Synthesis and in vitro bioactivity of human growth hormone-releasing factor analogs substituted at position-1

Synthesis and biological evaluation of novel structure-related hGHRH agonistic analogs

Activity and half-life play key roles in the application of GHRH analogues. The GHRH monomers produced in a solid synthesizer were incubated, respectively, in NH4OH solution and lyophilized to obtain their dimers. The activities, specificities, and receptor affinities of the GHRH dimers were evaluated in rGH release/inhibition, rACTH/LH/PRL release, pituitary homogenate binding, and fluorescent staining. Compared to hGHRH(1-44)NH2 (S), PP-hGHRH(1-44)-GGC-CGG-hGHRH(44-1)-PP (2D), P-hGHRH(1-44)-GGC-CGG-hGHRH(44-1)-P (2E), (1)P-hGHRH(2-44)-GGC-CGG-hGHRH(44-2)-(1)P (2F), or hGHRH(1-44)-GGC-CGG-hGHRH(44-1) (2Y) had potency of 104 ± 16.7%, 94 ± 32.6%, 114 ± 16.6%, or 122 ± 14.5% and similar specificities. The inhibition effect of GHIH on rGH stimulated by GHRH dimer was in dose-/time-dependent manner. The staining of FITC-labeled dimer showed cytomembrane distribution and the binding ranking was 2F>2D>2Y>2E>S. 2F presents the strongest activity and the highest affinity to pituitary cells. The dimer with (1)Pro-GHRH stimulates stronger rGH release than that with (1)Tyr-GHRH and the N-terminal single cyclic amino acid is required for the stimulation.

Synthesis of new potent agonistic analogs of growth hormone-releasing hormone (GHRH) and evaluation of their endocrine and cardiac activities

In view of the recent findings of stimulatory effects of GHRH analogs, JI-34, JI-36 and JI-38, on cardiomyocytes, pancreatic islets and wound healing, three series of new analogs of GHRH(1-29) have been synthesized and evaluated biologically in an endeavor to produce more potent compounds. "Agmatine analogs", MR-356 (N-Me-Tyr(1)-JI-38), MR-361(N-Me-Tyr(1), D-Ala(2)-JI-38) and MR-367(N-Me-Tyr(1), D-Ala(2), Asn(8)-JI-38), in which Dat in JI-38 is replaced by N-Me-Tyr(1), showed improved relative potencies on GH release upon subcutaneous administration in vivo and binding in vitro. Modification with N-Me-Tyr(1) and Arg(29)-NHCH3 as in MR-403 (N-Me-Tyr(1), D-Ala(2), Arg(29)-NHCH3-JI-38), MR-406 (N-Me-Tyr(1), Arg(29)-NHCH3-JI-38) and MR-409 (N-Me-Tyr(1), D-Ala(2), Asn(8), Arg(29)-NHCH3-JI-38), and MR-410 (N-Me-Tyr(1), D-Ala(2), Thr(8), Arg(29)-NHCH3-JI-38) resulted in dramatically increased endocrine activities. These appear to be the most potent GHRH agonistic analogs so far developed. Analogs with Apa(30)-NH2 such as MR-326 (N-Me-Tyr(1), D-Ala(2), Arg(29), Apa(30)-NH2-JI-38), and with Gab(30)-NH2, as MR-502 (D-Ala(2), 5F-Phe(6), Ser(28), Arg(29),Gab(30)-NH2-JI-38) also exhibited much higher potency than JI-38 upon i.v. administration. The relationship between the GH-releasing potency and the analog structure is discussed. Fourteen GHRH agonists with the highest endocrine potencies were subjected to cardiologic tests. MR-409 and MR-356 exhibited higher potency than JI-38 in activating myocardial repair in rats with induced myocardial infarction. As the previous class of analogs, exemplified by JI-38, had shown promising results in multiple fields including cardiology, diabetes and wound healing, our new, more potent, GHRH agonists should manifest additional efficacy for possible medical applications.

