Hormone-receptor interactions. The message sequence of alpha-melanotropin: demonstration of two active sites

Development of a Therapeutic Peptide for Cachexia Suggests a Platform Approach for Drug-like Peptides

During the development of a melanocortin (MC) peptide drug to treat the condition of cachexia (a hypermetabolic state producing lean body mass wasting), we were confronted with the need for peptide transport across the blood-brain barrier (BBB): the MC-4 receptors (MC4Rs) for metabolic rate control are located in the hypothalamus, i.e., behind the BBB. Using the term "peptides with BBB transport", we screened the medical literature like a peptide library. This revealed numerous "hits"-peptides with BBB transport and/or oral activity. We noted several features common to most peptides in this class, including a dipeptide sequence of nonpolar residues, primary structure cyclization (whole or partial), and a Pro-aromatic motif usually within the cyclized region. Based on this, we designed an MC4R antagonist peptide, TCMCB07, that successfully treated many forms of cachexia. As part of our pharmacokinetic characterization of TCMCB07, we discovered that hepatobiliary extraction from blood accounted for a majority of the circulating peptide's excretion. Further screening of the literature revealed that TCMCB07 is a member of a long-forgotten peptide class, showing active transport by a multi-specific bile salt carrier. Bile salt transport peptides have predictable pharmacokinetics, including BBB transport, but rapid hepatic clearance inhibited their development as drugs. TCMCB07 shares the general characteristics of the bile salt peptide class but with a much longer half-life of hours, not minutes. A change in its C-terminal amino acid sequence slows hepatic clearance. This modification is transferable to other peptides in this class, suggesting a platform approach for producing drug-like peptides.

Structure and chemistry of the peptide hormones of the intermediate lobe

The peptide hormones of the intermediate lobe are derived from a common precursor protein and are therefore biogenetically and structurally related. They represent a group of linear, flexible peptides which elicit a variety of physiological response. Structure-activity studies have shown that different segments of adjacent amino acid residues have a specific function (e.g. address, message, potentiation) in the interaction of each of these hormones with its receptor(s). This kind of organization of hormonal information is called sychnologic; it is the basic for the pleiotropic action of the opiomelanocortin peptides, i.e. the ability of related peptides to interact with different types of receptors in different target cells. Labelled peptide hormones with radioactive, fluorescent, or photolabile groups at defined sites are a prerequisite for studying hormone-receptor interaction. Multi-labelled derivatives of alpha-MSH are suitable for degradation and intracellular incorporation studies. Photoaffinity labelling of melanophore receptors with azidophenyl-containing analogues of alpha-MSH produces an irreversible stimulation of pigment cells. Covalent conjugates between peptide hormone receptors. These conjugates exhibit remarkable properties such as superpotency, strongly enhanced receptor affinity and prolonged action.

Peptidology: short amino acid modules in cell biology and immunology

Short amino acid motifs, either linear sequences or discontinuous amino acid groupings, can interact with specific protein domains, so exerting a central role in cell adhesion, signal transduction, hormone activity, regulation of transcript expression, enzyme activity, and antigen-antibody interaction. Here, we analyze the literature for such critical short amino acid motifs to determine the minimal peptide length involved in biologically important interactions. We report the pentapeptide unit as a common minimal amino acid sequence critically involved in peptide-protein interaction and immune recognition. The present survey may have implications in defining the dimensional module for peptide-based therapeutical approaches such as the development of novel antibiotics, enzyme inhibitors/activators, mimetic agonists/antagonists of neuropeptides, thrombolitic agents, specific anti-viral agents, etc. In such a therapeutical context, it is of considerable interest that low molecular weight peptides can easily cross biological barriers, are less susceptible to protease attacks, and can be administered at high concentrations. In addition, small peptides are a rational target for strategies aimed at antigen-specific immunotherapeutical intervention. As an example, specific short peptide fragments might be used to elicit antibodies capable of reacting with the full-length proteins containing the peptide fragment's amino acid sequence, so abolishing the risk of cross-reactivity.

