Peroxisome proliferators induce apoptosis and decrease DNA synthesis in hepatoma cell lines

Modulating responses of indicator genes in cellular homeostasis, immune defense and apoptotic process in the Macrophthalmus japonicus exposed to di(2-ethylhexyl) phthalate as a plastic additive

Di(2-ethylhexyl) phthalate (DEHP), have been increasingly used as plasticizers to manufacture soft and flexible materials and ubiquitously found in water and sediments in the aquatic ecosystem. The aim of the present study was to evaluate the effect of DEHP exposure on cellular homeostasis (HSF1 and seven HSPs), immune responses (ILF), and apoptotic responses (p53, BAX, Bcl-2). DEHP exposure upregulated the expression of HSF1 and ILF. Moreover, it altered the expression levels of HSPs (upregulation of HSP70, HSP90, HSP40, HSP83, and HSP67B2 and downregulation of HSP60 and HSP21) in conjunction with HSF1 and ILF in the gills and hepatopancreas of M. japonicus exposed to DEHP. At the protein level, DEHP exposure changed apoptotic signals in both tissues of M. japonicus. These findings indicate that chronic exposures to several DEHP concentrations could disturb cellular balance, damage the inflammatory and immune systems, and induce apoptotic cell death, thereby affecting the survival of M. japonicus.

DBP induced autophagy and necrotic apoptosis in HepG2 cells via the mitochondrial damage pathway

Phthalate esters (PAEs) are widely present in human tissues and pose significant health risks. In this study, HepG2 cells were treated with 0.0625, 0.125, 0.25, 0.5 and 1 mM Dibutyl phthalate (DBP) for 48 h to investigate mitochondrial toxicity. The results showed that DBP caused mitochondrial damage, autophagy, apoptosis and necroptosis; Transcriptomics analysis identified that MAPK and PI3K were significant factors in the cytotoxic changes induced by DBP; N-Acetyl-L-cysteine (NAC), SIRT1 activator, ERK inhibitor, p38 inhibitor and ERK siRNA treatments counteracted the changes of SIRT1/PGC-1α and Nrf2 pathway-related proteins, autophagy and necroptotic apoptosis proteins induced by DBP. While PI3K and Nrf2 inhibitors exacerbated the changes in SIRT1/PGC-1α, Nrf2-associated proteins and autophagy and necroptosis proteins induced by DBP. In addition, the autophagy inhibitor 3-MA alleviated the increase in DBP-induced necroptosis proteins. These results suggested that DBP-induced oxidative stress activated the MAPK pathway, inhibited the PI3K pathway, which in turn inhibited the SIRT1/PGC-1α pathway and Nrf2 pathway, thereby causing cell autophagy and necroptosis.

Effect of DEHP and DnOP on mitochondrial damage and related pathways of Nrf2 and SIRT1/PGC-1α in HepG2 cells

Di-2-ethylhexyl phthalate (DEHP) and Dioctyl phthalate (DnOP) are widely used as plasticizers in various industries for which the consequent health problems are of great concern. In this context, we treated HepG2 cells with DEHP or DnOP for 48 h. The results showed that DEHP and DnOP caused increase in oxygen species (ROS), malondialdehyde (MDA), Alanine aminotransferase (ALT) and Aspartate transaminase (AST). The proteins NF⁃E2-related factor 2 (Nrf2) and haemeoxygenase-1 (HO-1), were significantly down-regulated. Subsequently, the mitochondrial structure was disrupted, and the ATP content, the mitochondrial copy number as well as the expression of the corresponding mitochondrial genes were also reduced. The expression of sirtuin 1(SIRT1), PPAR gamma co-activator 1 alpha (PGC-1α), Nuclear respiratory factor 1(Nrf1), Mitochondrial transcription factor A (TFAM) on the SIRT1/PGC-1α pathway were significantly reduced. Finally, neither DEHP nor DnOP was found to induce apoptosis, but could significantly up-regulate Light chain 3 II (LC3II) levels. In conclusion, DEHP and DnOP could induce HepG2 cell damage via mitochondria, probably by causing oxidative stress, inhibiting the Nrf2 pathway and inhibiting the mitochondrial biogenesis pathway, which leads to excessive autophagy and cell death. DEHP and DnOP differ in the Nrf2 pathway, autophagic pathway and MAPK pathway, which may be structurally related.

