BACKGROUND: Glucagon-like peptide-2 (GLP-2) has been demonstrated to stimulate bone formation and increase bone mass. Conversely, aberrant expression of fibroblast growth factor 23 (FGF23), a crucial bone-derived hormone that regulates phosphate metabolism and mineralization, is implicated in the pathogenesis of osteoporosis. This study aimed to elucidate the mechanisms by which GLP-2 ameliorates postmenopausal osteoporosis, focusing on whether it exerts bone-protective effects through downregulation of FGF23 expression via the PI3K/AKT/FOXO1 signalling pathway.
METHODS: An ovariectomized (OVX) mouse model was established to mimic postmenopausal osteoporosis. Mice were treated with GLP-2, the PI3K inhibitor LY294002, or a combination of both. The bone mineral density (BMD) of the femur and lumbar spine, trabecular microarchitecture (BV/TV, Tb.N, Tb.Sp, Tb.Pf), and expression of Runx2 and FGF23 were assessed. RNA sequencing and KEGG pathway enrichment analyses revealed key signalling pathways. In vitro, MC3T3-E1 osteoblasts were subjected to GLP-2R overexpression or knockdown, as well as PI3K/AKT and FOXO1 modulation, to explore the underlying mechanisms.
RESULTS: Compared with the sham, control mice, the OVX mice exhibited significantly lower femoral and lumbar BMD (P < 0.01), lower BV/TV and Tb.N, and greater Tb.Sp and Tb.Pf (P < 0.01). Runx2 expression was downregulated, whereas FGF23 levels in bone and serum were markedly elevated (P < 0.01). GLP-2 treatment significantly increased the BMD (femur + 18%, lumbar + 22%), increased the BV/TV and Tb.N, reduced Tb.Sp and Tb.Pf (all P < 0.05), upregulated Runx2 expression, and downregulated FGF23 expression. RNA-seq revealed enrichment of the PI3K/AKT pathway in the differentially expressed genes. LY294002 partially reversed the effects of GLP-2, lowering the BV/TV by 12% and increasing the level of FGF23 by 30% (P < 0.05). In MC3T3-E1 cells, GLP-2 increased p-AKT and p-FOXO1 levels (P < 0.01) and decreased FGF23 mRNA and protein levels (40% reduction, P < 0.01); these effects were abolished by PI3K/AKT inhibition. Inhibition of FOXO1 can reduce FGF23 expression, whereas the overexpression of FOXO1 leads to an increase in FGF23 expression, suggesting that the transcription of the FGF23 gene requires the participation of FOXO1.
CONCLUSIONS: GLP-2 ameliorates postmenopausal osteoporosis by activating the PI3K/AKT/FOXO1 pathway in osteoblasts, leading to FGF23 downregulation. These findings provide novel molecular insights and potential therapeutic targets for the use of GLP-2 in osteoporosis management.
