Extraordinarily stable disulfide-linked homodimer of human growth hormone

Improved immunostaining of nanostructures and cells in human brain specimens through expansion-mediated protein decrowding

Proteins are densely packed in cells and tissues, where they form complex nanostructures. Expansion microscopy (ExM) variants have been used to separate proteins from each other in preserved biospecimens, improving antibody access to epitopes. Here, we present an ExM variant, decrowding expansion pathology (dExPath), that can expand proteins away from each other in human brain pathology specimens, including formalin-fixed paraffin-embedded (FFPE) clinical specimens. Immunostaining of dExPath-expanded specimens reveals, with nanoscale precision, previously unobserved cellular structures, as well as more continuous patterns of staining. This enhanced molecular staining results in observation of previously invisible disease marker-positive cell populations in human glioma specimens, with potential implications for tumor aggressiveness. dExPath results in improved fluorescence signals even as it eliminates lipofuscin-associated autofluorescence. Thus, this form of expansion-mediated protein decrowding may, through improved epitope access for antibodies, render immunohistochemistry more powerful in clinical science and, perhaps, diagnosis.

Validation of the male-specific ORF of the paternally-transmitted mtDNA in Mytilus edulis as a protein-coding gene

There appears to be an additional set of sex-specific mtDNA-encoded proteins in bivalve species with doubly uniparental mitochondrial inheritance that may be involved in the transmission of the female and male mitogenomes. In the marine mussel Mytilus edulis, the translation of the female-specific open reading frame (F-ORF) was demonstrated but the translation of the male-specific ORF (M-ORF) remains to be shown. Here we validate the male-specific ORF of the paternal mitogenome in M. edulis as a protein-coding gene. The M-ORF protein was detected only in male gonads and localized in sperm mitochondria and acrosome, suggesting that it is involved in a key sperm function in Mytilus edulis.

A small protein coded within the mitochondrial canonical gene nd4 regulates mitochondrial bioenergetics

BACKGROUND

Mitochondria have a central role in cellular functions, aging, and in certain diseases. They possess their own genome, a vestige of their bacterial ancestor. Over the course of evolution, most of the genes of the ancestor have been lost or transferred to the nucleus. In humans, the mtDNA is a very small circular molecule with a functional repertoire limited to only 37 genes. Its extremely compact nature with genes arranged one after the other and separated by short non-coding regions suggests that there is little room for evolutionary novelties. This is radically different from bacterial genomes, which are also circular but much larger, and in which we can find genes inside other genes. These sequences, different from the reference coding sequences, are called alternatives open reading frames or altORFs, and they are involved in key biological functions. However, whether altORFs exist in mitochondrial protein-coding genes or elsewhere in the human mitogenome has not been fully addressed.

RESULTS

We found a downstream alternative ATG initiation codon in the + 3 reading frame of the human mitochondrial nd4 gene. This newly characterized altORF encodes a 99-amino-acid-long polypeptide, MTALTND4, which is conserved in primates. Our custom antibody, but not the pre-immune serum, was able to immunoprecipitate MTALTND4 from HeLa cell lysates, confirming the existence of an endogenous MTALTND4 peptide. The protein is localized in mitochondria and cytoplasm and is also found in the plasma, and it impacts cell and mitochondrial physiology.

CONCLUSIONS

Many human mitochondrial translated ORFs might have so far gone unnoticed. By ignoring mtaltORFs, we have underestimated the coding potential of the mitogenome. Alternative mitochondrial peptides such as MTALTND4 may offer a new framework for the investigation of mitochondrial functions and diseases.

Pseudophosphorylation of Arabidopsis jasmonate biosynthesis enzyme lipoxygenase 2 via mutation of Ser600 inhibits enzyme activity

Jasmonates are oxylipin phytohormones critical for plant resistance against necrotrophic pathogens and chewing herbivores. An early step in their biosynthesis is catalyzed by non-heme iron lipoxygenases (LOX; EC 1.13.11.12). In Arabidopsis thaliana, phosphorylation of Ser⁶⁰⁰ of AtLOX2 was previously reported, but whether phosphorylation regulates AtLOX2 activity is unclear. Here, we characterize the kinetic properties of recombinant WT AtLOX2 (AtLOX2^WT). AtLOX2^WT displays positive cooperativity with α-linolenic acid (α-LeA, jasmonate precursor), linoleic acid (LA), and arachidonic acid (AA) as substrates. Enzyme velocity with endogenous substrates α-LeA and LA increased with pH. For α-LeA, this increase was accompanied by a decrease in substrate affinity at alkaline pH; thus, the catalytic efficiency for α-LeA was not affected over the pH range tested. Analysis of Ser⁶⁰⁰ phosphovariants demonstrated that pseudophosphorylation inhibits enzyme activity. AtLOX2 activity was not detected in phosphomimics Atlox2^S600D and Atlox2^S600M when α-LeA or AA were used as substrates. In contrast, phosphonull mutant Atlox2^S600A exhibited strong activity with all three substrates, α-LeA, LA, and AA. Structural comparison between the AtLOX2 AlphaFold model and a complex between 8R-LOX and a 20C polyunsaturated fatty acid suggests a close proximity between AtLOX2 Ser⁶⁰⁰ and the carboxylic acid head group of the polyunsaturated fatty acid. This analysis indicates that Ser⁶⁰⁰ is located at a critical position within the AtLOX2 structure and highlights how Ser⁶⁰⁰ phosphorylation could affect AtLOX2 catalytic activity. Overall, we propose that AtLOX2 Ser⁶⁰⁰ phosphorylation represents a key mechanism for the regulation of AtLOX2 activity and, thus, the jasmonate biosynthesis pathway and plant resistance.

