Pituitary peptide

60 YEARS OF POMC: Purification and biological characterisation of melanotrophins and corticotrophins

The remarkable conservation of the primary structures and anatomical location of dogfish α-melanocyte-stimulating hormone (MSH), corticotrophin-like intermediate lobe peptide (CLIP) and adrenocorticotrophic hormone (ACTH) compared with mammals reinforced the tissue-specific processing hypothesis of ACTH peptides in the pituitary gland. The cloning of dogfish pro-opiomelanocortin (POMC) led to the identification of δ-MSH and simultaneously revealed the high conservation of the γ-MSH sequence during evolution. These studies have also shown that β-MSH is much less conserved during evolution and in some species is not even processed from β-LPH. Human pro-γ-MSH potentiates the corticosteroidogenic activity of ACTH and peptides generated from its N-terminal, in particular big-γ-MSH, appear to have adrenal mitogenic activity. Human big-γ-MSH (from the zona intermedia) may also cause the adrenache. The review finishes with a cautionary note with regard to the misdiagnosis of the ectopic ACTH syndrome in which partial processing of ACTH can result in large concentrations of α-MSH and CLIP, which can interfere in the performance of two-site immunoassays, and the problem of the correct disulphide bridge arrangement in synthetic N-POMC peptides is also discussed.

The placenta is simply a neuroendocrine parasite

This paper will document the early scientific observations that kindled my neuroendocrinological interest in pre-eclampsia, a life-threatening disease that affects both mother and baby. My interest in this subject started with the placental origin of melanotrophin activity, moving on, through corticotrophin-releasing factor and its binding protein, to a tachykinin modified specifically in the placenta by phosphocholine, a post-translational moiety normally used by parasites to avoid immune surveillance and rejection. This work may finally have led to an understanding of the identity of the elusive placental factor that, whilst attempting to compensate for the poor implantation of the placenta, causes the many symptoms seen in the mother during pre-eclampsia.

Molecular evolution of the polypeptide hormones

Any biological function is at least bimolecular and its evolution therefore is at least dual, with variations in two lines of molecules. The hormone specificity results from a particular fit between the three-dimensional structure of the agent and that of the receptor but, because receptors are not known at the structural level, a discussion on the evolution of the polypeptide hormones is mainly limited to the possible progressive changes of the latter. As for other proteins (enzymes, oxygen carriers etc.) two degrees of complexity can be distinguished according to whether the hormone comprises one or several polypeptide chains. Protein assembly can bring new biological properties, each subunit playing a particular role. In this case, the 'internal' evolution (chain-chain interactions) overlaps the 'external' evolution (hormone-receptor contacts). The 'monomeric' hormones present the following problems: evolution of the prohormone and of the converting enzyme (for insulin), duplication and differentiation of two lines of hormones either by amino acid substitutions (neurohypophysial hormones and neurophysins) or by substitutions and size modifications (corticotropin and lipotropin), duplication and fusion leading to internal homology in the single polypeptide chain (somatotropin, prolactin, placental lactogen). The 'dimeric' hormones lead to several problems: successive duplications giving different subunits, selective associations between subunits, unequal rates of evolution of the subunits, the function of each subunit (lutropin, follitropin, thyrotropin, choriogonadotropin). An attempt is made to integrate the evolution of polypeptide hormones in the frame of the evolution of proteins.

Biosynthesis of insulin and glucagon: a view of the current state of the art

It is now well established that insulin biosynthesis proceeds through a precursor molecule, proinsulin. This single polypeptide chain form has been identified as a ribosomal product in the microsomal fraction from islet tissues. The newly synthesized peptide chain, after folding and thiol oxidation, is transferred to the Golgi apparatus where it begins to undergo proteolytic processing to insulin and packaging into secretory granules. The secretion from the cells of significant amounts of newly synthesized material by exocytosis begins only one hour or more after biosynthesis and this process is regulated by several factors, including glucose. Foci of current attention discussed in this paper include (1) the possible existence of larger precursor forms than proinsulin, especially short-lived biosynthetic transients with extended NH2-termini analogous to the recently described immunoglobulin L chain and proparathyroid hormone precursors; (2) the large-scale production of insulin by chemical or genetic engineering approaches; (3) isolation of beta-cell plasma membranes; (4) regulatory mechanisms for the biosynthesis and secretion of insulin, the possible role of mRNA modification in this process, and effects of somatostatin on insulin biosynthesis and secretion; (5) studies on the secretion, metabolism and clinical usefulness of the proinsulin C-peptide; (6) finally, the biosynthesis of glucagon and other peptide hormones and the general significance of precursor forms.