Fenofibrate induces human hepatoma Hep3B cells apoptosis and necroptosis through inhibition of thioesterase domain of fatty acid synthase

This study demonstrated that fenofibrate, a lipid-lowering drug, induced a significant time-dependent cytotoxicity of hepatoma Hep3B cells. Hep3B cells are significantly more sensitive to cell killing by fenofibrate than hepatoma HepG2, lung cancer CH27 and oral cancer HSC-3 cells. From the result of docking simulation, fenofibrate can bind excellently to the thioesterase domain of fatty acid synthase (FASN) binding site as orlistat, a FASN inhibitor, acts. The fenofibrate-induced cell cytotoxicity was protected by addition of palmitate, indicating that the cytotoxic effect of fenofibrate is due to starvation of Hep3B cells by inhibiting the formation of end product in the FASN reaction. Inhibition of lipid metabolism-related proteins expression, such as proteins containing thioesterase domain and fatty acid transport proteins, was involved in the fenofibrate-induced Hep3B cell death. Fenofibrate caused S and G2/M cell cycle arrest by inducing cyclin A/Cdk2 and reducing cyclin D1 and E protein levels in Hep3B cells. The anti-tumor roles of fenofibrate on Hep3B cells by inducing apoptosis and necroptosis were dependent on the expression of Bcl-2/caspase family members and RIP1/RIP3 proteins, respectively. These results suggest that fenofibrate has an anti-cancer effect in Hep3B cells and inhibition of lipid metabolism may be involved in fenofibrate-induced Hep3B cells apoptosis and necroptosis.

Differential effects of triclosan on the activation of mouse and human peroxisome proliferator-activated receptor alpha

Triclosan is an anti-bacterial agent used in many personal care products, household items, medical devices, and clinical settings. Liver tumors occur in mice exposed to triclosan, a response attributed to peroxisome proliferator-activated receptor alpha (PPARα) activation; however, the effects of triclosan on mouse and human PPARα have not been fully evaluated. We compared the effects of triclosan on mouse and human PPARα using PPARα reporter assays and on downstream events of PPARα activation using mouse hepatoma Hepa1c1c7 cells and human hepatoma HepG2 cells. PPARα transcriptional activity was increased by triclosan in a mouse PPARα reporter assay and decreased in a human PPARα reporter assay. Concentrations of triclosan inhibiting 50% cell growth were similar in both human and mouse hepatoma cells. Western blotting analysis showed that triclosan increased acyl-coenzyme A oxidase (ACOX1), a PPARα target, in Hepa1c1c7 cells but decreased the level in HepG2 cells. Treatment of Hepa1c1c7 cells with triclosan enhanced DNA synthesis and suppressed transforming growth factor beta-mediated apoptosis. This did not occur in HepG2 cells. These data demonstrate that triclosan had similar cytotoxicity in Hepa1c1c7 and HepG2 cells, but differential effects on the activation of PPARα, the expression of ACOX1, and downstream events including DNA synthesis and apoptosis.

Chemical constituents from aerial parts of Caryopteris incana and cytoprotective effects in human HepG2 cells

An ethyl acetate fraction of the aerial parts of Caryopteris incana (Verbenaceae) showed potent cytoprotective effects against damage to HepG2 cells induced by tert-butylhydroperoxide (t-BHP). To search for hepatoprotective components of C. incana, various chromatographic separations of the ethyl acetate soluble fraction of C. incana led to isolation of three phenylpropanoid glycosides, 6‴-O-feruloylincanoside D, 6‴-O-sinapoylincanoside D and caryopteroside, and two iridoid glycosides, incanides A and B, together with 17 known compounds. Structures of these compounds were determined by spectroscopic analyses. The absolute stereochemistry of the caryopteroside was established with the help of circular dichroism data and in comparison with literature data. All isolated substances were determined for their cytoprotective effects against t-BHP-induced toxicity in HepG2 cells. Among the tested compounds, 6'-O-caffeoylacteoside exhibited the most potent cytoprotective activity with an IC50 value of 0.8±0.1 μM against t-BHP-induced toxicity. Structure-activity relationships of the assay results indicated an important role of the catechol moiety in phenylpropanoid, iridoid and flavonoid derivatives in eliciting cytoprotective effects.

