Insensitivity of Human Prolactin Receptors to Nonhuman Prolactins: Relevance for Experimental Modeling of Prolactin Receptor-Expressing Human Cells

Abstract

Prolactin (PRL) receptors are expressed in a broad range of human cell types and in a majority of human breast and prostate cancers. Experimentally, normal and malignant human cells are typically cultured in vitro in media containing bovine PRL (bPRL) from fetal bovine serum or as xenotransplants in vivo in the presence of murine PRL (mPRL). The biological efficacy of bPRL toward hPRL receptors (hPRLR) is controversial, and hPRLR are insensitive to mPRL, but the mechanism is not known. To clarify limitations of current in vitro and in vivo experimental model systems for studies of hPRLR-expressing cells, we tested human and relevant subprimate prolactins in multiple hPRLR bioassays. bPRL and ovine PRL were 10-fold less potent hPRLR agonists than hPRL, although maximal responses at high ligand concentrations (efficacies) equaled that of hPRL. mPRL and rat PRL had greater than 50-fold lower potencies toward hPRLR than hPRL and had 50% reduced efficacies. In fact, mPRL and rat PRL were less effective hPRLR agonists than murine GH. Unexpectedly, mPRL was an effective competitive inhibitor of hPRL binding to hPRLR with an inhibitory constant of 1.3 nm and showed partial antagonist activity, suggesting reduced site-2 binding. Collectively, low bioactivities of bPRL and mPRL toward hPRLR suggest that existing laboratory cancer cell lines grown in 10% bovine serum-supplemented media or in mice are selected for growth under lactogen-depleted conditions. The biology and drug responsiveness of existing human cell lines may therefore not be representative of clinical cancers that are sensitive to circulating PRL.

Prolactin-Induced Prostate Tumorigenesis

“…Interestingly, lobe-specific differences in Stat5 activation are not observed when wild-type mice are acutely injected with PRL and analyzed shortly after ~ 1 h, suggesting that, at least in Pb-Prl mice, the lobe-specific variations in Stat5 activation must rely on regulatory mechanisms resulting from long-term exposure to Prl. Prostate lobe differences in response to chronic hyperprolactinemia have also been described in rat models [81] with no real understanding of the underlying mechanisms.…”

Assessment of Prolactin and Quantitative Milk Production After Induction of Lactation in Barren Jennies (Equus asinus): A Pilot Study

“…Several causes may contribute to assay variability, starting with the different standards used by individual assays, as not all nonhuman PRL formulations are equally recognized by assay antibodies ePRL, likewise porcine PRL shared uniquely low potencies to human PRL (hPRL) receptor, but efficacies at high ligand concentrations were similar to those of hPRL. Intriguingly, ePRL and porcine PRL have the highest overall percent amino acid identity, with hPRL (80%) among multiple species of PRL tested (mouse: 59%, rat: 62%, ovine, bovine: 73% PRL) [35].…”

Modeling Prolactin Actions in Breast Cancer In Vivo: Insights from the NRL-PRL Mouse

“…In combination with the other lactogenic hormones (placental lactogens, primate growth hormone) [38,39], it is clear that examination of circulating PRL alone underestimates the exposure of mammary tissue to PRLR agonists.…”