Prolactin-like activity of anti-prolactin receptor antibodies on casein and DNA synthesis in the mammary gland

Anti-Idiotypic Agonistic Antibodies: Candidates for the Role of Universal Remedy

Anti-idiotypic antibodies (anti-IDs) were discovered at the very beginning of the 20th century and have attracted attention of researchers for many years. Nowadays, there are five known types of anti-IDs: α, β, γ, ε, and δ. Due to the ability of internal-image anti-IDs to compete with an antigen for binding to antibody and to alter the biologic activity of an antigen, anti-IDs have become a target in the search for new treatments of autoimmune illnesses, cancer, and some other diseases. In this review, we summarize the data about anti-IDs that mimic the structural and functional properties of some bioregulators (autacoids, neurotransmitters, hormones, xenobiotics, and drugs) and evaluate their possible medical applications. The immune system is potentially able to reproduce or at least alter the effects of any biologically active endogenous or exogenous immunogenic agent via the anti-idiotypic principle, and probably regulates a broad spectrum of cell functions in the body, being a kind of universal remedy or immunacea, by analogy to the legendary ancient goddess of universal healing Panacea (Πανάκεια, Panakeia in Greek) in the treatment and prevention of diseases, possibly including non-infectious somatic and even hereditary ones.

Development of a Novel, Anti-idiotypic Monoclonal Anti-prolactin Antibody That Mimics the Physiological Functions of Prolactin

In this work, we prepared a panel of monoclonal anti-idiotypic antibodies to ovine prolactin (oPRL) by the hybridoma technique. Among these antibodies, one anti-idotypic antibody (designated B7) was chosen for further characterization by a series of experiments. We first demonstrated that B7 behaved as a typical Ab2β based on a series of enzyme-linked immunosorbent assays. Subsequently, the results of a competitive receptor-binding assay confirmed that B7 could specifically bind to the prolactin receptor (PRLR) expressed on target cells. Finally, we examined its biological activities in CHO-PRLR and Nb2 cells and observed that B7 could activate Janus kinase 2-signal transducer and activator of transcription signalling in CHO-PRLR and Nb2 cells and induce BaF3 proliferation. The present study suggests that i) B7 can serve as a PRLR agonist or PRL mimic and has potential applications in regulating mammary gland development, milk production and maintenance of lactation in domestic animals and ii) B7 may be a biological reagent that can be used to explore the mechanism of PRLR-mediated intracellular signalling.

Different intracellular signalling pathways triggered by an anti-prolactin receptor (PRLR) antibody: Implication for a signal-specific PRLR agonist

In this work, we prepared a panel of monoclonal antibodies directed against prolactin receptor (PRLR) using the hybridoma technique. Of these monoclonal antibodies (Mabs), the Mab designated B6 was chosen for further characterization based on its biological activity. We first demonstrated that B6 can specifically bind to the prolactin receptor (PRLR) expressed on target cells by immunoprecipitation and Western blotting analysis. Subsequently, epitope mapping studies using a competitive receptor-binding assay indicated that B6 epitopes partially overlapped with those of prolactin (PRL). We then examined the resulting signal transduction pathways activated by this antibody in T-47D and CHO-PRLR cells and found that B6 induced different intracellular signalling compared with prolactin, which activates serine-threonine kinase (AKT), extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription1 (STAT1) and STAT3 but not STAT5. The present study suggests that: (a) B6 may be a signal-specific prolactin receptor (PRLR) agonist; (b) B6 may be a biological reagent that can be used to explore the mechanism of PRLR-mediated intracellular signalling. In addition, this work also implies a strategy for preparing signal-specific cytokine agonists.

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.

