Role of nuclear protein denaturation and aggregation in thermal radiosensitization

Hyperthermia at temperatures above 41 degrees C denatures a set of thermolabile cellular proteins located in all parts of the cell. Non-histone nuclear proteins, including those comprising the nuclear matrix, appear to be particularly thermolabile. Heating isolated nuclear matrices of Chinese hamster lung (CHL) V79 cells to 46 degrees C at 1 degree C/min results in approximately 15% denaturation. Protein unfolding during denaturation exposes buried hydrophobic residues, which increases protein-protein interactions and results in the co-aggregation of denatured thermolabile proteins and native, aggregative-sensitive nuclear proteins. This aggregated protein, the majority of which is native, is insoluble and resistant to extraction during isolation of nuclei and is responsible for the increased protein content, usually expressed as an increased protein:DNA ratio, of nuclei isolated from heated cells. A large fraction of the aggregated protein is found to be associated with the nuclear matrix, distributed throughout the fibre network and nucleolus. Three general consequences of nuclear protein denaturation and aggregation of relevance to cellular damage are: (1) protein (enzyme) inactivation, both direct inactivation of thermolabile proteins and indirect inactivation due to co-aggregation; (2) reduced accessibility and altered physical properties of DNA due to masking by aggregated protein; and (3) protein redistribution into and out of the nucleus. Functional impairment of the nucleus appears to be due to one or a combination of these basic mechanisms.