Prolactin Receptor Expression is an Independent Favorable Prognostic Marker in Human Breast Cancer.

Prolactin (PRL) hormone plays an important role in the development of the mammary gland and terminal differentiation of the mammary epithelial cells. While initial studies suggested that PRL may contribute to the development of breast cancer through PRL/prolactin receptor (PRLR) autocrine function, mounting evidence indicate a different role for PRL, highlighting this hormone as a regulator of epithelial plasticity and as a potential tumor suppressor. To gain further insights into the role of PRL in human breast carcinogenesis, immunohistochemistry analyses of PRLR protein expression levels using tissue microarray of 102 cases were done in comparison with various clinical/pathologic parameters and molecular subtypes. In addition, gene expression level of PRLR was also evaluated in relation to intrinsic molecular subtypes, tumor grade, and patient outcome using GOBO database for 1881 breast cancer patients. Interestingly, PRLR expression was found to be significantly downregulated in invasive breast cancer (21.4%) in comparison with normal/benign (80%) and in situ carcinoma (60%) (P=0.003498). Moreover, PRLR expression was associated with lymph node negativity and low-grade well-differentiated tumors. PRLR expression was strongest in luminal A subtype, and was virtually undetectable in the worse prognosis triple-negative breast cancer subtype (P=0.00001). Furthermore, PRLR expression was independent of ER, PR, HER-2, and P53 status. Finally, PRLR expression was significantly (P<0.01) associated with prolonged distant metastasis-free survival in breast cancer patients. In conclusion, our results highlight PRLR as an independent predictor of favorable prognosis in human breast cancer.