Troglitazone induces a rapid drop of mitochondrial membrane potential in liver HepG2 cells

Sensitivity of liver injury in heterozygous Sod2 knockout mice treated with troglitazone or acetaminophen

Recently, it was reported that the intraperitoneal administration of 30 mg/kg/day troglitazone to heterozygous superoxide dismutase 2 gene knockout (Sod2+/-) mice for twenty-eight days caused liver injury, manifested by increased serum ALT activity and hepatic necrosis. Therefore, we evaluated the reproducibility of troglitazone-induced liver injury in Sod2+/- mice, as well as their validity as an animal model with higher sensitivity to mitochondrial toxicity by single-dose treatment with acetaminophen in Sod2+/- mice. Although we conducted a repeated dose toxicity study in Sod2+/- mice treated orally with 300 mg/kg/day troglitazone for twenty-eight days, no hepatocellular necrosis was observed in our study. On the other hand, six hours and twenty-four hours after an administration of 300 mg/kg acetaminophen, plasma ALT activity was significantly increased in Sod2+/- mice, compared to wild-type mice. In particular, six hours after administration, hepatic centrilobular necrosis was observed only in Sod2+/- mice. These results suggest that Sod2+/- mice are valuable as an animal model with higher sensitivity to mitochondrial toxicity. On the other hand, it was suggested that the mitochondrial damage alone might not be the major cause of the troglitazone-induced idiosyncratic liver injury observed in humans.

Troglitazone-induced hepatic mitochondrial proteome expression dynamics in heterozygous Sod2(+/-) mice: two-stage oxidative injury

The determinants of susceptibility to troglitazone-induced idiosyncratic liver injury have not yet been determined; however, troglitazone has been shown to target mitochondria and induce mitochondria-mediated hepatocellular injury in vitro. The aim of this study was to use a systems approach to analyze the dynamics of mitochondrial changes at the proteome level and more clearly define the mechanisms and time course of troglitazone hepatotoxicity by using a previously characterized mouse model that is highly sensitized to troglitazone hepatotoxicity. Mice heterozygous in mitochondrial superoxide dismutase-2 (Sod2(+/-)) were injected intraperitoneally with troglitazone (30 mg/kg/day) or vehicle daily for 2 or 4 weeks. Hepatic mitochondria were isolated, purified, and subjected to two-dimensional difference gel electrophoresis (2D-DIGE). We found that among the ~1500 resolved hepatic mitochondrial proteins, 70 exhibited significantly altered abundance after troglitazone treatment. MALDI-TOF/TOF MS/MS analysis revealed that early changes (2 weeks) included increased levels of heat shock protein family members (mortalin, HSP7C), Lon protease, and catalase, indicating induction of a mitochondrial stress response. In contrast, after 4 weeks, a number of critical proteins including ATP synthase beta-subunit, aconitase-2, and catalase exhibited decreased abundance, and total protein carbonyls were significantly increased, suggesting uncompensated oxidative damage. Aconitase-2 (ACO2) was decreased at both time points, making this protein a potential sensitive and early biomarker for mitochondrial oxidant stress. These results show that, in this murine model of underlying clinically silent mitochondrial stress, superimposed troglitazone induces a two-stage response: an initial adaptive response, followed by a toxic response involving oxidant injury to mitochondrial proteins.

The significance of mitochondrial toxicity testing in drug development

Mitochondrial dysfunction is increasingly implicated in the etiology of drug-induced toxicities. Members of diverse drug classes undermine mitochondrial function, and among the most potent are drugs that have been withdrawn from the market, or have received Black Box warnings from the FDA. To avoid mitochondrial liabilities, routine screens need to be positioned within the drug-development process. Assays for mitochondrial function, cell models that better report mitochondrial impairment, and new animal models that more faithfully reflect clinical manifestations of mitochondrial dysfunction are discussed in the context of how such data can reduce late stage attrition of drug candidates and can yield safer drugs in the future.

