Extracellular nicotinamide phosphoribosyltransferase (NAMPT/visfatin) inhibits insulin-like growth factor-1 signaling and proteoglycan synthesis in human articular chondrocytes

Human articular chondrocytes isolated from normal ankle cartilage were pretreated with eNAMPT (0.1 to 5.0 μg/ml) overnight followed by stimulation with IGF-1 (50 ng/ml) for 24 hours, and proteoglycan synthesis was measured by [35S]sulfate incorporation. Chondrocytes were pretreated with eNAMPT overnight followed by IGF-1 for 10 minutes, and the cell lysates were immunoblotted for various signaling proteins that are activated by IGF-1 using phosphospecific antibodies. In addition, chondrocytes were pretreated with mitogen-activated protein kinase kinase inhibitor (U0126) prior to stimulation with eNAMPT and IGF-1.Pretreatment of chondrocytes with eNAMPT inhibited IGF-1-stimulated proteoglycan synthesis in a dose-dependent manner. Treatment of chondrocytes with eNAMPT inhibited IGF-1-induced phosphorylation of signaling molecules, including insulin receptor substrate-1 and AKT. Interestingly, pretreatment of chondrocytes with eNAMPT did not inhibit IGF-1-mediated phosphorylation of the IGF-1 receptor; however, it stimulated a sustained phosphorylation of the extracellular signal-regulated kinase (ERK)/mitogen activated protein kinase (MAPK) signaling pathway. Inhibition of the ERK/MAPK signaling pathway restored IGF-1-mediated insulin receptor substrate-1 and AKT phosphorylation.Our study demonstrates that eNAMPT/visfatin inhibits IGF-1 function in articular chondrocytes by activating the ERK/MAPK pathway independent of the IGF-1 receptor. Since eNAMPT levels are elevated in the synovial fluid of OA patients, the signaling pathway activated by eNAMPT could contribute to IGF-1 resistance in OA.Obesity is a major risk factor for the development of osteoarthritis (OA) [1,2]. Emerging data have shown that metabolic factors associated with obesity, including adipokines, play an important role in the progression of OA, prompting some to classify OA as a metabolic disease. Several adipokines, including leptin, resistin, and adiponectin, have been found in synovial fluid from