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Short Communications |




From the Department of Physiology and
Pharmacology*
and the Department of
Pathology,
Lundberg Laboratory for Cancer
Research, Göteborg University, Göteborg; and
AstraZeneca,
R&D Mölndal,
Mölndal, Sweden
| Abstract |
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| Introduction |
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| Materials and Methods |
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Immunohistochemistry
All immunostains were performed according to standardized protocols using the TechMate Horizon immunostainer (DAKO, Copenhagen, Denmark). For detection of GHR, the mouse mAb 263 (AGEN Biomedical, Brisbane, Australia) was used at a dilution of 1:500 with application of tyramide signal amplification system (NEN Life Science Products, Boston, MA). All primary tumors were also analyzed for estrogen receptor (ER; clone 105, DAKO), progesterone receptor (PR; clone PgR 636, DAKO), MIB-1 (Immunotech, Marseille, France), and p53 (D07, DAKO). The immunoreactions for mAb 263 were graded as negative (0), weakly positive (1), moderately positive (2), or strongly positive (3). For ER, PR, MIB-1, and p53, the estimated percentage of positive tumor cell nuclei was recorded.
RT-PCR
Preparation of RNA was performed.13 cDNA was synthesized from 0.5 µg RNA with 5 U avian myeloblastosis virus reverse transcriptase (Promega, Madison, WI) and 0.5 µg oligo-(dt) primer (Promega). For the PCR reaction, the following primers were used: 5'-GCTGCTGTTGACCTTGGC-3' (sense) and 5'-ACCTCATCTGTCCAGTGG-3' (antisense) (Scandinavian Gene Synthesis, Köping, Sweden) located in exons 2 and 4, respectively. These primers will amplify a 201-bp fragment corresponding to nucleotides 58258 of the human GHR cDNA. If the GHR lacks the nucleotides corresponding to exon 3, the amplified fragment will be 135 bp. PCR was performed following a standard protocol. Samples were amplified for 30 cycles at an annealing temperature of 55°C. Specificity of the PCR products was verified by specific cleavage with the restriction enzyme Bsp 1286 I (Promega), which cleaves the GHR in exon 3, rendering two fragments 119 and 82 bp in size when PCR products containing exon 3 are digested, whereas PCR products without exon 3 are left intact. Specificity of PCR products was also confirmed by Southern hybridization (not shown).
Western Blotting
The antibody used, GHR06, was a mouse monoclonal raised against amino acids 396407 of the extracellular part of human GHR. It recognizes the human GHR protein by Western blotting, by immunoprecipitation, and by fluorescence-activated cell sorting (G Norstedt, unpublished data).
Soluble tissues were prepared by homogenization in PE buffer (10 mmol/L potassium phosphate buffer, pH 6.8 and 1 mmol/L EDTA) containing 6 mg/ml 3-(3-cholamidopropyl)dimethyl-ammonio 1-propane sulfate (CHAPS), aprotinin (200 kallikrein inhibitory units per milliliter), leupeptin (10 µg/ml), pepstatin (10 µg/ml), and Pefabloc (1 mg/ml; Boehringer Mannheim, Mannheim, Germany). After sonication and centrifugation, supernatants were collected and protein concentrations were determined by the Bio-Rad method. Supernatants were stored at -70°C until analysis. The samples were diluted in SDS sample buffer and denatured before loading on a SDS-polyacrylamide gel (8% Tris-glycine; NOVEX, San Diego, CA). Fifty micrograms of total protein were loaded into each lane. A prestained standard (SeeBlue, NOVEX) was used as weight marker. After electrophoresis, the proteins were transferred to a polyvinyldifluoride (PVDF) membrane (Amersham, Buckinghamshire, UK) using a standard electroblotting system, followed by incubation with GHR06, diluted 1:1000. Immunoreactive protein was visualized by chemiluminescence using an ALP-conjugated secondary antibody (goat-anti-mouse, SIGMA), diluted 1:30000 and CDP Star (Tropix, Bedford, MA) as substrate. Membranes were exposed to ECL film (Amersham) for 10 seconds to 3 minutes and developed in a Curix 60 developing machine (AGFA). Autoradiograms were scanned and the bands corresponding to GHR protein were analyze by densitometry. Quantitative analyses were performed using ImageQuant software (Molecular Dynamics, Sunnyvale, CA).
Statistical Analyses
Correlation between GHR expression as determined both by mAb 263 immunostaining and by Western blot analysis, and patient age, tumor size, tumor grade, tumor type, axillary lymph node status, estrogen receptor expression, progesterone receptor expression, MIB-1 immunoreactivity, and p53 immunoreactivity was evaluated using Fishers permutation test. Differences in GHR expression between tumors and adjacent normal mammary tissue as determined by Western blot analysis were evaluated using a paired t-test. P values <0.05 were considered significant.
| Results |
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The GHR (mAb 263) immunostaining results are summarized in Table 1
. A variable degree of cytoplasmic staining of tumor cells was seen in
all but two of the 47 analyzed tumors (Figure 1, ac)
. Weak focal nuclear staining was
observed in a few instances. Weak or moderate cytoplasmic staining also
occurred in some stromal cells (Figure 1b)
. Adjacent normal breast
tissue was negative (Figure 1d)
, or weakly positive, sharply
contrasting with mostly strongly positive tumors. A significant inverse
correlation was found between GHR expression and tumor grade
(P < 0.05).