Discovery of prolactin-like in lamprey: Role in osmoregulation and new insight into the evolution of the growth hormone/prolactin family

We used a representative of one of the oldest extant vertebrate lineages (jawless fish or agnathans) to investigate the early evolution and function of the growth hormone (GH)/prolactin (PRL) family. We identified a second member of the GH/PRL family in an agnathan, the sea lamprey (Petromyzon marinus). Structural, phylogenetic, and synteny analyses supported the identification of this hormone as prolactin-like (PRL-L), which has led to added insight into the evolution of the GH/PRL family. At least two ancestral genes were present in early vertebrates, which gave rise to distinct GH and PRL-L genes in lamprey. A series of gene duplications, gene losses, and chromosomal rearrangements account for the diversity of GH/PRL-family members in jawed vertebrates. Lamprey PRL-L is produced in the proximal pars distalis of the pituitary and is preferentially bound by the lamprey PRL receptor, whereas lamprey GH is preferentially bound by the lamprey GH receptor. Pituitary PRL-L messenger RNA (mRNA) levels were low in larvae, then increased significantly in mid-metamorphic transformers (stage 3); thereafter, levels subsided in final-stage transformers and metamorphosed juveniles. The abundance of PRL-L mRNA and immunoreactive protein increased in the pituitary of juveniles under hypoosmotic conditions, and treatment with PRL-L blocked seawater-associated inhibition of freshwater ion transporters. These findings clarify the origin and divergence of GH/PRL family genes in early vertebrates and reveal a function of PRL-L in osmoregulation of sea lamprey, comparable to a role of PRLs that is conserved in jawed vertebrates.

Thrombin Cleaves Prolactin Into a Potent 5.6-kDa Vasoinhibin: Implication for Tissue Repair

Vasoinhibin is an endogenous prolactin (PRL) fragment with profibrinolytic, antivasopermeability, and antiangiogenic effects. The fact that blood clotting, vascular permeability, and angiogenesis are functionally linked during the wound-healing process led us to investigate whether thrombin, a major protease in tissue repair, generates vasoinhibin. Here, we have incubated human PRL with thrombin and analyzed the resulting proteolytic products by Western blot, mass spectrometry, high-performance liquid chromatography purification, recombinant production, and bioactivity. We unveil a main thrombin cleavage site at R48-G49 that rapidly (< 10 minutes) generates a 5.6-kDa fragment (residues 1-48) with full vasoinhibin activity, that is, it inhibited the proliferation, invasion, and permeability of cultured endothelial cells and promoted the lysis of a fibrin clot in plasma with a similar potency to that of a conventional 14-kDa vasoinhibin (residues 1-123). The R48-G49 cleavage site is highly conserved throughout evolution and precedes the intramolecular disulfide bond (C58-C174), thereby allowing the 5.6-kDa vasoinhibin to be released without a reduction step. Furthermore, the 5.6-kDa vasoinhibin is produced by endogenous thrombin during the clotting process. These findings uncover the smallest vasoinhibin known, add thrombin to the list of PRL-cleaving proteases generating vasoinhibin, and introduce vasoinhibin as a thrombin-activated mechanism for the regulation of hemostasis, vasopermeability, and angiogenesis in response to tissue injury.

Synthesis and Assembly of Hepatitis B Virus-Like Particles in a Pichia pastoris Cell-Free System

Virus-like particles (VLPs) are supramolecular protein assemblies with the potential for unique and exciting applications in synthetic biology and medicine. Despite the attention VLPs have gained thus far, considerable limitations still persist in their production. Poorly scalable manufacturing technologies and inconsistent product architectures continue to restrict the full potential of VLPs. Cell-free protein synthesis (CFPS) offers an alternative approach to VLP production and has already proven to be successful, albeit using extracts from a limited number of organisms. Using a recently developed Pichia pastoris-based CFPS system, we have demonstrated the production of the model Hepatitis B core antigen VLP as a proof-of-concept. The VLPs produced in the CFPS system were found to have comparable characteristics to those previously produced in vivo and in vitro. Additionally, we have developed a facile and rapid synthesis, assembly and purification methodology that could be applied as a rapid prototyping platform for vaccine development or synthetic biology applications. Overall the CFPS methodology allows far greater throughput, which will expedite the screening of optimal assembly conditions for more robust and stable VLPs. This approach could therefore support the characterization of larger sample sets to improve vaccine development efficiency.

