In a cohort of individuals with type 2 diabetes using the drug sulfasalazine, HbA1c lowering is associated with haematological changes.

OBJECTIVES: Several small studies indicate the sulphonamide component of the drug sulfasalazine lowers HbA1c. We investigated reduction of HbA1c following incident prescription of sulfasalazine and related aminosalicylates, lacking the sulphonamide group, in an observational cohort. RESEARCH DESIGN AND METHODS: Individuals in the Scottish Care Information Diabetes Collaboration (SCI-Diabetes) with type 2 diabetes and incident prescription for an aminosalicylate drug (sulfasalazine, mesalazine, olsalazine or balsalazide) were identified. Baseline and 6-month HbA1c were required for eligibility, to calculate HbA1c response. To investigate association with haemolysis, change in components of full blood count was assessed. Paired t-tests compared difference in baseline and treatment HbA1c measures and other clinical variables. RESULTS: In all, 113 individuals treated with sulfasalazine and 103 with mesalazine (lacking the sulphonamide group) were eligible, with no eligible individuals treated with olsalazine or balsalazide. Baseline characteristics were similar. Mean (SD) HbA1c reduction at 6 months was -9 ± 16 mmol/mol (-0.9 ± 1.4%) (p < 0.0001) in those taking sulfasalazine with no reduction in those taking mesalazine (2 ± 16 mmol/mol (0.2 ± 1.4%). Sulfasalazine but not mesalazine was associated with a mean (SD) increase in mean cell volume of 3.7 ± 5.6 fl (p < 0.0001) and decrease in red cell count of -0.2 ± 0.4 × 10-12 /L (p < 0.0001). CONCLUSIONS: In this observational, population-based study, sulfasalazine initiation was associated with a 6-month reduction in HbA1c . This correlated with haematological changes suggesting haemolytic effects of sulfasalazine. Haemolysis is proposed to contribute to HbA1c lowering through the sulphonamide pharmacophore. This suggests that HbA1c is not a reliable measure of glycaemia in individuals prescribed sulfasalazine.

Anti-Inflammatory Effects of Metformin Irrespective of Diabetes Status.

RATIONALE: The diabetes mellitus drug metformin is under investigation in cardiovascular disease, but the molecular mechanisms underlying possible benefits are poorly understood. OBJECTIVE: Here, we have studied anti-inflammatory effects of the drug and their relationship to antihyperglycemic properties. METHODS AND RESULTS: In primary hepatocytes from healthy animals, metformin and the IKKß (inhibitor of kappa B kinase) inhibitor BI605906 both inhibited tumor necrosis factor-α-dependent IκB degradation and expression of proinflammatory mediators interleukin-6, interleukin-1ß, and CXCL1/2 (C-X-C motif ligand 1/2). Metformin suppressed IKKα/ß activation, an effect that could be separated from some metabolic actions, in that BI605906 did not mimic effects of metformin on lipogenic gene expression, glucose production, and AMP-activated protein kinase activation. Equally AMP-activated protein kinase was not required either for mitochondrial suppression of IκB degradation. Consistent with discrete anti-inflammatory actions, in macrophages, metformin specifically blunted secretion of proinflammatory cytokines, without inhibiting M1/M2 differentiation or activation. In a large treatment naive diabetes mellitus population cohort, we observed differences in the systemic inflammation marker, neutrophil to lymphocyte ratio, after incident treatment with either metformin or sulfonylurea monotherapy. Compared with sulfonylurea exposure, metformin reduced the mean log-transformed neutrophil to lymphocyte ratio after 8 to 16 months by 0.09 U (95% confidence interval, 0.02-0.17; P=0.013) and increased the likelihood that neutrophil to lymphocyte ratio would be lower than baseline after 8 to 16 months (odds ratio, 1.83; 95% confidence interval, 1.22-2.75; P=0.00364). Following up these findings in a double-blind placebo controlled trial in nondiabetic heart failure (trial registration: NCT00473876), metformin suppressed plasma cytokines including the aging-associated cytokine CCL11 (C-C motif chemokine ligand 11). CONCLUSION: We conclude that anti-inflammatory properties of metformin are exerted irrespective of diabetes mellitus status. This may accelerate investigation of drug utility in nondiabetic cardiovascular disease groups. CLINICAL TRIAL REGISTRATION: Name of the trial registry: TAYSIDE trial (Metformin in Insulin Resistant Left Ventricular [LV] Dysfunction). URL: https://www.clinicaltrials.gov. Unique identifier: NCT00473876.

