Radiation-induced apoptosis in human non-small-cell lung cancer cell lines is secondary to cell-cycle progression beyond the G2-phase checkpoint.

PURPOSE

To characterize the relationship between cell-cycle progression and radiation-induced apoptosis in NSCLC cell lines with different p53 status.

MATERIALS AND METHODS

Cell lines with functional (H460, A549) and non-functional p53 (H661 and H520) were irradiated with 20 Gy. Multiparameter flow-cytometry was used to follow the progression of synchronized cells through the cell cycle after irradiation.

RESULTS

Delayed apoptosis was observed after cell-cycle progression beyond the G2 block, either in the late G2/M-phase of the same cell cycle being irradiated (H661, H520) or in the G1-phase of the subsequent cell cycle (H460, A549). The apoptotic fraction in H661 and H520 was 60-80% at 144h after irradiation, higher than in A549 and H460 (5 and 35%, respectively). As an alternative to apoptosis in cells cycling beyond the G2 restriction point, hyperploid cells were generated by all cell lines. Inhibition of cell-cycle progression through the G2/M-phase efficiently reduced the induction of late apoptosis. After irradiation in S-phase, 50-60% of cells with functional p53 remained arrested at the G2 restriction point until 144 h post-irradiation, while only 20% of the H661 or H520 did so.

CONCLUSIONS

These data characterize radiation-induced apoptosis in NSCLC cell lines as a removal pathway of clonogenically inactivated cells secondary to cell-cycle progression beyond G2/M, and is unlikely to be a critical factor for cellular radiation sensitivity.