Decreased Activity of the Ghrhr and Gh Promoters Causes Dominantly Inherited GH Deficiency in Humanized GH1 Mouse Models

Isolated growth hormone deficiency type II (IGHD2) is mainly caused by heterozygous splice-site mutations in intron 3 of the GH1 gene. A dominant-negative effect of the mutant GH lacking exon 3 on wild-type GH secretion has been proposed; however, the molecular mechanisms involved are elusive. To uncover the molecular systems underlying GH deficiency in IGHD2, we established IGHD2 model mice, which carry both wild-type and mutant copies of the human GH1 gene, replacing each of the endogenous mouse Gh loci. Our IGHD2 model mice exhibited growth retardation along with intact cellular architecture and mildly activated endoplasmic reticulum stress in the pituitary gland, caused by decreased GH-releasing hormone receptor (Ghrhr) and Gh gene promoter activities. Decreased Ghrhr and Gh promoter activities were likely caused by reduced levels of nuclear CREB3L2, which was demonstrated to stimulate Ghrhr and Gh promoter activity. To our knowledge, this is the first in vivo study to reveal a novel molecular mechanism of GH deficiency in IGHD2, representing a new paradigm that differs from widely accepted models.

A pull-down and slot blot-based screening system for inhibitor compounds of the podoplanin-CLEC-2 interaction

Podoplanin, a transmembrane glycoprotein, is overexpressed in certain types of tumors and induces platelet aggregation by binding to C-type lectin-like receptor 2 (CLEC-2) on the platelet membrane. Activated platelets release granule components, which in turn, trigger epithelial-mesenchymal transition and confer invasive capacity to the tumor cells. Therefore, blocking the podoplanin-CLEC-2 interaction by a small-molecule compound is a potential therapeutic strategy to prevent cancer metastasis and invasion. To effectively identify such inhibitory compounds, we have developed a pull-down-based inhibitory compound screening system. An immunoglobulin Fc domain-CLEC-2 fusion protein was used as a bait to capture podoplanin derived from podoplanin-overexpressing HeLa cells in the presence and absence of the test compound. The protein complex was then pulled down using protein A beads. To shorten the turnaround time, increase throughput, and decrease the workload for the operators, centrifugal filter units were employed to separate free and bound podoplanin, instead of using customary aspiration-centrifugation washing cycles. Slot blotting was also utilized in lieu of gel electrophoresis and electrical transfer. Thus, the use of our pull down screening system could facilitate the effective selection of potential inhibitor compounds of the podoplanin-CLEC-2 interaction for cancer therapy. Importantly, our methodology is also applicable to targeting other protein-protein interactions.

Expression of bovine interleukin 15 in Pichia pastoris and study on its biological activity: a T-cell activator

Interleukin 15 (IL-15) plays crucial role as stimulatory cytokine in generation and proliferation of CD8⁺ T memory cells. The present study was initiated to analyze the role of yeast-expressed bovine IL-15 in inducing the CD8⁺ T memory cells. The bIL-15 was cloned in pPICZαA expression vector. The expressed bovine IL-15 was purified by anion exchange chromatography and further characterized using SDS-PAGE and Western blotting. Biological activity of the purified protein was evaluated in vitro through induction of Bcl2 in IL-15 stimulated PBMCs was measured using qPCR and the phosphorylation of STAT3 was confirmed by immuno-staining of HEK273 cells. From the recent research studies, it was evident that the fatty acid oxidation catalyzed by Carnitine Palmitoyl Transferase 1a (Cpt1a) is a critical step during the conversion of effector CD8⁺ T cells to memory CD8⁺ T cells which is induced by IL-15. There were no research studies revealed the role of IL-15 on activating memory CD8⁺ T cells by influencing the Cpt1a in Bovines was documented; yet and in the present study, we evaluated that bovine IL-15 could induce the expression of Cpt1a in IL-15 treated bovine PBMCs and helps in memory cell induction.

