Antigenic, receptor-binding and mitogenic activity of proteolytic fragments of human growth hormone.

Analysis of the subtilisin-digested, two-chain form of human growth hormone (hGH) and its constituent polypeptide fragments has been aided by the use of monoclonal antibodies which bind specifically to four distinct epitopes on the native hormone. Using the SDS-polyacrylamide immunoblotting technique, it was shown that one epitope (shared with human chorionic somatomammotropin) detected by EB1 (or EB3) antibody was expressed to a similar extent by both the N-terminal (15 K) and C-terminal (7 K) polypeptides. This epitope is unique in that it represents a repeating determinant within the single chain structure of the hormone. Another three epitopes detected by monoclonal antibodies QA68/NA27, NA71 or NA39/EB2 were absent from the 7-K fragment but were expressed on the 15-K fragment to a similar extent to that on unmodified growth hormone. Binding of NA71 antibody was demonstrated only by radioimmunoassay since this, presumably conformational epitope could not be detected by immunoblotting. The functional importance of the 15-K peptide was demonstrated by its ability to bind specifically to hormone receptors on IM9 human lymphoblastoid cells and by its retention of mitogenicity for the NB2 rat lymphoma cell line. However, all tested monoclonal antibodies inhibited the binding of [125I]15-K to IM9 cell receptors by either steric hindrance or by an allosteric mechanism and therefore could not be further related topographically to the receptor-binding moiety of hGH.     

An antigenic determinant common to human chorionic somatomammotropin and human growth hormone revealed by limited proteolysis of immune complexes.

An epitope of human chorionic somatomammotropin for one of the monoclonal antibodies raised against the whole antigen has been identified. We compared the release of peptides from limited proteolysis of the antigen in the presence and absence of the related antibody. Using enzymes of different specificity, we could determine the amino acid sequence that can be considered at least inclusive of the epitope. The monoclonal antibody selected is 100% cross-reactive with human growth hormone, so the antigenic determinant identified is shared by the two protein hormones.     

Human pituitary growth hormone: solubility in ammonium sulfate solutions

The solubility of human pituitary growth hormone in ammonium sulfate solutions has been investigated and compared under similar conditions with that of three related proteins: ovine pituitary growth hormone, bovine pituitary growth hormone, and human chorionic somatomammotropin.     

Reaction of human chorionic somatomammotropin and human pituitary growth hormone with tetranitromethane at 0 degrees C

The comparative reactivity of the eight tyrosine residues which occur at homologous positions in human chorionic somatomammotropin and human pituitary growth hormone has been investigated by their reaction with tetranitromethane at 0 °C. The derivatives were characterized by circular dichroism spectra, spectrophotometric titrations, rate of tryptic digestion, and immunodiffusion. Pigeon crop-sac stimulating activities were fully retained in these derivatives. The extent of modification for human chorionic somatomammotropin and human pituitary growth hormone was 2.5 and 4.2 out of 8 residues, respectively. The location of each modified tyrosine residue in the derivatives was determined by amino acid analysis of isolated nitrated peptides after cyanogen bromide cleavage and enzymatic digestion. It was found that tyrosine-143 was highly reactive in the pituitary hormone but unreactive in the placental hormone.     

A circular antibody-antigen complex is responsible for increased affinity shown by mixtures of monoclonal antibodies to human chorionic gonadotropin

Mixtures of some pairs of monoclonal antibodies that have separate epitopes on the beta-subunit of hCG have increased affinity for the hormone relative to that of either antibody alone. A mathematical model developed to explain the phenomenon predicted that a circular tetrameric complex composed of each antibody and two molecules of hCG was responsible for the effect. This structure has now been identified experimentally by the following criteria: 1) the m.w. of the complex observed by electrophoresis (370,000 g/mol) and gel filtration (440,000 g/mol) was in agreement with the m.w. expected for a tetramer composed of two molecules of antibody and two molecules of hCG (i.e., 376,000 g/mol); 2) the ratio of individual antibodies to hCG measured with the use of 131I and 125I-labeled antibodies and/or hCG was 1:1:2; and 3) the complex failed to adhere to affinity columns containing either antibodies or hCG covalently coupled to Sepharose. These columns adsorbed B101, B102, hCG, and mixtures of B101 plus hCG or B102 plus hCG. The observations made with the affinity resins are compatible with a circular model for antigen-antibody complex in which the epitopes of the antigen and the binding site of the antibodies were mutually and completely obscured. Although not studied in detail, a similar complex was formed when the beta-subunit of hCG was substituted for the intact hormone. In addition, a mixture of antibodies that bound to the alpha- and beta-subunits of hCG (i.e., A102 and B102) and that had a higher affinity for the hormone than either antibody also gave rise to a similar species that could be detected after electrophoresis. A pair of antibodies that bind to separate epitopes on the beta-subunit (i.e., B101 and B103) and do not show enhanced affinity for hCG failed to form a stable complex that could be identified as a separate species after electrophoresis. Thus, the studies reported here confirm earlier theoretical predictions linking the increase in affinity observed on mixing monoclonal antibodies to the formation of a circular complex.     