Glitazones induce astroglioma cell death by releasing reactive oxygen species from mitochondria: modulation of cytotoxicity by nitric oxide

The glitazones (or thiazolidinediones) are synthetic compounds used in type-2 diabetes, but they also have broad antiproliferative and anti-inflammatory properties still not well understood. We described previously the apoptotic effects of glitazones on astroglioma cells ( J Biol Chem 279: 8976-8985, 2004 ). At certain concentrations, we found a selective lethality on glioma cells versus astrocytes that was dependent on a rapid production of reactive oxygen species (ROS) and seemed unrelated to the receptor peroxisome proliferator activated receptor-gamma. The present study was aimed at characterizing the oxygen derivatives induced by ciglitazone, rosiglitazone, and pioglitazone in C6 glioma cells and to investigate their intracellular source. We examined the interaction of ROS with nitric oxide (NO) and its consequences for glioma cell survival. Fluorescence microscopy and flow cytometry showed that glitazones induced superoxide anion, peroxynitrite, and hydrogen peroxide, with ciglitazone being the most active. ROS production was completely prevented by uncoupling of the electron transport chain and by removal of glucose as an energy substrate, whereas it was unaffected by inhibition of NADPH-oxidase and xanthine-oxidase. Moreover, glitazones inhibited state 3 respiration in permeabilized cells, and experiments with mitochondrial inhibitors suggested that complex I was the likely target of glitazones. Therefore, these results point to the mitochondrial electron transport chain as the source of glitazone-induced ROS in C6 cells. Glitazones also depolarized mitochondria and reduced mitochondrial pH. NO synthase inhibitors revealed that superoxide anion combines with NO to yield peroxynitrite and that the latter contributes to the cytotoxicity of glitazones in astroglioma cells. Future antitumoral strategies may take advantage of these findings.

Mechanism of thiazolidinedione-dependent cell death in Jurkat T cells

Thiazolidinediones are synthetic agonists for the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) and are therapeutically used as insulin sensitizers. Besides therapeutical benefits, potential side effects such as the induction of cell death by thiazolidinediones deserve consideration. Although PPARgamma-dependent and -independent cell death in response to thiazolidinediones has been described, we provide evidence supporting a new mechanism to account for thiazolidinedione-initiated but PPARgamma-independent cell demise. In Jurkat T cells, ciglitazone and troglitazone provoked rapid and dose-dependent cell death, whereas rosiglitazone did not alter cell viability. We found induction of apoptosis by troglitazone, whereas ciglitazone caused necrosis. Because preincubation with the reactive oxygen species (ROS) scavengers manganese (III) tetrakis(4-benzoic acid) porphyrin and vitamin C significantly inhibited ciglitazone- and partially troglitazone-mediated cell death, we suggest that ROS contribute to cytotoxicity. Assuming that ROS originate from mitochondria, studies in submitochondrial particles demonstrated that all thiazolidinediones inhibited complex I of the mitochondrial respiratory chain. However, only ciglitazone and troglitazone lowered complex II activity as well. Pharmacological inhibition of complexes I and II documented that complex II inhibition in Jurkat cells caused massive apoptotic cell death, whereas inhibition of complex I provoked only marginally apoptosis after 4-h treatment. Therefore, inhibition of complex II by ciglitazone and troglitazone is the main trigger of cell death. ATP depletion by ciglitazone, in contrast to troglitazone, is responsible for induction of necrosis. Our results demonstrate that despite their similar molecular structure, thiazolidinediones differently affect cell death, which might help to explain some adverse effects occurring during thiazolidinedione-based therapies.

Mitochondrial abnormalities--a link to idiosyncratic drug hepatotoxicity?

