The beta cell glucocorticoid receptor protects against hyperglycaemia by modulating insulin secretion during glucocorticoid rhythm disruption in mice
Aims/hypothesis: Hyperinsulinaemia is typically viewed as a secondary, compensatory response to insulin resistance or elevated glycaemia. However, we previously found that disrupting the daily glucocorticoid rhythm in mice rapidly increases circulating insulin several-fold while fasting glucose remains normal. This raised the question of what generates and sustains the hyperinsulinaemia. Because glucocorticoids act directly on beta cells through the glucocorticoid receptor (GR), we tested whether beta cell GR is required for this rise in insulin and whether elevated insulin is necessary to maintain glucose homeostasis. Methods: Glucocorticoid rhythms were disrupted in male C57BL/6J mice by subcutaneously implanting corticosterone pellets that raise the trough and lower the peak while maintaining near-physiological mean glucocorticoid exposure, a manipulation we refer to as GC-flattening. Placebo-treated mice served as controls, and high-fat-diet-fed mice provided a metabolic comparison. Beta cell function was assessed by dynamic glucose stimulated insulin secretion and beta cell specific Ca2+ imaging. The requirement for beta cell GR was tested using adult-inducible beta cell specific GR knockout mice, thereby limiting developmental effects of constitutive GR deletion. Combined beta cell and hepatocyte GR knockout mice were used to test the consequences of further reducing systemic insulin availability. Insulin sensitivity and glucose tolerance were assessed in vivo. Insulin clearance was assessed from plasma C-peptide:insulin ratios, direct measurement of the disappearance of intravenously administered human insulin, and hepatic insulin-degrading enzyme abundance and activity. Results: GC-flattening produced rapid, sustained hyperinsulinaemia while blood glucose remained normal, distinct from the more gradual hyperinsulinaemia and hyperglycaemia observed in high-fat-diet-fed mice. Islets from GC-flattened mice retained enhanced insulin secretion and Ca2+ responses to glucose after isolation, indicating a persistent increase in beta cell glucose responsiveness. During GC-flattening, beta cell GR deletion reduced cumulative circulating insulin exposure by approximately 40% (p < 0.001) and worsened glycaemic control despite similar or greater insulin sensitivity, demonstrating that the GR-dependent rise in insulin helps maintain glucose homeostasis. Direct measurement on Day 3 confirmed reduced insulin clearance in GC-flattened mice. This was accompanied by a reduced plasma C-peptide-to-insulin ratio and decreased hepatic insulin-degrading enzyme abundance and activity. Further lowering circulating insulin by combined beta cell and hepatocyte GR deletion worsened glycaemic control further. Conclusions/interpretation: Disruption of glucocorticoid rhythmicity directly initiates hyperinsulinaemia by enhancing glucose-stimulated insulin secretion through beta cell GR signalling and by reducing insulin clearance. The resulting increase in insulin is required to maintain glucose homeostasis, demonstrating that hyperinsulinaemia can be an early adaptive response to altered endocrine timing rather than simply a consequence of insulin resistance or hyperglycaemia.