The desert gerbil Psammomys obesus as a model for metformin-sensitive nutritional type 2 diabetes to protect hepatocellular metabolic damage: Impact of mitochondrial redox state

Abstract

Introduction: While metformin (MET) is the most widely prescribed antidiabetic drug worldwide, its beneficial effects in Psammomys obesus (P. obesus), a rodent model that mimics most of the metabolic features of human diabetes, have not been explored thoroughly. Here, we sought to investigate whether MET might improve insulin sensitivity, glucose homeostasis, lipid profile as well as cellular redox and energy balance in P. obesus maintained on a high energy diet (HED).

Materials and methods: P. obesus gerbils were randomly assigned to receive either a natural diet (ND) consisting of halophytic plants (control group) or a HED (diabetic group) for a period of 24 weeks. MET (50 mg/kg per os) was administered in both animal groups after 12 weeks of feeding, i.e., the time required for the manifestation of insulin resistance in P. obesus fed a HED. Parallel in vitro experiments were conducted on isolated hepatocytes that were shortly incubated (30 min) with MET and energetic substrates (lactate + pyruvate or alanine, in the presence of octanoate).

Results: In vivo, MET lowered glycemia, glycosylated haemoglobin, circulating insulin and fatty acid levels in diabetic P. obesus. It also largely reversed HED-induced hepatic lipid alterations. In vitro, MET increased glycolysis but decreased both gluconeogenesis and ketogenesis in the presence of glucogenic precursors and medium-chain fatty acid. Importantly, these changes were associated with an increase in cytosolic and mitochondrial redox states along with a decline in respiration capacity.

Conclusions: MET prevents the progression of insulin resistance in diabetes-prone P. obesus, possibly through a tight control of gluconeogenesis and fatty acid β-oxidation depending upon mitochondrial function. While the latter is increasingly becoming a therapeutic issue in diabetes, the gut microbiota is another promising target that would need to be considered as well.

Fig 1. Effect of metformin on both cytosolic and mitochondrial redox states (NADH/NAD⁺) in P. obesus fed ND or HED.

Hepatocytes were incubated for 30 min with energy substrates in the absence or presence of metformin at the indicated concentrations before calculating the lactate/pyruvate ratio (A) and β-hydroxybutyrate/acetoacetate (β-HB/AcAc) ratio (B and C). Data are means ± SEM of 15 separate experiments. *p < 0.05; **p < 0.01 compared with the group without metformin for each corresponding dietary condition.

Fig 2. Effect of metformin on both cytosolic and mitochondrial phosphate potential (ATP/ADP) in P. obesus fed ND or HED.

Hepatocytes were incubated for 30 min with alanine + octanoate (A and B) or lactate + pyruvate + octanoate (C and D) in the absence or presence of metformin at the indicated concentrations, before assaying the content in adenine nucleotides within each compartment. Data are means ± SEM of 15 separate experiments. *p < 0.05; **p < 0.01; ***p < 0.001 compared with the group without metformin for each corresponding dietary condition.

Fig 3. Summary of the metformin effects on diabetic syndrome evolution in P. obesus fed a HED.

P. obesus is a unique gerbil species predisposed to develop morbid obesity, ingestion of HED induces a high flux of free fatty acids which first activate gluconeogenesis (causing glucose intolerance followed by hyperglycemia), and later become toxic for the animals (leading to dyslipidemia, ketoacidosis and energy deficit). Metformin is then able to hamper this cascade of detrimental events or processes by mainly acting at the intracellular level (i.e., mitochondria), with positive influence on systemic metabolism homeostasis through improvement of insulin resistance. See the text for details.