Idiosyncratic drug-induced liver injury (DILI) is a major clinical problem and poses a considerable challenge for drug development as an increasing number of successfully launched drugs or new potential drugs have been implicated in causing DILI in susceptible patient subsets. Although the incidence for a particular drug is very low (yet grossly underestimated), the outcome of DILI can be serious. Unfortunately, prediction has remained poor (both for patients at risk and for new chemical entities). The underlying mechanisms and the determinants of susceptibility have largely remained ill-defined. The aim of this review is to provide both clinical and experimental evidence for a major role of mitochondria both as a target of drugs causing idiosyncratic DILI and as mediators of delayed liver injury. We develop a unifying hypothesis that involves underlying genetic or acquired mitochondrial abnormalities as a major determinant of susceptibility for a number of drugs that target mitochondria and cause DILI. The mitochondrial hypothesis, implying gradually accumulating and initially silent mitochondrial injury in heteroplasmic cells which reaches a critical threshold and abruptly triggers liver injury, is consistent with the findings that typically idiosyncratic DILI is delayed (by weeks or months), that increasing age and female gender are risk factors and that these drugs are targeted to the liver and clearly exhibit a mitochondrial hazard in vitro and in vivo. New animal models (e.g., the Sod2(+/-) mouse) provide supporting evidence for this concept. However, genetic analyses of DILI patient samples are needed to ultimately provide the proof-of-concept.

Dephosphorylation of ribosomal protein P0 in response to troglitazone-induced cytotoxicity

Troglitazone (TRO)-induced cytotoxicity was investigated in HepG2 cells. The cells were exposed to TRO as well as rosiglitazone (RSG) at concentrations of 0, 25, 50 and 75 microM for 48 h. Total proteins were separated by two-dimensional electrophoresis and visualized by silver staining. We focused on a protein spot at an approximate molecular weight of 35 kDa and isoelectric point (pI) of 5.7, which appeared only with the cytotoxic concentrations (50 and 75 microM) of TRO, but not with the low concentration (25 microM) of TRO or any concentrations of RSG. This protein spot was subjected to amino acid sequence analysis and identified as ribosomal protein P0 (P0). Interestingly, without any significant induction of its protein and mRNA, P0 was dephosphorylated depending on the concentration- and time-dependent manner of TRO-induced cytotoxicity. Pretreatment with a general caspase inhibitor, Z-VAD.fmk, prevented cleavage of caspase-3 but demonstrated a slight improvement of cytotoxicity induced by TRO. Thus, these effects could not prevent the dephosphorylation of P0. Our results strongly suggest that a post-translational modification, dephosphorylation, of P0 is associated with TRO-induced cytotoxicity.

Mitochondrial permeability transition as a potential determinant of hepatotoxicity of antidiabetic thiazolidinediones

Troglitazone, a thiazolidinedione class of antidiabetic agent, causes serious idiosyncratic hepatotoxicity. Troglitazone is metabolized to a reactive metabolite that covalently binds to cellular macromolecules, but the role of the covalent adduct in the hepatotoxicity is controversial. Because troglitazone has been found to cause cytotoxicity to hepatocytes along with mitochondrial dysfunction, we investigated the effects of troglitazone and other thiazolidinediones on mitochondrial function by using liver mitochondria fraction isolated from male CD-1 mice. Incubation of energized mitochondria with succinate in the presence of Ca2+ and troglitazone induced mitochondrial swelling, and the swelling was partially inhibited by cyclosporin A. Troglitazone also induced decreases in mitochondrial membrane potential and mitochondrial Ca2+ accumulation. These results demonstrate that troglitazone induces mitochondrial permeability transition (MPT). Similar results were obtained for ciglitazone, whereas rosiglitazone and pioglitazone, which are less hepatotoxic than troglitazone, had little effect on these mitochondria functions. It is therefore possible that the troglitazone-induced opening of MPT pore, which is not induced by rosiglitazone or pioglitazone, may contribute to the hepatotoxicity induced specifically by troglitazone.

