Endocytosis as a regulator of Glucagon Receptor Signaling

By: Jan Mikhale Cajulao and Eduardo Hernandez

Department: Cellular & Molecular Biology

Faculty Advisors: Dr. Erica L. Sanchez and Dr. Mark von Zastrow (UCSF)

Eukaryotic cells express receptors on the cell surface used for detecting environmental stimuli. G Protein-Coupled Receptors are the largest class of surface receptors on eukaryotic cells, and upon stimulation at the surface, internalize (via endocytosis) and traffic to various intracellular compartments to promote signaling. It is now thought that intracellular G Protein-Coupled Receptors are active, as measured by generation of the second messenger cAMP, and transcription of relevant genes. The Glucagon Receptor (GCGR) is a G Protein-Coupled Receptor that mediates the liver’s response to low blood sugar (hypoglycemia) and is stimulated by the peptide hormone Glucagon to activate glucose synthesis (gluconeogenesis). Here, we show that GCGR undergoes dynamin-dependent endocytosis in HEK293 and Huh-7-Lunet cells upon glucagon activation within 5 minutes, and localizes to EEA1-marked endosomes, shown previously to be sites of production of the second messenger cAMP. We further show that endocytosis potentiates cytoplasmic cAMP elevation produced by GCGR, and expression of GCGR’s target genes, including the gluconeogenic enzyme PCK1. Together, these results implicate the endosomal signaling paradigm in metabolic regulation by glucagon.