Accumulation of p21(Cip1/WAF1) during hyperoxic lung injury in mice

Hyperoxic lung injury results in decreased cell proliferation, DNA damage, and cell death. Because the cyclin-dependent kinase inhibitor p21(Cip1/WAF1 ) (p21) inhibits cell proliferation in G1/S, enhances DNA repair, and regul ates apoptosis in some cells, we hypothesized that the expression of p21 wo uld increase in lungs of C57B1/6J male mice exposed to and recovered from > 95% oxygen. A low level of p21 messenger RNA (mRNA) expression was detected by Northern blot analysis of room air-exposed lungs. Exposure to hyperoxia resulted in a modest increase in p21 mRNA expression by 24 h, followed by a marked induction by 48 to 72 h. In situ hybridization revealed that p21 m RNA abundance increased in bronchiolar epithelium and in resident alveolar cells, but not in: smooth-muscle cells or large airway epithelium. Hyperoxi a increased the expression of p21 protein by 24 h and continued to increase at 48 and 72 h. Immunohistochemical staining showed that p21 protein accum ulated in the bronchiolar epithelium and in alveolar regions that had incre ased p21 mRNA expression. Ire contrast, the expression of the cyclin-depend ent kinase inhibitor p27(Kip1) was not altered by hyperoxia. To determine w hether p21 expression was altered during the repair process, mice were expo sed to hyperoxia for 64 h and allowed to recover for up to 4 d in room air. The abundance of p21 mRNA and protein decreased by 1 to 2 d of recovery an d returned to room air-exposed levels by 3 to 3 d of recovery. These findin gs support the concept that bronchiolar epithelial and alveolar cells damag ed by hyperoxia express molecules such as p21, which may participate in reg ulating cell proliferation, DNA repair, and cell death.