Multiple isoforms of the oestrogen receptor in endometrial cancer

Abstract

We evaluated the presence and variability of oestrogen receptor (ER) isoforms in endometrial cancer by using [3H]oestradiol-labelled ERs and the H222 monoclonal antibody obtained from the Abbott enzyme immunoassay kit. Using isoelectric focusing (IEF), endometrial ER was shown to be composed of four different species, with pI values of 6.1, 6.3, 6.6 and 6.8, indistinguishable from the isoforms found in normal rat uterus, and human breast and larynx carcinomas. The isoforms at pI 6.3, 6.6 and 6.8, all sedimenting at 4S by sucrose gradient fractionation, showed, on two-dimensional SDS electrophoresis, relative masses of 50, 70 and 65 kDa respectively, equal to the masses previously found in breast cancer. These isoforms did not alter their pI values during IEF fractionation performed in a linear gradient of urea, while the pI 6.1, sedimenting at 8S, generated a new isoform at about 9 mol/l urea with pI 7.2 and a relative mass of 65 kDa. The urea-dissociated isoform (pI 7.2) was able approximately to double the antibody binding with respect to the nondissociated oligomer, which suggested that some epitopes are 'masked', i.e. not accessible to the antibodies when ER is present in its complexed form. The evidence thus suggested that the oligomer at pI 6.1 contained a single 65 kDa ER form which, as a monomer, focused at pI 7.2. The variability in the ER isoform profile found in endometrial cancer was similar to the variability previously reported in breast and larynx carcinomas. The balance between these isoforms could be a dynamic parameter involved in the functionality of this receptor and consequently in cell transformation.

Detection of oestrogen receptor variants in endometrium, myometrium, leiomyoma and peripheral blood mononuclear cells: comparison to variants present in breast cancer

“…For example, variant with exons 3 and 7 deleted may act as dominant negative regulators of wild-type ER (Wang and Miksicek 1991;Fuqua et al 1992), whereas exon-5-deleted ER has ligand-independent transcriptional activity . A variability in the oestrogen receptor isoform pro®le has also been found in endometrial cancer (Marsigliante et al 1995). The majority of ER-positive breast cancers contain two isoforms of ER protein, whereas only 12% possess four isoforms (Marsigliante et al 1991), implying that a loos of isoforms takes place gradually in cancerous tissues.…”

“…Multiple isoforms of the ER, possibly caused by the translation of variant mRNA into stable proteins, have also been detected in endometrial carcinomas, but their signi®cance in the aetiology of endometrial cancer has not been documented (Marsigliante et al 1991(Marsigliante et al , 1995.…”