Non-Clinical Pharmacology and Safety Evaluation of TH9507, a Human Growth Hormone-Releasing Factor Analogue

TH9507, an analogue of human growth hormone-releasing factor (hGRF1-44NH2) minimally modified by addition of a trans-3-hexenoyl moiety to Tyr1 of the amino acid sequence, was found to be resistant to dipeptidyl aminopeptidase-IV deactivation. Compared to natural hGRF1-44NH2, the modification slowed the in vitro degradation of the peptide in rat, dog and human plasma and prolonged the in vivo plasma elimination kinetics of immunoreactive TH9507. Plasma growth hormone and insulin-like growth factor-1 (IGF-1) markedly increased in pigs, rats and dogs after daily repeat intravenous or subcutaneous injections of TH9507 at doses up to 600 microg/kg. Subchronic toxicity studies in rats and dogs with TH9507 treatment for up to 4 months showed a significant, but not dose-related, increase in body weight gain associated with increased biomarker response. Although TH9507 was well tolerated by both rats and dogs, a more pronounced anabolic effect and more evident (reversible) adverse effects (liver and kidney findings, anaemia, clinical chemistry changes, organ weight effects) were observed in dogs after repeat daily subcutaneous injections, which were attributed to prolonged exposure to supraphysiological levels of growth hormone and/or IGF-1. In both rats and dogs, toxicokinetic evaluations indicated that exposure to immunoreactive TH9507 was dose related after both routes of administration. The apparent elimination t1/2 in dogs ranged from 21 to 45 min. In conclusion, TH9507 is a modified hGRF peptide having enhanced potency and duration of action. The adverse treatment-related effects in dogs appear to be associated with sustained exposure to supraphysiological levels of growth hormone and IGF-1 induced by prolonged TH9507 treatment.

Study on the constructions and activities of three novel hGHRH analogs with N-terminal prolyl modulation+

Growth hormone releasing hormone is one of the hormones secreted by the hypothalamus. Because of its potential applications in agriculture and medicine, its short half-life and its expensive chemical synthesis, an analog with high GHRH activity and prolonged half-life has been looked for. The fusion partner gene with 127 amino acid residues of the C-terminus from L-asparaginase was recombined respectively with asp-pro-pro-hGHRH(1-44), asp-pro-hGHRH(1-44) or asp-1pro-GHRH(2-44) genes synthesized by PCR method to form three kinds of fusion proteins with unique acid labile linker Asp-Pro. The Pro-Pro-hGHRH(1-44), Pro-hGHRH(1-44), and 1Pro-GHRH(2-44) peptides were purified to homogeneity by means of cell disruption, washing of inclusion body, ethanol fraction precipitation, acid hydrolysis, SP-Sephadex C-25 and Sephadex G-10 column chromatography. The peptide molecular mass of 5235, 5139 or 4975 Da was determined by ESI mass spectroscopy and purity was determined by SDS-PAGE. In the study of in vitro activity, the antiserum kit against human GH and peptide doses of 0.1, 1.0 and 10 microg/ml were used. These peptides obviously increased GH releases both from human pituitary and from rat pituitary. The activity comparisons showed that there was significant difference between Pro-Pro-hGHRH(1-44)-Gly-Gly-Cys and Pro-Pro-hGHRH(1-44) at 1.0 microg/ml, or between 1Pro-hGHRH(2-44) and Pro-Pro-hGHRH(1-44) or Pro-hGHRH(1-44) at 10 microg/ml. The structure-activity relationships showed that at the original C-terminus, for rat pituitary the activity of the GHRH analog with 1Tyr-->Pro was more than that of Pro-Pro-hGHRH(1-44) or Pro-hGHRH(1-44). The results showed that the analogs had good GH-releasing activity and species specificity.

Production and enhanced biological activity of a novel GHRH analog, hGHRH with an N-terminal Pro–Pro extension

Growth Hormone Releasing Hormone (GHRH) is one of the most important hormones in life. Because of its potential clinical importance, its short half-life, and its expensive chemical synthesis, an analog of hGHRH with a prolonged half-life and better activity has been studied for clinical application, especially for the treatment of muscle wasting, type II diabetes, or sleep disorders. The Pro-Pro-hGHRH(1-44) peptide has better activity. The fusion partner gene with 127 amino acid residues of the C-terminus from l-asparaginase was recombined with asp-pro-pro-hGHRH(1-44) gene synthesized by PCR method to form a fusion protein with the unique acid labile linker Asp-Pro. The recombinant protein was expressed to high levels in Escherichia coli BL21 (DE3). The Pro-Pro-hGHRH(1-44) peptide was purified to homogeneity by means of cell disruption, washing, ethanol precipitation, acid hydrolysis, and SP-Sephadex C-25, and Sephadex G-25 column chromatography. The fold of the purification was about 88 times and the yield was 1.1% of the total protein weight of the inclusion body. The peptide molecular mass of 5235.25 Da was determined by ESI mass spectroscopy. Its purity was determined by SDS-PAGE. In the study of the activity, we measured GH release of rat pituitary by using the antiserum kit against human GH. The peptide doses of 0.01, 0.1, 1.0, 7.72, and 20.9 microg/ml used, respectively, released the GH values of 0.1+/-0.1, 12.5+/-7.3, 16.6+/-5.8, 49.8+/-7.6, and 79.5+/-5.7 ng/ml whereas their blank controls, respectively, were 0.5+/-0.8, 4.1+/-2.6, 3.1+/-3.1, 4.7+/-1.8, and 1.2+/-0.3 ng/ml. The activity results of all dose groups except 0.01 microg/ml Pro-Pro-hGHRH(1-44) group and hGHRH(1-40) group showed that there were significant differences between GH released by the peptide and that by its blank control. With the increase of dosage, the differences were more significant. hGHRH(1-40) showed no measured GH release when the dose was up to 2 microg/ml. The activity results show that the Pro-Pro-hGHRH(1-44) peptide is a potential GH releasing analog.

