Glucocorticoid rhythm disruption drives hyperinsulinaemia in mice through beta cell glucocorticoid receptor signalling.

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 Ca 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 Ca 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.