Combined radiation-protective and radiation-sensitizing agents. IV: Measurement of intracellular protector concentrations

Green tea polyphenol protection against 4-nitroquinoline 1-oxide-induced bone marrow lipid peroxidation and genotoxicity in Wistar rats

4-Nitroquinoline 1-oxide (4-NQO) a potent oral carcinogen, widely used for induction of oral carcinogenesis, has been found to induce lipid peroxidation in vivo and in vitro. Green tea contains a high content of polyphenols, which are potent antioxidants. Thus green tea polyphenols (GTP) might be expected play a protective role against 4-NQO induced lipid peroxidation and bone marrow toxicity. In the present study, a dose of 200 mg of GTP/kg b.wt/day was given orally for a week, simultaneously animals received 0.2 ml of 0.5% 4-NQO in propylene glycol (5 mg/ml) injected intramuscularly for three times/week. Oxidants and antioxidants such as malendialdehyde (MDA) and thiols, glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD) and catalase (CAT) were significantly decreased in 4-NQO induced animals except MDA, and these parameters were brought back to near normalcy on treatment with GTP. The results suggest that GTP treatment offers significant protection against 4-NQO induced lipid peroxidation and bone marrow toxicity and might be a promising potential candidate for prevention of mutations leading to cancer.

Therapeutic efficacy of green tea polyphenols on cellular thiols in 4-Nitroquinoline 1-oxide-induced oral carcinogenesis

In cancer, a high flux of oxidants not only depletes the cellular thiols, but damages the whole cell as well. Epidemiological studies suggest green tea may mitigate cancers in human and animal models for which several mechanisms have been proposed. In the present investigation, the levels of cellular thiols such as reduced glutathione (GSH), oxidised glutathione (GSSG), protein thiols (PSH), total thiols, lipid peroxidation product conjugated dienes and the activity of gamma glutamyl transferase (GGT) were assessed in tongue and oral cavity. In 4-Nitroquinoline 1-oxide- (4-NQO) induced rats, there was a decrease in the levels of GSH, PSH and total thiols and an increase in the levels of GSSG, conjugated dienes and the activity of GGT. On supplementation of green tea polyphenols (GTP) for 30 days (200 mg/kg) for the oral cancer-induced rats, there was a moderate increase in the levels of GSH, PSH and total thiols and a decrease in the levels of GSSG, conjugated dienes and the activity of GGT. Thus, GTP reduces the oxidant production thereby maintains the endogenous low molecular weight cellular thiols in oral cancer-induced rats. From the results, it can be concluded that GTP supplementation enhances the cellular thiol status thereby mitigate oral cancer.

The K(m) for Radiosensitization of Human Tumor Cells by Oxygen is Much Greater than 3 mmHg and is Further Increased by Elevated Levels of Cysteine

We studied the role of cysteine as an intracellular radiation protector under conditions in which both oxygen and thiols were monitored at 37 degrees C. In HCT-116 human colon cancer cells, the intracellular cysteine content affects the radiation survival dramatically at intermediate oxygen levels, but not at zero or high oxygen levels. Using a spin-through-oil method with a dual radioactive label detection system, we measured intracellular cysteine and glutathione (GSH) levels for cells in suspension culture. A comparison of the cysteine levels of monolayer cells lysed in situ and of trypsinized monolayer cells in suspension (Horan et al., Cytometry 29, 76-82, 1997) revealed that, upon trypsinization from monolayer culture and transfer to a spinner apparatus at 37 degrees C, HCT-116 cells lose most of their intracellular cysteine. Over the 60-min time course of control experiments, these cells do not recover intracellular cysteine despite the availability of cystine (the disulfide of cysteine) in the medium. When cells in spinner culture are provided with exogenous cysteine, they initially concentrate it to 10-fold the extracellular concentration, with the concentration factor decreasing to about 5-fold over the course of an hour. The intracellular GSH concentration changes little throughout this period, regardless of the changes in cysteine levels. The same apparatus was used to assess the survival of HCT-116 cells irradiated at 37 degrees C under conditions of constant pO(2) monitoring. For cells without added cysteine, the oxygen concentration for half-maximal radiation sensitivity was about 7.5 mmHg (intermediate hypoxia), more than twice the commonly accepted value (3 mmHg). At 7.5 mmHg, cells with added cysteine (intracellular concentration 3.5 mM) were almost as radioresistant as severely hypoxic cells (approximately 0.005% oxygen). Cells in parallel experiments in which the cells were grown in monolayers on glass Petri dishes had intermediate cysteine values and also intermediate radiosensitivity. We conclude that the radiation response of cells at intermediate oxygen levels is controlled predominantly by intracellular cysteine levels and that the cysteine levels commonly found in tumors may increase the K(m) for radiosensitivity to values much higher than suggested previously.

