Expression of prolactin receptors in normal canine mammary tissue, canine mammary adenomas and mammary adenocarcinomas

Abstract

Background: Mammary tumors represent the most common neoplastic disease in female dogs. Recently, the promoting role of prolactin (PRL) in the development of human breast carcinoma has been shown. Possible proliferative, anti-apoptotic, migratory and angiogenic effects of PRL on human mammary cancer cells in vitro and in vivo were suggested. The effects of PRL are mediated by its receptor, and alterations in receptor expression are likely to play a role in tumor development. Currently, not much data is available about prolactin receptor (PRLR) expression in canine mammary tumors. To set the basis for investigations on the role of PRL in mammary tumorigenesis in this species, prolactin receptor expression was evaluated by semi-quantitative real time PCR and immunohistochemistry on 10 formalin-fixed, paraffin-embedded samples each of canine non-neoplastic mammary tissue, mammary adenomas and adenocarcinomas.

Results: The highest PRLR expression levels were found in normal mammary tissue, while adenomas, and to an even higher degree adenocarcinomas, showed a significant decrease in prolactin receptor expression. Compared to normal tissue, PRLR mRNA was reduced 2.4 fold (p = 0.0261) in adenomas and 4.8 fold (p = 0.008) in adenocarcinomas. PRLR mRNA expression was significantly lower in malignant than in benign lesions (p = 0.0165). Immunohistochemistry demonstrated PRLR expression in all three tissue types with signals mostly limited to epithelial cells.

Conclusions: Malignant transformation of mammary tissue was associated with a decline in prolactin receptor expression. Further studies are warranted to address the functional significance of this finding.

Figure 1

Expression of PRLR in normal canine mammary tissue, benign (mammary adenomas) and malignant (mammary adenocarcinomas) lesions as determined by Real Time (TaqMan) RT-PCR (normalized to GAPDH expression): mean (Xg x DF ± 1); bars with one asterisk differ at (P < 0.05); bars with two asterisks differ at (P < 0.01).

Figure 2

Immunohistochemical labeling of PRLR in canine mammary tissues. a) Non-neoplastic mammary tissue. Signal can be detected in epithelial cell cytoplasm and in alveolar secrete. Connective tissue cells are mostly negative. b) Non-neoplastic mammary tissue. Epithelial cells are flattened and exhibit a strong signal. c) Mammary adenoma. The adjacent non-neoplastic tissue shows more intense positive staining than the neoplastic one. This tissue is from an original block used for the study. d) Mammary adenoma. Signal is present in epithelial cells. e) Mammary adenoma. Staining is mainly localized to the basal epithelial cell layer. f) Mammary adenocarcinoma. Signal is present in epithelial cells and rarely in some fibrous tissue cells. g) Mammary adenocarcinoma. An intense signal is present in epithelial cells. h) Mammary adenocarcinoma. Only a weak signal signal is present in epithelial cells. Immunoperoxidase staining, anti-human PRLR antibody.