 0.1 mol/L triethanolamine pH 8.0 was used. Sections were then dehydrated in gradient ethanol and dilapidated in chloroform for 5 minutes, slightly rehydrated in 100 and 95% ethanol and dried at 37°C for 2 hours. The sections were washed, incubated in prehybridization solution (20 mmol/L Tris-HCl pH 7.4, 1 nmol/L EDTA, 300 mmol/L sodium chloride, 50% formamide, 10% dextran sulfate and 1X Denhardt’s solution) at 42°C for 1 hour, drained, and incubated at 42°C for 16 hours in the hybridization solution. Every milliliter of hybridization solution contained 830 µl of prehybridization solution and 20 ng of cRNA Dig-labeled probe diluted in 45 µl of Ribomix (100 µg/ml salmon sperm DNA, 250 µg/ml yeast total RNA, and 250 µg/ml yeast total tRNA), 50 µl 2 mol/L DTT, 10 µl of 10% sodium thiophosphate, and 10 µl of 10% SDS. The coverslips were removed and sections were washed in decreasing concentrations of SSC (4X to 0.1X) containing 1 mmol/L DTT, equilibrated in 100 mmol/L Tris-HCl pH 7.5, 150 mmol/L NaCl for 5 minutes., and blocked with the same buffer, now containing 2% sheep whole serum and 0.3% triton X-100, for 1 hour at room temperature. Hybridized Dig-labeled cRNA probe was visualized by reaction with anti-Dig-POD conjugated polyclonal antibody diluted 1:100 in the same Tris buffer but containing 1% sheep whole serum and 0.1% triton X-100 for 1 hour and developed with chromogen 3,3'-diaminobenzidine (DAB) and hydrogen peroxide for 1 hour. Tissues were counterstained with hematoxylin, dehydrated and coverslipped.
Follow-Up
One hundred three cases were available for follow-up. They were followed postoperatively at least once every 6 months. Relapsed-free survival (RFS) was calculated as the period from surgery until the date of the first recurrence (RFS). Mean follow-up was 39 months (range, 12 to 68 months).
Statistical Analysis
Statistical analysis was performed using the SPSS statistical
software program (SPSS Inc, Chicago, IL). The analysis of the
association between gene expression and prognostic factors for breast
carcinoma was performed by the 
2
test for
categorical variables and by the analysis of variance test for
continuous variables (natural log transformed when necessary).
Relapse-free survival was estimated by the Kaplan and Meier method, and
survival curves were compared with the log rank test. Cox’s
proportional hazards survival analysis was used to determine the
relative risk in multivariate analysis.
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Results |
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, Table 1
). Similar clusterin expression was
present in high-grade (7 of 14, 50%) and low-grade (10 of 21, 48%)
DCIS. In invasive carcinomas, clusterin expression showed a highly
significant increase compared to benign lesions
(P < 0.001). Thus, 53% (60 of 114) of primary
carcinomas were positive for clusterin expression (Table 1)
. In
invasive carcinomas containing areas of in situ disease
(31 cases), similar expression was detected in both components.
Clusterin expression also showed a significant increase from primary
carcinomas to metastatic carcinomas in lymph nodes where only 8 of 40
cases were negative for the expression of clusterin
(P = 0.004) (Figure 2
; Table 1
). Staining intensity and
cellular distribution in the lymph nodes was similar to that of the
primary tumors. In 10 of 15 cases (67%) in which the primary carcinoma
was negative, lymph node metastases became positive, whereas most (22
of 25, 88%) of the primary carcinomas with clusterin expression also
showed immunostaining in the lymph node metastases (Figure 2)
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suggesting that clusterin may be contained within secretory vesicles.
Nuclear staining was only noted in three cases.
We found a negative correlation between the extent of clusterin
immunoreaction and estrogen and progesterone receptor expression
(P = 0.02 and P = 0.001,
respectively). The mean diameter of clusterin-positive tumors was
larger than that of negative ones (3.01 versus 2.28)
(P = 0.04). Concerning the degree of
differentiation, most poorly differentiated tumors were positive for
clusterin expression (9 of 12) although without achieving statistical
significance when compared to the expression in well and moderately
differentiated tumors. We did not detect any correlation between
clusterin immunoreactivity and cancer stage, menopausal status (pre-
and postmenopausal), histological type or Ki67 immunostaining (Table 2)
.
