Inhibition of pentose cycle of A549 cells by 6-aminonicotinamide: consequences for aerobic and hypoxic radiation response and for radiosensitizer action.

Metabolism of glucose via the pentose cycle is a principal source of NADPH, an important cellular reducing species. Both aerobic and hypoxic irradiation stimulate the pentose cycle activity of A549 human lung carcinoma cells, which indicates that NADPH is utilized during irradiation, either as a direct hydrogen donor or as a cofactor for enzymatic repair of radiation damage. To evaluate the role of the pentose cycle in radiation response, we treated A549 cells with 6-aminonicotinamide (6-AN), which blocks the oxidative limb of this pathway in some cell lines. We found 6-AN to be a very effective inhibitor of pentose cycle activity, as indicated both by accumulation of 6-phosphogluconate in A549 cells and by the inability of nitrofurazone or peroxide to stimulate release of 14CO2 from 14C-1-labeled glucose after 6-AN treatment. Effects of 6-AN were time and concentration dependent; it caused partial inhibition of glycolysis but had no effect on respiratory rate or on intracellular glutathione levels. Effects of 6-AN on radiation response were examined under two conditions: 1) after treatment with 0.3 mM drug for 5 hours, which inhibited pentose cycle activity by 50%, and 2) after treatment for 15 hours, which completely inhibited pentose cycle activity. Neither treatment affected aerobic radiation response, but both increased hypoxic sensitivity to a similar extent, with the oxygen enhancement ratio reduced from 3.0 to 2.0 at a 0.05 surviving fraction. Treatment of A549 cells with 6-AN caused an increase in hypoxic cell radiosensitization by misonidazole, but effects of the combined agents were not more than additive.(ABSTRACT TRUNCATED AT 250 WORDS)