Mutational analysis of the ligand-binding domain of the prolactin receptor

Cloning, expression of, and evidence of positive selection for, the prolactin receptor gene in Chinese giant salamander (Andrias davidianus)

Prolactin receptor (PRLR) is a protein associated with reproduction in mammals and with osmoregulation in fish. In this study, the complete length of Chinese giant salamander Andrias davidianus prolactin receptor (AD-prlr) was cloned. Andrias davidianus prlr expression was high in the kidney, pituitary, and ovary and low in other examined tissues. The AD-prlr levels were higher in ovary than in testis, and increased in ovaries with age from 1 to 6 years. To determine effect of exogenous androgen and aromatase inhibitor on AD-prlr expression, methyltestosterone (MT) and letrozole (LE) were injected, resulting in decreased AD-prlr in both brain and ovary, with MT repressing prlr transcription more rapidly than did LE. The molecular evolution of prlr was assessed, and found to have undergone a complex evolution process. The obranch-site test detected four positively selected sites in ancestral lineages prior to the separation of mammals and birds. Fourteen sites underwent positive selection in ancestral lineages of birds and six were positively selected in amphibians. The site model showed that 16, 7, and 30 sites underwent positive selection in extant mammals, amphibians, and birds, respectively. The positively selected sites in amphibians were located outside the transmembrane domain, with four in the extracellular and three in the intracellular domain, indicating that the transmembrane region might be conserved and essential for protein function. Our findings provide a basis for further studies of AD-prlr function and molecular evolution in Chinese giant salamander. J. Exp. Zool. (Mol. Dev. Evol.) 324B: 707-719, 2015. © 2015 Wiley Periodicals, Inc.

Extra-pituitary prolactin (PRL) and prolactin-like protein (PRL-L) in chickens and zebrafish

It is generally believed that in vertebrates, prolactin (PRL) is predominantly synthesized and released by pituitary lactotrophs and plays important roles in many physiological processes via activation of PRL receptor (PRLR), including water and electrolyte balance, reproduction, growth and development, metabolism, immuno-modulation, and behavior. However, there is increasing evidence showing that PRL and the newly identified 'prolactin-like protein (PRL-L)', a novel ligand of PRL receptor, are also expressed in a variety of extra-pituitary tissues, such as the brain, skin, ovary, and testes in non-mammalian vertebrates. In this brief review, we summarize the recent research progress on the structure, biological activities, and extra-pituitary expression of PRL and PRL-L in chickens (Gallus gallus) and zebrafish (Danio rerio) from our and other laboratories and briefly discuss their potential paracrine/autocrine roles in non-mammalian vertebrates, which may promote us to rethink the broad spectrum of PRL actions previously attributed to pituitary PRL only.

Molecular mechanisms of prolactin and its receptor

Prolactin and the prolactin receptors are members of a family of hormone/receptor pairs which include GH, erythropoietin, and other ligand/receptor pairs. The mechanisms of these ligand/receptor pairs have broad similarities, including general structures, ligand/receptor stoichiometries, and activation of several common signaling pathways. But significant variations in the structural and mechanistic details are present among these hormones and their type 1 receptors. The prolactin receptor is particularly interesting because it can be activated by three sequence-diverse human hormones: prolactin, GH, and placental lactogen. This system offers a unique opportunity to compare the detailed molecular mechanisms of these related hormone/receptor pairs. This review critically evaluates selected literature that informs these mechanisms, compares the mechanisms of the three lactogenic hormones, compares the mechanism with those of other class 1 ligand/receptor pairs, and identifies information that will be required to resolve mechanistic ambiguities. The literature describes distinct mechanistic differences between the three lactogenic hormones and their interaction with the prolactin receptor and describes more significant differences between the mechanisms by which other related ligands interact with and activate their receptors.

Molecular characterization, mRNA expression of prolactin receptor (PRLR) gene during pregnancy, nonpregnancy in the yak (Bos grunniens)

Prolactin (PRL) plays central roles in a wide range of body functions in mammals, and the actions are mediated by the specific cell surface receptor, the prolactin receptor (PRLR). To better understand the role of PRL in the yak (Bos grunniens), in the present study, we first cloned yak PRLR cDNA, and compared its mRNA expression in several tissues with cattle (Bos taurus). By reverse transcriptase-polymerase chain reaction (RT-PCR) strategy, we obtained full-length of yak PRLR cDNA sequence comprised of an open reading frame of 1746bp encoding a 581 amino acid protein, and contained a signal sequence and a transmembrane region. The intracellular domain had two pairs of cysteine residues and a WSXWS motif. The cytoplasmic domain comprised 323 residues and contained box 1 sequence. The yak PRLR shared 66.0-98.5% protein sequence identity with mammalian homologs. Real-time PCR analysis revealed that PRLR mRNA was higher in mammary tissue than in ovary and endometrium (P<0.01). During pregnancy, the ovary and mammary PRLR mRNA expression increased by 33- and 2.9-fold in yak, respectively, and increased by 46- and 3.8-fold in cattle, respectively. PRLR mRNA expression was higher (P<0.05) in mammary tissue and ovary of pregnant cow than that of pregnant yak. It is proposed that the increased ovarian and mammary PRLR mRNA expression during pregnancy may be associated with corpus luteum function for maintenance of pregnancy and mammary development for subsequent lactation.

