In This Issue of Diabetes

Mitochondria, widely regarded as the energy source of cells, and specifically changes in their architecture and how they deal with fusion and fission, might be a key feature of type 2 diabetes and obesity. And, on the basis of new data reported this month, a potential opportunity might exist to target a specific gene that controls mitochondrial architecture to improve the effects of metformin. According to Kulkarni et al. (p. 3552), who report a series of controlled mice experiments, deleting the liver-specific gene mitofusin 1 (Mfn1) led to a dramatic fragmentation of mitochondrial networks. This subsequently resulted in enhanced cellular lipid sizes and an enhanced preference for lipids as the main energy source in the mice. However, despite the fragmentation of the networks, the actual function in terms of respiratory capacity of the mitochondria increased as a result of the gene deletion. The authors report that, overall, the modifications resulted in the mice with the deletion having protection against glucose intolerance and insulin resistance induced by a high-fat diet and the mice were more sensitive towards metformin intervention, i.e., that the hypoglycemic effects of metformin were enhanced. However, a more fundamental observation of the authors is that mitochondrial fragmentation might actually be an adaptive response to better handle lipid overflow from the diet and that it might not be deleterious per se. They stress, however, that fission and fragmentation of mitochondria are not exclusively under the control of Mfn1 and that other targets should be considered if such a mechanism is used to enhance metformin effectiveness and thus glycemic control. The enhanced effects of metformin could clearly be of interest in diseases such as type 2 diabetes and obesity, and the authors go on to detail some of the ways that could be used to target Mfn1 levels.

Kulkarni et al. Mfn1 deficiency in the liver protects against diet-induced insulin resistance and enhances the hypoglycemic effect of metformin. Diabetes 2016;65:3552–3560

An investigation into the metabolic consequences of metformin treatment in patients with type 2 diabetes has revealed more putative metabolites involved in the action of the drug. Specifically the studies show that a compound called citrulline, an α-amino acid, has reduced relative concentrations in blood, whereas another unidentified metabolite termed X-21365 is raised following metformin treatment. The findings reported by Adam et al. (p. 3776) come from the KORA (Cooperative Health Research in the Region of Augsburg) cohort (follow-up survey no. 4) and involve the application of a nontargeted metabolomics approach to examine serum samples from patients with type 2 diabetes taking metformin or not. Reportedly, the authors used the approach to examine 353 metabolites in serum samples, and following the application of multivariable linear regression models, they managed to identify the two compounds as being significantly associated with metformin intake. Further analysis (using less stringent statistical criteria) also revealed ∼44 additional compounds that might also be associated with metformin intake. To confirm their observations, the authors then used longitudinal samples (two time points) from the same individuals in the KORA cohort and also a translational approach involving four tissues of db/db mice. Consistently, citrulline was significantly associated metformin treatment in mouse plasma, skeletal muscle, and epididymal adipose tissue but not in liver. According to the authors, the reduced citrulline is likely the result of metformin’s action on the nitric oxide and urea cycles. They also suggest that the increased levels of the unidentified compound, which they speculate may be 5-trimethylaminovalerate, might be related to the actions of the gastrointestinal microbiota. Commenting more widely on the study, author Rui Wang-Sattler told Diabetes: “Our analysis indicates that the activation of the AMPK pathway by metformin affects nitrogen and urea metabolism through eNOS [endothelial nitric oxide synthase], which thus lowers the citrulline levels. We speculate that the additional intake of citrulline could have a positive effect on the cardiovascular system in patients being treated with metformin.”

Adam et al. Metformin effect on nontargeted metabolic profiles in patients with type 2 diabetes and in multiple murine tissues. Diabetes 2016;65:3776–3785

The effects of metformin include improved cardiovascular outcomes in type 2 diabetes and yet the exact mechanisms behind the effects remain debatable. The application of a relatively new imaging technique has revealed that metformin likely does have significant effects on cardiac redox state and pyruvate metabolism. The study by Lewis et al. (p. 3544) involved both acute intravenous metformin infusion and chronic administration of metformin to rats to study pyruvate metabolism via an approach called hyperpolarized [1-¹³C]pyruvate magnetic resonance spectroscopy. Reportedly, both administration routes resulted in a significantly increased lactate:pyruvate ratio in both cardiac and liver tissues and also a significant increase in redox state. The approach was also able to localize the effects to the left ventricular myocardium, implying that metformin was having a direct effect in the heart. According to the authors, the data suggest that the interconversion of pyruvate and lactate is likely cytosol based and probably mediated via a redox-dependent mechanism. They suggest that the major mechanism by which metformin induces effects on hyperglycemia is therefore likely to be primarily via inhibition of gluconeogenesis rather than any other route. Perhaps the biggest outcome of the study though is the demonstration that the approach works and that it is possible to assess such redox shifts in vivo, which the authors say has considerable translational potential. Assessing the effects of metformin in humans is one step that could bring considerable insights for type 2 diabetes and potentially for the off-label use of metformin to heart failure in general. Commenting more widely on the study, author Damian J. Tyler said: “Our work shows the potential for hyperpolarized pyruvate magnetic resonance to provide more information on the cardiovascular effects of both existing and novel therapeutics. The ongoing work we are undertaking to translate this technology into clinical studies is opening up a new approach to studying metabolic changes in the human heart.”

Lewis et al. Assessment of metformin-induced changes in cardiac and hepatic redox state using hyperpolarized [1-¹³C]pyruvate. Diabetes 2016;65:3544–3551

The ability to detect renal function loss in type 2 diabetes is not optimal according to Saulnier et al. (p. 3744), but a solution may exist in measuring serum advanced glycation end products. If proven, this may mean a simple blood test performed over sequential years should be able to detect the risk of kidney failure ahead of it actually happening. The outcome is the result of a study that focused on the Pima Indians in the U.S., a population with a very high prevalence of type 2 diabetes. Following a long-term study of the population, a subpopulation was followed up to track the correlation between renal function loss and levels of serum advanced glycation end products. The authors also looked at the correlation between these compounds and the differences in kidney structure among those at different stages of kidney disease. A variety of compounds did correlate with renal function loss. The major compound to correlate was methylglyoxal hydroimidazolone, as well as other compounds that included carboxyethyl lysine. Indeed, each doubling of carboxyethyl lysine and methylglyoxal hydroimidazolone suggested an increased hazard of renal function loss of between 30% and 60%. A range of the compounds also correlated with specific renal lesions, supporting the direct association with renal function loss. Commenting on the wider implications of the study, senior author Paul J. Beisswenger said: “Patients with diabetes and their caregivers have not had a test to identify the risk of eventual kidney failure during the long ‘silent period’ when diabetic kidney disease is not apparent. Finding that specific advanced glycation end products predicted both clinical and morphological manifestations of diabetic kidney disease in the Pima Indian Community is consistent with prior findings in type 1 diabetes and confirms that these products reflect important pathobiological processes. These accumulating data support the potential of a new test that could identify risk and change the current paradigm for prevention of this devastating complication of diabetes.”

Saulnier et al. Advanced glycation end products predict loss of renal function and correlate with lesions of diabetic kidney disease in American Indians with type 2 diabetes. Diabetes 2016;65:3744–3753