PDK1-FoxO1 pathway in AgRP neurons of arcuate nucleus promotes bone formation via GHRH-GH-IGF1 axis.

OBJECTIVE: In the hypothalamic arcuate nucleus (ARC), orexigenic agouti-related peptide (AgRP) neurons regulate feeding behavior and energy homeostasis, functions connected to bone metabolism. The 3-phosphoinositide-dependent protein kinase-1 (PDK1) serves as a major signaling molecule particularly for leptin and insulin in AgRP neurons. We asked whether PDK1 in AGRP neurons also contributes to bone metabolism.

METHODS: We generated AgRP neuron-specific PDK1 knockout ( Agrp Pdk1 -/- ) mice and those with additional AgRP neuron-specific expression of transactivation-defective FoxO1 ( Agrp Pdk1 -/- Δ256Foxo1 ). Bone metabolism in KO and WT mice was analyzed by quantitative computed tomography (QCT), bone histomorphometry, measurement of plasma biomarkers, and qPCR analysis of peptides.

RESULTS: In Agrp Pdk1 -/- female mice aged 6 weeks, compared with Agrp Cre mice, both stature and femur length were shorter while body weight was unchanged. Cortical bone mineral density (BMD) and cancellous BMD in the femur decreased, and bone formation was delayed. Furthermore, plasma GH and IGF-1 levels were reduced in parallel with decreased mRNA expressions for GH in pituitary and GHRH in ARC. Osteoblast activity was suppressed and osteoclast activity was enhanced. These changes in stature, BMD and GH level were rescued in Agrp Pdk1 -/- Δ256Foxo1 mice, suggesting that the bone abnormalities and impaired GH release were mediated by enhanced Foxo1 due to deletion of PDK1.

CONCLUSIONS: This study reveals a novel role of PDK1-Foxo1 pathway of AgRP neurons in controlling bone metabolism primarily via GHRH-GH-IGF-1 axis.