Crystal structure of an affinity-matured prolactin complexed to its dimerized receptor reveals the topology of hormone binding site 2

We report the first crystal structure of a 1:2 hormone.receptor complex that involves prolactin (PRL) as the ligand, at 3.8-A resolution. Stable ternary complexes were obtained by generating affinity-matured PRL variants harboring an N-terminal tail from ovine placental lactogen, a closely related PRL receptor (PRLR) ligand. This structure allows one to draw up an exhaustive inventory of the residues involved at the PRL.PRLR site 2 interface, consistent with all previously reported site-directed mutagenesis data. We propose, with this description, an interaction model involving three structural components of PRL site 2 ("three-pin plug"): the conserved glycine 129 of helix alpha3, the hydrogen bond network involving surrounding residues (glycine cavity), and the N terminus. The model provides a molecular basis for the properties of the different PRL analogs designed to date, including PRLR antagonists. Finally, comparison of our 1:2 PRL.PRLR(2) structure with those of free PRL and its 1:1 complex indicates that the structure of PRL undergoes significant changes when binding the first, but not the second receptor. This suggests that the second PRLR moiety adapts to the 1:1 complex rather than the opposite. In conclusion, this structure will be a useful guiding tool for further investigations of the molecular mechanisms involved in PRLR dimerization and activation, as well as for the optimization of PRLR antagonists, an emerging class of compounds with high therapeutic potential against breast and prostate cancer.

Intramolecular disulfide bonds of the prolactin receptor short form are required for its inhibitory action on the function of the long form of the receptor

The short form (S1b) of the prolactin receptor (PRLR) silences prolactin-induced activation of gene transcription by the PRLR long form (LF). The functional and structural contributions of two intramolecular disulfide (S-S) bonds within the extracellular subdomain 1 (D1) of S1b to its inhibitory function on the LF were investigated. Mutagenesis of the paired cysteines eliminated the inhibitory action of S1b. The expression of the mutated S1b (S1bx) on the cell surface was not affected, indicating native-like folding of the receptor. The constitutive JAK2 phosphorylation observed in S1b was not present in cells expressing S1bx, and JAK2 association was disrupted. BRET(50) (BRET(50) represents the relative affinity as acceptor/donor ratio required to reach half-maximal BRET [bioluminescence resonance energy transfer] values) showed decreased LF/S1bx heterodimeric-association and increased affinity in S1bx homodimerization, thus favoring LF homodimerization and prolactin-induced signaling. Computer modeling based on the PRLR crystal structure showed that minor changes in the tertiary structure of D1 upon S-S bond disruption propagated to the quaternary structure of the homodimer, affecting the dimerization interface. These changes explain the higher homodimerization affinity of S1bx and provide a structural basis for its lack of inhibitory function. The PRLR conformation as stabilized by S-S bonds is required for the inhibitory action of S1b on prolactin-induced LF-mediated function and JAK2 association.

Prolactin-dependent modulation of organogenesis in the vertebrate: Recent discoveries in zebrafish

The scientific literature is replete with evidence of the multifarious functions of the prolactin (PRL)/growth hormone (GH) superfamily in adult vertebrates. However, little information is available on the roles of PRL and related hormones prior to the adult stage of development. A limited number of studies suggest that GH functions to stimulate glucose transport and protein synthesis in mouse blastocytes and may be involved during mammalian embryogenesis. In contrast, the evidence for a role of PRL during vertebrate embryogenesis is limited and controversial. Genes encoding GH/PRL hormones and their respective receptors are actively transcribed and translated in various animal models at different time points, particularly during tissue remodeling. We have addressed the potential function of GH/PRL hormones during embryonic development in zebrafish by the temporary inhibition of in vivo PRL translation. This treatment caused multiple morphological defects consistent with a role of PRL in embryonic-stage organogenesis. The affected organs and tissues are known targets of PRL activity in fish and homologous structures in mammalian species. Traditionally, the GH/PRL hormones are viewed as classical endocrine hormones, mediating functions through the circulatory system. More recent evidence points to cytokine-like actions of these hormones through either an autocrine or a paracrine mechanism. In some situations they could mimic actions of developmentally regulated genes as suggested by experiments in multiple organisms. In this review, we present similarities and disparities between zebrafish and mammalian models in relation to PRL and PRLR activity. We conclude that the zebrafish could serve as a suitable alternative to the rodent model to study PRL functions in development, especially in relation to organogenesis.

