Inhibition by trioxalen (psoralen) plus near-ultraviolet light of the induction of ornithine decarboxylase in Chinese-hamster cells

Ultraviolet light inactivation of zinc-mediated metallothionein induction in normal and repair-deficient human cells

Synthesis of the low molecular weight, thiol-rich, metal-binding metallothioneins (MTS) is undetectable in normal human (NF) or xeroderma pigmentosum (XP) fibroblasts grown in the absence of excess ZnCl2. Addition of 200 microM ZnCl2 to the growth medium produces an increased MT synthesis rising from the basal rate to a rate at least 50-fold greater than basal rate within 7 h. MT induction kinetics in confluent and in exponentially growing subconfluent monolayers were indistinguishable. Zn2+-mediated MT induction is sensitive to actinomycin D suggesting that the induction process is under transcriptional control. Ultraviolet light irradiation causes a dose-dependent inactivation of Zn2+-mediated MT induction in both NF and XP cells. Post-irradiation incubation of UV-irradiated cells using liquid holding techniques leads to reactivation of Zn2+-mediated MT induction in NF cells but not in XP cells. These findings suggest the utility of MT induction produce transcription-terminating lesions, and (b) in evaluating cellular repair capacity for this class of DNA lesions.

Inhibition of induced ornithine decarboxylase activity in Chinese hamster cells by gamma radiation, far ultraviolet light and psoralen plus near-ultraviolet light: a comparative study

Inhibition by radiation of the transcriptionally controlled induction of ornithine decarboxylase (ODC) was studied in plateau-phase Chinese hamster cells. The effects of gamma radiation, far ultraviolet (254nm, UV-C) light and psoralen plus near-U.V. light (360nm, PUV-A) were compared. Increasing doses of all three treatments caused an exponential decrease in the capacity for ODC induction. This was preceded by a large shoulder region in the case of PUV-A. There was no shoulder for gamma radiation and possibly none or a small one for UV-C. When compared on the basis of cell killing UV-C is 13 times more efficient in inhibiting ODC induction than PUV-A and 69 times more than gamma radiation. The doses required to produce an inactivating hit for inhibition of ODC induction (D0) are 106 krad for gamma radiation, 2400 J m-2 for PUV-A and 14 J m-2 for UV-C. These doses produce about one gamma ray lesion, one psoralen adduct and one pyrimidine dimer in the structural gene of ODC. However, the number of lesions per transcription unit may be up to 10, depending on the contribution to the effect by lesions in transcribed and non-transcribed spacers and introns. Thus, assuming that most of the inhibition is due to effects on RNA synthesis, the various DNA lesions appear to have a similar efficiency in terminating RNA transcription, in spite of their greatly different efficiency in cell killing.

Gamma radiation inhibits the appearance of induced ornithine decarboxylase activity in Chinese hamster cells

Ornithine decarboxylase activity of Chinese hamster cells (ODC, EC 4.1.1.17) can be induced in plateau phase by change of medium. Exposure of the cells to gamma radiation before induction reduces the amount of ODC activity induced. The dose-response curve is exponential with a D0 of 106 krad. Exposure of BUdR-substituted cells is more effective in reducing ODC induction at high doses, with a D0 of 38 krad. Cells can recover from the reduction incurred by 74 krad if enzyme induction is delayed for 2 hours after exposure. Treatment of the cells with psoralen-plus-light completely inhibits RNA synthesis without affecting protein synthesis (Heimer, Ben-Hur and Riklis 1977, 1978). Using this procedure it is shown that the effect of gamma radiation on inducible ODC activity is due not only to DNA damage by also involves a post-transcriptional effect. This conclusion is supported by employing a heat shock to inhibit protein synthesis prior to gamma-irradiation of log-phase cells. In such cells the increased activity of ODC upon transfer to 37 degrees C is due primarily to enzyme synthesis using pre-existing RNA species during the first few hours. A low concentration of actinomycin D, which inhibits rRNA synthesis, applied during the recovery period, prevents the recovery of the cells' capacity for maximal ODC induction. This may indicate that, in order to recover, the cells have to repair damage to the ribosomes as well as to DNA.

An apparent correlation between the inhibition of induced ornithine decarboxylase activity by gamma radiation and the capacity for DNA repair synthesis in normal and ataxia telangiectasia human fibroblasts: no correlation with cell survival

Exposure of normal human fibroblasts (F107) in stationary phase to gamma radiation inhibited the appearance of induced ornithine decarboxylase (ODC) activity. Skin fibroblasts derived from two ataxia telangiectasia (AT) patients (F184 and F182) displayed a similar response. The level of DNA repair synthesis was also similar in the three cell strains. Fibroblasts from another apparently normal donor (F196) were very sensitive to inhibition of induced ODC activity by gamma radiation and were also deficient in radiation-induced DNA repair synthesis. However, the two strains derived from normal donors displayed the same degree of cellular sensitivity towards X-rays, whereas the two AT strains showed the typical hypersensitivity to the cytotoxic effect of X-irradiation. The results suggest a possible correlation between the inhibition of induced ODC activity by gamma radiation and the extent of DNA repair synthesis at high radiation doses, but there is no correlation between these two parameters and cellular survival at low radiation doses.