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Inhibition of insulin signaling and adipogenesis by rapamycin: effect on phosphorylation of p70 S6 kinase vs eIF4E-BP1

International Journal of Obesity volume 28, pages 191–198 (2004)Cite this article

Abstract

OBJECTIVE: Insulin-responsive adipogenic signaling molecules include insulin receptor substrates (IRS)-1 and -2, phosphoinositide 3-kinase (PI3K), and protein kinase B (PKB; also known as Akt). Mammalian target of rapamycin (mTOR) is a PKB substrate, and regulates p70 S6 kinase (p70 S6K). Since p70 S6K is an insulin-responsive kinase downstream of PI3K and PKB, its potential role in adipogenic insulin signaling was investigated.

DESIGN: We measured the effect of rapamycin, a specific inhibitor of mTOR, on insulin-induced 3T3-L1 adipogenesis and on insulin-stimulated p70 S6K activation.

RESULTS: Rapamycin partially reduced differentiation, measured by Oil Red O staining, triacylglycerol accumulation (by up to 46%), and peroxisome proliferator-activated receptor γ protein expression (by 50%). In contrast, rapamycin completely inhibited insulin-stimulated p70 S6K activation, assessed by phosphorylation of p70 S6K and its substrate, S6. Expression of a constitutively activated form of p70 S6K did not promote 3T3-L1 adipogenesis. The considerable residual differentiation in the presence of rapamycin, despite the complete blockade of p70 S6K activation, prompted us to measure the phosphorylation of another rapamycin-sensitive protein, eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). Insulin-stimulated 4E-BP1 phosphorylation in 3T3-L1 preadipocytes was only partially affected by rapamycin, consistent with the differentiation data. Phosphorylation of eIF4E itself, an expected consequence of 4E-BP1 phosphorylation, was also only partially inhibited.

CONCLUSION: Our data suggest that adipogenic mTOR signaling occurs via the 4E-BP1/eIF4E pathway, rather than through p70 S6K.

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Acknowledgements

This work was supported by a grant from the Canadian Institutes of Health Research. AS is a Career Investigator of the HSFO. AG is supported by a Premier's Research Excellence Award held by AS. DE-C was supported by a John D Schultz Science Student Scholarship of the HSFO. We thank Thet Crapper for technical assistance.

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  1. Department of Medicine and Biochemistry, Microbiology & Immunology, University of Ottawa, Ottawa Health Research Institute, Ottawa, K1Y 4E9, Canada

    D El-Chaâr, A Gagnon & A Sorisky

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  1. D El-Chaâr
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  2. A Gagnon
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  3. A Sorisky
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Correspondence to A Sorisky.

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El-Chaâr, D., Gagnon, A. & Sorisky, A. Inhibition of insulin signaling and adipogenesis by rapamycin: effect on phosphorylation of p70 S6 kinase vs eIF4E-BP1. Int J Obes 28, 191–198 (2004). https://doi.org/10.1038/sj.ijo.0802554

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