Human growth hormone-variant demonstrates a receptor binding profile distinct from that of normal pituitary growth hormone

We have recently established that the human growth hormone-variant (hGH-V) gene is functional in vivo by documenting its expression in the placenta. We have subsequently generated transformed murine cell lines stably expressing the genes for normal pituitary growth hormone (hGH-N), hGH-V, and each of two chimeric genes generated by exon 3 exchanges, hGH-NV3 and hGH-VN3. In the present study, we utilize these cell lines as sources of hormone to characterize and compare the receptor binding profiles of hGH-N with hGH-V. hGH-V was found to displace 125I-ovine prolactin bound to rat liver microsomes (lactogen binding) and to displace 125I-hGH bound to rabbit liver microsomes (somatogen binding). Therefore, hGH-V would be predicted to display both somatogenic and lactogenic bioactivity, a dual specificity previously thought to be unique to hGH-N. The concentrations of hormone necessary to displace 50% (IC50) of the 125I-hGH from somatogen receptors and 125I-ovine prolactin from lactogen receptors was expressed as a ratio, IC50 somatogen: IC50 lactogen, for each hormone tested. A 7.4-fold difference in this ratio was observed for hGH-N compared to hGH-V, suggesting significantly greater selectivity by hGH-V in binding to the somatogen receptor. The intermediate binding ratios of the hGH-NV3 and hGH-VN3 chimeric proteins confirmed the distinct receptor binding profiles of the two parent hormones and served to identify three amino acids of potential importance in defining their respective receptor binding specificities.     

Alteration in the receptor binding specificity of human growth hormone by genomic exon exchange

The biological activities of the GH-PRL family of hormones are mediated by selective binding to two classes of cell membrane receptors, somatogen and lactogen. Primate GH such as human GH (hGH) are unusual in that they bind to both classes of receptors. Replacement of exons 3 or 4 of the hGH gene by the corresponding exons of the rat PRL or rat GH genes results in the synthesis of chimeric proteins which retain the ability to bind to lactogen receptors but can no longer bind to somatogen receptors. This selective loss of somatogen receptor binding in the chimeric proteins suggests that certain of the structural determinants of somatogen and lactogen receptor binding activities in hGH are distinct and can be separately modified by a limited number of amino acid substitutions.     

The characteristics of hGH binding to the liver macrophages

Macrophages isolated from female rat liver as well as hepatocytes bind 125I-hGH. This study compares the effect of sex of the rat, hypophysectomy (hypox) and preincubation of the cells with oPrl on the binding of 125I-hGH to the cells. The percent of 125I-hGH to the hepatocytes was decreased in cells from hypox female and male rats, and hepatocytes preincubated with oPrl to 0.43, 0.21 and 0.39, respectively, of that observed in hepatocytes from normal female rats. In the hepatocytes from normal female, hypox female, and male rats, hGH was the most effective competitor for 125I-hGH binding with an ID50 of 0.73-0.99 nM. The concentration of oPrl, bGH and rGH that produced half-maximal inhibition (ID50) of 125I-hGH binding to hepatocytes from female rat liver was 6.3, 100, and 420 nM respectively. In hepatocytes from male and hypox female rats, and hepatocytes preincubated with oPrl, the ID50 for bGH and rGH varied from 2.1 to 15.9 nM. The percent of 125I-hGH bound by the macrophages from hypox female and male rats, and macrophages preincubated with oPrl was 0.06, 0.15 and 0.18, respectively, of that bound by macrophages from normal female rat liver. In contrast to hGH binding to the hepatocytes, the ID50 for hGH was 6 to 180-fold greater in macrophages from hypox female and male rats, and macrophages preincubated with oPrl compared to that observed in macrophages from normal female rats, Rat GH was the most effective competitor for 125I-hGH binding in the macrophages from the hypox female and male rat liver with ID50 of 5.5 and 85 respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification of the rat hepatic alpha 2-macroglobulin receptor as an approximately 440-kDa single chain protein