Expression of a monoclonal antibody-defined aminoterminal epitope of human apoC-I on native and reconstituted lipoproteins

Nine distinct mouse monoclonal antibodies were produced in two fusions using holo-human very low density lipoprotein (VLDL) as antigen. On immunoblotting first with human VLDL and then with isolated human apoC-I, seven of the antibodies, representing three isotypes, manifested specificity for apoC-I. Two antibodies were directed against apoB. To assess whether the seven anti-apoC-I antibodies were directed against the same or distinctively different epitopes, cross-competition assays were performed wherein 125I-labeled monoclonal antibodies were made to compete with unlabeled antibodies for occupancy on immobilized VLDL-associated apoC-I. All antibodies cross-competed to varying extents implying that they were directed against closely spaced epitopes, but based on these experiments three different epitopes were defined. On immunoblotting with CNBr fragments, all of the epitopes were assigned to the CNBr I fragment of human apoC-I (amino acids 1-38) suggesting that the NH2-terminal region of apoC-I is more immunogenic in mice than other parts of the molecule when apoC-I is associated with VLDL. A competitive solid-phase radioimmunoassay (RIA) was developed employing one of the anti-apoC-I antibodies (A3-4). VLDL was adsorbed to plastic microtiter wells, and a limiting amount of the antibody was reacted with the adsorbed VLDL. The amount of monoclonal antibody that bound to the immobilized VLDL-apoC-I was determined with a 125I-labeled goat anti-mouse IgG antibody. The addition of competitor apoC-I complexed with lipids resulted in reduced binding of the anti-apoC-I antibody to the immobilized VLDL-apoC-I. Competitor complexes consisted of an artificial lipid emulsion (Intralipid) incubated with apoC-I at phospholipid/apoC-I ratios of 1:1 to 60:1 (w/w). As the lipid/protein ratios were increased, the competitive displacement curves produced by the complexes become progressively steeper, while isolated lipid-free apoC-I produced curves with very shallow slopes, suggesting that a conformation-dependent epitope was being probed. Other apoproteins (C-II, C-III, A-I, A-II, and E) whether lipid-free or complexed with lipids did not compete. Fractionation of the 30:1 apoC-I-Intralipid complex by gel permeation chromatography suggested that apoC-I bound to phospholipids was the most effective competitor. This was confirmed by testing of apoC-I-DMPC complexes, which yielded curves that paralleled those produced by apoC-I-Intralipid.(ABSTRACT TRUNCATED AT 400 WORDS)     

Enhancement of insulin secretion by human chorionic somatomammotropin and related hormones.

Hypophysectomized rats were treated for 6 days with 200 mug per day of either human chorionic somatomammotropin, human pituitary growth hormone, plasmin-modified human pituitary growth hormone, or ovine prolactin. All hormone preparations except ovine prolactin enhanced the ability of the pancreases of hypophysectomized rats to secrete insulin in the isolated pancreas perfusion system.     

Inhibition of hCG, alpha hCG and progesterone release from human placental tissue in vitro by a GnRH antagonist

A dramatic suppression of hCG, alpha hCG and progesterone release from midgestation, human placentas in vitro was effected when incubated with 1 microgram/ml of an antagonist to GnRH. This inhibition of hormonal release occurred rapidly and was partially restored by the addition of GnRH. Human chorionic somatomammotropin was also suppressed, but only two days following the decline of the other hormones. These data demonstrate that an antagonist to GnRH can rapidly inhibit human placental hormone release.     

