Hyperoxia causes a reproducible pattern of lung injury and repair in rodents, in which proliferation of alveolar epithelial cells (AEC) and fibroblasts is observed during recovery. We postulated that if quiescent cells are stimulated to reenter the cell cycle, then cyclin expression and cyclin-dependent protein kinase activity would be reactivated in AEC during the repair process after hyperoxic lung injury. To test this hypothesis, we exposed adult rats to short-term hyperoxia, followed by recovery for various times in room air. Cellular proliferation in vivo was confirmed by 1) flow cytometric analysis of DNA content (FACS) of freshly isolated AEC and 2) immunohistochemistry of proliferating cell nuclear antigen (PCNA) and bromodeoxyuridine (BrdU) incorporation into DNA on lung sections. The percentage of freshly isolated AEC in S phase and G2/M phase on FACS analysis increased twofold to a maximum of 16.5%, after 48 h in 100% oxygen and 48 h recovery in air. Cyclins A and D and p34cdc2 protein expression were also increased during the recovery period; while p33cdk2 and p34cdk4 increased only slightly. p34cdc2 histone H1 kinase activity, both in whole lung and in AEC, decreased initially after 48 h in oxygen. However, a marked increase in p34cdc2 kinase activity was observed at 48 h recovery in whole lung and returned to baseline by 72 h. In isolated and cultured AEC, p34cdc2 kinase activity was maximal at 24 h of recovery in air. We conclude that cyclins A and D and p34cdc2 protein expression and p34cdc2 kinase activity are increased in vivo during recovery from hyperoxic lung injury in both adult rat lungs and in AEC isolated from these lungs. We speculate that the induction of cyclin-dependent protein kinase activity is a key event in mediating the proliferative cellular repair response to lung injury.
Mitogen regulation of c-Raf-1 protein kinase activity toward mitogen-activated protein kinase-kinase