Comparison of rat anterior and intermediate pituitary in tissue culture: corticotropin (ACTH) and beta-endorphin

The forms of immunoreactive beta-endorphin-sized material in extracts of anterior and intermediate-posterior pituitary from the rat examined by the ion exchange chromatography method of Zakarian & Smyth. The anterior pituitary primarily contained material that co-migrated with synthetic camel beta-endorphin(1-31), whereas the intermediate-posterior pituitary contained relatively little such material. The majority of immunoactive beta-endorphin-sized peptides in the intermediate pituitary eluted at lower concentrations of NaCl than did camel beta-endorphin. Conditions were developed for the stable, long-term tissue culture of dissociated intermediate-posterior pituitary cells. Extracts of cells maintained in tissue culture for 18 h or nine days had the same content of immunoreactive beta-endorphin, 16k fragment, ACTH(18-39) (or CLIP) and ACTH(17-24). Throughout the nine days in culture, characteristic cells that could be immunostained with antibodies to various regions of pro-ACTH/endorphin were present; during the time in culture, non-reactive background cells multiplied rapidly. The major proteolytic processing of pro-ACTH/endorphin remained characteristic of intermediate pituitary tissue throughout the nine days in tissue culture, and did not become similar to the simpler pattern of proteolytic processing found in the anterior pituitary.

Prohormones of beta-melanotropin (beta-melanocyte-stimulating hormone, beta-MSH) and corticotropin (adrenocorticotropic hormone, ACTH): structure and activation

It is proposed that all peptide hormones and releasing factors are biosynthesized in the form of precursor molecules which are biologically inactive. Enzymic activation may take place by hydrolytic cleavage to release a terminal COOH group or by transmidation to form a COOH-terminal amide. Studies with pituitary prohormones and hormones are providing data that support this hypothesis. Evidence has been obtained that the 91 residue beta-lipotropin (beta-LPH) is the prohormone of beta-melanotropin (beta-MSH). The specificity of the pituitary enzymes involved in release of the hormone was demonstrated by the isolation of five constituent fragments of LPH, which were obtained in homogeneous form from the pituitary gland of the pig. The enzymes have specificities similar to trypsin and carboxypeptidase B; carboxypeptidase A and aminopeptidase activities do not appear to be involved. Mild digestion of beta-LPH by trypsin in vitro has confirmed the susceptibility of the peptide bond on the carboxy side of the paired basic residues at positions 59 and 60, adjacent to the COOH-terminus of beta-MSH, and tryptic digestion of a model peptide demonstrated the same specificity. The paired basic residues at positions 39 and 40 adjacent to the NH2-terminus of beta-MSH were more resistant to tryptic attack, both in LPH and in a model peptide. In the gland it is apparent that LPH is cleaved on the carboxy side of the paired lysyl residues at positions 39 and 40, whereas in the synthetic peptide cleavage takes place in between these residues. The activating enzyme may differ from trypsin; alternatively, explanation may be found in the conformation of the prohormone. Prediction of secondary indicates that both pairs of basic residues lie adjacent to beta-bends on the surface of the molecule and occupy sites accessible to enzymic attack. It seems likely that alpha-MSH and corticotropin (ACTH) share a common pro hormone. The release of ACTH could involve cleavage of a -Gly-Ser- bond in the prohormone to expose the NH2-terminus of the hormone. With alpha-MSH, a concerted acetylation and cleavage may take place to form the N-acetylserine residue; the COOH-terminus may be released as an amide by direct transamidation of a -Val-Gly- bond in the prohormone. Release of either hormone would be accompanied by the release of contiguous fragments of the prohormone. We have isolated two novel polypeptides from pig pituitary in substantial quantity and have determined the primary structures. They may represent fragments of a prohormone to alpha-MSH or ACTH.

Molecular cloning of prepro-thyrotropin-releasing hormone cDNA from medaka (Oryzias latipes)

The cDNA encoding prepro-thyrotropin-releasing hormone (ppTRH) in a teleost, medaka (Oryzias latipes) was isolated and characterized. The medaka ppTRH cDNA codes for 270 amino acid residues including eight TRH progenitor sequences (-Lys/Arg-Arg-Gln-His-Pro-Gly-Lys/Arg-Arg-). In silico analyses of the medaka genome database predicted that the structure of the medaka ppTRH gene is similar to the ppTRH genes of the other vertebrate species studied to date; consisting of three exons and two introns. Identity of the medaka ppTRH with the other vertebrates is rather low except the sockeye salmon. A molecular phylogenic tree showed that the ppTRH sequences reflected the predicted pattern of species classification. RT-PCR analysis demonstrated ppTRH gene expression in the brain and retina. These results gave some insight into the molecular evolution of ppTRH and physiological functions of TRH in vertebrates.

Isolation and structure-bioactivity characterization of glycosylated N-pro-opiomelanocortin isoforms