Characterization of a rat uterine cell line, U(III) cells: prolactin (PRL) expression and endogenous regulation of PRL-dependent genes; estrogen receptor beta, alpha(2)-macroglobulin, and decidual PRL involving the Jak2 and Stat5 pathway

Decidualization of endometrial stroma in the rat induces the expression and secretion of rat decidual PRL (rdPRL). Recently, we have generated a nontransformed rat uterine stromal cell line (U(III)) that decidualizes spontaneously in culture. In this report, we have established by immunocytochemistry, RT-PCR, Western blot analysis, labeled amino acid incorporation and RIA that these cells express the rat PRL messenger RNA as well as synthesize and secrete PRL. We have also cloned by RT-PCR a 403-bp complementary DNA fragment whose sequence is identical with that of rat pituitary PRL. In addition, U(III) cells express the PRL receptor (PRL-R) long form, all the components involved in the PRL signal transduction pathway, estrogen receptor beta (ER beta) and alpha(2)-macroglobulin (alpha(2)-MG), which are known to be PRL-regulated genes. However, when U(III) cells were treated with PRL, no regulation of these genes was observed. Moreover, in these cells, the PRL signaling components: the tyrosine kinase Jak2 and the transcription factor Stat5 were endogenously phosphorylated and their phosphorylation states were not enhanced in the presence of exogenous PRL. To examine whether the endogenously secreted PRL affects the expression of PRL-regulated genes, U(III) cells were treated with either an anti-PRL receptor antibody or a Jak2 inhibitor, AG490. The anti-PRL receptor antibody decreased alpha(2)-MG expression. AG490 inhibited Jak2 and Stat5 phosphorylation, prevented Stat5 binding to its DNA consensus sequence, and also caused a dose-dependent down-regulation of alpha(2)-MG and ER beta expression. In contrast, AG490 enhanced PRL mRNA levels. In summary, we have established that the U(III) stromal cells of uterine origin produce PRL. Furthermore, we have shown for the first time that decidual PRL may act locally to activate the Jak2/Stat5 pathway and up-regulate important genes involved in decidual growth and placentation.

Immunological manipulation of body composition

Hormonal growth promoters (e.g., growth hormone [GH], beta-adrenergic agonists, steroids), which improve growth rate and/or lean:fat ratios in the carcass, have received considerable adverse publicity and are either banned or have no licence for their use in the EC. This has led to the development of a number of techniques, involving the use of antibodies, aimed at regulating metabolic processes involved in determining growth and body composition. A different approach to the problem of excess fat deposition involves the use of antibodies directed against the plasma membranes of adipocytes in order to elicit their destruction and thereby limit the capacity for fat storage. This technique has been demonstrated in rats, sheep and pigs in both passive and active immunisation techniques. This promising approach is, however, limited by the lack of a commercially suitable vaccine. The identification of individual membrane proteins that are antigenic has been achieved and this affords the prospect of producing recombinant DNA-derived vaccines. Whether these new approaches will be perceived as acceptable to the general public remains a serious concern and a potential limitation to their development as many would-be sponsors withdraw their support for research in these areas.

Growth hormone treatment induces mammary gland hyperplasia in aging primates

The decline of growth hormone (GH) and insulin-like growth factor I (IGF-I) production during aging has been likened to the decrease in gonadal steroids in menopause. The repletion of GH/IGF-I levels in aging individuals is suggested to restore the lean tissue anabolism characteristic of youth. In addition to anabolic effects on musculo-skeletal tissues, GH also stimulates mammary glandular growth in some species, although its effects on primate mammary growth remain unclear. Some clinical observations implicate GH in human mammary growth, for example, gynecomastia occurs in some children treated with GH (ref. 6), and tall stature and acromegaly are associated with an increased incidence of breast cancer. To investigate the effects of GH/IGF-I augmentation on mammary tissue in a model relevant to aging humans, we treated aged female rhesus monkeys with GH, IGF-I, GH + IGF-I or saline diluent for 7 weeks. IGF-I treatment was associated with a twofold increase, GH with a three- to fourfold increase, and GH + IGF-I with a four'-to fivefold increase in mammary glandular size and epithelial proliferation index. These mitogenic effects were directly correlated with circulating GH and IGF-I levels, suggesting that either GH or its downstream effector IGF-I stimulates primate mammary epithelial proliferation.

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.

