Progesterone receptor isoforms in the mammary gland of cats and dogs.

Progesterone exerts its effect by binding to specific progesterone receptors (PR) within the cell. In dogs and cats, no data are available on PR isoforms as found in other species. We therefore investigated the sequence of the PR gene and encoded protein in dogs and cats, the expression of PR isoforms in mammary tissue using Western blots and the presence of PR in mammary tissue using immunohistochemistry. Comparison of the amino acid sequence of the canine and feline PR with human PR revealed major differences in the PR-B-specific upstream segment (BUS). However, the essential activation function 3 (AF3) domain was intact in the cat but mutated in the dog. The DNA and ligand-binding domains were highly similar among the species. In cats with fibroadenomatous hyperplasia (FAH), high expression of PR mRNA together with growth hormone (GH), GH receptor (GHR) and IGF-I mRNA was found in comparison with feline mammary carcinomas. Immunohistochemical analysis showed strong nuclear as well as cytoplasmic staining for PR in FAH. Western blot analysis revealed expression of the PR-A and PR-B isoforms in the feline mammary gland. In canine mammary tissue, the most abundant PR staining was found in proliferative zones of the mammary gland. Western blot analyses showed mainly staining for PR-A with lower PR-B staining. It is concluded that in dogs and cats both PR isoforms are expressed. The role of mutations found in the canine PR-B is discussed.

Gene expression profiles of progestin-induced canine mammary hyperplasia and spontaneous mammary tumors.

Spontaneous mammary tumors are the most prevalent type of neoplasms in women as well as in female dogs. Although ovarian hormones estrogen and progesterone are known to play a key role in mammary tumorigenesis, conflicting reports have been obtained from in vivo and in vitro studies concerning the role of especially progesterone in mammary tumorigenesis. Prolonged exposure to high concentrations of progesterone during the unusually long luteal phase of the estrous cycle is suspected to be the key event in canine mammary tumorigenesis. Accordingly, previous studies have shown the development of mammary hyperplasia in dogs upon prolonged progestin administration. In this study, a dog-specific cDNA microarray was used to identify oncogenic determinants in progestin-induced canine hyperplasia (CMH) and spontaneous mammary tumors (CMC) by comparing expression profiles to those obtained from mammary glands of healthy dogs. The CMH profile showed elevated expression of genes involved in cell proliferation such as PCNA, NPY, RAN and also alterations in expression of transcription factors and cell adhesion molecules. Whereas in CMC, major alterations to the expression of genes involved in cell motility, cytoskeletal organization and extra cellular matrix production was evident besides differential expression of cell proliferation inducing genes. The overall gene expression profile of CMH was related to cell proliferation where as that of CMC was associated with both cell proliferation as well as neoplastic transformation. In conclusion, our findings support a strong cell proliferation inducing potential of progestins in the canine mammary gland. Moreover, deregulated genes identified in CMC are potentially involved in their malignant and may serve as prospective therapeutic targets.