Deletion of RalGAPβ protects pancreatic β cells and improves glycemic control

Pancreatic β cell homeostasis involving their proliferation, growth, and apoptosis is critical for postprandial insulin levels and glycaemic control. The regulatory mechanisms governing β cell homeostasis and insulin secretion are incompletely understood. This study aims to elucidate the role of Ral-GTPase activating protein (RalGAP) complexes in β cell homeostasis and insulin secretion.

Pancreatic β cell-specific RalGAPβ knockout mice and RalGAPβ-knockdown INS-1 cells were constructed for studying the pancreatic function of RalGAP complexes. Metabolic phenotyping, immunoblotting, RT-QPCR, glucose-stimulated insulin secretion (GSIS), proliferation and apoptosis assays, RNA-sequencing and metabolomic profiling were performed.

High-fat diet increased RalGAP complex and concomitantly decreased GTP-bound active RalA in pancreatic islets. Deletion of RalGAPβ, a common regulatory subunit of RalGAP complexes, destabilised RalGAPα1/α2 and elevated GTP-RalA in β cells. RalGAPβ deficiency promoted proliferation of β cells, increased their sizes, and inhibited streptozotocin-induced β cell apoptosis. Moreover, β cell-specific deletion of RalGAPβ enhanced KCl-stimulated insulin secretion in isolated islets as well as GSIS in vivo. Consequently, RalGAPβ deletion in β cells improved glucose tolerance in mice fed a chow or high-fat diet. At the molecular level, RalGAPβ deletion may reset the glucostat consisting of GLUT1 and GLUT2 to modulate glycemic set point. Furthermore, RalGAPβ deficiency elevated p70 S6-kinase in islets, whose inhibition abrogated RalGAPβ deficiency-induced β cell proliferation.

RalGAP complexes function as a critical regulator of both β cell homeostasis and insulin secretion. Our results have implications for the discovery of drugs targeting RalGAP complexes to protect β cells in type 2 diabetes.