Estrogen receptor (ER) belongs to the steroid receptor superfamily of nuclear receptors. It is a protein with 553 amino acids. The receptor molecule has three domains, i.e. the central DNA-binding domain, the hormone-binding domain at the C-terminal, and the transcription-activating domain at the N-terminal. ER mediates regulatory functions of female sex steroids, mainly 17 (E2), on growth, differentiation and function in several target tissues, including female and male reproductive tract, mammary gland, and skeletal and cardiovascular systems. Recently, a second estrogen receptor, termed ER?, was discovered. Human ER? shares a high structural homology with the previously known human ER, now termed ER?, especially in the DNA- and hormone binding domains. Both receptors bind hormones with similar affinity and their transcriptional activation is identical. The tissue distribution of ER? is similar to that of ER? with some differences. In normal and malignant human breast tissue ER? is expressed in stromal cells in addition to epithelia. Only limited data are available on the role of ER? in normal and neoplastic tissues.
ER? is mainly expressed in tumours of female sex steroid hormone responsive tissues such as the mammary gland, endometrium, and ovary. ER? protein is expressed in 60-70% of female breast cancers (ER+/PR- 19-22%; ER+/PR+ 49-53%). Other tumours expressing ER? are meningiomas, salivary gland tumours, some neuroendocrine tumours, and some colorectal and hepatocellular carcinomas.
The applications of immunohistochemical demonstration of ER? are two-fold. The main clinical use of ER? immunohistochemistry is prediction of response to therapy in breast carcinoma. Tumours expressing both ER? and PR react positively to antiestrogen therapy in 50-70% of cases as against below 10% of those negative for ER? and PR. Based on these facts and a number of meta-analyses, adjuvant antiestrogen treatment is administered in most countries to postmenopausal women with ER+ breast cancer. In addition to predicting treatment response, the ER? (and PR) status can be used to estimate disease-free and overall survivals of breast carcinoma patients. In newer studies with immunohistochemical assay, positive steroid hormone status has predicted favourable overall, survival, independently of hormonal treatment. Secondly, ER? can be used as a tumour marker (see Neoplasms above), preferentially in combination with an antibody to Progesterone receptor, e.g., in the classification of adenocarcinomas.
Uterine cervix is found to be an appropriate positive tissue control for ER staining: In optimal protocols, virtually all epithelial cells throughout the layers of the squamous epithelium and in the glands showes a moderate to strong and distinct nuclear staining reaction. In the stromal compartment, moderate to strong nuclear staining reaction is seen in most cells except endothelial and lymphatic cells. Tonsil is also found to be highly recommendable as a tool to monitor the analytical sensitivity for the IHC demonstration of ER and is in fact superior to uterine cervix. Dispersed germinal centre cells (most likely macrophages) and squamous epithelial cells are distinctively demonstrated in virtually all protocols providing an optimal result. If the germinal centre macrophages are negative, a reduced proportion of ER positive cells is seen in the other tissues and a too weak or even false negative staining can bee seen. Simultaneously, tonsil can be used as supplementary negative tissue control, as B-cells in mantle zones and within germinal centres must be negative.
To validate the specificity of the IHC protocol further, an ER negative breast carcinoma must be included as primary negative tissue control, in which only remnants of normal epithelial and stromal cells must be ER positive, serving as internal positive tissue control. Positive staining reaction of the stromal cells in breast tissue indicates that a highly sensitive protocol is being applied, whereas the sensitivity cannot be evaluated in normal epithelial cells in mamma as they express high levels of ER.