Development of new prolactin analogs acting as pure prolactin receptor antagonists

Discovery and characterization of prolactin neutralizing monoclonal antibodies for the treatment of female-prevalent pain disorders

Prolactin (PRL) has recently been demonstrated to elicit female-selective nociceptor sensitization and increase pain-like behaviors in female animals. Here we report the discovery and characterization of first-in-class, humanized PRL neutralizing monoclonal antibodies (PRL mAbs). We obtained two potent and selective PRL mAbs, PL 200,031 and PL 200,039. PL 200,031 was engineered as human IgG1 whereas PL 200,039 was reformatted as human IgG4. Both mAbs have sub-nanomolar affinity for human PRL (hPRL) and produce concentration-dependent and complete inhibition of hPRL signaling at the hPRL receptor (hPRLR). These two PRL mAbs are selective for hPRL as they do not inhibit other hPRLR agonists such as human growth hormone or placental lactogen. They also cross-react with non-human primate PRL but not with rodent PRL. Further, both mAbs show long clearance half-lives after intravenous administration in FcRn-humanized mice. Consistent with their isotypes, these mAbs only differ in binding affinities to Fcγ receptors, as expected by design. Finally, PL 200,019, the murine parental mAb of PL 200,031 and PL 200,039, fully blocked stress-induced and PRL-dependent pain behaviors in female PRL-humanized mice, thereby providing in vivo preclinical proof-of-efficacy for PRL mAbs in mechanisms relevant to pain in females.

Suppression of Prolactin Secretion Partially Explains the Antidiabetic Effect of Bromocriptine in ob/ob Mice

Previous studies have shown that bromocriptine mesylate (Bromo) lowers blood glucose levels in adults with type 2 diabetes mellitus; however, the mechanism of action of the antidiabetic effects of Bromo is unclear. As a dopamine receptor agonist, Bromo can alter brain dopamine activity affecting glucose control, but it also suppresses prolactin (Prl) secretion, and Prl levels modulate glucose homeostasis. Thus, the objective of the current study was to investigate whether Bromo improves insulin sensitivity via inhibition of Prl secretion. Male and female ob/ob animals (a mouse model of obesity and insulin resistance) were treated with Bromo and/or Prl. Bromo-treated ob/ob mice exhibited lower serum Prl concentration, improved glucose and insulin tolerance, and increased insulin sensitivity in the liver and skeletal muscle compared with vehicle-treated mice. Prl replacement in Bromo-treated mice normalized serum Prl concentration without inducing hyperprolactinemia. Importantly, Prl replacement partially reversed the improvements in glucose homeostasis caused by Bromo treatment. The effects of the Prl receptor antagonist G129R-hPrl on glucose homeostasis were also investigated. We found that central G129R-hPrl infusion increased insulin tolerance of male ob/ob mice. In summary, our findings indicate that part of Bromo effects on glucose homeostasis are associated with decrease in serum Prl levels. Because G129R-hPrl treatment also improved the insulin sensitivity of ob/ob mice, pharmacological compounds that inhibit Prl signaling may represent a promising therapeutic approach to control blood glucose levels in individuals with insulin resistance.

Development of a new anti-prolactin receptor (PRLR) antibody, F56, which can serve as a PRLR antagonist

In this work, we developed a new prolactin receptor (PRLR) antagonist using the hybridoma technique. A series of monoclonal antibodies against prolactin receptor (PRLR) was prepared, from which we characterized and selected one anti-PRLR antibody, F56. Epitome mapping showed that F56 and prolactin (PRL) share a common binding epitope on PRLR, and therefore, F56 could compete with prolactin (PRL) for binding to PRLR. Subsequent experiments indicated that F56 could effectively neutralize PRLR-mediated intracellular signalling molecules, such as signal transducer and activator of transcription (STAT) and extracellular signal-regulated kinase-1 and kinase 2 (ERK1/2), either by endogenously expressed PRLR or in a cell model transfected with PRLR. In addition, further experiments showed that F56 could effectively inhibit PRL-induced cell proliferation. The current study suggests that F56 has potential applications in PRLR-related studies.

