Chemical agents that promote chromatin compaction radiosensitize tumour cells

Previous studies indicated that cells whose chromatin is naturally compacted at the time of radiation are hypersensitive to radiation-induced killing, primarily by single-hit inactivation. Some chemicals that are known to promote chromatin compaction in interphase cells are here investigated for their radiosensitizing potential.

Okadaic acid (OA), a protein phosphatase inhibitor, fostriecin (FC), a topoisomerase II inhibitor and trichostatin A (TSA), a histone deacetylase inhibitor, were reported to promote chromatin compaction in mammalian cells. Asynchronous populations of HT-29 (human colon carcinoma) cells were exposed to various concentrations of OA, FC and TSA for various times before irradiation with various doses of Cs-137 gamma-rays and toxicity and radiosensitization were measured. Induced chromatin compaction was visualized by electron microscopy (EM). Histone 1 (H1) and histone 3 (H3) phosphorylation was measured by Western blotting, whole-cell fluorescence microscopy and confocal microscopy.

OA and FC produced significant radiosensitization at 2 Gy after short (2 h) exposures. These chemical treatments also produced increased phosphorylation of H3 and increased chromatin compaction as measured by EM. A 2-h exposure of cells to TSA had no effect on cell radiosensitivity, histone phosphorylation or chromatin condensation. However, a 16-h exposure to TSA produced significant radiosensitization, histone phosphorylation and chromatin condensation, presumably by secondary mechanisms.

These data are consistent with the hypothesis that compacted chromatin is a hypersensitive target for radiation killing. Furthermore, the modulation of chromatin conformation by drugs selectively in tumour cells might radiosensitize tumours whose cells are intrinsically radioresistant.