The epitopes of two complex-specific monoclonal antibodies, related to the receptor recognition site, map to the COOH-terminal end of human alpha 2-macroglobulin

The elucidation of the molecular structure of the receptor recognition site of human alpha 2-macroglobulin (alpha 2M) was approached by mapping the epitopes of two monoclonal antibodies (F2B2 and F12A3). These antibodies were shown to be complex-specific, defining neo-antigenic sites not detectable on native alpha 2M and thereby mimicking the specificity of the receptor expressed on macrophages and fibroblasts. The antibodies inhibited binding of alpha 2M-trypsin complexes to the receptor. The epitopes of both monoclonal antibodies are shown here to be located on the Mr 60,000 heat-induced fragment of partially reduced alpha 2M. Limited proteolysis of alpha 2M-methylamine with lysine-specific bacterial endoproteinase was examined by rate electrophoresis and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to correlate the loss of the epitopes with the generation of defined fragments. 14C-Labeled alpha 2M-methylamine was used as an internal marker for the position of the thioesters. Finally, the epitopes were protected toward proteolysis by subjecting immune complexes of alpha 2M-methylamine with the monoclonal antibodies to proteolysis under the same conditions as uncomplexed alpha 2M-methylamine. The results obtained allowed us to map the epitopes of both the monoclonal antibodies to within a distance of about Mr 20,000 from the COOH-terminal end of human alpha 2M.     

Immunochemical similarity of the human plasma growth hormone-binding protein and the rabbit liver growth hormone receptor

We probed the (immunochemical) relationship between the recently discovered growth hormone binding protein in human plasma and the growth hormone receptor using monoclonal and polyclonal antibodies raised against rabbit liver growth hormone receptor. The human binding protein was recognized by these antibodies; its immunological crossreactivity compared to the rabbit receptor was 1-2%. These data suggest a) that the binding protein and the receptor are structurally related and b) that rabbit and human growth hormone receptors share some but not all epitopes.     

Lipolytic action of human chorionic somatomammotropin.

Human chorionic somatomammotropin extracted and purified from placenta at term was proved to have a lipolytic action in the epididymal fat pad of rats. The following mechanism appears to be involved in the lipolytic action of the hormone; human chorionic somatomammotropin activates adenyl cyclase, thereby increasing the concentration of cyclic AMP in the tissue, which, in turn, activates protein kinase to lead to the activation of hormone sensitive lipase.     

Affinity enhancement of bispecific antibody against two different epitopes in the same antigen.

For the enhancement of antibody binding affinity, a bispecific antibody against two different epitopes in human chorionic gonadotropin hormone, one is in alpha-subunit and the other is in beta-subunit, was prepared by chemical recombination using 5,5'-dithiobis(2-nitrobenzoic acid). The epitopes recognized by antibodies were investigated by competitive radioimmunoassay, two-site sandwich radioimmunoassay and additivity assay and a proper epitope pair was chosen for preparation of the bispecific antibody. This bispecific antibody has dual specificity and as much as 17.2-fold higher affinity than that of monoclonal antibody with higher affinity by dual antigen binding radioimmunoassay and Scatchard plot analysis.     

Human pituitary growth hormone. Intrinsic lipolytic activity on rabbit fat cells.

The stimulation of lipolysis in isolated rabbit fat cells by human growth hormone was investigated in detail. The action of the hormone on rabbit adipocytes is very similar to that of adrenocorticotropin and the melanotropins. The effect is rapid, requires Ca²⁺, appears to be mediated by cyclic AMP, and is not blocked by inhibitors of protein synthesis. The lipolytic action of human growth hormone was neutralized by antisera to itself and to human chorionic somatomammotropin. Several lines of evidence indicate that the rapid lipolytic activity of the growth hormone in rabbit fat cells in an intrinsic property of the hormone, although the physiological significance of this activity remains obscure.     

Fixation of cryo-sections under HIV-1 inactivating conditions: integrity of antigen binding sites and cell surface antigens

