Endocrine Abstracts

Introduction: Thiazolidinediones (TZDs) are synthetic insulin-sensitizing drugs used in the treatment of type 2 diabetes. They work as agonists of peroxisome proliferator-activated receptor γ (PPARg). TZDs action via PPARg does not exclude independent PPARg-activation effects. Our research group has recently reported that TZDs also execute direct effects implicated in glucose homeostasis. However, its mitochondrial action PPARg-independent has not been fully studied yet.

Aim: To investigate direct mitochondrial effect of pioglitazone (PIO) and rosiglitazone (ROSI) on isolated liver mitochondria.

Materials and methods: Liver mitochondria were obtained from fed male Wistar rats according to standard differential centrifugation procedures. Oxygen consumption rates were measured polarographically at 37 °C using a Clark-type oxygen electrode, whereas ROS (free radicals) production was assayed by incubating mitochondria in a stirred 2 ml volume with 10 UI HRP and 2 μM Amplex Red, and was calibrated against H₂O₂ standards. Moreover, activity of mitochondrial respiratory chain complexes I, II and III were spectrometrically determined.

Results: Both TZDs, at all tested doses (10, 25, 50, and 100 μM), inhibited oxygen consumption (JO₂) in a dose-dependent manner when glutamate and malate (GM) was used as substrate. This inhibition affected the state 3 of respiration (in the presence of ADP) and the uncoupled state (after addition of dinitrophenol). Maximal JO₂ inhibition was about 50% with the highest dose of glitazones. On the other hand, we found a constant inhibition of JO₂ in a non does-dependent manner when we employed succinate and malate (SM) as substrate. PIO and ROSI dramatically reduced ROS production from mitochondria energized with GM or SM by 50 and 80%, respectively. Finally, by investigating the direct effect of TZDs on the activity of respiratory complexes (I, II and III), we found that both glitazones specifically inhibited the activity of complexes I and III by 20 and 25%, respectively.

Conclusions: Pioglitazone and rosiglitazone reduce both mitochondrial respiration and ROS production acutely and PPARg-independent way, through inhibition of respiratory complexes I and III activities. This new finding could contribute to their antidiabetic properties.