Inhibition of c-Myc expression sensitizes hepatocytes to tumor necrosis factor-induced apoptosis and necrosis

The Connection Between Cell Fate and Telomere

Abolition of telomerase activity results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Telomere shortening leads to the attainment of the "Hayflick limit", and the transition of cells to state of senescence. If senescence is bypassed, cells undergo crisis through loss of checkpoints. This process causes massive cell death concomitant with further telomere shortening and spontaneous telomere fusions. In functional telomere of mammalian cells, DNA contains double-stranded tandem repeats of TTAGGG. The Shelterin complex, which is composed of six different proteins, is required for the regulation of telomere length and stability in cells. Telomere protection by telomeric repeat binding protein 2 (TRF2) is dependent on DNA damage response (DDR) inhibition via formation of T-loop structures. Many protein kinases contribute to the DDR activated cell cycle checkpoint pathways, and prevent DNA replication until damaged DNA is repaired. Thereby, the connection between cell fate and telomere length-associated telomerase activity is regulated by multiple protein kinase activities. Contrarily, inactivation of DNA damage checkpoint protein kinases in senescent cells can restore cell-cycle progression into S phase. Therefore, telomere-initiated senescence is a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. In this review, in addition to the above mentioned, the choice of main repair pathways, which comprise non-homologous end joining and homologous recombination in telomere uncapping telomere dysfunctions, are discussed.

Impaired macrophage autophagy increases the immune response in obese mice by promoting proinflammatory macrophage polarization

Recent evidence that excessive lipid accumulation can decrease cellular levels of autophagy and that autophagy regulates immune responsiveness suggested that impaired macrophage autophagy may promote the increased innate immune activation that underlies obesity. Primary bone marrow-derived macrophages (BMDM) and peritoneal macrophages from high-fat diet (HFD)-fed mice had decreased levels of autophagic flux indicating a generalized impairment of macrophage autophagy in obese mice. To assess the effects of decreased macrophage autophagy on inflammation, mice with a Lyz2-Cre-mediated knockout of Atg5 in macrophages were fed a HFD and treated with low-dose lipopolysaccharide (LPS). Knockout mice developed systemic and hepatic inflammation with HFD feeding and LPS. This effect was liver specific as knockout mice did not have increased adipose tissue inflammation. The mechanism by which the loss of autophagy promoted inflammation was through the regulation of macrophage polarization. BMDM and Kupffer cells from knockout mice exhibited abnormalities in polarization with both increased proinflammatory M1 and decreased anti-inflammatory M2 polarization as determined by measures of genes and proteins. The heightened hepatic inflammatory response in HFD-fed, LPS-treated knockout mice led to liver injury without affecting steatosis. These findings demonstrate that autophagy has a critical regulatory function in macrophage polarization that downregulates inflammation. Defects in macrophage autophagy may underlie inflammatory disease states such as the decrease in macrophage autophagy with obesity that leads to hepatic inflammation and the progression to liver injury.

c-Myc quadruplex-forming sequence Pu-27 induces extensive damage in both telomeric and nontelomeric regions of DNA

Quadruplex-forming DNA sequences are present throughout the eukaryotic genome, including in telomeric DNA. We have shown that the c-Myc promoter quadruplex-forming sequence Pu-27 selectively kills transformed cells (Sedoris, K. C., Thomas, S. D., Clarkson, C. R., Muench, D., Islam, A., Singh, R., and Miller, D. M. (2012) Genomic c-Myc quadruplex DNA selectively kills leukemia. Mol. Cancer Ther. 11, 66-76). In this study, we show that Pu-27 induces profound DNA damage, resulting in striking chromosomal abnormalities in the form of chromatid or chromosomal breaks, radial formation, and telomeric DNA loss, which induces γ-H2AX in U937 cells. Pu-27 down-regulates telomeric shelterin proteins, DNA damage response mediators (RAD17 and RAD50), double-stranded break repair molecule 53BP1, G2 checkpoint regulators (CHK1 and CHK2), and anti-apoptosis gene survivin. Interestingly, there are no changes of DNA repair molecules H2AX, BRCA1, and the telomere maintenance gene, hTERT. ΔB-U937, where U937 cells stably transfected with deleted basic domain of TRF2 is partially sensitive to Pu-27 but exhibits no changes in expression of shelterin proteins. However, there is an up-regulation of CHK1, CHK2, H2AX, BRCA1, and survivin. Telomere dysfunction-induced foci assay revealed co-association of TRF1with γ-H2AX in ATM deficient cells, which are differentially sensitive to Pu-27 than ATM proficient cells. Alt (alternating lengthening of telomere) cells are relatively resistant to Pu-27, but there are no significant changes of telomerase activity in both Alt and non-Alt cells. Lastly, we show that this Pu-27-mediated sensitivity is p53-independent. The data therefore support two conclusions. First, Pu-27 induces DNA damage within both telomeric and nontelomeric regions of the genome. Second, Pu-27-mediated telomeric damage is due, at least in part, to compromise of the telomeric shelterin protein complex.