Prolactin actions

Molecular genetics and other contemporary approaches have contributed to a better understanding of prolactin (PRL) actions at the cellular and organismal levels. In this review, several advances in knowledge of PRL actions are highlighted. Special emphasis is paid to areas of progress with consequences for understanding of human PRL actions. The impacts of these advances on future research priorities are analyzed.

Growth hormone and prolactin regulate human neural stem cell regenerative activity

We have previously shown that the growth hormone (GH)/prolactin (PRL) axis has a significant role in regulating neuroprotective and/or neurorestorative mechanisms in the brain and that these effects are mediated, at least partly, via actions on neural stem cells (NSCs). Here, using NSCs with properties of neurogenic radial glia derived from fetal human forebrains, we show that exogenously applied GH and PRL promote the proliferation of NSCs in the absence of epidermal growth factor or basic fibroblast growth factor. When applied to differentiating NSCs, they both induce neuronal progenitor proliferation, but only PRL has proliferative effects on glial progenitors. Both GH and PRL also promote NSC migration, particularly at higher concentrations. Since human GH activates both GH and PRL receptors, we hypothesized that at least some of these effects may be mediated via the latter. Migration studies using receptor-specific antagonists confirmed that GH signals via the PRL receptor promote migration. Mechanisms of receptor signaling in NSC proliferation, however, remain to be elucidated. In summary, GH and PRL have complex stimulatory and modulatory effects on NSC activity and as such may have a role in injury-related recovery processes in the brain.

Structural characterization of the stem-stem dimerization interface between prolactin receptor chains complexed with the natural hormone

The most promising approach to targeting the tumor-growth-promoting actions of prolactin (PRL) mediated by its autocrine/paracrine pathway has been the development of specific PRL receptor (PRLR) antagonists. However, the optimization of such antagonists requires a thorough understanding of the activation mechanism of PRLR. We have thus conducted a systematic X-ray crystallographic study in order to visualize the successive steps of PRLR activation by PRL. We report here the structure at 3.35 Å resolution of the 1:2 complex between natural PRL and two PRLR chains (PRLR1 and PRLR2), corresponding to the final activated state of PRLR. Further than our previously published structure involving an affinity-matured PRL variant, this structure allowed to visualize for the first time the loop L5 spanning PRLR2 residues Thr133-Phe140, revealing its central implication for the three intermolecular interfaces of the complex. We equally succeeded in obtaining a comprehensive picture of the PRLR-PRLR dimerization interface, also called stem-stem interface. Site-directed mutagenesis was conducted to probe the energetic importance of stem-stem contacts highlighted by the structure. Surprisingly, in spite of significant structural differences between the PRL/PRLR(2) complex and the 1:2 growth hormone/growth hormone receptor complex, our mutational data suggest that hot-spot residues that stabilize the receptor dimerization interface are equivalent in the two complexes. This study provides a new overall picture of the structural features of PRLR involved in stabilizing its complex with PRL.

Insensitivity of human prolactin receptors to nonhuman prolactins: relevance for experimental modeling of prolactin receptor-expressing human cells

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.

From bench to bedside: future potential for the translation of prolactin inhibitors as breast cancer therapeutics

A role for prolactin (PRL) in the pathogenesis of breast cancer has been confirmed at the cellular level in vitro, with multiple transgenic and knockout models in vivo, and within sizable patient populations through epidemiologic analysis. It is the obvious "next step" that these findings are translated into meaningful therapies to block PRL/PRLr function in human breast cancer. Several broad categories of PRL/PRLr antagonists are discussed in their pre-clinical context, including inhibitors of endocrine PRL elaboration, mutant ligand antagonists, ligand chimeras, and inhibitors of PRL-induced signaling and transactivation. The clinical potential for GHr antagonists are also discussed. These varied approaches all have demonstrated as proof-of-principle that PRL/PRLr antagonism can inhibit the in vitro and in vivo growth of breast cancer. Further pre-clinical development is required for most, however, before translation to clinical trials in breast cancer patients can occur.

