Chemical agents that promote chromatin compaction radiosensitize tumour cells.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Biade
S
S
Department of Radiation Oncology, Fox Chase Cancer Center, 7701 Burholme Avenue, Philadelphia, PA 19111, USA.
Stobbe
C C
CC
Boyd
J T
JT
Chapman
J D
JD
D000475:Alkenes / Q000494:pharmacology; D002470:Cell Survival / Q000528:radiation effects; D002843:Chromatin / Q000737:chemistry / Q000187:drug effects*; D019073:HT29 Cells; D006657:Histones / Q000032:analysis; D006801:Humans; D006877:Hydroxamic Acids / Q000494:pharmacology; D007150:Immunohistochemistry; D009369:Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D019319:Okadaic Acid / Q000494:pharmacology; D011090:Polyenes; D011753:Pyrones; D011836:Radiation Tolerance
D000475:Alkenes; D002843:Chromatin; D006657:Histones; D006877:Hydroxamic Acids; D011090:Polyenes; D011753:Pyrones; D019319:Okadaic Acid; C012589:trichostatin A; C040313:fostriecin