Glucose-6-phosphate dehydrogenase and the oxidative pentose phosphate cycle protect cells against apoptosis induced by low doses of ionizing radiation.
The initial and rate-limiting enzyme of the oxidative pentose phosphate shunt, glucose-6-phosphate dehydrogenase (G6PD), is inhibited by NADPH and stimulated by NADP(+). Hence, under normal growth conditions, where NADPH levels exceed NADP(+) levels by as much as 100-fold, the activity of the pentose phosphate cycle is extremely low. However, during oxidant stress, pentose phosphate cycle activity can increase by as much as 200-fold over basal levels, to maintain the cytosolic reducing environment. G6PD-deficient (G6PD(-)) cell lines are sensitive to toxicity induced by chemical oxidants and ionizing radiation. Compared to wild-type CHO cells, enhanced sensitivity to ionizing radiation was observed for G6PD(-) cells exposed to single-dose or fractionated radiation. Fitting the single-dose radiation response data to the linear-quadratic model of radiation-induced cytotoxicity, we found that the G6PD(-) cells exhibited a significant enhancement in the alpha component of radiation-induced cell killing, while the values obtained for the beta component were similar in both the G6PD(-) and wild-type CHO cell lines. Here we report that the enhanced alpha component of radiation-induced cell killing is associated with a significant increase in the incidence of ionizing radiation-induced apoptosis in the G6PD(-) cells. These data suggest that G6PD and the oxidative pentose phosphate shunt protect cells from ionizing radiation-induced cell killing by limiting the incidence of radiation-induced apoptosis. The sensitivity to radiation-induced apoptosis was lost when the cDNA for wild-type G6PD was transfected into the G6PD(-) cell lines. Depleting GSH with l-BSO enhanced apoptosis of K1 cells while having no effect in the G6PD(-) cell line
Tuttle
S
S
Radiation Oncology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
Stamato
T
T
Perez
M L
ML
Biaglow
J
J
D000818:Animals; D017209:Apoptosis / Q000528:radiation effects*; D016466:CHO Cells; D006224:Cricetinae; D005720:Gamma Rays; D005954:Glucosephosphate Dehydrogenase / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism; D010084:Oxidation-Reduction; D010427:Pentose Phosphate Pathway; D011829:Radiation Dosage
D005954:Glucosephosphate Dehydrogenase; D005978:Glutathione