Characterization of a growth hormone-releasing hormone binding site in the rat renal medulla

Receptor binding analysis was performed in the renal medulla from 2-month-old rats, an extrapituitary tissue containing the highest level of GHRH receptor mRNA. At 4 degrees C, in the presence of a cocktail of protease inhibitors, binding of [125I-Tyr(10)]hGHRH (1-44)NH(2) to medullary homogenates was specific, time-dependent, reversible and saturable (K(d): 28 nM; B(max): 30 fmol/mgprot.). In these experimental conditions, no change of binding parameters could be detected in the course of aging. The structure-affinity profile was different in the two tissues and chemical cross-linking revealed the presence of 65-, 55- and 38-kDa 125I-GHRH-labeled complexes in the renal medulla compared to 65-, 47- and 28-kDa radioactive complexes in the anterior pituitary. It is suggested that GHRH binding sites, and possibly the receptor, may be different in the two tissues.

Characterization of a growth hormone-releasing hormone binding site in the rat renal medulla

Receptor binding analysis was performed in the renal medulla from 2-month-old rats, an extrapituitary tissue containing the highest level of GHRH receptor mRNA. At 4 degrees C, in the presence of a cocktail of protease inhibitors, binding of [125I-Tyr(10)]hGHRH NH(2) to medullary homogenates was specific, time-dependent, reversible and saturable (K(d): 28 nM; B(max): 30 fmol/mg prot.). In these experimental conditions, no change of binding parameters could be detected in the course of aging. The structure-affinity profile was different in the two tissues and chemical cross-linking revealed the presence of 65-, 55- and 38-kDa 125I-GHRH-labeled complexes in the renal medulla compared to 65-, 47- and 28-kDa radioactive complexes in the anterior pituitary. It is suggested that GHRH binding sites, and possibly the receptor, may be different in the two tissues.

Sequence and expression of cDNA for pituitary adenylate cyclase activating polypeptide (PACAP) and growth hormone-releasing hormone (GHRH)-like peptide in catfish

Growth hormone-releasing hormone (GHRH) and pituitary adenylate cyclase activating polypeptide (PACAP) are two neuropeptides that are associated with the release of pituitary growth hormone. Here a cDNA of 2501 base pairs encoding both a PACAP and a GHRH-like peptide was isolated from a brain cDNA library made from Thai catfish (Clarias macrocephalus). The organization is unlike that of the mammalian gene where PACAP and PACAP-related peptide (PRP) are encoded in one gene, and the GHRH peptide is on a separate gene. Northern analysis of catfish brain mRNA indicated that PACAP/GHRH-like mRNA has three sizes; bands of 6000, 2500, and 1000 bases suggest alternative splicing of the gene. Reverse transcriptase/PCR assay detected PACAP/GHRH-like mRNA in tissues from the brain, testis, ovary, and stomach, but not from the pancreas, pituitary, muscle, and liver. Our hypothesis that the two mammalian genes encoding GHRH or PACAP originated from a gene duplication between fish and tetrapods is supported by the present findings of similar mRNA organization and pattern of expression for the one fish gene and two mammalian genes.

Rational design, synthesis, and biological evaluation of novel growth hormone releasing factor analogues

Since its initial discovery in 1982, growth hormone-releasing factor (GRF) has been the subject of intense investigation. This interest was prompted by the potential application of GRF for stimulating growth in dwarf humans and for performance enhancement in livestock. Substantial research has been focused upon the development of potent, long-acting analogs as therapeutics. Herein is described a summary of the cumulative efforts of various laboratories endeavoring in this quest. The rationale utilized in GRF analog development is discussed: 1) determination of bioactive core, 2) evaluation of secondary structure, and 3) elucidation of degradation pathways (chemical and enzymatic). Using this information, several series of linear (unnatural and natural sequence) and cyclic GRF analogs were designed, synthesized, and evaluated. Stimulated by the constraints of commercial production, innovative, alternative methods of synthesis were explored: solid-phase, solution-phase, enzymatic, and recombinant. To date, the most promising candidate for drug development is [His1, Val2, Gln8, Ala15, Leu27]-hGRF(1-32)-OH. This natural sequence analog, consisting of rodent and human sequences, incorporates the bioactive core, preferred secondary structure, resistance to chemical and enzymatic degradation; with the added benefit of amenability to large-scale recombinant synthesis.