Metabolic and Non-Metabolic Factors Determining Troglitazone Hepatotoxicity: A Review

Troglitazone (TGZ), a thiazolidinedione class of antidiabetic agent, causes serious idiosyncratic hepatotoxicity. TGZ is metabolized into reactive metabolites that covalently bind to cellular macromolecules, one of which is oxidation at the chromane ring, a unique structure of TGZ, and another involves oxidative cleavage of the thiazolidinedione ring, a structure common to less hepatotoxic antidiabetics, rosiglitazone and pioglitazone. TGZ is cytotoxic to HepG2 cells and rat and human hepatocytes. However, the role of the reactive metabolite on the TGZ toxicity is controversial, because there was no correlation of the generation of the reactive metabolites with susceptibility to the TGZ cytotoxicity, and chemical inhibitors of drug metabolizing enzymes could not protect the cells against the toxicity. Mitochondrial dysfunction, especially mitochondrial permeability transition, may be a pathophysiological event, which is mediated by TGZ itself and is a major non-metabolic factor. Other events such as apoptosis and PPARgamma-dependent steatosis could be also mediated by TGZ, while inhibition of bile salt export pump, a cause of TGZ-induced cholestasis, may be caused by the TGZ sulfate. In conclusion, although the TGZ is biotransformed into chemically reactive metabolites, there is currently no potential evidence for involvement of the reactive metabolite in the TGZ-induced liver injury.

Receptor-independent actions of PPAR thiazolidinedione agonists: is mitochondrial function the key?

Agonists of the peroxisome proliferator activated receptor gamma (PPAR(gamma)) are currently used for treatment of type 2 diabetes due to their insulin sensitizing and glucose metabolism stabilizing effects. More recently some of these same agonists were shown to exert anti-inflammatory and anti-proliferative effects as well. Although PPAR(gamma) agonists can operate via receptor-mediated events occurring at the genomic level, thereby causing long lasting changes in gene expression patterns, recent studies demonstrate non-genomic as well as genomic actions, and receptor-dependent as well as receptor-independent effects of the thiazolidinedione (TZD) class of PPAR(gamma) agonists. In this review we will summarize data describing some of these novel, receptor independent actions of TZDs, review evidence that TZDs directly influence mitochondrial function, and attempt to reconcile how changes in mitochondrial function could contribute to other receptor-independent actions of these drugs.

Peroxisome proliferator-activated receptor gamma ligands induce cell cycle arrest and apoptosis in human renal carcinoma cell lines

AIM

To study the effect of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on cell proliferation and apoptosis in human renal carcinoma cell lines.

METHODS

The expression of PPARgamma was investigated by reverse transcriptase polymerase chain reaction (RT-PCR), Western blot and immunohistochemistry. The effect of thiazolidinedione (TZD) PPARgamma ligands on growth of renal cell carcinoma (RCC) cells was measured by MTT assay and flow cytometric analysis. Cell death ELISA, Hoechst 33342 fluorescent staining and DNA ladder assay were used to observe the effects of PPAR gamma ligands on apoptosis. Regulatory proteins of cell cycle and apoptosis were detected by Western blot analysis.

RESULTS

PPARgamma was expressed at much higher levels in renal tumors than in the normal kidney (2.16+/-0.85 vs 0.90+/-0.73; P<0.01). TZD PPARgamma ligands inhibited RCC cell growth in a dose-dependent manner with IC50 values of 7.08 micromol/L and 11.32 micromol/L for pioglitazone, and 5.71 micromol/L and 8.38 micromol/L for troglitazone in 786-O and A498 cells, respectively. Cell cycle analysis showed a G0/G1 arrest in human RCC cells following 24-h exposure to TZD. Analysis of cell cycle regulatory proteins revealed that TZD decreased the protein levels of proliferating cell nuclear antigen, pRb, cyclin D1, and Cdk4 but increased the levels of p21 and p27 in a time-dependent manner. Furthermore, high doses of TZD induced massive apoptosis in renal cancer cells, with increased Bax expression and decreased Bcl-2 expression.

CONCLUSION

TZD PPAR gamma ligands showed potent inhibitory effect on proliferation, and could induce apoptosis in RCC cells. These results suggest that ligands for PPAR gamma have potential antitumor effects on renal carcinoma cells.