Focal adhesion kinase, Rap1, and transcriptional induction of vascular endothelial growth factor

Background: Signals from a cell's environment are sensed by receptors, which activate pathways that, in turn, transmit the signals to the nucleus, informing decisions on growth, angiogenesis, and other cell functions. Transcription of vascular endothelial growth factor (VEGF), a potent angiogenic factor, can be induced by cell-cell contact. In the current work, we sought to determine if this induction is dependent on transformation of cells to a malignant phenotype and subsequently to determine which signaling molecules mediate activation of VEGF transcription. Methods: Normal and transformed prostate epithelial cell lines were examined at various cell densities to simulate the effect of increased cell contact on expression of VEGF messenger RNA. Transformed cells were also cotransfected with a VEGF promoter-reporter construct and with constructs that express dominant negative or activated versions of signal transduction proteins hypothesized to be involved in the cell-cell contact process, and reporter activity was assessed at various cell densities. All P values are two-sided, Results: Direct cell-cell contact, but not extracellular matrix components, resulted in transcriptional activation of a VEGF promoter-reporter construct in malignant (P<.0001) but not in nonmalignant (P = .37) prostate cells, This process was mediated via a mitogen-activated protein kinase (MAPK); it required the activity of focal adhesion kinase (FAK), Rap1, and Raf and was Ras independent. In addition, transcriptional activation of a Ras-sensitive Elk-1 chimeric reporter by cell-cell contact suggests that Rap1 is a key factor in regulating the specificity of convergent MAPK-signaling pathways arising from different upstream extracellular stimuli. Conclusions: Cell contact induction of VEGF transcription via FAK and Rap1 provides a novel Ras-independent, but transformation-dependent, mechanism for stimulus-specific regulation of tumor VEGF expression via MAPK.