The N-terminal fragment of mouse pro-opiomelanocortin (N-POMC) was isolated from AtT-20 cell-conditioned medium on the basis of immunoreactivity to an anti-POMC1-50 monoclonal antibody by a concentration step, a cation exchange step, reversed phase high-performance liquid chromatography (HPLC) and size exclusion HPLC. Two groups of N-POMC isoforms with a molecular weight (MW) of approximately 11 kDa and 13 kDa, respectively, were identified by mass spectrometry and N-terminal amino acid sequencing. C-terminal sequencing indicated that 11 kDa isoforms correspond to POMC1-74 and 13 kDa isoforms to POMC1-95. Isoforms from both groups enhanced the prolactin mRNA content (measured by means of TaqMan real-time reverse transcription-polymerase chain reaction) in cultured rat pituitary cell aggregates in a dose-dependent manner, but not all of them showed this activity. POMC1-74 compounds were significantly more potent than POMC1-95 isoforms. The observed effects were abolished by coincubation with the monoclonal anti-POMC1-50 antibody, showing the specificity of this biological action. Incorporation of bromodeoxyuridine into DNA of immunostained lactotrophs was enhanced by only a minor part of the isoforms. Some of these had no effect on prolactin mRNA expression. The N-POMC isoforms appeared to be N- and at least in part O-glycosylated. After enzymatic N-deglycosylation of selected N-POMC isoforms, the stimulatory effect on the prolactin mRNA level was depressed (in case of the POMC1-95 isoforms) or totally abolished (in case of the POMC1-74 isoforms). The present findings show that N-POMC is a mixture of differentially glycosylated isoforms, that the isoforms of POMC1-74 are the biologically more effective forms and that different isoforms induce different biological responses in the same cell population. The data also show the essential role of N-glycosylation in the biological response.

Analysis of the aphthovirus 2A/2B polyprotein 'cleavage' mechanism indicates not a proteolytic reaction, but a novel translational effect: a putative ribosomal 'skip'

The 2A region of the aphthovirus foot-and-mouth disease virus (FMDV) polyprotein is only 18 aa long. A 'primary' intramolecular polyprotein processing event mediated by 2A occurs at its own C terminus. FMDV 2A activity was studied in artificial polyproteins in which sequences encoding reporter proteins flanked the 2A sequence such that a single, long, open reading frame was created. The self-processing properties of these artificial polyproteins were investigated and the co-translational 'cleavage' products quantified. The processing products from our artificial polyprotein systems showed a molar excess of 'cleavage' product N-terminal of 2A over the product C-terminal of 2A. A series of experiments was performed to characterize our in vitro translation systems. These experiments eliminated the translational or transcriptional properties of the in vitro systems as an explanation for this imbalance. In addition, the processing products derived from a control construct encoding the P1P2 region of the human rhinovirus polyprotein, known to be proteolytically processed, were quantified and found to be equimolar. Translation of a construct encoding green fluorescent protein (GFP), FMDV 2A and beta-glucuronidase, also in a single open reading frame, in the presence of puromycin, showed this antibiotic to be preferentially incorporated into the [GFP2A] translation product. We conclude that the discrete translation products from our artificial polyproteins are not produced by proteolysis. We propose that the FMDV 2A sequence, rather than representing a proteolytic element, modifies the activity of the ribosome to promote hydrolysis of the peptidyl(2A)-tRNA(Gly) ester linkage, thereby releasing the polypeptide from the translational complex, in a manner that allows the synthesis of a discrete downstream translation product to proceed. This process produces a ribosomal 'skip' from one codon to the next without the formation of a peptide bond.

Immunosuppressive effects of corticotropin and melanotropin and their possible significance in human immunodeficiency virus infection

The activation of human granulocytes and invertebrate immunocytes was found to be suppressed by corticotropin (ACTH) and melanotropin (MSH). In spontaneously active granulocytes both neuropeptides caused significant conformational changes indicative of inactivity plus a reduction in their locomotion. Significant inactivation of human granulocytes by ACTH required 2 hr, that by MSH only 20 min. The addition to the incubation medium of phosphoramidon, a specific inhibitor of neutral endopeptidase 24.11, blocked inactivation of granulocytes by ACTH. Radioimmunoassay for MSH of supernatant fluids from granulocytes incubated with ACTH demonstrated a time-dependent increase in MSH. These data strongly indicate that the effect of ACTH is largely due to its conversion to MSH by granulocyte-associated neutral endopeptidase. Parallel experiments with immunocytes from the mollusc Mytilus edulis gave similar results, indicating the universality of this phenomenon. Our finding that the human immunodeficiency virus, among several viruses, induces ACTH and MSH production in H9 T-lymphoma cells suggests an important role of these neuropeptides in the immunosuppression characteristic of such infections.

Study of frog (Rana esculenta) proopiomelanocortin processing in the intermediate pituitary. Identification of alpha-melanotropin, beta-melanotropin, Lys-gamma-melanotropin, and corticotropin-like intermediate lobe peptide

The proteolytic processing of frog (Rana esculenta) proopiomelanocortin in melanotropic cells of the intermediate pituitary gland has been examined through purification of the mature fragments by reverse-phase high-pressure liquid chromatography and microsequencing of isolated peptides. alpha-Melanotropin, beta-melanotropin, Lys-gamma-melanotropin, corticotropin-like intermediate lobe peptide, and hinge peptide have been isolated and chemically characterized. The results show a high preservation in the processing sites of frog proopiomelanotropin when compared to bovine counterparts. They reveal also a great conservation of the processing enzyme equipment of melanotropic cells in tetrapods species. Identification of Lys-gamma-melanotropin suggests the occurrence of an endopeptidase able to cleave between two basic residues. On the other hand alpha-melanotropin does not appear to be N-acetylated, as previously found in the clawed-toad Xenopus laevis, and this feature might distinguish amphibian from mammalian proopiomelanocortin processing.