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10% in 17/46 tumors, >10% and
<50% in 18/46, and
50% in 11/46 tumors. p53 immunostaining >20%
was seen in 14/47 tumors. There was a positive correlation between GHR
expression and progesterone receptor expression
(P < 0.01), and an inverse correlation between
GHR expression and level of MIB-1 immunoreactivity
(P < 0.05). RT-PCR
Amplified fragments of the expected sizes (201 or 135 bp)
were detected in all 36 tumor samples examined (Figure 2a
and Table 1
). To verify their
identity, the PCR products were digested by the restriction enzyme Bsp
1286 I (Figure 2b)
. Twenty-one of 36 tumors expressed the GHR form
containing exon 3, seven tumors expressed the GHR form without exon 3,
and eight tumors expressed both forms. In 17 cases, GHR mRNA expression
was also assessed in adjacent normal mammary tissue. GHR mRNA was
detected in all normal mammary tissues and was always of the same form
as the corresponding tumor.
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GHR protein was detected in all 28 tumors analyzed. The amount of
GHR varied between different tumors (Figure 3a
and Table 1
). The relative protein
levels ranged between 0.22 and 1.31 arbitrary units (median, 0.71). In
adjacent normal breast tissue (n = 17), the
relative protein levels ranged between 0.09 and 0.75 arbitrary units
(median, 0.26). Comparison of GHR expression levels in 16 tumors and
adjacent normal mammary tissues (Figure 3b)
revealed significantly
higher levels of GHR in tumors (P < 0.01,
paired t-test; Figure 3c
). No significant correlation was
found between GHR expression levels using Western blotting and any of
the clinical, morphological, or immunophenotypic parameters recorded.
However, there was a positive correlation between expression levels of
GHR detected by Western blotting and the intensity of GHR
immunoreactivity (P < 0.05).
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| Discussion |
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The predominantly cytoplasmic GHR immunoreactivity found in our study supports previous observations.14-16 Weak nuclear staining was occasionally seen; this has also been previously reported17 and is of interest since nuclear translocation of GHR may be induced by GH stimulation.18 Another interesting observation was the detection of GHR in stromal cells of the breast carcinomas. The stromal compartment of the normal mammary gland is suggested to be the site of action for GH during normal mammary development in rodents.19,20 GH, possibly via local insulin-like growth factor (IGF)-I production, acts synergistically with estradiol during normal mammary gland development in rodents21,22 and primates.23
In this study of human breast cancer, there was an inverse correlation between GHR expression as determined by immunohistochemistry and proliferative activity (MIB-1 immunostaining) as well as tumor grade, whereas there was a positive correlation between GHR expression and progesterone receptor expression. These findings, as well as the previous detection of GHR in benign epithelial proliferations of the breast, all suggest that increased GHR expression does not correspond to aggressive biological behavior per se. Additional points of interest are the detection of GHR expression by RT-PCR and Western blotting, and strong immunoreactivity for GHR in both stromal and epithelial cells of a benign phyllodes tumor not included in this series.
Despite the significant correlation between GHR expression levels detected by Western blotting and immunostaining intensity, a significant correlation between progesterone receptor expression, tumor grade, and proliferative activity (MIB-1) was found only with immunostaining intensity. This apparent discrepancy could be explained by the relatively small number of cases analyzed using Western blotting.
The detection of an additional GHR cDNA lacking exon 324 (in this series, seen as the sole form in 7/36 tumors; 21/36 tumors expressed the full-length form; 8/36 expressed both forms) was originally believed to be due to an alternative splicing event. Subsequent studies suggested tissue-specific and individual-specific expression patterns.25,26 The expression of GHR cDNA without exon 3, however, has recently been shown to be the result of a deletion of this part of the GHR gene, which, in turn, is due to a recombination of two retro-elements flanking exon 3.27 Thus, our detection of the same GHR cDNA in tumors and normal breast tissues from the same individual supports these findings. Interestingly, the invasive ductal carcinomas of the breast with the exon 3-deleted GHR occurred in patients who were significantly younger than those who had full-length GHR (mean age 41 vs. 59 years). However, analysis of a larger series is required to draw any conclusions.
This study indicates that GH and GHR play a role in human breast cancer, but the exact mechanisms involved remain unclear. Autocrine/paracrine mechanisms have been suggested based on the detection of local GH production in normal breast tissue and carcinoma,28 and transfection studies of GH expression plasmids in MCF-7 breast cancer cells in which increased growth response was recorded with autocrine/paracrine stimulation compared to exogenous GH administration.8 Because GH-mediated stimulation of IGF-I production occurs in normal breast development, a similar phenomenon could occur in the development of breast cancer.
GH is closely related to PRL; although both hormones have specific receptors, GH also activates PRL receptors (PRLR) in primates.29 Thus, in many studies of GH action, it is not possible to discriminate between GHR- and PRLR-mediated effects. Tumor-promoting effects have been shown to be mediated via PRLR and not GHR in mice.30 The situation in human breast cancer, however, is probably more complex. Both GHR and PRLR are expressed in most human breast carcinomas11,31 with higher expression levels of PRLR and GHR in breast cancers compared to normal breast tissues 32 (and this study). Local production of PRL, as well as GH, has also been demonstrated in breast carcinomas.28,33 More potent mitogenic signals are suggested to be a result of PRLR activation rather than GHR activation.7
In conclusion, this study, which provides evidence of GHR expression and up-regulation in human breast cancer, indicates a role for GHR signaling in human breast cancer. To further understand the mechanisms involved, additional studies are necessary, including analyses of benign breast lesions and precancerous conditions.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the Swedish Cancer Society, the Ingabritt and Arne Lundberg Foundation, the Assar Gabrielsson Foundation, the Syskonen Svensson Foundation, the Wilhelm and Martina Lundgren Foundation, and the Göteborg Medical Society.
Accepted for publication January 8, 2001.
| References |
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