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

Background: Signals from a cell's environment are sensed by receptors, whic h activate pathways that, in turn, transmit the signals to the nucleus, inf orming decisions on growth, angiogenesis, and other cell functions. Transcr iption of vascular endothelial growth factor (VEGF), a potent angiogenic fa ctor, can be induced by cell-cell contact. In the current work, we sought t o 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 s imulate the effect of increased cell contact on expression of VEGF messenge r RNA. Transformed cells were also cotransfected with a VEGF promoter-repor ter construct and with constructs that express dominant negative or activat ed versions of signal transduction proteins hypothesized to be involved in the cell-cell contact process, and reporter activity was assessed at variou s cell densities. All P values are two-sided, Results: Direct cell-cell con tact, 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 vi a a mitogen-activated protein kinase (MAPK); it required the activity of fo cal adhesion kinase (FAK), Rap1, and Raf and was Ras independent. In additi on, transcriptional activation of a Ras-sensitive Elk-1 chimeric reporter b y cell-cell contact suggests that Rap1 is a key factor in regulating the sp ecificity of convergent MAPK-signaling pathways arising from different upst ream extracellular stimuli. Conclusions: Cell contact induction of VEGF tra nscription via FAK and Rap1 provides a novel Ras-independent, but transform ation-dependent, mechanism for stimulus-specific regulation of tumor VEGF e xpression via MAPK.