The thyrotropin-releasing hormone-like peptides pGlu-Phe-Pro amide and pGlu-Glu-Pro amide increase plasma triiodothyronine levels in the mouse; the activity is sensitive to testosterone

The Thyrotropin-Releasing Hormone-Degrading Ectoenzyme, a Therapeutic Target?

Thyrotropin releasing hormone (TRH: Glp-His-Pro-NH2) is a peptide mainly produced by brain neurons. In mammals, hypophysiotropic TRH neurons of the paraventricular nucleus of the hypothalamus integrate metabolic information and drive the secretion of thyrotropin from the anterior pituitary, and thus the activity of the thyroid axis. Other hypothalamic or extrahypothalamic TRH neurons have less understood functions although pharmacological studies have shown that TRH has multiple central effects, such as promoting arousal, anorexia and anxiolysis, as well as controlling gastric, cardiac and respiratory autonomic functions. Two G-protein-coupled TRH receptors (TRH-R1 and TRH-R2) transduce TRH effects in some mammals although humans lack TRH-R2. TRH effects are of short duration, in part because the peptide is hydrolyzed in blood and extracellular space by a M1 family metallopeptidase, the TRH-degrading ectoenzyme (TRH-DE), also called pyroglutamyl peptidase II. TRH-DE is enriched in various brain regions but is also expressed in peripheral tissues including the anterior pituitary and the liver, which secretes a soluble form into blood. Among the M1 metallopeptidases, TRH-DE is the only member with a very narrow specificity; its best characterized biological substrate is TRH, making it a target for the specific manipulation of TRH activity. Two other substrates of TRH-DE, Glp-Phe-Pro-NH2 and Glp-Tyr-Pro-NH2, are also present in many tissues. Analogs of TRH resistant to hydrolysis by TRH-DE have prolonged central efficiency. Structure-activity studies allowed the identification of residues critical for activity and specificity. Research with specific inhibitors has confirmed that TRH-DE controls TRH actions. TRH-DE expression by β2-tanycytes of the median eminence of the hypothalamus allows the control of TRH flux into the hypothalamus-pituitary portal vessels and may regulate serum thyrotropin secretion. In this review we describe the critical evidences that suggest that modification of TRH-DE activity in tanycytes, and/or in other brain regions, may generate beneficial consequences in some central and metabolic disorders and identify potential drawbacks and missing information needed to test these hypotheses.

TRH-like peptides

TRH-like peptides are characterized by substitution of basic amino acid histidine (related to authentic TRH) with neutral or acidic amino acid, like glutamic acid, phenylalanine, glutamine, tyrosine, leucin, valin, aspartic acid and asparagine. The presence of extrahypothalamic TRH-like peptides was reported in peripheral tissues including gastrointestinal tract, placenta, neural tissues, male reproductive system and certain endocrine tissues. Work deals with the biological function of TRH-like peptides in different parts of organisms where various mechanisms may serve for realisation of biological function of TRH-like peptides as negative feedback to the pituitary exerted by the TRH-like peptides, the role of pEEPam such as fertilization-promoting peptide, the mechanism influencing the proliferative ability of prostatic tissues, the neuroprotective and antidepressant function of TRH-like peptides in brain and the regulation of thyroid status by TRH-like peptides.

Occurrence of the free and Peptide forms of pyroglutamic acid in plasma from the portal blood of rats that had ingested a wheat gluten hydrolysate containing pyroglutamyl peptides

In order to determine pyroglutamic acid levels in plasma, we developed a method based on precolumn derivatization of the carboxyl group of pyroglutamic acid with 2-nitrophenylhydrazine. Eight-week-old male SD strain rats were administered 200 mg of an acidic peptide fraction obtained from a commercial wheat gluten hydrolysate containing 0.63 mmol/g pyroglutamyl peptide. After administration, significant amounts of free pyroglutamic acid were observed in the ethanol-soluble fraction of the plasma from the portal vein. In addition, pyroglutamate aminopeptidase digestion of the ethanol-soluble fraction liberated significant amounts of pyroglutamic acid, which indicated the presence of the pyroglutamyl peptide. The presence of the pyroglutamyl peptide in the plasma was further confirmed by size exclusion chromatography. The levels of free and peptide forms of pyroglutamic acid increased significantly and reached a maximum (approximately 40 nmol/mL) at 15 and 30 min after administration, respectively.

