Rac1-dependent transcriptional up-regulation of p27Kip1 by homophilic cell–cell contact in vascular endothelial cells

The mechanism for the transcriptional up-regulation of p27Kip1 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 p27Kip1 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 p27Kip1 up-regulation and delayed the cell cycle arrest. Rac1 was therefore suggested to mediate the contact-induced transcriptional up-regulation of p27Kip1. The role of Rac1 in the regulation of the p27Kip1 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 p27Kip1 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 p27Kip1 in vascular endothelial cells.