Mono-2-ethylhexyl phthalate induced loss of mitochondrial membrane potential and activation of Caspase3 in HepG2 cells

L02 and HepG2 cells were exposed to mono-(2-ethylhexyl) phthalate (MEHP) at concentrations of 6.25-100μM. After 48h treatment, MEHP decreased HepG2 cell viability in a concentration-dependent manner and L02 cell viability in the 50 and 100μM groups (p<0.01). Furthermore, at 24 and 48h after treatment, MEHP decreased the glutathione levels of HepG2 cells in all treatment groups and in the ΔΨ(m) in L02 and HepG2 cells with MEHP≥25μM (p<0.05 or p<0.01). At 24h after treatment, MEHP induced activation of caspase3 in all treated HepG2 and L02 cells (p<0.05 or p<0.01) except the 100μM MEHP treatment group. The increase in the Bax to Bcl-2 ratio suggests that Bcl-2 family involved in the control of MEHP-induced apoptosis in these two cell types. The data suggest that MEHP could induce apoptosis of HepG2 cells through mitochondria- and caspase3-dependent pathways.

Clofibrate treatment in pigs: effects on parameters critical with respect to peroxisome proliferator-induced hepatocarcinogenesis in rodents

Background

In rodents treatment with fibrates causes hepatocarcinogenesis, probably as a result of oxidative stress and an impaired balance between apoptosis and cell proliferation in the liver. There is some debate whether fibrates could also induce liver cancer in species not responsive to peroxisome proliferation. In this study the effect of clofibrate treatment on peroxisome proliferation, production of oxidative stress, gene expression of pro- and anti-apoptotic genes and proto-oncogenes was investigated in the liver of pigs, a non-proliferating species.

Results

Pigs treated with clofibrate had heavier livers (+16%), higher peroxisome counts (+61%), higher mRNA concentration of acyl-CoA oxidase (+66%), a higher activity of catalase (+41%) but lower concentrations of hydrogen peroxide (-32%) in the liver than control pigs (P < 0.05); concentrations of lipid peroxidation products (thiobarbituric acid-reactive substances, conjugated dienes) and total and reduced glutathione in the liver did not differ between both groups. Clofibrate treated pigs also had higher hepatic mRNA concentrations of bax and the proto-oncogenes c-myc and c-jun and a lower mRNA concentration of bcl-X_L than control pigs (P < 0.05).

Conclusion

The data of this study show that clofibrate treatment induces moderate peroxisome proliferation but does not cause oxidative stress in the liver of pigs. Gene expression analysis indicates that clofibrate treatment did not inhibit but rather stimulated apoptosis in the liver of these animals. It is also shown that clofibrate increases the expression of the proto-oncogenes c-myc and c-jun in the liver, an event which could be critical with respect to carcinogenesis. As the extent of peroxisome proliferation by clofibrate was similar to that observed in humans, the pig can be regarded as a useful model for investigating the effects of peroxisome proliferators on liver function and hepatocarcinogenesis.

HMG-CoA reductase and PPARalpha are involved in clofibrate-induced apoptosis in human keratinocytes

Contrasting data have been reported on the effects of clofibrate, a PPARalpha agonist and hypolipidemic drug. The carcinogenic and anti-apoptotic effects have been demonstrated especially in rodents in both "in vivo" and "in vitro" experiments. In contrast, in rat and human hepatoma cell lines, several reports have shown its concentration-dependent pro-apoptotic effect. No epidemiological data exist about its carcinogenetic effect in man. This study shows that clofibrate also induced apoptosis in a human non-tumour cell line, NCTC 2544, which shares the characteristic of proliferation with tumour cells. Both HMG-CoA reductase and PPARalpha were found to be involved in the signal transduction pathway inducing apoptosis, the former being the principal target: HMG-CoA reductase decreased and PPARalpha increased. Changes in HMG-CoA reductase expression caused activation of parameters leading to apoptosis via the mitochondria pathway. Clofibrate must be considered a pro-apoptotic molecule at concentrations of 0.25 mM and above: the effect is exercised not only on tumour cells but also on normal human proliferating cells. Clofibrate should thus be regarded as a potential drug to reduce the number of proliferating cells in pathological conditions.