Identification and functional activity of prolactin receptors in thymic epithelial cells

Thymic epithelial cells (TECs), the major component of the thymic microenvironment, can be modulated by pituitary hormones. We have shown previously that prolactin (PRL) can influence the endocrine activity of TECs and stimulate TEC proliferation as well as cytokeratin expression, suggesting the existence of PRL receptors on TECs. Using a series of monoclonal antibodies (mAbs) to the extracellular domain of the rat liver PRL receptor, we have demonstrate that rat TECs bear specific receptors for PRL, as assessed by immunoblotting as well as by immunocytochemistry experiments. Using a probe specific for the long form of PRL receptor, mRNAs of 6.7 and 10.1 kilobases were detected, although by immunoblot the major protein in TECs had a molecular mass of 43 kDa. Functionally, these mAbs were able to modulate thymulin secretion, as well as TEC proliferation. Moreover, the mAbs cross-reacted with human TECs and were able to mimic the action of PRL on these cells. These data bring further support for the general concept of the neuroendocrine immune circuit and extend the notion for a pleiotropic role of PRL as an immunomodulatory hormone.

Partial proteolytic digestion of the mammary prolactin receptor: identification of smaller prolactin binding fragments

Partial proteolytic digestion of the mammary prolactin (PRL) receptor was used to generate receptor fragments and analyze their immunoreactivity and PRL binding properties. Tryptic digestion of the PRL receptor produced two immunoreactive fragments (Mr approximately 30,000 and approximately 15,000) that reacted with a monoclonal anti-PRL receptor antibody and still specifically bound PRL, while the complete immunoreactive PRL binding unit (Mr approximately 42,000) disappeared. Neither chymotrypsin nor V8 protease were able to generate any immunoreactive receptor fragments. These receptor fragments may represent smaller PRL binding receptor form(s) of biological significance.

Monoclonal antibody detection of prolactin-binding subunits in the rabbit mammary gland

The structure of prolactin (PRL) receptor in the rabbit mammary gland was examined using a receptor-specific monoclonal antibody (MAb). The PRL receptor preparation used was purified by making use of a PRL-affinity column. MAb inhibited the binding of PRL to the receptor, in a dose-dependent manner and completely at a high concentration. Using the receptor directly labelled by 125I, the preparation was incubated with MAbs and the immune complex was collected by Pansorbin and examined by SDS/polyacrylamide-gel electrophoresis. The autoradiography showed that three species with apparent Mr values of 77,000, 41,000 and 25,000 specifically reacted with MAbs. The pattern changed little in the presence or absence of dithiothreitol. Western blot analysis showed that two species (Mr 77,000 and 41,000) reacted with MAb. Affinity labelling of the receptor with labelled PRL revealed three bands with Mr values of 96,000, 60,000 and 43,000 on SDS gels. The high-Mr complex (Mr greater than 200,000) was always present at the top of the gel. These results show that the mammary gland contains at least three PRL-binding subunits. The differences in Mr before and after PRL binding were close to the Mr of PRL. This would suggest that each PRL binding subunit reacts with one PRL molecule.

Anti-growth action on mouse mammary and prostate glands of a monoclonal antibody to prolactin receptor

Monoclonal antibody (PrR-7A) against purified PRL receptor was used in the following studies. When PRL receptor was chromatographed on affinity columns containing PrR-7A antibody or monoclonal antibody against hemocyanin, which served as a control, PRL receptor was bound to the column containing PrR-7A antibody, but not to the column containing control antibody. When solubilized PRL receptor was incubated with PrR-7A antibody, the specific binding of the receptor was reduced 52%. Female mice were treated with the carcinogen, 7,12-dimethylbenz[a]anthracene, and during the succeeding 48 weeks were treated weekly with PrR-7A antibody or control antibody. In the control group 13% developed mammary carcinomas, and 16% developed moderate-to-severe intraductal hyperplasia. No mammary carcinomas were found in the mice treated with PrR-7A antibody, and only 8% of the mice had moderate-to-severe intraductal hyperplasia. Male mice made hyperprolactinemic by implanted pituitary glands were treated weekly with PrR-7A or control antibody. After 7 weeks of treatment, the mean weight of the prostates of mice treated with PrR-7A antibody was 8 +/- 1.1 mg (mean +/- SE), and that of mice treated with control antibody was 27 +/- 3.6 mg. Similar differences were seen in the protein and DNA content of the prostates. These results indicate that PrR-7A antibody is directed against PRL receptor and that immunization with this antibody reduces the incidence of PRL-dependent mammary tumors and preneoplastic ductal hyperplasia and prevents PRL-induced hyperplasia of the prostate.