Enhanced stability and potency of novel growth hormone-releasing factor (GRF) analogues derived from rodent and human GRF sequences

Native human GRF(1-44)-NH2(hGRF44) is subject to biological inactivation by both enzymatic and chemical routes. In plasma, hGRF44 is rapidly degraded via dipeptidylpeptidase IV (DPP-IV) cleavage between residues Ala2 and Asp3. The hGRF44 is also subject to chemical rearrangement (Asn8-->Asp8, beta-Asp8 via aminosuccinimide formation) and oxidation [Met27-->Met(O)27] in aqueous environments, greatly reducing its bioactivity. It is therefore advantageous to develop long-acting GRF analogues using specific amino acid replacements at the amino-terminus (to prevent enzymatic degradation): residue 8 (to reduce isomerization) and residue 27 (to prevent oxidation). Inclusion of Ala15 substitution (for Gly15), previously demonstrated to enhance receptor binding affinity, would be predicted to improve GRF analogue potency. Substitution of [His1,Val2]-(from the mouse GRF sequence) for [Tyr1,Ala2]-(human sequence) in [Ala15,Leu27]hGRF(1-32)-OH analogues completely inhibited (24-h incubation) DPP-IV cleavage and greatly increased plasma stability in vitro. Additional substitution of Thr8 (mouse GRF sequence), Ser8 (rat GRF sequence), or Gln8 (not naturally occurring) for Asn8 (human GRF sequence) resulted in analogues with enhanced aqueous stability in vitro (i.e., decreased rate of isomerization). These three highly stable and enzymatically resistant hGRF(1-32)-OH analogues, containing His1, Val2, Thr/Gln8, Ala15, and Leu27 replacements, were then bioassayed for growth hormone (GH)-releasing activity in vitro (rat pituitary cell culture) and in vivo (SC injection into pigs). Enhanced bioactivity was observed with all three hGRF(1-32)-OH analogues. In vitro, these analogues were approximately threefold more potent than hGRF44, whereas in vivo they were eleven- to thirteenfold more potent.(ABSTRACT TRUNCATED AT 250 WORDS)

Two salmon neuropeptides encoded by one brain cDNA are structurally related to members of the glucagon superfamily

A cDNA that codes for two peptides in the glucagon superfamily has been isolated from sockeye salmon brain. The first peptide is related to growth hormone-releasing hormone (GHRH), which has high sequence similarity with PACAP-related peptide. The second peptide is structurally related to vasoactive intestinal peptide, which is also related to a newly identified peptide in mannals, pituitary adenylate-cyclase-activating polypeptide (PACAP). The salmon precursor contains 173 amino acids and has dibasic and monobasic enzyme-processing sites for cleavage of a 45-amino-acid GHRH-like peptide with a free C-terminus and a 38-amino-acid PACAP with an amidated C-terminus. The salmon GHRH-like peptide has 40% amino acid sequence identity with a human GHRH and 56% identity with human PACAP-related peptide. The 38-amino-acid salmon PACAP is highly conserved (89-92% identity) with only three or four amino acid substitutions compared with the human, ovine and rat 38-amino-acid PACAP. Not previously reported for mammalian species, a short precursor coding for only one peptide exists in salmon in addition to the long precursor coding for two peptides. In the short precursor, the coding region for GHRH is deleted leaving the PACAP-coding region in a correct reading frame. This provides one possible control mechanism for an increased expression of one peptide (PACAP) without the concomitant increase in the other peptide (GHRH) as occurs in a double-peptide precursor. The importance of the 3' non-translated region of the salmon GHRH/PACAP precursor in the regulation of translation is suggested by its 70% nucleotide sequence identity to the 3' non-translated regions of the mammalian PACAP precursors. The structural organization of the salmon GHRH/PACAP precursor provides a possible evolutionary scheme for precursors that contain tandem peptides in the glucagon superfamily.