Gonadins, a novel family of glutamyl-tripeptide amides present in the testis with activity in the hypophyseal–gonadal axis

Here we present a new family of endogenous peptides identified in rat testis with structure of glutamyl-tripeptide amides which are also present in plasma. These peptides have different activities in the hypophyseal-gonadal axis. Evidences showing the endocrine activities of some of the peptides are presented. In this communication we demonstrate the presence of peptides with a common structure Glu-X-Pro amide, where X can be one of the following amino acids: glutamic acid, glutamine, aspartic acid, asparagine, phenylalanine or tyrosine. These peptides have been identified by a series of chromatographies and by mass spectrometry. Some of the peptides where tested for its biological activity observing that subcutaneous administration of the peptides Glu-Glu-Pro amide, Glu-Gln-Pro amide and Glu-Phe-Pro amide were able to reduce plasma levels of testosterone and luteinizing hormone (LH) without modification of the levels of follicle stimulating hormone (FSH). The peptide Glu-Asp-Pro amide, however, produced an increase in the levels of testosterone without modifying LH or FSH levels. It is proposed that the glutamyl-tripeptide amides that reduce the levels of testosterone and LH are released from the testis and act in the pituitary via circulation in an endocrine manner. The specific inhibition of LH release is similar to that produced by inhibin on FSH release. On the other hand the peptide that increases the levels of testosterone is produced in the testis and seems to act directly in the testis in a paracrine or autocrine manner. It is proposed here a new mechanism of regulation of hypophyseal-gonadal axis, a negative feedback exerted by the glutamyl-tripeptide amides in the pituitary. Also it is proposed the generic name of gonadins for the novel family of glutamyl-tripeptide amides. We suggest that gonadins could be used in the future as drugs for treatment of different endocrine disorders, hormone-dependent cancer and as contraceptives.

Characterization of a circulating N-extended form of the thyrotropin-releasing hormone-like peptide pGlu-Glu-Pro amide in human plasma

The TRH-like peptides pGlu-Glu-Pro amide, pGlu-Phe-Pro amide, and pGlu-Gln-Pro amide were isolated and identified some years ago, and these peptides have been proven to be present in many tissues and fluids. The presence of TRH-like immunoreactivity distinct from TRH in blood has been observed previously. In the present study, the presence of N-extended forms of TRH-like peptides in plasma has been investigated. Peripheral blood samples of human, rat, and rabbit were obtained and plasma was extracted. The peptides were separated in several steps of chromatography, including gel filtration, cation and anion exchange, and HPLC. The concentrations of the TRH-like peptides in the column fractions were measured by RIA with TRH antibody. The N-extended forms of TRH-like peptides were determined by RIA after trypsin digestion. In human plasma it was observed an N-extended form of TRH-like peptides in substantial concentration. After trypsin and heating, the N-extended forms of TRH-like peptides were rechromatographed on Sephadex G-50. This showed that the TRH-like peptides released have a similar size to TRH. The peptides were then separated by cation exchange chromatography, and the major fraction was unretained, indicating a neutral or acidic nature. Part of the unretained fraction was then chromatographed on anion exchange column in which the major fraction was retained, demonstrating the acidic nature of the peptides. Similar results have been observed in rat and rabbit. The other part of the unretained fraction from cation exchange chromatography of human plasma was purified on HPLC. The results demonstrated that the major component observed by HPLC cochromatographed with synthetic pGlu-Glu-Pro amide. This study represents the first demonstration of a circulating N-extended form of any TRH-like peptide.

Kinetic investigation of the specificity of porcine brain thyrotropin-releasing hormone-degrading ectoenzyme for thyrotropin-releasing hormone-like peptides

Evidence indicates that neuronally released thyrotropin-releasing hormone (TRH) is selectively inactivated by TRH-degrading ectoenzyme (TRH-DE) (EC ). TRH-DE inhibitors may be used to enhance the therapeutic actions of TRH and to investigate the functions of TRH and TRH-DE in the central nervous system. Although TRH-DE appears to exhibit a high degree of specificity toward TRH, systematic specificity studies, which would facilitate inhibitor design, have not been previously conducted for this enzyme. In this paper we present the first description of TRH-DE specificity across a directed peptide library in which the histidyl (P(1)') residue of TRH was replaced by a series of amino acids. Peptides were synthesized using standard solid phase chemistry. Kinetic parameters were measured either by continuous or discontinuous fluorometric assays or by quantitative high pressure liquid chromatography. The P(1)' residue was found to influence significantly both the ability of the peptides to bind to TRH-DE, as measured by their K(i) values, and the ability of TRH-DE to catalyze their hydrolysis. Moderately bulky, uncharged P(1)' residues were found to bind preferentially to TRH-DE. Results from this screen provide valuable information for the development of TRH-DE inhibitors and have led to the identification of two potent, reversible TRH-DE inhibitors, l-pyroglutamyl-l-asparaginyl-l-prolineamide (K(i) = 17.5 micrometer) and Glp-Asn-Pro-7-amido-4-methyl coumarin (K(i) = 0.97 micrometer).