Oxygen dependence of cellular uptake of EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)a cet amide] : analysis of drug adducts by fluorescent antibodies vs bound radioactivity

The present studies were initiated to quantitate the oxygen dependence of bioreductive metabolism-induced binding of EF5, a pentafluorinated derivative of the 2-nitroimidazole, etanidazole. Two different assays were compared: first, radioactive drug incorporation into cell lysates, which provides a direct measure of drug metabolism or uptake; second, monoclonal antibody detection of cellular macromolecular adducts of EF5 after whole cell permeabilisation and fixing. The antibodies (a single clone designated ELK3-51) were conjugated with the fluorescent dye Cy3, with fluorescence determined by fluorescence microscopy and flow cytometry. For the two cell lines tested (V79 Chinese hamster fibroblasts and 9L rat glioma), the oxygen dependence of binding was found to be the same for the two techniques. Using the antibody binding technique, the fluorescence signal was highly reproducible between experiments, resistant to light or chemical bleaching and stable over time following cell or tissue staining. Flow cytometric analysis of cells from rat 9L tumours treated with EF5 in vivo or in vitro showed a distribution of fluorescent signal which was very compatible, on both a relative and absolute basis, with the in vitro results. Our results indicate that immunofluorescent techniques provide a quantitative assay for bioreductive drug adducts, and therefore may be able to measure the absolute oxygen concentration distribution in cell populations and tissues of interest.

Influence of plasma GSH level on acute radiation mucositis of the oral cavity

PURPOSE

To see how pretreatment plasma GSH level influences the severity of acute radiation mucositis of the oral cavity during therapeutic irradiation in patients with oral cancer.

METHODS AND MATERIALS

Thirteen patients with squamous cell carcinoma of the oral cavity form the subject material. Radical radiotherapy (60 Gy in 25 fractions over 5 weeks) was given using telecobalt. Pretreatment plasma GSH level was measured by Beutler's method. The normal tissue reaction during radiotherapy was monitored and graded.

RESULTS

The GSH levels ranged from 10.6-90.5 microM/L (mean 30.6 microM/L). Those who had higher GSH levels developed less severe mucositis. The mean GSH levels in the groups with different severity of reactions were: Grade 2 (four patients) = 50.7 microM/L; Grade 3 (five patients) = 26.1 microM/L; Grade 4 (two patients) = 20.4 microM/L and Grade 5 (two patients) = 26.1 microM/L; Grade 4 (two patients) = 20.4 microM/L and Grade 5 (two patients) = 13.6 microM/L.

CONCLUSION

Plasma GSH estimation has the potential to predict individual sensitivity to acute radiation mucositis and may particularly be useful in hyperfractionated regimes. The study also affirms the radioprotective role of GSH and suggests that this effect is either due to protection against membrane lipid peroxidation (since GSH does not enter the cell freely) or DNA damage (fractionated radiotherapy may permit freer entry of GSH into cell).

Pharmacodynamics of prolonged treatment with L,S-buthionine sulfoximine

PURPOSE

To develop dosing criteria for the use of L-buthionine-S-sulfoximine (active diastereoisomer) as a glutathione depletor in the clinic, using a pharmacodynamic and pharmacokinetic in vitro-in vivo approach.