Unmodified Prolactin (PRL) and S179D PRL-Initiated Bioluminescence Resonance Energy Transfer between Homo- and Hetero-Pairs of Long and Short Human PRL Receptors in Living Human Cells

We have used bioluminescence resonance energy transfer (BRET) to examine the interaction between human prolactins (PRLs) and the long (LF) and two short isoforms (SF1a and SF1b) of the human PRL receptor in living cells. cDNA sequences encoding the LF, SF1a, and SF1b were subcloned into codon-humanized vectors containing cDNAs for either Renilla reniformis luciferase (Rluc) or a green fluorescent protein (GFP2) with a 12- or 13-amino acid linker connecting the parts of the fusion proteins. Transfection into human embryonic kidney 293 cells demonstrated maintained function of Rluc and GFP2 when linked to the receptors, and confocal microscopy demonstrated the localization of tagged receptors in the plasma membrane by 48 h after transfection. All three tagged receptors transduced a signal, with the LF and SF1a stimulating, and SF1b inhibiting, promoter activity of an approximately 2.4-kb β-casein-luc construct. Both unmodified PRL (U-PRL) and the molecular mimic of phosphorylated PRL, S179D PRL, induced BRET with all combinations of long and short receptor isoforms except SF1a plus SF1b. No BRET was observed with the site two-inactive mutant, G129R PRL. This is the first demonstration, 1) that species homologous PRL promotes both homo- and hetero-interaction of most long and short PRLR pairs in living cells, 2) that both U-PRL and S179D PRL are active in this regard, and 3) that there is some aspect of SF1a-SF1b structure that prevents this particular hetero-receptor pairing. In addition, we conclude that preferential pairing of different receptor isoforms is not the explanation for the different signaling initiated by U-PRL and S179D PRL.

Seasonal environmental changes modulate the prolactin receptor expression in an eurythermal fish

Eurythermal fish have evolved compensatory responses to the cyclical seasonal changes of the environment. The complex adaptive mechanisms include the transduction of the physical parameters variations into molecular signals. Studies in carp have indicated that prolactin and growth hormone expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in the generation of the homeostatic rearrangement that occurs in this adaptive process. Here, we report the cloning and characterization of a full-length carp prolactin receptor cDNA, which codes for the long form of the protein resembling that found in mammalian prolactin receptors. We identified up to three receptor transcript isoforms in different tissues of the teleost and assessed cell- and temporal-specific transcription and protein expression in carp undergoing seasonal acclimatization. The distinctive pattern of expression that carp prolactin receptor (cPRLr) depicts upon seasonal acclimatization supports the hypothesis that prolactin and its receptor are clearly involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat.

Characterization of a novel and functional human prolactin receptor isoform (deltaS1PRLr) containing only one extracellular fibronectin-like domain

Prolactin (PRL)-dependent signaling occurs as the result of ligand-induced homodimerization of the PRL receptor (PRLr). To date, short, intermediate, and long human PRLr isoforms have been characterized. To investigate the expression of other possible human PRLr isoforms, RT-PCR was performed on mRNA isolated from the breast carcinoma cell line T47D. A 1.5-kb PCR fragment was isolated, subcloned, and sequenced. The PCR product exhibited a nucleotide sequence 100% homologous to the human long isoform except bp 71-373 were deleted, which code for the S1 motif of the extracellular domain. Therefore, this isoform was designated the deltaS1 PRLr. Northern analysis revealed variable deltaS1 PRLr mRNA expression in a variety of tissues. Transfection of Chinese hamster ovary cells with deltaS1 cDNA showed the isoform is expressed at the protein level on the cell surface with a molecular mass of approximately 70 kDa. Kinetic studies indicated the deltaS1 isoform bound ligand at a lower affinity than wild-type receptor. The deltaS1 PRLr was also shown to activate the proximal signaling molecule Jak2 upon addition of ligand to transfected cells, and, unlike the long PRLr, high concentrations of ligand did not function as a self-antagonist to signaling during intervals of PRL serum elevation, i.e. stress and pregnancy. Given its apparent widespread expression, this PRLr isoform may contribute to PRL action. Furthermore, the functionality of this receptor raises interesting questions regarding the minimal extracellular domain necessary for ligand-induced receptor signaling.

