Targeting N-cadherin through fibroblast growth factor receptor-4: distinct pathogenetic and therapeutic implications.

Several molecular aberrations have been implicated in the pathogenesis of pituitary tumors, but few have proven thus far to be of therapeutic value. Pituitary tumor-derived fibroblast growth factor receptor-4 (ptd-FGFR4) is an alternatively transcribed cytoplasmic isoform lacking most of the extracellular domain. This oncogene recapitulates the morphological features of human pituitary tumors in transgenic mice. To investigate the therapeutic potential of targeting ptd-FGFR4, we examined the impact of FGFR4 tyrosine kinase inhibition in xenografted mice. GH4 pituitary cells expressing ptd-FGFR4 develop into invasive tumors. Systemic treatment of mice bearing ptd-FGFR4 tumors with the FGFR-selective inhibitor PD173074 resulted in recovery of membranous N-cadherin staining and a significant reduction in tumor volume with less invasive growth behavior. Mutation of tyrosine Y754F in ptd-FGFR4 abrogated the effect of PD173074-mediated inhibition. The pivotal role of N-cadherin as a mediator of this pituitary cell growth was demonstrated by small interfering RNA mediated down-regulation, which promoted invasive growth in xenografted mice. To validate this model in primary human pituitary tumors, we examined the expression of ptd-FGFR4, N-cadherin, and clinical behavior. Loss of membranous N-cadherin correlated with cytoplasmic FGFR4 expression and with tumor invasiveness in surgically resected human pituitary tumors. Primary human pituitary tumor cells treated with PD173074 showed restoration of N-cadherin to the membrane with dephosphorylation of retinoblastoma protein. These data highlight the pathogenetic significance of N-cadherin misexpression and emphasize the importance of FGFR partnership in mediating its functions.