alpha 2-Macroglobulin-trypsin complex (alpha 2M.T) and alpha 2M-methylamine bind in a Ca2+-dependent way to a 400- to 500-kDa receptor in rat and human liver membranes (Gliemann, J., Davidsen, O., and Moestrup, S. K. (1989) Biochim. Biophys. Acta 980, 326-332). Here we report the preparation of alpha 2M receptors from rat liver membranes solubilized in 3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonic acid (CHAPS) dihydrate and incubated with Sepharose-immobilized alpha 2M-methylamine. The receptor preparation eluted with EDTA (pH 6.0) contained a protein larger than the 360-kDa alpha 2M (nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and some minor contaminants. The reduced large protein was about 440 kDa using reduced laminin (heavy chain: 400 kDa) as a standard. About 10 micrograms of receptor protein was obtained from 100 mg of liver membranes. The receptor preparation immobilized on nitrocellulose sheets bound 125I-alpha 2M.T, and the binding activity co-eluted with the 440-kDa protein. 125I-Labeled rat alpha 1-inhibitor-3 (alpha 1I3), a 200-kDa analogue of the alpha 2M subunit which binds to the alpha 2M receptors, was cross-linked to the 440-kDa protein. The receptor preparation was iodinated, and the 125I-labeled 440-kDa protein was isolated. It showed Ca2+-dependent saturable binding to alpha 2M-methylamine. In conclusion, we have purified the major hepatic alpha 2M receptor as an approximately 440-kDa single chain protein.     

Multiple growth hormone-binding proteins are expressed on insulin-producing cells

The insulin-producing rat islet tumor cell line, RIN-5AH, expresses somatogen binding sites and responds to GH by increased proliferation and insulin production. Affinity cross-linking shows that RIN-5AH cells contain two major GH-binding subunits of Mr 100-130K (110K), which appear to exist as disulfide-linked multimers of Mr 270-350K (300K). In addition, a minor Mr 180K GH-binding protein is identified which does not appear to be associated with other proteins by disulfide bridges. A plasma membrane-enriched fraction accounts for 86% of the RIN-cell GH-binding activity while cytosol and intracellular organelles are low in GH-binding activity. The plasma membrane-bound activity is soluble in Triton X-100 with intact hormone binding characteristics. The apparent KD in detergent solution is estimated to 18 ng/ml (8 x 10(-10) M). 125I-hGH-affinity cross-linking to intact and detergent-solubilized membranes as well as hGH-affinity purified protein reveals labeled proteins of Mr 180K and Mr 285-350K. In contrast to the cross-linked Mr 300K complexes of intact cells those of disintegrated cellular material are resistant to reduction with dithiothreitol, and it is speculated that this is due to intersubunit cross-linking of the disulfide-linked Mr 110K GH-binding subunits. The GH-binding proteins are purified approximately 100-fold by one cycle of hGH-affinity chromatography and five major proteins of Mr 180K, 94K, 86K, 64K, and 54K are identified by silver staining in the purified fraction. It is concluded that the RIN-5AH cells have multiple GH-binding proteins which may mediate signals for either proliferation and/or insulin production.     

Association of a lower molecular weight protein to the mu-opioid receptor demonstrated by (125)I-beta-endorphin cross-linking studies

