Effects of hyperthermia on chromatin condensation and nucleoli disintegration as visualized by induction of premature chromosome condensation in interphase mammalian cells.

The effects of hyperthermia on chromatin condensation and nucleoli disintegration, as visualized by induction of premature chromosome condensation in interphase mammalian cells, was studied in exponentially growing and plateau phase Chinese hamster ovary cells. Exposure to heat reduced the ability of interphase chromatin to condense and the ability of the nucleolar organizing region to disintegrate under the influence of factors provided by mitotic cells when fused to interphase cells. Based on these effects treated cells were classified in three categories. Category 1 contained cells able to condense their chromatin and disintegrate the nucleolar organizing region. Category 2 contained cells able to only partly condense their chromatin and unable to disintegrate the nucleolar organizing region. Category 3 contained cells unable to condense their chromatin and unable to disintegrate the nucleolar organizing region. The fraction of cells with nondisintegrated nucleoli increased with increasing exposure time at 45.5 degrees C and reached a plateau at almost 100% after about 20 min. Exponentially growing and plateau phase cells showed similar response. Recovery from the effects of heat on chromatin condensation and disintegration of the nucleolar organizing region depended upon the duration of the heat treatment. For exposures up to 15 min at 45.5 degrees C, a gradual reduction in the fraction of cells with nondisintegrated nucleoli was observed when cells were allowed for repair at 37 degrees C. However, only a very limited amount of repair was observed after a 30-min exposure to 45.5 degrees C. The repair times observed at the chromosome level were similar to those reported for the removal of excess protein accumulating in chromatin or the nuclear matrix, suggesting a causal relationship between the two phenomena. It is proposed that nuclear protein accumulation on chromatin or in the nuclear matrix reduces the accessibility of chromatin to enzymes responsible for the phosphorylation reactions necessary for chromatin condensation and disintegration of the nucleolus.