Urea can inhibit efficient reduction and alkylation of protein dimers in solution demonstrated by the beta subunit of alpha glucosidase II

Protein reduction and alkylation is routinely used for analysis of protein dimers and protein complexes in cell fractions using two dimensional gel electrophoresis and mass spectrometry. To resolve the heterogeneity of a high molecular weight protein band that is highlighted by an antibody to the beta subunit of alpha glucosidase II (GIIβ), we performed reduction and alkylation of cytosolic proteins extracted from mouse brain. The presence of urea in the reduction/alkylation buffer inhibited the chemical processes. It is thus recommended that protein reduction/alkylation be performed both in the presence and absence of urea for the separation of mono-/hetero-mers.

Identification of Myricetin as an Ebola Virus VP35-Double-Stranded RNA Interaction Inhibitor through a Novel Fluorescence-Based Assay

Ebola virus (EBOV) is a filovirus that causes a severe and rapidly progressing hemorrhagic syndrome; a recent epidemic illustrated the urgent need for novel therapeutic agents because no drugs have been approved for treatment of Ebola virus. A key contribution to the high lethality observed during EBOV outbreaks comes from viral evasion of the host antiviral innate immune response in which viral protein VP35 plays a crucial role, blocking interferon type I production, first by masking the viral double-stranded RNA (dsRNA) and preventing its detection by the pattern recognition receptor RIG-I. Aiming to identify inhibitors of the interaction of VP35 with the viral dsRNA, counteracting the VP35 viral innate immune evasion, we established a new methodology for high-yield recombinant VP35 (rVP35) expression and purification and a novel and robust fluorescence-based rVP35-RNA interaction assay ( Z' factor of 0.69). Taking advantage of such newly established methods, we screened a small library of Sardinian natural extracts, identifying Limonium morisianum as the most potent inhibitor extract. A bioguided fractionation led to the identification of myricetin as the component that can inhibit rVP35-dsRNA interaction with an IC₅₀ value of 2.7 μM. Molecular docking studies showed that myricetin interacts with the highly conserved region of the VP35 RNA binding domain, laying the basis for further structural optimization of potent inhibitors of VP35-dsRNA interaction.

The product of the γ-secretase processing of ephrinB2 regulates VE-cadherin complexes and angiogenesis

Presenilin-1 (PS1) gene encodes the catalytic component of γ-secretase, which proteolytically processes several type I transmembrane proteins. We here present evidence that the cytosolic peptide efnB2/CTF2 produced by the PS1/γ-secretase cleavage of efnB2 ligand promotes EphB4 receptor-dependent angiogenesis in vitro. EfnB2/CTF2 increases endothelial cell sprouting and tube formation, stimulates the formation of angiogenic complexes that include VE-cadherin, Raf-1 and Rok-α, and increases MLC2 phosphorylation. These functions are mediated by the PDZ-binding domain of efnB2. Acute downregulation of PS1 or inhibition of γ-secretase inhibits the angiogenic functions of EphB4 while absence of PS1 decreases the VE-cadherin angiogenic complexes of mouse brain. Our data reveal a mechanism by which PS1/γ-secretase regulates efnB2/EphB4 mediated angiogenesis.

The Minor Capsid Protein VP11 of Thermophilic Bacteriophage P23-77 Facilitates Virus Assembly by Using Lipid-Protein Interactions

Thermus thermophilus bacteriophage P23-77 is the type member of a new virus family of icosahedral, tailless, inner-membrane-containing double-stranded DNA (dsDNA) viruses infecting thermophilic bacteria and halophilic archaea. The viruses have a unique capsid architecture consisting of two major capsid proteins assembled in various building blocks. We analyzed the function of the minor capsid protein VP11, which is the third known capsid component in bacteriophage P23-77. Our findings show that VP11 is a dynamically elongated dimer with a predominantly α-helical secondary structure and high thermal stability. The high proportion of basic amino acids in the protein enables electrostatic interaction with negatively charged molecules, including nucleic acid and large unilamellar lipid vesicles (LUVs). The plausible biological function of VP11 is elucidated by demonstrating the interactions of VP11 with Thermus-derived LUVs and with the major capsid proteins by means of the dynamic-light-scattering technique. In particular, the major capsid protein VP17 was able to link VP11-complexed LUVs into larger particles, whereas the other P23-77 major capsid protein, VP16, was unable to link VP11-comlexed LUVs. Our results rule out a previously suggested penton function for VP11. Instead, the electrostatic membrane association of VP11 triggers the binding of the major capsid protein VP17, thus facilitating a controlled incorporation of the two different major protein species into the assembling capsid.

IMPORTANCE

The study of thermophilic viruses with inner membranes provides valuable insights into the mechanisms used for stabilization and assembly of protein-lipid systems at high temperatures. Our results reveal a novel way by which an internal membrane and outer capsid shell are linked in a virus that uses two different major protein species for capsid assembly. We show that a positive protein charge is important in order to form electrostatic interactions with the lipid surface, thereby facilitating the incorporation of other capsid proteins on the membrane surface. This implies an alternative function for basic proteins present in the virions of other lipid-containing thermophilic viruses, whose proposed role in genome packaging is based on their capability to bind DNA. The unique minor capsid protein of bacteriophage P23-77 resembles in its characteristics the scaffolding proteins of tailed phages, though it constitutes a substantial part of the mature virion.

