Apoptosis and multistage carcinogenesis in rat liver

Nuclear receptor CAR-mediated liver cancer and its species differences

Introduction: The nuclear receptor CAR plays an important role in the regulation of hepatic responses to xenobiotic exposure, including the induction of hepatocyte proliferation and chemical carcinogenesis. Phenobarbital, a well-known liver cancer promoter, has been found to promote hepatocyte proliferation via CAR activation. However, the molecular mechanisms by which CAR induces liver carcinogenesis remain unknown. In addition, it is believed that CAR-mediated liver carcinogenesis shows a species difference; phenobarbital treatment induces hepatocyte proliferation and liver cancer in rodents but not in humans. However, the mechanisms are also unknown.Areas covered: Several reports indicate that the key oncogenic signaling pathways Wnt/β-catenin and Hippo/YAP are involved in CAR-mediated liver carcinogenesis. We introduce current data about the possible molecular mechanisms involved in CAR-mediated liver carcinogenesis and species differences by focusing on these two signaling pathways.Expert opinion: CAR may activate both the Wnt/β-catenin and Hippo/YAP signaling pathways. The synergistic activation of both signaling pathways seems to be important for CAR-mediated liver cancer development. Low homology between the ligand binding domains of human CAR and rodent CAR might cause species differences in the interactions with proteins that control the Wnt/β-catenin and Hippo/YAP pathways as well as liver cancer induction.

Mechanisms of DNA-reactive and epigenetic chemical carcinogens: applications to carcinogenicity testing and risk assessment

Chemicals with carcinogenic activity in either animals or humans produce increases in neoplasia through diverse mechanisms. One mechanism is reaction with nuclear DNA. Other mechanisms consist of epigenetic effects involving either modifications of regulatory macromolecules or perturbation of cellular regulatory processes. The basis for distinguishing between carcinogens that have either DNA reactivity or an epigenetic activity as their primary mechanism of action is detailed in this review. In addition, important applications of information on these mechanisms of action to carcinogenicity testing and human risk assessment are discussed.

Attenuation of steatohepatitis, fibrosis, and carcinogenesis in mice fed a methionine-choline deficient diet by CCAAT/enhancer-binding protein homologous protein deficiency

BACKGROUND AND AIMS

Hepatic steatosis is a metabolic liver disease with the potential to progress to steatohepatitis, cirrhosis, and hepatocellular carcinoma (HCC). The aim of this study was to investigate the impact of CCAAT/enhancer-binding protein homologous protein (CHOP) deficiency in the development of steatosis-associated progression of HCC.

METHODS

Eight-week-old wild-type (WT) and CHOP knockout (CHOP-/-) mice were fed a normal or methionine-choline-deficient (MCD) diet. Mice were sacrificed after 3 weeks, and steatosis, inflammation, apoptosis, and liver damage were assessed. We also evaluated fibrosis after 8 weeks of nutrition intervention. To explore the role of CHOP in liver carcinogenesis, 25 mg/kg of diethylnitrosamine (DEN) was injected intraperitoneally into 2-week-old mice, which were then fed the aforementioned diets from 8 to 24 weeks of age. CHOP expression in HCC patient livers was also evaluated.

RESULTS

CHOP deficiency did not affect steatosis but significantly reduced apoptotic cells, inflammation scores, and serum liver enzymes. It also significantly suppressed total serum bilirubin levels, fibrotic area size, and messenger RNA expression of profibrotic cytokines. DEN-initiated carcinogenesis was promoted by the MCD diet, while CHOP deficiency significantly attenuated the total number and maximum diameter of tumors and the Ki-67 labeling index. In human livers, CHOP expression was enhanced in parallel with non-alcoholic steatohepatitis-to-HCC progression.

CONCLUSIONS

CHOP deficiency attenuated apoptosis, inflammation, fibrosis, and tumorigenesis under fat-loading conditions, indicating that a therapeutic strategy targeting CHOP might be effective for fat-induced liver injury and protecting against promotion of carcinogenesis in patients with liver steatosis.

Adventures in hepatocarcinogenesis

Neoplasia is a heritably altered, relatively autonomous growth of tissue. Hepatocarcinogenesis, the pathogenesis of neoplasia in liver, as modeled in the rat exhibits three distinct, quantifiable stages: initiation, promotion, and progression. Simple mutations and/or epigenetic alterations may result in the irreversible stage of initiation. The stage of promotion results from selective enhancement of cell replication and selective inhibition of cellular apoptosis of initiated cells dependent on the genetic and/or epigenetic alterations of the latter. The irreversible stage of progression results from initial karyotypic alterations that evolve into greater degrees of genomic instability. The initial genomic alteration in the transition from promotion to progression may involve primarily epigenetic mechanisms driven by epigenetic and genetic alterations fixed during the stage of promotion.

