Adipocyte iron regulates adiponectin and insulin sensitivity
J. Scott Gabrielsen, … , William T. Cefalu, Donald A. McClain
J. Scott Gabrielsen, … , William T. Cefalu, Donald A. McClain
Published September 10, 2012
Citation Information: J Clin Invest. 2012;122(10):3529-3540. https://doi.org/10.1172/JCI44421.
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Research Article Metabolism

Adipocyte iron regulates adiponectin and insulin sensitivity

Abstract

Iron overload is associated with increased diabetes risk. We therefore investigated the effect of iron on adiponectin, an insulin-sensitizing adipokine that is decreased in diabetic patients. In humans, normal-range serum ferritin levels were inversely associated with adiponectin, independent of inflammation. Ferritin was increased and adiponectin was decreased in type 2 diabetic and in obese diabetic subjects compared with those in equally obese individuals without metabolic syndrome. Mice fed a high-iron diet and cultured adipocytes treated with iron exhibited decreased adiponectin mRNA and protein. We found that iron negatively regulated adiponectin transcription via FOXO1-mediated repression. Further, loss of the adipocyte iron export channel, ferroportin, in mice resulted in adipocyte iron loading, decreased adiponectin, and insulin resistance. Conversely, organismal iron overload and increased adipocyte ferroportin expression because of hemochromatosis are associated with decreased adipocyte iron, increased adiponectin, improved glucose tolerance, and increased insulin sensitivity. Phlebotomy of humans with impaired glucose tolerance and ferritin values in the highest quartile of normal increased adiponectin and improved glucose tolerance. These findings 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.

Authors

J. Scott Gabrielsen, Yan Gao, Judith A. Simcox, Jingyu Huang, David Thorup, Deborah Jones, Robert C. Cooksey, David Gabrielsen, Ted D. Adams, Steven C. Hunt, Paul N. Hopkins, William T. Cefalu, Donald A. McClain

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Figure 3

Transcriptional regulation of adiponectin by iron.

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Transcriptional regulation of adiponectin by iron. (A) Media adiponectin...
(A) Media adiponectin levels in 3T3-L1 cells 12 hours following 12-hour pretreatment. P < 0.0001. (B) RT-PCR quantification of adiponectin mRNA levels in 3T3-L1 adipocytes treated with no iron or 100 μM FeSO4 for 24 hours, normalized to cyclophilin A. *P = 0.02. (C) Adiponectin promoter-driven luciferase activity in the presence or absence of 100 μM FeSO4. ‡P = 0.0025. (D) Western blot for acetylated FOXO1 (Ac-FOXO1), phosphorylated FOXO1 (P-FOXO1), total FOXO1 (Tot-FOXO1), and β-actin in 3T3-L1 adipocytes treated with no iron or 100 μM FeSO4 for 8 hours. (E) Quantitation of Western blots (total n = 6 independent determinations) normalized to β-actin. *P < 0.05. (F) Quantitation of Western blots for phosphorylated AKT in 3T3-L1 adipocytes treated with no iron or 100 μM FeSO4 for 8 hours and insulin (10 nM) for 1 hour. (G) ChIP showing FOXO1 occupancy of adiponectin promoter FOXO1 sites and PPRE and PPARγ occupancy of PPRE in 3T3-L1 adipocytes (n = 3 experiments each assayed in duplicate, *P < 0.05). (H) Immunoprecipitation of 3T3-L1 adipocyte extracts, treated overnight in the presence or absence of 100 μM FeSO4, by antibodies to FOXO1, followed by immunoblotting for FOXO1 (t-FOXO1) and C/EBPα (0.58 ± 0.15 density units for control, 0.61 ± 0. 26 density units for iron-treated extracts, P = 0.93).