Abstract
This study explores the role of ERK activation in regulating G 1 and S-G 2 /M delays during hyperoxia. We demonstrate here that exposing A549 human alveolar type 2 adenocarcinoma cells to hyperoxia (95% O 2 ) for 0.5-24 h time-dependently increases phospho-ERK, phospho-p53 Ser15 , p53, and p21 CIP1 protein levels. Decreasing phospho-ERK with the pharmacological inhibitors, PD98059 and U0126, markedly suppresses hyperoxia-stimulated phospho-p53 Ser15 , p53, and p21 CIP1 , and also restores the hyperoxia-reduced kinase activities of cyclin D1/E1-Cdks. Our results suggest that ERK activation during hyperoxia contributes to the p53/p21-mediated G 1 checkpoint. However, inhibition of ERK signaling during hyperoxia further delays S-phase entry and progression. Hyperoxia induces significant expression of cyclin A/B1 and translocation of cyclin A into nuclei while marginally decreasing cyclin A/B1-Cdks kinase activities, which may be related to nuclear association with p21. Interestingly, inhibition of ERK signaling markedly suppresses the elevation of cyclin A/B1 proteins and cyclin A/B1-Cdks kinase activities during hyperoxia. Taken together, the results presented here suggest that hyperoxia-activated ERK acts upstream of p53 and p21 to suppress G 1 -Cdk activities; however, it is also required for induction of cyclin A/B1 and maintenance of cyclin A/B1-Cdk activities that oppose delays in S-phase entry and progression. © 2004 Elsevier Inc. All rights reserved.