Fenofibrate induces apoptotic injury in cultured human hepatocytes by inhibiting phosphorylation of Akt

Fibric acid derivatives have a potent and effective lipid-lowering action, however, the use of these compounds is sometimes limited due to the occurrence of hepatic injury. In the present study, we characterized cell injury induced by fenofibrate in cultured human hepatocytes. Fenofibrate caused a loss of cell viability and nuclear damage as assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling or by DNA electrophoresis, in which caspase activation is involved. The cell injury was accompanied by the shrinkage and the translocation of phosphatidyl serine from inner membrane to the outer membrane as determined by annexin V stain. The mRNA expression for bcl-2 was reduced by fenofibrate. An immunofluorescent stain with antiserum raised against phosphorylated Akt revealed that fenofibrate inhibited insulin-stimulated phosphorylation of Akt. Like fenofibrate, several compounds that inhibit the phosphorylation of Akt, including wortmannin, SH-6 and a high concentration (100 microM) of SB203580, reduced the viability of cultured human hepatocytes. Both nuclear damage and cell injury induced by fenofibrate were reversed by insulin in a concentration-dependent manner. In contrast, bezafibrate or 8(S)-hydroxyeicosatetraenoic acid had no hepatotoxic action. These findings suggest that fenofibrate causes caspase-dependent apoptosis in human hepatocytes by inhibiting phosphorylation of Akt, in which PPARalpha is not involved.

Synthesis of 4-hydroxycoumarin and 2,4-quinolinediol derivatives and evaluation of their effects on the viability of HepG2 cells and human hepatocytes culture

We report here the synthesis of aromatic coumarins and aromatic alpha-quinolones which were evaluated in vitro for their protective potentialities against tert-butyl hydroperoxide (t-BHP)-induced oxidative damage on human liver cell death, i.e., human hepatoma HepG2 cell line and human hepatocytes in primary culture. We found that the presence of a benzylidene at the 3-position or a heterocycle with N and S heteroatoms on the benzopyrone or quinolone system was essential for the protective effect of these compounds against t-BHP-induced decrease in viability of cells. We found also that a methoxy group on the aromatic ring systems decreased this potential. t-BHP-induced cytotoxicity in primary cultures of human hepatocytes could be therefore prevented by these compounds suggesting that they could display hepatoprotective effects in humans.

PPARalpha agonists clofibrate and gemfibrozil inhibit cell growth, down-regulate hCG and up-regulate progesterone secretions in immortalized human trophoblast cells

We studied effects of PPARalpha agonists clofibric acid and gemfibrozil on cell growth and functions of immortalized human extravillous trophoblast cells. Levels of DNA and protein gradually increased during incubation for 4 days. Gemfibrozil (>0.25mM) and clofibric acid (2.5mM) suppressed the rate of increase in DNA and protein. Specific activities of fatty acyl-CoA oxidase and catalase were increased to about 1.2-2.0 times the control value by 0.05mM gemfibrozil and 1.0 and 2.5mM clofibric acid after incubation for 3 days. Acid phosphatase activity showed a small increase in response to both agents, but esterase activity changed little. The secretion of progesterone from the cells into the medium was increased by 0.25mM gemfibrozil and 1.0 and 2.5mM clofibric acid after incubation for 3 days, but that of human chorionic gonadotropin (hCG) was decreased by 0.35mM gemfibrozil and 2.5mM clofibric acid. The specific activity of lactate dehydrogenase in the cells was hardly changed at all after incubation for 3 days. These results suggest that gemfibrozil and clofibric acid inhibit the proliferation of trophoblast cells. Cell metabolism is probably affected by both agents. The two agents may down-regulate hCG and up-regulate progesterone secretions.

