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From the Departments of Pathology and Laboratory
Medicine*
and Epidemiology and
Biostatistics,
European Institute of Oncology,
University of Milan School of Medicine, Milan, Italy; and the
Department of Pathology,
Yale University
School of Medicine, New Haven, Connecticut
| Abstract |
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| Introduction |
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and ß cytoplasmic domains modulate
the effects of integrins, and the occurrence of several alternatively
spliced variants of these domains possibly accounts for the variety of
biological functions of these receptors. Among the five known alternatively spliced forms of the integrin ß1 subfamily, ß1C contains an unique unspliced intervening 116-bp sequence (exon C), which codes for a peculiar 48-amino-acid COOH-terminal sequence.2 This integrin has been shown to inhibit cell proliferation in vitro,3-6 and its expression in epithelial cells of the prostate and bile ducts correlated with a nonproliferating, differentiated phenotype. It is interesting that ß1C integrin is down-regulated in prostatic adenocarcinoma6,7 and in some non-small-cell lung carcinomas.8
Using an affinity-purified rabbit antibody,7 we have documented consistent immunoreactivity for ß1C of the epithelial (luminal) cells of the normal mammary ducts and acini. Prompted by this observation, we have extended our investigation to a series of breast carcinomas to ascertain the actual prevalence of ß1C expression in these tumors and define its pathological correlates.
| Materials and Methods |
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We analyzed 283 consecutive and previously untreated invasive cancers of the female breast, which were diagnosed and treated at the European Institute of Oncology (Milan, Italy). The tumors were histologically classified by the World Health Organization Histological Classification of Breast Tumors,9 as modified by Rosen and Oberman.10 There were 227 invasive duct carcinomas not otherwise specified, 25 invasive lobular carcinomas, and 31 other tumor types (medullary, tubular, cribriform, and mucinous).
Hematoxylin and eosin (H&E)-stained sections were reviewed to evaluate
tumor type and grade. Grading of tumors was by the method of Elston and
Ellis.11
Sections of all cases already immunostained as
previously reported12
for estrogen (ER) and progesterone
receptors (PgR) and for the Ki-67 antigen (using the MIB-1 monoclonal
antibody) were also reevaluated. The pathological classification of the
tumors was retrieved from the original pathological reports and
recorded (Table 1)
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Consecutive serial sections were cut from the same formalin-fixed, paraffin-embedded tissue blocks used for the immunohistochemical evaluation of the hormone receptor status and of the proliferative fraction and were immunostained for ß1C with an affinity-purified polyclonal antibody.7
Described briefly, the procedure was next to dewax and rehydrate the sections. Endogenous peroxidase was quenched with 3% H2O2 in distilled water for 5 minutes at room temperature. The sections were then sequentially incubated with the primary antibody (1.7 µg/ml) and the Dako EnVision detection system (Dako, Glostrup, Denmark), by the manufacturers instructions.
Negative control sections were immunostained with the affinity-purified antibody to ß1C preabsorbed for 30 minutes at 4°C with 10 µg/ml ß1C 785808 peptide, and the specific immunoreactivity was abolished. Double immunostaining experiments for the simultaneous localization of ß1C and Ki-67 antigen were performed in selected cases, as previously described.13
The immunostaining results were independently evaluated by three investigators and scored in three classes by the percentage (< 10%; 1050%; > 50%) of cells showing definite immunoreactivity over the total number of neoplastic cells in 10 randomly selected high-power (x400) fields.
Messenger RNA Amplification by Reverse Transcriptase-Polymerase Chain Reaction
Total cellular RNA was isolated from frozen tissue samples of six breast carcinomas (two exhibiting intense and diffuse immunoreactivity for ß1C and four devoid of ß1C immunoreactivity), using TRIzol reagent (Life Technologies, Inc., Gaithersburg, MD) by the manufacturers instructions. RNA (1 µg) was reverse transcribed for 1 hour at 37°C in 20 µl of 1x first-strand buffer (Life Technologies), 10 mmol/L dithiothreitol (Life Technologies), 1 mmol/L dNTPs (Pharmacia, Uppsala, Sweden), 2.5 mmol/L MgCl2 (Perkin Elmer, Foster City, CA), 2.5 µmol/L Random Hexamers (Perkin Elmer), 20 U RNase inhibitor (Perkin Elmer), and 200 U mouse murine leukemia virus-reverse transcriptase (Life Technologies).