Prolactin expression is induced in Jurkat T-cells by beta-catenin LEF-1, AP-1 and cAMP

Prolactin (PRL) in humans is produced in the pituitary as well as in extra-pituitary sites. A proximal promoter that requires the Pit-1 transcription factor controls pituitary PRL expression, whereas a distal (upstream) promoter located at 5.8 kb upstream of the pituitary start site regulates extra-pituitary PRL synthesis. We have previously reported that cAMP regulates PRL transcription in Jurkat lymphocytes in part through a cAMP responsive element. Here we demonstrate that additional PRL regulatory elements corresponding to LEF-l and AP-1 transcription factor binding sites appear important for PRL expression, since factor binding by EMSA and reporter gene expression are reduced when these sites are deleted or mutated. Interestingly, over-expression of a constitutively active form of beta-catenin increases PRL expression of Jurkat cells. This effect occurs through both LEF-dependent and -independent pathways. Our studies identify the distal PRL promoter as a target for beta-catenin, and reveal novel pathways regulating extra-pituitary PRL expression.

Therapeutic potential of S179D prolactin – from prostate cancer to angioproliferative disorders: the first selective prolactin receptor modulator

Increasing evidence suggests an important role for autocrine/paracrine prolactin in breast and prostate cancers and other disease states. Prolactin production in these extrapituitary sites is not governed by dopamine agonists, a finding that has spurred the production of prolactin receptor antagonists. This review focuses on one such antagonist, S179D prolactin, which was produced by mimicking a natural antagonist, phosphorylated prolactin. S179D prolactin is a very effective growth antagonist, partly because it inhibits signalling from unmodified prolactin and partly because it produces its own intracellular signal. This signal results in cell differentiation, cell-cycle arrest or apoptosis depending on dose, duration of treatment and cellular context. S179D prolactin is also a potent antiangiogenic and initial studies have shown it to be a potent anti-inflammatory agent. In light of these additional modes of action, it is suggested that S179D prolactin should now be more aptly referred to as a selective prolactin receptor modulator.

Prolactin modulates TRPV1 in female rat trigeminal sensory neurons

Sex dependency in pain perception is well documented and is thought to be attributable to the effect of reproductive hormones on nociceptive processing. In the present study, we evaluated whether estradiol alters gene transcription in the trigeminal ganglia (TG) of ovariectomized rats (OVX). These experiments demonstrated a dramatic (40-fold) upregulation of prolactin (PRL) expression in TG by 17-beta-estradiol (E2). PRL expression was restricted to TG neurons and was highly overlapped with transient potential receptor vanilloid type 1 (TRPV1) (approximately 90%) in TG. Additionally, PRL is released from neurons during stimulation. Both forms of PRL receptors (PRLRs), short and long, were also present in TG neurons. Moreover, expression of the long PRLRs was under control of estradiol. We next evaluated the novel hypothesis that PRL acts as a neuromodulator of sensory neurons. PRL pretreatment significantly enhanced capsaicin-evoked inward currents, calcium influx, and immunoreactive calcitonin gene-related peptide release from cultured TG neurons. This PRL modulation of capsaicin responses was abolished by withdrawal of E2 from TG cultures. Biochemical analysis demonstrated that PRL increased (>50%) phosphorylation levels of TRPV1 in TG. In a behavioral test, PRL pretreatment significantly potentiated capsaicin-evoked nocifensive behavior in female rats at proestrous and in OVX rats after E2 treatment. The in vivo potentiating effect of PRL on capsaicin responses was also dependent on E2. Collectively, these data demonstrate that PRL is a novel modulator of sensory neurons tightly regulated by E2. These findings are consistent with the hypothesis that PRL could contribute to the development of certain pain disorders, possibly including those modulated by estrogen.