Comparison of basal gene expression profiles and effects of hepatocarcinogens on gene expression in cultured primary human hepatocytes and HepG2 cells

Toxicogenomics is a relatively new discipline of toxicology. Microarrays and bioinformatics tools are being used successfully to understand the effects of toxicants on in vivo and in vitro model systems, and to gain a better understanding of the relevance of in vitro models commonly used in toxicological studies. In this study, cDNA filter arrays were used to determine the basal expression patterns of human cultured primary hepatocytes from different male donors; compare the gene expression profile of HepG2 to that of primary hepatocytes; and analyze the effects of three genotoxic hepatocarcinogens; aflatoxin B(1) (AFB(1)), 2-acetylaminofluorene (2AAF), and dimethylnitrosamine (DMN), as well as one non-gentoxic hepatotoxin, acetaminophen (APAP) on gene expression in both in vitro systems. Real-time PCR was used to verify differential gene expression for selected genes. Of the approximately 31,000 genes screened, 3-6% were expressed in primary hepatocytes cultured on matrigel for 16 h. Of these genes, 867 were expressed in cultured hepatocytes from all donors. HepG2 cells expressed about 98% of the genes detectable in cultured primary hepatocytes, however, 31% of the HepG2 transcriptome was unique to the cell line. A number of these genes are expressed in human liver but expression is apparently lost during culture. There was considerable variability in the response to chemical carcinogen exposure in primary hepatocytes from different donors. The transcription factors, E2F1 and ID1 mRNA were increased three-fold and six-fold (P < 0.05, P < 0.01), respectively, in AFB(1) treated primary human hepatocytes but were not altered in HepG2. ID1 expression was also increased by dimethylnitrosamine, acetylaminofluorene and acetaminophen in both primary hepatocytes and HepG2. Identification of genes that are expressed in primary hepatocytes from most donors, as well as those genes with variable expression, will aid in understanding the variability in human reactions to drugs and chemicals. This study suggests that identification of biomarkers of exposure to some chemicals may be possible in the human through microarray analysis, despite the variability in responses.

Dissociation of oxidant production by peroxisome proliferator-activated receptor ligands from cell death in human cell lines

Ligands of peroxisome proliferator-activated receptors (PPARs) come from a diverse group of chemicals that include pharmaceutical drugs, phthalate plasticizers, steroids, and pesticides. PPAR ligands exhibit a number of effects, including an ability to induce apoptosis in some systems. The mechanism(s) underlying the induction of apoptosis is not known. The current study examined the ability of Wy14643, a fibrate and PPARalpha agonist, and ciglitazone, a thiazolidinedione and PPARgamma agonist, to induce apoptosis as well as the production of oxidants in human Jurkat T cells that express all PPAR isoforms. Treatment with increasing doses of Wy14643 caused a substantial time-dependent increase in the overall oxidant status (as reflected by increased dichlorofluorescein fluorescence) of Jurkat cells without any change in viability except at the highest dose and longest time. Ciglitazone also caused a dose- and time-dependent increase in oxidant production. However, although the extent of this production was less than that seen with Wy14643, ciglitazone caused a dose- and time-dependent increase in apoptosis that could not be inhibited by antioxidants. Confocal micrographs of Jurkat cells loaded with dichlorofluorescein diacetate or dihydrorhodamine 123 and treated with Wy14643 or ciglitazone revealed a punctate pattern of fluorescence at early time points suggestive of a mitochondrial origin for these oxidants. Rotenone and antimycin A prevented Wy14643- but not ciglitazone-induced oxidant production. Other relatively specific PPARgamma agonists (15delta-PGJ2, and troglitazone), but not nonspecific agonists (bezafibrate and conjugated linoleic acid), were also able to induce oxidant production in Jurkat cells. These data, as well as the findings that oxidant production could be induced by Wy14643 in A549 cells that lack PPARalpha, and could not be blocked in Jurkat cells by the PPARalpha inhibitor MK886, indicate oxidant formation is unrelated to PPARalpha. These data also suggest that oxidant production induced by PPARalpha ligands originates in the mitochondria.