FXR regulates liver repair after CCl4-induced toxic injury

Liver repair is key to resuming homeostasis and preventing fibrogenesis as well as other liver diseases. Farnesoid X receptor (FXR, NR1H4) is an emerging liver metabolic regulator and cell protector. Here we show that FXR is essential to promote liver repair after carbon tetrachloride (CCl(4))-induced injury. Expression of hepatic FXR in wild-type mice was strongly suppressed by CCl(4) treatment, and bile acid homeostasis was disrupted. Liver injury was induced in both wild-type and FXR(-/-) mice by CCl(4), but FXR(-/-) mice had more severe defects in liver repair than wild-type mice. FXR(-/-) livers had a decreased peak of regenerative DNA synthesis and reduced induction of genes involved in liver regeneration. Moreover, FXR(-/-) mice displayed increased mortality and enhanced hepatocyte deaths. During the early stages of liver repair after CCl(4) treatment, we observed overproduction of TNFalpha and a strong decrease of phosphorylation and DNA-binding activity of signal transducer and activator of transcription 3 in livers from FXR(-/-) mice. Exogenous expression of a constitutively active signal transducer and activator of transcription 3 protein in FXR(-/-) liver effectively reduced hepatocyte death and liver injury after CCl(4) treatment. These results suggest that FXR is required to regulate normal liver repair by promoting regeneration and preventing cell death.

Chronic oxidative stress sensitizes hepatocytes to death from 4-hydroxynonenal by JNK/c-Jun overactivation

Sustained activation of the c-Jun NH(2)-terminal kinase (JNK) signaling pathway mediates the development and progression of experimental diet-induced nonalcoholic fatty liver disease (NAFLD). Delineating the mechanism of JNK overactivation in the setting of a fatty liver is therefore essential to understanding the pathophysiology of NAFLD. Both human and experimental NAFLD are associated with oxidative stress and resultant lipid peroxidation, which have been proposed to mediate the progression of this disease from simple steatosis to steatohepatitis. The ability of oxidants and the lipid peroxidation product 4-hydroxynonenal (HNE) to activate JNK signaling suggested that these two factors may act synergistically to trigger JNK overactivation. The effect of HNE on hepatocyte injury and JNK activation was therefore examined in cells under chronic oxidant stress from overexpression of the prooxidant enzyme cytochrome P450 2E1 (CYP2E1), which occurs in NAFLD. CYP2E1-generated oxidant stress sensitized a rat hepatocyte cell line to death from normally nontoxic concentrations of HNE. CYP2E1-overexpressing cells underwent a more profound depletion of glutathione (GSH) in response to HNE secondary to decreased gamma-glutamylcysteine synthetase activity. GSH depletion led to overactivation of JNK/c-Jun signaling at the level of mitogen-activated protein kinase kinase 4 that induced cell death. Oxidant stress and the lipid peroxidation product HNE cause synergistic overactivation of the JNK/c-Jun signaling pathway in hepatocytes, demonstrating that HNE may not be just a passive biomarker of hepatic oxidant stress but rather an active mediator of hepatocellular injury through effects on JNK signaling.

