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

• Published in: International Journal of Obesity 2004-02, Vol.28 (2), p.191-198
• Main Authors: El-Chaâr, D, Gagnon, A, Sorisky, A
• Format: Article
• Language: English
• Subjects: Adipocytes; Biological and medical sciences; Epidemiology; General and cellular metabolism. Vitamins; Health Promotion and Disease Prevention; Insulin; Internal Medicine; Kinases; Medical research; Medical sciences; Medicine; Medicine & Public Health; Metabolic Diseases; Obesity; Ophthalmology; Penicillin; Pharmacology. Drug treatments; Phosphorylation; Proteins; Public Health; Retinopathies
• ISSN: 0307-0565; 1476-5497
• DOI: 10.1038/sj.ijo.0802554

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.