Isolation, characterization, and regulation of the prolactin receptor

Characterization of Δ7/11, a functional prolactin-binding protein

Prolactin is essential for normal mammary gland development and differentiation, and has been shown to promote tumor cell proliferation and chemotherapeutic resistance. Soluble isoforms of the prolactin receptor (PrlR) have been reported to regulate prolactin bioavailability by functioning as 'prolactin-binding proteins'. Included in this category is Δ7/11, a product of alternate splicing of the PrlR primary transcript. However, the direct interactions of prolactin with Δ7/11, and the resulting effect on cell behavior, have not been investigated. Herein, we demonstrate the ability of Δ7/11 to bind prolactin using a novel proximity ligation assay and traditional immunoprecipitation techniques. Biochemical analyses demonstrated that Δ7/11 was heavily glycosylated, similar to the extracellular domain of the primary PrlR, and that glycosylation regulated the cellular localization and secretion of Δ7/11. Low levels of Δ7/11 were detected in serum samples of healthy volunteers, but were undetectable in human milk samples. Expression of Δ7/11 was also detected in six of the 62 primary breast tumor biopsies analyzed; however, no correlation was found with Δ7/11 expression and tumor histotype or other patient demographics. Functional analysis demonstrated the ability of Δ7/11 to inhibit prolactin-induced cell proliferation as well as alter prolactin-induced rescue of cell cycle arrest/early senescence events in breast epithelial cells. Collectively, these data demonstrate that Δ7/11 is a novel regulatory mechanism of prolactin bioavailability and signaling.

Progesterone induces expression of the prolactin receptor gene through cooperative action of Sp1 and C/EBP

Prolactin (Prl) and progesterone (P) cooperate synergistically during mammary gland development and tumorigenesis. We hypothesized that one mechanism for these effects may be through mutual induction of receptors (R). EpH4 mouse mammary epithelial cells stably transfected with PR-A express elevated levels of PrlR mRNA and protein compared to control EpH4 cells that lack the PR. Likewise, T47D human breast cancer cells treated with P overexpress the PrlR and activate PrlR promoter III. PrlR promoter III does not contain a classical P response element but contains several binding sites for transcription proteins, including C/EBP, Sp1 and AP1, which may also interact with the PR. Using promoter deletion and site directed mutagenesis analyses as well as gel shift assays, cooperative activation of the C/EBP and adjacent Sp1A, but not the Sp1B or AP1, sites by P is shown to confer P responsiveness leading to increased PrlR transcription.

Regulation of prolactin receptor glycosylation and its role in receptor location

In unstimulated mammary epithelial cells from virgin mice, the prolactin receptor exists as two isoforms: a 78 and a 70 kDa species. Both proteins are reduced to a single 61 kDa molecule after N-glycanase F treatment, indicating that their size difference is solely a result of carbohydrate content. Membrane fractionation experiments reveal that the smaller species is exclusively intracellular, while the larger one is located on the cell surface. Nitric oxide (NO) stimulates the migration of prolactin receptors from an internal pool to the plasmalemma in only 30 min and this redistribution is associated with an increase in molecular weight. Redistribution is blocked by swainsonine, but not by castanospermine or 1-deoxymannojirimycin, suggesting that the glycosylation step involved with translocation is either alpha-mannosidase II or N-acetylglucosamine (NAG) transferase I. The former is unaffected by NO but the activity of the latter is doubled 30 min after exposure to NO. These data suggest that prolactin receptors are retained intracellularly because of their incomplete N-glycosylation and that NO triggers their redistribution by stimulating the completion of this process, in part by increasing NAG transferase I activity.

Characteristics of a membrane-associated antilactogen binding site for tamoxifen

The antilactogen binding site (ALBS) is a membrane associated protein to which tamoxifen (TAM) and related non-steroidal antiestrogens, but not estrogen, bind. It is through this site that TAM inhibits lactogen binding to the prolactin (Prl) receptor and subsequent Prl induced growth and differentiation in target tissues. Binding of lactogens to the Prl receptor is inhibited by TAM or 4-hydroxy-TAM at 4 degrees C as well as room temperature, thus suggesting that the ALBS is not an enzyme. TAM acts by inhibiting the binding of lactogens to the receptor rather than promoting dissociation of the hormone-receptor complex. Lactogens bind to mammary gland membranes with an Kd of 4.3-8.2 x 10(-10) M. In the presence of 10(-7) M TAM the affinity decreased to a Kd of 0.8-1.6 x 10(-9) M. Binding of 3H-TAM to mammary gland membranes was effectively inhibited by an anti-Prl receptor antibody, thus suggesting a close relationship between the Prl receptor and the ALBS. Separate affinity purification of the ALBS and the Prl receptor resulted in peak fractions demonstrating specific binding activity for both TAM and lactogenic hormones. Re-isolation of the affinity purified Prl receptor on a TAM-Sepharose affinity resin again resulted in co-elution of both binding activities. The isolates from both affinity resins contained primarily a single band with an apparent molecular mass of 90 kDa. This band was precipitated with the anti-Prl receptor antibody and specifically bound the affinity label ring-3H-TAM aziridine.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a monoclonal antibody against human prolactin receptors