Autophagy regulates adipose mass and differentiation in mice

The relative balance between the quantity of white and brown adipose tissue can profoundly affect lipid storage and whole-body energy homeostasis. However, the mechanisms regulating the formation, expansion, and interconversion of these 2 distinct types of fat remain unknown. Recently, the lysosomal degradative pathway of macroautophagy has been identified as a regulator of cellular differentiation, suggesting that autophagy may modulate this process in adipocytes. The function of autophagy in adipose differentiation was therefore examined in the current study by genetic inhibition of the critical macroautophagy gene autophagy-related 7 (Atg7). Knockdown of Atg7 in 3T3-L1 preadipocytes inhibited lipid accumulation and decreased protein levels of adipocyte differentiation factors. Knockdown of Atg5 or pharmacological inhibition of autophagy or lysosome function also had similar effects. An adipocyte-specific mouse knockout of Atg7 generated lean mice with decreased white adipose mass and enhanced insulin sensitivity. White adipose tissue in knockout mice had increased features of brown adipocytes, which, along with an increase in normal brown adipose tissue, led to an elevated rate of fatty acid, beta-oxidation, and a lean body mass. Autophagy therefore functions to regulate body lipid accumulation by controlling adipocyte differentiation and determining the balance between white and brown fat.

CYP2E1 and oxidative liver injury by alcohol

Ethanol-induced oxidative stress seems to play a major role in mechanisms by which ethanol causes liver injury. Many pathways have been suggested to contribute to the ability of ethanol to induce a state of oxidative stress. One central pathway seems to be the induction of cytochrome P450 2E1 (CYP2E1) by ethanol. CYP2E1 metabolizes and activates many toxicological substrates, including ethanol, to more reactive, toxic products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions and after acute and chronic alcohol treatment. CYP2E1 is also an effective generator of reactive oxygen species such as the superoxide anion radical and hydrogen peroxide and, in the presence of iron catalysts, produces powerful oxidants such as the hydroxyl radical. This review article summarizes some of the biochemical and toxicological properties of CYP2E1 and briefly describes the use of cell lines developed to constitutively express CYP2E1 and CYP2E1 knockout mice in assessing the actions of CYP2E1. Possible therapeutic implications for treatment of alcoholic liver injury by inhibition of CYP2E1 or CYP2E1-dependent oxidative stress will be discussed, followed by some future directions which may help us to understand the actions of CYP2E1 and its role in alcoholic liver injury.

Loss of macroautophagy promotes or prevents fibroblast apoptosis depending on the death stimulus

Macroautophagy has been implicated as a mechanism of cell death. However, the relationship between this degradative pathway and cell death is unclear as macroautophagy has been shown recently to protect against apoptosis. To better define the interplay between these two critical cellular processes, we determined whether inhibition of macroautophagy could have both pro-apoptotic and anti-apoptotic effects in the same cell. Embryonic fibroblasts from mice with a knock-out of the essential macroautophagy gene atg5 were treated with activators of the extrinsic and intrinsic death pathways. Loss of macroautophagy sensitized these cells to caspase-dependent apoptosis from the death receptor ligands Fas and tumor necrosis factor-alpha (TNF-alpha). Atg5-/- mouse embryonic fibroblasts had increased activation of the mitochondrial death pathway in response to Fas/TNF-alpha in concert with decreased ATP levels. Fas/TNF-alpha treatment failed to up-regulate macroautophagy, and in fact, decreased activity at late time points. In contrast to their sensitization to Fas/TNF-alpha, Atg5-/- cells were resistant to death from menadione and UV light. In the absence of macroautophagy, an up-regulation of chaperone-mediated autophagy induced resistance to these stressors. These results demonstrate that inhibition of macroautophagy can promote or prevent apoptosis in the same cell and that the response is governed by the nature of the death stimulus and compensatory changes in other forms of autophagy. Experimental findings that an inhibition of macroautophagy blocks apoptosis do not prove that autophagy mediates cell death as this effect may result from the protective up-regulation of other autophagic pathways such as chaperone-mediated autophagy.

