The rodent non-genotoxic hepatocarcinogen nafenopin suppresses apoptosis preferentially in non-cycling hepatocytes but also elevates CDK4, a cell cycle progression factor

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

Predictive toxicogenomics approaches reveal underlying molecular mechanisms of nongenotoxic carcinogenicity

Toxicogenomics technology defines toxicity gene expression signatures for early predictions and hypotheses generation for mechanistic studies, which are important approaches for evaluating toxicity of drug candidate compounds. A large gene expression database built using cDNA microarrays and liver samples treated with over one hundred paradigm compounds was mined to determine gene expression signatures for nongenotoxic carcinogens (NGTCs). Data were obtained from male rats treated for 24 h. Training/testing sets of 24 NGTCs and 28 noncarcinogens were used to select genes. A semiexhaustive, nonredundant gene selection algorithm yielded six genes (nuclear transport factor 2, NUTF2; progesterone receptor membrane component 1, Pgrmc1; liver uridine diphosphate glucuronyltransferase, phenobarbital-inducible form, UDPGTr2; metallothionein 1A, MT1A; suppressor of lin-12 homolog, Sel1h; and methionine adenosyltransferase 1, alpha, Mat1a), which identified NGTCs with 88.5% prediction accuracy estimated by cross-validation. This six genes signature set also predicted NGTCs with 84% accuracy when samples were hybridized to commercially available CodeLink oligo-based microarrays. To unveil molecular mechanisms of nongenotoxic carcinogenesis, 125 differentially expressed genes (P<0.01) were selected by Student's t-test. These genes appear biologically relevant, of 71 well-annotated genes from these 125 genes, 62 were overrepresented in five biochemical pathway networks (most linked to cancer), and all of these networks were linked by one gene, c-myc. Gene expression profiling at early time points accurately predicts NGTC potential of compounds, and the same data can be mined effectively for other toxicity signatures. Predictive genes confirm prior work and suggest pathways critical for early stages of carcinogenesis.

PPAR trilogy from metabolism to cancer

PURPOSE OF REVIEW

This review highlights recent advances related to malignancies in the field of peroxisome proliferator-activated receptors (PPARs). It also discusses the implications of cancer research and therapy.

RECENT FINDINGS

In the last few years, genetic evidence has implicated the PPARs, specifically PPARgamma and PPARbeta/delta, in tumorigenesis. Also, new insights into the regulation of the nuclear hormone receptors have emerged.

SUMMARY

Exciting research in PPAR biology has established these nuclear factors as key regulators of metabolism and energy homeostasis. Evidence indicates that PPARs can also affect the pathogenesis and development of tumors. However, the type of effects observed thus far appears to depend on the experimental context. As a result, the findings are generating much debate, as PPAR agonists are widespread targets in the treatment of metabolic disorders such as diabetes and dyslipidemia. Here, we summarize the most recent advances in this field, outline the conflicting reports and discuss their overall implications in cancer research.

Apoptosis induced by clofibrate in Yoshida AH-130 hepatoma cells: role of HMG-CoA reductase

Clofibrate is a hypolipidemic drug belonging to the peroxisome proliferator (PP) family. PPs are well-recognized hepatocarcinogens, though only for rodents and not for humans. Their oncogenicity is usually ascribed to mitogenic or antiapoptotic action. However, we have reported that clofibrate can trigger fast and extensive apoptosis in rodent and human tumor cell lines. The present study examines the possible mechanisms involved in clofibrate-induced apoptosis in AH-130 hepatoma cells. The results show that the apoptogenic effect of clofibrate does not depend on induction of peroxisome proliferator activated receptors (PPARs), but on interference with HMG-CoA reductase (HMGR), a key enzyme that regulates cholesterol biosynthesis and production of isoprenoid units for protein farnesylation. The level and activity of HMGR mRNA are reduced in clofibrate-treated AH-130 cells and apoptosis can be partially prevented by addition of mevalonate. Moreover, cholesterol and cholesterol ester content decreases early in mitochondria, and cytocrome c is released in the cytosol. On the contrary, perturbations at the level of protein farnesylation are not important in determining the fast apoptogenic effect, since treatment of AH-130 cells with an inhibitor of farnesyltransferase induces apoptosis only after 4 h. In conclusion, inhibition of HMGR and decreased cholesterol content are crucial events in clofibrate-induced apoptosis in AH-130 hepatoma cells.

