342-LB: Iron Regulatory Protein 2 Regulates Proinsulin Production in Pancreatic Beta Cells through Iron-Dependent Cdkal1-Catalyzed tRNA Modification

Iron regulatory protein 2 (Irp2) is a key regulator of cellular iron metabolism in vertebrates. Irp2 controls the post-transcriptional expression of the iron-uptake protein transferrin receptor 1 (TfR1) and the iron-storage protein ferritin. Here we show that Irp2^-/-mice develop diabetes characterized by glucose intolerance and impaired glucose-stimulated insulin secretion from pancreatic β-cells. Impaired glucose-stimulated insulin secretion in Irp2^-/-β-cells is not due to insulin resistance but rather to a reduction in insulin stores caused by impaired proinsulin processing. Loss of Irp2 reduces TfR1 expression and iron content, and increases ferritin expression and iron sequestration causing cellular iron deficiency that impairs mitochondrial Fe-S cluster biosynthesis. This in turn reduces the function of Cdk5-regulated subunit-associated protein 1 like (Cdkal1), a 4Fe-4S cluster enzyme that catalyzes the 2-methythio (ms) modification of t⁶A37 of tRNALys(UUU)to ms²t⁶A37, which is critical for efficient translation of lysine codons in proinsulin. Irp2-deficient β cells show reduced ms²t⁶A37 levels and lysine incorporation into proinsulin. Increasing cellular iron content in Irp2-deficient β cells normalizes mitochondrial function, ms²t⁶A37 levels, proinsulin and insulin content, and insulin secretion. These study reveals a previously unidentified link between insulin processing and cellular iron homeostasis, and show that Irp2 is required for normal β cell function and whole body glucose homeostasis.