Cross-linking experiments using the (125)I-beta-endorphin revealed the presence of several receptor-related species in cell lines expressing endogenous opioid receptors, including a small molecular mass protein (approximately 22 kDa). Previous reports have suggested that this 22-kDa (125)I-beta-endorphin cross-linked protein could be the degradative product from a higher molecular mass species, i.e., a fragment of the receptor. To determine if this protein is indeed a degraded receptor fragment, (125)I-beta-endorphin was cross-linked to the (His)(6) epitope-tagged mu-opioid receptor (His-mu) stably expressed in the murine neuroblastoma Neuro(2A) cells. Similar to earlier reports with cell lines expressing endogenous receptors, two major bands of 72- and 25-kDa proteins were specifically cross-linked. Initial cross-linking experiments indicated the absolute requirement of the high-affinity (125)I-beta-endorphin binding to the mu-opioid receptor prior to the appearance of the low molecular weight species, suggesting that the 22-kDa protein could be a degraded fragment of the receptor. However, variations in the ratios of these protein bands being cross-linked by several homo- or heterobifunctional cross-linking agents were observed. Although neither the carboxyl terminus mu-opioid receptor-specific antibodies nor the antibodies against the epitope at the amino terminus of the receptor could recognize the 22-kDa protein, this (125)I-beta-endorphin cross-linked species could be coimmunoprecipitated with the receptor antibodies or could be isolated with a nickel resin affinity chromatography. The direct physical association of the 22-kDa protein with the receptor was demonstrated also by the observation that the 22-kDa protein could not bind to the nickel resin alone, but that its binding to the nickel resin was restored in the presence of the His-mu. Taken together, these results suggest that the 22-kDa protein cross-linked by (125)I-beta-endorphin is not a degradative product, but a protein located within the proximity of the mu-opioid receptor, and that it is tightly associated with the receptor.     

Induction of specific human growth hormone binding sites in rat liver by human growth hormone.

125I-Labeled hGH was bound to liver plasma membranes which were obtained from female rats. The binding was displaced by hGH, hPRL, bPRL, rPRL and bGH but not by rGH. This result indicated that hGH was bound to lactogenic binding sites in rat livers. After hypophysectomy, the binding was markedly decreased. Treatment of hypophysectomized rats with hGH (80 micrograms/day) for 10 days increased the binding sites for hGH. These binding sites were different from those found in normal female rat livers because of their high affinity and specificity for hGH. These results indicate that hGH induces specific binding sites for hGH in rat livers.     

Binding and structural characteristics of a soluble lactogen-binding protein from rabbit mammary-gland cytosol.

Specific receptors for prolactin (PRL) are known to be present on plasma membranes and intracellular membranes of mammary gland. We now report, however, the detection and characterization of a soluble lactogen-specific binding protein in high-speed (200,000 g) cytosolic preparations from pregnant- and non-pregnant-rabbit mammary gland. The binding protein was not detectable by poly(ethylene glycol) precipitation; instead, bound and free 125I-labelled human growth hormone (hGH; a potent lactogen) was separated using a mini-gel filtration technique. Specific binding of 125I-hGH reached an apparent equilibrium between 10 and 14 h at 21-23 degrees C. It was dependent on mammary-gland protein concentration and, partially, on Ca2+ or Mg2+ concentrations. Scatchard analysis revealed steep curvilinear plots, the high-affinity component having a KA of approximately 3 X 10(10) M-1. Gel filtration on calibrated Ultrogel AcA34 columns of 125I-hGH-cytosol complexes or of cytosol alone, followed by measurement of 125I-hGH binding in each eluted fraction, indicated that the binding protein had an Mr of 33,000-43,000. A specific binding protein of the same size was observed when 125I-ovine or -human PRL, but not 125I-bovine GH, was used as ligand. The apparent lactogenic specificity was confirmed by a lack of cross-reactivity of the binding protein with an anti-[GH receptor (rabbit liver)] monoclonal antibody. Polyacrylamide-gel electrophoresis of 125I-hGH covalently cross-linked to cytosol with disuccinimidyl suberate revealed binding proteins of Mr 35,000 (non-reduced) and 37,000 (reduced), results comparable with those obtained by gel filtration and indicating an absence of inter-subunit disulphide bonds. These studies have shown the presence of an apparently naturally soluble lactogen-binding protein in the cytosolic fraction of rabbit mammary gland. The relationship between this binding protein and the membrane PRL receptor is not yet known.     