To check for the integrity of amplifiable complementary DNA templates, the samples were first amplified for ß-actin complementary DNA, using the specific primers BACT-2284F and BACT-3000R (5'-GAAATCGTGCGTGACATTAGG-3', and 5'-CTAGAAGCATTTGCGGTGGA-3').
Two consecutive rounds of polymerase chain reaction (PCR) with ß1C-specific primers (forward, 5'-TCTGTCGCCCAGCCTGGAGTG-3'; reverse, 5'-TTTCCCTCATACTTCGGATTG-3') were then carried on to amplify a 172-bp sequence of ß1C complementary DNA. Described briefly, 4 µl of complementary DNA were subjected to a first round of amplification in 50 µl of 1x PCR Buffer II (Perkin Elmer), 100 µmol/L dNTPs (Pharmacia), 2 mmol/L MgCl2 (Perkin Elmer), 0.025 U/µl AmpliTaq Polymerase (Perkin Elmer), 0.1 µmol/L forward primer, and 0.4 µmol/L reverse primer. Twenty cycles of 30 seconds at 94°C and 1 minute at 68°C were run. Of the first amplification product, 4 µl were then subjected to a second amplification round performed at the same conditions except for the annealing/extension step (touchdown). During the first nine cycles, the annealing/extension temperature was lowered from 67°C to 58°C and the time from 90 to 45 seconds (decreasing the annealing temperature 1°C and the time 5 seconds for each cycle). Eleven cycles of 30 seconds at 94°C, 45 seconds at 58°C, and 1 minute at 72°C were finally run.
ß1A messenger RNA (mRNA) amplification was performed in the same reaction buffer as for ß1C, using 0.1 µmol/L of specific primers (5'-AGAATCCAGAGTGTCCCACTGG-3' and 5'-TTTCCCTCATACTTCGGATTG-3') and 30 cycles of 30 seconds at 94°C, 1 minute at 55°C, and 1.5 minutes at 72°C. All of the PCR reactions were performed with a Perkin Elmer GeneAmp PCR System 2400. The PCR products were analyzed on a 2% agarose gel, stained with ethidium bromide.
Immunoblotting
Frozen tissue samples of five of the six breast cancers subjected to reverse transcriptase-PCR (RT-PCR) analysis (the two immunohistochemically positive samples and three of the four tumors nonimmunoreactive for ß1C), cryopreserved at -80°C in OCT, were homogenized as previously described.5 The protein concentration, for each lysate was quantitated using the DC protein assay kit (BIORAD, Hercules, CA), according to the manufacturers instructions.
To detect ß1C and ß1A integrins, 200 µg of lysate were electrophoresed on a 7.5% sodium dodecyl sulfate-polyacrylamide gel under reducing conditions and transferred to nitrocellulose (Schleicher & Schuell, Keene, NH) for 5 hours at 300 mA in 25 mmol/L Tris (pH 8.3; BDH, Dorset, U.K.), 192 mmol/L glycine (Sigma), and 20% methanol. Immunostaining was performed as previously described,7 with 7.5 µg/ml affinity-purified antibody to ß1C, 7.5 µg/ml nonimmune rabbit immunoglobulin G, or 5 µg/ml monoclonal antibody to ß1A integrin (mAb 13; Becton Dickinson, San Jose, CA). Briefly described, after an incubation in blocking buffer (20 mmol/L Tris, pH 7.5, 150 mmol/L NaCl) containing 0.2% Nonidet P-40 (Calbiochem, San Diego, CA) and 5% nonfat dry milk, the membrane was incubated with the primary antibody (1 hour at room temperature for ß1A and overnight at 4°C for ß1C). The filter was then washed three times in phosphate-buffered saline containing 0.2% Nonidet P-40 and incubated with the secondary peroxidase-conjugated antibodies for 1 hour, at room temperature. After three washes in phosphate-buffered saline0.2% Nonidet P-40, proteins were visualized using the Amersham-ECL system (Amersham, Arlington Heights, IL), according to the manufacturers instructions.
Statistical Analysis
ß1C immunoreactivity was categorized into three categories (< 10%, 1050%, and > 50% neoplastic cells exhibiting immunostaining of the same intensity as the normal cell counterpart), Ki-67 immunoreactivity was divided into six categories (< 10%, 1019%, 2029%, 3039%, 4049%, and > 50% immunolabeled neoplastic cells), and ER and PgR immunoreactivities were dichotomized into two categories, using the staining of less than 20% or of 20% or more neoplastic cells as the cut-off value.
