The role of phenylalanine at position 6 in glucagon's mechanism of biological action: multiple replacement analogues of glucagon

The Molecular Pharmacology of Glucagon Agonists in Diabetes and Obesity

Within the past couple decades glucagon receptor agonism has drawn attention as a therapeutic tool for the treatment of type 2 diabetes and obesity. In both mice and humans glucagon-induced enhancements in energy expenditure and suppression of food intake suggest promising utility, therefore interest has advanced in the synthetic optimization of glucagon-based pharmacology to further resolve the physiological and cellular underpinnings. Modifications within the glucagon peptide sequence have allowed for greater solubility, stability, circulating half-life, and understanding of the structure-function potential behind partial and "super"-agonists. This knowledge gained from such modifications has provided a basis for the development of long-acting therapeutically useful glucagon analogues, chimeric unimolecular dual- and tri-agonists, and novel strategies for the targeting of nuclear hormones into glucagon receptor-expressing tissues. In this review, we summarize the peptide path leading to these glucagon-based developments in the field of anti-diabetes and anti-obesity pharmacology, while highlighting the associated biological and therapeutic effects.

Optimization of Truncated Glucagon Peptides to Achieve Selective, High Potency, Full Antagonists

Antagonism of glucagon's biological action is a proven strategy for decreasing glucose in diabetic animals and patients. To achieve full, potent, and selective suppression, we chemically optimized N-terminally truncated glucagon fragments for the identification and establishment of the minimum sequence peptide, [Glu9]glucagon(6-29) amide (11) as a full antagonist in cellular signaling and receptor binding (IC₅₀ = 36 nM). Substitution of Phe6 with l-3-phenyllactic acid (Pla) produced [Pla6, Glu9]glucagon(6-29) amide (21), resulting in a 3-fold improvement in receptor binding (IC₅₀ = 12 nM) and enhanced antagonist potency. Further substitution of Glu9 and Asn28 with aspartic acid yielded [Pla6, Asp28]glucagon amide (26), which demonstrated a further increase in inhibitory potency (IC₅₀ = 9 nM), and improved aqueous solubility. Peptide 26 and a palmitoylated analogue, [Pla6, Lys10(γGluγGlu-C16), Asp28]glucagon(6-29) amide (31), displayed sustained duration in vivo action that successfully reversed glucagon-induced glucose elevation in mice.

Secretion expression of recombinant glucagon inEscherichia coli

A novel approach for the preparation of recombinant human glucagon was described. An expression vector pAGluT, containing phoA promoter, phoA signal peptide and glucagon gene, was constructed by means of genetic engineering.Escherichia coli strain YK537 was transformed with pAGluT. High-level secretory expression of recombinant human glucagon was achieved. The expression yield of recombinant human glucagon was found to be 80 mg/L, approximately 30% of the total proteins in supernatant. The biological activities and the physicochemical properties of the purified recombinant human glucagon were found to be the same as that of native glucagon. In addition, our results suggested that phoA expression system may be suitable for the expression of other small peptides.

Design of a long acting peptide functioning as both a glucagon-like peptide-1 receptor agonist and a glucagon receptor antagonist

The closely related peptides glucagon-like peptide (GLP-1) and glucagon have opposing effects on blood glucose. GLP-1 induces glucose-dependent insulin secretion in the pancreas, whereas glucagon stimulates gluconeogenesis and glycogenolysis in the liver. The identification of a hybrid peptide acting as both a GLP-1 agonist and a glucagon antagonist would provide a novel approach for the treatment of type 2 diabetes. Toward this end a series of hybrid peptides made up of glucagon and either GLP-1 or exendin-4, a GLP-1 agonist, was engineered. Several peptides that bind to both the GLP-1 and glucagon receptors were identified. The presence of glucagon sequence at the N terminus removed the dipeptidylpeptidase IV cleavage site and increased plasma stability compared with GLP-1. Targeted mutations were incorporated into the optimal dual-receptor binding peptide to identify a peptide with the highly novel property of functioning as both a GLP-1 receptor agonist and a glucagon receptor antagonist. To overcome the short half-life of this mutant peptide in vivo, while retaining dual GLP-1 agonist and glucagon antagonist activities, site-specific attachment of long chained polyethylene glycol (PEGylation) was pursued. PEGylation at the C terminus retained the in vitro activities of the peptide while dramatically prolonging the duration of action in vivo. Thus, we have generated a novel dual-acting peptide with potential for development as a therapeutic for type 2 diabetes.