Tamoxifen is the first line of therapy for most human breast cancers. It not only works through the estrogen receptor but also can directly affect the binding of prolactin to its receptor. To define this latter mechanism, the nature of the prolactin receptor needs to be clearly defined. Monoclonal antibody (MAb) B6.2, and IgG1 raised against a membrane-enriched fraction from metastatic human breast cancer cells, was as effective as polyclonal anti-prolactin receptor antibody in inhibiting the binding of prolactin to membranes from human tissue and to T47D human breast cancer cells. Control MAbs, MOPC-2I and the anti-NCA B1.1 MAb, had no effect on binding. Epidermal growth-factor receptors on these same cells were unaffected by B6.2. Prolactin-induced growth of the T47D cells was blocked by addition of B6.2 to the media while the control antibodies were without effect. Specific binding of B6.2 to the cells was completely inhibited by prolactin. Binding of both prolactin and B6.2 was inhibited by growing the T47D cells in the presence of tunicamycin A1 under conditions where protein synthesis was not affected but glycosylation of proteins was. An affinity column of B6.2 was used to purify its antigen from T47D cells. The primary purification product, a M(r) 90,000 protein, specifically bound the lactogenic hormones human prolactin, human growth hormone and ovine prolactin but not the somatogenic hormone, bovine growth hormone and was precipitated by the polyclonal anti-prolactin receptor antibody but not by control MAbs. When tryptic and V8 digests of the B6.2 antigen and purified prolactin receptors were compared, identical electrophoretic profiles were obtained. Mouse 3T3 cells, when stably transfected with the gene for the long form of the human prolactin receptor, reacted with B6.2 and polyclonal anti-prolactin receptor antibody. Parental 3T3 cells, devoid of prolactin receptors, were negative for all antibodies tested. Thus, MAb B6.2 provides a useful tool for further studies on purification and characterization of these receptors from human tissues and may provide new insights into treatment for breast cancer.

Effects of epidermal growth factor, estrogen, and progestin on DNA synthesis in mammary cells in vivo are determined by the developmental state of the gland

Estrogen (E), progesterone (P), and epidermal growth factor (EGF) are involved in the growth and development of the normal mammary gland. While studies have been carried out to investigate the in vivo effects of EGF in the immature mammary gland, nothing is known about the growth effects of EGF or its potential interactions with E and/or P in the adult mammary gland. The present studies were undertaken to investigate the effects of EGF, E, and P on mammary cell proliferation in immature, peripubertal vs. adult, sexually mature mice. We have found that EGF promotes epithelial and stromal cell proliferation in both the immature and adult mammary glands. In the immature gland, the end bud epithelium is most responsive to the proliferative effects of EGF and there is no apparent interaction between EGF, E, and/or P. In contrast, in the mature gland EGF adds to the proliferative effects of E+P in the ductal epithelium resulting in more extensive ductal sidebranching. Thus these results demonstrate that the developmental state of the mammary gland determines the nature and extent of the interactions between EGF, E, and P in growth and development.

Solubilization and characterization of a lactogenic receptor from human placental chorion membranes

Prolactin has a wide range of actions, including osmoregulation and the control of mammary gland development and lactation. These effects are mediated through a high-affinity cell surface receptor, which has been well characterized in a number of animal tissues. The molecular characteristics of the human receptor are unknown, however. The present studies were initiated, therefore, to determine the binding and molecular characteristics of the lactogenic receptor of human placental chorion membranes. Subcellular fractionation studies showed that the bulk of the receptor sedimented in the microsomal fraction at 45,000gav. Endogenous ligand was dissociated from the receptor with 3.5 M MgCl2 or 0.05 M acetate buffer (pH 4.8) with preservation of binding activity. The microsomal receptor bound human growth hormone (hGH), human prolactin (hPRL), ovine prolactin (oPRL), and human placental lactogen (hPL) but not non-primate growth hormones, indicating a narrow specificity for lactogenic hormones. The binding was only partially reversible in agreement with the known binding kinetics of animal lactogenic receptors. The receptor was solubilized with 45% yield from the microsomes using 16 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulphonate (CHAPS) detergent-250 mM NaCl, and the binding activity was fully restored by a two-fold dilution in the binding reaction to reveal a KD of 0.8 nM for hGH and a binding capacity of 200 fmol of specifically bound hGH per mg of microsomal protein. Gel filtration chromatography indicated the minimum molecular weight of the ligand-receptor complex was approximately 60,000 daltons, and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of covalently cross-linked 125I-hGH-receptor complexes revealed a molecular size of 58,000 daltons. When account was taken of the contribution of the ligand, a molecular weight of 36,000 for the receptor's binding domain was obtained. These data indicate that the chorion lactogenic receptor has very similar binding and molecular characteristics to the lactogenic receptors from other mammalian species. Chorion membranes are thus a convenient source of material for the further purification and characterization of the human lactogenic receptor.

