Date Published: July 22, 2012
Publisher: Impact Journals LLC
Author(s): Bruna Corominas-Faja, Rosa Quirantes-Piné, Cristina Oliveras-Ferraros, Alejandro Vazquez-Martin, Sílvia Cufí, Begoña Martin-Castillo, Vicente Micol, Jorge Joven, Antonio Segura-Carretero, Javier A. Menendez.
Metabolomic fingerprint of breast cancer cells treated with the antidiabetic drug metformin revealed a significant accumulation of 5-formimino-tetrahydrofolate, one of the tetrahydrofolate forms carrying activated one-carbon units that are essential for the de novo synthesis of purines and pyrimidines. De novo synthesis of glutathione, a folate-dependent pathway interconnected with one-carbon metabolism was concomitantly depleted in response to metformin. End-product reversal studies demonstrated that thymidine alone leads to a significant but incomplete protection from metformin’s cytostatic effects. The addition of the substrate hypoxanthine for the purine salvage pathway produces major rightward shifts in metformin’s growth inhibition curves. Metformin treatment failed to activate the DNA repair protein ATM kinase and the metabolic tumor suppressor AMPK when thymidine and hypoxanthine were present in the extracellular milieu. Our current findings suggest for the first time that metformin can function as an antifolate chemotherapeutic agent that induces the ATM/AMPK tumor suppressor axis secondarily following the alteration of the carbon flow through the folate-related one-carbon metabolic pathways.
The exact site of action and molecular mechanism by which the anti-diabetic, biguanide metformin exerts its recently recognized anticancer activity is poorly understood and remains largely controversial [1-5]. Most of the cell-autonomous mechanisms describing metformin’s anticancer activity have been associated with its ability to activate the metabolic rheostat AMP-activated protein kinase (AMPK) [6-10].
Metformin has been repeatedly proposed to function as a “mitochondrial poison” by inhibiting complex I of the respiratory chain, which consequently leads to an imbalance of the AMP:ATP ratio, which is an extremely sensitive indicator of intracellular energy status that molecularly monitored by AMPK [13-16]. Because catabolism “charges the battery” by synthesizing ATP, while most other cellular processes consume ATP and tend to “discharge the battery”, AMPK’s ability to detect and react to fluctuations in the AMP:ATP ratio ensures that the ATP synthesis rate closely aligns with the ATP consumption rate in most cells [13-16]. In the mitochondrial model describing metformin’s activity, metformin-induced metabolic stress may cause the catabolic rate to be insufficient to respond to the ATP consumption rate, thereby causing ADP levels to rise and ATP levels to fall. ADP would be converted into AMP by adenylate kinase, and this process, when combined with the drop in ATP levels, would eventually active AMPK. Following AMPK activation, it is well known that catabolic pathways, such as fatty acid oxidation and glycolysis, become activated, whereas ATP-consuming pathways, such as lipogenesis, are inactivated. Although AMPK activation induces energy conservation and ultimately promotes the survival of normal cells, rapidly growing cancer cells cannot sustain this AMPK-induced limit on the utilization of available bioenergetic resources. Indeed, because the “metabolic transformation” of cancer cells commonly involves the oncogenetically driven addiction to nutrients (e.g., glucose) in the presence of genetically or functionally inactivated metabolic checkpoints, such as p53 deficiency, AMPK inactivation, and/or mTOR hyperactivation [71-73], it is not entirely unexpected that metformin-induced AMPK “re-activation” may induce specific tumor growth-inhibitory effects. However, we are now aware that the concept that metformin poisons mitochondrial respiratory enzyme activity can no longer be considered as the pivotal mechanism underlying metformin’s ability to activate AMPK.