Evaluation of tumour promoting potency of fish borne toxaphene residues, as compared to technical toxaphene and UV-irradiated toxaphene

In this study the potential impact of food chain-based biotransformation and physico-chemical weathering of toxaphene on its tumour promoting potential was investigated in vitro and in vivo. Human exposure to toxaphene is mainly through consumption of contaminated fish, therefore fish-borne residues of toxaphene (cod liver extract, CLE) were prepared by exposing cod to technical toxaphene (TT) for 63 days. UV-irradiated toxaphene (uvT) was included to represent a physico-chemical weathered toxaphene mixture. In vitro, TT, uvT and CLE all showed a dose- and time-dependent inhibition of gap junctional intercellular communication (GJIC) with a relative potency of CLE>TT=uvT. Tumour promoting potency was further studied in vivo in a medium term two-stage initiation/promotion bioassay in female Sprague-Dawley rats, using an increase in altered hepatic foci positive for glutathione-S-transferase-P (AHF-GST-P) as read out. No increase in AHF-GST-P occurred following exposure to either TT, uvT, or CLE, except for the positive control group (2,3,7,8-TCDD). Based on this study the no observed adverse effect level (NOAEL) for tumour promoting potency is at least 12.5mg/kg/week, or higher for CLE. Considering current human exposure levels in Europe it is doubtful that consumption of fish at current levels of toxaphene contamination give rise to human health risk.

Microarray analysis of gene expression during epithelial-mesenchymal transformation

One of the most fundamental biological processes in development, as well as a primary mechanism for tumor metastasis, is epithelial-mesenchymal transformation (EMT). To gain a greater understanding of this transition, we have obtained a genomic profile of the critical stages before and during this rapid change in morphology in the developing mouse palate. By isolating the medial edge epithelium of each palatal shelf, we were able to obtain pure gene expression data without contamination from surrounding mesenchymal cells. Our results support the important role of the TGF-beta/Smad signal transduction pathway in the stimulation of EMT by means of up-regulation of the EMT-inducing gene, LEF-1. We document changes in gene expression profiles during palatal adherence and subsequent transformation of the medial edge epithelial seam that suggests a high number of LEF-1 target genes promote cellular transformation to mesenchyme. These include genes involved in cell adhesion, polarity, cytoskeletal dynamics, migration, and intracellular signaling. This knowledge of the changes in gene expression levels during palatogenesis should lead to a better understanding of the mechanisms of EMT.

Loss of hepatic NF-kappa B activity enhances chemical hepatocarcinogenesis through sustained c-Jun N-terminal kinase 1 activation

A major link between inflammation and cancer is provided by NF-kappaB transcription factors. Ikkbeta(Deltahep) mice, which specifically lack IkappaB kinase beta (IKKbeta), an activator of NF-kappaB, in hepatocytes, are unable to activate NF-kappaB in response to proinflammatory stimuli, such as TNF-alpha. Surprisingly, Ikkbeta(Deltahep) mice are hypersusceptible to diethylnitrosamine (DEN)-induced hepatocarcinogenesis. Because defective NF-kappaB activation promotes sustained c-Jun N-terminal kinase (JNK) activation in cells exposed to TNF-alpha, whose expression is induced by DEN, and JNK activity is required for normal hepatocyte proliferation, we examined whether increased susceptibility to DEN-induced hepatocarcinogenesis in Ikkbeta(Deltahep) mice requires JNK activation. Hepatocytes express both JNK1 and JNK2, but previous studies indicate that JNK1 is more important for hepatocyte proliferation. We therefore investigated this hypothesis using mice homozygous for a JNK1 deficiency either in wild-type or Ikkbeta(Deltahep) backgrounds. In both cases, mice lacking JNK1 were much less susceptible to DEN-induced hepatocarcinogenesis. This impaired tumorigenesis correlated with decreased expression of cyclin D and vascular endothelial growth factor, diminished cell proliferation, and reduced tumor neovascularization. Whereas hepatocyte-specific deletion of IKKbeta augmented DEN-induced hepatocyte death and cytokine-driven compensatory proliferation, disruption of JNK1 abrogated this response. In addition to underscoring the importance of JNK1-mediated hepatocyte death and compensatory proliferation, these results strongly suggest that the control of tissue renewal through the IKK and JNK pathways plays a key role in liver carcinogenesis.

