Endocrine Abstracts

Two progesterone receptor (PR) isoforms, the full-length PR-B and the N-terminally truncated PR-A, have distinct reproductive functions, with PR-B being required for a proper mammary gland development. The functional distinction between PR-B and PR-A has partially been attributed to the additional activation function (AF3) domain, localized within the PR-B-unique sequence (PR-BUS). Three highly conserved sequence motifs within AF3 domain are required for the strong transactivation potential of human PR-B on most target gene promoters. In dogs, however, these motifs have multiple amino acid replacements, implicating a non-functional AF3 domain. Hence, we hypothesized that canine PR-B (cPR-B) has a weakened transactivation potential.

In order to test this, human and canine PR isoforms, and a chimera cPR-B with human PR-BUS (hBUS-cPR-B) were transiently transfected in human mammary (T47D-Y), chinese hamster ovary (CHO-K1) and canine mammary (CNMp) cell lines. Transactivation of PR target gene promoters (MMTV- and PRE2-luciferase reporter plasmids) was subsequently measured after 20 nM progesterone treatment. In addition, to determine the evolutionary origin of changes within cPR-B, PR-BUS regions of additional dog-related species (cat, ferret, seal and wolf) were sequenced.

Our results indicate that cPR-B is a poor transactivator on PRE-containing promoters when compared to hPR-B and even PR-A. Moreover, the attenuated transcriptional activity of cPR-B is dependent on cPR-BUS. Preliminary data suggests that the observed amino acid replacements within the AF3 domain have occurred in a common ancestor of dogs and wolfs. Interestingly, dogs are not distinct from other mammals regarding progesterone-dependent mammary development, but they show a very high incidence of spontaneous mammary tumors and endometrial lesions. The lack of functional AF3 domain in cPR-B may, therefore, suggest that the function of AF3 domain is not required for the normal mammary gland development but rather has a tumor-protective role.