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  • A recent study revealed that in human


    A recent study revealed that in human glioma cells, DNA-PKcs interacted with the regulatory γ subunit of AMPK and positively regulated AMPK phosphorylation and activation under glucose-deprived conditions. This suggested that DNA-PK is an important regulator of AMPK activation in response to energy depletion [25]. To elucidate the role of DNA-PK in regulating AMPK activation in response to irradiation, we treated LMP1-positive YM178 with NU7026, a specific DNA-PK inhibitor, and found that the phosphorylation of AMPKα (Thr172) was further decreased, suggesting that DNA-PK is a positive regulator of AMPK activation in response to DNA damage. We also observed that the catalytic subunit of DNA-PK physically interacted with the α-subunit of AMPK under DNA damage conditions, whereas LMP1 disrupted this interaction. These data indicated that the interaction between AMPK and DNA-PK promotes the phosphorylation of AMPKα at Thr172 and LMP1 might affect the phosphorylation by disrupting this interaction. Recently, a series of our studies confirmed that LMP1 contributes to the radioresistance of NPC. LMP1 might promote radioresistance by regulating tumor angiogenesis through the JNKs/HIF-1 pathway or glycolysis through the up-regulation of hexokinase 2 (HK2), a rate-limiting enzyme of glycolysis, or by inhibiting telomerase activity in NPC cells [41], [42], [43]. Other studies showed that LMP1-induced cancer stem cell (CSC)-like properties might contribute to radioresistance in NPC cells [44]. Here we found that reactivation of AMPK by metformin could substantially reverse radioresistance of LMP1-positive NPC cells both in vitro and in vivo, suggesting that AMPK plays an important role in regulating LMP1-mediated radioresistance. AMPK is considered a negative regulator of aerobic glycolysis and recent findings revealed that the lactate concentration was significantly correlated with tumor response to fractionated irradiation, probably due to its anti-oxidative capacity [45], [46], [47]. We found that LMP1-positive NPC cells displayed dramatically increased glucose consumption and lactate production after irradiation, a metabolic signature consistent with the Warburg effect. When AMPK was activated with metformin, glucose consumption and lactate production in LMP1-positive NPC cells dropped back to a similar level as observed in LMP1-negative NPC cells. Apoptosis is a primary means of death after DNA damage. We found that LMP1 profoundly inhibited irradiation-induced apoptosis, but apoptosis increased dramatically in LMP1-positive cells after reactivation of AMPK. Together these findings suggested that high concentrations of lactate and resistance to apoptosis may contribute to radioresistance in LMP1-positive NPC cells, whereas AMPK reactivation reversed radioresistance by negatively regulating aerobic glycolysis and promoting apoptosis. A low level of phosphorylated AMPK is common in primary NPC specimens and significantly correlates with the expression of LMP1 [48]. Our data revealed that a lower level of phosphorylated AMPKα (Thr172) predicted a poorer 5-year overall survival rate in NPC patients. A previous study showed that DNA-PKcs overexpression was observed in 36.8% of NPC tumor specimens and was highly correlated to advanced clinical stages and poor survival [49]. Another study revealed that negative expression of DNA-PKcs was detected in 35 of 87 (40.2%) NPC patients and was significantly associated with poor patient survival [50]. A separate study by Lee et al., however, showed no association between DNA-PKcs overexpression and the clinical outcome of NPC [51]. Herein, we found no significant relationship between phosphorylated DNA-PKcs (Thr2609) expression and the 5-year overall survival rate of NPC patients, which might possibly be due to the small sample size.
    Conflict of interest
    Introduction The DNA-dependent protein kinase (DNA-PK) is a DNA-activated serine/threonine protein kinase, and abundantly expressed in almost all mammalian cells. DNA-PK has been mainly known as a critical component in the DNA-damage repair pathway, including non-homologous end-joining (NHEJ) repair and homologous recombinant repair. Homozygous knock-out mice of DNA-Pk catalytic subunit (DNA-PKcs−/−) are hypersensitive to radiation and chemical treatment, and have defects in V(D)J recombination. Although DNA-PK is highly expressed in human cells, the other NHEJ factors are not as abundant. The high levels of DNA-PK in human cells are also somewhat paradoxical in that it does not impart any increased ability to repair DNA damage. If the amount of expressed DNA-PK essentially exceeds the demand for DNA-damage repair, why do human cells universally express such high levels of this huge complex?