Apoptosis of Mesothelial Cells Caused by Unphysiological Characteristics of Peritoneal Dialysis Fluids

There is an ongoing debate as to which peritoneal dialysis fluids (PDFs) provide the best preservation of peritoneal cells. To investigate this topic further, we measured apoptosis and necrosis of cultured mesothelial cells (MCs) after exposure to different single unphysiological features of PDFs and PDFs for whole. MCs were incubated in buffers containing plasticizers, high osmolarity by sodium chloride, low pH, and high glucose for 0.5, 4, and 24 h. The same procedure was repeated with different PDFs. Apoptosis and necrosis were measured by FACS-analysis (annexin-FITC and propidium iodide). We found that plasticizers were clearly able to induce apoptosis after 24 h (18 +/- 4%). The same result was observed with high osmolarity by sodium chloride (17 +/- 5%), but not for high glucose (9 +/- 8%). All fluids with low pH (5.2) caused severe and almost complete necrosis (after 4 and 24 h). Incubation in neutral, two-compartment PDFs (glucose 4.25%) without plasticizers for 4 h showed no significant necrosis (3%), but after 24 h apoptosis was detectable in 10 +/- 9% and necrosis in 29 +/- 8% of MCs. In conclusion, after improving PDFs and introducing neutral fluids, further attention should be drawn to inducers of apoptosis. Apoptosis can be detected quite early (24 h) and is caused by plasticizers and high osmolarity.

Mechanisms involved in growth inhibition induced by clofibrate in hepatoma cells

Low concentrations of some peroxisome proliferators have been found to decrease apoptosis in rat liver cells, whereas higher but pharmacological concentrations have been found to inhibit cell proliferation or to induce apoptosis in human and rat hepatoma cells. The highly deviated JM2 rat hepatoma cell line was used to examine the mechanisms underlying the inhibitory effect on cell proliferation. Clofibrate chiefly inhibited cell proliferation in these cells. Parallel to the decrease in cell proliferation there was an increase of peroxisome proliferator activated receptor (PPAR) gamma and of protein phosphatase 2A, whose importance was confirmed, respectively, by using antisense oliginucleotides (AS-ODN) or okadaic acid. The increase of protein phosphatase 2A induced by PPARgamma caused a decrease of MAPK, an intracellular signaling transduction pathway, as shown by evaluation of Erk1,2 and c-myc. In light of these results, clofibrate, like conventional synthetic ligands of PPARgamma, may be regarded as a possible prototype anti-tumour drug.

Tumor necrosis factor induces apoptosis in hepatoma cells by increasing Ca(2+) release from the endoplasmic reticulum and suppressing Bcl-2 expression

Tumor necrosis factor (TNF) plays an import role in the control of apoptosis. The most well known apoptotic pathway regulated by TNF involves the TNFR1-associated death domain protein, Fas-associated death domain protein, and caspase-8. This study examines the mechanism of TNF-induced apoptosis in FaO rat hepatoma cells. TNF treatment significantly increased the percentage of apoptotic cells. TNF did not activate caspase-8 but activated caspase-3, -10, and -12. The effect of TNF on the expression of different members of the Bcl-2 family in these cells was studied. We observed no detectable changes in the steady-state levels of Bcl-X(L), Bax, and Bid, although TNF suppresses Bcl-2 expression. Dantrolene suppressed the inhibitory effect of TNF on Bcl-2 expression. TNF induced release of Ca(2+) from the endoplasmic reticulum (ER) that was blocked by dantrolene. Importantly, the expression of Bcl-2 blocked TNF-induced apoptosis and decreased TNF-induced Ca(2+) release. These results suggest that TNF induces apoptosis by a mechanism that involves increasing Ca(2+) release from the ER and suppression of Bcl-2 expression.

Thiazolidinedione activation of peroxisome proliferator-activated receptor gamma can enhance mitochondrial potential and promote cell survival

Thiazolidinediones (TZDs) are widely used for treatment of type 2 diabetes mellitus. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is the molecular target of TZDs and is believed to mediate the apoptotic effects of this class of drugs in a variety of cell types, including B and T lymphocytes. The finding that TZDs induce lymphocyte death has raised concerns regarding whether TZDs might further impair immune functions in diabetics. To address this issue, we investigated the roles of PPAR gamma and TZDs in lymphocyte survival. PPAR gamma was up-regulated upon T cell activation. As previously reported, PPAR gamma agonists induced T cell death in a dose-dependent manner. However, the concentrations of TZD needed to cause T cell death were above those needed to induce PPAR gamma-dependent transcription. Surprisingly, at concentrations that induce optimal transcriptional activation, TZD activation of PPAR gamma protected cells from apoptosis following growth factor withdrawal. The survival-enhancing effects depended on both the presence and activation of PPAR gamma. Measurements of mitochondrial potential revealed that PPAR gamma activation enhanced the ability of cells to maintain their mitochondrial potential. These data indicate that activation of PPAR gamma with TZDs can promote cell survival and suggest that PPAR gamma activation may potentially augment the immune responses of diabetic patients.