PRL-induced ERalpha gene expression is mediated by Janus kinase 2 (Jak2) while signal transducer and activator of transcription 5b (Stat5b) phosphorylation involves Jak2 and a second tyrosine kinase

In the rat corpus luteum of pregnancy, PRL stimulation of ER expression is a prerequisite for E2 to have any luteotropic effect. Previous work from our laboratory has established that PRL stimulates ERalpha expression at the level of transcription and that the transcription factor Stat5 (signal transducer and activator of transcription 5) mediates this stimulation. Since it is well established that PRL activates Stat5 through the tyrosine kinase, Janus kinase 2 (Jak2), the role of Jak2 in PRL regulation of ERalpha expression was investigated. In primary luteinized granulosa cells, the general tyrosine kinase inhibitors, genistein and AG18, and the Jak2 inhibitor, AG490, prevented PRL stimulation of ERalpha mRNA levels, suggesting that PRL signaling to the ERalpha gene requires Jak2 activity. However, using an antibody that recognizes the tyrosine-phosphorylated forms of both Stat5a and Stat5b (Y694/Y699), it was found that AG490 could inhibit PRL-induced Stat5a phosphorylation only and had little or no effect on Stat5b phosphorylation. These effects of AG490 were confirmed in COS cells overexpressing Stat5b. Also in COS cells, a kinase-negative Jak2 prevented PRL stimulation of ERalpha promoter activity and Stat5b phosphorylation while a constitutively active Jak2 could stimulate both in the absence of PRL. Furthermore, kinase-negative-Jak2, but not AG490, could inhibit Stat5b nuclear translocation and DNA binding. Therefore, it seems that in the presence of AG490, Stat5b remains phosphorylated, is located in the nucleus and capable of binding DNA, but is apparently transcriptionally inactive. These findings suggest that PRL may activate a second tyrosine kinase, other than Jak2, that is capable of phosphorylating Stat5b without inducing transcriptional activity. To investigate whether another signaling pathway is involved, the src kinase inhibitor PP2 and the phosphoinositol-3 kinase inhibitor (PI3K), LY294002, were used. Neither inhibitor alone had any major effect on PRL regulation of ERalpha promoter activity or on PRL-induced Stat5b phosphorylation. However, the combination of AG490 and LY294002 largely prevented PRL-induced Stat5b phosphorylation. These findings indicate that PRL stimulation of ERalpha expression requires Jak2 and also that PRL can induce Stat5b phosphorylation through two tyrosine kinases, Jak2 and one downstream of PI3K. Furthermore, these results suggest that the role of Jak2 in activating Stat5b may be through a mechanism other than simply inducing Stat5b phosphorylation.

Cloning, characterization, and tissue distribution of prolactin receptor in the sea bream (Sparus aurata)

The prolactin receptor (PRLR) was cloned and its tissue distribution characterized in adults of the protandrous hermaphrodite marine teleost, the sea bream (Sparus aurata). An homologous cDNA probe for sea bream PRLR (sbPRLR) was obtained by RT-PCR using gill mRNA. This probe was used to screen intestine and kidney cDNA libraries from which two overlapping clones (1100 and 2425 bp, respectively) were obtained. These clones had 100% sequence identity in the overlapping region (893 bp) and were used to deduce the complete amino acid sequence of sbPRLR. The receptor spans 2640 bp and encodes a protein of 537 amino acids. Features characteristic of PRLR, two pairs of cysteines, WS box, hydrophobic transmembrane domain, box 1, and box 2, were identified and showed a high degree of sequence identity to PRLRs from other vertebrate species. SbPRLR is 29 and 32% identical to tilapia (Oreochromis niloticus) and goldfish (Carassius auratus) PRLRs, respectively. In the sea bream two PRLR transcripts of 2.8 and 3.2 kb were detected in the intestine, kidney, and gills and a single transcript of 2.8 kb was detected in skin and pituitary by Northern blot. Spermiating gonads (more than 95% male tissue; gonado-somatic index of 0.6) contained, in addition to the 2.8-kb transcript, three more transcripts of 1.9, 1.3, and 1.1 kb. RT-PCR, which is a far more sensitive method than Northern blot, detected PRLR mRNA in gills, intestine, brain, pituitary, kidney, liver, gonads, spleen, head-kidney, heart, muscle, and bone. Immunohistochemistry using specific polyclonal antibodies raised against an oligopeptide from the extracellular domain of sbPRLR detected PRLR in several epithelial tissues of juvenile sea bream, including the anterior gut, renal tubule, choroid membrane of the third ventricle, saccus vasculosus, branchial chloride cells, and branchial cartilage.