Regulation of hepatic glucose production and AMPK by AICAR but not by metformin depends on drug uptake through the equilibrative nucleoside transporter 1 (ENT1).

AIM: Recently we have observed differences in the ability of metformin and AICAR to repress glucose production from hepatocytes using 8CPT-cAMP. Previous results indicate that, in addition to activating protein kinase A, 8CPT-modified cAMP analogues suppress the nitrobenzylthioinosine (NBMPR)-sensitive equilibrative nucleoside transporter ENT1. We aimed to exploit 8CPT-cAMP, 8CPT-2-Methyl-O-cAMP and NBMPR, which is highly selective for a high-affinity binding-site on ENT1, to investigate the role of ENT1 in the liver-specific glucose-lowering properties of AICAR and metformin. METHODS: Primary mouse hepatocytes were incubated with AICAR and metformin in combination with cAMP analogues, glucagon, forskolin and NBMPR. Hepatocyte glucose production (HGP) and AMPK signalling were measured, and a uridine uptake assay with supporting LC-MS was used to investigate nucleoside depletion from medium by cells. RESULTS: AICAR and metformin increased AMPK pathway phosphorylation and decreased HGP induced by dibutyryl cAMP and glucagon. HGP was also induced by 8CPT-cAMP, 8CPT-2-Methyl-O-cAMP and NBMPR; however, in each case this was resistant to suppression by AICAR but not by metformin. Cross-validation of tracer and mass spectrometry studies indicates that 8CPT-cAMP, 8CPT-2-Methyl-O-cAMP and NBMPR inhibited the effects of AICAR, at least in part, by impeding its uptake into hepatocytes. CONCLUSIONS: We report for the first time that suppression of ENT1 induces HGP. ENT1 inhibition also impedes uptake and the effects of AICAR, but not metformin, on HGP. Further investigation of nucleoside transport may illuminate a better understanding of how metformin and AICAR each regulate HGP.

The mechanisms of action of metformin.

Metformin is a widely-used drug that results in clear benefits in relation to glucose metabolism and diabetes-related complications. The mechanisms underlying these benefits are complex and still not fully understood. Physiologically, metformin has been shown to reduce hepatic glucose production, yet not all of its effects can be explained by this mechanism and there is increasing evidence of a key role for the gut. At the molecular level the findings vary depending on the doses of metformin used and duration of treatment, with clear differences between acute and chronic administration. Metformin has been shown to act via both AMP-activated protein kinase (AMPK)-dependent and AMPK-independent mechanisms; by inhibition of mitochondrial respiration but also perhaps by inhibition of mitochondrial glycerophosphate dehydrogenase, and a mechanism involving the lysosome. In the last 10 years, we have moved from a simple picture, that metformin improves glycaemia by acting on the liver via AMPK activation, to a much more complex picture reflecting its multiple modes of action. More work is required to truly understand how this drug works in its target population: individuals with type 2 diabetes.

Investigation of salicylate hepatic responses in comparison with chemical analogues of the drug.

Anti-hyperglycaemic effects of the hydroxybenzoic acid salicylate might stem from effects of the drug on mitochondrial uncoupling, activation of AMP-activated protein kinase, and inhibition of NF-κB signalling. Here, we have gauged the contribution of these effects to control of hepatocyte glucose production, comparing salicylate with inactive hydroxybenzoic acid analogues of the drug. In rat H4IIE hepatoma cells, salicylate was the only drug tested that activated AMPK. Salicylate also reduced mTOR signalling, but this property was observed widely among the analogues. In a sub-panel of analogues, salicylate alone reduced promoter activity of the key gluconeogenic enzyme glucose 6-phosphatase and suppressed basal glucose production in mouse primary hepatocytes. Both salicylate and 2,6 dihydroxybenzoic acid suppressed TNFα-induced IκB degradation, and in genetic knockout experiments, we found that the effect of salicylate on IκB degradation was AMPK-independent. Previous data also identified AMPK-independent regulation of glucose but we found that direct inhibition of neither NF-κB nor mTOR signalling suppressed glucose production, suggesting that other factors besides these cell signalling pathways may need to be considered to account for this response to salicylate. We found, for example, that H4IIE cells were exquisitely sensitive to uncoupling with modest doses of salicylate, which occurred on a similar time course to another anti-hyperglycaemic uncoupling agent 2,4-dinitrophenol, while there was no discernible effect at all of two salicylate analogues which are not anti-hyperglycaemic. This finding supports much earlier literature suggesting that salicylates exert anti-hyperglycaemic effects at least in part through uncoupling.