Effects of a Shuangling Fuzheng anticancer preparation on the proliferation of SGC-7901 cells and immune function in a cyclophosphamide-treated murine model

AIM: To study the inhibitory effects of a Shuangling Fuzheng anticancer preparation (SFAP) on the human gastric cancer cell line SGC-7901 in vitro as well as its immune-modulated effects in a cyclophosphamide-treated murine model.

METHODS: MTT experiments and immunocytochemistry ABC experiments were performed for detecting the proliferation of SGC-7901 cells in vitro and protein expression of c-myc. The staphylococcal protein A (SPA) rosette test was utilized for measuring the ratio of T-lymphocyte subsets from peripheral blood in a cyclophosphamide-treated murine model. Enzyme-linked immunosorbant assay (ELISA) was performed for measuring the levels of serum sIL-2R in treated mice, while immunoturbidimetry was used for measuring the levels of immunoglobulins (Ig).

RESULTS: SFAP (40-640 mg/L, 48 h) inhibited the proliferation of SGC-7901 cells, and a positive correlation was noted between inhibitory effects and dosage. At a dosage of 160-320 mg/L in cultured cells, the expression of c-myc was decreased. SFAP (50-200 mg/kg) increased the percentage of CD3⁺ and CD4⁺ T-lymphocytes, the ratio of CD4/CD8, and the contents of Ig such as IgM, IgG or IgA, but decreased the levels of serum sIL-2R in peripheral blood from cyclophosphamide-treated mice.

CONCLUSION: SFAP can inhibit the proliferation of SGC-7901 cells via the c-myc gene. In addition, SFAP can modulat the cellular and humoral immunity in cyclophosphamide-induced immunosuppressed mice.

Smad4 cooperates with lymphoid enhancer-binding factor 1/T cell-specific factor to increase c-myc expression in the absence of TGF-beta signaling

The c-myc protooncogene is a key regulator of cell proliferation whose expression is reduced in normal epithelial cells in response to the growth inhibitory cytokine TGF-beta. Smad4 mediates this inhibitory effect of TGF-beta by forming a complex with Smad3, E2F4/5, and p107 at the TGF-beta inhibitory element (TIE) element on the c-myc promoter. In contrast, cell proliferation and c-myc expression are increased in response to Wnt ligands; this effect is mediated through the lymphoid enhancer-binding factor 1/T cell-specific factor (LEF/TCF) family of transcription factors on the c-myc promoter LEF/TCF-binding elements (TBE1 and TBE2). We report that a peptide aptamer designed to inhibit the binding between Smad4 and LEF/TCF reduced c-myc expression and the growth rate of HepG2 cells. Further analysis demonstrated that, in the absence of TGF-beta, Smad4 was bound to the positive regulatory element TBE1 from the c-myc promoter and activated c-myc promoter activity. Smad4 binding to the positive TBE1 c-myc element was reduced by TGF-beta, consistent with Smad4's inhibitory role on c-myc expression in response to TGF-beta. Reduction of Smad4 levels by RNAi knockdown also reduced c-myc expression levels and sensitized hepatocytes to cell death by serum deprivation. Two tumor-derived mutant Smad4 proteins that fail to mediate TGF-beta responses were still competent to cooperate with LEF1 to activate the c-myc promoter. These results support a previously unreported TGF-beta-independent function for Smad4 in cooperating with LEF/TCF to activate c-myc expression.

Tumor necrosis factor-induced toxic liver injury results from JNK2-dependent activation of caspase-8 and the mitochondrial death pathway