Newt prolactin and its involvement in reproduction

The amino acid sequence of newt (Cynops pyrrhogaster) prolactin deduced from the nucleotide sequence of its cDNA showed a relatively high homology with sequences of chicken and sea turtle prolactins as well as with those of anuran prolactins. Cynops prolactin receptor transcripts were detected in various tissues and organs, suggesting that prolactin plays multiple roles in urodeles. Urodele prolactin was purified from the pituitaries of C. pyrrhogaster. Antiserum against this prolactin was used for radioimmunoassay of plasma prolactin and immunoneutralization experiments. Endogenous prolactin was shown to induce migration to water, courtship behavior, and cessation of spermatocytogenesis in the Cynops newt. The hormone was found to be involved in the development of cloacal glands such as the lateral and abdominal glands, growth of the tail and Mauthner neurons, secretion of oviducal jelly, and enhanced synthesis of a female attracting pheromone (sodefrin), and responsiveness of the olfactory epithelium to sodefrin. In most of these cases, prolactin was found to act synergistically or antagonistically with sex steroids. We also discovered that hypersecretion of prolactin in the newts subjected to cold temperature was induced by hypothalamic stimulation rather than release from hypothalamic inhibition.Key words: prolactin, newts, reproduction.

Sequence and functional characterisation of the marmoset monkey (Callithrix jacchus) prolactin receptor: comparative homology with the human long-form prolactin receptor

This study demonstrates the cloning and in-vitro characterisation of the marmoset monkey (Callithrix jacchus) prolactin receptor cDNA. The marmoset prolactin receptor cDNA was generated by reverse transcription-polymerase chain reaction using adrenal RNA and primers designed from prolactin receptor conserved regions. Sequence analysis predicts a mature protein of 598 amino acids exclusive of the 24 amino acid signal peptide. The marmoset prolactin receptor cDNA shares 93 and 61% base pair, and 89 and 61% amino acid sequence homologies with the long form human and rat prolactin receptor cDNA, respectively. The marmoset prolactin receptor cDNA sequence retains all the receptor sequences that have been shown previously to be essential for ligand binding, structural integrity and signal transduction. Transfection of human 293 fibroblast cells with the marmoset prolactin receptor cDNA (three independent experiments) confirmed the expression of a receptor that has high binding affinity to human growth hormone (K(a)=3.6+/-0.07 nM(-1) and B(max)=7.55+/-2.06x10(-11) M) and human prolactin (K(a)=3.1+/-0.12 nM(-1) and B(max)=2.87+/-0.66x10(-11) M). Functionality of the receptor was assessed by co-transfection of 293 fibroblast cells with marmoset prolactin receptor cDNA and the Jak2 cDNA, or marmoset prolactin receptor and a Stat5 responsive element linked to the luciferase coding sequence. Incubation of the cells with 18 nM ovine prolactin resulted in rapid phosphorylation of Jak2 as ascertained by Western blotting. In addition, the marmoset prolactin receptor cDNA led to 9.06+/-0.47-fold induction of luciferase gene activity. This was comparable with the induction observed following transfection with the human prolactin receptor cDNA (8.55+/-0. 5-fold). In-vivo prolactin receptor expression in the marmoset monkey was assessed by ribonuclease protection assay and detected in a number of tissues including female reproductive organs. These data confirm the cloning and functionality of the marmoset prolactin receptor cDNA. The marmoset prolactin receptor shares a high sequence homology with the long-form human prolactin receptor, and both receptors bind hormones with comparable affinity and confer a similar intracellular response. The marmoset monkey may provide a useful tool to investigate the role of prolactin in primate reproduction.

Binding of leukemia inhibitory factor (LIF) to mutants of its low affinity receptor, gp190, reveals a LIF binding site outside and interactions between the two cytokine binding domains

The gp190 transmembrane protein, the low affinity receptor for the leukemia inhibitory factor (LIF), belongs to the hematopoietin family of receptors characterized by the cytokine binding domain (CBD). gp190 is one of the very few members of this family to contain two such domains. The membrane-proximal CBD (herein called D2) is separated from the membrane-distal one (called D1) by an immunoglobulin-like (Ig) domain and is followed by three fibronectin type III repeats. We used truncated gp190 mutants and a blocking anti-gp190 monoclonal antibody to study the role of these repeats in low affinity receptor function. Our results showed that the D1Ig region was involved in LIF binding, while D2 appeared to be crucial for the proper folding of D1, suggesting functionally important interactions between the two CBDs in the wild-type protein. In addition, a point mutation in the carboxyl terminus of the Ig region strongly impaired ligand binding. These findings suggest that at least two distinct sites, both located within the D1Ig region, are involved in LIF binding to gp190, and more generally, that ligand binding sites on these receptors may well be located outside the canonical CBDs.

