Distribution of prolactin receptors suggests an intraductal role for prolactin in the mouse and human mammary gland, a finding supported by analysis of signaling in polarized monolayer cultures

Polarity in breast development and cancer

Mammary gland development and breast cancer progression are associated with extensive remodeling of epithelial tissue architecture. Apical-basal polarity is a key feature of epithelial cells that coordinates key elements of epithelial morphogenesis including cell organization, proliferation, survival, and migration. In this review we discuss advances in our understanding of how apical-basal polarity programs are used in breast development and cancer. We describe cell lines, organoids, and in vivo models commonly used for studying apical-basal polarity in breast development and disease and discuss advantages and limitations of each. We also provide examples of how core polarity proteins regulate branching morphogenesis and lactation during development. We describe alterations to core polarity genes in breast cancer and their associations with patient outcomes. The impact of up- or down-regulation of key polarity proteins in breast cancer initiation, growth, invasion, metastasis, and therapeutic resistance are discussed. We also introduce studies demonstrating that polarity programs are involved in regulating the stroma, either through epithelial-stroma crosstalk, or through signaling of polarity proteins in non-epithelial cell types. Overall, a key concept is that the function of individual polarity proteins is highly contextual, depending on developmental or cancer stage and cancer subtype.

Multiple cell types in the oviduct express the prolactin receptor

Little is known about the physiological role of prolactin in the oviduct. Examining mRNA for all four isoforms of the prolactin receptor (PRLR) in mice by functional oviduct segment and stage of the estrous cycle, we found short form 3 (SF3) to be the most highly expressed, far exceeding the long form (LF) in highly ciliated areas such as the infundibulum, whereas in areas of low ciliation, the SF3 to LF ratio was ~1. SF2 expression was low throughout the oviduct, and SF1 was undetectable. Only in the infundibulum did PRLR ratios change with the estrous cycle. Immunofluorescent localization of SF3 and LF showed an epithelial (both mucosal and mesothelial) distribution aligned with the mRNA results. Despite the high SF3/LF ratio in densely ciliated regions, these regions responded to an acute elevation of prolactin (30 min, intraperitoneal), with LF-tyrosine phosphorylated STAT5 seen within cilia. Collectively, these results show ciliated cells are responsive to prolactin and suggest that prolactin regulates estrous cyclic changes in ciliated cell function in the infundibulum. Changes in gene expression in the infundibulum after prolonged prolactin treatment (7-day) showed prolactin-induced downregulation of genes necessary for cilium development/function, a result supporting localization of PRLRs on ciliated cells, and one further suggesting hyperprolactinemia would negatively impact ciliated cell function and therefore fertility. Flow cytometry, single-cell RNAseq, and analysis of LF-td-Tomato transgenic mice supported expression of PRLRs in at least a proportion of epithelial cells while also hinting at additional roles for prolactin in smooth muscle and other stromal cells.

Suppression of Breast Cancer by Small Molecules That Block the Prolactin Receptor

Simple Summary

Unabated tumor growth, metastasis, and resistance to hormone therapy and/or to chemotherapy constitute serious impediments for combating breast cancer (BC). With the exception of targeted anti-HER2/neu therapy and combination therapies, there have been no radical changes in the standard of care for BC patients in the past two decades. In addition, there are only limited options for treating BC-derived brain metastases that cause high morbidity and mortality. This report describes the use of high throughput screening (HTS) for identifying novel small molecules that blocked the prolactin receptor (PRLR) and suppressed BC in a laboratory setting. These small molecules have a great potential to become effective therapeutics in patients with BC.