Isolation and full structural characterisation of six adrenocorticotropin-like peptides from porcine pituitary gland. Identification of three novel fragments of adrenocorticotropin and of two forms of a novel adrenocorticotropin-like peptide

A partially purified fraction of extracted porcine pituitary glands which possesses lipolytic and adrenocorticotropic activity has been characterised. It consists of six adrenocorticotropin(ACTH)-like peptides (five of which have not been previously described) which were each purified by sequential reverse-phase (rp) HPLC. Their complete primary structures were determined following amino acid compositional analysis, extensive peptide mapping and partial sequencing. Four of the fragments represent the following ACTH fragments; ACTH(1-31), ACTH(7-34), ACTH(7-36) and ACTH(7-38). By combined analytical rpHPLC and an ACTH radioimmunoassay (with an antiserum exhibiting full cross-reaction with all six ACTH variants isolated here), evidence was obtained from analysis of extracts of whole pituitary that these fragments of ACTH exist in significant amounts relative to intact ACTH(1-39). This suggests that ACTH can undergo more extensive differential proteolytic processing than previously thought. These peptides were found to possess reduced or a complete absence of ACTH-like biological activity. Therefore the biological significance of this processing needs to be resolved. The other two fragments also resembled fragments of ACTH but each possessed the same, single amino acid substitution: a threonine replacing the arginine at the position corresponding to position 8 in the ACTH sequence and had the structures [Thr8]ACTH(1-31) and [Thr8]ACTH(7-31). They possess little ACTH-like biological activity. If these variants are derived from a variant ACTH, this would be a significant finding in view of the site of the amino acid substitution and the highly conserved nature of the ACTH primary structure. The possible physiological and genetic implications are briefly discussed. In this study attempts were also made to identify the DNA coding for the mutant ACTH sequence.

Evolutionary specificity of hydrins, new hydroosmotic neuropeptides: occurrence of hydrin 2 (vasotocinyl-Gly) in the toad Bufo marinus but not in the viper Vipera aspis

Hydrin 2 (vasotocinyl-Gly), a hydroosmotic peptide resulting from differential processing of provasotocin and recently identified in frog neurohypophysis, has been looked for in the pituitary gland of an exotic toad (Bufo marinus) and of a reptile (Vipera aspis). Hydrin 2 has been found in the amphibian but not in the reptile. This result confirms the evolutionary specificity of hydrin 2 that has been identified only in frogs and toads but not in birds and reptiles. Occurrence of hydrin 2 is explained by its regulatory function on the water permeability of the skin of anurans.

Purification and sequencing of neuropeptides contained in neuron R15 of Aplysia californica

R15 is a large identified neuron present in the abdominal ganglion of the mollusc Aplysia. Previous studies have indicated that this neuron may play a role in water balance and possibly renovascular functions. A peptidic factor contained in the neuron R15 has been shown to increase the water content of Aplysia. To determine the structure of the peptides contained in R15, we purified the extracts of 820 R15 cells by means of two steps of reverse-phase HPLC. The purification yielded a number of peptides, only one of which, R15 alpha 1, resulted in water uptake when injected into animals. Determination of the amino acid content and sequence analysis of the R15 alpha 1 peptide demonstrated that this peptide contains 38 residues, including two cysteines. The peptide failed to react with iodoacetate, indicating that the two cysteines are connected by a disulfide bridge. To confirm the assigned structure, the peptide was synthesized with a disulfide bridge. The chromatographic properties and bioactivity of the synthetic material were identical to those of the native peptide. Several other R15 peptides were inactive in the bioassay for water uptake. The sequence of one of these peptides (R15 beta) was determined, and it was established that the peptide contains 28 residues. Amino acid analysis of three other peaks was performed. One of these peaks contained a peptide (R15 beta f) whose amino acid composition suggests that it is a fragment of the R15 beta peptide. The other two peaks contained peptides with identical amino acid compositions, suggesting that they are variants of a single peptide (R15 gamma). The amino acid sequences of all the peptides identified in neuron R15 correspond to stretches of a polyprotein encoded by a recently sequenced R15 cDNA.

Particular processing of pro-opiomelanocortin in Xenopus laevis intermediate pituitary. Sequencing of alpha- and beta-melanocyte-stimulating hormones

alpha- and beta-melanocyte-stimulating hormones (alpha-MSH and beta-MSH) have been isolated from Xenopus laevis neurointermediate pituitary and microsequenced. Intracellular alpha-MSH is not N-acetylated after proteolytic processing of pro-opiomelanocortin in contrast to mammalian alpha-MSHs. There is a high preservation of the melanotropic amino acid sequence common to all MSHs although in Xenopus beta-MSH a histidine residue replaces the glutamic acid residue found in position 8 of mammalian beta-MSHs.