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Prolactin: a hormone at the crossroads of neuroimmunoendocrinology

Prolactin (PRL), secreted by the pituitary, decidua, and lymphoid cells, has been shown to have a regulatory role in reproduction, immune function, and cell growth in mammals. The effects of PRL are mediated by a membrane-bound receptor that is a member of the superfamily of cytokine receptors. Formation of a trimer, consisting of one molecule of ligand and two molecules of receptor, appears to be a necessary prerequisite for biological activity. The function of these receptors is mediated, at least in part, by two families of signaling molecules: Janus tyrosine kinases (JAKs) and signal transducers and activators of transcription (STATs). To study these receptors, we have used two approaches: mutational analysis of their cytoplasmic domains coupled with functional tests and inactivation (knockout) of the receptor gene by homologous recombination in mice. We have produced mice by gene targeting in embryonic stem cells carrying a germline null mutation of the prolactin receptor gene. Heterozygous (+/-) females show almost complete failure to lactate, following their first, but not subsequent pregnancies. Homozygous (-/-) females are infertile as a result of multiple reproductive abnormalities, including ovulation of premiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Half of the homozygous males are infertile or show reduced fertility. In view of the wide-spread distribution of PRL receptors, other phenotypes including those on the immune system, are currently being evaluated in -/- animals. This study establishes the prolactin receptor as a key regulator of mammalian reproduction and provides the first total ablation model to further study the role of the prolactin receptor and its ligands.

N-glycosylation of the prolactin receptor is not required for activation of gene transcription but is crucial for its cell surface targeting

The functional importance of the three oligosaccharide chains linked to Asn35, Asn80 and Asn108, of the long form of the PRL receptor (PRLR) was investigated by individual or multiple substitutions of asparagyl residues using site-directed mutagenesis and transient transfection of these mutated forms of PRLR in monkey kidney cells, Chinese hamster ovary, and human 293 fibroblast cells that exhibit different levels of protein expression. Scatchard analysis performed on monkey kidney cells revealed that the mutants possess the same affinity for PRL as compared with wild-type PRLR. A strong reduction (90%) of the aglycosylated PRLR expression at the cell surface of monkey kidney or human 293 cells was observed. Immunohistochemistry experiments using an anti-PRLR monoclonal antibody showed an accumulation of the deglycosylated receptor in the Golgi area of transfected monkey kidney cells. Upon PRL stimulation, the aglycosylated PRLR associated with Janus kinase 2 was phosphorylated and was able to activate a beta-casein gene promoter in transfected 293 fibroblast cells. The active form of the PRLR was thus acquired independently of glycosylation. By contrast, no functional activity was detectable in transfected Chinese hamster ovary cells that expressed low levels of PRLR. These studies demonstrate that the glycosylation on the asparagyl residues of the extracellular domain of the PRLR is crucial for its cell surface localization and may affect signal transduction, depending on the cell line.

Identification of cytoplasmic motifs required for short prolactin receptor internalization

Cloning of rat prolactin receptor (PRLR) cDNAs revealed the existence of two isoforms, termed short and long according to the length of their cytoplasmic domain. Internalization studies show, first, that PRLR internalization is hormone-dependent and, second, that ligand-receptor complexes of the short PRLR are internalized to a larger extent compared to the long form. In order to identify regions within the cytoplasmic domain of the short PRLR required for efficient internalization, serial truncations of the cytoplasmic tail were performed by inserting a stop codon in place of those encoding residues 282, 273, 262, 253, 244, or 237 (wild type short PRLR contains 291 amino acids). Our data show that two motifs, lying within residues 253-261 and 273-281, are involved in internalization. Both regions contain a consensus feature identified within other receptors as internalization signals, namely a di-leucine peptide (amino acids 259-260) and a tetrapeptide predicted to adopt a beta-turn structure (amino acids 276-279). We propose these two motifs are involved in PRLR endocytosis. Finally, we show that alpha-adaptin, a component of adaptor protein AP-2, coprecipitates with short PRLR complexes upon PRL stimulation, which strongly suggests that PRLR internalization is mediated by the clathrin-coated pits endocytotic pathway.

Null mutation of the prolactin receptor gene produces multiple reproductive defects in the mouse

Mice carrying a germ-line null mutation of the prolactin receptor gene have been produced by gene targeting in embryonic stem cells. Heterozygous females showed almost complete failure of lactation attributable to greatly reduced mammary gland development after their first, but not subsequent, pregnancies. Homozygous females were sterile owing to a complete failure of embryonic implantation. Moreover, they presented multiple reproductive abnormalities, including irregular cycles, reduced fertilization rates, defective preimplantation embryonic development, and lack of pseudopregnancy. Half of the homozygous males were infertile or showed reduced fertility. This work establishes the prolactin receptor as a key regulator of mammalian reproduction, and provides the first total ablation model to further study the role of the prolactin receptor and its ligands.

