Production of recombinant rat proopiomelanocortin1-74 and characterization of its mitogenic action on pituitary lactotrophs

Detection and identification of arginine modifications on methylglyoxal-modified ribonuclease by mass spectrometric analysis

Analysis of the broad range of trace chemical modifications of proteins in biological samples is a significant challenge for modern mass spectrometry. Modification at lysine and arginine residues, in particular, causes resistance to digestion by trypsin, producing large tryptic peptides that are not readily sequenced by mass spectrometry. In this work, we describe the analysis of ribonuclease (RNase) modified by methylglyoxal (MGO) under physiological conditions. For detection of modifications, we use comparative analysis of the single combined spectra extracted from the full-scan MS data of the tryptic digests from native and modified proteins. This approach revealed 11 ions unique to MGO-modified RNase, including a 32-amino acid peptide containing a modified Arg-85 residue. Sequential digestion of MGO-modified RNase by endoproteinase Glu-C and trypsin was required to obtain peptides that were amenable to sequencing analysis. Arg-39 was identified as the main site of modification (35% modification) on MGO-modified Rnase, and the dihydroxyimidazolidine and hydroimidazolone derivatives were the main adducts formed, with minor amounts of the tetrahydropyrimidine and argpyrimidine derivatives. For identification of these products, we used variations in source voltage and collision energy to obtain the dehydration and decarboxylation products of the tetrahydropyrimidine-containing peptides and dehydration of the dihydroxyimidazoline-containing peptides. The resultant spectra were dependent on the cone voltage and collision energy, and analysis of spectra at various settings permitted structural assignments. These studies illustrate the usefulness of single combined mass spectra extracted from full-scan data and variations in source and collision cell voltages for detection and structural characterization of chemical adducts on proteins.

A critical review of the methods for cleavage of fusion proteins with thrombin and factor Xa

Expression and purification of proteins in recombinant DNA systems is a powerful and widely used technique. Frequently there is the need to express the protein of interest as a fusion protein or chimeric protein. Fusion protein technology is frequently used to attach a "signal" which can be used for subsequent localization of the protein or a "carrier" which can be used to deliver a "therapeutic" such as a radioactive molecule to a specific site. In addition to these applications, fusion protein technology can be employed for several other useful purposes. Of these, the most frequent reason is to provide a 'tag' or 'handle' which will aid in the purification of the protein. Another useful purpose is to improve the expression or folding of the protein of interest. In these latter two situations, it is often necessary to remove the fusion partner before the recombinant protein of interest can be used for further studies. This removal process involves the insertion of a unique amino acid sequence that is susceptible to cleavage by a highly specific protease. Thrombin and factor Xa are the most frequently used proteases for this application. The purpose of this review is to discuss the application of thrombin and factor Xa for the cleavage of fusion proteins. It is emphasized that while these enzymes are quite specific for cleavage at the inserted cleavage site, proteolysis can frequently occur at other site(s) in the protein of interest. It is necessary to characterize the protein of interest after cleavage from the affinity label to assure that there are no changes in the covalent structure of the protein of interest. Examples are presented which describe the proteolysis of the protein of interest by either factor Xa or thrombin.

Occurrence of beta-endorphin binding sites in the pituitary of the frog Rana esculenta: effect of beta-endorphin on luteinizing hormone secretion

The possible effect of proopiomelanocortin-derived peptide, beta-endorphin on frog gonadotrope cells was investigated. Binding and internalization of beta-endorphin to pituitary pars distalis cultured cells were visualized by immunofluorescence and analyzed by means of confocal laser scanning microscopy. Using biotinylated endorphin, the time-course of beta-binding showed that this opioid was internalized through receptor-mediated endocytosis, the mechanism in which actin and clathrin were involved; then, the lysosomal degradation program occurred at later stages. The beta-endorphin binding was well antagonized by Naloxone, the opiate receptor antagonist, and up-regulated since more rapid response was obtained in the previously primed cells. The double immunostaining reaction for beta-endorphin and LH beta-subunit revealed that half the beta-endorphin labeled cell population was positively immunostained for LH beta-subunit, and beta-endorphin was able to induce an increasing trend of LH secretion in cultured pars distalis cells. Therefore, it seems that beta-endorphin acts directly on pituitary pars distalis and influences gonadotropin secretion through the interaction with its own receptor.

