Significance of the 8S complex in oestrogen receptor recognition

Current status of estrogen receptors

Increasing knowledge on structure and function of estrogen receptors is providing information on the mechanism of action of estrogen agonists, as well as antagonists, and in understanding their tissue-selective action. However, there are still many factors associated with estrogen response which are poorly understood. Therefore, the task of designing a tissue-selective estrogen for use as a pharmaceutical in estrogen-dependent disorders remains an uncertain game. This review provides information on the current status of estrogen receptors for a better understanding.

Human larynx expresses isoforms of the oestrogen receptor

Commercially available enzyme immunoassays (EIAs) were used for oestrogen (ER) and progesterone (PR) receptor determination in the cytosol fraction of 118 human larynx cancer specimens and in the corresponding histologically proven non-malignant tissues. Fifty-one ER positive cancerous samples had corresponding non-cancerous tissues also expressing the receptor. A high resolution isoelectric focusing (IEF) technique followed by immunoblotting with the H222 anti-ER monoclonal antibody was used to evaluate the presence of ER isoforms in the 51 ER positive human larynx cancer specimens and in their corresponding non-malignant tissues. In both tissues, four ER isoforms were detected, with isoelectric points (pI) similar to those obtained in breast and endometrium carcinomas (6.1, 6.3, 6.6 and 6.8). A significant difference in the expression of ER isoforms between cancerous and non-cancerous tissue was found; precisely, the 94.1% of the ER positive non-malignant specimens co-expressed the four isoforms while they were detected in only the 35.5% of the malignant specimens (P < 0.0001 by Fisher's exact test). In larynx cancer, the concentration values of ER and PR did not correlate, nevertheless tumours co-expressing the four ER isoforms had PR levels significantly higher than those which did not (P = 0.02 by Mann-Whitney Wilcoxon sum rank test). To investigate the possibility that the isoforms of the monomeric 4S form of the ER (those with pI 6.3, 6.6, and 6.8) could dimerise, a cold agarose gel electrophoresis technique was used on IEF-separated ER isoforms. In summary, the evidence shows that all the isoforms are able to form homodimers and that the isoforms at pI 6.3 and 6.8 are able to dimerise with that at pI 6.6 but, under the same experimental conditions, they do not form the 6.3/6.8 heterodimer. It was concluded that: (1) the four isoforms of the ER are co-expressed by the non-malignant human larynx and the cancer loses the capacity to express some of them; (2) the complete complement of ER isoforms (all four) is needed for PR expression; (3) the monomeric 4S isoform with pI 6.6 has the capacity to form homo- and heterodimers, while the remaining two are only able to homodimerise.

Oestrogen receptor isoforms, their distribution and relation to progesterone receptor levels in breast cancer samples

Oestrogen receptors (ER) in breast cancer tumours are highly heterogeneous. In this study, the variability in the profile of ER isoforms and its relation to progesterone receptor (PgR) levels in breast tumours has been studied. Using high resolution isoelectric focusing (IEF) 4 ER isoforms can be detected with pI values of 6.1 (corresponding to the 8S ER), and 6.3, 6.6 and 6.8 (all of which have a sedimentation pI values of 6.1 (corresponding to the 8S ER), and 6.3, 6.6 and 6.8 (all of which have a sedimentation coefficient of approximately 4S in sucrose density gradients). Data were obtained on the soluble receptors from supernatants of 66 ER-positive primary breast tumour homogenates using high resolution IEF. In 43 of these samples PgR levels were also measured. The isoform at pI 6.6 was present in 97.0% of tumours, the isoform at pI 6.1 in 83.3%, the pI 6.3 isoform 39.4% of tumours and the pI 6.8 isoform in only 33.3% of tumours. Only 12.1% of tumours studied contained the full complement of ER isoforms (pI 6.1, 6.3, 6.6 & 6.8). The ER isoforms at pI 6.1 & 6.8 were only found in PgR-positive (> 10 fmol PgR/mg protein) tumours. Some tumours contained only a single ER isoform at pI 6.6 or 6.1, but those at pI 6.3 and 6.8 were never found singly. Tumours containing 3 or 4 ER isoforms had significantly higher levels of PgR (> 90 fmol/mg protein) than those with only 1 or 2 (P < 0.001). The presence of ER isoforms at pI 6.3 and pI 6.8 also significantly correlated with high levels of PgR (P < 0.001). This variability in the ER isoform profile of breast tumours and their correlation with PgR levels may have a bearing on prognosis and tumour response to endocrine therapy.

Type II oestrogen binding site is associated with the major 4S oestrogen receptor isoform in breast tumours

Using high resolution isoelectric focusing we have been able to identify a low affinity/high capacity oestrogen binding protein, which exhibits an apparent pI of 7.0. Using this system it can be separated from the previously described high affinity oestrogen receptor (ER) isoforms which focus at pI 6.1, 6.3, 6.6 and 6.8. The pI 7.0 protein was detected in 30/30 breast tumours analysed and had the binding characteristics of the cytoplasmic Type II ER (Kd = 88 +/- 8 nM). The concentration of this protein was shown to be significantly correlated with the concentration of the pI 6.6 species, which represents the major 4S isoform. It is not related to any other isoform of ER, and is expressed independently of the progesterone receptor. The importance of this observed relationship with respect to ER function remains obscure, but it may provide new insights into the role of the Type II oestrogen binding site in breast cancer.

The biosynthesis of aldosterone

The cellular mechanisms for aldosterone biosynthesis are incompletely understood. Although the enzymes involved are now well characterized, the dynamics of aldosterone secretion in a variety of rat adrenal preparations are not consistent with the concept that freshly synthesized corticosterone is an important intermediate. In whole glomerulosa tissue preparations, aldosterone is more readily formed from endogenous precursors than from an added radioactive precursor, such as [3H]pregnenolone, and in the in situ perfused gland preparation, aldosterone responses to stimulation, for example by ACTH, are significantly more rapid than those of corticosterone, suggesting a tissue source of steroid substrate for aldosterone production other than corticosterone. The only steroid which is stored in rat adrenal glomerulosa tissue to any extent is 18-hydroxydeoxycorticosterone (18-OH-DOC), and this pool has been located in plasma membrane fractions. It is lost on preparation of collagenase dispersed glomerulosa cells. Since dispersed glomerulosa cell preparations produce significantly less aldosterone, relative to corticosterone, than incubated intact whole glomerulosa, it is plausible that this tissue pool (which is not found in the inner zones) is the immediate precursor for aldosterone formation. Further evidence shows that trypsin, which stimulates aldosterone (and 18-hydroxycorticosterone) production in rat intact glomerulosa tissue, but not in dispersed cells, stimulates translocation of protein kinase C to the plasma membrane. It is plausible that one function of protein kinase C in the rat adrenal zona glomerulosa is to mobilize membrane sequestered 18-OH-DOC for conversion to aldosterone.