β-Catenin mediates glucose-dependent insulinotropic polypeptide increases in lysyl oxidase expression in osteoblasts

Abstract

Osteoblast lysyl oxidase (LOX) is a strongly up-regulated mRNA and protein by the glucose-dependent insulinotropic polypeptide (GIP). LOX is critically required for collagen maturation, and was shown to be dramatically down-regulated in a mouse model of type 1 diabetes, consistent with known low collagen cross-linking and poor bone quality in diabetic bone disease in humans and in mouse models. GIP is a gastric hormone released by the gut upon consumption of nutrients, which then stimulates insulin release from β-cells in the pancreas. GIP is directly anabolic to osteoblasts and to bone, while gut-derived dopamine attenuates effects of GIP on osteoblast anabolic pathways, including LOX expression. GIP-stimulation of LOX expression was shown to be dependent on increased cAMP levels and protein kinase A activity, consistent with the fact that GIP receptors are G protein coupled receptors. Downstream signaling events resulting in increased LOX expression remain, however, unexplored. Here we provide evidence for β-catenin mediation of signaling from GIP to increase LOX expression. Moreover, we have identified a TCF/LEF element in the Lox promoter that is required for GIP-upregulation of LOX. These findings will be of importance in designing potential therapeutic approaches to address deficient LOX production in diabetic bone disease by pointing to the importance of exploring strategies to stimulate β-catenin signaling in osteoblasts under diabetic conditions as potential therapeutic strategies.