In vitro studies of hepatocytes have implicated over-activation of c-Jun N-terminal kinase (JNK) signaling as a mechanism of tumor necrosis factor-alpha (TNF)-induced apoptosis. However, the functional significance of JNK activation and the role of specific JNK isoforms in TNF-induced hepatic apoptosis in vivo remain unclear. JNK1 and JNK2 function was, therefore, investigated in the TNF-dependent, galactosamine/lipopolysaccharide (GalN/LPS) model of liver injury. The toxin GalN converted LPS-induced JNK signaling from a transient to prolonged activation. Liver injury and mortality from GalN/LPS was equivalent in wild-type and jnk1-/- mice but markedly decreased in jnk2-/- mice. This effect was not secondary to down-regulation of TNF receptor 1 expression or TNF production. In the absence of jnk2, the caspase-dependent, TNF death pathway was blocked, as reflected by the failure of caspase-3 and -7 and poly(ADP-ribose) polymerase cleavage to occur. JNK2 was critical for activation of the mitochondrial death pathway, as in jnk2-/- mice Bid cleavage and mitochondrial translocation and cytochrome c release were markedly decreased. This effect was secondary to the failure of jnk2-/- mice to activate caspase-8. Liver injury and caspase activation were similarly decreased in jnk2 null mice after GalN/TNF treatment. Ablation of jnk2 did not inhibit GalN/LPS-induced c-Jun kinase activity, although activity was completely blocked in jnk1-/- mice. Toxic liver injury is, therefore, associated with JNK over-activation and mediated by JNK2 promotion of caspase-8 activation and the TNF mitochondrial death pathway through a mechanism independent of c-Jun kinase activity.

c-Myc phosphorylation is required for cellular response to oxidative stress

Aside from the well-established roles of c-Myc in the regulation of cell cycle, differentiation, and apoptosis, a recent picture is beginning to emerge linking c-Myc to the regulation of metabolic pathways. Here, we define a further function for c-Myc in determining cellular redox balance, identifying glutathione (GSH) as the leading molecule mediating this process. The link between c-Myc and GSH is gamma-glutamyl-cysteine synthetase (gamma-GCS), the rate-limiting enzyme catalyzing GSH biosynthesis. Indeed, c-Myc transcriptionally regulates gamma-GCS by binding and activating the promoters of both gamma-GCS heavy and light subunits. Exposure to H2O2 enhances c-Myc recruitment to gamma-GCS regulatory regions through ERK-dependent phosphorylation. Phosphorylation at Ser-62 is required for c-Myc recruitment to gamma-GCS promoters and determines the cellular response to oxidative stress induced by different stimuli. Thus, the c-Myc phosphorylation-dependent activation of the GSH-directed survival pathway can contribute to oxidative stress resistance in tumor cells, which generally exhibit deregulated c-Myc expression.

Inhibitory effect of c-Myc on p53-induced apoptosis in leukemia cells. Microarray analysis reveals defective induction of p53 target genes and upregulation of chaperone genes

We have previously demonstrated that c-Myc impairs p53-mediated apoptosis in K562 human leukemia cells, which lack ARF. To investigate the mechanisms by which c-Myc protects from p53-mediated apoptosis, we used K562 cells that conditionally express c-Myc and harbor a temperature-sensitive allele of p53. Gene expression profiles of cells expressing wild-type conformation p53 in the presence of either uninduced or induced c-Myc were analysed by cDNA microarrays. The results show that multiple p53 target genes are downregulated when c-Myc is present, including p21WAF1, MDM2, PERP, NOXA, GADD45, DDB2, PIR121 and p53R2. Also, a number of genes that are upregulated by c-Myc in cells expressing wild-type conformation p53 encode chaperones related to cell death protection as HSP105, HSP90 and HSP27. Both downregulation of p53 target genes and upregulation of chaperones could explain the inhibition of apoptosis observed in K562 cells with ectopic c-Myc. Myc-mediated impairment of p53 transactivation was not restricted to K562 cells, but it was reproduced in a panel of human cancer cell lines derived from different tissues. Our data suggest that elevated levels of Myc counteract p53 activity in human tumor cells that lack ARF. This mechanism could contribute to explain the c-Myc deregulation frequently found in cancer.

c-Myc regulates cell size and ploidy but is not essential for postnatal proliferation in liver

The c-Myc protein is a transcription factor implicated in the regulation of multiple biological processes, including cell proliferation, cell growth, and apoptosis. In vivo overexpression of c-myc is linked to tumor development in a number of mouse models. Here, we show that perinatal inactivation of c-Myc in liver causes disorganized organ architecture, decreased hepatocyte size, and cell ploidy. Furthermore, c-Myc appears to have distinct roles in proliferation in liver. Thus, postnatal hepatocyte proliferation does not require c-Myc, whereas it is necessary for liver regeneration in adult mice. These results show novel physiological functions of c-myc in liver development and hepatocyte proliferation and growth.

