Detection of progesterone receptor forms A and B by immunohistochemical analysis

Aim-The measurement of progesterone receptors (PR) is recommended as part o f the clinical management of breast and endometrial. cancers, and immunohis tochemistry on formalin fixed tissue is now the method of choice. PR is exp ressed as two isoforms, PRA and PRB, and although both these proteins are e xpressed in hormone dependent cancers, there is evidence that a large propo rtion of tumours express a predominance of one isoform. Therefore, it is es sential to document the individual detection of PRA and PRB by the presentl y available anti-PR antibodies. The aim of this study is to investigate the detection of PR isoforms A and B in formalin fixed, paraffin wax embedded cell lines and tissue sections by immunohistochemistry, using a panel of co mmercial and in house antibodies to human PR. Methods-PR negative cell lines stably transfected to express only PRA (MDA- MB-231/PRB) or PRB (MDA-MB-231/PRB), and tissue sections of human breast ca rcinoma and normal endometrium were stained using an immunoperoxidase metho d. A panel of primary PR specific antibodies was evaluated for ability to d etect both PRA and PRB proteins, and for intensity and distribution of posi tive staining under optimal conditions. Results-Of the 11 antibodies assessed, only four recognised PRA and PRB sim ilarly. Six recognised PRA proteins but were unable to detect PRB expressio n in the cell lines expressing only PRA or PRB. In tissues expressing high amounts of PRA and PRB, all antibodies tested demonstrated positive PR stai ning. However, in tissues expressing a predominance of PRB, differential st aining patterns were observed, with variations in staining intensity and in the proportion of cells positive for PR. Conclusions-Most PR specific antibodies tested failed to detect PRB in form alin fixed tissue by immunohistochemical techniques, despite their ability to do so by immunoblot analysis. These observations suggest that there are conformational differences between PRA and PRB that mask epitopes on the PR B protein recognised by most anti-PR antibodies. The selection of antibodie s that recognise both PRB and PRA in formalin fixed tissue is essential for the accurate evaluation of PR positivity in clinical specimens.