FIGURE 4.

C-terminal domain of FGFRL1 activates the MEK/MAPK signaling pathway independent of receptor-ligand association. A, representative phospho-ERK1/2 (pERK1/2) and total ERK1/2 immunoblots, and B, mean fold-change in phospho-ERK1/2 responses revealed a significant increase in activity associated with overexpression of full-length FGFRL1 (RL1), both in the absence (−) and presence (+) of FGF2 ligand (10 ng/ml for 10 min). Independent of ligand stimulation, ERK1/2 phosphorylation was reduced to comparable Venus control levels when either the C-terminal domain (RL1-ΔC) or histidine-rich region (RL1-ΔHis) was removed. *, p < 0.05 compared with Venus, RL1-ΔC, and RL1-ΔHis controls using one-way ANOVA. n = 4. C and D, enhanced phosphorylation of ERK1/2 in FGFRL1-overexpressing cells (FGFRL1 −/−) was reduced to control levels when cells were pretreated with the MEK inhibitor U0126 (FGFRL1 +/− compared with Venus −/−). Pretreatment with U0126 also reduced FGF-2-stimulated phosphorylation to control levels for both control (Venus +/+ compared with Venus −/−) and FGFRL1 (FGFRL1 +/+ compared with FGFRL1 −/−) cells. Representative blots are shown; each phospho-ERK1/2 blot was stripped and reprobed for total ERK1/2 to assess sample loading integrity and determine ERK1/2 activation (pERK1/2/ERK1/2 intensity ratios). Data are plotted as the mean fold-change in phospho-ERK1/2 response ± S.E. compared with Venus control. n = three separate experiments.