Cytokine receptor signal transduction and the control of hematopoietic cell development

The cytokine receptor superfamily is characterized by structural motifs in the exoplasmic domain and by the absence of catalytic activity in the cytosolic segment. Activated by ligand-triggered multimerization, these receptors in turn activate a number of cytosolic signal transduction proteins, including protein tyrosine kinases and phosphatases, and affect an array of cellular functions that include proliferation and differentiation. Molecular study of these receptors is revealing the roles they play in the control of normal hematopoiesis and in the development of disease.

Identification of a critical ligand binding determinant of the human erythropoietin receptor. Evidence for common ligand binding motifs in the cytokine receptor family

The erythropoietin receptor (EPOR) is a member of a family of cytokine and growth factor receptors that share conserved features in their extracellular and cytoplasmic domains. We have used site-specific mutagenesis within the extracellular domain of the EPOR to search for amino acid residues involved in erythropoietin (EPO) binding. Mutant proteins were expressed in bacteria as soluble EPO binding proteins (EBP) and characterized for EPO binding activity in a number of different assays. Substitution of phenylalanine at position 93 (Phe93) with alanine (F93A mutation) resulted in a drastic reduction in EPO binding in the EBP. More conservative tyrosine or tryptophan substitutions at Phe93 resulted in much less dramatic effects on EPO binding. Biophysical studies indicated that the F93A mutation does not result in gross structural alterations in the EBP. Furthermore, the F93A mutation in full-length EPOR expressed in COS cells abolished detectable EPO binding. This was not a result of processing or transport defects, since mutant receptor was present on the surface of the cells. Mutations in the region immediately around Phe93 and in residues homologous to other reported ligand binding determinants of the cytokine receptor family had small to moderate effects on EPO binding. These data indicate that Phe93 is a critical EPO binding determinant of the EPOR. Furthermore, since Phe93 aligns with critical ligand binding determinants in other receptors of the cytokine receptor family, these data suggest that receptors of this family may use common structural motifs to bind their cognate ligands.

Preparation and characterization of recombinant prolactin receptor extracellular domain from rat

Complementary (c)DNA of the extracellular domain of rat prolactin receptor (rPRLR-ECD) was cloned in the prokaryotic expression vector pTrc99A, and expressed in Escherichia coli following induction with isopropyl-b-D-thiogalactopyranoside. The expressed rPRLR-ECD protein, contained within the refractile body pellet was solubilized in 4.5 M urea, refolded and purified on a Q-Sepharose column by stepwise elution with NaCl. Only approximately 10% of the expressed protein refolded as a monomeric fraction, yielding 5-6 mg/l of induced culture. The purified protein was over 98% homogeneous, as shown by SDS-PAGE in the presence or absence of reducing agent, and by chromatography on a Superdex column. Its molecular mass, determined by SDS-PAGE in the absence of reducing agent, was 28 kDa, and by gel filtration, 25.6 kDa. Binding experiments indicated high affinity for bovine placental lactogen (bPL) and human growth hormone (hGH) as compared to ovine (o) or rat PRLs. Gel filtration was used to determine the stoichiometry of rPRLR-ECD's interaction with these hormones. At a 5 microM initial concentration of the hormones, formation of 2:1 (ECD:ligand) complexes was detected with bPL, hGH and oPRL whereas only 1:1 complex was formed with rPRL. Dilution (25-fold) of these complexes did not affect the stoichiometry with bPL, whereas with hGH a clear tendency towards dissociation of the initial 2:1 complex to 1:1 complex was observed. This tendency was even stronger in the case of oPRL. Although all four hormones exhibited nearly identical activities in the Nb2-11C lymphoma cell bioassay, the ability of the purified rat or rabbit PRLR-ECD to inhibit hormonal mitogenic activity generally reflected their affinity for the respective hormones. In view of these and former results, we suggest that unlike in the GH:GHR-ECD interaction, the inability of lactogenic hormones to form a 1:2 complex with soluble recombinant PRLR-ECDs does not necessarily predicts lack of biological activity.

