Glycoprotein hormones: structure and function

Characterization and nucleotide sequence of the gene for the common alpha subunit of the bovine pituitary glycoprotein hormones

The gene coding for the common alpha subunit of the bovine pituitary glycoprotein hormones was isolated from a bovine genomic library. The gene spans roughly 16.5 kbp, contains three intervening sequences, and codes for a message of approximately 730 nucleotides. The complete coding region of the gene was sequenced as well as 315 nucleotides of 5' flanking sequence and the entire intron C. Only a single base difference was found when the sequence of the gene was compared with that of the cDNA. Genomic blotting experiments suggest the presence of a single alpha subunit gene. Comparison of the bovine and human alpha subunit genes indicated that the high level of homology observed in the coding regions has been maintained throughout the 5' and 3' untranslated regions, and at least 90 nucleotides of the 5'flanking regions. Additionally, there is an 18 base pair sequence present in both the 5' flanking and 5' untranslated regions of the gene that is homologous to a region of the chick ovalbumin gene. This ovalbumin sequence has been suggested as a binding site for the progesterone receptor-complex.

Subunits of an avian (ostrich) follicle-stimulating hormone and their hybridization with subunits of mammalian gonadotropins

Follicle-stimulating hormone (FSH) from the ostrich, Struthio camelus, was dissociated by methods previously used to prepare subunits from mammalian gonadotropins. Two chemically dissimilar subunits were obtained from the ostrich FSH and these resembled the alpha- and beta-subunits of mammalian FSH in amino acid composition. The subunits were relatively inactive in radioimmunoassay, radioreceptorassay, and bioassay when tested alone, but significant activity was regenerated upon their recombination. Incubations of mixtures of one of the ostrich subunits with the opposing subunit of ovine FSH also resulted in a significant regeneration of binding and biological activity in FSH assays. These hybrid recombinants demonstrated that the species specificity of the FSH molecule is conferred entirely by the source of the beta-subunit; in fact, hybrids formed between ovine LH-alpha and either ovine or ostrich FSH-beta were more potent in FSH assays than were those containing even two homologous FSH subunits. In contrast, there was little regeneration of LH bioactivity when FSH subunits (from ostrich or sheep) were combined with the subunits of ovine LH, although some LH binding activity was obtained when the mixture contained either one of the LH subunits; i.e., even LH-alpha + FSH-beta showed significant enhancement of binding activity for LH receptors. Thus, the subunits of avian FSH show biochemical and functional homologies to those of mammalian FSH, but there is only limited interchangeability with ovine LH subunits.

Correlation of glycosylation forms with position in amino acid sequence

We surveyed published reports on about 50 glycoproteins whose amino acid sequence, glycosylation sites, and type of glycosylation at a particular site have been established. We note that high-mannose substances were rarely found at the N-terminal side of a previously glycosylated complex site. There was a very definite distribution of complex sites about the N-terminal region. Furthermore, secreted glycoproteins usually contained only complex oligosaccharides whereas membrane proteins contained both types. We suggest that the position of the glycosylation site with respect to the N-terminus affects the extent of oligosaccharide processing and subsequent presentation of complex or high-mannose structures in the mature glycoprotein. This review relates glycosylation type to its position in the known sequence of given proteins and discusses these observations in light of known glycosylation processing reactions.

Nucleotide sequence of the cDNA encoding the precursor of the beta subunit of rat lutropin

We have determined the nucleotide sequences of cDNAs encoding the precursor of the beta subunit of rat lutropin, a polypeptide hormone that regulates gonadal function, including the development of gametes and the production of steroid sex hormones. The cDNAs were prepared from poly(A)+ RNA derived from the pituitary glands of rats 4 weeks after ovariectomy and were cloned in bacterial plasmids. Bacterial colonies containing transfected plasmids were screened by hybridization with a 32P-labeled cDNA encoding the beta subunit of human chorionic gonadotropin, a protein that is related in structure to lutropin. Several recombinant plasmids were detected that by nucleotide sequence analyses contained coding sequences for the precursor of the beta subunit of lutropin. Complete determination of the nucleotide sequences of these cDNAs, as well as of cDNA reverse-transcribed from pituitary poly(A)+ RNA by using a synthetic pentadecanucleotide as a primer of RNA, provided the entire 141-codon sequence of the precursor of the beta subunit of rat lutropin. The precursor consists of a 20 amino acid leader (signal) peptide and an apoprotein of 121 amino acids. The amino acid sequence of the rat lutropin beta subunit shows similarity to the beta subunits of the ovine/bovine, porcine, and human lutropins (81, 86, and 74% of amino acids identical, respectively). Blot hybridization of pituitary RNAs separated by electrophoresis on agarose gels showed that the mRNA encoding the lutropin beta subunit consists of approximately 700 bases. The availability of cDNAs for both the alpha and beta subunits of lutropin will facilitate studies of the regulation of lutropin expression.