Clinical Stage of Breast Cancer by Parity, Age at Birth, and Time Since Birth: A Progressive Effect of Pregnancy Hormones?

Breast cancer diagnosed during pregnancy or 1 to 2 years after birth often occurs at a late stage. Little is known about tumor characteristics in the high-risk period shortly after a childbirth. We here explore whether stage of disease differs according to timing of births. Results are based on 22,351 Norwegian breast cancer patients of parity 0 to 5, ages 20 to 74 years. The proportion of stage II to IV tumors was considerably higher among parous than nulliparous women at age <30 years (52.7% versus 36.8%, P=0.009), but similar or lower in other age groups (P(interaction)=0.029). In general, the largest proportion of stage II to IV tumors was found among women diagnosed during pregnancy or <2 years after birth. However, among women with late-age births (first or second birth >or=30 years, third birth >or=35 years), as well as women with an early second birth (<25 years), the proportion with advanced disease was rather similar or even higher among those diagnosed 2 to 6 years after birth (49.3-56.0%). The association between clinical stage and time since birth reached statistical significance among women with a late first or second birth and among all triparous women (P <or= 0.032). The subgroups with a high proportion of advanced disease 2 to 6 years after birth corresponded quite well to those previously found to have the most pronounced transient increase in risk after birth. Thus, pregnancy hormones may have a progressive effect on breast cancer tumors in addition to a possible promoting effect. A potential effect of prolactin is discussed.

Solution structure of human prolactin

We report the solution structure of human prolactin determined by NMR spectroscopy. Our result is a significant improvement over a previous structure in terms of number and distribution of distance restraints, regularity of secondary structure, and potential energy. More significantly, the structure is sufficiently different that it leads to different conclusions regarding the mechanism of receptor activation and initiation of signal transduction. Here, we compare the structure of unbound prolactin to structures of both the homologue ovine placental lactogen and growth hormone. The structures of unbound and receptor bound prolactin/placental lactogen are similar and no noteworthy structural changes occur upon receptor binding. The observation of enhanced binding at the second receptor site when the first site is occupied has been widely interpreted to indicate conformational change induced by binding the first receptor. However, our results indicate that this enhanced binding at the second site could be due to receptor-receptor interactions or some other free energy sources rather than conformational change in the hormone. Titration of human prolactin with the extracellular domain of the human prolactin receptor was followed by NMR, gel filtration and electrophoresis. Both binary and ternary hormone-receptor complexes are clearly detectable by gel filtration and electrophoresis. The binary complex is not observable by NMR, possibly due to a dynamic equilibrium in intermediate exchange within the complex. The ternary complex of one hormone molecule bound to two receptor molecules is on the contrary readily detectable by NMR. This is in stark contrast to the widely held view that the ternary prolactin-receptor complex is only transiently formed. Thus, our results lead to improved understanding of the prolactin-prolactin receptor interaction.

Prolactin and reproductive medicine

PURPOSE OF REVIEW

This review aims to summarize current knowledge about prolactin, and outlines recent information that affects the management of patients with hyperprolactinaemia.

RECENT FINDINGS

The actions of prolactin have been clarified by studies of prolactin-receptor-deficient mice, which have a clear phenotype of reproductive failure at multiple sites. The treatment of patients with hyperprolactinaemia or prolactinoma is largely achieved using dopamine agonist drugs, which induce the shrinkage of pituitary prolactinomas as well as control of the endocrine syndrome. Recent findings indicate that successful cabergoline treatment may be able to induce long-term remission, allowing drug withdrawal in a substantial proportion of patients.

SUMMARY

At present, dopamine agonist drugs remain the best treatment for hyperprolactinaemic patients, and can help most affected women achieve pregnancy. Future work is likely to help understand the basis of long-term remission in patients with pituitary prolactinomas.