Immunocytochemical localization of prolactin receptors in rat liver cells: I. Dependence on sex and sex steroids

The peculiarities of cellular and tissue distribution of prolactin receptors (PRLR) in the liver of female and male rats with different sex steroid status were investigated in paraplast-embedded tissue with the indirect immunoperoxidase technique. Two clones of antibodies directed outside the PRL-binding site (U6) or to the PRL-binding site (T6) of the receptor were used. PRLR-specific immunoreactivity was identified essentially in hepatocytes. PRLR can be visualized in sinusoidal domains of cellular membranes, in cytoplasmic granules and sometimes in the perinuclear area of hepatocytes. The staining characteristics were similar with both antibodies. There were no prominent differences in the intensity of PRLR-positive staining among hepatocytes of different zones of hepatic lobules with the exception of some hepatocytes around central veins. Sex differences in the intensity of immunostaining (strong in females, and faint in males) but not in the amount and distribution of PRLR-containing cells were observed. Gonadectomy of animals caused the disappearance of sex differences in the intensity of PRLR-positive staining as a result of its decrease in females and increase in males. The essential elevation in the intensity of PRLR-specific immunoreactivity was revealed in hepatocytes of gonadectomized females and males after prolonged estradiol administration (10 micrograms for 14 days). The cytoplasmic staining of some hepatocytes surrounding central veins was much more pronounced in estrogenized animals. PRLR-specific immunoreactivity in the perinuclear area was identified in these cells. Androgen treatment (3 mg of testosterone-propionate for 3 days) of gonadectomized animals caused a decrease in the intensity of hepatocyte PRLR-positive staining similarly in both sexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the full-length rabbit prolactin receptor and its specific domains in baculovirus infected insect cells

The prolactin receptor is a membrane protein mainly involved in the development of the mammary gland and in lactation in mammals. We used specific cDNA constructs and the insect/baculovirus expression system and produced independently and in large amounts several recombinant forms of the rabbit mammary gland prolactin receptor: the full-length receptor (L1, L2), a truncated membrane form (S), a secretable form of the extracellular domain (E) and two forms of the intracellular domain (I1, I2). Of these forms, the L1 and L2 are associated with the membrane fraction, the E is predominantly secreted into the medium and the I1 and I2 are expressed as soluble proteins and surprisingly, a great portion accumulates in the culture medium. The molecular mass (94 kDa) of the expressed full-length receptor corresponds to the translation product of the entire cDNA coding region. The receptor biochemically identified in the rabbit mammary gland is however much shorter. Thus, in the mammary gland, the receptor presumably undergoes post-translational modifications. The receptor forms L1, L2 and S bind prolactin with specificity and affinity similar to those reported for the native receptor. They also interact with two monoclonal antibodies, M110 and A917, specific for the native conformation of the hormone-binding site. The I1 and I2 forms do not bind prolactin, whereas the E form does. Thus, the hormone binding site is located in the extracellular domain which can function autonomously as a PRL-binding soluble protein. However, the E form binds prolactin with a higher affinity than the native receptor and it does not bind one of the two antireceptor monoclonal antibodies, known to be hormone binding-site specific. Thus, the conformation of the native receptor and that of the E form differ.

Molecular cloning of the bovine prolactin receptor and distribution of prolactin and growth hormone receptor transcripts in fetal and utero-placental tissues

We have isolated a bovine prolactin (bPRL) receptor cDNA from an endometrial cDNA library, which predicts a 557 amino acid transmembrane protein similar to the long forms of other characterized prolactin receptors. The predicted cytoplasmic domain is slightly truncated primarily by a stop codon located 36 codons 5' from the stop utilized in the human hepatic transcript. When expressed in COS cells, this cDNA was shown to encode a protein which bound bovine placental lactogen (bPL) and bPRL with nearly equal affinity (KD for bPL, 2.03 x 10(-10) M; bPRL, 3.07 x 10(-10) M). Northern analysis demonstrated multiple transcripts, with maternal liver, corpus luteum, intestine, endometrium and fetal liver containing a major transcript of about 3.8 kb, and maternal corpus luteum and endometrium, a second sized transcript of apparently equal abundance of 4.4 kb. This difference did not appear to be within the coding region. Primer extension analysis of maternal hepatic and endometrial transcripts revealed considerable heterogeneity. Examination of the distribution of prolactin and growth hormone receptor transcripts at mid-pregnancy by semi-quantitative reverse transcriptase polymerase chain reaction showed that both are widespread in bovine fetal and placental tissues. This isolation of bovine prolactin receptor cDNA, and description of receptor distribution will facilitate study of the action of the placental and pituitary members of this gene family during pregnancy.

Protein hormones and their receptors

Technological advances in the isolation and characterization of novel receptors have led to a significant increase in our understanding of protein-ligand binding to receptors and the means by which responses are triggered. Hormones and their receptors are composed of structurally conserved domains, and several ligands appear to use similar surface regions for receptor binding. A key event in signal transduction is the aggregation by the ligand of one or more receptor subunits, and this can include the sharing of subunits between different ligands. These findings have allowed the design of ligands with receptor-antagonist properties.