Proteomic characterization of the effects of clofibrate on protein expression in rat liver

Clofibrate is a peroxisome proliferator known to induce liver tumours in rats. A proteomics study was conducted to provide new insights into the molecular mechanisms of clofibrate-induced non-genotoxic hepatocarcinogenesis. Rats were treated with 250 mg/kg day clofibrate orally and sacrificed after 7 days. Proteins extracted from the liver were analysed by 2-DE using DIGE technology. The protein identification performed by MS showed that clofibrate induced up-regulation of 77 proteins and down-regulation of 27 proteins. The highest expression ratios corresponded to proteins involved in a series of biochemical pathways such as lipid metabolism, fatty acid metabolism, amino acid metabolism, protein metabolism, citric acid cycle, xenobiotic detoxification and oxidative stress. Proteins implicated in cell proliferation and apoptosis, such as prohibitin, 10-formyl tetrahydrofolate dehydrogenase, senescence marker protein-30, pyridoxine 5'-phosphate oxidase and vimentin, were also identified as being regulated. These results provide leads for further investigations into the molecular mechanisms of liver tumours induced by clofibrate. In addition, MS results showed that a series of regulated proteins were detected as several spots corresponding to different pI and/or M(r). Differential effects on those variants could result from specific PTM and could be a specific molecular signature of the clofibrate-induced protein expression modulation in rat liver.

Low dose DDT inhibition of hepatocarcinogenesis initiated by diethylnitrosamine in male rats: possible mechanisms

Previously we reported a tendency for reduction of the development of glutathione-S-transferase placental form (GST-P) positive foci, recognized as preneoplastic changes in rat liver, by a low dose of 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), which belongs to the same group of hepatic cytochrome P-450 inducers as phenobarbital and is itself a non-genotoxic hepatocarcinogen. In order to clarify the biological significance of this phenomenon, we investigated the reproducibility and changes in other parameters using an initiation-promotion model in which male F344 rats were treated with DDT at doses of 0, 0.005, 0.5, 500 ppm in the diet for 11 or 43 weeks after initiation of hepatocarcinogenesis with N-diethylnitrosamine (DEN). When 500 ppm DDT was applied, the formation of GST-P positive foci and tumor were markedly elevated. In contrast, induction of GST-P positive foci and liver tumors tended to be inhibited at a dose of 0.005 ppm, correlating with protein levels of cytochrome P450 2B1 and 3A2 (CYP2B1 and 3A2) and generation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative DNA damage. mRNA levels for 8-oxoguanine glycosylase 1 (OGG1), an 8-OHdG repair enzyme, connexin 32 (Cx32), a major component of Gap junctions, and hepatic nuclear factor 1alpha (HNF-1alpha), a Cx32 regulator, were inversely correlated with GST-P positive foci and tumor formation. These results indicate that low dose DDT may indeed exhibit inhibitory effects on chemically initiated-rat hepatocarcinogenicity, in contrast to the promotion observed with high doses, and that this is related to changes in metabolizing enzymes, cell communication, and DNA damage and its repair.

Carcinogenic risks of dioxin: Mechanistic considerations

Dioxins and dioxin-like chemicals demonstrate high affinity binding to the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor, which mediates most, if not all, of the toxic responses of these agents. Since dioxins are not directly genotoxic their carcinogenic effect is likely the result of their tumor promoting activity produced by activation of the AhR. For the purpose of risk assessment extrapolation from effects in the observable high dose range to background dietary exposure is necessary. In the present review, we discuss various aspects of low-dose-response of receptor-mediated processes in general, including threshold phenomena with regard to tumor promotion during multi-stage carcinogenesis. In this connection the reversibility of tumor promotion plays an important role but this may not be valid for dioxins due to their long half life. The relevance of cytochrome P 4501 A-induction as biomarker for prediction of carcinogenic effects of dioxins at low doses is considered. Dioxins may act in concert with endogenous ligands of the AhR, an effect which becomes particularly relevant at low toxicant concentrations. At present, however, the nature and role of these postulated ligands are unknown. Furthermore, it is unclear whether dioxins produce synergistic tumor promotional effects with non-dioxin-like chemicals to which humans are also exposed. Dioxins and, e.g., non-dioxin-like PCBs act through different receptors and there is, albeit yet limited, experimental evidence from experimental studies to suggest that they may act on different target cell populations within the same target organ. From the available data the existence of a (physiological) threshold of effects cannot be proven and may not even exist. For regulatory purposes the application of a so called "practical threshold" for the carcinogenic effect of dioxins is proposed. Further mechanistic studies should be conducted to get insight into the dose-response characteristics of relevant events of dioxin-like and non-dioxin-like agents and into the consequences of potential interactions between both group of compounds during carcinogenesis.

