Biological activities of binding site specific monoclonal antibodies to prolactin receptors of rabbit mammary gland

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.

Role of protein kinases in the prolactin-induced intracellular calcium rise in Chinese hamster ovary cells expressing the prolactin receptor

There is still only limited understanding of the early steps of prolactin signal transduction in target cells. It has been shown that prolactin actions are associated with cell protein phosphorylation, Ca2+ increases, and so on. However, the link between the activation of kinases and calcium influx or intracellular Ca2+ mobilization has not yet been clearly established. Chinese hamster ovary (CHO) cells, stably transfected with the long form of rabbit mammary gland prolactin receptor (PRL-R) cDNA were used for PRL-R signal transduction studies. Spectrofluorimetric techniques were used to measure intracellular calcium ([Ca2+]i) in cell populations with Indo1 as a calcium fluorescent probe. We demonstrate that, although protein kinase C activation (PMA or DiC8) caused a calcium influx in CHO cells, prolactin-induced PKC activation was not responsible for the early effect of prolactin on [Ca2+]i. Activation of protein kinase A (PKA) or protein kinase G did not modify [Ca2+]i and inhibition of PKA pathway did not affect the prolactin response. In the same way, phosphatidylinositol-3 kinaseinhibition had no effect on the prolactin-induced Ca2+ increase. On the other hand, tyrosine kinase inhibitors (herbimycin A, lavendustin A, and genistein) completely blocked the effect of prolactin on [Ca2+]i (influx and release). W7, a calmodulin-antagonist, and a specific inhibitor of calmodulin kinases (KN-62), only blocked prolactin-induced Ca2+ influx but had no significant effect on Ca2+ release. Using pharmacological agents, we present new data concerning the involvement of protein phosphorylations in the early effects of prolactin on ionic channels in CHO cells expressing the long form of PRL-R. Our results suggest that, at least in the very early steps of prolactin signal transduction, serine-threonine phosphorylation does not participate in the prolactin-induced calcium increase. On the other hand, tyrosine phosphorylation is a crucial, very early step, since it controls K+ channel activation, calcium influx, and intracellular calcium mobilization. Calmodulin acts later, since its inhibition only blocks the prolactin-induced Ca2+ influx.

Prolactin and prolactin receptors in the lacrimal gland

Light and electron microscopic immunocytochemistry, in situ hybridization and Dot Blot analysis revealed intracellular localization of prolactin-like molecules and prolactin mRNA in epithelial cells of the lacrimal glands of rabbits. There was also positive immunostaining for prolactin receptors on acinar cells and some interstitial cells. On Western blots of homogenates of whole lacrimal gland, isolated lacrimal acinar cells, isolated lacrimal interstitial cells and peripheral blood lymphocytes, prolactin antibody consistently labeled protein bands migrating at approximately 36 and 50 kD. These data confirm that lacrimal gland acinar cells produce endogenous prolactin-like molecules, but also express prolactin receptors. Since prolactin immunoreactivity has been detected in tear fluid and we found no accumulations of immunogold label in endocytic or transport vesicles, we hypothesize that the prolactin-like molecules in tear fluid originate primarily from synthesis within the acinar cells. We hypothesize further that prolactin from pituitary and other non-acinar cell origin has a modulating influence on acinar cell activity as well as immune function in the lacrimal gland, and that some of the prolactin-like molecules produced by the acinar cells contribute to these functions by autocrine/paracrine mechanisms.

Activation of chimeric and full-length growth hormone receptors by growth hormone receptor monoclonal antibodies. A specific conformational change may be required for full-length receptor signaling

