The Recombinant Rat Glucagon-Like Peptide-1 Receptor, Expressed in an α-Cell Line, Is Coupled to Adenylyl Cyclase Activation and Intracellular Calcium Release

 

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Abstract

The glucagon-like peptide-1 (GLP-1) receptor is expressed on α-cells, though its functional significance is unknown. The endogenous β-cell GLP-1 receptor is coupled to adenylyl cyclase, cell depolarization, activation of voltage-dependent Ca²⁺ channels (VDCC) and extracellular Ca²⁺ influx ([Lu et al. 1993 b]). In contrast, the signaling pathways of the GLP-1 receptor in α-cells are poorly understood. To determine the signaling mechanisms of the α-cell GLP-1 receptor, we established a stable pancreatic islet α-cell line expressing the recombinant rat GLP-1 receptor (INR1-SF2), using INRl-G9 cells. These INRl-G9 cells do not express endogenous GLP-1 receptor. In INR1-SF2 cells, GLP-1 bound to the recombinant receptor (K_d = 0.9 nM) and increased cAMP (ED₅₀ = 0.6 nM). GLP-1 increased the free cytosolic Ca²⁺ ([Ca²⁺]_i) (ED₅₀ = 50 nM) by release from intracellular stores, but did not affect INR1-SF2 cell phosphoinositol turnover. Despite expressing VDCC, the INR1-SF2 cells were not depolarized by GLP-1, even in the presence of glucose. This contrasts with the depolarizing action of GLP-1 in β-cells in the presence of glucose ([Lu et al., 1993 b]).

This study establishes that a single GLP-1 receptor species can mediate the effects of GLP-1 through multiple signaling pathways, including the adenylyl cyclase system and intracellular Ca²⁺ release, in an α-cell type. Furthermore, since GLP-1 is unable to cause cellular depolarization or activate VDCC in INR1-SF2 cells, these data suggest that glucose-induced membrane depolarization may be crucial for GLP-1 to further activate VDCC and potentiate glucose-stimulated insulin release in β-cells. Finally this study describes a cell line that can be used as a model system for evaluation of GLP-1 signaling in α-cells.

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Joseph Dillon

Division of Endocrinology
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University of Iowa

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Phone: + 3193380581 ext. 7640

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Email: joseph-dillon@uiowa.edu