S-100 protein in clonal astroglioma cells is released by adrenocorticotropic hormone and corticotropin-like intermediate-lobe peptide

S-100 protein in clonal GA-1 and C6 rat glioma cell lines was released in serum-free medium supplemented with adrenocorticotropic hormone (ACTH). The induction of S-100 protein release by ACTH was dose-dependent, showing a half-maximal release at about 5 microM, and the S-100 protein concentration in the medium increased sharply within 3 min, but slightly during further incubation. The S-100 protein release was apparently accompanied by a decrease in the membrane-bound form of S-100 protein in the cell. The S-100 protein release was induced not by the ACTH1-24 fragment, which exhibits the known effects of ACTH, but by the ACTH18-39 fragment, which is designated as corticotropin-like intermediate-lobe peptide (CLIP). These results indicate that the C-terminal half of ACTH is responsible for the S-100 protein release. The enhancement of S-100 protein release by ACTH was also observed in normal rat glioblasts. The release induced by ACTH was apparently specific to S-100 protein, because little release of the cytoplasmic enzymes, creatine kinase, and enolase was observed under the same conditions. High concentrations (5 mM) of dibutyryl cyclic AMP or dibutyryl cyclic GMP were also found to induce S-100 protein release; however, catecholamines (epinephrine, norepinephrine, isoproterenol, and dopamine), acetylcholine, and glutamic acid did not enhance the release.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteolytic enzymes in the post-translational processing of polypeptide hormone precursors

Selective and limited proteolysis is a key step in the post-translational modification of peptide hormone precursors. This process appears to involve a proteolytic machinery including highly specific endoproteases. Some of the enzyme systems possibly involved in the processing of pro-neuropeptides will be described and their mechanism of action discussed. Special emphasis will be on the following: i) the physico-chemical characteristics of proteolytic enzymes which are believed to be involved in the processing of some of these polypeptide hormone precursors; ii) the bio-specificity of these enzymes toward the substrates; iii) the importance of both secondary and tertiary structures of the cleavage domain in recognition by the selective proteases. These properties will be discussed in connection with the possible importance of the maturation enzymes in the in vivo regulation of hormone biosynthesis.

Endocrine cells producing regulatory peptides

Recent data on the immunolocalization of regulatory peptides and related propeptide sequences in endocrine cells and tumors of the gastrointestinal tract, pancreas, lung, thyroid, pituitary (ACTH and opioids), adrenals and paraganglia have been revised and discussed. Gastrin, xenopsin, cholecystokinin (CCK), somatostatin, motilin, secretin, GIP (gastric inhibitory polypeptide), neurotensin, glicentin/glucagon-37 and PYY (peptide tyrosine tyrosine) are the main products of gastrointestinal endocrine cells; glucagon, CRF (corticotropin releasing factor), somatostatin, PP (pancreatic polypeptide) and GRF (growth hormone releasing factor), in addition to insulin, are produced in pancreatic islet cells; bombesin-related peptides are the main markers of pulmonary endocrine cells; calcitonin and CGRP (calcitonin gene-related peptide) occur in thyroid and extrathyroid C cells; ACTH and endorphins in anterior and intermediate lobe pituitary cells, alpha-MSH and CLIP (corticotropin-like intermediate lobe peptide) in intermediate lobe cells; met- and leu-enkephalins and related peptides in adrenal medullary and paraganglionic cells as well as in some gut (enterochromaffin) cells; NPY (neuropeptide Y) in adrenaline-type adrenal medullary cells, etc.. Both tissue-appropriate and tissue-inappropriate regulatory peptides are produced by endocrine tumours, with inappropriate peptides mostly produced by malignant tumours.

A gonadotropin-like thermostable peptide, prepared from bovine anterior pituitary, inducing spermiation in the frog Rana esculenta (L.) and binding to a rat ovarian FSH receptor

A thermostable peptide, inducing sperm release in Rana esculenta L. and having immunological properties which resemble those of the gonadotropins but nonidentical with any one of the gonadotropins or their subunits, has been prepared from pars distalis of bovine anterior pituitaries by means of acetic acid extraction, heating, ethanol precipitation, and reverse-phase high-performance liquid chromatography (HPLC). During the purification, the biological activity was regularly followed by means of the potency of collected fractions to induce sperm release in frogs. Using HPLC (5 micron C18 silica), the activity was eluted with 38% of acetonitrile:water:trifluoroacetic acid (80:19.5:0.5) and 62% of 0.5% trifluoroacetic acid, pH 3.3. The active principle was also found to bind to a rat ovarian follitropin (FSH) receptor. In contrast, bovine FSH and LH treated identically to the peptide lost all such affinity, indicating that the peptide is not created from either of these during the purification procedure. Further, the peptide was, like FSH, also shown to stimulate aromatase activity in intact Sertoli cells from immature male rats in vitro.

The primary structure of the fourth component of human complement (C4)-C-terminal peptides

C-terminal CNBr peptides of the three polypeptide chains of C4 were obtained and sequenced. These results supplement previously obtained data, notably the protein sequence derived from cDNA sequencing of pro-C4 (Belt KT, Carroll MC & Porter RR (1984) Cell 36, 907-914) and the N-terminal sequences of the three polypeptides (Gigli I, von Zabern I & Porter RR (1977) Biochem. J. 165, 439-446), to define the complete primary structure of the plasma form of C4. The beta (656 residues), alpha (748 residues), and gamma (291 residues) chains are found in positions 1-656, 661-1408, and 1435-1725 in the pro-C4 molecule.

Are prohormones converted to hormones during secretion?

