Pancreatic β-cells expressing the Arg64 variant of the β3-adrenergic receptor exhibit abnormal insulin secretory activity

Abstract

The Arg64 β3-adrenergic receptor (β3AR) variant is associated with an earlier age of onset of diabetes and lower levels of insulin secretion in humans. The aims of this study were to investigate whether β3AR is expressed by islet cells, if receptor binding affects insulin secretion and, finally, if the β3AR Arg64 variant induces abnormal insulin secretory activity. Human pancreas extracts were subjected to RT-PCR, Western blotting and immunostaining analyses. DNA sequencing and Western blotting demonstrated that the β3AR gene is transcribed and translated in the human pancreas; immunostaining showed that it is expressed by the islets of Langerhans. Cultured rat β-cells responded to human β3AR agonists in a dose- and time-dependent manner. Transfection of cultured rat β-cells with the wild-type human β3AR produced an increased baseline and ligand-dependent insulin secretion compared with parental cells. On the other hand, cells transfected with the Arg64 variant of the β3AR secreted less insulin, both spontaneously and after exposure to human β3AR agonists. Furthermore, while transfection with the wild-type β3AR preserved the glucose-dependent secretion of insulin, expression of the variant receptor rendered the host cells significantly less responsive to glucose. In summary, cells express the β3AR, and its activation contributes to the regulation of insulin secretion. These findings may help explain the low levels of insulin secretion in response to an i.v. glucose tolerance test observed in humans carrying the Arg64 polymorphism.