Rac1-dependent transcriptional up-regulation of p27^Kip1 by homophilic cell-cell contact in vascular endothelial cells

The mechanism for the transcriptional up-regulation of p27^Kip1 due to the formation of the cell-cell contact was investigated in vascular endothelial cells. The induction of the cell-cell contact by adding an extra number of endothelial cells activated Rac1, up-regulated p27^Kip1 mRNA and protein, and also facilitated the cell cycle arrest. Transduction of the Rac1 inhibitor protein using the cell-penetrating peptide or treatment with a Rac1 inhibitor NSC23766 inhibited the p27^Kip1 up-regulation and delayed the cell cycle arrest. Rac1 was therefore suggested to mediate the contact-induced transcriptional up-regulation of p27^Kip1. The role of Rac1 in the regulation of the p27^Kip1 promoter activity was next examined with a luciferase reporter assay. The promoter activity was increased by inducing the cell-cell contact, which was significantly inhibited by the Rac1 inhibitory protein and NSC23766. The evaluation of various truncated promoter regions determined region - 620 to - 573 nucleotides from the initiation codon to be responsible for the contact-induced, Rac1-dependent activation of the p27^Kip1 promoter. The present study thus demonstrated for the first time that the activation of Rac1 due to the cell-cell contact plays a critical role in the transcriptional up-regulation of p27^Kip1 in vascular endothelial cells.