A number of lines of evidence suggest that the polypeptide prohormone converting enzyme is a trypsin-like serine protease. A model is proposed in which the converting enzyme is activated from an inactive zymogen during secretion. Converting enzyme then activates co-secreted prohormone proteolytically. An important feature of the model lies in the geometry of the secretory granule immediately after exocytosis. It is suggested that initially diffusion of the granule contents is limited enough to allow extensive proteolysis to occur. Conversion of prohormone to hormone is terminated by diffusion of converting enzyme and prohormone from the site of release.

Novel variants of adrenocorticotrophic hormone in porcine anterior pituitary

Extracts of porcine anterior pituitary contain several corticotrophic variants of ACTH 1-39. They were isolated by adsorption chromatography, ion-exchange chromatography and reverse-phase high-performance liquid chromatography. Four variants were then identified as starting and ending at positions corresponding to ACTH 1-38, 1-37, 7-39 and 7-38. Several of these fragments were recovered in chromatographically multiple forms. Although all fragments isolated had corticotrophic activity (measured on isolated rat adrenal cells) those with a shortened N-terminal region had a lower potency than those with an intact N-terminal region. Corticotrophic activity of porcine ACTH 7-38 was detected in a preparation with a beta-aspartyl shift at position 25. However, in agreement with previous studies, synthetic human ACTH 7-38, in which an intact Asn-Gly bond was structurally proven, possessed no such activity. The results indicate that position 25 as well as positions 1-6 are important for corticotrophic activity, and that a deamidative beta-aspartyl shift at position 25 can influence the activity.

An ultrastructural study of retinal photoreceptor degeneration associated with bronchial carcinoma

We studied both eyes of a 66-year-old man with retinal degeneration and oat cell carcinoma of the bronchus. Retinal degeneration was most marked peripheral to the parafovea where photoreceptor cells and their outer segments were absent. Within the parafovea, photoreceptor cells remained but rod outer segments were absent and cone outer segments were fragmented and disorganized. The retinal pigment epithelium contained many immature melanin granules within melanolysosomes, suggesting abnormal melanin synthesis and resorption. We suggest that a pharmacologically active substance resembling a hormone produced by the tumor increased melanin synthesis in the pigment epithelium and that the increased melanin content in these cells compromised their ability to phagocytose and maintain normal turnover of photoreceptor outer segments. We believe these changes led to photoreceptor outer segment loss and subsequent degeneration of the photoreceptor cells.

Double immunostaining reveals distinctions among opioid peptidergic neurons in the medial basal hypothalamus

We adapted an uncommon peroxidase substrate, ortho-dianisidine, to double immunostaining methodology in order to analyze the degree of neuronal coexistence of pro-opiomelanocortin-derived peptides and to compare the distribution of neurons containing beta-endorphin immunoreactivity (ir) with those containing enkephalin-ir and dynorphin-ir in the medial basal hypothalamus. Double immunostaining demonstrated co-localization of beta-endorphin-ir, adrenocorticotropic hormone-ir, and gamma 3-melanocyte stimulating hormone-ir in medial basal hypothalamus (MBH) neurons. However, while all perikarya containing beta-endorphin-ir invariably contained adrenocorticotropic hormone-ir, not all of these cell bodies contained gamma 3-melanocyte stimulating hormone-ir. In contrast, separate neuronal cell bodies containing beta-endorphin-ir, enkephalin-ir, and dynorphin-ir were distributed throughout the rostral-caudal extent of the MBH. Fibers containing enkephalin-ir closely surrounded cell bodies containing beta-endorphin-ir, suggesting axosomatic contacts between these two opioid peptidergic neuronal populations.

A study of corticotrophic and melanotrophic activities in the pituitary and brain of the lamprey Lampetra fluviatilis

Biological and radioimmunological assay methods were used in an attempt to detect and determine corticotrophic and melanotrophic activity in the pituitary gland and the central nervous system of the lamprey, Lampetra fluviatilis. The activities of various corticotrophin (1-39 ACTH)-related peptides in the cytochemical assay for ACTH could be readily distinguished on the basis of their dose-response lines. Extracts of the pituitary gland and brain but not of spinal cord were also active in this test. The dose-response lines of the pars distalis extracts were parallel with those of 1-39 ACTH; those of the neurointermediate lobe and the brain resembled des-Ac-alpha-MSH and alpha-MSH, respectively, although subsequent tests showed that the active molecules in the extracts differed from these standard peptides in other respects. Pituitary extracts were active in the Anolis bioassay for melanotrophin but not in a radioimmunoassay for alpha-MSH. Moreover, the electrophoretic Rf value of lamprey melanotrophic activity was distinct from both alpha-MSH and des-Ac-alpha-MSH. Brain extracts showed only low melanotrophic bioactivity and again no immunoreactivity. The results suggest that small amounts of corticotrophin occur in the pars distalis of the lamprey and that the melanotrophic factors in the neurointermediate lobe and brain are not identical to mammalian alpha-MSH and des-Ac-alpha-MSH.