Paracrine control of lactotrope proliferation and differentiation

Lactotropes in the pituitary gland might be useful models of how a cell type develops, differentiates, proliferates and regresses under the control of paracrine and autocrine signals. Lactotrope development during embryonic life is determined by a well-defined sequence of temporal and positional actions of locally produced members of the bone morphogenetic protein, hedgehog and fibroblast growth factor families. Transforming growth factor alpha (TGF-alpha), TGF-beta and galanin mediate the action of estrogen on the postnatal expansion of the lactotrope cell population and expression of the gene encoding prolactin in an autocrine/paracrine manner. Moreover, the classic hormone precursor pro-opiomelanocortin generates differentially glycosylated isoforms of its N-terminal fragment as paracrine controllers, which each induce distinct aspects of lactotrope differentiation and growth.

The pro-opiomelanocortin gene of the zebrafish (Danio rerio)

The cDNA and the gene for pro-opiomelanocortin (POMC) in the zebrafish (Danio rerio) were isolated and analyzed. The gene consists of three exons and two short introns and has a similar overall structural organization as in Homo sapiens. Intron 1 (339 bp) divides the 5(') untranslated region from the coding region while intron 2 (1522 bp) is located between the signal peptide and the sequence encoding ACTH. Transcription starts 26 bp downstream of a TATA box and there is one polyadenylation signal in the 3(') untranslated region. The cDNA comprises of 964 bp with an open reading frame encoding a 222 amino acid hormone prepropeptide that is split into six putative hormones. Sequence comparison of zebrafish POMC to sequences of various other vertebrate species reveals four regions that are highly conserved during the evolution of vertebrates-the N-terminal region, ACTH, beta-MSH, and beta-endorphin, whereas the connecting peptides show a much higher degree of variability. Phylogenetic analysis of the POMC sequences of various vertebrate species resulted in the expected pattern of species evolution. In situ hybridization demonstrated POMC expression in a cluster of cells (corticotrophs) in the pituitary of the zebrafish as early as 23 h after fertilization. These findings will facilitate the use of the zebrafish as a model organism in the study of the physiological role of POMC-derived peptides.

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.

Effects of melanocortins on cardiovascular regulation in rats

1. Of the melanocortin peptides, gamma(2)-melanocyte-stimulating hormone (MSH) has been attributed a cardiovascular effect, inducing an increase in blood pressure and heart rate. Although still controversial, this effect, based on pharmacological blockade experiments, is supposed to be mediated through sympathetic activation. 2. The aims of the present study were to identify the N-terminal pro-opiomelanocortin (N-POMC) fragments and melanocortins that influence blood pressure and heart rate and to investigate the real-time changes in baroreflex sensitivity and in sympathetic and vagal modulation underlying cardiovascular effects in conscious rats without the use of pharmacological blockade. 3. Intracerebroventricular administration of different melanocortins and N-POMC induced a long-lasting dose- dependent pressor response from 1 nmol onwards, with only a small initial bradycardic response with the highest dose. 4. Coinciding with this pressor response, an elicitation of the low-frequency (LF) component was observed in spectral analysis of both blood pressure variability (BPV) and heart rate variability (HRV), followed by the high-frequency (HF) component in at least BPV. Baroreflex sensitivity remained unchanged. 5. After intravenous administration, gamma(2)-MSH produced a short-lasting dose-dependent pressor and cardioaccelerator response with very rapid onset with concentrations from 1 nmol onwards. 6. Continuous infusion of gamma(2)-MSH depressed baroreflex sensitivity and simultaneously increased both components of BPV, with a radical reduction of the LF component and a preserved vagal HF component in HRV. 7. Of all the intravenously administered melanocortins, only gamma(2)-MSH was active. The central effect is likely to depend on an increase of (alpha-)sympathetic outflow. 8. For the peripheral effect, gamma(2)-MSH appeared to act as a baroreceptor reflex-blocking agent, being compatible with a role in the acute stress response.