Biomolecular mode of action of metformin in relation to its copper binding properties.

Metformin (Metf), the most commonly used type 2 diabetes drug, is known to affect the cellular housekeeping of copper. Recently, we discovered that the structurally closely related propanediimidamide (PDI) shows a cellular behavior different from that of Metf. Here we investigate the binding of these compounds to copper, to compare their binding strength. Furthermore, we take a closer look at the electronic properties of these compounds and their copper complexes such as molecular orbital interactions and electrostatic potential surfaces. Our results clearly show that the copper binding energies cannot alone be the cause of the biochemical differentiation between Metf and PDI. We conclude that other factors such as pKa values and hydrophilicity of the compounds play a crucial role in their cellular activity. Metf in contrast to PDI can occur as an anion in aqueous medium at moderate pH, forming much stronger complexes particularly with Cu(II) ions, suggesting that biguanides but not PDI may induce easy oxidation of Cu(I) ions extracted from proteins. The higher hydrophobicity and the lack of planarity of PDI may further differentiate it from biguanides in terms of their molecular recognition characteristics. These different properties could hold the key to metformin's mitochondrial activity because they suggest that the drug could act at least in part as a pro-oxidant of accessible protein-bound Cu(I) ions.

Defining Activity Thresholds Triggering a "Stand Hour" for Apple Watch Users: Cross-Sectional Study.

BACKGROUND: Sedentary behavior (SB) is one of the largest contributing factors increasing the risk of developing noncommunicable diseases, including cardiovascular disease and type 2 diabetes. Guidelines from the World Health Organization for physical activity suggest the substitution of SB with light physical activity. The Apple Watch contains a health metric known as the stand hour (SH). The SH is intended to record standing with movement for at least 1 minute per hour; however, the activity measured during the determination of the SH is unclear. OBJECTIVE: In this cross-sectional study, we analyzed the algorithm used to determine time spent standing per hour. To do this, we investigated activity measurements also recorded on Apple Watches that influence the recording of an SH. We also aimed to estimate the values of any significant SH predictors in the recording of a SH. METHODS: The cross-sectional study used anonymized data obtained in August 2022 from 20 healthy individuals gathered via convenience sampling. Apple Watch data were extracted from the Apple Health app through the use of a third-party app. Appropriate statistical models were fitted to analyze SH predictors. RESULTS: Our findings show that active energy (AE) and step count (SC) measurements influence the recording of an SH. Comparing when an SH is recorded with when an SH is not recorded, we found a significant difference in the mean and median AE and SC. Above a threshold of 97.5 steps or 100 kJ of energy, it became much more likely that an SH would be recorded when each predictor was analyzed as a separate entity. CONCLUSIONS: The findings of this study reveal the pivotal role of AE and SC measurements in the algorithm underlying the SH recording; however, our findings also suggest that a recording of an SH is influenced by more than one factor. Irrespective of the internal validity of the SH metric, it is representative of light physical activity and might, therefore, have use in encouraging individuals through various means, for example, notifications, to reduce their levels of SB.

Molecular mechanism of action of metformin: old or new insights?

Metformin is the first-line drug treatment for type 2 diabetes. Globally, over 100 million patients are prescribed this drug annually. Metformin was discovered before the era of target-based drug discovery and its molecular mechanism of action remains an area of vigorous diabetes research. An improvement in our understanding of metformin's molecular targets is likely to enable target-based identification of second-generation drugs with similar properties, a development that has been impossible up to now. The notion that 5' AMP-activated protein kinase (AMPK) mediates the anti-hyperglycaemic action of metformin has recently been challenged by genetic loss-of-function studies, thrusting the AMPK-independent effects of the drug into the spotlight for the first time in more than a decade. Key AMPK-independent effects of the drug include the mitochondrial actions that have been known for many years and which are still thought to be the primary site of action of metformin. Coupled with recent evidence of AMPK-independent effects on the counter-regulatory hormone glucagon, new paradigms of AMPK-independent drug action are beginning to take shape. In this review we summarise the recent research developments on the molecular action of metformin.

