UMMS Affiliation

Program in Molecular Medicine

Date

5-2012

Document Type

Article

Medical Subject Headings

3T3 Cells; Adipocytes; Animals; Cell Communication; Coculture Techniques; Cytokines; Humans; Insulin; Insulin Resistance; JNK Mitogen-Activated Protein Kinases; Macrophage Activation; Macrophages; Membrane Proteins; Mice; Retinol-Binding Proteins, Plasma; Signal Transduction; Toll-Like Receptor 4; Vitamin A

Disciplines

Biochemistry | Cell Biology | Cellular and Molecular Physiology | Molecular Biology

Abstract

Retinol-binding protein 4 (RBP4), the sole retinol transporter in blood, is secreted from adipocytes and liver. Serum RBP4 levels correlate highly with insulin resistance, other metabolic syndrome factors, and cardiovascular disease. Elevated serum RBP4 causes insulin resistance, but the molecular mechanisms are unknown. Here we show that RBP4 induces expression of proinflammatory cytokines in mouse and human macrophages and thereby indirectly inhibits insulin signaling in cocultured adipocytes. This occurs through activation of c-Jun N-terminal protein kinase (JNK) and Toll-like receptor 4 (TLR4) pathways independent of the RBP4 receptor, STRA6. RBP4 effects are markedly attenuated in JNK1-/- JNK2-/- macrophages and TLR4-/- macrophages. Because RBP4 is a retinol-binding protein, we investigated whether these effects are retinol dependent. Unexpectedly, retinol-free RBP4 (apo-RBP4) is as potent as retinol-bound RBP4 (holo-RBP4) in inducing proinflammatory cytokines in macrophages. Apo-RBP4 is likely to be physiologically significant since RBP4/retinol ratios are increased in serum of lean and obese insulin-resistant humans compared to ratios in insulin-sensitive humans, indicating that higher apo-RBP4 is associated with insulin resistance independent of obesity. Thus, RBP4 may cause insulin resistance by contributing to the development of an inflammatory state in adipose tissue through activation of proinflammatory cytokines in macrophages. This process reveals a novel JNK- and TLR4-dependent and retinol- and STRA6-independent mechanism of action for RBP4.

Rights and Permissions

Citation: Mol Cell Biol. 2012 May;32(10):2010-9. doi: 10.1128/MCB.06193-11. Epub 2012 Mar 19. Link to article on publisher's site

Comments

Copyright © American Society for Microbiology. Publisher PDF posted as allowed by publisher's author rights policy at http://journals.asm.org/site/misc/ASM_Author_Statement.xhtml.

Related Resources

Link to Article in PubMed

PubMed ID

22431523

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