Platelet-derived growth factor induces interleukin-6 transcription in osteoblasts through the activator protein-1 complex and activating transcription factor-2

Platelet-derived growth factor (PDGF) BE, a mitogen that stimulates bone re sorption, increases the expression of interleukin-6 (IL-6), a cytokine that induces osteoclast recruitment. The mechanisms involved in IL-6 induction by PDGF BE are poorly understood. We examined the effect of PDGF BE on IL-6 expression in cultures of osteoblasts from fetal rat calvariae (Ob cells). PDGF BE increased IL-6 mRNA and heterogeneous nuclear RNA levels, the rate of transcription, and the activity of base pairs (bp) -2906 to +20 IL-6 pr omoter fragments transiently transfected into Ob cells. Deletion analysis r evealed two responsive regions, one containing an activator protein-1 (AP-1 ) site located between bp -276 and -257, and a second, less well defined, d ownstream of -257. Targeted mutations of a cyclic AMP-responsive element (C RE), and nuclear factor-IL-6 and nuclear factor-kappa B binding sites in a bp -257 to +20 IL-6 construct that was transfected into Ob cells, revealed that the CRE also contributed to IL-6 promoter induction by PDGF BE. Electr ophoretic mobility shift assay revealed AP-1 and CRE nuclear protein comple xes that were enhanced by PDGF BE. Supershift assays revealed binding of Ju n and Fos to AP-1 and CRE sequences and binding of activating transcription factor-a to CRE. In conclusion, PDGF BE induces IL-6 transcription in oste oblasts by regulating nuclear proteins of the AP-1 complex and activating t ranscription factor-2.