Extracellular nicotinamide phosphoribosyltransferase inhibits insulin-like growth factor-1-stimulated proteoglycan synthesis. (A) Normal human chondrocytes were treated with (0 to 5 μg/ml) extracellular nicotinamide phosphoribosyltransferase (eNAMPT)/visfatin overnight, followed by 24-hour stimulation with 50 ng/ml insulin-like growth factor-1 (IGF-1). (B) Cells were stimulated with 5 μg/ml eNAMPT or 50 ng/ml IGF-1 overnight. Proteoglycan (PG) synthesis was measured using [35S]sulfate incorporation and DNA was analyzed with a PicoGreen assay. Results are expressed as the fold change versus unstimulated control. Data presented as mean ± standard deviation of three independent experiments.