METHODS AND MATERIALS

In vitro: L-buthionine-S-sulfoximine uptake was determined in human glioblastoma cells (T98G) and NIH-3T3 cells using 35S-labeled drug. Dose response relationships were derived for inhibition of glutathione synthesis in CHO cells, and for depletion of glutathione in exponentially growing T98G and CHO cells, as a function of extracellular L-buthionine-S-sulfoximine concentration. Steady-state glutathione levels for CHO and NIH-3T3 cells were measured using an enzymatic assay, while glutathione synthesis rates in CHO cells were determined using a flow cytometric assay. In vivo: L-buthionine-S-sulfoximine biodistribution was determined in male nude mice carrying human glioblastomas (T98G) intracranially, using 35S-labeled drug infused subcutaneously by osmotic pump. Tissue glutathione levels were measured using an enzymatic assay.

RESULTS AND CONCLUSION

The observed cellular uptake t1/2 of approximately 55 min, coupled with a previously reported, rapid in vivo clearance of buthionine sulfoximine, suggest that continuous infusion would be preferable to bolus dosing. Effective concentrations of L-buthionine-S-sulfoximine (24 h exposure), required to lower cellular glutathione content to 50% of control (EC50), were under 1 mM for both cell lines. The amount of L-buthionine-S-sulfoximine in tissues (estimated from 35S drug disposition) reached steady state within 8 h and was proportional to the rate of infusion. Brain tumors were depleted to approximately 50% of control glutathione by a infusion rate of 0.25 mumoles/h (25 g mice). At lower infusion rates an increase in glutathione content was noted in certain nude mouse tissues including brain tumor xenografts.

Extracellular: intracellular and subcellular concentration gradients of thiols

Chinese hamster V79 cells in Eagle's minimum essential medium in vitro at room temperature were incubated with the aminothiol, WR-1065, or glutathione (GSH) at extracellular concentrations of approximately 1 mmol dm-3. Average intracellular concentrations of GSH, cysteine, and WR-1065 were measured by high performance liquid chromatography, and the effective reducing environment near DNA probed by staining the cells with acridine orange (AO) and measuring the delayed fluorescence. Exposure to either thiol resulted in a rapid, 10-fold increase in average intracellular cysteine concentrations (to about 1 mmol dm-3). Adding extracellular GSH after prior depletion of GSH by treatment with L-buthionine sulfoximine (BSO) did not restore intracellular GSH, but intracellular cysteine was elevated 10-fold. These results are ascribed to thiol/disulfide exchange with cystine in the medium. WR-1065 slowly concentrated intracellularly to approximately 160% of the extracellular concentration. Chemical conjugation of GSH in cells decreased the reducing environment near DNA, but BSO treatment altered the uptake of AO. The electrostatic attraction of WR-1065 toward isolated DNA was markedly affected by ionic strength.

Radiolytic reduction of protein and nonprotein disulfides in the presence of formate: a chain reaction

We have reported recently that the disulfide groups in bovine serum albumin can be reduced by a radiolytic chain reaction which occurs in deoxygenated solutions containing formate ions. This reaction, which involves the reduction of disulfide groups by hydrated electrons and carbon dioxide radical anions, has now been studied in greater detail and compared with an analogous reaction in small, disulfide containing molecules over a range of pH values and substrate concentrations. A two-step reaction is proposed to account for the reduction of disulfides in reactions which can have chain lengths of 20 or more. Thiols produced by the disulfide reduction are stable to the conditions of the reaction. For example, a biological assay showed that the integrity of glutathione was maintained even at radiation doses much larger than those required to achieve complete reduction of glutathione disulfide. It was found that the extent of disulfide reduction could easily be controlled by varying the radiation dose delivered to the solutions. Radiolytic reduction is a very useful way of reducing protein and low molecular weight disulfides without the use of excess quantities of reagents such as dithiothreitol. In many cases, the reaction solutions could be used directly for subsequent reactions and this may be of considerable value in modifying the structure of hormones, enzymes, membrane receptors, and other disulfide containing proteins. If ammonium formate is used, freeze drying is an effective way to remove the formate salt, should this be required.

Increase in intracellular cysteine after exposure to dithiothreitol: implications in radiobiology

Dithiothreitol reduces cystine in tissue-culture medium, and the resulting cysteine is accumulated in Chinese hamster V79 cells to an extent which should be taken into account in evaluating models of radiosensitivity and the effects of thiol radioprotectors.