Glucagon receptor signaling — glucagon acting through GCGR, Gs, adenylyl cyclase 5/6 and cAMP/PKA, with GRK/beta-arrestin desensitization

Glucagon is the counter-regulatory hormone to insulin: released from pancreatic alpha cells when blood glucose falls, it acts chiefly on liver and kidney through the glucagon receptor (GCGR), a class B G protein-coupled receptor. Agonist-occupied GCGR acts as a guanine nucleotide exchange factor for the stimulatory G protein; GTP-bound G alpha s (GNAS) activates the membrane adenylyl cyclases ADCY5 and ADCY6, raising cAMP and activating protein kinase A (PKA). PKA then drives the hepatic glucose-output programme — activation of glycogenolysis, inhibition of glycogen synthesis and glycolysis, and promotion of gluconeogenesis — which lies downstream of this module. Glucagon itself is one of several peptides cut from the preproglucagon precursor encoded by GCG; the convertase that makes glucagon in alpha cells is part of hormone processing rather than of receptor signalling. As for the other Gs-coupled hormone receptors, signalling is limited by G protein-coupled receptor kinase phosphorylation of the occupied receptor and subsequent beta-arrestin binding.

Derived from the RGD Pathway Ontology diagram PW:0000676 through the RGD_PATHWAY_ONTOLOGY project, as one of four mutually consistent Gs-coupled hormone receptor modules (crh_receptor_signaling, beta_adrenergic_signaling, pth_receptor_signaling, glucagon_receptor_signaling) that share one structure: ligand -> receptor -> Gs -> adenylyl cyclase/cAMP -> PKA -> termination by GRK phosphorylation and beta-arrestin binding. Molecular functions were taken from each protein's experimental GO annotations in QuickGO and from the completed GCG gene review, and verified against go-basic (releases/2026-07-26): GCGR GO:0004967 IDA; GNAS P63092 GO:0003925 IDA; ADCY5 GO:0004016 EXP/IDA; ADCY6 GO:0004016 EXP/IDA; GRK2 GO:0004703 IDA; ARRB1/ARRB2 GO:0030674 IDA. Ligand. The ligand annoton is the glucagon peptide, the GCG cleavage product UniProtKB:P01275-PRO_0000011256, with hormone activity only and no process role, as in the human GO-CAM gomodel:68d5ebd600000298 and following the substrate-versus-participant rule. The completed GCG gene review (genes/human/GCG, P01275) carries GO:0005179 hormone activity as a core function, so the claim is consistent with it; the review is keyed to the parent accession, which is why the module validator reports one CONTEXT_REQUIRED warning for this annoton: the chain-level assertion cannot be matched automatically to the gene-level review. The warning is expected and the claim is the review's own core function. Processing of preproglucagon by PCSK2 in alpha cells, and the alternative PCSK1 products (GLP-1, GLP-2, oxyntomodulin) with their own receptors, are hormone production and separate pathways, outside the module. Cyclase modelled locally. Unlike the three sibling modules, RGD names the cyclase isoforms here (Adcy5 and Adcy6), so the cyclase step is modelled as a ONE_OR_MORE variant set of those two human enzymes rather than taken by reference from modules/gpcr_camp_pka_signaling.yaml; the PKA step is still a module_ref to that module. The mouse GO-CAM for this process uses Adcy8, a third isoform that RGD does not draw and that is not modelled here. Outputs out of scope. Hepatic glycogenolysis, gluconeogenesis and the inhibition of glycolysis and glycogen synthesis are RGD subpathways (PW:0000534, PW:0000641, PW:0000640, PW:0000532) reached through PKA; they are downstream outputs, not parts of receptor signalling, and are kept out of the module. Gq coupling, which the RGD text raises as possible and for which the cloned receptor's calcium response (PMID:8384375) is the evidence, is not asserted. Termination. RGD's glucagon diagram draws no GRK or arrestin, but the brief for this batch models termination in all four modules. The step here is therefore marked optional and grounded only on the generic mechanism (GRK2 phosphorylation of the occupied receptor and beta-arrestin binding, PMID:2163110, established on beta-adrenergic receptors); no publication or cached GO-CAM tying GRK2 or an arrestin to GCGR was found, which is recorded as a knowledge gap rather than asserted. GCGR is not among the receptors in the GRK/arrestin desensitization model gomodel:67f85f2b00004426 either, so that model is linked for gene and function only. GO-CAM grounding. Glucagon, GCGR and GNAS are linked to the human Gs model gomodel:68d5ebd600000298, and the receptor, G protein and ligand steps also to the mouse "Cellular response to glucagon stimulus" model gomodel:636d9ce800000147 where the orthologous gene carries the same molecular function (Gcgr GO:0004967, Gnas GO:0003925, Gcg GO:0048018 receptor ligand activity, the parent of which the human model types as GO:0005179 hormone activity). The second candidate model, gomodel:65288b2200000062, is the GPR174 signaling pathway: it shares only the generic GNAS and ADCY6 activities and no glucagon-specific gene, so it is not linked. A second human model, gomodel:68d5ebd600000330, gives GCGR a Gi-coupled arm; that arm is not modelled here for the same reason as the Gq route.