Immunological recognition of the prolactin receptor: identification of a single binding unit of molecular weight approximately 42,000

Different polyclonal and monoclonal antibodies against the rabbit mammary prolactin (PRL) receptor were previously obtained that totally inhibited PRL binding in the rabbit mammary gland. Only polyclonal antibodies were shown to immunoprecipitate preformed PRL--receptor complexes in solubilized mammary membranes suggesting that they also recognized domains outside of the PRL binding site of the receptor. When partially purified PRL receptor preparations from both rabbit and pig mammary tissues were iodinated, immunoprecipitated and subsequently analyzed by SDS--PAGE, a single component of molecular weight approximately 42,000 was specifically recognized by all the anti-PRL receptor antibodies. This unit was the only component immunoprecipitated by the monoclonal antibody M 110. Its identification was not impaired by using reducing or non-reducing conditions. Moreover, a further purification of the [125I]-labeled receptor preparations from both species by a second PRL affinity chromatography selected a single binding unit of the same molecular weight. In contrast, polyclonal antibodies immunoprecipitated additional components apart from the 42,000 unit, especially one unit of molecular weight 70,000-80,000 in both species. We conclude that rabbit and pig mammary PRL receptors exhibit striking immunological similarities. Both contain a single binding unit of molecular weight approximately 42,000 that is not linked to other units via disulfide bridges. This binding unit could be associated with a larger component of MW 70,000-80,000 in the holo receptor.

Hormonal action controlling mammary activity

Mammary gland differentiation includes multiplication of cells, activation of genes specific to milk synthesis, and activation of "house-keeping" genes. These events are controlled by multiple hormones, the roles of which are not known in detail. Prolactin induction of milk synthesis is accompanied by accumulation of casein messenger ribonucleic acid resulting from acceleration of casein gene transcription as well as stabilization of messenger ribonucleic acid. Prolactin also favors translation of casein messenger ribonucleic acid. Glucocorticoids amplify and progesterone inhibits prolactin action on transcription of casein genes.

Target cell prolactin, II

Is the entry hypothesis compatible with all the existing data about "the" second messenger for prolactin listed in Section II? All of these messengers, in some way either participate in, or modify, prolactin's actions or, in an end point-dependent manner, may actually mimic prolactin. There remains considerable uncertainty as to whether these findings reflect phenomena, some independent of and others quite dependent upon entry, on the one hand, or merely portions of a relatively large number of molecular cascades, some (but not necessarily all) begun initially at the plasmalemma and many (if not all) orchestrated toward completion by intracellular prolactin or agonist-receptor complex.

Characterization of prolactin receptors in pig mammary gland

Prolactin receptors present in the particulate fraction of lactating pig mammary gland were solubilized by 7.5mM-3-[(3-cholamidopropyl)dimethylammonio]-1-propane-su lph onic acid (Chaps) and purified by affinity chromatography on prolactin coupled to Affi-Gel 10. Nearly 30% of the particulate receptors were solubilized by the detergent and over a 1000-fold purification from homogenates was achieved. A water-soluble fraction rich in receptors was observed during the preparation of membranes, although this fraction has not yet been purified. Prolactin binding to the receptors was a time-dependent, reversible and saturable reaction in particulate, Chaps-solubilized and purified receptors. In all forms, receptors showed the same specificity to peptide hormones. Prolactin and human growth hormone bound to the same receptors, whereas bovine growth hormone, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone and insulin failed to bind. After solubilization, the dissociation constant (Kd) for prolactin was decreased 5-fold from 9.8 X 10(-11) M in the particulate receptors to 1.8 X 10(-11) M in solubilized and purified receptors, being due principally to an increase in the association rate constant from 1.0 X 10(9)M-1 X h-1 to (3.9-4.6) X 10(9)M-1 X h-1, respectively, with the dissociation rate constant remaining unchanged at (1.1-1.3) X 10(-2)h-1. Isoelectric focusing of the prolactin-receptor complex revealed two peaks, one at a pI of 5.5-5.6 and the other at 5.2-5.3. Microsomal receptors were covalently cross-linked to 125I-labelled ovine prolactin with ethylene glycol bis(succinimidyl succinate) and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Autoradiography of the gel revealed a major subunit of Mr 28 000-35 000 and a minor one of Mr 67 000-69 000. Anti-(prolactin receptor) antibodies raised against rabbit mammary gland prolactin receptors were equally effective in inhibiting prolactin binding to particulate, solubilized and affinity-purified receptors, suggesting that purified prolactin receptors have a structure indistinguishable immunologically from particulate receptors and rabbit mammary gland prolactin receptors. The present demonstration shows that particulate prolactin receptors from a domestic animal can be solubilized and purified without losing the original properties of high affinity and binding specificity for hormones.