A simple and sensitive solid-phase radioimmunoassay for the assay of human TSH antibody. Use in screening hybridoma cell lines

A simple and sensitive radioimmunoassay procedure is described for the screening and detection of specific antibodies in hybridoma cell lines. The specific procedure was developed to screen for antibodies against human thyrotropin (hTSH), but the procedure is applicable to screening for any desired antibodies. The immunoglobulin G(IgG) fraction of goat anti-mouse IgG is used to coat wells of microtiter plates. Anti-hTSH antibodies are measured by incubating antiserum dilutions in the coated wells and detecting the bound IgG with radioiodinated hTSH. Unlabeled hTSH may also be detected by its ability to inhibit binding of 125I-hTSH to the coated wells. This assay technique meets the demands of simplicity, sensitivity, reproducibility, and rapidity as a screening assay of hybridoma cell lines capable of secreting anti h-TSH.

Estradiol regulates mRNAs encoding precursors to rat lutropin (LH) and follitropin (FSH) subunits

Anterior pituitary mRNA was prepared using a microscaled method and translated in a wheat-germ cell-free system in the presence of [35S] labeled cysteine and methionine. Translation products, immunologically related to LH beta, FSH beta and the common subunit alpha, were isolated as precursors with antisera to denatured subunits and characterized by SDS-polyacrylamide gel electrophoresis and fluorography. The radioactive bands were excised from the gel and counted for quantitative evaluation. Our data show that translation of pituitary mRNAs from ovariectomized (ovx) rats results in precursor levels increased by 10 fold for alpha and 14 fold for LH beta as compared to the levels in normal rats. The increase in FSH beta precursor was impossible to evaluate as a specific immunoprecipitation product was undetectable in the case of normal rats. Estradiol, but not progesterone, administered in vivo to ovariectomized rats, reversed the stimulatory effect of ovariectomy on the expression of mRNAs coding for gonadotropin subunit precursors 48 h after injection. These results suggest that estradiol, but not progesterone, negatively regulates the synthesis of the pituitary gonadotropins in the female rats via changes in specific mRNA levels. This hormonal control probably occurs at the transcriptional level.

Studies of the histidine residues of human and bovine glycoprotein hormones by nuclear magnetic resonance

Titration curves of the histidine residues in lutropin, thyrotropin, follitropin and chorionic gonadotropin have been assigned using imidazole C-2 proton nuclear magnetic resonance spectra and their estimated pK values determined. Spectra of reassociated hormone preparations, in which one or the other of their two subunits (alpha or beta) have had their accessible histidines exchanged with deuterium, permitted assignment of C-2 resonance to specific residues. Similar titration curves were found for residues which are conserved from one hormone to another. However, these conserved histidines do not have identical pK values, indicating that differences in the conformation or microenvironment around these residues occur in these hormones. Changes in some pK values also occur as a function of subunit association. The most dramatic change seen in all cases is the exposure to solvent of histidine alpha-83; in isolated alpha subunits this residue is unavailable for titration over a wide pH range. This change appears to be a general consequence of the association of the two subunits in any of these hormones. The data show that all histidines in the intact hormones are accessible to the environment, including those proposed to be in domains involved in subunit-subunit interaction.

Cloning of cDNA encoding the pre-beta subunit of mouse thyrotropin

Double-stranded cDNA was synthesized from sucrose gradient-purified poly(A)+ mRNA from a mouse thyrotropic tumor, inserted into the Pst I site of plasmid pBR322 by using poly(dC) . poly(dG) homopolymeric extensions, and cloned in Escherichia coli RRI. Plasmids containing cDNA sequences coding for the beta subunit of thyrotropin (TSH) were identified by cell-free translation of hybrid-selected mRNA and immunoprecipitation with specific antibody to TSH beta subunit. Determination of the nucleotide sequence of one cDNA, 595 base pairs in length, allowed us to deduce the entire amino acid sequence of the mouse TSH beta subunit. The pre-beta subunit contains a 20-amino acid amino-terminal signal sequence followed by a 118-amino acid mature TSH beta subunit. There is 85-90% homology in amino acid sequence between mouse TSH beta subunit and subunits from man, pig, and cow; however, the mouse subunit contains an additional 5 or 6 amino acids at its carboxyl terminus compared to the bovine or human and pig subunits, respectively. TSH beta-subunit mRNA from mouse thyrotropic tumor was estimated to be 750 nucleotides in length by hybridization with labeled TSH beta-subunit cDNA.