Mutational analysis and protein engineering of receptor-binding determinants in human placental lactogen

Human placental lactogen (hPL) shares 85% sequence identity to human growth hormone (hGH) yet has some very different receptor-binding properties. For example, hPL binds 2300-fold weaker than hGH to the hGH receptor, yet these two hormones have similar affinities for prolactin receptors. We have expressed hPL in Escherichia coli, and we show that, like hGH, hPL requires zinc for tight binding to the extracellular domain of the human prolactin receptor (hPRLbp). In fact, hPL contains virtually the same receptor-binding determinants and zinc ligands (His-18, His-21, and Glu-174) that hGH uses for coordinating zinc in the hGH.hPRLbp complex. As with hGH, mutation of Glu-174 to Ala in hPL reduces the affinity for the hPRLbp by 1400-fold. We can increase the affinity of hPL by over 200-fold for the hGHbp by installing four hGH receptor determinants that are not conserved in hPL. By simultaneously introducing E174A, we produced a pentamutant whose binding affinity for the hGHbp is only 1.6-fold weaker than hGH, but whose binding affinity for the hPRLbp is weaker by greater than 1000-fold relative to wild-type hPL. Thus, we have identified an hPRLbp epitope in hPL, "recruited" an hGHbp epitope into hPL, and produced receptor selective analogs of hPL that are designed to bind tightly to either, neither, or both receptors. Such variants should be important molecular probes to link specific receptor-binding, activation, and biological events.     

Identification of retinal insulin receptors using site-specific antibodies to a carboxyl-terminal peptide of the human insulin receptor alpha-subunit. Up-regulation of neuronal insulin receptors in diabetes

Insulin receptor-specific polyclonal antipeptide serum was generated against a synthetic pentadecapeptide (residues 657-670) of the deduced amino acid sequence of human insulin proreceptor cDNA for use in the analysis of insulin receptors in the retina. The affinity-purified antibodies recognized peptide antigen but not keyhole limpet hemocyanin as determined by dot blot analysis and solid phase radioimmunoassay. Addition of either synthetic peptide or the affinity-purified serum had no effect on 125I-insulin binding to placental membranes or to cells in culture. alpha-Subunits of approximately 125 kDa from human placental membranes and liver membranes were labeled by immunoblot analysis with this antiserum. In membranes isolated from human retina and brain, two classes of alpha-subunits of approximately 125 and 115 kDa were detectable. The 115-kDa subunit was neuraminidase resistant whereas the 125-kDa subunit was digested to a band of 115 kDa, indicating that these bands represent peripheral and neuronal receptors, respectively. Analysis of human retinas obtained from type I diabetic donors revealed an increased level of neuronal receptor as compared with normal retinas. These data indicate that human retina expresses neuronal insulin receptor subtypes that are up-regulated in diabetes.     

Mitogenic and binding properties of monoclonal antibodies to the prolactin receptor in Nb2 rat lymphoma cells. Selective enhancement by anti-mouse IgG

Three monoclonal antibodies (mAbs) (T6, U5, and U6) against prolactin (PRL) receptors in rat liver were studied in the rat lymphoma lactogen-dependent (Nb2-11C) and autonomous (Nb2-SP) cell lines. The mAbs had strong affinity for lactogen receptors (Ka = 12-14 nM-1), similar to that of human growth hormone (hGH) which is a lactogenic hormone. T6 and hGH competed for the same binding site, while U5 and U6 interacted with another epitope. The 125I-hGH-receptor complex could be immunoprecipitated by either U5 or U6, but not by T6. Affinity labeling and immunoblotting revealed that hGH and U6 bind to a protein of 63-65 kDa. T6, U5, and U6 were mitogenic in Nb2-11C cells but their respective potencies were 185-, 70-, and 4700-fold lower than that of hGH. Anti-mouse IgG enhanced the mitogenic effect of all three mAbs and almost completely abolished the differences between them, although their mitogenic activity was still 60-120-fold lower than hGH. Des-13-hGH, a competitive antagonist of hGH which hardly effected the binding of 125I-U5, inhibited the U5-stimulated proliferation of Nb2-11C cells in a noncompetitive manner, indicating that simultaneous binding of both ligands fixed the receptor in a nonactive conformation. A Fab fragment of T6 was not mitogenic, and inhibited the hGH-induced mitogenesis in a competitive manner, but its mitogenicity could be restored by anti-mouse IgG. We suggest that the dimerization or oligomerization of the lactogen receptor in Nb2-11C cells is an obligatory step in the transduction of the mitogenic signal. It may be induced by binding of the mAb to a site, which can be either identical or may even be distinct from that which binds the lactogenic hormone.     