Synthesis and biological activity of novel C-terminal-extended and biotinylated growth hormone-releasing factor (GRF) analogs

A series of novel hGRF(1-29)-NH2 analogs were synthesized and biotinylated. The immunological and biological activities of these analogs were then characterized. To distance the biotin moiety from the putative bioactive core, a C-terminal spacer arm consisting of -Gly-Gly-Cys-NH2 (-GGC) was added to hGRF(1-29)-NH2 (hGRF29) and analogs, with subsequent biotinylation performed at the cysteine residue. Neither addition of the C-terminal spacer arm nor biotinylation affected affinity of these analogs for GRF antibody. Relative to hGRF(1-44)-NH2 (hGRF44: potency = 1.0), the biotinylated analogs were equipotent in vitro to their nonbiotinylated, parent compounds: [desNH2Tyr1,D-Ala2,Ala15]hGRF29-GGC-(tpBiocyt in)-NH2 (4.7) = [Ala15]hGRF29-GGC-(tpBiocytin)-NH2 (3.9) greater than hGRF29-GGC-(tpBiocytin)-NH2 (0.8). Based upon cumulative GH release data in vivo (0-60 min postinjection), [desNH2Tyr1,D-Ala2,Ala15]hGRF29-GGC-(tpBiocyt in)-NH2, [Ala15]hGRF29-GGC-(tpBiocytin)-NH2, and hGRF29-GGC-(tpBiocytin)-NH2 displayed 8.6, 5.5, and 0.8 times, respectively, the potency of hGRF44. These in vivo potency values were not significantly different from the corresponding parent compounds (i.e., with or without the C-terminal spacer arm). In summary, biotinylated hGRF analogs have been developed that retain full immunoreactivity and potent bioactivity (in vitro and in vivo), thus permitting their use in GRF receptor isolation, ELISA, and histochemical procedures.

Production and biological activity of hybrid growth hormone-releasing hormone propeptides

Growth hormone-releasing hormone (GHRH), a hypothalamic hormone that stimulates the synthesis and release of growth hormone (GH) from anterior pituitary cells, has been previously produced by synthetic peptide chemistry and recombinant DNA procedures. GHRH is capable of stimulating growth as well as eliciting other anabolic effects on animals and thus may have potential applications in agriculture and human medicine. However, economical production of GHRH by recombinant DNA process has been difficult since GHRH is degraded rapidly by endogenous E. coli proteases. We report here an efficient process to produce hybrid GHRH analogs of higher molecular weight. These hybrid GHRH propeptides (proGHRH) are comprised of an analog of GHRH (44 aa) and the human GHRH carboxy-terminal peptide (33 aa). In E. coli K-12 RV308, the expression levels of the proGHRH analogs were estimated to be 10% of the total cellular protein. An in vitro assay to measure the release of rat growth hormone by GHRH analogs using crude E. coli lysates was also developed. This assay showed that the proGHRH analogs produced in E. coli efficiently stimulated GH release from rat anterior pituitary cells. One proGHRH analog, [alao]-proGHRH, was purified ans shown to efficiently elevate plasma GH levels in wether lambs. Our data indicate that the hybrid proGHRH peptides, unlike other hormone propeptides such as proinsulin, are remarkably bioactive.

Potent agonists of growth hormone-releasing hormone. Part I

Analogs of the 29 amino acid sequence of growth hormone-releasing hormone (GH-RH) with agmatine (Agm) in position 29 have been synthesized by the solid phase method, purified, and tested in vitro and in vivo. The majority of the analogs contained desaminotyrosine (Dat) in position 1, but a few of them had Tyr1, or N-MeTyr1. Some peptides contained one or more additional L- or D-amino acid substitutions in positions 2, 12, 15, 21, 27, and/or 28. Compared to the natural sequence of GH-RH(1-29)NH2, [Dat1,Ala15]GH-RH(1-28)Agm (MZ-3-191) and [D-Ala2,Ala15]GH-RH(1-28)Agm (MZ-3-201) were 8.2 and 7.1 times more potent in vitro, respectively. These two peptides contained Met27. Their Nle27 analogs, [Dat1,Ala15,Nle27]GH-RH(1-28)Agm(MZ-2-51), prepared previously (9), and [D-Ala2,Ala15,Nle28]GH-RH(1-28)Agm(MZ-3-195) showed relative in vitro potencies of 10.5 and 2.4, respectively. These data indicate that replacement of Met27 by Nle27 enhanced the GH-releasing activity of the analog when the molecule contained Dat1-Ala2 residues at the N-terminus, but peptides containing Tyr1-D-Ala2 in addition to Nle27 showed decreased potencies. Replacement of Ser28 with Asp in multi-substituted analogs of GH-RH(1-28)Agm resulted in a decrease in in vitro potencies compared to the parent compound. Thus, the Ser28-containing MZ-2-51, and [Dat1,Ala15,D-Lys21,Nle27]GH-RH(1-28)Agm, its Asp28 homolog (MZ-3-149), possessed relative activities of 10.5 and 5.6, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of growth hormone releasing factor analog expression in Saccharomyces cerevisiae