Mechanism of action of glycopeptide hormones and cholera toxin: what is the role of ADP-ribosylation?

The cultured murine Leydig tumor cell line MLTC-1 and the normal rat thyroid strain FRTL have adenylate cyclase activities that are stimulated by human chorionic gonadotropin (hCG) and thyrotropin, respectively. Both cell types also respond to choleragen. Activation of adenylate cyclase in membranes by choleragen required NAD whereas stimulation of the enzyme by hormones did not. With [alpha-32P]NAD as a donor, ADP-ribosylation of membrane proteins was determined under the same conditions used to assay adenylate cyclase activity. Under these conditions, choleragen, but not the hormones, caused the ADP-ribosylation of subunits of the regulatory component (G/F) of adenylate cyclase in both FRTL and MLTC-1 membranes. In the absence of any effectors, several membrane proteins became labeled but the hormones did not cause the specific labeling of these or any other membrane proteins. Pretreatment of intact MLTC-1 cells with hCG did not block the ability of choleragen to ADP-ribosylate G/F in isolated membranes; labeling was actually enhanced in a manner related to the length of exposure to hCG. In contrast, pretreatment of the cells with choleragen inhibited ADP-ribosylation of G/F by the toxin in isolated membranes. Extracts of membranes from untreated, hCG-treated, and choleragen-treated MLTC-1 cells were used to reconstitute adenylate cyclase activity in membranes from the cyc- variant of S49 lymphoma cells which lacks a functional G/F. Toxin but not hormone treatment caused an increase in the basal activity of adenylate cyclase in the reconstituted system. Our results indicate that ADP-ribosylation of the regulatory component of adenylate cyclase is required for choleragen action but not for hormone action.

Biological activity of anti-thyrotropin anti-idiotypic antibody

Rabbit anti-rat anti-human thyrotropin anti-idiotypic antibodies have been raised. These antibodies were active at the thyrotropin (TSH) receptor, in that they inhibited 125I-labeled bovine TSH binding to thyroid plasma membranes, stimulated adenylate cyclase activity through a guanyl nucleotide-dependent mechanism, augmented radioiodide transport into isolated porcine thyroid follicular cells and induced such cultured cells to organize into follicles. Aside from substantiating the expectation that anti-hormone anti-idiotypic antibodies may possess properties of the original hormone, this work raised the possibility that thyroid-stimulating antibodies which cause the hyperthyroidism of Graves' disease may be anti-TSH anti-idiotypic antibodies.

Combination of rat lutropin subunits occurs early in the secretory pathway

The combination of lutropin (LH; luteinizing hormone) alpha and beta subunits was examined in rat pituitaries incubated with [35S]methionine or [35S]sulfate. Combination was assessed by using antiserum directed against the beta subunit. The data show that combination of most of the subunits proceeds rapidly, well before the addition of sulfate and prior to the processing of asparagine-linked oligosaccharides to the complex form. Thus, combination appears to initiate in the endoplasmic reticulum and does not require those post-translational modifications. We observed that two forms of the LH-alpha subunit were processed--one that is secreted into the medium not associated with the LH beta subunit and another secreted as part of the alpha-beta dimer. Both forms of the alpha subunit are sulfated, and the data suggest that subsequent to sulfate addition, secretion of free alpha subunit and the dimer occur independently by separate pathways.