Immunohistochemical detection of activated caspases in apoptotic hepatocytes in rat liver

In our study we tested the utility of antibodies that specifically recognize the cleaved large (active) subunits of caspase-3 and caspase-9 for immunohistochemical detection of apoptotic hepatocytes in rat liver sections using archival material from cyproterone acetate (CPA)-treated and control rats. CPA blocks apoptosis of hepatocytes and discontinuation of CPA treatment results in a syncronized wave of hepatocyte apoptosis. By comparing liver sections from CPA-treated and control rats with high and low rates of apoptosis we observed a close correlation between the occurrence of cleaved caspase-positive apoptotic figures and H&E-stained apoptotic bodies when evaluated in parallel sections. Caspase-stained figures were either immuno-positive apoptotic bodies or pre-apoptotic hepatocytes showing cytoplasmic and/or nuclear caspase-staining with otherwise normal cellular appearance. In extension of these observations we developed a double-immunohistochemical staining procedure which enables the detection of caspase-3-positive apoptotic hepatocytes within glutathione-S-transferase-P (GST-P)-positive preneoplastic liver foci. By use of this technique, inhibition of apoptosis by 2,3,7,8-tetrachlorodibenzo-p-dioxin as detected by counting of H&E-stained apoptotic bodies was found to correlate with a strong reduction of cleaved caspase-positive hepatocytes in GST-P-positive preneoplastic foci. In summary, this study demonstrates that cleaved caspase-positive apoptotic hepatocytes could be reliably identified and quantified both in normal and neoplastically transformed liver tissue.

Induction of apoptosis by a hepatocyte conditioned medium

Incubation of primary cultures of parenchymal hepatocytes in a conditioned medium (CM), collected over the first 3 h of serum-free rat hepatocyte culture (CM(0-3)), induces a time dependent increase of the frequency of apoptotic cells which is accompanied by prominent changes of cell morphology. Short-term treatment with CM(0-3) for the first 3 h of culture is sufficient to significantly (P < 0.05) increase the frequency of apoptotic cells, however, the effect is more pronounced upon long-term treatment. Although apoptosis induction by CM(0-3) is independent of the timepoint when cultivation in CM(0-3) starts, our results suggest that the sensitivity for apoptosis induction by CM(0-3) is increased during the phase of attachment. Purification of CM(0-3) resulted in a fraction which significantly (P < 0.05) induced apoptosis at concentrations >/=10 ng/ml. Exposure of cultures to concentrations >/=1 microg/ml of purified CM(0-3) gave rise to a prominent cytotoxic effect as indicated by the massive occurrence of necrotic cells. Biochemical analysis showed that the purified fraction of CM(0-3) contains acidic ferritins with molecular weight of 23 and 43 kDa. Strikingly, both share homologies with placental isoferritins (PLF), for which growth inhibitory and immunosuppressive effects have been demonstrated by several investigations. Therefore, our results provide evidence that rat hepatocytes produce PLF or PLF-related acidic isoferritins which are able to induce apoptosis.

Small-Molecule Anthracene-Induced Cytotoxicity and Induction of Apoptosis through Generation of Reactive Oxygen Species