Regulation of inhibin beta chains and follistatin mRNA levels during rat hepatocyte growth induced by the peroxisome proliferator di-n-butyl phthalate

Peroxisome proliferators stimulate hepatocyte growth in rat liver in vivo. Activin A, a homodimer of inhibin betaA, inhibits DNA synthesis in hepatocytes. The inhibitory action of activin A is suppressed by follistatin, an activin-binding protein. In this paper, we investigated whether administration of di-n-butyl phthalate (DBP), a peroxisome proliferator, modifies the production of activin A and follistatin in rat liver by hourly monitoring of inhibin betaA and follistatin mRNA levels by reverse transcriptase polymerase chain reaction analysis. The mRNA levels of the other inhibin beta chains (inhibin betaB and betaC) were examined in a similar manner. The inhibin betaA mRNA level decreased to about 30% by 3 h after DBP administration (8.6 mmol/kg body weight), remained low until 12 h, and returned to its original level by 24 h. The follistatin mRNA level increased to about 2 times by 6 h, and returned to its original level by 24 h. The inhibin betaB mRNA had started to increase by 1 h, peaked at 6 h at about 4 times its initial level, and returned to its original level by 12 h. The inhibin betaC mRNA level had doubled by 6 h and it returned to its original level. These results indicate that the growth stimulatory action of peroxisome proliferators may be mediated via the decrease in activin A level and activity and suggest that the increases in follistatin as well as inhibin betaB and betaC chains may play a role in peroxisome proliferator-stimulated hepatocyte growth.

Induction of DNA replication by peroxisome proliferators is independent of both tumour necrosis factor (alpha) priming and EGF-receptor tyrosine kinase activity

Peroxisome proliferators (PPs) cause hepatocyte proliferation and tumorigenesis in rodent liver. PPs induce hepatocyte DNA synthesis although the mechanism is unclear. Tumour necrosis factor (alpha) (TNF(alpha)) and epidermal growth factor (EGF) have been implicated in mediating this growth response since these factors induce a threefold and 17.2-fold increase, respectively, in DNA synthesis in rat primary hepatocyte cultures. Previously, others have suggested that TNF(alpha) acts as a primer to sensitise hepatocytes to the proliferative effects of growth factors. Indeed, here we show that costimulation with TNF(alpha) and a suboptimal (4-20% of optimal) concentration of EGF permits an 11.7-fold increase in DNA synthesis in rat primary hepatocyte cultures. The PP nafenopin induced a 2. 3-fold increase in DNA synthesis but there was no further increase upon co-administration of either TNF(alpha) or a suboptimal concentration of EGF. Furthermore, there was no gross dysregulation of the CDK and cyclin protein expression profile upon stimulation with nafenopin. Using a specific epidermal growth factor receptor tyrosine kinase inhibitor (4-(3-chloro-4-fluorophenylamino)-7-methoxy-6- (3-?1-pyrolidino)-propoxyquinazoline, EGFR-TKI), we show that signalling through EGF-R is not required for nafenopin-induced DNA synthesis. The EGFR-TKI also prevented progression into S phase upon stimulation with TNF(alpha), but DNA synthesis was not reduced to control levels, indicating that TNF(alpha) has a mitogenic activity in the absence of EGF signalling. Therefore, although TNF(alpha) can act as a priming factor for growth factors such as EGF, nafenopin does not appear to act via this mechanism.