The distribution of some proopiomelanocortin-related peptides in the pituitary gland of the rainbow trout, Salmo gairdneri

Acid extracts of neurointermediate lobe (NIL) and pars distalis (PD) of the pituitary gland of the rainbow trout were radioimmunoassayed for alpha-melanocyte-stimulating hormone (alpha-MSH), beta-melanocyte-stimulating hormone (beta-MSH), adrenocorticotrophin (ACTH), and endorphin. The majority of the alpha-MSH, beta-MSH, and endorphin was present in the NIL; the small amount of these peptides present in the PD may be native to this lobe, or may be due to slight contamination with NIL material--it is not possible to distinguish between these possibilities at present. All the ACTH was present in the PD. These results support the hypothesis that throughout the vertebrates proopiomelanocortin is cleaved to smaller peptides in the NIL than it is in the PD.

Biological and immunological characterization of alpha-melanocyte-stimulating hormone (alpha-MSH) in two neuronal systems of the rat brain

Recent immunocytochemical studies have demonstrated the existence of two different neuronal systems containing alpha-MSH-like material in the brain: one originating from the arcuate nucleus and the other one from the dorsolateral hypothalamus. The aim of the present study was to further characterize alpha-MSH in these two discrete regions of the rat diencephalon. Intracerebroventricular administration of colchicine resulted in a marked decrease in the number of ACTH and beta-endorphin nerve fibers and a significant reduction in ACTH and beta-endorphin content in the dorsolateral hypothalamus. Conversely, colchicine treatment did not alter alpha-MSH, ACTH or beta-endorphin content in the arcuate nucleus and did not significantly affect alpha-MSH concentration in the dorsal region. Reverse-phase high-performance liquid chromatography showed that the major alpha-MSH-like compound localized in the dorsal hypothalamus co-migrated exactly with synthetic alpha-MSH, whereas the arcuate nucleus contained 5 peptides cross-reacting with alpha-MSH antibodies, 4 of them being different from standard alpha-MSH. Significant amounts of biologically active melanotropin, which migrated on Sephadex G-25 columns like synthetic alpha-MSH, were also detected in both the arcuate nucleus and dorsolateral hypothalamus. Taken together, these data demonstrate that the alpha-MSH cell bodies located in the dorsolateral hypothalamus specifically produce authentic alpha-MSH, whereas the alpha-MSH cell bodies in the arcuate nucleus also contain ACTH, beta-endorphin and several peptides immunologically related but not identical to alpha-MSH.

Evidence that the insulin secretagogue, beta-cell-tropin, is ACTH22-39

The pituitary neurointermediate lobe of genetically obese (ob/ob) mice contains a hormone which stimulates insulin release and which cross-reacts with a -COOH-terminal ACTH antiserum, suggesting that it is related to corticotropin-like intermediate lobe peptide (CLIP), the 18-39 fragment of ACTH. The hormone, which we have called beta-cell-tropin, has been shown to be present in the plasma of the ob/ob mouse and to potentiate glucose induced insulin secretion. We have now shown that ACTH22-39 prepared by tryptic digestion of human synthetic CLIP behaves similarly on Biogel chromatography and on reverse-phase HPLC to the naturally occurring beta-cell-tropin. Furthermore, beta-cell-tropin and ACTH22-39 have indistinguishable antigenic and insulin releasing properties.

Characterization of pro-opiocortin-converting activity in purified secretory granules from rat pituitary neurointermediate lobe

Lysates of secretory granules from rat pituitary neurointermediate lobes were incubated with [3H]arginine- or [3H]phenylalanine-labeled toad pro-opiocortin. The processed products formed were identified by immunoprecipitation with adrenocorticotropin (ACTH) and endorphin antisera and by migration behavior on acid/urea/polyacrylamide gels. Pro-opiocortin was cleaved by the proteolytic activity in the secretory granule fraction to approximately 21,000 Mr ACTH, approximately 13,000 Mr ACTH, alpha-melanotropin, 16,000 Mr NH2-terminal glycopeptide, beta-lipotropin, and an endorphin-related peptide. Characterization of this pro-opiocortin-converting activity shows that it (i) is present in membrane and soluble fractions of the granule lysates, (ii) has a pH optimum of 5.0, (iii) appears to cleave at pairs of basic amino acid residues in the precursor, and (iv) is inhibited by leupeptin, pepstatin A, and p-chloromercuribenzoate but not diisopropyl fluorophosphate, N alpha-p-tosyl-L-lysine chloromethyl ketone hydrochloride, chloroquine, or EDTA. These inhibitor studies suggest that the converting-enzyme activity is due to an acid thiol, arginyl protease, distinct from any known cathepsin B-like activity.

Adrenocorticotropin-like and alpha-melanotropin-like peptides in a subpopulation of human gastrin cell granules: bioassay, immunoassay, and immunocytochemical evidence

Adrenocorticotropin (ACTH)-like and alpha-melanotropin (alpha-MSH)-like peptides have been localized to a subpopulation of cytoplasmic (secretory) granules of human antropyloric gastric cells and of fetal and neoplastic gastrin cells. These granules also store gastrin and belong to the electron-dense variety of gastrin cell granules. Gastrin cells also contain granules of low to medium electron density; these store only gastrin and do not react with ACTH or alpha-MSH antisera. The alpha-MSH immunoreactive peptide was shown also to display alpha-MSH bioactivity by a combined immunosorbent-bioassay technique. This peptide cochromatographs with synthetic alpha-MSH in several systems and is not detected in oxyntic mucosa or in gastric muscle wall. As in the pituitary intermediate lobe, the alpha-MSH-like peptide may be formed by cleavage of ACTH-like peptides also in gastrin cells. These data provide additional evidence for local formation of ACTH/alpha-MSH-related peptides in gastrin cells and suggest a heterogenous peptide make-up of endocrine cell granules.