Molecular determinants of glucagon receptor signaling

A 29-amino acid polypeptide hormone, glucagon has been one of the most prolific models in the study of hormone action. The key biologic function of glucagon is to counterbalance the actions of insulin and maintain a normal level of serum glucose. Diabetes mellitus can thus be considered a bihormonal disorder with an excess of glucagon contributing to the hyperglycemic state. The effects of glucagon are mediated by the glucagon receptor, which is itself a prototypical member of a distinct category called family B receptors within the G protein-coupled superfamily of seven-helical transmembrane receptors (GPCRs). At the structural level, the peptide ligands of family B receptors are highly homologous, in particular in the N-terminal region of the molecules. The mechanism by which highly homologous peptide ligands selectively recognize their receptors involves distinct molecular interactions that are gradually being elucidated. This review focuses on structural determinants of the glucagon receptor that are important for its activity with respect to interaction with its ligand and G proteins. Information about the glucagon receptor is presented within the context of what is known about other members of the family B GPCRs.

Roles of specific extracellular domains of the glucagon receptor in ligand binding and signaling

To identify structural determinants of ligand binding in the glucagon receptor, eight receptor chimeras and additional receptor point mutants were prepared and studied. Amino acid residues 103-117 and 126-137 in the extracellular N-terminal tail and residues 206-219 and 220-231 in the first extracellular loop of the glucagon receptor were replaced with the corresponding segments of the glucagon-like peptide-1 receptor or the secretin receptor. Specific segments of both the N-terminal tail and the first extracellular loop of the glucagon receptor are required for hormone binding. The 206-219 segment of the first loop appears to be important for both glucagon binding and receptor activation. Functional studies with a synthetic chimeric peptide consisting of the N-terminal 14 residues of glucagon and the C-terminal 17 residues of glucagon-like peptide 1 suggest that hormone binding specificity may involve this segment of the first loop. The binding selectivity may arise in part from aspartic acid residues in this segment. Mutation of R-202 located at the junction between the second transmembrane helix and the first loop resulted in a mutant receptor that failed to bind glucagon or signal. We conclude that high-affinity glucagon binding requires multiple contacts with residues in the N-terminal tail and first extracellular loop domain of the glucagon receptor, with hormone specificity arising primarily from the amino acid 206-219 segment. The data suggest a model whereby glucagon first interacts with the N-terminal domain of the receptor followed by more specific interactions between the N-terminal half of the peptide and the first extracellular loop of the receptor, leading to activation.

Identification of alkylidene hydrazides as glucagon receptor antagonists

High throughput screening of our small molecule combinatorial library identified a class of benzoylnaphthalenehydrazones with modest affinity for the human glucagon receptor. Optimization of this initial hit through a series of targeted libraries and traditional medicinal chemistry led to ligands with nanomolar affinities. Pharmacological evaluation demonstrated that these ligands were competitive glucagon receptor antagonists. Intravenous administration of a representative benzoylnaphthalenehydrazone into rats attenuated glucagon-stimulated glucose levels.

Development of potent glucagon antagonists: structure-activity relationship study of glycine at position 4

We examined the functional role of glycine at position 4 in the potent glucagon antagonist [desHis(1), Glu(9)]glucagon amide, by substituting the L- and D-enantiomers of alanine and leucine for Gly(4) in this antagonist. The methyl and isobutyl side-chain substituents were introduced to evaluate the preference shown by the glucagon receptor, if any, for the orientation of the N-terminal residues. The L-amino acids demonstrated only slightly better receptor recognition than the D-enantiomers. These results suggest that the Gly(4) residue in glucagon antagonists may be exposed to the outside of the receptor. The enhanced binding affinities of analogs 1 and 3 compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide, may have resulted from the strengthened hydrophobic patch in the N-terminal region and/or the increased propensity for a helical conformation due to the replacement of alanine and leucine for glycine. Thus, as a result of the increased receptor binding affinities, antagonist activities of analogs 1-4 were increased 10-fold compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide. These potent glucagon antagonists have among the highest pA(2) values of any glucagon analogs reported to date.

Development of potent truncated glucagon antagonists

In pursuit of truncated glucagon analogues that can interact with the glucagon receptor with substantial binding affinity, 23 truncated glucagon analogues have been designed and synthesized. These truncated analogues consist of several fragments of glucagon with 11 or 12 amino acid residues (1-4), conformationally constrained analogues containing the sequence of the middle region of glucagon (5-15), and truncated analogues containing the sequence of the C-terminal region (16-23). Biological assays of these analogues showed that the truncated glucagon analogues with the sequence of the C-terminal region possess significantly better binding affinity compared to the truncated analogues with the sequence of the middle region, and these analogues (17-23) demonstrated potent antagonistic activity (pA(2) values between 6.5 and 7.5). On the basis of these results, it can be suggested that glucagon interacts with its receptor with two hydrophobic patches located in the middle and the C-terminal regions of glucagon, and both hydrophobic patches are necessary for significant receptor recognition. These two hydrophobic binding motifs, located in two different regions of glucagon, appear to be the reason why the earlier attempts to obtain truncated analogues with good binding affinity did not result in any success. Long peptide hormones such as glucagon seem to require more than one binding pocket on the receptors for maximal interaction.

