Insulin-like actions of glucagon-like peptide-1: a dual receptor hypothesis

Abstract

GLP-1 (9-36)amide is the cleavage product of GLP-1(7-36) amide, formed by the action of diaminopeptidyl peptidase-4 (Dpp4), and is the major circulating form in plasma. Whereas GLP-1(7-36)amide stimulates glucose-dependent insulin secretion, GLP-1(9-36)amide has only weak partial insulinotropic agonist activities on the GLP-1 receptor, but suppresses hepatic glucose production, exerts antioxidant cardioprotective actions and reduces oxidative stress in vasculature tissues. These insulin-like activities suggest a role for GLP-1 (9-36)amide in the modulation of mitochondrial functions by mechanisms independent of the GLP-1 receptor. In this paper, we discuss the current literature suggesting that GLP-1(9-36)amide is an active peptide with important insulin-like actions. These findings have implications in nutrient assimilation, energy homeostasis, obesity, and the use of Dpp4 inhibitors for the treatment of diabetes.

Figure 1

Proglucagon is a protein precursor of glucagon and glucagon-like peptides (GLPs). (a) The multifunctional proglucagon is cleaved by (b) site-selective proteases (PC2, PC1/3) in the pancreas to liberate (i) glucagon as the active peptide and (ii) GLP-1 and GLP-1 in the intestine.. The prohormone convertases PC2 and PC1/3 selectively cleave proglucagon to produce glucagon and GLPs, respectively. (iii) The insulinotropic GLP-1 peptides are GLP-1(7-36)amide and a glycine-extended form GLP-1(7-37). They are released from the intestine in response to feeding and stimulate glucose-dependent insulin secretion. These insulinotropic GLP-1s are rapidly converted after their secretion to (iv) insulinomimetic hormones GLP-1(9-36)amide and GLP-1(9-37) by removal of the N-terminal two amino acids by the diaminopeptidyl peptidase-4 (Dpp4). These may be further cleaved by neutral endopeptidases (NEP) to produce (v) small C-terminal peptides; a nonapeptide and pentapeptide, GLP-1(28-36)amide and GLP-1(32-36)amide, and the corresponding decapeptide and hexapeptide GLP-1(28-37) and GLP-1(32-37). It is proposed that these small peptides may target mitochondria and modulate oxidative phosphorylation, glucose and fatty acid metabolism, and energy expenditure resulting in an attenuation of oxidative stress (ROS formation) and the promotion of cell survival.

Figure 2

Amino acid sequences of Glucagon-like peptides-1 and sub-peptides generated by selective modifications by specific proteases. The parent insulinotropic peptide is cleaved out from the proglucagon precursor by the actions of prohormone convertase type 1/3. The carboxylterminus of GLP-1 is modified at the time of synthesis by the actions of peptide amidating monooxygenase (PAM) which removes the carboxyl terminal glycine (G) and amidates the penultimate arginine (R). Within minutes after their secretion into the circulation, the GLP-1(7-36)amide and GLP-1(7-37) are modified by removal of the amino-terminal amino acids, histidine (H) and alanine (A) (black font) by the diaminopeptidyl peptidase-4 (Dpp4). Cleavage by Dpp4 yields the insulinomimetic peptides, GLP-1(9-36)amide and GLP-1(9-37). It is proposed that additional cleavages within the carboxy-terminal domains of the insulinotropic peptides by endopeptidases (NEP), such as the selective cleavages by neutral endopeptidase 24.11, generates small penta- (red font) to deca-peptides (purple + red font) that target mitochondria and thereby modulate oxidative phosphorylation, energy expenditure, and apoptosis.

Figure 3

Model depicting two hypothetical cell signaling pathways by which GLP-1 exerts insulinomimetic actions on insulin-sensitive target tissues. (a) In one mechanism, GLP-1(7-36)amide acts on its GPCR receptor, GLP-1R, to activate cAMP-dependent protein kinase A (PKA) and phosphoinositol 3 kinase (PI3K)-dependent Akt, the pro-survival kinase. Both PKA and Akt are known to target to mitochondria and modulate cytochrome and caspase-dependent apopotosis. In the absence of the expression of the GLP-1 receptor, it is hypothesized that some other G-protein-coupled receptor related to the GLP-1R, such as the glucagon receptor, may convey signaling in response to GLP-1 agonists. (b) In the absence of the GLP-1R, e.g. in the liver, GLP-1(9-36) binds to a novel translocator receptor, such as the pattern-reading scavenger receptor CD36/FAT, that transports the peptide into the cell where it is cleaved internally by selective endopeptidases, e.g. the neutral endopeptidase NEP 24.11, to liberate a small nonapeptide GLP-1(28-36)amide and a pentapeptide GLP-1(32-36)amide that target to mitochondria. The epsilon amino group on Lysine-34 may be fatty-acylated by the fatty-acylCoA ligase. These small peptides are proposed to contain consensus mitochondrial targeting sequences, LVKGRamde or LVKGRGR, and to have antioxidant actions by suppression of fatty acid oxidation and gluconeogenesis in the liver. Transport of these peptides into the matrix of the mitochondria may be facilitated by fatty acylation of the epsilon amino group on the lysine-34 by fatty-acylCoA ligase/synthase.