Inhibition of peroxisome-proliferator-activated receptor (PPAR)alpha by MK886

Although MK886 was originally identified as an inhibitor of 5-lipoxygenase activating protein (FLAP), recent data demonstrate that this activity does not underlie its ability to induce apoptosis [Datta, Biswal and Kehrer (1999) Biochem. J. 340, 371--375]. Since FLAP is a fatty-acid binding protein, it is conceivable that MK886 may affect other such proteins. A family of nuclear receptors that are activated by fatty acids and their metabolites, the peroxisome-proliferator-activated receptors (PPARs), have been implicated in apoptosis and may represent a target for MK886. The ability of MK886 to inhibit PPAR-alpha, -beta and -gamma activity was assessed using reporter assay systems (peroxisome-proliferator response element--luciferase). Using a transient transfection system in monkey kidney fibroblast CV-1 cells, mouse keratinocyte 308 cells and human lung adenocarcinoma A549 cells, 10--20 microM MK886 inhibited Wy14,643 activation of PPAR alpha by approximately 80%. Similar inhibition of PPAR alpha by MK886 was observed with a stable transfection reporter system in CV-1 cells. Only minimal inhibitory effects were seen on PPAR beta and PPAR gamma. MK886 inhibited PPAR alpha by a non-competitive mechanism as shown by its effects on the binding of arachidonic acid to PPAR alpha protein, and a dose-response study using a transient transfection reporter assay in COS-1 cells. An assay assessing PPAR ligand-receptor interactions showed that MK886 prevents the conformational change necessary for active-complex formation. The expression of keratin-1, a protein encoded by a PPAR alpha-responsive gene, was reduced by MK886 in a culture of mouse primary keratinocytes, suggesting that PPAR inhibition has functional consequences in normal cells. Although Jurkat cells express all PPAR isoforms, various PPAR alpha and PPAR gamma agonists were unable to prevent MK886-induced apoptosis. This is consistent with MK886 functioning as a non-competitive inhibitor of PPAR alpha, but may also indicate that PPAR alpha is not directly involved in MK886-induced apoptosis. Although numerous PPAR activators have been identified, the results show that MK886 can inhibit PPAR alpha, making it the first compound identified to have such an effect.

Reactive Oxygen Species and Mitochondria Mediate the Induction of Apoptosis in Human Hepatoma HepG2 Cells by the Rodent Peroxisome Proliferator and Hepatocarcinogen, Perfluorooctanoic Acid

We have previously shown that one of the most potent rodent hepatocarcinogens, perfluorooctanoic acid (PFOA), induces apoptosis in human HepG2 cells in a dose- and time-dependent manner. In this study we have investigated the involvement of reactive oxygen species (ROS), mitochondria, and caspase-9 in PFOA-induced apoptosis. Treatment with 200 and 400 microM PFOA was found to cause a dramatic increase in the cellular content of superoxide anions and hydrogen peroxide after 3 h. Measurement of the mitochondrial transmembrane potential (Delta Psi(m)) after PFOA treatment showed a dissipation of Delta Psi(m) at 3 h. Caspase-9 activation was seen at 5 h after treatment with 200 microM PFOA. In order to evaluate the importance of these events in PFOA-induced apoptosis, cells were cotreated with PFOA and N-acetylcysteine (NAC), a precursor of glutathione, or Cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore (MPT pore). NAC reduced Delta Psi(m) dissipation, caspase 9 activation, and apoptosis, indicating a role for PFOA-induced ROS. In addition, CsA also reduced Delta Psi(m) dissipation, caspase 9 activation, and apoptosis, indicating a role for PFOA-induced opening of the MPT pore. In summary, we have delineated a ROS and mitochondria-mediated pathway for induction of apoptosis by PFOA.