Estrogen receptor isoforms and progestin hormone dependence in a mouse mammary tumor model

The close interaction between receptors and other transcription factors suggests that their corresponding transducing signals can trigger functional and structural changes in other related molecules. The effect of a progestinic agent, medroxyprogesterone acetate (MPA), on some of the estrogen-receptor (ER) parameters was studied in 2 murine mammary tumor sublines with different progestin hormone dependence for their respective growth. The relative binding affinity of estradiol and tamoxifen for the ER, the receptor content and the ER isoforms studied by HPLC were determined in the hormone-autonomous (HA) and the hormone-dependent (HD) tumor sublines. In the HA subline administration of MPA did not modify the tumor growth rate, whereas this was accelerated in the HD subline. The ER content was clearly increased in the HD tumor subline, but not in the HA subline, compared with the untreated controls. In contrast, the E2 and tamoxifen relative binding affinity for the ER and the isoform profiles were affected by MPA treatment in the HA, but not in the HD tumor subline. The functional change (decrease in relative binding affinity) can be attributed to the appearance of a lower-molecular-size ER isoform under the progestinic treatment. Modifications in one receptor molecule by the action of ligands corresponding to another type of receptor show the interconection between transcription factors and the necessity of broadening conventional concepts regarding hormone dependence in mammary tumorigenesis.