A series of anthracene derivatives have been synthesized, and their potential individual cytotoxicity was evaluated using Jurkat T cells and peripheral blood mononuclear cells (PBMCs) in vitro. These compounds, except for 2l, showed less cytotoxicity in PBMCs than mitoxantrone. We also analyzed the antiproliferative activity of these derivatives using the annexin V/propidium iodide assay. These synthetic compounds induced apoptosis, thus leading to antitumor effects. Compounds 2b, 2e, 2f, 2g, 2h, 2i, 2j, and mitoxantrone produced dose-dependent cytotoxicity, while the antiproliferative activity of the anthracene pharmacophore was retained in Jurkat T cells base on the detection of DNA degradation and membrane unpacking. These clearly indicate a correlation between cytotoxicity and antitumor activity. Unlike mitoxantrone, cytotoxic properties were observed, as documented by the reactivity of these novel compounds against Jurkat T cells and PBMCs as normal cells, respectively. Various concentrations of 2b, 2e, 2f, 2g, 2h, 2i, and 2j preparations also inhibited Jurkat T cell proliferation and induced apoptosis of Jurkat T cells, potentially confirmed through the detection of DNA degradation and membrane unpacking. In the present report we also investigated the antiinflammatory activity against phorbol-12-myristate-13-acetate induced superoxide anion production, a marker for an inflammatory mediator produced by neutrophils, with IC(50) (microM) values of 2b, 2h, 2l, and 2o of 4.28+/-0.89, 3.31+/-0.88, 4.38+/-0.25, and 5.45+/-1.78, respectively. These results suggest that, in addition to the specific chromosomal aberrations and cell death, elevated apoptosis could also be a marker for exposure to anthracene derivatives.

Agaricus blazei (Himematsutake) does not alter the development of rat diethylnitrosamine-initiated hepatic preneoplastic foci

The modifying potential of crude extracts of the mushroom Agaricus blazei Murrill (Himematsutake) on the development and growth of glutathione S-transferase placental form (GST-P)-positive liver foci (liver preneoplastic lesion) was investigated in adult male Wistar rats. Six groups of animals were used. Groups 2 to 5 were given a single i.p. injection of 200 mg/kg b.w. of diethylnitrosamine (DEN) and groups 1 and 6 were treated with saline at the beginning of the experiment. After 2 weeks, animals of groups 3 to 6 were orally treated with three dose levels of aqueous extracts of the mushroom A. blazei (1.2, 5.6, 11.5, and 11.5 mg/ml of dry weight of solids) for 6 weeks. All animals were subjected to two-thirds partial hepatectomy at week 3 and sacrificed at week 8. Two hours before sacrifice, ten animals of each group were administered a single i.p injection of 100 mg/kg of bromodeoxyuridine (BrdU). Apoptotic bodies and BrdU-positive hepatocyte nuclei were quantified in liver sections stained for hematoxylin and eosin (H&E) (eosinophilic foci) and simultaneously stained for GST-P expression (GST-P-positive foci), respectively. The 6-week treatment with A. blazei did not alter the development (number and size) of GST-P-positive foci and did not affect the growth kinetics of liver normal parenchyma or foci in DEN-initiated animals. Our results indicate that the treatment with aqueous extracts of the mushroom A. blazei during the post-initiation stage of rat liver carcinogenesis does not exert any protective effect against the development of GST-P-positive foci induced by DEN.

Imperatorin, a furanocoumarin from Angelica dahurica (Umbelliferae), induces cytochrome c-dependent apoptosis in human promyelocytic leukaemia, HL-60 Cells

Imperatorin, a biologically active furanocoumarin from the roots of Angelica dahurica (Umbelliferae), was found to induce apoptosis in human promyelocytic leukaemia, HL-60 cells. DNA fragmentation assay, morphology-based evaluation, and flow cytometric analysis demonstrated that imperatorin at micromolar concentrations was able to trigger apoptosis of HL-60 cells. Neither necrosis nor differentiation was observed at cytotoxic micromolar concentrations of imperatorin. Further studies showed that the cytochrome c/caspase-9 pathway was responsible for imperatorin-induced apoptosis; i.e., mitochondrial membrane was depolarized, Bcl-2 was down-regulated, cytochrome c was released from mitochondria, caspase-9 and caspase-3 were activated, and poly(ADP-ribose) polymerase was cleaved. Furthermore, imperatorin-induced apoptosis was significantly blocked by Z-VAD-FMK (a broad spectrum caspase inhibitor), Z-LEHD-FMK (a caspase-9 inhibitor) and Ac-DMQD-CHO (a caspase-3 inhibitor), but not by Z-IEDT-FMK (a caspase-8 inhibitor).