Signal transduction by the growth hormone receptor (GHR) occurs through growth hormone (GH)-induced dimerization of two GHRs to form a trimeric complex. It is thought that dimerization alone is sufficient for signaling, since monoclonal antibodies (mAbs) against the extracellular domain of the GHR elicit proliferation of FDC-P1 cells transfected with a chimeric receptor comprising the extracellular domain of the GHR and the fibronectin and cytoplasmic domains of the murine granulocyte colony-stimulating factor receptor. We have screened 14 GHR mAbs for proliferative activity against characterized FDC-P1 and BaF-B03 cell lines stably expressing the full-length human, rabbit, or rat GHR, or the chimeric human GHR/granulocyte colony-stimulating factor receptor, and for transactivation of the c-fos promoter and STAT activation. With the chimeric receptor, eight mAbs were able to elicit proliferation, although there was no correlation between inhibition of hormone binding and agonist activity. In contrast, no mAbs were able to act as agonists with the full-length GHR FDC-P1 cell lines, although nine competed with GH for binding. A weak proliferative response was observed in the BaF-B03 cell lines with two of the mAbs (263 and 1C9), and the addition of anti-mouse F(ab)2 resulted in increased signaling in the hGHR BaF-B03 cell line to a plateau of 28 +/- 4% of the GH maximum for mAb 263. These data could indicate considerable stringency in the ability of mAbs to correctly dimerize the full-length GHR. However, the ability of mAb 263 to stimulate a mutant hGHR altered in the F'-G' loop of domain 2 was nearly abolished, concurrent with an increased affinity of this mAb for the receptor. Since the F'-G' loop undergoes a conformational change on GH binding and is necessary for full proliferative signaling, we propose that in addition to promoting receptor dimerization, mAb 263 may induce specific changes in receptor conformation similar to GH, which are required for the biological response.

Role of tyrosine phosphorylation in potassium channel activation. Functional association with prolactin receptor and JAK2 tyrosine kinase

Chinese hamster ovary (CHO) cells, stably transfected with the long form of the prolactin (PRL) receptor (PRL-R) cDNA, were used for PRL-R signal transduction studies. Patch-clamp technique in whole cell and cell-free configurations were employed. Exposure of transfected CHO cells to 5 nM PRL led to the increase of Ca(2+)- and voltage-dependent K+ channel (KCa) activity. The effect was direct as it was observed also in excised patch experiments. A series of tyrosine kinase inhibitors was studied to investigate the possible involvement of protein tyrosine kinases in KCa functioning and its stimulation by PRL. Genistein, lavendustin A, and herbimycin A decreased in a concentration and time-dependent manner the amplitude of the KCa current in whole cell and the open probability of KCa channels in cell-free experiments. The subsequent application of PRL was ineffective. The protein tyrosine phosphatase inhibitor orthovanadate (1 mM) stimulated KCa channel activity in excised patches, indicating that channels can be modulated in opposite directions by protein tyrosine kinase and protein tyrosine phosphatase. Moreover, in whole cell experiments as well as in excised patch recordings, anti-JAK2 tyrosine kinase antibody decreased the KCa conductance and the open probability of the KCa channels. Subsequent application of PRL was no longer able to stimulate KCa conductance. Immunoblotting studies using the same anti-JAK2 antibody, revealed the constitutive association of JAK2 kinase with PRL-R. Preincubation of anti-JAK2 antibody with the JAK2 Immunizing Peptide abolished the effects observed using anti-JAK2 antibody alone in both electrophysiological and immunoblotting studies. We conclude from these findings that these KCa channels are regulated through tyrosine phosphorylation/dephosphorylation; JAK2 tyrosine kinase, constitutively associated with PRL-R, is implicated in PRL stimulation of KCa channels.

Prolactin receptor antagonists that inhibit the growth of breast cancer cell lines

We investigated the mechanism of action of the human prolactin (hPRL) receptor on four different breast cancer cell lines, T-47D, MCF-7, BT-474, and SK-BR3, that express elevated levels of the receptor compared with normal cells. Cells treated with human growth hormone (hGH), which binds and activates the hPRL receptor, exhibited bell-shaped dose-response growth curves consistent with the sequential dimerization mechanism proposed for the hPRL receptor (Fuh, G., Colosi, P., Wood, W.I., and Wells, J.A. (1993) J. Biol. Chem. 268, 5376-5381). Growth stimulation was enhanced by Zn2+, which preferentially increases the affinity of hGH for the hPRL receptor. Furthermore, receptor-selective variants of hGH that bind the hPRL receptor but not the hGH receptor were agonistic, providing additional support that specific binding to the hPRL receptor can stimulate these breast cancer cells to grow. On this basis we produced variants of hGH and human placental lactogen (hPL) that were potential antagonists because they bind but do not dimerize the hPRL receptor. The hPL-based antagonist was less potent than the hGH-based antagonist toward the growth of MCF-7 cells, consistent with the lower affinity of hPL for hPRL receptor than for hGH. However, the hPL-based antagonist was more potent than the hGH antagonist for BT-474 cells. Antibodies to the hPRL receptor inhibited growth of FDC-P1 cells transfected with the hPRL receptor; these also inhibited MCF-7 cells and T47D cells but not BT-474 cells. A unique feature of BT-474 cells was found when screening its cDNA revealed the presence of a novel alternative splice of the hPRL receptor that codes for the soluble extracellular domain; this may explain these differential inhibitory effects. These studies provide further molecular insight into the potential role of the hPRL receptor in breast cancer and demonstrate that hPRL receptor antagonists can inhibit the growth of breast cancer cells.

