Since its first description as a major v-Src-substrate in Rous sarcoma virus-transformed chicken embryo fibroblasts, Caveolin-1 has been considered as a presumable mediator of transformation by oncogenic tyrosine kinases [22
]. The contribution of Caveolin-1 to carcinogenesis and tumour progression has been intensively evaluated.
In order to examine the potential clinical relevance of Caveolin-1 in premalignant and malignant breast disease, we studied Caveolin-1 protein expression in tissue probes of healthy breast tissue, benign breast disease, DCIS, and invasive breast cancer, using immunohistochemistry. We found Caveolin-1 expression in epithelial tumour cells in 32 of 109 cases (29.4%) of invasive breast carcinomas. In contrast, when evaluating 108 cases of DCIS specimens, 236 cases of benign breast disease and five cases of healthy breast tissue, no Caveolin-1 expression could be found in the epithelial component. Caveolin-1 expression was consistently detected in ductal and lobular myoepithelial cells, in vascular smooth muscle cells, and in endothelial cells in non-malignant breast tissue samples, which is in concordance with previous reports [14
Several comprehensive immunohistochemical studies have reported on Caveolin-1 expression in human breast cancer; Yang et al
], examined Caveolin-1 protein expression in 15 cases of invasive breast cancer, 15 cases of intraductal breast cancer, and 9 cases of lymph node metastasis. They reported significantly higher expression of Caveolin-1 in both intraductal carcinomas (p > 0.001) and infiltrating ductal carcinomas (93.3%, p < 0.001) as well as in lymph node metastases (p < 0.001) relative to normal breast epithelium. However, even in normal breast epithelial cells, minimal staining was observed. In contrast, Hurlstone et al
], supported our observations in that they could not detect Caveolin-1 expression within the epithelial cell component of human mammary normal ducts or terminal ductal lobular units of 10 breast reduction specimens. Instead, high Caveolin-1 expression levels were again observed in mammary myoepithelial cells. In the most recent immunohistochemical study, Savage et al
], studied the frequency and cellular distribution of Caveolin-1 expression in normal breast, benign breast lesions, breast cancer precursors, and breast carcinomas. Using a monoclonal antibody, the authors corroborated our results in that no expression of Caveolin-1 could be observed in the epithelial cell component of normal breast tissue or in luminal epithelial cells of benign breast lesions such as radial scars. However, luminal epithelial cells demonstrated Caveolin-1 expression in 13.4% of DCIS and 9.4% of invasive breast cancer specimens. The authors observed an inverse correlation between Caveolin-1 expression and expression of ER, PR, HER2, and cyclin D1, as well as an association with the expression of EGFR, cytokeratins 5/6, 14, and 17, high MIB-1 expression, and p53 expression. Furthermore, they described a significant association between Caveolin-1 expression and both shorter disease-free and overall survival as well as with the so-called 'basal-like' immunophenotype, which also has been repeatedly associated with adverse clinical outcome [21
]. Interestingly, an association between basal-like phenotype and Caveolin-1 expression has been described in another report. Pinilla et al
], examined Caveolin-1 expression in 509 cases of sporadic and 47 cases of hereditary breast cancers using a monoclonal Caveolin-1 antibody. Caveolin-1 expression was observed among 4.6% of sporadic cases, but among as many as 10.6% of hereditary cases. Caveolin-1 positivity was again significantly associated with lack of ER, PR, and HER2 expression and presence of cytokeratin 5/6 and EGFR expression. Lack of expression of ER and HER2 expression and presence of cytokeratin 5/6 and/or EGFR expression were taken as surrogate markers indicating a basal-like phenotype. Accordingly, 52% of Caveolin-1 positive cases were classified as basal-like subtype. These results are in striking contrast to observation by Sagara et al
., The group examined 162 breast cancer specimens using the same monoclonal anti-Caveolin-1 antibody and realtime-PCR. They described a significant positive correlation between Caveolin-1-mRNA expression in breast cancer and positive oestrogen receptor-status as well as reduced tumour size [20
]. In our study, neither oestrogen receptor status, nor tumour stage, nor other clinical or pathological parameters, besides multifocality (p = 0.008), correlated with Caveolin-1 expression. Furthermore, no significant correlation with either disease free survival or overall survival could be demonstrated.
