Adipocyte iron regulates leptin and food intake

Iron overload has been proposed to be a component of the insulin-resistance syndrome. On the other hand, dietary iron supplementation is associated with growth and increased appetite in children. We found the concentration of circulating ferritin is associated with low serum adiponectin, the insulin-sensitizing adipokine, and leptin, the adipokine that is responsible for regulating food intake. Similarly, mice fed a high iron diet and 3T3-L1 adipocytes treated with iron exhibited decreased adiponectin and leptin mRNA and protein. Iron negatively regulates transcription of adiponectin promoter-driven luciferase activity in a FOXO1-dependent manner. However, iron decreases the inactivated form of FOXO1, acetyl-FOXO1, while leaving phosphorylated FOXO1 and total FOXO1 unaffected. The mechanism of iron's effect on adiponectin and promotion of metabolic syndrome is demonstrated via the binding affinity of multiple transcription factors in the adiponectin promoter using ChIP studies. The higher activation of FOXO1 in iron-treated cells contributes to more inactivation of PPARγ through direct interaction, leading to decreased adiponectin transcription and expression. These findings directly demonstrate a causal role for iron as a risk factor for metabolic syndrome and a role for adipocytes in modulating metabolism through adiponectin in response to iron stores. We found iron negatively regulated leptin transcription via cAMP response element binding protein (CREB) activation. Two potential CREB-binding sites were identified in the mouse leptin promoter region. Mutation of both sites completely blocked the effect of iron on promoter activity. We also found enrichment of phospho-CREB iv binding to those two sites by ChIP in 3T3-L1 adipocytes treated with iron. CREB is modified and destabilized by O-GlcNAc modification. Iron also negatively regulates O-GlcNAc modification both in 3T3-L1 and epididymal fat, and mice heterozygous for deletion of the gene encoding the enzyme that removes O-GlcNAc, O-GlcNAcase, showed significantly increased serum leptin compared to wild-type mice. Glucosamine treatment rescued high iron-induced decrease of leptin promoter activity. These results suggest that the effects of iron on leptin may be via crosstalk of O-GlcNAcylation and phosphorylation of CREB. These findings indicate that levels of dietary iron play an important role in regulation of appetite and fat metabolism through CREB-dependent modulation of leptin expression.