Isolation and characterization of the bovine corticotropin/beta-lipotropin precursor gene

The entire bovine corticotropin/beta-lipotropin precursor gene has been isolated as a set of overlapping genomic DNA fragments which extend over a length of approximately 17000 base pairs. Restriction mapping of the cloned DNA fragments and nucleotide sequence analysis of the whole mRNA-coding segments and their surrounding regions have established that the corticotropin/beta-lipotropin precursor gene is approximately 7300-base-pairs long and contains two intervening sequences; one with an approximate length of 4000 base pairs is located within the segment encoding the 5'-untranslated region of the mRNA, and the other with an approximate length of 220 base pairs interrupts the protein-coding sequence near the signal peptide region. Sequence analysis of more than 200 base pairs preceding the proximal end of the corticotropin/beta-lipotropin precursor gene has revealed a 'Hogness box' and a variant of the model sequence d(G-G-TC-C-A-A-T-C-T) as well as palindrome structures as observed in other eukaryotic genes. Furthermore, some sequence similarities in the 5'-flanking region are found between the corticotropin/beta-lipotropin precursor gene and the mouse alpha-globin and beta-globin genes, all of which are negatively regulated by glucocorticoids. At least four homologous repetitive sequences are distributed at 3000-5000-base-pair distances in the corticotropin/beta-lipotropin precursor gene region; two such sequences are located in the 5'-flanking region, and one within each intervening sequence. Blot hybridization analysis of bovine pituitary nuclear RNA has indicated that the entire corticotropin/beta-lipotropin precursor gene is transcribed into a primary hnRNA product, which is then spliced to form the mature mRNA.

Immunocytochemical detection of anti-ACTH reactivity in pancreatic islet cells of normal and steroid diabetic rats

Morphologically different pancreatic islet cells of the rat reveal immunoreactivity under incubation with anti-ACTH 1-39. In normal rats, anti-ACTH reactivity is detectable predominantly in peripheral, polygonal cells. Paraffin serial section treated alternately with anti-ACTH and anti-glucagon show that the major portion of the anti-ACTH reactivity is localized in A cells. A few A cells, however, lack anti-ACTH reactivity. Anti-ACTH reactivity was also detected in individuals, round-to-oval, occasionally quite large and more centrally situated cells of pancreatic islets. In contrast, islets of steroid diabetic rats reveal a different distribution islets. In contrast, islets of steroid diabetic rats reveal a different distribution of anti-ACTH reactive cells. The number of peripheral reacting cells is greatly reduced; whereas there is an increase in the number of immunoreactive, large, round or polygonal cells, which are distributed throughout the islet. Preliminary investigations indicate that adsorption of ACTH 18-39 and glucagon onto anti-ACTH 1-39 reduces immunoreactivity in normal and steroid diabetic rats, whereas adsorption of ACTH 1-24 does not.

5'-terminal nucleotide sequence of the messenger RNA coding for bovine corticotropin/beta-lipotropin precursor

The complete 5'-terminal nucleotide sequence of the MRNA coding for the bovine common precursor of corticotropin and beta-lipotropin has been determined. The 5'-32P-labelled, 21-nucleotides-long, single-stranded DNA fragment complementary to a portion of the 5'-noncoding region of the mRNA was prepared from a cDNA clone and elongated by reverse transcriptase reaction with the mRNA as template. The DNA transcript formed was sequenced by the procedure of Maxam and Gilbert, and the resultant sequence was cross-checked by two-dimensional electrophoretic analysis of the partial alkaline digest of the 5'-32P-labelled mRNA. The 5'-terminal nucleotide residue was determined by two-dimensional thin-layer chromatography of the complete hydrolysis product of the 5'-32P-labelled mRNA. The nucleotide sequence determined, which partially overlaps the known sequence of the cloned cDNA, reveals the complete 5'-terminal sequence of the mRNA. This, in conjunction with our previous data, defines the complete primary structure of the mRNA. The mRNA is composed of 1098 nucleotides, including an unusually long 5'-noncoding sequence of 128 nucleotides. The presence of a 'cap' structure at the 5' terminus of the mRNA is suggested. The 5'-terminal 48 nucleotide residues of the mRNA are extremely purine-rich, having an A + G content of 83%, whereas all pyrimidine-rich segments are located downstream from there. Because the 5'-noncoding region of the mRNA contains three segments of potential secondary structure which partially overlap, it can exist in a number of alternative base-pairing configurations. However, its interaction with the 3'-terminal segment of 18-S rRNA at the site of maximal complementarity would fix the mRNA configuration in such a way as to bring the possible site of ribosome binding near the initiation codon.