Abstract

Prolactin (PRL) is a protein hormone which in humans is secreted by pituitary lactotrophs as well as by many normal and malignant non-pituitary sites. Many lines of evidence demonstrate that both circulating and locally produced PRL increase breast cancer (BC) growth and metastases and confer chemoresistance. Our objective was to identify and then characterize small molecules that block the tumorigenic actions of PRL in BC. We employed three cell-based assays in high throughput screening (HTS) of 51,000 small molecules and identified two small molecule inhibitors (SMIs), named SMI-1 and SMI-6. Both compounds bound to the extracellular domain (ECD) of the PRL receptor (PRLR) at 1–3 micromolar affinity and abrogated PRL-induced breast cancer cell (BCC) invasion and malignant lymphocyte proliferation. SMI-6 effectively reduced the viability of multiple BCC types, had much lower activity against various non-malignant cells, displayed high selectivity, and showed no apparent in vitro or in vivo toxicity. In athymic nude mice, SMI-6 rapidly and dramatically suppressed the growth of PRL-expressing BC xenografts. This report represents a pre-clinical phase of developing novel anti-cancer agents with the potential to become effective therapeutics in breast cancer patients.

Prolactin Receptor Isoforms as the Basis of Tissue-Specific Action of Prolactin in the Norm and Pathology

The review describes functional and structural features of different isoforms of prolactin receptor, mechanisms of signaling pathway activation, and molecular messengers involved in the transmission and termination of signal from the prolactin receptor isoforms. Changes in the ratio between prolactin receptor isoforms, key mediators of prolactin signal transduction and termination in various organs and tissues, are analyzed. Special attention is given to the role of molecular mediators and the ratio between the isoforms in normal physiological functions and pathologies. Approaches for therapeutic correction of prolactin signaling impairments are discussed.

Inhibition of prolactin with bromocriptine for 28days increases blood-brain barrier permeability in the rat

The blood-brain barrier (BBB) is necessary for the proper function of the brain. Its maintenance is regulated by endogenous factors. Recent evidences suggest prolactin (PRL) regulates the BBB properties in vitro, nevertheless no evidence of these effects have been reported in vivo. The aim of this study was to evaluate the role of PRL in the maintenance of the BBB in the rat. Male Wistar rats were treated with Bromocriptine (Bromo) to inhibit PRL production for 28days in the absence or presence of lipopolysaccharide (LPS). BBB permeability was evaluated through the Evans Blue dye and fluorescein-dextran extravasation as well as through edema formation. The expression of claudin-5, occludin, glial fibrillary acidic protein (GFAP) and the PRL receptor (PRLR) was evaluated through western blot. Bromo reduced the physiological levels of PRL at 28days. At the same time, Bromo increased BBB permeability and edema formation associated with a decrement in claudin-5 and occludin and potentiated the increase in BBB permeability induced by LPS. However, no neuroinflammation was detected, since the expression of GFAP was unchanged, as well as the expression of the PRLR. These data provide the first evidence that inhibition of PRL with Bromo affects the maintenance of the BBB through modulating the expression of tight junction proteins in vivo.

A major prolactin-binding complex on human milk fat globule membranes contains cyclophilins A and B: the complex is not the prolactin receptor

Prolactin (PRL) in milk influences maturation of gastrointestinal epithelium and development of both the hypothalamo-pituitary and immune systems of offspring. Here, we demonstrate that most PRL in human milk is part of a novel, high-affinity, multicomponent binding complex found on the milk fat globule membrane and not in whey. To examine properties of the complex, a sensitive ELISA was developed such that human PRL (hPRL) binding to the complex was measured by loss of hPRL detectability; thus, as much as 50 ng of hPRL was undetectable in the presence of 10 μl of human milk. Using the same methodology, no comparable complex formation was observed with human serum or amniotic fluid. hPRL complexation in milk was rapid, time dependent, and cooperative. Antibodies to or competitors of the hPRL receptor (placental lactogen and growth hormone) showed the hPRL receptor was not involved in the complex. However, hPRL complexation was antagonized by cyclosporine A and anti-cyclophilins. The complex was very stable, resisting dissociation in SDS, urea, and dithiothreitol. Western analysis revealed an ∼75-kDa complex that included hPRL, cyclophilins A and B, and a 16-kDa cyclophilin A. Compared with noncomplexed hPRL, complexed hPRL in whole milk showed similar activation of STAT5 but markedly delayed activation of ERK. Alteration of signaling suggests that complex formation may alter hPRL biological activity. This is the first report of a unique, multicomponent, high-capacity milk fat reservoir of hPRL; all other analyses of milk PRL have utilized defatted milk.