Mechanisms of MEOX1 and MEOX2 Regulation of the Cyclin Dependent Kinase Inhibitors p21CIP1/WAF1 and p16INK4a in Vascular Endothelial Cells

Senescence, the state of permanent cell cycle arrest, has been associatedwith endothelial cell dysfunction and atherosclerosis. The cyclin dependentkinase inhibitors p21CIP1/WAF1 and p16INK4a govern theG1/S cell cycle checkpoint and are essential for determining whethera cell enters into an arrested state. The homeodomain transcription factorMEOX2 is an important regulator of vascular cell proliferation and is a directtranscriptional activator of both p21CIP1/WAF1 and p16INK4a.MEOX1 and MEOX2 have been shown to be partially functionally redundant duringdevelopment, suggesting that they regulate similar target genes invivo. We compared the ability of MEOX1 and MEOX2 to activate p21CIP1/WAF1and p16INK4a expression and induce endothelial cell cycle arrest.Our results demonstrate for the first time that MEOX1 regulates the MEOX2target genes p21CIP1/WAF1 and p16INK4a. In addition,increased expression of either of the MEOX homeodomain transcription factorsleads to cell cycle arrest and endothelial cell senescence. Furthermore, weshow that the mechanism of transcriptional activation of these cyclin dependentkinase inhibitor genes by MEOX1 and MEOX2 is distinct. MEOX1 and MEOX2 activatep16INK4a in a DNA binding dependent manner, whereas they inducep21CIP1/WAF1 in a DNA binding independent manner.