The anti-neurodegenerative agent clioquinol regulates the transcription factor FOXO1a.

Many diseases of aging including AD (Alzheimer's disease) and T2D (Type 2 diabetes) are strongly associated with common risk factors, suggesting that there may be shared aging mechanisms underlying these diseases, with the scope to identify common cellular targets for therapy. In the present study we have examined the insulin-like signalling properties of an experimental AD 8-hydroxyquinoline drug known as CQ (clioquinol). The IIS [insulin/IGF-1 (insulin-like growth factor-1) signalling] kinase Akt/PKB (protein kinase B) inhibits the transcription factor FOXO1a (forkhead box O1a) by phosphorylating it on residues that trigger its exit from the nucleus. In HEK (human embryonic kidney)-293 cells, we found that CQ treatment induces similar responses. A key transcriptional response to IIS is the inhibition of hepatic gluconeogenic gene expression, and, in rat liver cells, CQ represses expression of the key gluconeogenic regulatory enzymes PEPCK (phosphoenolpyruvate carboxykinase) and G6Pase (glucose-6-phosphatase). The effects on FOXO1a and gluconeogenic gene expression require the presence of Zn2+ ions, reminiscent of much earlier studies examining diabetogenic properties of 8-hydroxyquinolines. Comparative investigation of the signalling properties of a panel of these compounds demonstrates that CQ alone exhibits FOXO1a regulation without diabetogenicity. Our results suggest that Zn2+-dependent regulation of FOXOs and gluconeogenesis may contribute to the therapeutic properties of this drug. Further investigation of this signalling response might illuminate novel pharmacological strategies for the treatment of age-related diseases.

Metformin: evidence from preclinical and clinical studies for potential novel applications in cardiovascular disease.

INTRODUCTION: For a long time, metformin has been the first-line treatment for glycemic control in type 2 diabetes; however, the results of recent cardiovascular outcome trials of sodium-glucose co-transporter 2 inhibitors and glucagon-like peptide 1 receptor agonists have caused many to question metformin's position in the guidelines. Although there are several plausible mechanisms by which metformin might have beneficial cardiovascular effects, for example, its anti-inflammatory effects and metabolic properties, and numerous observational data suggesting improved cardiovascular outcomes with metformin use, the main randomized clinical trial data for metformin was published over 20 years ago. Nevertheless, the overwhelming majority of participants in contemporary type 2 diabetes trials were prescribed metformin. AREAS COVERED: In this review, we will summarize the potential mechanisms of cardiovascular benefit with metformin, before discussing clinical data in individuals with or without diabetes. EXPERT OPINION: Metformin may have some cardiovascular benefit in patients with and without diabetes, however the majority of clinical trials were small and are before the use SGLT2 inhibitors and GLP1-RAs. Larger contemporary randomized trials, with metformin evaluating its cardiovascular benefit are warranted.

Inflammation as a therapeutic target in heart failure with preserved ejection fraction.

Heart failure with preserved ejection fraction (HFpEF) accounts for around half of all cases of heart failure and may become the dominant type of heart failure in the near future. Unlike HF with reduced ejection fraction there are few evidence-based treatment strategies available. There is a significant unmet need for new strategies to improve clinical outcomes in HFpEF patients. Inflammation is widely thought to play a key role in HFpEF pathophysiology and may represent a viable treatment target. In this review focusing predominantly on clinical studies, we will summarise the role of inflammation in HFpEF and discuss potential therapeutic strategies targeting inflammation.

AMPK integrates metabolite and kinase-based immunometabolic control in macrophages.

OBJECTIVE: Previous mechanistic studies on immunometabolism have focused on metabolite-based paradigms of regulation, such as itaconate. Here, we, demonstrate integration of metabolite and kinase-based immunometabolic control by AMP kinase. METHODS: We combined whole cell quantitative proteomics with gene knockout of AMPKα1. RESULTS: Comparing macrophages with AMPKα1 catalytic subunit deletion with wild-type, inflammatory markers are largely unchanged in unstimulated cells, but with an LPS stimulus, AMPKα1 knockout leads to a striking M1 hyperpolarisation. Deletion of AMPKα1 also resulted in increased expression of rate-limiting enzymes involved in itaconate synthesis, metabolism of glucose, arginine, prostaglandins and cholesterol. Consistent with this, we observed functional changes in prostaglandin synthesis and arginine metabolism. Selective AMPKα1 activation also unlocks additional regulation of IL-6 and IL-12 in M1 macrophages. CONCLUSIONS: Together, our results validate AMPK as a pivotal immunometabolic regulator in macrophages.