Relevance of the C-terminal Arg-Phe sequence in gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH) for inducing cardiovascular effects in conscious rats

1. The cardiovascular effects by gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH) are probably not due to any of the well-known melanocortin subtype receptors. We hypothesize that the receptor for Phe-Met-Arg-Phe-amide (FMRFa) or Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide (neuropeptide FF; NPFFa), other Arg-Phe containing peptides, is the candidate receptor. Therefore, we studied various Arg-Phe containing peptides to compare their haemodynamic profile with that of gamma(2)-MSH(6 - 12), the most potent fragment of gamma(2)-MSH. 2. Mean arterial pressure (MAP) and heart rate (HR) changes were measured in conscious rats after intravenous administration of gamma(2)-MSH related peptides. 3. Phe-Arg-Trp-Asp-Arg-Phe-Gly (gamma(2)-MSH(6 - 12)), FMRFa, NPFFa, Met-enkephalin-Arg-Phe-amide (MERFa), Arg-Phe-amide (RFa), acetyl-Phe-norLeu-Arg-Phe-amide (acFnLRFa) and desamino-Tyr-Phe-norLeu-Arg-Phe-amide (daYFnLRFa) caused a dose-dependent increase in MAP and HR. gamma(2)-MSH(6 - 12) showed the most potent cardiovascular effects (ED(50)=12 nmol kg(-1) for delta MAP; 7 nmol kg(-1) for delta HR), as compared to the other Arg-Phe containing peptides (ED(50)=177 - 292 nmol kg(-1) for delta MAP; 130 - 260 nmol kg(-1) for delta HR). 4. Peptides, which lack the C-terminal Arg-Phe sequence (Lys-Tyr-Val-Met-Gly-His-Phe-Arg-Trp-Asp-Arg-Pro-Gly (gamma(2)-pro(11)-MSH), desamino-Tyr-Phe-norLeu-Arg-[L-1,2,3,4 tetrahydroisoquinoline-3-carboxylic acid]-amide (daYFnLR[TIC]a) and Met-enkephalin (ME)), were devoid of cardiovascular actions. 5. The results indicate that the baroreceptor reflex-mediated reduction of tonic sympathetic activity due to pressor effects is inhibited by gamma(2)-MSH(6 - 12) and that its cardiovascular effects are dependent on the presence of a C-terminal Arg-Phe sequence. 6. It is suggested that the FMRFa/NPFFa receptor is the likely candidate receptor, involved in these cardiovascular effects.

New aspects on the melanocortins and their receptors

Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.

Evidence for increased alpha MSH receptor binding in the F1 variant of B16 melanoma cells grown in dialyzed fetal calf serum

The specific binding of an alpha MSH analogue (Ac-[Nle4, D-Phe7] alpha MSH4-11 NH2) was enhanced in the presence of 10% dialyzed fetal calf serum (FCS) as compared with 10% FCS (nondialyzed) in the F1 variant of B16 melanoma cells. The replenishment of dialyzed serum with adrenocorticotrophic hormone (ACTH) or insulin had no effect on the increased level of alpha MSH receptor binding in these cells. However, 10 nM alpha MSH or 1 microM ACTH under identical conditions significantly decreased the level of alpha MSH binding. Competitive binding studies involving the alpha MSH analogue revealed that the specificity of the receptor was restricted to the complete molecule of alpha MSH, our analogue, beta MSH and ACTH1-24, ACTH4-10, which contains the amino acid sequence responsible for biological activity, showed a very low affinity for the receptor. Furthermore, we observed an interesting phenomenon unique to dialyzed FCS in that once the cells were grown to confluence and melanin was produced, the cells were no longer viable. However, in McCoy's medium, which is deficient in tyrosine, the cells did not produce melanin and remained viable.