Site-directed mutations of human growth hormone that selectively modify its lactogenic activity and binding properties

Two novel analogs of human (h) GH, 1) Des-7-hGH (Arg8Met, Asp11Ala) in which the Arg8 was substituted by Met and Asp11 by Ala, and 2) bovine (b) GH/hGH hybrid II (MetAla 1-13/14-191, Ala11Asp) composed of 13 N-terminal amino acid of bGH and elongated by two amino acids (Met-Ala-1-13) and 14-191 amino acids of hGH, were constructed and expressed in Escherichia coli. CD spectra indicated that the alpha-helix content of the purified proteins was similar to that of the native hormone. Both analogs retained their full ability to stimulate the proliferation of Nb2 lymphoma cells, and their binding to the lactogen receptors in homogenate of Nb2 cells and in microsomal fraction from bovine lactating mammary gland was only slightly reduced. However, their ability to bind to the somatogen receptors in intact IM-9 lymphocytes and bovine liver was reduced by 7- to 11-fold (bGH/hGH hybrid II) and 20- to 30-fold (Des-7-hGH). Both analogs were able to down-regulate the respective lactogen and somatogen receptors in intact Nb2 and IM-9 cells. The galactopoietic activity of both analogs in the lactating bovine mammary explants bioassay was almost completely abolished, and the bGH/hGH hybrid II exhibited a remarkable antagonistic activity. These results further indicate that the lactogen receptors in different species or organs are not identical. We have shown that the new recombinant analogs of hGH that recognize both somatogen and lactogen receptors but have modified postreceptor effects are helpful in elucidating these differences.     

Characterization of multiple forms of the Ah receptor: recognition of a dioxin-responsive enhancer involves heteromer formation.

We have employed a combination of gel retardation, protein-DNA cross-linking, and protein-protein cross-linking techniques to further examine the 2,3,7,8-tetrachlorodibenzo-p- dioxin-(TCDD-) dependent changes in the Ah receptor that result in a DNA-binding conformation. Gel retardation analysis of DNA-Sepharose chromatographic fractions of rat hepatic cytosol indicated that TCDD-dependent and sequence-specific DNA binding coeluted with a 200-kDa form of the Ah receptor (peak 2) previously characterized as being multimeric and having high affinity for calf thymus DNA. The TCDD-bound, 100-kDa form of the receptor (peak 1) bound weakly to the DNA recognition motif. These results indicated that the DNA-binding form of the Ah receptor is a multimer. SDS-polyacrylamide gel electrophoresis of peak 2 cross-linked to a bromodeoxyuridine-substituted DNA recognition motif indicated that this form of the receptor present in rat hepatic cytosol is composed of at least two DNA-binding proteins of approximately 100 and 110 kDa. Using the chemical cross-linking agent dimethyl pimelimidate, we further established that the 100-kDa form of the receptor (peak 1) associates with a different protein to generate the receptor form (peak 2) that binds to the dioxin-responsive enhancer. Photoaffinity-labeling studies indicated that only the 100-kDa protein (peak 1), and not the 110-kDa protein, binds ligand. Together, these observations imply that the DNA-binding form of the Ah receptor exists as a heteromer.     

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.     

