Figure 5.

Model for inhibitory effect of extracellular nicotinamide phosphoribosyltransferase on insulin-like growth factor-1 function in chondrocytes. In this hypothetical model, sustained activation of extracellular signal-regulated kinases (ERK) by extracellular nicotinamide phosphoribosyltransferase (eNAMPT) promotes increased phosphorylation of insulin receptor substrate-1 (IRS-1) (serine-312), which negatively regulates the downstream insulin-like growth factor-1 (IGF-1) signaling pathway and thereby matrix synthesis. IGF-1R, insulin-like growth factor-1 receptor.