Somatotropic axis genes are expressed before pituitary onset during zebrafish and sea bass development

The somatotropic axis, or growth hormone-insulin-like growth factor-1 (GH-IGF-1) axis, of fish is involved in numerous physiological process including regulation of ionic and osmotic balance, lipid, carbohydrate and protein metabolism, growth, reproduction, immune function and behavior. It is thought that GH plays a role in fish development but conflicting results have been obtained concerning the ontogeny of the somatotropic axis. Here we investigated the developmental expression of GH, GH-receptor (GHR) and IGF-1 genes and of a GH-like protein from fertilization until early stages of larval development in two Teleosts species, Danio rerio and Dicentrarchus labrax, by PCR, in situ hybridization and Western blotting. GH, GHR and IGF-1 mRNA were present in unfertilized eggs and at all stages of embryonic development, all three displaying a similar distribution in the two species. First located in the whole embryo (until 12 hpf in zebrafish and 76 hpf in sea bass), the mRNAs appeared then distributed in the head and tail, from where they disappeared progressively to concentrate in the forming pituitary gland. Proteins immunoreactive with a specific sea bass anti-GH antibody were also detected at all stages in this species. Differences in intensity and number of bands suggest that protein processing varies from early to later stages of development. The data show that all actors of the somatotropic axis are present from fertilization in these two species, suggesting they plays a role in early development, perhaps in an autocrine/paracrine mode as all three elements displayed a similar distribution at each stage investigated.

Recombinant expression, purification and dimerization of the neurotrophic growth factor Artemin for in vitro and in vivo use

Artemin (ARTN) is a neurotrophic growth factor of the GDNF ligand family that signals through the specific GFRα-3 coreceptor/cRet tyrosine kinase-mediated signaling cascade. Its expression and signaling action in adults are restricted to nociceptive sensory neurons in the dorsal root ganglia. Consequently, Artemin supports survival and growth of sensory neurons and has been studied as a possible treatment for neuropathic pain paradigms. In this paper, we describe the development of an efficient method for the recombinant bacterial production of large quantities of highly pure, biologically active ARTN for in vitro and in vivo studies. Using Escherichia coli expression of an NH(2)-terminal SUMO-Artemin fusion protein and subsequent refolding from inclusion bodies followed by cleavage of the SUMO fusion part, mature Artemin with a native NH(2)-terminal amino acid sequence was obtained at high purity (>99%). Experiments using the reducing agent dithiothreitol (DTT) demonstrated that the intermolecular disulphide bridge in the cysteine knot is dispensable for dimerization of stable ARTN monomers. Our production method could facilitate in vitro and in vivo experimentation for the possible development of Artemin as a therapeutic agent for neuropathic pain.

High molecular weight isoforms of growth hormone in cells of the immune system

A substantial body of research exists to support the idea that cells of the immune system produce growth hormone (GH). However, the structure and mechanism of action of lymphocyte-derived GH continues to remain largely unknown. Here we present the results of Western analysis of whole cell extracts showing that different molecular weight isoforms of GH of approximately 100, 65, and 48 kDa can be detected in primary mouse cells of the immune system and in the mouse EL4 cell line. The identity of the 65 and 48 kDa isoforms of GH were confirmed by mass spectrometry. The various isoforms were detected in both enriched T and B spleen cell populations. The large molecular weight isoform appears to reside primarily in the cytoplasm, whereas the lower molecular weight 65 and 48 kDa isoforms were detected primarily in the nucleus. These results also suggest that GH isoforms are induced by oxidative stress. In EL4 cells overexpressing GH, the expression of luciferase controlled by a promoter containing the antioxidant response element is increased almost threefold above control. The data suggest that the induction of isoforms of the GH molecule in cells of the immune system may be an important mechanism of adaptation and/or protection of lymphoid cells under conditions of oxidative stress.

Discrimination of recombinant from natural human growth hormone using DNA aptamers