Choice of primary antibody and scoring system has been shown to have a substantial impact on the results of immunoreactivity. For example, Kersting et al
., determined epidermal growth factor receptor (EGFR) immunoreactivity in 302 cases of soft tissue sarcomas using five different commercially available antibodies, and EGFR amplification status in 283 cases using fluorescence in situ hybridisation (FISH). Depending on the antibody and scoring method used, EGFR expression frequency varied between 0.3% and 52.9%. EGFR gene amplification was determined in 3.5% of tumours showed and correlated with EGFR expression for only three antibodies [25
]. Of note, Yang et al
., used a polyclonal antiserum to determine Caveolin-1 expression status in human prostate and breast malignancies. They reported positive staining in as many as 80% of cases of intraductal carcinomas and also minimal Caveolin-1 expression in normal breast epithelium. In contrast, when using a monoclonal antibody, Hurlstone et al
., corroborated our results, in that no Caveolin-1 expression was observed among normal breast epithelial cells [14
]. Different scoring methods might explain some of the discrepancies between our results and those of Savage et al
]. The group applied a semiquantitative consensus score of both distribution and intensity of Caveolin-1 immunostaining. Based on a cutoff score of ≥ 4 they reported Caveolin-1 expression in 9.4% of primary breast cancers. In our study we applied a 4-tired semiquantitative score to describe intensity of Caveolin-1 expression. In face of the lack of Caveolin-1 expression in normal breast epithelial cells, we considered any Caveolin-1 staining as positive and combined weak, moderate and strong Caveolin-1 expression (scores "1" to "3") to represent Caveolin-1 positivity. Thus, we observed Caveolin-1 expression in 32 of 109 cases of invasive breast carcinomas (29.4%). However, if we had only regarded moderate and strong expression, we would have observed Caveolin-1 expression in only 7.3% of cases, which is in the range of the results by Savage et al.
Interestingly, Savage et al
., reported Caveolin-1 immunostaining in 2 of 15 cases (13.4%) of DCIS [21
]. This is in striking contrast to our results. We examined 108 cases of DCIS and could not find Caveolin-1 positivity among these cases. In concordance with the methodology of Savage et al
., entrapped blood vessels were used as internal positive controls in order to ensure robustness of the data. The TMAs in this study included endothelial cells in both malignant tumour specimens as well as adjacent normal breast tissue. Endothelial cells were consistently found to be Caveolin-1 positive.
The role of Caveolin-1 in mammary carcinogenesis is still far from being completely understood. Scientific evidence of a tumour suppressive role of Caveolin-1 in breast cancer supported by some researchers [26
] is contrasted by recent results which strengthen the role of Caveolin-1 overexpression to promote certain steps of tumourigenesis: Caveolin-1 has been shown to inhibit anoikis in MCF7 breast cancer cells [27
]. Furthermore, Caveolin-1 has been demonstrated to mediate medroxyprogesterone acetate-(MPA)-induced breast cancer cell growth [28
]. Inflammatory breast cancer represents a highly aggressive form of invasive breast cancer. Among these cancers, Caveolin-1 expression is upregulated compared to expression levels in non-inflammatory carcinomas [29
]. In face of this controversy one has to assume that the role of Caveolin-1 as both tumour suppressor and promoter might be context-depending. While being downregulated in early stage malignancies and thereby mediating growth promoting effects, upregulation of Caveolin-1 in late stage disease might promote resistance against chemotherapeutic agents in colon cancer as well as metastatic properties in prostate cancer [30
]. It seems reasonable that both the conflicting data on Caveolin-1 expression frequencies and the lack of a clear prognostic impact in breast cancer mirror the variety of functions, which Caveolin-1 is believed to obtain in breast cancer pathogenesis. Caveolin-1 has been shown to determine the function of caveolae as a platform to preassemble distinct components of cellular pathways, and therefore both to render signal transduction more efficient and to enable appropriate interaction between distinct pathways [31
]. This allows placing the protein components in close proximity to each other. Thus, its distinct role in cellular processes may depend on the combination of proteins expressed in the cells rather than on Caveolin-1 expression itself.
Importantly, recent results have revealed a potential therapeutic relevance of Caveolin-1 since the Caveolin-1-promoter has been hypothesized to be used as a specific target in gene therapy of prostate carcinoma in the nearer future [32
]. Bortezomib, an antibody against the 26-S-proteasome, has been shown to target Caveolin-1 among a variety of other proteins in studies in various cancer entities [33