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Leptin-Stimulated Endothelial Nitric-Oxide Synthase via an Adenosine 5'-Monophosphate-Activated Protein Kinase/Akt Signaling Pathway Is Attenuated by Interaction with C-Reactive Protein

Department of Experimental and Clinical Medicine (C.P., F.A., E.L., F.A., M.L.H., F.P., G.S.), University Magna Græcia of Catanzaro, 88100 Catanzaro, Italy; and Istituto di Endocrinologia ed Oncologia Sperimentale-Consiglio Nazionale delle Ricerche (F.B., A.C.), 80131 Naples, Italy

Address all correspondence and requests for reprints to: Giorgio Sesti, University Magna Græcia of Catanzaro, Department of Experimental and Clinical Medicine, Viale Europa, Germaneto, 88100 Catanzaro, Italy. E-mail: sesti{at}unicz.it.

The AMP-activated protein kinase (AMPK) lies upstream of Akt in the pathway leading to endothelial NO synthase (eNOS) activation. Whether leptin promotes eNOS activation via AMPK-dependent activation of Akt, and which of the two AMPK catalytic subunits is involved, remains unknown. Leptin resistance may be partly attributed to interaction between leptin and C-reactive protein (CRP). We hypothesized that leptin effect on eNOS activation in human aortic endothelial cells might be blunted by direct interaction with human recombinant CRP. Small interfering RNAs (siRNAs) were used to knock down expression of 1- or 2-AMPK in transient transfection assay to evaluate which is involved in this pathway and whether leptin effect on eNOS activation in human aortic endothelial cells might be blunted by direct interaction with human CRP. siRNA-mediated down-regulation of AMPK1, but not AMPK2, abolished leptin-induced Akt-Ser⁴⁷³ phosphorylation, eNOS-Ser¹¹⁷⁷ phosphorylation, eNOS activation, and cGMP accumulation. By contrast, siRNA-mediated knockdown of Akt1 did not affect AMPK1 phosphorylation, but it abolished leptin-induced phosphorylation of Akt-Ser⁴⁷³ and eNOS-Ser¹¹⁷⁷, suggesting that Akt functions downstream of AMPK1. Preincubation of leptin with human recombinant CRP impaired leptin-induced AMPK activation, eNOS-Ser¹¹⁷⁷ phosphorylation, eNOS activity, and intracellular cGMP accumulation. The data are consistent with a model implicating an AMPK1AkteNOS pathway leading to NO production in response to leptin supporting the idea that interaction between leptin and CRP may have a role in impairing leptin effect on eNOS activation, suggesting a link between leptin resistance, low-grade inflammation, and endothelial dysfunction.