Species variation in the binding of hGH to hepatic membranes

The binding of 125I-labeled human growth hormone (hGH) to liver membranes from several different species was studied to determine the lactogenic or somatotropic hormone nature of the receptors. Liver membranes from several species of the class of Mammalia bound significant quantities of 125I-hGH. Goat, sheep, rat, mouse, and rabbit liver membranes exhibited the highest binding with cow, pig, human, and hamster liver membranes exhibiting severalfold less binding. The binding of the dog and cat liver membranes exhibited relatively high nonspecific binding. Fish and chicken liver membranes did not bind appreciable quantities of 125I-hGH. In all species except for dog and cat in which 125I-hGH bound to the membranes, hGH was the most effective competitor for binding. The mean ID50 for hGH and all membranes was 2.4 X 10(-9) M. Human liver membranes exhibited the smallest ID50, 4.9 X 10(-10) M. In sheep liver membranes, bovine growth hormone (bGH) was equipotent to hGH in competing for 125I-hGH binding. bGH also demonstrated significant competition for 125I-hGH binding in pig and cow membranes. Ovine prolactin (oPrl) exhibited significant competition for 125I-hGH only in rodent membranes. The ID50 for oPrl was 3- to 10-fold greater than for hGH in the rat, hamster, and mouse liver membranes. The ID50 for oPrl in the sheep liver membranes was 13-fold greater than that of hGH. We conclude the following: (1) There appears to be a species specificity of hGH binding that may be phylogenetically significant and may result from variations in the structure of the hormone or the receptor. (2) The competitive binding properties of hGH are fairly consistent within phylogenetic orders. (3) The simple designation of lactogenic or somatotropic for hormones and receptors is insufficient to characterize the binding properties of this group of hormones.     

Characterization of the recombinant rat 175-kDa hyaluronan receptor for endocytosis (HARE)

Hyaluronan (HA) and chondroitin sulfate (CS) clearance from lymph and blood in mammals is mediated by the HA receptor for endocytosis (HARE), which is present as two isoforms in rat and human (175/300 kDa and 190/315 kDa, respectively) in the sinusoidal endothelial cells of liver, spleen, and lymph nodes (Zhou, B., McGary, C. T., Weigel, J. A., Saxena, A., and Weigel, P. H. (2003) Glycobiology 13, 339-349). The small rat and human HARE proteins are not encoded directly by mRNA but are derived from larger precursors. Here we characterize the specificity and function of the 175-kDa HARE, expressed in the absence of the 300-kDa species, in stably transfected SK-Hep-1 cells. The HARE cDNA was fused with a leader sequence to allow correct orientation of the membrane protein. The recombinant rHARE contained approximately 25 kDa of N-linked oligosaccharides and, like the native protein, was able to bind HA in a ligand blot assay, even after de-N-glycosylation. SK-HARE cell lines demonstrated specific 125I-HA endocytosis, receptor recycling, and delivery of HA to lysosomes for degradation. The Kd for the binding of HA (number-average molecular mass approximately 133 kDa) to the 175-kDa HARE at 4 degrees C was 4.1 nm with 160,000 to 220,000 HA-binding sites per cell. The 175-kDa rHARE binds HA, dermatan sulfate, and chondroitin sulfates A, C, D, and E, but not chondroitin, heparin, heparan sulfate, or keratan sulfate. Surprisingly, recognition of glycosaminoglycans (GAGs) other than HA by native or recombinant HARE was temperature-dependent. Although competition was observed at 37 degrees C, none of the other GAGs competed for 125I-HA binding to SK-HARE cells at 4 degrees C. Anti-HARE monoclonal antibody-174 showed a similar temperature-dependence in its ability to block HA endocytosis. These data suggest that temperature-induced conformational changes may alter the GAG specificity of HARE. The results confirm that the 175-kDa rHARE does not require the larger HARE isoform to mediate endocytosis of multiple GAGs.     

Interactions of a laminin-binding peptide from a 33-kDa protein related to the 67-kDa laminin receptor with laminin and melanoma cells are heparin-dependent.

A laminin-binding peptide (peptide G), predicted from the cDNA sequence for a 33-kDa protein related to the 67-kDa laminin receptor, specifically inhibits binding of laminin to heparin and sulfatide. Since the peptide binds directly to heparin and inhibits interaction of another heparin-binding protein with the same sulfated ligands, this inhibition is due to direct competition for binding to sulfated glycoconjugates rather than an indirect effect of interaction with the binding site on laminin for the 67-kDa receptor. Direct binding of laminin to the peptide is also inhibited by heparin. This interaction may result from contamination of the laminin with heparan sulfate, as binding is enhanced by the addition of substoichiometric amounts of heparin but inhibited by excess heparin and two heparin-binding proteins. Furthermore, laminin binds more avidly to a heparin-binding peptide derived from thrombospondin than to the putative receptor peptide. Adhesion of A2058 melanoma cells on immobilized peptide G is also heparin-dependent, whereas adhesion of the cells on laminin is not. Antibodies to the beta 1-integrin chain or laminin block adhesion of the melanoma cells to laminin but not to peptide G. Thus, the reported inhibition of melanoma cell adhesion to endothelial cells by peptide G may result from inhibition of binding of laminin or other proteins to sulfated glycoconjugate receptors rather than from specific inhibition of laminin binding to the 67-kDa receptor.     