Interferon receptor interaction. Internalization of interferon alpha 2 and modulation of its receptor on human cells

Studies reported earlier [ Joshi et al. (1982) J. Biol. Chem. 257, 13884-13887] have indicated that human interferon-alpha 2 (HuIFN-alpha 2) binds to a specific macromolecular receptor on human cells as identified by cross-linking with bifunctional cross-linking reagents and analysis by polyacrylamide gel electrophoresis. We have carried out experiments to investigate the fate of the interferon-receptor complex on the cell surface under conditions which lead to cellular response. As analyzed by cross-linking and gel electrophoresis, the interferon-receptor complex, formed on incubation with 125I-IFN-alpha 2 at 4 degrees C, persisted at the cell surface for several hours at 4 degrees C; however, if the cells were switched to 37 degrees C, there was a rapid decline in the complex, apparently due to a loss of the interferon receptors from the cell surface. This was associated with an internalization of the 125I-interferon as indicated by the fact that, on incubation at 37 degrees C, an appreciable fraction of the cell-associated interferon (approximately equal to 50%) became resistant to trypsin digestion, or dissociation on incubation in growth medium or low-pH buffer. A large fraction of the trypsin-resistant (internalized) 125I-labeled material migrated as intact interferon in polyacrylamide gels, and it was immunoprecipitated by anti-(HuIFN-alpha)antibodies but not by anti-(HuIFN-beta)antibodies. The bulk of the internalized 125I-interferon was recovered in a particulate fraction and, on cross-linking with disuccinimidyl suberate, a 150000-Mr complex could be detected. The results suggest that interferon may be internalized as a complex with the receptor, which may account for the loss of the interferon-receptors on the cell surface. This modulation of the IFN-alpha/beta receptors was induced by HuIFN-alpha and HuIFN-beta but not by HuIFN-gamma. The recovery of the IFN-alpha/beta receptors, lost upon incubation with HuIFN-alpha, took several hours and required protein synthesis. The significance of the results is discussed.

The effect of age on the number of pituitary cells immunoreactive to growth hormone and prolactin

The results of a combined immunocytochemical, morphometric, and clinicopathologic analysis of growth-hormone-producing and prolactin-producing pituitary cells in 28 subjects ranging in age from 16 to 90 are reported. There was a significant age-related decline in the number and size of growth-hormone-producing cells, which was most marked in the transition from youth to middle age. There was also a significant age-related decline in the number of pituitary parenchymal cells but not in pituitary weight. Prolactin cells did not show a significant decline in number with age.

Immunoreactive prolactin in subcellular fractions from bovine mammary tissue

Mammary tissue from lactating Holsteins at slaughter was homogenized and fractionated into subcellular compartments. Bovine prolactin was quantified in each subcellular fraction and in preslaughter milk by radioimmunoassay procedures validated for this purpose. Prolactin concentrations were ng prolactin/mg protein for all samples. Immunoreactive prolactin was in rough endoplasmic reticulum (24.8 ng/mg), Golgi apparatus (18.8 ng/mg), and secretory vesicle (10.2 ng/mg). All of these concentrations exceeded that in samples of milk (2.7 ng/mg). Nuclear fractions contained a material that reacted in the prolactin radioimmunoassay, but criteria for valid quantification could not be satisfied. These results confirm prolactin inside bovine mammary cells, and they are consistent with an intracellular pathway for transfer of prolactin from plasma into milk.