The endothelial mTORC2-Foxo1 axis serves as an iron-responsive sensor governing systemic iron homeostasis

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Abstract
Liver sinusoidal endothelial cells (LSECs) are essential for maintaining liver function by actively sensing nutrients and producing angiocrine factors. LSECs also regulate systemic iron metabolism by secreting bone morphogenetic proteins (BMPs), which are key modulators of systemic iron homeostasis. However, the mechanism by which LSECs sense iron to regulate iron metabolism remains unclear. Here, we identify that the endothelial transcriptional factor forkhead box protein O1 (Foxo1) and its upstream protein kinase, mechanistic target of rapamycin complex 2 (mTORC2), as critical iron sensors. In response to iron, Foxo1 undergoes acute and dynamic nuclear translocation to activate the transcription of Bmp2 and Bmp6, thereby stimulating the synthesis of iron-regulatory hormone hepcidin in adjacent hepatocytes. Foxo1 directly binds evolutionally conserved Foxo binding sites within the Bmp2 and Bmp6 promoters to mediate this response. Mechanistically, iron triggers the lysosomal degradation of the mTORC2-specific component rapamycin-insensitive companion of mTOR (Rictor), enhancing Foxo1 activation. Endothelial-specific Foxo1 deletion reduces the expressions of hepatic Bmp2/6 and hepcidin, leading to systemic iron overload, whereas endothelial Rictor deletion increases the expressions of hepatic Bmp2/6 and hepcidin, producing an iron-deficient phenotype. Moreover, endothelial-targeted lipid nanoparticles expressing endothelial-specific and constitutively active Foxo1 alleviate iron overload in a murine model of hereditary hemochromatosis. Collectively, our study establishes the endothelial mTORC2-Foxo1 axis as an iron-responsive regulator of Bmp2 and Bmp6 expression and identifies it as a promising target for iron-related disorders.

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