The rabbit mammary gland prolactin receptor is tyrosine-phosphorylated in response to prolactin in vivo and in vitro

We report the first in vivo study demonstrating tyrosine phosphorylation of mammary gland proteins including the prolactin receptor, in response to the injection of prolactin. Immunoblotting of mammary gland membrane extracts revealed that subunits of 200, 130, 115, 100, 90, 70, and 45 kDa display increased tyrosine phosphorylation within 5 min of prolactin administration. The 100-kDa component was identified as the full-length prolactin receptor by a variety of means including immunoprecipitation and immunoblotting with monoclonal (U5, 917, 110, and 82) and polyclonal (46) antibodies to the prolactin receptor. Maximal receptor phosphorylation was seen within 1 min of hormone injection, and to obtain a strong response it was necessary to deprive rabbits of their endogenous prolactin for 36 h. Rapid tyrosine phosphorylation of the full-length receptor was verified by its demonstration in Chinese hamster ovary cells stably transfected with rabbit prolactin receptor cDNA. Both in vivo and in vitro, the phosphorylation signal was transient, being markedly reduced within 10 min of exposure to prolactin. Tyrosine-phosphorylated receptor was shown to be associated with JAK 2 by immunoblotting of receptor immunoprecipitated from transfected Chinese hamster ovary cells with polyclonal 46. A 48-kDa ATP-binding protein was also shown to be associated with the mammary gland receptor by U5 or polyclonal 46 immunoprecipitation of receptor complexes following covalent labeling with [alpha-32P]azido-ATP. Our demonstration of prolactin receptor tyrosine phosphorylation raises the possibility of signaling pathways regulated by receptor/SH2 protein interaction, which would facilitate prolactin specific responses. The fact that a period of hormone deprivation is needed for significant hormone triggered receptor phosphorylation indicates that the mammary gland receptor exists in a largely desensitized state in vivo, analogous to the related growth hormone receptor.

Prolactin induces growth inhibition and promotes differentiation of CHO cells stably transfected with prolactin receptor complementary DNA

We have characterized a stable and functional transfectant of the rabbit prolactin receptor in Chinese hamster ovary cells, and investigated the action of prolactin (PRL) on the growth and differentiation of this transfectant (clone E32). PRL induced a significant inhibition of E32 cell proliferation. Growth inhibition correlated with gene induction of the molecular marker of ovarian differentiation cholesterol side chain cleavage P450 (P450scc). Both effects were inversely proportional to cell confluence. The limits and potential development of such transfected cellular systems are discussed.

Prolactin-immune interactions in carcinogen-induced rat mammary tumors

Because many mammary tumors are prolactin (PRL) dependent, tumor-bearing animals are immunocompromised, and PRL directly affects the immune system, we examined the endocrine and immune systems of rats initiated with nitrosomethylurea (NMU) to cause mammary tumors. We tested: a) PRL cells in the pituitary; b) pituitary PRL as detected by radioimmunoassay (RIA), Nb2 bioassay, and induction of interleukin-2 receptors on splenocytes; c) induction of IL-2R on lymphocytes in response to a standard PRL; d) CD phenotype of the splenocytes and tumor infiltrating lymphocytes. We found that 80% of all NMU-treated animals developed mammary tumors 10 to 13 weeks post-injection. PRL cell number, size, and granule content were unaffected. When tested by RIA or by the Nb2 bioassay, there appeared to be approximately 50% less PRL secreted (2 weeks post-injection) by cells of the NMU-treated than the vehicle-treated animals. However, when tested by IL-2R assay, PRL cells of NMU-treated animals secreted 50% more activity. Splenocytes from the treated animals, 2-6 weeks post-injection, expressed fewer IL-2R in response to standard PRL. NMU treatment (12 wks post-injection) increased the numbers of T-cytotoxic cells by 49%, had no effect on T-helpers, and increased the number of IL-2R positive splenocytes by 81%. Our interpretation is that NMU treatment interferes with the feedback of lymphokines on the pituitary with a decrease in the form of PRL detected by the RIA and Nb2 assays and an increase in the form which activates splenocytes, and thus changes the composition and function of the immune system.

