Fibroblast growth factor receptors (FGFR) play important roles in many biological processes. Nothing is presently known about possible roles of the human FGFR1-IIIb mRNA splice variant. In this study, we characterized for the first time the effects of FGFR1-IIIb expression on the transformed phenotype of human pancreatic cancer cells. The full-length FGFR1-IIIb cDNA was generated and stably expressed in PANC-1 and MIA PaCa-2 pancreatic cancer and TAKA-1 pancreatic ductal cells. FGFR1-IIIb-expressing cells synthesized a glycosylated 110-kDa protein enhancing tyrosine phosphorylation of FGFR substrate-2 on FGF-1 stimulation. The basal anchorage-dependent and anchorage-independent cell growth was significantly inhibited. These effects were associated with a marked reduction of p44/42 mitogen-activated protein kinase (MAPK) phosphorylation in combination with enhanced activity of p38 MAPK and c-Jun NH(2)-terminal kinase. FGFR1-IIIb expression inhibited single-cell movement and in vitro invasion as determined by time-lapse microscopy and Boyden chamber assay as well as in vivo tumor formation and growth in nude mice. Microscopic analysis of the xenograft tumors revealed a reduced Ki-67 labeling and a lower amount of tumor necrosis in FGFR1-IIIb-expressing tumors. Our results show that FGFR1-IIIb is a functional FGFR that inhibits the transformed phenotype of human pancreatic cancer cells.