Amino acid homeostasis is a target of metformin therapy.

OBJECTIVE: Unexplained changes in regulation of branched chain amino acids (BCAA) during diabetes therapy with metformin have been known for years. Here we have investigated mechanisms underlying this effect. METHODS: We used cellular approaches, including single gene/protein measurements, as well as systems-level proteomics. Findings were then cross-validated with electronic health records and other data from human material. RESULTS: In cell studies, we observed diminished uptake/incorporation of amino acids following metformin treatment of liver cells and cardiac myocytes. Supplementation of media with amino acids attenuated known effects of the drug, including on glucose production, providing a possible explanation for discrepancies between effective doses in vivo and in vitro observed in most studies. Data-Independent Acquisition proteomics identified that SNAT2, which mediates tertiary control of BCAA uptake, was the most strongly suppressed amino acid transporter in liver cells following metformin treatment. Other transporters were affected to a lesser extent. In humans, metformin attenuated increased risk of left ventricular hypertrophy due to the AA allele of KLF15, which is an inducer of BCAA catabolism. In plasma from a double-blind placebo-controlled trial in nondiabetic heart failure (trial registration: NCT00473876), metformin caused selective accumulation of plasma BCAA and glutamine, consistent with the effects in cells. CONCLUSIONS: Metformin restricts tertiary control of BCAA cellular uptake. We conclude that modulation of amino acid homeostasis contributes to therapeutic actions of the drug.

Factors affecting resting heart rate in free-living healthy humans.

Resting heart rate (RHR) is a potential cardiac disease prevention target because it is strongly associated with cardiac morbidity and mortality, yet community-based monitoring of RHR remains in its infancy. Recently, smartwatches have become available enabling measurement with non-intrusive devices of relationships between RHR and other factors outside the laboratory. We carried out cross-sectional observational retrospective analysis of anonymised smartwatch data obtained by participants in their everyday lives between 2016 and 2021 in a single centre community-based study, using convenience sampling. Between participants, overall RHR means strongly or moderately inversely correlated with means of stand hour (SH), calculated VO2 max, walking and running distance (WRD), steps and flights climbed (FC). Within participants, in quarterly averages, RHR inversely correlated moderately with frequency of standing (stand hours, SH). RHR also inversely correlated moderately with heart rate variability (HRV), consistent with the known impact of increasing parasympathetic dominance on RHR. These within participant correlations suggest that RHR might be modifiable by changes in SH and HRV within individuals. Indeed, analysing paired daily data, relationships between these three categories were dose dependent. 15 SH versus 5 SH associated with a reduction of 10 beats per minute in mean RHR and increase in mean HRV of 14 ms, respectively. We conclude that within individuals, RHR inversely correlates with frequency of standing and HRV, with paired daily measurements indicating effects are mediated that day. RHR also inversely correlates with fitness and activity measures between participants. Our findings provide initial community-based observational evidence supporting further prospective interventional investigation of frequency of standing or HRV modifiers, alongside more familiar interventions, for cardiac disease prevention.

Left Ventricular Hypertrophy in Diabetic Cardiomyopathy: A Target for Intervention.

Heart failure is an important manifestation of diabetic heart disease. Before the development of symptomatic heart failure, as much as 50% of patients with type 2 diabetes mellitus (T2DM) develop asymptomatic left ventricular dysfunction including left ventricular hypertrophy (LVH). Left ventricular hypertrophy (LVH) is highly prevalent in patients with T2DM and is a strong predictor of adverse cardiovascular outcomes including heart failure. Importantly regression of LVH with antihypertensive treatment especially renin angiotensin system blockers reduces cardiovascular morbidity and mortality. However, this approach is only partially effective since LVH persists in 20% of patients with hypertension who attain target blood pressure, implicating the role of other potential mechanisms in the development of LVH. Moreover, the pathophysiology of LVH in T2DM remains unclear and is not fully explained by the hyperglycemia-associated cellular alterations. There is a growing body of evidence that supports the role of inflammation, oxidative stress, AMP-activated kinase (AMPK) and insulin resistance in mediating the development of LVH. The recognition of asymptomatic LVH may offer an opportune target for intervention with cardio-protective therapy in these at-risk patients. In this article, we provide a review of some of the key clinical studies that evaluated the effects of allopurinol, SGLT2 inhibitor and metformin in regressing LVH in patients with and without T2DM.

