Chuck C.-K. Chao

Cross-resistance to UV radiation of a cisplatin-resistant human cell line: overexpression of cellular factors that recognize UV-modified DNA.

A human cell line selected for cisplatin resistance (CPR) was irradiated with UV light and showed cross-resistance to UV light. Applying a modified chloramphenicol acetyltransferase assay, we observed that CPR cells acquired enhanced host cell reactivation of a transfected plasmid carrying UV damage. Gel mobility shift analysis indicated that two nuclear factors that recognize UV-modified DNA were overexpressed in CPR cells. In addition, factors that bind UV-modified DNA were independent from the factors that bind cisplatin-modified DNA. The significance of the identified binding factors, possibly DNA repair enzymes, is discussed.

Induction of sister-chromatid exchanges in ICR 2A frog cells exposed to 265-313 nm monochromatic ultraviolet wavelengths and photoreactivating light.

Exposure of ICR 2A frog cells to 265 nm, 289 nm, 302 nm or 313 nm monochromatic ultraviolet (UV) wavelengths induced the formation of sister-chromatid exchanges (SCEs). However, treatment of cells with photoreactivating light (PRL) following the UV irradiations resulted in a lower level of SCEs compared with cells incubated in the dark. Hence, it can be concluded that pyrimidine dimers are the principal photoproducts responsible for the induction of SCEs in cells exposed to 265-313 nm UV due to the specificity of DNA photolyase for the light-dependent monomerization of dimers in DNA. It was also found that the maximum yield of induced SCEs in 313 nm-irradiated cells was only about 7 SCEs per cell whereas the plateau values for the shorter wavelengths were approximately 15-20 SCEs per cell. In addition, treatment of cells with 313 nm plus 265 nm light resulted in a lower level of SCEs than in cells exposed to 265 nm UV alone. These results can be interpreted in the context of a replication model for SCE, in which the high level of non-dimer damages produced in the DNA of 313 nm-irradiated cells inhibits the induction of SCEs by the pyrimidine dimers that are also produced by this wavelength.

Regulation of photorepair in growing and arrested frog cells in response to ultraviolet light

The constitutive and inducible levels of enzymatic photorepair (EPR) in growing and arrested ICR 2A frog cells were studied using clonogenic assays. Both arrested and growing cells exhibited an equal level of constitutive EPR following ultraviolet irradiation. However, only arrested, but not growing, cells treated with a low fluence of UV (90% survival) developed an enhanced EPR. The induced process developed transiently with a peak 3 days after pre-irradiation, and was totally blocked by a nontoxic concentration of cycloheximide. In addition, the induced EPR is unique to low fluences of UV.

Loss of inducible photorepair in a frog cell line hypersensitive to solar UV light

The induction of enzymatic photorepair (EPR) in ICR 2A frog cells and a derived mutant cell line DRP36 hypersensitive to solar UV was studied. Using clonogenic assays, when induced wild‐type cells demonstrated an 8‐fold increase of EPR the mutant cells displayed a near‐background level of inducible EPR. The constitutive EPR in mutant cells, however, was the same as in wild‐type cells. A mixed culture of ICR 2A and DRP36 cells showed an intermediate inducible EPR depending upon the cell ratio. Inducible EPR was also detected at the DNA level in wild‐type cells, but not in mutant cells.