An analog of growth hormone releasing factor (GRF), [Leu27]GRF(1-40)-OH, has been expressed and secreted in Saccharomyces cerevisiae under the control of the alpha-factor gene promoter and prepro sequence. A single pair of consecutive basic residues served as a processing site between the alpha-factor sequences and the GRF sequences. [Leu27]GRF(1-40)-OH from fermentor broth containing 20-30 mg/L of immunoreactive peptides was shown to be correctly processed and to possess biological activity as measured in vitro and in vivo. Additional peptides purified from broth appear to result from proteolytic degradation of the original translation product. Analysis of the amino acid compositions and sequences of these peptides suggests that processing enzymes may be responsible for some of the degradation.

Synthesis and biological evaluation of mouse growth hormone-releasing factor

The recently described mouse growth hormone-releasing factor (mGRF) was synthesized by the solid phase procedure, purified by 2 stages of preparative high performance liquid chromatography and fully characterized. The biologic activity of the 42-amino acid peptide (H-His-Val-Asp-Ala-Ile-Phe- Thr-Thr-Asn-Tyr- Arg-Lys-Leu-Leu-Ser-Gln-Leu-Tyr-Ala-Arg-Lys-Val-Ile-Gln-Asp-Ile-Met-Asn- Lys- Gln-Gly-Glu-Arg-Ile- Gln-Glu-Gln-Arg-Ala-Arg-Leu-Ser-OH) was assessed in primary cultures of both mouse and rat anterior pituitary cells and compared to synthetic rat (rGRF) and human (hGRF) growth hormone-releasing factors. mGRF was equipotent to rGRF in mouse somatotrophs but slightly less potent in rat somatotrophs, while hGRF was 3-5 times less potent in both rodent species.

Expression of a cytomegalovirus-human growth hormone-releasing hormone precursor fusion gene in transfected GH3 cells

Pituitary GH3 cells were transfected with a human growth hormone-releasing hormone (hGRH) precursor minigene fused to the promoter region of either a cytomegalic immediate early gene (pCMV) or the mouse metallothionein-1 gene (mMT) to examine the molecular heterogeneity of the translation products. Expression of the hGRH message occurred following transfection of the cells with each fusion gene. Extracts of pCMV-hGRH-transfected GH3 cells as well as the culture medium contained detectable levels of immunoreactive (ir)-hGRH peptides. Analysis of molecular heterogeneity by reverse-phase high performance liquid chromatography and radioimmunoassay indicated that both mature forms of hGRH (hGRH(1-44)-NH2 and hGRH(1-40)-OH) were synthesized in the cells, although hGRH(1-44)-NH2 was the primary form secreted into the medium. A high molecular weight form of ir-hGRH, believed to represent the hGRH precursor (or a partially processed form of the precursor) was detected in cells and, in smaller amounts, in the medium. Several ir-hGRH peptides, presumed cleavage products of the mature forms of hGRH, were also found. The efficiency of processing of the hGRH precursor and metabolism of the mature hormonal forms in transfected cells grown in the presence of four different peptidase inhibitors varied with the inhibitor present. Transfected GH3 cells, therefore, possess all of the necessary enzymes for and are capable of processing the hGRH precursor to mature GRH and provide a model to study hGRH biosynthesis.

Neuropeptide families: an evolutionary perspective

Changes in the structure and function of five neuropeptide families during evolution are considered. The families of gonadotropin-releasing hormone (GnRH), corticotropin-releasing factor (CRF), growth hormone-releasing hormone (GH-RH), somatostatin (SS), and vasopressin/oxytocin (VP/Oxy) are used as models to illustrate the importance of a phylogenetic approach in understanding neuropeptide structure/activity relationships, precursors, processing, gene duplication, novel locations and functions, and gene-associated peptides.