Crystal structure and site 1 binding energetics of human placental lactogen

In primates, placental lactogen (PL) is a pituitary hormone with fundamental roles during pregnancy involving fetal growth, metabolism, and stimulating lactation in the mother. Human placental lactogen (hPL) is highly conserved with human growth hormone (hGH) and both hormones bind to the hPRLR extracellular domain (ECD), the first step in receptor homodimerization, in a Zn2+-dependent manner. A modified surface plasmon resonance method was developed to measure the kinetics for hPL and hGH binding to the hPRLR ECD, with and without Zn2+ and showed that hPL has about a tenfold higher affinity for the hPRLR ECD1 than hGH. The crystal structure of the free state of hPL has been determined to 2.0 A resolution showing the molecule possesses an overall structure similar to other long chain four-helix bundle cytokines. Comparison of the free hPL structure with the 1:1 complex structure of hGH bound to the hPRLR ECD1 suggests that two surface loops undergo conformational changes >10 A upon binding. An 18 residue Ala-scan was used to characterize the binding energy epitope for the site 1 interface of hPL. Individual alanine substitutions at five positions reduced binding affinity by a DeltaDeltaG > or = 3 kcal mol(-1). A comparison of the hPL site 1 epitope with that previously determined for hGH indicates contributions of individual residues track reasonably well between hPL and hGH. In particular, residues involved in the zinc-binding site and Lys172 constitute the principal binding determinants for both hormones. However, several residues that are identical between hPL and hGH contribute quite differently to the binding of the hPRLR ECD1. Additionally, the overall magnitudes of the DeltaDeltaG changes observed from the Ala-scan of hPL were markedly larger than those determined in the comparative scan of hGH to the hPRLR ECD1. The structural and biophysical data presented here show that subtle changes in the structural context of an interaction can lead to significantly different effects at the individual residue level.     

Variations in prolactin and growth hormone production during cellular growth in clonal strains of rat pituitary cells.

A permanent, clonal strain of rat pituitary tumor cells (GH3-cells) spontaneously synthesizes and secretes prolactin (rPRL) and growth hormone (rGH) into the culture medium. The rates of hormone production (microng extracellular hormone/mg cell protein/24 hours) and synthesis (vida infra) as well as the rate of [3H]thymidine incorporation into DNA (DNA synthesis) have been studied. During logarithmic growth rPRL and rGH production increased to 160 and 250% of the value at day 2 after plating, while during the plateau phase of cell growth hormone production decreased to initial values. The fluctuations in rPRL production could be fully explained by variations in the rate of rPRL synthesis: [3H]eucine incorporated into rPRL as measured with immunoprecipitation and polyacryl-amide gel electrophoresis. Also the rates of synthesis and production of rGH showed parallel changes during exponential and plateau phase of growth, but this hormone was probably degraded intracellularly. The relative reduction in the rate of synthesis of rPRL and rGH during the plateau of growth corresponded closely to the fall in the rate of DNA synthesis. The reduction in rPRL synthesis could not be explained through an inhibition by extra-cellular rPRL accumulation or by cell to cell interaction occurring in dense cultures. The intracellular concentrations of both hormones were unaltered during logarithmic growth, but rose to 500% for rPRL and 200% for rGH during the plateau phase. In spite of the marked variations in basal rPRL and rGH production the GH3 cultures of different ages were equally able to increase rPRL and decrease rGH production in response to thyrotropin releasing hormone (3 X 10(-7) M) and 17beta-estradiol (10(-8)M).