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We also examined clusterin mRNA by in situ hybridization in
30 randomly chosen carcinomas to confirm the specific cellular pattern
of clusterin expression in breast tumors. The qualitative results of
immunohistochemical detection of clusterin in breast carcinoma cells
paralleled the changing levels of clusterin mRNA revealed by in
situ hybridization (Figure 2)
. In fact, only 2 cases of 30
examined by this method were positive by ISH and negative by
immunohistochemistry. In all cases the sense probe gave no
hybridization signal, demonstrating the specificity of the labeling
observed.
By univariate analysis, the variables associated with short
relapse-free survival were high histological grade
(P < 0.001), large tumor size
(P = 0.01), presence of lymph node metastasis
(P < 0.001), estrogen and progesterone receptor
negativity (P = 0.02 in both cases) and a high
apoptotic index (P = 0.004) (Table 3)
. No differences were observed between
clusterin-positive and negative tumors. In a multivariate analysis only
axillary nodal status (relative risk, 2.99; 95% confidence interval,
1.41 to 6.35), histological grade (2.33; 1.22 to 4.43) and extent of
apoptosis (1.88; 1.11 to 3.18) were of independent prognostic value.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The present study is the first, to our knowledge, to examine the potential role of clusterin in the development of breast carcinoma. Our findings demonstrate that the up-regulation of this gene is closely associated with the different steps of tumor progression from normal tissue to premalignant and malignant breast lesions, with a greater expression in lymph node metastasis. Overexpression of clusterin may thus represent an acquired phenotypic feature which facilitates the local invasion and dissemination of breast tumor cells, in fact, our results also show a positive correlation between clusterin expression and tumor size. Therefore, clusterin could be an important factor in determining the aggressive nature of a given breast tumor.
Clusterin over-expression correlated significantly with negative estrogen and progesterone receptor status. In addition, nuclear clusterin expression was detected in three invasive carcinomas which were also negative for the expression of estrogen and progesterone receptors. In a previous study, Akakura et al 24 reported nuclear clusterin staining in recurrent androgen-independent tumor cells, and these authors suggest that the presence of clusterin in the nucleus may serve to inhibit early events in the apoptotic process.
We also found that clusterin gene expression is not directly associated with programmed cell death, in accordance with other reports.12,15,25 In fact, we found a lower apoptotic index in clusterin positive tumors than in negative ones. One study has reported a significant increase in cell death following transfection of antisense oligonucleotides to clusterin.14 This inverse correlation may support the hypothesis that protection from apoptotic death may account, in part, for biologically aggressive tumor behavior. However, the precise biological function of clusterin in mechanisms of cell death/survival remain to be defined. In this sense, a recent study reported different isoforms of clusterin in apoptotic and surviving cells in regressing rat ventral prostate.26 Apoptosis-associated isoforms were predominantly localized to perinuclear regions while surviving associated isoforms were localized to the cytoplasm as is found in most of the primary and metastatic breast carcinomas included in our study. Further studies are needed to determine the exact function of the different isoforms of clusterin and how changes in their expression may modify properties of tumor cells.
Clusterin mediated inhibition of complement-induced cytolysis probably protects carcinoma cells from complement-mediated lysis and contributes to the highly metastatic phenotype of those cells. Its presence at cell surfaces and its ubiquitous presence in virtually all types of biological fluids may protect cell membranes exposed to deleterious components. This protective effect may favor the metastatic phenotype of tumor cells which may migrate through tissues and fluids without excessive damage.2 Thus, an attractive hypothesis is that clusterin induction is a reactive response to environmental changes rather than a causative factor in cell death. In this manner, clusterin could be thought of as an extracellular version of a heat shock protein.26,27
In conclusion, the detection of clusterin in breast carcinoma suggests that this protein may have a role in breast tumorigenesis and progression although it has not been shown to possess clinical utility as a survival prognostic factor. Further studies are needed to distinguish whether clusterin reflects a cause or an effect of increased tumor progression.
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Acknowledgements |
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Footnotes |
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Supported by grants from Fondo de Investigaciones Sanitarias (FIS 97/414), Junta de Andalucía (98/154), and Fundacion Rey Fahd, Spain.
Accepted for publication April 23, 2000.
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