Dipeptidylpeptidase IV and trypsin-like enzymatic degradation of human growth hormone-releasing hormone in plasma

The plasma enzyme responsible for primary proteolytic cleavage of growth hormone-releasing hormone (GRH) at the 2-3 amino acid bond was characterized. Native GRH[GRH(1-44)-NH2 and GRH(1-40)-OH], and COOH-terminally shortened fragments [GRH(1-32)-NH2 and GRH(1-29)-NH2] were rapidly cleaved, while GRH(2-32)-NH2 was not degraded at this site. Moreover, degradation to GRH(3-44)-NH2 was unaffected by an aminopeptidase inhibitor, indicating that this metabolite was generated from a single step cleavage by a dipeptidylpeptidase (DPP) rather than sequential aminopeptidase cleavages. Conversion to GRH(3-44)-NH2 was blocked by diprotin A, a DPP type IV (DPP IV) competitive inhibitor. D-Amino acid substitution at either position 1 or 2 also prevented hydrolysis, characteristic of DPP IV. Analysis of endogenous plasma GRH immunoreactivity from a human GRH transgenic pig revealed that the major peak coeluted with GRH(3-44)-NH2. Native GRH exhibited trypsin-like degradation at the 11-12 position but cleavage at the 12-13 site occurred only with GRH(1-32)-NH2 and GRH(1-29)-NH2. Formation of these metabolites was independent of prior DPP IV hydrolysis but was greatly reduced by trypsin inhibitors. Evaluation of plasma stability of potential GRH super analogues, designed to resist degradation by these enzymes, confirmed that GRH degradation in plasma occurs primarily by DPP IV, and to a lesser extent by trypsin-like enzyme(s).

Synthesis, biological activity and conformational analysis of cyclic GRF analogs

A novel cyclic GRF analog, cyclo(Asp8-Lys12)-[Asp8,Ala15]-GRF(1-29)-NH2, i.e. cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2, was synthesized by the solid phase procedure and found to retain significant biological activity. Solid phase cyclization of Asp8 to Lys12 proceeded rapidly (approximately 2 h) using the BOP reagent. Substitution of Ala2 with D-Ala2 and/or NH2-terminal replacement (desNH2-Tyr1 or N-MeTyr1) in the cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2 system resulted in highly potent analogs that were also active in vivo. Conformational analysis (circular dichroism and molecular dynamics calculations based on NOE-derived distance constraints) demonstrated that cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2 contains a long alpha-helical segment even in aqueous solution. A series of cyclo8,12 stereoisomers containing D-Asp8 and/or D-Lys12 were prepared and also found to be highly potent and to retain significant alpha-helical conformation. The high biological activity of cyclo8,12[N-MeTyr1,D-Ala2,Asp8,Ala15]-GRF(1-29)- NH2 may be explained on the basis of retention of a preferred bioactive conformation.

Potent long-acting growth hormone releasing factor analogues

Amidated fragments 1 to 29 of human growth hormone releasing factor, (GRF1-29NH2), were designed to encompass modifications that could prevent degradation by exopeptidases. The NH2-terminus was blocked by either N-methylation of Tyr1 or by introduction of NMeDAla2 while the COOH-terminus was blocked by an N-ethylamide. Other substitutions such as Nle in position 27 for methionine conferred stability toward air oxidation while Asn28, an amino acid substitution found in rat GRF, seemed to confer stronger binding affinity to the GRF receptor. Potency in vitro and duration of action in vivo of [NMeTyr1,Nle27,Asn28]hGRF1-29NHEt (4SG-29) were compared to those of hGRF1-40OH. 4SG-29 was found to be both ten times more potent than hGRF1-40)OH and exhibit significantly extended duration of action.

Rapid enzymatic degradation of growth hormone-releasing hormone by plasma in vitro and in vivo to a biologically inactive product cleaved at the NH2 terminus

The effect of plasma on degradation of human growth hormone-releasing hormone (GRH) was examined in vitro and in vivo using high performance liquid chromatography (HPLC), radioimmunoassay (RIA), and bioassay. When GRH(1-44)-NH2 was incubated with human plasma, the t1/2 of total GRH immunoreactivity was 63 min (RIA). However, HPLC revealed a more rapid disappearance (t1/2, 17 min) of GRH(1-44)-NH2 that was associated with the appearance of a less hydrophobic but relatively stable peptide that was fully immunoreactive. Sequence analysis indicated its structure to be GRH(3-44)-NH2. Identity was also confirmed by co-elution of purified and synthetic peptides on HPLC. Biologic activity of GRH(3-44)-NH2 was less than 10(-3) that of GRH(1-44)-NH2. After intravenous injection of GRH(1-44)-NH2 in normal subjects, a plasma immunoreactive peak with HPLC retention comparable to GRH(3-44)-NH2 was detected within 1 min and the t1/2 of GRH(1-44)-NH2 (HPLC) was 6.8 min. The results provide evidence for GRH inactivation by a plasma dipeptidylaminopeptidase that could limit its effect on the pituitary.