Detection of athletes who use synthetic human growth hormone (hGH; or somatotropin) to enhance physical strength and obtain an advantage in competitive sports is a formidable problem, as rhGH is virtually identical to the natural pituitary hormone. However, some post-translational and other modifications have been documented by chromatographic separation and mass spectrometry (MS) in a small percentage of rhGH. In the present work, development of DNA aptamers against research-grade rhGH and natural hGH with adsorption of the rhGH aptamers against natural hGH was shown to produce a small family of aptamer sequences that bound consistently with greater affinity to rhGH over a low nanogram-to-microgram range in ELISA-like microplate assays. This collection of rhGH discriminatory aptamer sequences shared some short sequence segments and secondary structural features. The top rhGH discriminatory aptamers also appeared to cross-react with human myoglobin and BSA but not with bone collagen peptides and an unrelated viral envelope peptide. The cross-reactivity results suggested several strings of up to five consecutive amino acids that might serve as common epitopes for aptamer binding. SDS-PAGE revealed that the rhGH existed largely as a 45-kDa dimer, and the natural hGH was almost exclusively monomeric. The existence of the rhGH dimer suggests that a discontinuous "bridge" epitope may exist on the rhGH, which spans the subunits, thereby accounting somewhat for the difference in detection. Overall, these results suggest that aptamers might be useful for routine, presumptive laboratory screening to identify athletes who are potentially cheating by administration of rhGH.

Human growth hormone: 45-kDa isoform with extraordinarily stable interchain disulfide links has attenuated receptor-binding and cell-proliferative activities

BACKGROUND

Human growth hormone (hGH) is a complex mixture of molecular isoforms. Gaps in our knowledge exist regarding the structures and biological significances of the uncharacterized hGH molecular variants. Mercaptoethanol-resistant 45-kDa human growth hormone (MER-45 kDa hGH) is an extraordinarily stable disulfide-linked hGH homodimer whose biological significance is unknown.

OBJECTIVES

To elucidate the pharmacokinetic abilities of dimeric MER-45-kDa hGH to bind to GH and prolactin (PRL) receptors and to elucidate its abilities to stimulate cell proliferation in lactogen-induced and somatogen-induced in vitro cell proliferation bioassays.

DESIGN

The binding of MER-45-kDa hGH to GH and PRL receptors was tested in radioreceptor assays (RRAs). Competitive displacements of [(125)I]-bovine GH from bovine liver membranes, [(125)I]-ovine PRL from lactating rabbit mammary gland membranes and [(125)I]-hGH from human IM-9 lymphocytes by unlabelled GHs, PRLs or dimeric MER-45-kDa hGH were evaluated. The abilities of dimeric MER-45-kDa hGH to stimulate proliferation of lactogen-responsive Nb2 lymphoma cells and to stimulate proliferation of somatogen-responsive T47-D human breast cancer cells were assessed by incubation of cells with GHs or PRLs and subsequently measuring growth using the MTS cell proliferation assay.

RESULTS

Dimeric MER-45-kDa hGH, compared to monomeric hGH, had reduced binding affinities to both GH and prolactin receptors. In a bovine liver GH radioreceptor assay its ED(50) (197.5 pM) was 40.8% that of monomeric hGH. In a human IM-9 lymphocyte hGH RRA its ED(50) (2.96 nM) was 26.2% that of monomeric hGH. In a lactating rabbit mammary gland prolactin RRA its ED(50) (3.56 nM) was 16.8% that of a monomeric hGH. Dimeric MER-45-kDa hGH, compared to monomeric hGH, had a diminished capacity to stimulate proliferation of cells in vitro. In a dose-response relationship assessing proliferation of Nb2 lymphoma cells its ED(50) (191 pM) was 18.0% that of monomeric hGH. While monomeric hGH stimulated a 2.2-fold proliferation of T47-D human breast cancer cells above vehicle control, dimeric MER-45-kDa hGH was unable to stimulate the cells to proliferate and slightly inhibited their proliferation to 77.6% that of control.

CONCLUSIONS

The topological arrangement of monomeric hGHs to form an unusually stable disulfide-linked dimer markedly diminishes hGH's binding affinities to both GH and PRL receptors and also drastically attenuates its ability to stimulate proliferation of cells in vitro.

O-Glycosylated 24 kDa human growth hormone has a mucin-like biantennary disialylated tetrasaccharide attached at Thr-60

MS was used to characterize the 24 kDa human growth hormone (hGH) glycoprotein isoform and determine the locus of O-linked oligosaccharide attachment, the oligosaccharide branching topology, and the monosaccharide sequence. MALDI-TOF/MS and ESI-MS/MS analyses of glycosylated 24 kDa hGH tryptic peptides showed that this hGH isoform is a product of the hGH normal gene. Analysis of the glycoprotein hydrolysate by high-performance anion-exchange chromatography with pulsed amperometric detection and HPLC with fluorescent detection for N-acetyl neuraminic acid (NeuAc) yielded the oligosaccharide composition (NeuAc(2), N-acetyl galactosamine(1), Gal(1)). After beta-elimination to release the oligosaccharide from glycosylated 24 kDa hGH, collision-induced dissociation of tryptic glycopeptide T6 indicated that there had been an O-linked oligosaccharide attached to Thr-60. The sequence and branching structure of the oligosaccharide were determined by ESI-MS/MS analysis of tryptic glycopeptide T6. The mucin-like O-oligosaccharide sequence linked to Thr-60 begins with N-acetyl galactosamine and branches in a bifurcated topology with one appendage consisting of galactose followed by NeuAc and the other consisting of a single NeuAc. The oligosaccharide moiety lies in the high-affinity binding site 1 structural epitope of hGH that interfaces with both the growth hormone and the prolactin receptors and is predicted to sterically affect receptor interactions and alter the biological actions of hGH.