Design and synthesis of conformationally constrained glucagon analogues

Glucagon was systematically modified by forming lactam bridges within the central region of the molecule to give conformationally constrained cyclic analogues. Six cyclic glucagon analogues have been designed and synthesized. They are c[Asp(9),Lys(12)][Lys(17,18), Glu(21)]glucagon-NH(2) (1), c[Asp(9),Lys(12)]glucagon-NH(2) (2), c[Lys(12),Asp(15)]glucagon-NH(2) (3), c[Asp(15), Lys(18)]glucagon-NH(2) (4), [Lys(17)-c[Lys(18), Glu(21)]glucagon-NH(2) (5), and c[Lys(12),Asp(21)]glucagon-NH(2) (6). The receptor binding potencies and receptor second messenger activities were determined by radio-receptor binding assays and adenylate cyclase assays, respectively, using rat liver plasma membranes. Most interestingly, analogues 1, 2, 3, and 4 were antagonists of glucagon stimulated adenylate cyclase activity, whereas analogues 5 and 6 were partial agonists in the functional assay. All of the cyclic analogues were found to have reduced binding potencies relative to glucagon. The structural features that might be responsible for these effects were studied using circular dichroism spectroscopy and molecular modeling. These results demonstrated the significant modulations of both receptor binding affinity and transduction (adenylate cyclase activity) that can accompany regional conformational constraints even in larger polypeptide ligands. These studies suggest that the entire molecular conformation, including the flexible middle portion, is important for molecular recognition and transduction at the hepatic glucagon receptor.

Discovery and structure-activity relationship of the first non-peptide competitive human glucagon receptor antagonists

The first non-peptide competitive human glucagon receptor antagonist, 2-(benzimidazol-2-ylthio)-1-(3,4-dihydroxyphenyl)-1-ethan one, NNC 92-1687 (2), is described. This antagonist has a binding affinity of 20 microM (IC50) and a functional Ki = 9.1 microM at the human glucagon receptor. A structure-activity relationship (SAR) was obtained on this compound, and the results show that only the benzimidazole part can be changed without complete loss of affinity. Analogues with tert-butyl or benzyloxy groups in the 5-position of the benzimidazole moiety were found to be equipotent or slightly more potent, all displaying binding affinities around 5-20 microM. Most of the changes to the catechol and the linker gave compounds without any affinity toward the human glucagon receptor. The 3-hydroxy group could, however, in the presence of a 4-hydroxy group be changed to a methoxy or a chloro group while retaining affinity.

Structure-function studies on positions 17, 18, and 21 replacement analogues of glucagon: the importance of charged residues and salt bridges in glucagon biological activity

We have designed and synthesized eight compounds 2-9 which incorporate various amino acid residues in positions 17, 18, and 21 of the glucagon molecule: 2, [Lys17]glucagon amide; 3, [Lys18]glucagon amide; 4, [Nle17,Lys18,Glu21]glucagon amide; 5, [Orn17,18, Glu21]glucagon amide; 6, [d-Arg17]glucagon; 7, [d-Arg18]glucagon; 8, [d-Phe17]glucagon; and 9, [d-Phe18]glucagon. Compared to glucagon (IC50 = 1.5 nM), analogues 2-9 were found to have binding affinity IC50 values (in nM) of 0.7, 4.1, 1.0, 2.0, 5.0, 25.0, 43.0, and 32.0, respectively. When these compounds were tested for their ability to stimulate adenylate cyclase (AC) activity, they were found to be full or partial agonists having maximum stimulation values of 100, 100, 100, 100, 87, 78, 94, and 100%, respectively. On the basis of the X-ray crystal structure of [Lys17,18,Glu21]glucagon amide reported here, the ability to form a salt bridge between Lys18 and Glu21 is probably key to their increased binding and second messenger activities. Among the eight analogues synthesized here, only analogue 4 preserves the ability to form a salt bridge between Lys18 and Glu21. However, since these modifications are minor they do not seem to change the amphiphilic character of the C-terminus, allowing these analogues to reach 78-100% stimulation in the adenylate cyclase assay. Biological data from analogues 6-9 supports the idea that position 18 of glucagon may influence binding only, while position 17 may influence both receptor recognition and transduction.