Cryostat-sections of biopsies from HIV-infected patients or HIV/SIV-infected experimental animals pose a biohazard risk to laboratory workers. The objective of this study was to select a procedure that appropriately fixes cryo-sections and reduces the risk of HIV-1 infectivity. This inactivation procedure should preserve antigen binding capacity of host-produced antibodies and the antigenic structure of epitopes present in these tissues, while retaining sufficient morphologic detail. We tested the effect of seven different established fixation-inactivation procedures for HIV-1 on the detection of specific antibodies and membrane markers, compared to acetone fixation as a reference. Frozen sections of spleens from mice immunized with trinitrophenyl (TNP)-Ficoll were incubated with TNP-alkaline phosphatase to detect specific antibody-forming cells and follicular immune complexes containing TNP-specific antibodies. In addition, sections were stained with monoclonal antibodies directed against IgM (187-1), T-cells (anti Thy-1), and marginal metallophilic macrophages (MOMA-1). Five procedures proved useful as they gave results similar to regular acetone fixation. In contrast, two procedures with a methanol-containing fixative obscured both antigen binding sites and membrane antigens. Subsequently, these five selected procedures were tested on glass slide preparations of HIV-1 infected cell lines, expressing HIV-1 determinants defined by monoclonal antibodies. Finally, the procedures were tested on sections of an HIV-1 infected human lymph node, for detection of HIV-specific B-cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monoclonal antibodies which preferentially bind to 22 K human growth hormone rather than its 20 K variant

We have established 13 hybridoma cell lines which secrete mouse IgG1 monoclonal antibodies (McAbs) to human growth hormone (hGH). Binding affinity and binding specificity of McAbs were analyzed by competitive radioimmunoassay. Among these McAbs, CL. B1 showed a high affinity of 9.8 x 10(8) l/mol, and all McAbs so far tested showed very weak cross-reactivity or none at all with human prolactin (hPRL) and human chorionic somatomammotropin (hCS; human placental lactogen). Analysis of binding sites of McAbs using hGH variant and fragments in both ELISA and RIA demonstrated that McAbs could be classified into two groups. All the McAbs obtained in this study bound to plasmin-digested fragment S2 (hGH 1-134 and 141-191) and fragment alpha 3 (hGH 1-134 and 147-191). However, five (such as 1D2) out of 13 McAbs bound to fragment F1 (hGH 1-134) and others (such as CL. B1) did not. The McAb CL. B1 in the latter group showed low affinity with 20 K hGH (residue 32-46 deleted in native 22 K hGH) in contrast to high affinity with hGH (22 K). This suggests that the former McAbs recognize an epitope located at the N-terminal two-third part of hGH. In contrast, the McAbs of the latter group are likely to recognize three-dimensional structure of native 22 K hGH.     

Circular dichroism studies on human chorionic somatomammotropin

The conformation of human chorionic somatomammotropin has been studied by means of circular dichroism spectra. The protein appears to contain about 45% α-helix in the native state. Circular dichroism bands in the region of side chain absorption have been assigned to phenylalanine and tryptophan residues. Tentative assignments has also been made to bands probably arising mostly from tyrosine residues. The stability of the native structure has been assessed by challenging the protein with four perturbing solvents. With the exception of 0.1 n NaOH which produced permanent denaturation, all conformational changes produced by the perturbants were fully reversible. In addition, the monomer molecular weight has been evaluated by gel filtration and osmotic pressure measurements. A value of 21,600 ± 900 was found by osmotic pressure at pH 8.4. The results have been compared with similar findings on human pituitary growth hormone and ovine pituitary lactogenic hormone.     

Molecular cloning and nucleotide sequence of the human growth hormone structural gene.

An almost complete cDNA copy of human growth hormone has been cloned and sequenced. The nucleotide sequence confirms the known protein sequence and predicts the sequence of a precursor region of 26 amino acids. We have compared the nucleotide sequence to that for the homolgous proteins, rat growth hormone and human chorionic somatomammotropin (Seeburg et al. and Shine et al., Nature 270, 486 (1977)). There appears to be evolutionary conservation of mRNA sequence features not related to protein structure.     

Preparation of peroxidase: antiperoxidase (PAP) complexes for immunohistological labeling of monoclonal antibodies

The production of mouse peroxidase:antiperoxidase (PAP) complexes suitable for immunohistological use in conjunction with monoclonal antibodies is described. Three approaches were explored: 1) production of conventional polyclonal PAP complexes; 2) conversion of rabbit PAP to "pseudo-mouse PAP" by incubation with monoclonal mouse anti-rabbit immunoglobulin; 3) formation of PAP complexes from monoclonal mouse antiperoxidase. PAP complexes prepared by the latter technique gave the best immunohistological labeling reactions, being stable on storage and compatible with a wide range of human monoclonal antibodies. Gel filtration revealed that monoclonal PAP is of lower molecular weight than conventional PAP complexes (fulfilling theoretical predictions based on the monospecificity of monoclonal antibodies).