Role of organic cation/carnitine transporter 1 in uptake of phenformin and inhibitory effect on complex I respiration in mitochondria.

Phenformin causes lactic acidosis in clinical situations due to inhibition of mitochondrial respiratory chain complex I. It is reportedly taken up by hepatocytes and exhibits mitochondrial toxicity in the liver. In this study, uptake of phenformin and [(14)C]tetraethylammonium (TEA) and complex I inhibition by phenformin were examined in isolated liver and heart mitochondria. Uptake of phenformin into isolated rat liver mitochondria was higher than that into heart mitochondria. It was inhibited by several cat ionic compounds, which suggests the involvement of multispecific transport system(s). Similar characteristics were also observed for uptake of TEA; however, uptake of phenformin into mitochondria of organic cation/carnitine transporter 1 (OCTN1) knockout mice was lower than that in wild-type mice, whereas uptake of TEA was comparable between the two strains, suggesting the involvement of distinct transport mechanisms for these two cations in mitochondria. Inhibition by phenformin of oxygen consumption via complex I respiration in isolated rat liver mitochondria was greater than that in heart mitochondria, whereas inhibitory effect of phenformin on complex I respiration was similar in inside-out structured submitochondrial particles prepared from rat livers and hearts. Lactic acidosis provoked by iv infusion of phenformin was weaker in octn1(-/-) mice than that in wild-type mice. These observations suggest that uptake of phenformin into liver mitochondria is at least partly mediated by OCTN1 and functionally relevant to its inhibition potential of complex I respiration. This study was, thus, the first to demonstrate OCTN1-mediated mitochondrial transport and toxicity of biguanide in vivo in rodents.

Multiple mechanisms underlying troglitazone-induced mitochondrial permeability transition.

Troglitazone, a thiazolidinedione class antidiabetic drug, was withdrawn from the market because of its severe idiosyncratic hepatotoxicity. It causes a mitochondrial permeability transition (MPT), which may in part contribute to its hepatotoxicity. In the present study, the mechanism of troglitazone mitochondrial toxicity was investigated in isolated rat liver mitochondria. Mitochondrial swelling induced by 10 µM troglitazone was attenuated by bromoenol lactone (BEL), an inhibitor of Ca²+-independent phospholipase A2 (iPLA2). In contrast, that induced by 50 µM troglitazone was exacerbated by BEL. This exacerbation was diminished by addition of 2mM glutathione, an antioxidant. Oxygen consumption by state 3 respiration in isolated mitochondria was also decreased by troglitazone, but it was not affected by BEL. Mitochondrial swelling induced by 10 µM troglitazone was completely attenuated in the absence of Ca²+ while that induced by 50 µM troglitazone was not affected. Addition of 1 µM cyclosporin A (CsA), an inhibitor of MPT pores, completely attenuated swelling induced by 10 µM troglitazone while it only partly diminished that induced by 50 µM troglitazone. Thus, the MPT induced by 10 and 50 µM troglitazone are regulated by different mechanism; the MPT induced by 10 µM troglitazone is regulated by the activation of iPLA2 and caused by the opening of CsA-regulating MPT pores followed by accumulation of Ca²+ in mitochondria, while that induced by 50 µM troglitazone is partly regulated by reactive oxygen species and mainly caused by the opening of CsA-insensitive MPT pores.