Bcl-2 expression inhibits liver carcinogenesis and delays the development of proliferating foci

Tumor development is thought to require both increased proliferation and inhibition of apoptosis. However, the relationship between cell replication and cell death in liver tumorigenesis is complex because both proliferation and apoptosis increase during hepatocarcinogenesis. To investigate the effect of the anti-apoptotic gene Bcl-2 in liver carcinogenesis, we established a line of double transgenic mice that express transforming growth factor-alpha (TGF-alpha), a liver mitogen, and Bcl-2. Double transgenic mice, TGF-alpha and Bcl-2 single transgenics, and wild type received an injection of diethylnitrosamine at 15 days of age. This alkylating agent induces liver carcinogenesis and its effect is greatly enhanced by TGF-alpha. We report that Bcl-2 expression inhibited diethylnitrosamine-induced liver carcinogenesis and counteracted the enhancing effect of TGF-alpha. Bcl-2 delayed the growth of proliferative foci at the early stages of carcinogenesis and inhibited cell proliferation in these foci. The effect of Bcl-2 on liver carcinogenesis is consistent with its reported ability to interfere with cell replication. The data demonstrate that the expression of an anti-apoptotic gene during liver carcinogenesis causes a delay rather than an increase in tumorigenesis.

Elevated calcium in preneoplastic cells activates NF-kappa B and confers resistance to apoptosis

Early preneoplastic cells (sup+) exhibit increased susceptibility to apoptosis, which is lost in late stage preneoplastic cells (sup-). Sup+ cells, which undergo apoptosis when cultured in low serum, show little or no DNA binding activity to nuclear factor (NF)-kappa B either in 10% or 0.2% serum. In contrast sup- cells, which are resistant to apoptosis in low serum, show a sustained constitutive activation of NF-kappa B. The constitutive activation of NF-kappa B observed in sup- cells is not due to loss of I kappa B alpha. We considered that the activation of NF-kappa B in sup- cells might be secondary to an increase in cytosolic Ca(2+), since sup- cells have a cytosolic Ca(2+) level that is double that in sup+ cells. In support of a role for Ca(2+), lowering cytosolic Ca(2+) in sup- cells by addition of the cell-permeable Ca(2+) chelator 1,2 bis(O-aminophenoxy)ethane-N, N, N', N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) reduced cytosolic Ca(2+) by approximately 31% relative to untreated sup- cells, concomitant with a 65% reduction in NF-kappa B DNA binding activity and a reduction in I kappa B kinase (IKK) activity. In sup- cells in low serum, addition of BAPTA-AM also resulted in a significant ( approximately 50%) increase in caspase-3 activity. Raising extracellular Ca(2+) in sup+ cells resulted in a slight activation of I kappa B kinase and in enhanced NF-kappa B DNA binding activity. Using proteasome and calpain inhibitors, we determined that the basal activity of NF-kappa B in sup- cells is largely proteasome-independent, but sensitive to calpain inhibitors. Taken together these data suggest that the elevated Ca(2+) in sup- cells causes a modest activation of IKK, which likely contributes to the enhanced basal activation of NF-kappa B in sup- cells; however, the predominant effect of Ca(2+) appears to be mediated by Ca(2+)-enhanced degradation by calpain.

Upregulation of lymphocyte apoptosis as a strategy for preventing and treating autoimmune disorders: a role for whole-food vegan diets, fish oil and dopamine agonists

Induced apoptosis of autoreactive T-lymphocyte precursors in the thymus is crucial for the prevention of autoimmune disorders. IGF-I and prolactin, which are lymphocyte growth factors, may have the potential to suppress apoptosis in thymocytes and thus encourage autoimmunity; conversely, dietary fish oil rich in omega-3 fats appears to upregulate apoptosis in lymphocytes. Since whole-food vegan diets may downregulate systemic IGF-I activity, it is proposed that such a diet, in conjunction with fish oil supplementation and treatment with dopamine agonists capable of suppressing prolactin secretion, may have utility for treating and preventing autoimmune disorders. This prediction is consistent with the extreme rarity of autoimmune disorders among sub-Saharan black Africans as long as they followed their traditional quasi-vegan lifestyles, and with recent ecologic studies correlating risks for IDDM and for multiple sclerosis mortality with animal product and/or saturated fat consumption. Moreover, there is evidence that vegan or quasi-vegan diets are useful in the management of rheumatoid arthritis, multiple sclerosis, and possibly SLE. The dopamine agonist bromocryptine exerts anti-inflammatory effects in rodent models of autoimmunity, and there is preliminary evidence that this drug may be clinically useful in several human autoimmune diseases; better tolerated D2-specific agonists such as cabergoline may prove to be more practical for use in therapy. The moderate clinical utility of supplemental fish oil in rheumatoid arthritis and certain other autoimmune disorders is documented. It is not unlikely that extra-thymic anti-inflammatory effects contribute importantly to the clinical utility of vegan diets, bromocryptine, and fish oil in autoimmunity. The favorable impact of low latitude or high altitude on autoimmune risk may be mediated by superior vitamin D status, which is associated with decreased secretion of parathyroid hormone; there are theoretical grounds for suspecting that parathyroid hormone may inhibit apoptosis in thymocytes. Androgens appear to up-regulate thymocyte apoptosis, may be largely responsible for the relative protection from autoimmunity enjoyed by men, and merit further evaluation for the management of autoimmunity in women. It will probably prove more practical to prevent autoimmune disorders than to reverse them once established; a whole-food vegan diet, coupled with fish oil and vitamin D supplementation, may represent a practical strategy for achieving this prevention, while concurrently lowering risk for many other life-threatening 'Western' diseases.

