Abstract
Toll-like receptor 4 (TLR4) has a key role in innate immunity by activating an inflammatory signaling pathway. Free fatty acids (FFAs) stimulate adipose tissue inflammation through the TLR4 pathway, resulting in insulin resistance1,2,3,4,5,6,7. However, current evidence suggests that FFAs do not directly bind to TLR48,9, but an endogenous ligand for TLR4 remains to be identified. Here we show that fetuin-A (FetA) could be this endogenous ligand and that it has a crucial role in regulating insulin sensitivity via Tlr4 signaling in mice. FetA (officially known as Ahsg) knockdown in mice with insulin resistance caused by a high-fat diet (HFD) resulted in downregulation of Tlr4-mediated inflammatory signaling in adipose tissue, whereas selective administration of FetA induced inflammatory signaling and insulin resistance. FFA-induced proinflammatory cytokine expression in adipocytes occurred only in the presence of both FetA and Tlr4; removing either of them prevented FFA-induced insulin resistance. We further found that FetA, through its terminal galactoside moiety, directly binds the residues of Leu100–Gly123 and Thr493–Thr516 in Tlr4. FFAs did not produce insulin resistance in adipocytes with mutated Tlr4 or galactoside-cleaved FetA. Taken together, our results suggest that FetA fulfills the requirement of an endogenous ligand for TLR4 through which lipids induce insulin resistance. This may position FetA as a new therapeutic target for managing insulin resistance and type 2 diabetes.
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Change history
11 January 2013
In the version of this article initially published, the groups presented in each key of the graphs for the insulin tolerance test results in Figure 1c,i were incorrectly assigned to the lines of the graphs. Also, the y-axis labels for the graphs were incorrect and misleading. The values along the graph lines for each group of animals presented represent the percentage of the initial value measured at time 0 for each group, not the percentage change. The errors have been corrected in the HTML and PDF versions of the article.
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Acknowledgements
This research was financially supported by a grant from the CSIR and Department of Science and Technology, Ministry of Science and Technology, New Delhi. We thank the National Centre for Cell Science, Pune, India, for providing the 3T3L1 cell line; A. Bandyopadhyay and S. Roy of the Indian Institute of Chemical Biology, Kolkata for their help in the confocal and SPR work, respectively. D.P. is thankful to CSIR, New Delhi, for the award of Senior Research Fellowship. S.D. thanks the University Grants Commission (UGC), New Delhi, for the award of a UGC-Dr. D.S. Kothari postdoctoral fellowship and CSIR-NEIST for a Quick Hire Fellowship. S.B. thanks the Indian National Science Academy for his Senior Scientist position. G.D. gratefully acknowledges the gift of Tlr4−/− mice from R. Medzhitov, Yale School of Medicine. The authors appreciate the use of facilities as extended to us by the head of the Department of Zoology, Visva-Bharati University, Santiniketan; the director of IICB, director of IPGME&R-SSKM Hospital, Kolkata; the director of the National Institute of Immunology, director of ICGEB, New Delhi and the director of NEIST, Jorhat, India.
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D.P. and S.D. designed and performed all the experiments, analyzed the data and wrote the manuscript; R.K. generated VMO–based FetA and Tlr4 knockdown mice; G.D. performed lipid infusion study in WT and FetAKD mice; S.R. provided nondiabetic and diabetic human blood and fat tissue; S.S.M. provided blood and tissue samples from WT and db/db mice; S. Maitra, G.D., S. Mukhopadhyay and S.S.M. wrote the manuscript; S.B. designed and supervised this study, analyzed the data and wrote the manuscript.
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Pal, D., Dasgupta, S., Kundu, R. et al. Fetuin-A acts as an endogenous ligand of TLR4 to promote lipid-induced insulin resistance. Nat Med 18, 1279–1285 (2012). https://doi.org/10.1038/nm.2851
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DOI: https://doi.org/10.1038/nm.2851
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