Metformin: still the sweet spot for CV protection in diabetes?

Metformin remains the first-line drug treatment for type 2 diabetes (T2D) in most guidelines not only because it achieves significant reduction in HbA1c but also because of a wealth of clinical experience regarding its safety and observational data that has shown that metformin use is associated with lower mortality rates when compared to sulphonylureas or insulin. Recently other diabetes drugs, particularly SGLT2 inhibitors (SGLT2i) and GLP1 receptor agonists (GLP1RA), have attracted considerable attention for their cardioprotective benefits reported in cardiovascular outcome trials (CVOTs). Randomised control trials on these newer drugs are on a larger scale but have shorter follow-up than UKPDS, the main study supporting metformin use. In a recent change to the European Society of Cardiology guidelines, metformin was replaced by SGLT2i and GLP1RA as first-line for T2D with atherosclerotic cardiovascular disease, whereas American Diabetes Association and UK-wide guidelines maintain metformin as first choice drug pharmacotherapy for all T2D. A definitive evidence-base for prioritisation of these drugs is currently missing because there are no head-to-head clinical trial data. Without such trials being forthcoming, innovative, pragmatic and low-cost 'real-world' trial approaches based on electronic health records may need to be harnessed to determine the correct priority, combinations of drugs and/or identify-specific patient populations most likely to benefit from each one.

Epigallocatechin gallate (EGCG) mimics insulin action on the transcription factor FOXO1a and elicits cellular responses in the presence and absence of insulin.

The green tea flavonoid epigallocatechin gallate (EGCG) is one of several compounds that have been reported to have insulin-like glucose-lowering properties in mammals. EGCG is understood to act at least in part by repression of gluconeogenic genes such as phosphoenolpyruvate carboxykinase but the transcription factors that are targeted to achieve this are unknown. We show here that EGCG induces phosphorylation of insulin-sensitive residues on the transcription factor FOXO1a. Like insulin, EGCG induced FOXO1a phosphorylation is abolished by the PtdIns 3-kinase inhibitor LY294002 but not by PD98059 (an inhibitor of mitogen-activated protein kinase cascade) or by rapamycin (an inhibitor of signalling to p70 S6 kinase). EGCG differs from insulin and IGF-1 however, in that its induction of FOXO1a phosphorylation is sensitive to scavengers of reactive oxygen species (ROS). These results indicate that EGCG exerts its insulin mimetic effects at least in part by phosphorylation of the FOXOs through a mechanism that is similar but not identical to insulin and IGF-1 induced FOXO phosphorylation. Our results suggest that agents acting in the manner of EGCG may be useful antidiabetic agents.

Metformin selectively targets redox control of complex I energy transduction.

Many guanide-containing drugs are antihyperglycaemic but most exhibit toxicity, to the extent that only the biguanide metformin has enjoyed sustained clinical use. Here, we have isolated unique mitochondrial redox control properties of metformin that are likely to account for this difference. In primary hepatocytes and H4IIE hepatoma cells we found that antihyperglycaemic diguanides DG5-DG10 and the biguanide phenformin were up to 1000-fold more potent than metformin on cell signalling responses, gluconeogenic promoter expression and hepatocyte glucose production. Each drug inhibited cellular oxygen consumption similarly but there were marked differences in other respects. All diguanides and phenformin but not metformin inhibited NADH oxidation in submitochondrial particles, indicative of complex I inhibition, which also corresponded closely with dehydrogenase activity in living cells measured by WST-1. Consistent with these findings, in isolated mitochondria, DG8 but not metformin caused the NADH/NAD+ couple to become more reduced over time and mitochondrial deterioration ensued, suggesting direct inhibition of complex I and mitochondrial toxicity of DG8. In contrast, metformin exerted a selective oxidation of the mitochondrial NADH/NAD+ couple, without triggering mitochondrial deterioration. Together, our results suggest that metformin suppresses energy transduction by selectively inducing a state in complex I where redox and proton transfer domains are no longer efficiently coupled.