Pituitary tumor-derived fibroblast growth factor receptor 4 isoform disrupts neural cell-adhesion molecule/N-cadherin signaling to diminish cell adhesiveness: a mechanism underlying pituitary neoplasia

We previously identified pituitary tumor-derived fibroblast growth factor receptor 4 (ptd-FGFR4), an alternatively transcribed N-terminally truncated cytoplasmic receptor isoform. Unlike wild-type FGFR4, ptd-FGFR4 facilitates cell transformation and results in pituitary tumor formation in transgenic mice. To investigate differences in the tumorigenic properties of FGFR4 and ptd-FGFR4, we examined their abilities to modulate cell adhesiveness. Introduction of ptd-FGFR4 into GH4 pituitary cells or NIH 3T3 fibroblasts resulted in significant reduction in cell adhesion to a collagen IV matrix compared with FGFR4- or empty vector-transfected cells. This adhesive difference was evident in the absence or presence of FGF stimulation. Furthermore, treatment with beta1-integrin neutralizing antibody markedly reduced adhesiveness in FGFR4-transfected cells but had little effect on the depressed adhesiveness of ptd-FGFR4-transfected cells. Unlike wild-type FGFR4, ptd-FGFR4 does not associate with neural cell-adhesion molecule (NCAM). Cells expressing FGFR4 demonstrate membranous N-cadherin with a noninvasive growth pattern identical to control GH4 cells when injected into immunodeficient mice. In contrast, ptd-FGFR4-expressing cells develop invasive tumors in vivo with marked loss of N-cadherin that localizes to the cytoplasm. Consistent with these changes, beta-catenin expression was diminished and its interaction with N-cadherin was disrupted in the presence of ptd-FGFR4, but both were intact in the presence of wild-type FGFR4. These data highlight the importance of membrane-anchored FGFR4 in assembling a multiprotein FGFR4 complex with NCAM and N-cadherin playing pivotal functions in maintaining normal cell adhesion. Disruption of distinct NCAM/N-cadherin proadhesive complexes by a tumor-derived FGFR4 isoform provides a novel mechanism beyond ligand independence that explains the pathobiology of proliferative and infiltrative but nonmetastatic neoplasms.