The association between ß1C immunoreactivity
and other clinico-pathological characteristics of the tumors was
analyzed by the Fishers exact test, and the strength of the linear
relationship between variables was measured by Pearsons correlation
coefficients. Logistic regression, with ß1C
immunoreactivity dichotomized as either < 50% or
50%
immunostained cells as the dependent variable, was used to assess the
independent association with all other available measures. All
P values were based on two-sided testing.
| Results |
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Double immunocytochemical experiments documented a mutually exclusive
expression of ß1C and Ki-67 antigen by the vast
majority of the neoplastic cells, with Ki-67labeled cells being most
often devoid of ß1C immunoreactivity (Figure 6, A and B)
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| Discussion |
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Down-regulation of ß1C has been reported in 54 to 79% of prostatic carcinomas6,7 and in some pulmonary carcinomas.8 We now document that approximately 40% of female breast carcinomas are characterized by a down-regulated expression of ß1C, with more than 50% of neoplastic cells failing to exhibit ß1C immunoreactivity. Immunoblotting experiments and mRNA amplification by RT-PCR have confirmed the reliability of the above immunohistochemical findings.
Down-regulation of ß1C is significantly correlated with prognostically relevant tumor parameters in invasive breast carcinomas. In univariate analyses, tumor grade, proliferative fraction, hormone receptor status, and pT classification were all significantly correlated with ß1C expression, whereas axillary node status was not. In multivariate analysis, however, only tumor grade, pT classification, and the proliferative fraction retained a significant association with ß1C expression, whereas the receptor status did not.
The strong inverse correlation of ß1C expression with tumor grade in breast carcinomas is at variance with the lack of such correlation in prostatic carcinomas.6,7 This might well reflect that the grading systems for breast and prostatic carcinomas are construed on different morphological parameters. Indeed, Gleasons criteria14 for grading prostatic cancer rely on the gland-forming capability of the neoplastic cells and on their infiltrative growth pattern, whereas the histological grading of breast carcinomas includes the evaluation of mitotic activity, in addition to tubule formation and nuclear atypia.11 The correlation of ß1C expression with tumor grade in breast carcinomas, however, is not justified by the higher proliferative fraction of high-grade carcinomas, because in multivariate analysis both grade and Ki-67 labeling were independently associated with ß1C immunoreactivity.
Our results strengthen the role of ß1C in the negative regulation of cell cycle progression and cell proliferation, which has been put forward by in vitro experiments.3-6 Indeed, we have shown a strong inverse correlation between ß1C expression and the percentage of neoplastic cells immunoreactive for the Ki-67 antigen, a consolidated marker of cycling cells. Double immunocytochemical experiments showed in most cases a mutually exclusive immunoreactivity for either antigen at the single-cell level. These data are consistent with a pivotal role in vivo of ß1C in the control of cell proliferation in invasive breast carcinomas.
A similar inverse correlation between ß1C immunoreactivity and Ki-67 labeling has been reported for squamous cell carcinomas and adenocarcinomas of the lung.8 Furthermore, down-regulation of ß1C expression has been previously noted in regenerative areas of hyperplastic prostates displaying hyperchromatic nuclei and cellular crowding.5
A molecular mechanism by which ß1C may inhibit cell proliferation has been recently highlighted.6 ß1C is coexpressed with p27kip1 in benign and neoplastic prostates, and both proteins are coordinately down-regulated in 54% of prostatic carcinomas. Furthermore, in vitro forced expression of ß1C is accompanied by increased levels of p27kip1, by selective inhibition of cyclin A-dependent kinase activity, and by increased p27kip1 association with cyclin A. These data point to a role for ß1C as an upstream regulator of p27kip1, whose loss is an adverse prognostic factor in breast cancer.15,16
Although the short follow-up time (23 years) of the current series of patients does not allow for survival analyses, the correlation of ß1C expression with tumor size, grade, and proliferative fraction and its alleged role as an upstream regulator of p27kip1 make this integrin variant a likely novel prognostic parameter for invasive carcinomas of the breast.
| Footnotes |
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Supported by grants from the Italian Ministry of Health, Rome, Italy, and the University of Milan, Milan, Italy (to G. V.), and by National Institutes of Health grant CA-71870 (to L. R. L.).
Accepted for publication September 27, 1999.
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