Interaction of lactogenic hormones with purified recombinant extracellular domain of rabbit prolactin receptor expressed in insect cells

The extracellular domain of rabbit prolactin receptor (rbPRLR-ECD) expressed in an insect/baculovirus expression system was purified by affinity chromatography on immobilized PRL followed by gel filtration. The purified protein was over 90% homogeneous as indicated 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 was 32 kDa, and by gel filtration, 27 kDa. Both values are higher than the 22.8 kDa deduced from the cDNA sequence, indicating extensive glycosylation. The Ka value for interaction with ovine (o) PRL was 25.4 nM-1, but even at high rbPRLR-ECD:hormone molar ratios, the stoichiometry of interaction with oPRL or human growth hormone indicated formation of only 1:1 complexes, in contrast to human growth hormone (hGH)-ECD which forms 2:1 complexes with hGH.

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.

Anti-substance P anti-idiotypic antibodies modulate the secretory process in the rat parotid gland in vitro

Anti-idiotypic antibodies (anti-Id) obtained in rabbits in response to immunization with polyclonal anti-substance P antibodies (anti-SP) were shown to bind specifically and with high affinity to membranes from rat parotid gland cells. Whereas substance P (SP) was unable to displace anti-Id from membrane binding sites, anti-Id partly inhibited the binding of radiolabelled substance P. Like substance P, anti-Id were able to trigger protein secretion by parotid cells i.e. to behave as physiological agonists of the neuropeptide. Under our experimental conditions, the biological effects of both ligands appear to be additive. Unlike substance P, however, anti-Id did not potentiate the secretory response induced by a beta-adrenoceptor agonist. Taken together, the present results might indicate that anti-Id interact with epitope(s) at or/and near the peptide-combining site on the substance P receptor. These data demonstrate further the possibility of raising pharmacologically active anti-receptor antibodies through the immunological anti-idiotypic approach.

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.

Binding proteins for growth hormone and prolactin in rabbit kidney cytosol

Two soluble, receptor-like binding proteins with apparent somatotrophic [growth hormone (GH)] and lactogenic [prolactin (PRL)] specificities, respectively, and that are present in rabbit kidney cytosol have now been examined in more detail using specific GH receptor and PRL receptor monoclonal antibodies (MAb). Gel chromatography of 125I-labeled human GH (125I-hGH) kidney cytosol complexes in the absence of these MAbs revealed two specifically bound regions of radioactivity at molecular weights (MW) of approximately 120,000 and approximately 60,000, which are similar in size to complexes formed by the native GH receptor of rabbit liver cytosol and the PRL receptor of mammary gland. Co-incubation with GH-receptor MAb inhibited 125I-hGH binding only to the higher MW (120,000) species, whereas the PRL-receptor MAb inhibited only the lower MW (60,000) species, thus establishing definitively the hormonal specificities of the two binding proteins. The presence of both GH- and PRL-specific binding subunits in cytosol was confirmed using covalent cross-linking techniques. No GH binding protein was detected in kidney membranes. The presence of naturally soluble, receptor-like binding proteins for GH and PRL in kidney cytosol preparations raises the possibility of their playing a role in the intracellular regulation of kidney function and/or metabolism.

Immunological relationship and binding capacity of prolactin receptors in cytosolic and membrane fractions of rabbit mammary gland

