ABSTRACT
Glucagon counters insulin's effects on glucose metabolism and serves as a rescue medicine in the treatment of hypoglycemia. Acute hypoglycemia, a common occurrence in insulin-dependent diabetes, is the central obstacle to correcting high blood glucose, a primary cause of long-term microvascular complications. As a result, there has been a resurgence of interest in improved glucagon therapy, including nonconventional liquid formulations, alternative routes of administration, and novel analogs with optimized biophysical properties. These options collectively minimize the complexity of glucagon delivery and enable its application in ways not feasible with conventional emergency rescue kits. These advances have indirectly promoted the integrated use of glucagon agonism with other hormones in a manner that runs counter to the long-standing pursuit of glucagon antagonism. This review summarizes novel approaches to glucagon optimization, methods with potential application to the broader family of therapeutic peptides, where biophysical challenges may be encountered.
Subject(s)
Glucagon/chemistry , Glucagon/therapeutic use , Amino Acid Sequence , Animals , Drug Delivery Systems , Humans , Hypoglycemia/drug therapy , Molecular Structure , Protein Stability , Solubility , Structure-Activity RelationshipABSTRACT
This report describes the chemical synthesis and biological characterization of novel three-chain insulin analogs with a destabilized secondary structure. The analogs, obtained by chemical synthesis via a single-chain precursor and selective enzymatic digestion, were used to investigate the role of the highly conserved 'insulin fold'. Biological characterization through in vitro biochemical signaling showed extremely low activity at each insulin receptor when compared with native insulin. We conclude that the 'insulin fold' is a structural foundation that supports insulin biological action.
Subject(s)
Antigens, CD/metabolism , Insulin/chemical synthesis , Insulin/metabolism , Receptor, Insulin/metabolism , Amino Acid Sequence , HEK293 Cells , Humans , Insulin/analogs & derivatives , Metalloendopeptidases/chemistry , Models, Molecular , Molecular Sequence Data , Protein Folding , Protein Stability , Protein Structure, Secondary , Signal Transduction , Solid-Phase Synthesis Techniques , Structure-Activity Relationship , Trypsin/chemistryABSTRACT
We studied interscapular brown adipose tissue (iBAT) activity in wild-type (WT) and glucagon-like peptide 1 receptor (GLP-1R)-deficient mice after the administration of the proglucagon-derived peptides (PGDPs) glucagon-like peptide (GLP-1), glucagon (GCG), and oxyntomodulin (OXM) directly into the brain. Intracerebroventricular injection of PGDPs reduces body weight and increases iBAT thermogenesis. This was independent of changes in feeding and insulin responsiveness but correlated with increased activity of sympathetic fibers innervating brown adipose tissue (BAT). Despite being a GCG receptor agonist, OXM requires GLP-1R activation to induce iBAT thermogenesis. The increase in thermogenesis in WT mice correlates with increased expression of genes upregulated by adrenergic signaling and required for iBAT thermogenesis, including PGC1a and UCP-1. In spite of the increase in iBAT thermogenesis induced by GLP-1R activation in WT mice, Glp1r(-/-) mice exhibit a normal response to cold exposure, demonstrating that endogenous GLP-1R signaling is not essential for appropriate thermogenic response after cold exposure. Our data suggest that the increase in BAT thermogenesis may be an additional mechanism whereby pharmacological GLP-1R activation controls energy balance.
Subject(s)
Adipose Tissue, Brown/metabolism , Central Nervous System/metabolism , Glucagon-Like Peptide 1/metabolism , Receptors, Glucagon/metabolism , Signal Transduction , Thermogenesis , Adipose Tissue, Brown/innervation , Animals , Glucagon/metabolism , Glucagon-Like Peptide 1/agonists , Glucagon-Like Peptide-1 Receptor , Insulin Resistance , Ion Channels/genetics , Ion Channels/metabolism , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Nerve Tissue Proteins/genetics , Nerve Tissue Proteins/metabolism , Neurons/metabolism , Oxyntomodulin/metabolism , Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha , Receptors, Glucagon/agonists , Receptors, Glucagon/genetics , Scapula , Sympathetic Nervous System/metabolism , Trans-Activators/genetics , Trans-Activators/metabolism , Transcription Factors , Uncoupling Protein 1 , Up-RegulationABSTRACT
BACKGROUND: Glucagon is a life-saving medication used in the treatment of hypoglycemia. It possesses poor solubility in aqueous buffers at or near physiological pH values. At low and high pH, at which the peptide can be formulated to concentrations of a milligram or more per milliliter, the chemical integrity of the hormone is limited, as evidenced by the formation of multiple degradation-related peptides. Consequently, the commercial preparation is provided as a lyophilized solid with an acidic diluent and directions for rendering it soluble at the time of use. Any unused material is recommended for disposal immediately after initial use. METHODS: A set of glucagon analogs was prepared by solid-phase peptide synthesis to explore the identification of a glucagon analog with enhanced solubility and chemical stability at physiological pH. The physical properties of the peptide analogs were studied by solubility determination, high-performance chromatography, and mass spectral analysis. The biochemical properties were determined in engineered human embryonic kidney cell line 293 (HEK293) cells that overexpressed either the human glucagon or glucagon-like peptide-1 (GLP-1) receptors linked to a luciferase reporter gene. RESULTS: We observed the previously characterized formation of glucagon degradation products upon incubation of the peptide in dilute acid for extended periods or elevated temperature. Lowering the isoelectric point of the hormone through the substitution of asparagine-28 with aspartic acid significantly increased the solubility at physiological pH. Similarly, the C-terminal extension (Cex) of the hormone with an exendin-based, 10-residue, C-terminal sequence yielded a peptide of dramatically enhanced solubility. These two glucagon analogs, D28 and Cex, maintained high potency and selectivity for the glucagon receptor relative to GLP-1 receptor. CONCLUSIONS: Glucagon presents unique structural challenges to the identification of an analog of high biological activity and selectivity that also possesses sufficient aqueous solubility and stability such that it might be developed as a ready-to-use medicine. The glucagon analogs D28 and Cex demonstrated all of the chemical, physical, and biochemical properties supportive of further study as potential clinical candidates for treatment of hypoglycemia.