Pleiotropic Effects of Biguanides on Mitochondrial Reactive Oxygen Species Production

Abstract

Metformin is widely prescribed as a first-choice antihyperglycemic drug for treatment of type 2 diabetes mellitus, and recent epidemiological studies showed its utility also in cancer therapy. Although it is in use since the 1970s, its molecular target, either for antihyperglycemic or antineoplastic action, remains elusive. However, the body of the research on metformin effect oscillates around mitochondrial metabolism, including the function of oxidative phosphorylation (OXPHOS) apparatus. In this study, we focused on direct inhibitory mechanism of biguanides (metformin and phenformin) on OXPHOS complexes and its functional impact, using the model of isolated brown adipose tissue mitochondria. We demonstrate that biguanides nonspecifically target the activities of all respiratory chain dehydrogenases (mitochondrial NADH, succinate, and glycerophosphate dehydrogenases), but only at very high concentrations (10−2–10−1 M) that highly exceed cellular concentrations observed during the treatment. In addition, these concentrations of biguanides also trigger burst of reactive oxygen species production which, in combination with pleiotropic OXPHOS inhibition, can be toxic for the organism. We conclude that the beneficial effect of biguanides should probably be associated with subtler mechanism, different from the generalized inhibition of the respiratory chain.

Mitochondrial targets of metformin—Are they physiologically relevant?

“…Metformin‐induced ROS production was dose‐dependent, but unlike typical inhibitors of respiratory chain, it increased measured ROS production using substrates for all respiratory chain dehydrogenases (complex I, II, and mGPDH). This suggests that metformin does not bind directly to the dehydrogenases and may affect respiratory chain dehydrogenases via indirect mechanism …”

“…The inhibition of NADH‐dependent respiration in SMPs was minor and even the prolonged treatment had no effect . The reported IC 50 for metformin on complex I activity are extremely high and range from 20 to 80 mmol/L . Moreover, the specific site of metformin binding to the complex I remains elusive as it was shown to influence both flavin and ubiquinone‐binding site.…”

“…Moreover, the specific site of metformin binding to the complex I remains elusive as it was shown to influence both flavin and ubiquinone‐binding site. Furthermore, in addition to complex I, metformin also inhibits other OXPHOS enzymes, which complicates interpretation of the data, if respiration is the measured outcome.…”

“…First, Gui et al demonstrate, that in permeabilized A549 cells, metformin (1 mmol/L) inhibits pyruvate/malate supported respiration, but fails completely to inhibit respiration supported by glycerophosphate . Furthermore, the direct inhibitory effect of metformin on mGPDH was also studied in brown adipose tissue (BAT) where the enzyme is highly expressed . Unlike the liver, the inhibition of mGPDH activity as well as cellular respiration in BAT was affected only by enormously high metformin concentrations.…”

“…16,35 The reported IC 50 for metformin on complex I activity are extremely high and range from 20 to 80 mmol/L. 33,38,40 Moreover, the specific site of metformin binding to the complex I remains elusive as it was shown to influence both flavin and ubiquinone-F I G U R E 1 Suggested molecular targets of metformin. Metformin is transported to the cell via organic cation transporter 1 (OCT1) and blocks mitochondrial respiratory chain (complex I, mGPDH) resulting in (a) higher lactate/pyruvate ratio and decreased gluconeogenesis, (b) apoptosis due to higher reactive oxygen species production, (c) higher AMP/ATP ratio and subsequent activation of AMP-activated kinase (AMPK) and reduced gluconeogenesis, lipogenesis, and cell growth.…”