Growth hormone isoforms

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms. The sources of this heterogeneity reside at the level of the genome, mRNA splicing, post-translational modification and metabolism. The GH gene cluster on chromosome 17q contains 2 GH genes (GH1 or GH-N and GH2 or GH-V) in addition to 2(-3) genes encoding the related chorionic somatomammotropin. Alternative mRNA splicing of the GH1 transcript yields two products: 22K-GH (the principal pituitary GH form) and 20K-GH. Post-translationally modified GH forms include N(alpha)-acylated, deamidated and glycosylated monomeric GH forms, as well as both non-covalent and disulfide-linked oligomers up to at least pentameric GH. GH fragments generated in the course of peripheral metabolism may be measured in immunoassays for GH. The GH-N gene is expressed in the pituitary, the GH-V gene in the placenta. Secretion of pituitary GH forms is pulsatile under control from the hypothalamus, whereas secretion of placental GH-V is tonic and rises progressively in maternal blood during the 2nd and 3rd trimester. Pituitary GH forms are co-secreted during a secretory pulse; no isoform-specific stimuli have been identified. There are minor differences in somatogenic and metabolic bioactivity among the GH isoforms, depending on species and assay system used. Both 20K-GH and GH-V have poor lactogenic activity. Oligomeric GH forms have variably diminished bioactivity compared to monomeric forms. GH isoforms cross-react in most immunoassays, but assays specific for 22K-GH, 20K-GH and GH-V have been developed. The metabolic clearance of 20K-GH and GH oligomers is delayed compared to that of 22K-GH. The heterogeneous mixture of GH isoforms in blood is further complicated by the presence of two GH-binding proteins, which form complexes with GH; isoform proportions also vary depending on the lag time from a secretory pulse because of different half-lives. GH forms excreted in the urine reflect monomeric GH isoforms in blood, but constitute only a minute fraction of the GH production rate. The heterogeneity of GH is one important reason for the notorious disparity among assay results. It also presents an opportunity for distinguishing endogenous from exogenous GH.

MAP dendrimer elicits antibodies for detecting rat and mouse GH-binding proteins

The membrane-bound rat GH-R and an alternatively spliced isoform, the soluble rat GH-BP, are comprised of identical N-terminal GH-binding domains; however, their C-terminal sequences differ. Immunological reagents are needed to distinguish between the two isoforms in order to understand their respective roles in mediating the actions of GH. Accordingly, a tetravalent MAP dendrimer with four identical branches of a C-terminal peptide sequence of the rat GH-BP (GH-BP(263-279)) was synthesized and used as an immunogen in rabbits. Solid-phase peptide synthesis of four GH-BP(263-279) segments onto a tetravalent Lys(2)-Lys-beta-Ala-OH core peptide was carried out using Fmoc chemistry. The mass of the RP-HPLC-purified synthetic product, 8398 Da, determined by ESI-MS, was identical to expected mass. Three anti-rat GH-BP(263-279) MAP antisera, BETO-8039, BETO-8040, and BETO-8041, at dilutions of 10(-3), recognized both the rat GH-BP(263-279) MAP and recombinant mouse GH-BP with ED(50)s within a range of 5-10 fmol, but did not cross-react with BSA in dot blot analyses. BETO-8041 antisera (10(-3) dilution) recognized GH-BPs of rat serum and liver having M(r)s ranging from 35 to 130 kDa, but did not recognize full-length rat GH-Rs. The antisera also detected recombinant mouse GH-BPs. In summary, the tetravalent rat GH-BP(263-279) MAP dendrimer served as an effective immunogenic antigen in eliciting high titer antisera specific for the C-termini of both rat and mouse GH-BPs. The antisera will facilitate studies aimed at improving our understanding of the biology of GH-BPs.