Genetic construction, expression, and melanoma-selective cytotoxicity of a diphtheria toxin-related alpha-melanocyte-stimulating hormone fusion protein

The structural gene for diphtheria toxin, tox, has been modified at its Sph I site by the introduction of an oligonucleotide linker encoding a unique Pst I restriction endonuclease site and a synthetic oligonucleotide encoding alpha-melanocyte-stimulating hormone (alpha-MSH). The resulting fusion gene directs the expression of a diphtheria toxin-related alpha-MSH hybrid protein in which the diphtheria toxin receptor-binding domain has been replaced with alpha-MSH sequences. The chimeric toxin has been partially purified from periplasmic extracts of recombinant Escherichia coli K-12 and has been found to be selectively toxic for alpha-MSH receptor-positive human malignant melanoma NEL-M1 cells in vitro.

Comparison of structural requirements of alpha-MSH and ACTH for inducing excessive grooming and pigment dispersion

alpha-MSH and ACTH-like peptides are known to play an important role in the adaptation of many vertebrates to a new environment. These peptides induce pigment dispersion in amphibian melanophores through a receptor-mediated mechanism. In this study we compared the structural requirements of these peptides for melanotropic activity on Xenopus laevis melanophores with those for inducing excessive grooming in the rat. With the exception of ACTH1-24 there is a close resemblance in structure-activity relationships of the fragments and analogs tested in the two bioassays. [Nle4,-D-Phe7]-alpha-MSH is extremely active in both assays. Weak agonists such as [Leu9]-alpha-MSH did not possess antagonistic properties either in the melanophore assay or in the excessive grooming test. The data suggest that the mechanism of action of alpha-MSH-like peptides in rat brain is receptor-mediated like their action on melanophores.

The enhanced recovery of sensorimotor function in rats is related to the melanotropic moiety of ACTH/MSH neuropeptides

The recovery of sensorimotor function in female rats was studied using a foot-flick response test after crushing the sciatic nerve. Every other day the animals received a subcutaneous injection of small ACTH/MSH-like peptides. Rats treated with ACTH-(4-10), ACTH-(4-9), ACTH-(4-9) analog ORG 2766, ACTH-(6-10) and alpha-MSH showed a faster recovery of sensorimotor function as compared to vehicle-treated rats. Treatment with ACTH-(4-7) and the tripeptide Phe7-D-Lys8-Phe9 (PDLP, the C-terminal part of the ORG 2766) remained ineffective. The effect of alpha-MSH was even stronger than that of the other peptides. The facilitation of the return of sensorimotor function by the ACTH-like peptides is discussed in relation to the corticotropic and melanotropic properties of these peptides. Furthermore, it was shown that treatment with ORG 2766 was effective not only in young adult animals (2--3 months) but also in one-year-old animals.

Stimulation of the adenylate cyclase of A B16 melanoma cell line by pro-opiocortin-related peptides--a structure-activity study

The ability of alpha-melanotrophin (alpha-MSH or ACTH 1-acetyl-13 amide) and other structurally related peptides derived from the common precursor, pro-opiocortin, to stimulate adenylate cyclase activity in a pigmented B16 mouse melanoma was investigated. The peptides ACTH 1-39, ACTH 1-24, alpha-MSH, ACTH 1-13 amide and beta-MSH all stimulated the enzyme to a similar maximal extent and with similar potency (ED50 = 1.3 . 10(-6) M) except that ACTH 1-39 was slightly less potent (ED50 = 5 . 10(-6) M). ACTH 4-10 (ED50 = 4 . 10(-5) M) and gamma-MSH (ED50 = 5 . 10(-6) M) were partial agonists. ACTH 1-10 was no more effective than ACTH 4-10 in stimulating the enzyme whereas ACTH 1-13 amide was a full agonist. The peptides beta-endorphin and its derivatives, Met-enkephalin and melanotrophin potentiating factor (MPF), failed to stimulate the enzyme. We suggest that the B16 melanoma requires not only the sequence ACTH 4-10 but also some part of the sequence ACTH 11-13, or a similar sequence in the terminal portion of beta-MSH, for full activation of the receptor-linked enzyme.