MULTIPLE ISOFORMS OF THE ESTROGEN-RECEPTOR IN ENDOMETRIAL CANCER

We evaluated the presence and variability of oestrogen receptor (ER) i soforms in endometrial cancer by using [H-3]oestradiol-labelled ERs an d the H222 monoclonal antibody obtained from the Abbott enzyme immunoa ssay kit. Using isoelectric focusing (IEF), endometrial ER was shown t o 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 ut erus, 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 b reast 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 wit h 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 res pect to the non-dissociated oligomer, which suggested that some epitop es are 'masked', i.e. not accessible to the antibodies when ER is pres ent in its complexed form. The evidence thus suggested that the oligom er at pi 6.1 contained a single 65 kDa ER form which, as a monomer, fo cused at pI 7.2. The variability in the ER isoform profile found in en dometrial cancer was similar to the variability previously reported in breast and larynx carcinomas. The balance between these isoforms coul d be a dynamic parameter involved in the functionality of this recepto r and consequently in cell transformation.