We have recently identified and partially characterized a specific lactogen binding protein in rabbit mammary gland cytosol. In this report, studies using pregnant or lactating rabbits are described which further characterize the cytosolic lactogen binding protein in relation to the membrane-bound lactogen receptor. The data show that in pregnant or lactating rabbits the binding capacity (fmol/mg protein) of membranes is at least double that of the cytosol preparation although when expressed on a tissue content basis (fmol/g tissue) there was no membrane-cytosol difference in receptor number. Treatment of lactating rabbits with CB-154, however, caused a marked increase (100-150%) in the binding capacity of membrane-bound receptors with comparatively little effect (+20%) on the cytosolic lactogram binding protein. There was also a marked difference in the association constants for 125I-hGH, with the cytosolic lactogen binding protein exhibiting a 6-fold higher affinity than the membrane-bound receptor. Three anti-prolactin receptor monoclonal antibodies (M110, A82 (antagonists) and A917 (agonist) have also been used to assess the relative immunological characteristics of the cytosolic lactogen binding protein and the membrane lactogen receptor. Each monoclonal antibody was able to inhibit the specific binding of 125I-hGH to both membranes and cytosol in a dose-dependent manner. However, the order of potency was not identical being M110 greater than A917 greater than A82 in membranes and M110 greater than A82 greater than A917 in cytosol. A917 was at least 10 times more active in membranes than cytosol whereas A82 was at least 10 times more active in cytosol.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin and antiprolactin receptor antibody inhibit steroidogenesis by purified rat Leydig cells in culture

The in vitro effects of ovine PRL (oPRL) on testicular testosterone synthesis were determined using isolated, collagenase-dispersed, adult rat Leydig cells in culture. oPRL (50-1000 ng/ml) had no effect either on basal or on LH (50, 100 or 2000 pg/ml)-stimulated testosterone secretion by Leydig cells in short-term culture (4 h). 125I-oPRL binding studies revealed a single class of high affinity sites (Ka 8.7 nM) with a low capacity (Bmax 6.7 fmol/mg protein identical to approximately 980 sites/Leydig cell). Isolated Leydig cells were further purified on a continuous Percoll gradient and cultured in serum-free medium, at 34 degrees C, in 5% CO2 and 95% air. After 3 days of culture, the media were collected, the cells washed and then stimulated with hCG (3 ng/ml) for 3 h. oPRL (1-1000 ng/ml) added at plating, caused a log dose-dependent inhibition of testosterone accumulation during the 3-day culture period; the highest and most consistent inhibition (31%) was with 500 ng/ml oPRL. hCG increased the sensitivity to the inhibitory effect of PRL, 10 ng/ml oPRL causing 40% inhibition and 100 ng/ml causing a maximal inhibition of 50%. PRL in fact caused a reduction in the maximal effect (efficacy) of hCG on steroidogenesis, without significantly affecting the ED50 (sensitivity). The effects of an antiPRL receptor antibody raised by the antiidiotypic route and previously shown to bind to rat testis PRL receptors were tested. The antiPRL receptor IgG (13 micrograms/ml) mimicked the PRL inhibitory effect and acted synergistically with PRL (100 ng/ml) in inhibiting both testosterone accumulation in 3-day cultured Leydig cells and their subsequent response to hCG. In summary, a clear inhibitory effect of PRL and a synergistic effect of antiPRL receptor antibody were demonstrated on testosterone synthesis by rat Leydig cells in 3-day culture.

Purification of prolactin receptors from sow mammary gland and polyclonal antibody production

After solubilization with Triton X-100 or 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS), prolactin receptors from mammary crude membranes of primiparous lactating sows (pretreated with bromocriptine) have been purified by affinity chromatography using ovine prolactin or a monoclonal antibody against rabbit prolactin receptor. Comparative analysis of these two methods of purification demonstrated that use of an immunoaffinity step allowed a great improvement of receptor yield (40%) compared to the hormone affinity method (10%). In addition, partially purified fractions obtained by immunoaffinity appeared more homogeneous and had much higher specific activity. Affinity labelling of prolactin receptors from crude membranes or solubilized extracts with iodinated ovine prolactin, followed by electrophoretic analysis (SDS-PAGE) and autoradiography, revealed one binding unit of approximately 45 kDa. When partially purified receptor preparations were labelled with 125I, submitted to an additional affinity chromatography and analyzed by SDS-PAGE, prolactin receptors appeared as a single form having a molecular weight of 42-45 kDa, which is not associated with itself or other subunits by disulfide linkages. Partially purified fractions were used to produce anti-prolactin receptor serum from goats. These polyclonal antibodies were able to completely inhibit the binding of lactogenic hormones in sow and rabbit mammary membranes. They were also able to recognize hormone-receptor complexes, but more specifically in sow mammary gland. These antisera could inhibit prolactin binding to its receptors in several organs of various species, suggesting that prolactin receptors shared numerous antigenic similarities between species and particularly between sow and rabbit. These similarities appeared to be located essentially on the part of the molecule more specifically involved in the recognition of the hormone.