Hepatocyte CYP2E1 overexpression and steatohepatitis lead to impaired hepatic insulin signaling

Insulin resistance and increased cytochrome P450 2E1 (CYP2E1) expression are both associated with and mechanistically implicated in the development of nonalcoholic fatty liver disease. Although currently viewed as distinct factors, insulin resistance and CYP2E1 expression may be interrelated through the ability of CYP2E1-induced oxidant stress to impair hepatic insulin signaling. To test this possibility, the effects of in vitro and in vivo CYP2E1 overexpression on hepatocyte insulin signaling were examined. CYP2E1 overexpression in a hepatocyte cell line decreased tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 in response to insulin. CYP2E1 overexpression was also associated with increased inhibitory serine 307 and 636/639 IRS-1 phosphorylation. In parallel, the effects of insulin on Akt activation, glycogen synthase kinase 3, and FoxO1a phosphorylation, and glucose secretion were all significantly decreased in CYP2E1 overexpressing cells. This inhibition of insulin signaling by CYP2E1 overexpression was partially c-Jun N-terminal kinase dependent. In the methionine- and choline-deficient diet mouse model of steatohepatitis with CYP2E1 overexpression, insulin-induced IRS-1, IRS-2, and Akt phosphorylation were similarly decreased. These findings indicate that increased hepatocyte CYP2E1 expression and the presence of steatohepatitis result in the down-regulation of insulin signaling, potentially contributing to the insulin resistance associated with nonalcoholic fatty liver disease.

Hepatocyte resistance to oxidative stress is dependent on protein kinase C-mediated down-regulation of c-Jun/AP-1

The prevention of injury from reactive oxygen species is critical for cellular resistance to many death stimuli. Resistance to death from the superoxide generator menadione in the hepatocyte cell line RALA255-10G is dependent on down-regulation of the c-Jun N-terminal kinase (JNK)/AP-1 signaling pathway by extracellular signal-regulated kinase 1/2 (ERK1/2). Because protein kinase C (PKC) regulates both oxidant stress and JNK signaling, the ability of PKC to modulate hepatocyte death from menadione through effects on AP-1 was examined. PKC inhibition with Ro-31-8425 or bisindolylmaleimide I sensitized this cell line to death from menadione. Menadione treatment led to activation of PKCmicro, or protein kinase D (PKD), but not PKCalpha/beta, PKCzeta/lambda, or PKCdelta/. Menadione induced phosphorylation of PKD at Ser-744/748, but not Ser-916, and translocation of PKD to the nucleus. PKC inhibition blocked menadione-induced phosphorylation of PKD, and expression of a constitutively active PKD prevented death from Ro-31-8425/menadione. PKC inhibition led to a sustained overactivation of JNK and c-Jun in response to menadione as determined by in vitro kinase assay and immunoblotting for the phosphorylated forms of both proteins. Cell death from PKC inhibition and menadione treatment resulted from c-Jun activation, since death was blocked by adenoviral expression of the c-Jun dominant negative TAM67. PKC and ERK1/2 independently down-regulated JNK/c-Jun, since inhibition of either kinase failed to affect activation of the other kinase, and simultaneous inhibition of both pathways caused additive JNK/c-Jun activation and cell death. Resistance to death from superoxide therefore requires both PKC/PKD and ERK1/2 activation in order to down-regulate proapoptotic JNK/c-Jun signaling.