Structure-activity relationship and signal transduction of gamma-MSH peptides in GH3 cells: further evidence for a new melanocortin receptor

The structure-activity relationship and signal transduction properties of the pro-opiomelanocortin (POMC)-derived gamma-MSH peptides in the GH3 cell line was compared with that described for the known melanocortin receptors (MCRs). Single alanine replacements showed that, unlike the classical MCRs, the His(5)-Phe(6)-Arg(7)-Trp(8) sequence in gamma2-MSH is not a core sequence for activating the gamma-MSH receptor in GH3 cells, whereas Met(3) is essential. gamma2-MSH increased binding of [35S]GTPgammaS to membrane preparations of GH3 cells. Blockade of protein kinase A abolished the [Ca(2+)](i) responses to gamma3-MSH, and low nanomolar doses of gamma3-MSH increased intracellular cAMP levels, which could be blocked by pertussis toxin (PTX). We conclude that the putative novel gamma-MSH receptor in GH3 cells is a GPCR, but with structure-activity and signal transduction features different from those of the classical MCRs.

Growth and differentiation factors derived from the N-terminal domain of pro-opiomelanocortin

1. We describe a novel paracrine control system in the pituitary gland, consisting of peptides derived from the N-terminal fragment of pro-opiomelanocortin (N-POMC), for example POMC(1-74) and gamma3-melanocyte-stimulating hormone (MSH). 2. By searching the target cells of these N-POMC fragments, using the rise of intracellular free calcium as a response system and single cell reverse transcription-polymerase chain reaction of hormone mRNA as a cell type identification method, we found that a considerable number of cells in normal rat pituitary display combinatorial expression of different pituitary hormone genes (further referred to as 'multihormone mRNA cells'), without indication that all these cells also produce or store the respective hormones translatable from these mRNA. The N-POMC fragments POMC(1-74) and gamma3-MSH preferentially target particular subsets of these multihormone mRNA cells. 3. We discovered a potentially novel receptor for gamma3-MSH on these cells; more precisely, on cells coexpressing growth hormone and prolactin. The putative novel receptor displays properties highly divergent from those of the known gamma3-MSH receptor (i.e. the melanocortin-3 receptor) and even of all other melanocortin receptors cloned today.

Stimulation of intracellular free calcium in GH3 cells by gamma3-melanocyte-stimulating hormone. Involvement of a novel melanocortin receptor?

The melanocortin (MC) gamma3MSH is a peptide that can be generated from the N-terminal domain of POMC and is believed to signal through the MC3 receptor. We recently showed that it induces a sustained rise in intracellular free calcium levels ([Ca(2+)](i)) in a subpopulation of pituitary cells, particularly in the lactosomatotroph lineage. In the present study we report that gamma3MSH and some analogs increase [Ca(2+)](i) in the GH- and PRL-secreting GH3 cell line and evaluate on the basis of pharmacological experiments and gene expression studies which MC receptor may be involved. A dose as low as 1 pM gamma3MSH induced an oscillating [Ca(2+)](i) increase in a significant percentage of GH3 cells. Increasing the dose recruited an increasing number of responding cells; a maximum was reached at 0.1 nM. gamma2MSH, alphaMSH, and NDP-alphaMSH displayed a similar effect. SHU9119, an MC3 and MC4 receptor antagonist, and an MC5 receptor agonist, did not affect the number of cells showing a [Ca(2+)](i) rise in response to gamma3MSH. SHU9119 had also no effect when added alone. MTII, a potent synthetic agonist of the MC3, MC4, and MC5 receptor as well as an N-terminally extended recombinant analog of gamma3MSH showed low potency in increasing [Ca(2+)](i) in GH3 cells, but high potency in stimulating cAMP accumulation in HEK 293 cells stably transfected with the MC3 receptor. In contrast, a peptide corresponding to the gamma2MSH sequence of POMC-A of Acipenser transmontanus increased [Ca(2+)](i) in GH3 cells, but was about 50 times less potent than gamma2- or gamma3MSH in stimulating cAMP accumulation in the MC3 receptor expressing HEK 293 cells. By means of RT-PCR performed on a RNA extract from GH3 cells, the messenger RNA of the MC2, MC3, and MC4 receptor was undetectable, but messenger RNA of the MC5 receptor was clearly present. These data suggest that the GH3 cell line does not mediate the effect of gamma3MSH through the MC3 receptor. The involvement of the MC5 receptor is unlikely, but cannot definitely be excluded. The findings animate the hypothesis that there exists a second, hitherto unidentified, MC receptor that displays high affinity for gamma3MSH.