G Protein‐Coupled Receptor Kinase 6 (GRK6) Regulation of Insulin Processing and Secretion

We performed exome sequencing in a Type 2 Diabetes (T2D) patient belonging to a T2D family containing two affected and five unaffected individuals revealing an autosomal dominant inheritance pattern. A mutation in the GRK6 gene, coding for G protein‐coupled receptor kinase 6, was identified that segregates with the phenotype. GRK6 belongs to a family of serine/threonine kinases that phosphorylate agonist occupied G protein‐coupled receptors (GPCRs) effectively modulating downstream signaling pathways including those involved in metabolism. To understand the potential role of GRK6 in T2D, we utilized pharmacological inhibition and genetic deletion of GRK6 by both shRNA and CRISPR in the mouse pancreatic beta‐cell line, MIN6. We then assayed the ability of these cells to process and secrete insulin in response to glucose. Small molecule inhibition of GRK5/GRK6 caused an increase in insulin secretion whereas insulin processing was reduced. Insulin processing was also diminished in GRK6 knockdown cells. Furthermore, GRK6 knockdown attenuated insulin secretion but enhanced proinsulin secretion consistent with a processing defect and the aforementioned patient phenotype. The insulin processing defect in GRK6 knockdown cells was also seen in CRISPR mediated GRK6 knockout cells. The altered insulin profile in MIN6 cells lacking GRK6 function appears to be caused by changes in the expression and activity of the proprotein convertases, the enzymes responsible for proinsulin to insulin conversion, as pharmacological inhibition, knockdown, and knockout of GRK6 all resulted in less expression and activity of these enzymes. To support this, we performed rescue experiments where GRK6 re‐expression in knockdown cells partially recovered insulin and proprotein convertase levels in cells. Taken together, our data show that GRK6 regulates insulin processing and provides novel insight into how GRK6 might contribute to T2D.