Close correlation between progesterone receptor concentration and hormonal sensitivity in DMBA-induced mammary tumours of the rat.

Cyclic administration of oestrogen alone and with progesterone to ovariectomized rats bearing DMBA-induced tumours resulted in more uniform cytosol oestrogen and progesterone receptor (ER and PgR) concentrations in the uteri and a stronger correlation between ER and PgR concentrations in the the tumours than in a group of naturally cycling animals. Using this model, the concentration of PgR was a better indicator of tumour response to ovariectomy and hormonal restimulation than the concentration of ER. In ten unequivocally responsive adenocarcinomas receptor values, expressed as fmol/mg cytosol protein, were: ER 47 +/- S.D. 19, PgR 246 +/- 134, PgR:ER 5.9 +/- 3.0, compared with ER 18 +/- 2, PgR 33 +/- 8 and PgR:ER 1.8 +/- 0.3 in three autonomous adenocarcinomas. All 3 parameters were significantly different between the two groups (0.02 less than P less than 0.05), but there was overlap in the ER values, whereas no overlap occurred in the PgR values or the PgR:ER ratios. Ten other adenocarcinomas showed a different sensitivity to restimulation than to ovariectomy and six tumours were fibroadenomas or of mixed histopathology. For the whole tumour population the response to hormonal restimulation at the time of excision suggested that a PgR concentration greater than 60 fm/mg protein combined with an ER concentration greater than 20 fm/mg protein is necessary for the maintenance of hormonal sensitivity. Using these criteria, response was related to receptor concentrations in 93% of the tumours.