Expression, translation, and localization of a novel, small growth hormone variant

A novel transcript of the GH gene has been identified in ocular tissues of chick embryos. It is, however, unknown whether this transcript (small chicken GH, scGH) is translated. This possibility was therefore assessed. The expression of scGH mRNA was confirmed by RT-PCR, using primers that amplified a 426-bp cDNA of its coding sequence. This cDNA was inserted into an expression plasmid to transfect HEK 293 cells, and its translation was shown by specific scGH immunoreactivity in extracts of these cells. This immunoreactivity was directed against the unique N terminus of scGH and was associated with a protein of 16 kDa, comparable with its predicted size. Most of the immunoreactivity detected was, however, associated with a 31-kDa moiety, suggesting scGH is normally dimerized. Neither protein was, however, present in media of the transfected HEK cells, consistent with scGH's lack of a signal sequence. Similar moieties of 16 and 31 kDa were also found in proteins extracted from ocular tissues (neural retina, pigmented epithelium, lens, cornea, choroid) of embryos, although they were not consistently present in vitreous humor. Specific scGH immunoreactivity was also detected in these tissues by immunocytochemistry but not in axons in the optic fiber layer or the optic nerve head, which were immunoreactive for full-length GH. In summary, we have established that scGH expression and translation occurs in ocular tissues of chick embryos, in which its localization in the neural retina and the optic nerve head is distinct from that of the full-length protein.

Evidence for structural constraint on ovulin, a rapidly evolving Drosophila melanogaster seminal protein

The egg-laying hormone ovulin (Acp26Aa) is among the most rapidly evolving proteins in the Drosophila genome. Against the background of ovulin's high sequence variability within and between species, we have identified highly conserved motifs that may play an important structural role. Using yeast two-hybrid and GST-pull-down assays, we show that ovulin interacts with itself. The C terminus of ovulin is necessary and sufficient for self-interaction, with its C-terminal 45 aa playing a major role. Under nonreducing conditions, ovulin participates in a high-molecular-mass complex, suggesting that it occurs in an oligomeric form. One or more of three predicted coiled-coil domains in the C terminus of ovulin may be involved in its self-interaction. These structural elements are conserved between species despite an overall rapid pace of evolution in ovulin's primary sequence. We therefore suggest that domains involved in ovulin's self-interaction form a conserved structural backbone for the protein, resulting in greater evolutionary flexibility at other sites.

Separation of proteins with a molecular mass difference of 2 kDa utilizing preparative double-inverted gradient polyacrylamide gel electrophoresis under nonreducing conditions: Application to the isolation of 24 kDa human growth hormone

A method for separating proteins with a molecular mass difference of 2 kDa using SDS-PAGE under nonreducing conditions is presented. A sample mixture containing several human growth hormone (hGH) isoforms was initially separated on a weak anion-exchange column. Fractions rich in 24 kDa hGH as determined by analytical SDS-PAGE were pooled and further separated by cation-exchange chromatography. The fractions pooled from the cation-exchange chromatography contained two hGH isoforms with a 2 kDa molecular mass difference according to SDS-PAGE analysis, 22 and 24 kDa hGH. The 22 and 24 kDa hGH were separated using continuous-elution preparative double-inverted gradient PAGE (PDG-PAGE) under nonreducing conditions. The preparative electrophoresis gel was composed of three stacked tubular polyacrylamide matrices, a 4% stacking gel, a 13-18% linear gradient gel, and a 15-10% linear inverted gradient gel. Fractions containing purified 24 kDa hGH were pooled and Western blot analysis displayed immunoreactivity to antihGH antibodies. PDG-PAGE provides researchers with an electrophoretic technique to preparatively purify proteins under nonreducing conditions with molecular mass differences of 2 kDa.

Two wrongs can make a right: dimers of prolactin and growth hormone receptor antagonists behave as agonists

Prolactin (PRL) and GH have two distinct binding sites (site 1 with high affinity; site 2 with low affinity) that each interact with a PRL receptor (PRLR) to form a functional receptor dimer that activates signal transduction. The G129R mutation in PRL and the G120R mutation in GH disrupt the structural integrity of site 2 such that the ligands retain the ability to bind to the first receptor with high affinity, but act as receptor antagonists. In this study, we examined the ability of monomeric and dimeric forms of these ligands, human (h) PRL and hGH, and their antagonists (hPRL-G129R and hGH-G120R) to 1) bind to PRLRs; 2) induce conformational changes in PRLRs; 3) activate signaling pathways associated with the PRLR; and 4) mediate cell proliferation in vitro. In contrast to monomeric hPRL-G129R, homodimeric hPRL-G129R induced PRLR dimerization; activated Janus family of tyrosine kinases 2/signal transducer and activator of transcription 5, Ras/Raf/MAPK kinase/Erk, and phosphatidylinositol 3-kinase/Akt signaling; and stimulated Nb2 cell proliferation. Similarly, homodimeric hGH-G120R was able to mediate signaling via the PRLR and to stimulate Nb2 cell proliferation. These experiments demonstrate that a ligand must have two functional binding sites, but that these may be site 1 plus site 2 or two site 1's, to elicit receptor-mediated signal transduction. The size of the ligand plays less of a role in receptor activation, suggesting that the extracellular portion of the PRLR (and possibly the GH receptor) is rather flexible and can accommodate larger ligands. These findings may have implications for designing multifunctional therapeutics that target this class of cytokine receptors.