Design and biological activity of analogs of growth hormone releasing factor with potential amphiphilic helical carboxyl termini

A 29-amino acid analog of growth hormone releasing factor (GHRF) was designed in which the sequence of the first six amino acids at the amino terminus was maintained while the postulated amphiphilic helical structure in the remainder of the molecule was optimized. The amino acid sequence of the analog differed from that of the first 29 residues of human GHRF by 13 residues. The peptide was synthesized by the solid-phase procedure in amide and free acid forms, both of which were tested for biological activity. When assayed for the ability to stimulate growth hormone secretion in primary cultures of rat anterior pituitary cells, the amide analog was 1.57 times as potent as GHRF-(1-40)-OH, which was used as the standard for comparison, while the free acid form was 1/6th as potent in the same assay. The two forms of the analog were also tested for stimulation of cAMP formation; they exhibited relative potencies similar to those observed for growth hormone secretion. The high activity of the analog provides good evidence for the importance of an amphiphilic helical structure in the carboxyl-terminal portion of the GHRF molecule.

Strategies in the design of synthetic agonists and antagonists of growth hormone releasing factor

Previous research on the favorable effects of mild conformational restriction in the N-terminal region of glucagon has led us to carry out analogue studies on the sequence-related 1-12 region of GRF(1-29)NH2. Replacement of each of the first 11 amino acids by its D-isomer in turn gave a total of 5 analogues exhibiting increases in potency. Other analogues containing multiple D-amino acid replacements were also examined and found to be highly potent, for instance: D-Tyr-1,D-Ala-2, 2630; His-1,D-Ala-2, 3440; Ac-His-1,D-Ala-2, 1574; D-Ala-2,Nle-27, 1840; D-Ala-2,D-Asn-8,Nle-27, 1580; D-Ala-2,D-Asp-3,D-Asn-8,Nle-27, 2000; D-Asp-3,D-Asn-8,Nle-27, 3810 (GRF(1-29) = 100%). It is possible that these results with D-isomers reflect the presence of reverse turns (beta-bends) in this region of GRF. Indeed, the qualitative predictive method of Chou and Fasman supports this theory and indicates reverse turns in the 1-5 and 6-10 sequences. Further studies were performed to test this hypothesis by introducing even more rigidity into the N-terminal region via disulfide bond formation between positions normally containing aromatic amino acids. None of the bridged peptides displayed biological activity which suggests that chain folding does not produce any proximity among N-terminal residues. We had shown previously that position 2(Ala) was extremely sensitive to both conformational and side-chain alterations. This observation was extended to analogues containing Sar and Pro, both of which were also inactive on GH release at the doses tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Radioimmunoassay for human growth hormone-releasing factor (hGRF 1-40): comparison of plasma immunoreactive GRF after intravenous and subcutaneous administration to rats

A homologous radioimmunoassay (RIA) system was developed for human GRF 1-40 and used to measure immunoreactive (IR) concentrations of the peptide in rats to determine some of its pharmacokinetic characteristics after intravenous (i.v.) and subcutaneous (s.c.) administration. A plot of the disappearance of IR-hGRF from plasma after a single intravenous injection was fitted by a biexponential curve, analysis of which gave a half-life of 3.2 +/- 0.2 min for the initial distribution phase and 57.3 +/- 1.5 min for the elimination phase. Comparison of areas under the plasma IR-hGRF/time curves after injection of identical doses of hGRF 1-40 showed that the amount detected in the circulation after it was injected s.c. was only 14-16% of the amount detected after i.v. administration. Such results may indicate degradation of a substantial proportion of the dose of the peptide at the site of injection or during its transfer to plasma; this should be borne in mind when undertaking s.c. administration for clinical purposes or in assessing the effect of GRF analogues.

Synthesis and in vitro bioactivity of C-terminal deleted analogs of human growth hormone-releasing factor

A series of C-terminal deleted analogs of human growth hormone-releasing factor (hGRF) with either an amidated or a free carboxylic acid C-terminus were synthesized by solid phase methodology. Their capacity to release growth hormone was tested on rat anterior pituitary cells in monolayer culture. A gradual decrease of bioactivity down to 23% relative to hGRF was noted when the C-terminal amino acids were deleted to hGRF (1-34)OH. Further deletions, however, did not decrease the bioactivity because the potencies of the fragments, hGRF(1-31)NH2, (1-30)NH2 and (1-29)NH2 remained at about 50% of that of hGRF. Continual deletion of residues to hGRF(1-23)NH2, (1-22)NH2 and (1-21)NH2 still yielded bioactive fragments with full intrinsic activity despite very low potency. Only with the deletion down to hGRF(1-19)NH2 did the bioactivity completely disappear. Thus, together with the data published in a previous paper (1), the minimal biologically active core of hGRF with full intrinsic activity comprises the fragment (3-21).