Liver toxicity and apoptosis: role of TGF-beta1, cytochrome c and the apoptosome

Transforming growth factor-beta1 (TGF-beta1), is involved in controlling liver size, by inducing apoptotic cell death in hepatocytes. However the mechanism by which TGF-beta(1) induces caspase activation and cell death is unknown. Apoptosis can be initiated either by receptor-mediated (e.g. Fas/CD95) or non-receptor chemically mediated (stress-induced) processes. With Fas/CD95 receptor mediated cell death, a multi-protein complex (DISC) is assembled at the plasma membrane, which activates the downstream caspases and cell death. In stress-mediated apoptosis, a cytosolic DISC equivalent, the apoptosome is formed that activates the effector caspases. We have characterised this complex in THP.1 cells, and shown that this is a cytochrome c dependent process that induces the formation of an approximately 700 kDa apoptosome caspase processing complex. This is formed by oligomerisation of apoptotic protease-activating factor 1 (Apaf-1), and recruitment and processing of caspase-9. We have now shown that TGF-beta1-induced apoptosis also occurs via the release of cytochrome c and the subsequent oligomerisation of Apaf-1 into an approximately 700 kDa apoptosome complex. Our studies show that, even though TGF-beta1 induction of apoptosis is a receptor-mediated event, it operates through the mitochondrial/Apaf-1 caspase activation pathway that appears to act as a common execution pathway for many diverse apoptotic stimuli.

Transforming growth factor-beta(1) induces apoptosis in rat FaO hepatoma cells via cytochrome c release and oligomerization of Apaf-1 to form a approximately 700-kd apoptosome caspase-processing complex

In mammalian cells, non receptor-mediated apoptosis occurs via the cytochrome c-dependent assembly of a approximately 700-kd apoptotic protease-activating factor 1 (Apaf-1)/caspase-9 containing apoptosome complex. This initiates the postmitochondrial-mediated effector caspase cascade. We now show that receptor mediated transforming growth factor beta(1) (TGF-beta(1))-induced apoptosis in rat hepatoma cells is accompanied by processing and activation of caspases-2, -3, -7, and -8. Furthermore, we show that caspase activation is mediated via the release of cytochrome c and the oligomerization of Apaf-1 into an approximately 700-kd apoptosome complex. Similarly, in vitro activation of hepatoma cell lysates with 2'-deoxyadenosine 5'-triphosphate (dATP) results in the formation of the approximately 700-kd apoptosome complex, which recruits and processes caspases-3 and -7. Z-VAD.FMK [benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone], the pan-caspase inhibitor totally inhibits dATP-stimulated caspase activation but does not block the assembly of the large Apaf-1 containing apoptosome complex. However, the recruitment and subsequent processing of caspases-3 and -7 to the apoptosome is blocked. Similarly, in intact cells, although Z-VAD.FMK blocked TGF-beta(1)-induced apoptosis, it did not prevent the oligomerization of Apaf-1 into the apoptosome. However, recruitment and processing of caspases-3 and -7 were prevented by Z-VAD.FMK. These data show that TGF-beta(1) induces apoptosis via release of cytochrome c and activation of the Apaf-1 apoptosome complex, which initiates the caspase cascade.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Cell death and cell proliferation in the control of normal and neoplastic tissue growth