Characterization of neutral TRH-like peptides in mammary gland, mammary tumors and milk

Three pyroglutamylpeptide amides, pGlu-Glu-Pro amide, pGlu-Phe-Pro amide and pGlu-Gln-Pro amide, with similar structures to thyrotropin-releasing hormone (TRH), have been identified previously in the male reproductive system. We report here that rat and human mammary gland contain neutral TRH-immunoreactive peptides which are not retained on cation or anion exchange chromatography and that similar peptides occur in the milk of rat, cow, ewe and sow. The TRH-like peptides in lyophilized milk from the cow were purified by gel exclusion chromatography, mini-column cation exchange chromatography and reversed phase high performance liquid chromatography (HPLC) and the chromatographed peptides were located by TRH radioimmunoassay (RIA). In each chromatographic system the major TRH-immunoreactive peptide from cow milk exhibited identical behavior to pGlu-Phe-Pro amide; in addition there were two minor TRH-immunoreactive components. The possible physiological role of the TRH-like peptides in the mammary gland is discussed. In a series of patients with breast carcinoma, mammary tumor tissue was shown to contain approximately four times more TRH-like peptide than normal mammary tissue from the same patient, raising the possibility that the TRH-like peptides may be implicated in tumor development.

Three TRH-like molecules are released from rat hypothalamus in vitro

TRH-like immunoreactivity distinct from TRH is present in various tissues and fluids. In order to determine whether TRH-like molecules are secreted by the hypothalamus, we analyzed tissues and media from hypothalamic slices incubated in Krebs Ringer bicarbonate. Media from basal or high KCl conditions contained 3 TRH-like molecules evidenced by reverse phase high performance liquid chromatography followed by TRH radioimmunoassay. Peak I corresponded to authentic TRH (73% of total immunoreactivity) and peaks II and III had a higher retention time. These additional TRH-like forms were neither detected in hypothalamic tissue nor in tissue or medium from olfactory bulb. Gel filtration analysis of hypothalamic media revealed only one TRH-like peak eluting as TRH, suggesting that the molecular weights of peaks II and III are similar to that of TRH. Peak II retention time was similar to that of pglu-phe-proNH2. We analysed if they could be produced by post secretory metabolism of TRH. Incubation of hypothalamic slices with [3H-Pro]-TRH did not produce radioactive species comigrating with peaks II or III. However, it induced rapid degradation to [3H-Pro]-his-prodiketopiperazine ([3H]-HPDKP). Inhibitor profile suggested that pyroglutamyl aminopeptidase II, but not pyroglutamyl aminopeptidase I, is responsible for [3H]-HPDKP production. These data are consistent with the hypothesis that pyroglutamyl aminopeptidase II is the main aminopeptidase degrading TRH in hypothalamic extracellular fluid. Furthermore, we suggest that the hypothalamus releases additional TRH-like molecules, one of them possibly pglu-phe-proNH2, which may participate in control of adenohypophyseal secretions.

Identification of the TRH-like peptides pGlu-Glu-Pro amide and pGlu-Phe-Pro amide in rat thyroid: regulation by thyroid status

Rat thyroid contains thyrotropin-releasing hormone (TRH) and TRH-like peptides which react with TRH antisera. We have identified the TRH-like peptides in the thyroid and examined whether their levels are influenced by thyroid status. The peptides were extracted from the thyroid glands of five hyperthyroid rats and purified by ion-exchange chromatography on SP-Sephadex C25 and reversed-phase high performance liquid chromatography. The principal TRH-immunoreactive component exhibited the same retention on HPLC as synthetic pGlu-Glu-Pro amide and a secondary component corresponded to synthetic pGlu-Phe-Pro amide. In agreement with these assignments the main peptide was shown to be acidic when chromatographed on DEAE-Sephadex A25 and the second peptide neutral. The levels of TRH and TRH-like peptides in the thyroid were investigated in hyper-, hypo- and euthyroid rats. Hyperthyroidism was induced by chronic subcutaneous administration of triiodothyronine (T3) and hypothyroidism was produced by addition of propylthiouracil (PTU) to the drinking water. The amounts of the peptides were determined by radioimmunoassay with a TRH-antiserum, carried out after extraction from the tissues and purification by ion exchange chromatography. The mean concentration of TRH-like peptides in the thyroids of the hyperthyroid rats was 95.5+/-25.5 pmol/g, the mean concentration in the hypothyroid rats was 11.7+/-3.4 pmol/g, and in the euthyroid rats 17.6+/-3.2 pmol/g. The concentrations of TRH were less influenced by thyroid status: the values in hyper-, hypo- and euthyroid rats were 47.5+/-9.4, 42.1+/-6.3, and 17.2+/-1.6 pmol/g respectively. The results show that the levels of the TRH-like peptides in rat thyroid are highly sensitive to thyroid status, suggesting a possible involvement in thyroid regulation.