Studies on pituitary follitropin. X. Biochemical, receptor binding and immunological properties of deglycosylated ovine hormone

Treatment of ovine pituitary follitropin with anhydrous hydrogen fluoride at 0 degrees C for 60 min resulted in the removal of approximately 80% of the sugars without affecting the polypeptide moiety. Fucose, sialic acid and N-acetylgalactosamine were eliminated completely while the loss of hexoses and N-acetylglucosamine amounted to 86% and 56% respectively. The treatment had no effect on the quaternary structure of the hormone. Consistent with the loss of carbohydrate, the elution volume of the hormone on Sephadex G100 was increased and the deglycosylated hormone lost its ability to interact with the lectin concanavalin-A immobilized on Sepharose. Deglycosylation rendered the hormone less acidic as reflected by altered electrophoretic mobility in polyacrylamide gels at pH 8.9 and 4.5. The UV absorption spectrum of the hormone was not affected by deglycosylation but tryptophan fluorescence was slightly decreased. The receptor-binding activity of the hormone as estimated by a specific radioreceptor assay using adult bovine testicular membranes and labeled 125I-FSH was increased by 250% following deglycosylation. In a conformation specific radioimmunoassay, the deglycosylated hormone showed 150% activity as compared to the native hormone. Thus, the full integrity of the carbohydrate moiety in ovine follitropin is not required for effective receptor binding and immunological activity.

Establishment of gonadotropin-responsive murine leydig tumor cell line

Several clonal Leydig tumor cell lines have been established by adapting the transplantable Leydig tumor, M548OP, to culture. One of these cell line, MLTC-1, has been characterized with regard to the gonadotropin-responsive adenylate cyclase system. The binding of 125I-labeled human chorionic gonadotropin (hCG) was blocked by excess unlabeled hCG and lutropin (LH) but not by follitropin, thyrotropin, or insulin, indicating the presence of specific receptors for hCG and LH. Based on the specific binding of hCG to isolated MLTC-1 membranes, the calculated dissociation constant was 1.0 +/- 0.2 X 10(-10) M. The receptors appeared identical to those from normal murine Leydig cells when analyzed by SDS PAGE and sucrose density gradient centrifugation. The molecular weight and sedimentation coefficient were 95,000 daltons and 8.5 S, respectively. MLTC-1 cells responded to hCG by accumulating cyclic AMP and producing progesterone. Cyclic AMP accumulation was time- and dose-dependent with a maximal accumulation occurring at approximately 0.2 nM hCG. At saturating levels of hCG, cAMP levels reached a maximum by 30 min and then declined very slowly. Adenylate cyclase activity in membranes prepared from MLTC-1 cells was stimulated by hCG, LH, NaF, cholera toxin, and guanyl-5'-ylimidodiphosphate, Additionally, choleragen was found to ADP-ribosylate a membrane protein of 54,000 daltons. This protein resembles the proposed guanine nucleotide regulatory component in both size and choleragen-dependent reactivity. These data suggest that MLTC-1 cells possess a gonadotropin-responsive adenylate cyclase system consisting of a specific hormone receptor, a regulatory component, and a catalytic subunit.

Metabolic labeling of lutropin with [35S]sulfate

Chemical analyses have previously detected sulfate linked to the oligosaccharides of lutropin isolated from bovine and human pituitaries. To determine whether lutropin could be metabolically labeled with sulfate, isolated bovine and rat pituitaries were incubated with [35S]sulfate. In both species, two major labeled products were immunoprecipitated with antisera specific to lutropin subunits. Incorporation into the subunits occurred posttranslationally since it was not blocked by cycloheximide, which did, however, block the incorporation of radiolabeled methionine. Metabolic labeling with [35S]sulfate provides a valuable approach for examining the biosynthetic processing of lutropin and the physiological role of sulfate in this hormone.

Recent advances in the control and function of the anterior pituitary

Major advances in our understanding of the synthesis and release of anterior pituitary hormones have been made over the past several years. Neurons of the hypothalamus have been found to serve as "neuroendocrine transducers" in that they have both electrical and secretory functions. Peptidergic neurons respond to appropriate stimuli with a release of hypothalamic factors into the hypophyseal-portal system. These factors or hormones ultimately control the endocrine function of anterior pituitary cells. Three hormones, Thyrotropin Releasing Hormone (TRH), Gonadotropin Releasing Hormone (GnRH or LHRH) and somatostatin have been identified, synthesized and tested for clinical applications. The clinical assessment of pituitary function has been greatly improved by new and improved radioimmunoassays. One of the recent clinical advances in the area of pituitary disease has been the determination of the relatively high frequency of prolactinomas. Prolactin secreting microadenomas are an important and treatable cause of amenorrhea and infertility in young women. In addition, many pituitary tumors previously believed to be non-functional or "chromophobe adenomas" appear to be prolactinomas. Many new diagnostic and therapeutic techniques are continuing to be developed to improve our management of patients with hypothalamic-pituitary disease.