Constitutive Activation of Proto-Oncogen Protein p21 Induces Cell Cycle Arrest in the G1 Phase in Contact- Inhibited Vascular Endothelial Cells

Abstract

In an attempt to find a strategy to modulate the proliferation of vascular endothelial cells, we examined whether constitutive activation of proto-oncogen protein p21 (Ras) induced the reentry of confluent human umbilical vascular endothelial cells (HUVECs) into the S phase. When an adenovirus construct expressing a constitutively active Ras mutant (Ad/RasG12V) was infected into HUVECs, their morphology changed strikingly and they appeared to be transformed. However, Ad/RasG12V-infected HUVECs did not enter the S phase, as determined by assessing ³H-thymidine incorporation. In accordance with the above results, the expression of cyclin A both at the transcript and protein levels did not increase in Ad/RasG12V-infected HU VECs relative to that in control cells, although the expression of cyclin D1 was induced in Ad/RasG12V-infected cells. Interestingly, the expression of the cyclin-dependent kinase (CDK) inhibitor p21^cip1 was remarkably increased while that of p27^kip1 did not decrease in Ad/RasG12V-infected HUVECs. Furthermore, CDK2 activity was not induced in Ad/RasG12V-infected HUVECs. These results suggested that the constitutive activation of Ras promoted the reentry of confluent HUVECs in the G0 phase into the G1 phase, but not into the S phase. The results also indicated that the constitutive activation of Ras might have induced the persistent expression of p21^cip1 and p27^kip1, and that this induction of p21^cip1 and p27^kip1 expression possibly caused the cell cycle arrest at the G1 phase. (Hypertens Res 2002; 25: 773-778)