The development of reliable methodology for the assessment of rates of cell replication and cell death has enabled the study of how these 2 fundamentally opposed processes work to form and maintain tissue and to remodel tissue following diseases resulting in cell loss. The balance between these 2 processes and the consequences of an imbalance are fundamental to a clearer understanding of how hyperplasia and neoplasia develop in tissues under the influence of chemicals and drugs. An understanding of the changes that occur in target organs and tissues following chemical or drug exposure has enabled a better understanding of the mechanism by which these chemicals are able to induce cancer after prolonged exposure. Studies of the control of cell replication and the changes that occur following drug exposure have defined 2 types of response, 1 in which the cell replicative response is sustained and the other in which the cell replicative response is transient and occurs during the first few days of exposure. Although regulatory and scientific opinion appears ready to accept sustained cell replicative processes as an increased risk factor in the development of cancer, the role played by transient increases in cell replication remains unclear. Concurrent events in target organs following treatment with chemicals that induce transient increases in cell replication have revealed that the rates of apoptosis are suppressed at the same time as the cell replication levels are induced. Additional evidence suggests that growth and antigrowth factors are central in controlling these responses. Escape from the regulatory action of these factors is postulated to be one of the ways in which nongenotoxic carcinogenic chemicals, such as the peroxisome proliferators and sodium phenobarbitone, may induce cancer, with apoptosis playing a key role in the process.

Apoptosis and cell proliferation in rat hepatocytes induced by barbiturates

To examine the effect on cell population in hepatocytes of phenobarbital (PB) and other barbiturates, PB, allobarbital (ALB), barbital sodium (BS) and barbituric acid (BA) were given orally to male rats for 7 consecutive days. Although there was no apparent change in non-promoting BA, hepatomegaly was induced by PB, BS and ALB, which are promoters of hepatocarcinogenesis. In PB- and BS-treated livers, hepatomegaly was attributable to hepatocyte proliferation and enzyme induction. In ALB-treated liver, it was attributable to enzyme induction. The level of cell proliferation was reduced to less than the control values following withdrawal of PB, ALB and BS. It seemed that the degree of suppression of cell proliferation following withdrawal of these compounds correlated to the degree of cell proliferation (PB>BS>ALB) during treatment. In PB-treated liver, apoptosis was induced during treatment, serving to eliminate the excess of hepatocytes. This suggests that short-term administration of PB neither induced suppression of apoptosis nor disturbed homeostasis of hepatocyte populations.

Upregulation of apoptosis with dietary restriction: implications for carcinogenesis and aging

The maintenance of cell number homeostasis in normal tissues reflects a highly regulated balance between the rates of cell proliferation and cell death. Under pathologic conditions such as exposure to cytotoxic, genotoxic, or nongenotoxic agents, an imbalance in these rates may indicate subsequent risk of carcinogenesis. Apoptotic cell death, as opposed to necrotic cell death, provides a protective mechanism by selective elimination of senescent, preneoplastic, or superfluous cells that could negatively affect normal function and/or promote cell transformation. The relative efficiency or dysfunction of the cell death program could therefore have a direct impact on the risk of degenerative or neoplastic disease. Dietary restriction of rodents is a noninvasive intervention that has been reproducibly shown to retard tumor development and most physiologic indices of aging relative to ad libitum-fed animals. As such, it provides a powerful model in which to study common mechanistic processes associated with both aging and cancer. In a recent study we established that chronic dietary restriction (DR) induces an increase in spontaneous apoptotic rate and a decrease in cell proliferation rate in hepatocytes of 12-month-old B6C3F1 DR mice relative to ad libitum (AL)-fed mice. This diet-induced shift in cell death/proliferation rates was associated with a marked reduction in subsequent development of spontaneous hepatoma and a marked increase in disease-free life span in DR relative to AL-fed mice. These results suggest that total caloric intake may modulate the rates of cell death and proliferation in a direction consistent with a cancer-protective effect in DR mice and a cancer-promoting effect in AL mice. To determine whether the increase in spontaneous apoptotic rate was maintained over the life span of DR mice, apoptotic rates were quantified in 12-, 18-, 24- and 30-month-old DR and AL mice. The rate of apoptosis was elevated with age in both diet groups; however, the rate of apoptosis was significantly and consistently higher in DR mice regardless of age. In double-labeling experiments, an age-associated increase in the glutathione S-transferase-II expression in putative preneoplastic hepatocytes in AL mice was rapidly reduced by apoptosis upon initiation of DR. Thus, intervention that promote a low-level increase in apoptotic cell death may be expected to protect genotypic and phenotypic stability with age. If during tumor promotion an adaptive increase in apoptosis effectively balances the dysregulated increase proliferation, the risk of permanent genetic error and carcinogenesis would be minimized.