ALTERED G(1) CHECKPOINT CONTROL DETERMINES ADAPTIVE SURVIVAL RESPONSES TO IONIZING-RADIATION

Adaptive survival responses (ASRs) are observed when cells become more resistant to a high dose of a cytotoxic agent after repeated low dose exposures to that agent or another genotoxic agent. Confluent (G(0)/G (1)) human normal (GM2936B, GM2937A, AG2603, IMR-90), cancer-prone (XP V2359), and neoplastic (U1-Mel, HEp-2, HTB-152) cells were primed with repeated low doses of X-rays (ranging from 0.05-10 cGy/day for 4 days ), then challenged with a high dose (290-450 cGy) on day 5. U1-Mel and HEp-2 cells showed greater than 2-fold transient survival enhancement when primed with 1-10 cGy. ASRs in U1-Mel or HEp-2 cells were blocked by cycloheximide or actinomycin D. Increases in cyclins A and D1 mRNA s were noted in primed compared to unirradiated U1-Mel and HEp-2 cells ; however, only cyclin A protein levels increased. Cyclin D1 and proli ferating cell nuclear antigen (PCNA) protein levels were constitutivel y elevated in HEp-2 and U1-Mel cells, compared to the other human norm al and neoplastic cells examined, and were not altered by low or high doses of radiation. Low dose primed U1-Mel cells entered S-phase 4-6 h faster than unprimed U1-Mel cells upon low-density replating. Similar responses in terms of survival recovery, transcript and protein induc tion, and altered cell cycle regulation were not observed in the other human normal, cancer-prone or neoplastic cells examined. We hypothesi ze that only certain human cells can adapt to ionizing radiation by pr ogressing to a point later in G(1) (the A point) where DNA repair proc esses and radioresistance can be induced. ASRs in human cells correlat ed well with constitutively elevated levels of PCNA and cyclin D1, as well as inducibility of cyclin A. We propose that a protein complex co mposed of cyclin D1, PCNA, and possibly cyclin A may play a role in ce ll cycle regulation and DNA repair, which determine ASRs in human cell s.