A novel alternatively spliced fibroblast growth factor receptor 3 isoform lacking the acid box domain is expressed during chondrogenic differentiation of ATDC5 cells

To determine the role of fibroblast growth factor (FGF).FGF receptor (FGFR) signaling in chondrogenesis, we analyzed the gene expression of alternativ ely spliced FGFRs during chondrogenic differentiation of ATDC5 cells in vit ro. Two isoforms of FGFR3 were expressed in these cells. One was the comple te form of FGFR3 (FGFR3) already reported, and the other was a novel one th at lacks the acid box domain (FGFR3 Delta AB). The gene of FGFR3 Delta AB w as expressed in undifferentiated ATDC5 cells. In contrast, the transcripts of FGFR3 were not detectable in undifferentiated cells but increased during cellular condensation, which is an obligatory step for chondrogenic differ entiation. FGFR1 and FGFR2 expression was higher than that of FGFR3 in undi fferentiated cells. The gene expression of cell cycle inhibitor p21 was ind uced during cell condensation and correlated best with the expression of FG FR3 among the FGFR isoforms expressed. The differential expression of FGFR3 isoforms during chondrogenesis suggests that these isoforms may play diffe rent roles in the regulation of growth and differentiation in chondrocytes. To define the mitogenic response of FGFR3 Delta AB and FGFR3 to FGFs, thei r cDNAs were stably transfected into mouse BaF3 pro-B cells. FGFR3 preferen tially mediates the mitogenic response to FGF1 and poor response to FGF2, I n contrast, FGFR3 Delta AB mediated a higher mitogenic response to FGF2 as well as to FGF1, In addition, FGFR3 Delta AB responds to FGF1 at lower conc entrations of heparin than FGFR3 does. These results suggest that the acid box plays an important role in the regulation of FGFR3 to mediate biologica l activities in response to FGFs.