Requirements for p53 and the ATM gene product in the regulation of G1/S and S phase checkpoints

We investigated the requirements for protein p53 and the ATM gene product in radiation-induced inhibition of DNA synthesis and regulation of the cyclin E/ and cyclin A/cyclin dependent kinases (Cdks), Wild mouse lung fibroblasts (MLFs), p53(-/-) knock-out, normal human skin fibroblasts (HSF-55), and human AT skin fibroblasts (GM02052) were used in the investigations, The absence of p53 had no significant effect on the inhibition or recovery of DNA synthesis throughout the S phase, as determined from BrdU labeling and flow cytometry, or the rapid inhibition of cyclin A/Cdks., gamma: radiation (8 Gy) inhibited DNA synthesis and progression into G2 during the first 3 h after irradiation, and the recovery of these processes occurred at similar rates in both WT and p53(-/-) MLFs., The cyclin A/Cdks were inhibited 55-70% at Ih after irradiation in both cell types, but 921(WAF1/Clp1) levels or p21 interaction with Cdk2 did not increase in the irradiated p53(-/-) MLFs., Although p53(-/-) MLFs do not exhibit prolonged arrest at a G1 checkpoint, radiation did induce a rapid 20% reduction and small super-recovery of cyclin E/Cdk2 within 1-2 h after irradiation, Similar inhibition and recovery of cyclin E/Cdk2 previously had been associated with regulation of transient G1 delay and the inhibition of initiation at an apparent G1/S checkpoint in Chinese hamster cells, In contrast, loss of the ATM gene product abrogated transient cyclin E/Cdk2 inhibition, most inhibition of DNA synthesis and all, but a 10-15% inhibition, of the cyclin A/Cdks, The results indicate that neither p53 nor p21 is required for transient inhibition of cyclin E/Cdk2 associated with the G1/S checkpoint or for inhibition of DNA synthesis at 'checkpoints' within the S phase, Conversely, the ATM gene product appears to be essential for regulation of the G1/S checkpoint and for inhibition of DNA replication the inhibition of cyclin A/Cdk2., of DNA synthesis inhibition among cell types are presented and discussed in the context of S phase checkpoints,.