Troglitazone induces cytotoxicity in part by promoting the degradation of peroxisome proliferator-activated receptor γ co-activator-1α protein

BACKGROUND AND PURPOSE Troglitazone (Tro), rosiglitazone (Rosi) and pioglitazone (Pio) are anti-diabetic thiazolidinediones that function as ligands for peroxisome proliferator-activated receptor gamma (PPAR gamma); however, Tro has been withdrawn from the market due to liver toxicity issues. Mitochondrial dysfunction induced by Tro has been suggested to be an important mechanism behind its cytotoxicity. Constitutively active nuclear hormone receptors, oestrogen-related receptor alpha and gamma are thought to regulate mitochondrial mass and oxidative phosphorylation together with their co-activators PPAR gamma co-activator-1 alpha and -1 beta (PGC-1 alpha and PGC-1 beta). Hence, in this study, we investigated whether Tro affects the expression and activity levels of these regulators. EXPERIMENTAL APPROACH Cellular viability was measured by an ATP-based assay. Mitochondrial mass and reactive oxygen species (ROS) were quantified by two different fluorogenic probes. Apoptosis was measured by an Annexin-V-based kit. Gene expression at the levels of mRNA and protein was measured by quantitative RT-PCR and Western analysis. Over-expression of PGC-1 alpha was mediated by an adenovirus. KEY RESULTS Tro, but not Rosi or Pio, selectively stimulated PGC-1 alpha protein degradation. As a result, Tro reduced mitochondrial mass, and superoxide dismutases 1 and 2 expressions, but induced ROS to initiate apoptosis. Using a ubiquitin-proteasome inhibitor MG132, it was established that blocking PGC-1 alpha degradation partially suppressed the reduction of mitochondrial mass. Importantly, over-expressing PGC-1 alpha partially restored the Tro-suppressed mitochondrial mass and attenuated the cytotoxic effects of Tro. CONCLUSIONS AND IMPLICATIONS Collectively, these results suggest that PGC-1 alpha degradation is an important mechanism behind the cytotoxic effects of Tro in the liver.