Porcine endometrial prolactin receptors detected by homologous radioreceptor assay

Heterologous radioreceptor assays, commonly using ovine prolactin, may generate inconsistent results since prolactin (PRL) from one species may be recognized as growth hormone in another. Homologous radioreceptor assays (RRA) are most similar to the in vivo hormone-tissue receptor environment; however, lactogenic homologous RRAs have been reported only for mouse hepatic membranes. In this study, an assay system was developed to investigate homologous binding for porcine PRL in porcine uterine tissue. The pig does not produce a decidual PRL or a placental lactogen; yet, PRL affects uterine physiology during reproductively important events. Optimal binding conditions established for porcine PRL homologous RRA include 150 micrograms membrane, 45,000 cpm labeled porcine PRL and 500 microliters sodium phosphate buffer pH 7.6, incubated at 25 degrees C for 24 h. Binding of porcine PRL tracer is very low (less than 3%); however, when tissue is treated with the chaotropic agent, MgCl2 (4 M), binding increases from 3 to 28%. Dissociation kinetics show a rate of 3.79 X 10(-6)/s initially, and then 1.63 X 10(-6)/s. Competition for labeled PRL on binding sites with unlabeled porcine PRL results in 80% displacement with unlabeled porcine prolactin (NSB) of 7% at 1000 ng. Affinity constant generated from homologous inhibition assays is 0.326 X 10(8) M-1. Porcine growth hormone (GH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) do not displace porcine PRL tracer. These data describe a lactogenic homologous RRA for porcine endometrial membranes similar to that previously reported for murine hepatic tissue. Homologous RRAs may allow elucidation of PRL receptor characteristics with more similarity to the in vivo hormone-receptor milieu.

Separation of rabbit mammary-gland prolactin receptors by ion-exchange chromatography, h.p.l.c.-gel filtration and ultracentrifugation

Rabbit mammary-gland prolactin (Prl) receptors in the microsomal fraction were solubilized in 7.5 mM-Chaps) or 1% Triton X-100 and analysed by ion-exchange chromatography using DEAE-Bio-Gel A. Prl receptors in the presence of 7.5 mM-Chaps were separated into two different fractions (Fr. A and B), both of which showed identical specificity of binding to peptide hormones as those in the Chaps or Triton extract. oPrl and human growth hormone (hGH) bound to the same site, but other non-lactogenic hormones (follicle-stimulating hormone, oGH, luteinizing hormone and insulin) failed to bind to the Prl receptors. The dissociation constant (Kd) for Prl binding to the receptors in Fr. A was about 50% of those in Fr. B, suggesting that the rabbit mammary gland contains two types of Prl receptors, one with a high, and one with a low, Kd for Prl binding. A decrease in the concentration of Chaps in the column buffer to 4 mM caused aggregation of the receptors in Fr. A. H.p.l.c.-gel filtration, using Shim pack 150 and 300 columns connected in series, separated the receptor as a protein with an Mr of 74,000 +/- 4,900 (mean +/- S.D.) in the presence of 5 mM-Chaps, or of 36,800 +/- 2,100 in the presence of 7.5 mM-Chaps. Sucrose-gradient-centrifugation analysis showed that the Prl-receptor complexes in the presence of 5 mM-Chaps were sedimented between gamma-globulin and bovine serum albumin (5.56 +/- 0.22 S). As the Chaps concentration was increased to 7.5 mM, a further peak of the Prl-receptor complexes (4.01 +/- 0.23 S) appeared below ovalbumin. The present data suggest that the binding subunit causes the monomeric subunit to aggregate with itself or with another specific associated protein of similar Mr.

Epidermal growth factor stimulates the growth of A431 tumors in athymic mice

Growth of the human epidermoid carcinoma cell line A431 in vitro is stimulated by low concentrations of epidermal growth factor (EGF; 0.1-10 pM) and inhibited by high concentrations (0.1-10 nM). This cell also grows as a solid tumor in female athymic mice. Sustained high levels of EGF in vivo can be achieved by the administration of testosterone to female mice via a cholesterol-based pellet inserted subcutaneously. This chronic elevation of EGF levels (serum concentration = 90 ng/ml), however, does not affect growth of the tumor. In contrast, low levels of the growth factor (0.5 micrograms/g body wt by injection 5 times/week; serum concentration = 8.25 ng/ml) stimulate growth of the tumor. These data suggest that the mechanism(s) involved in the inhibition of A431 cell growth by EGF in vitro does not function in vivo and the physiologically significant effect of EGF in vivo is growth promotion.