Mechanisms involved in the induced differentiation of leukemia cells

Despite the remarkable progress achieved in the treatment of leukemias over the last several years, many problems (multidrug resistance [MDR], cellular heterogeneity, heterogeneous molecular abnormalities, karyotypic instability, and lack of selective action of antineoplastic agents) still remain. The recent progress in tumor molecular biology has revealed that leukemias are likely to arise from disruption of differentiation of early hematopoietic progenitors that fail to give birth to cell lineage restricted phenotypes. Evidence supporting such mechanisms has been derived from studying bone marrow leukemiogenesis and analyzing differentiation of leukemic cell lines in culture that serve as models of erythroleukemic (murine erythroleukemia [MEL] and human leukemia [K562] cells) and myeloid (human promyelocytic leukemia [HL-60] cells) cell maturation. This paper reviews the current concepts of differentiation, the chemical/pharmacological inducing agents developed thus far, and the mechanisms involved in initiation of leukemic cell differentiation. Emphasis was given on commitment and the cell lineage transcriptional factors as key regulators of terminal differentiation as well as on membrane-mediated events and signaling pathways involved in hematopoietic cell differentiation. The developmental program of MEL cells was presented in considerable depth. It is quite remarkable that the erythrocytic maturation of these cells is orchestrated into specific subprograms and gene expression patterns, suggesting that leukemic cell differentiation represents a highly coordinated set of events that lead to irreversible growth arrest and expression of cell lineage restricted phenotypes. In MEL and other leukemic cells, differentiation appears to be accompanied by differentiation-dependent apoptosis (DDA), an event that can be exploited chemotherapeutically. The mechanisms by which the chemical inducers promote differentiation of leukemic cells have been discussed.

CYP2E1 overexpression alters hepatocyte death from menadione and fatty acids by activation of ERK1/2 signaling

Chronic oxidative stress induced by overexpression of the cytochrome P450 isoform 2E1 (CYP2E1) has been implicated in hepatocyte injury and death. However, the mechanism by which CYP2E1 overexpression may promote cell death is unknown. Acute oxidative stress activates mitogen-activated protein kinases (MAPK), suggesting that chronic oxidant generation by CYP2E1 may regulate cellular responses through these signaling pathways. The effect of CYP2E1 overexpression on MAPK activation and their function in altering death responses of CYP2E1-overexpressing hepatocytes were investigated. Chronic CYP2E1 overexpression led to increased extracellular signal-regulated kinase 1/2 (ERK1/2) activation constitutively and in response to oxidant stress from the superoxide generator menadione. CYP2E1-overexpressing cells were resistant to menadione toxicity through an ERK1/2-dependent mechanism. Similar to menadione, the polyunsaturated fatty acid (PUFA) arachidonic acid (AA) induced an increased activation of ERK1/2 in hepatocytes that overexpressed CYP2E1. However, CYP2E1-overexpressing cells were sensitized to necrotic death from AA and the PUFA gamma-linolenic acid, but not from saturated or monounsaturated fatty acids. Death from PUFA resulted from oxidative stress and was blocked by inhibition of ERK1/2, but not p38 MAPK or activator protein-1 signaling. CYP2E1 expression induced ERK1/2 activation through increased epidermal growth factor receptor (EGFR)/c-Raf signaling. Inhibition of EGFR signaling reversed CYP2E1-induced resistance to menadione and sensitization to AA toxicity. In conclusion, chronic CYP2E1 overexpression leads to sustained ERK1/2 activation mediated by EGFR/c-Raf signaling. This adaptive response in hepatocytes exposed to chronic oxidative stress confers differential effects on cellular survival, protecting against menadione-induced apoptosis, but sensitizing to necrotic death from PUFA.

Therapeutic mechanism of ginkgo biloba exocarp polysaccharides on gastric cancer

AIM: To study the therapeutic mechanism of Ginkgo biloba exocarp polysaccharides (GBEP) on gastric cancer.

METHODS: Thirty patients with gastric cancer were treated with oral GBEP capsules. The area of tumors was measured by electron gastroscope before and after treatment, then the inhibitory and effective rates were calculated. The ultrastructures of tumor cells were examined by transmissional electron microscope. Cell culture, MTT, flow cytometry were performed to observe proliferation, apoptosis and changes of relevant gene expression of human gastric cancer SGC-7901 cells.

RESULTS: Compared with the statement before treatment, GBEP capsules could reduce the area of tumors, and the effective rate was 73.4%. Ultrastructural changes of the cells indicated that GBEP could induce apoptosis and differentiation in tumor cells of patients with gastric cancer. GBEP could inhibit the growth of human gastric cancer SGC-7901 cells following 24-72 h treatment in vitro at 10-320 mg/L, which was dose- and time-dependent. GBEP was able to elevate the apoptosis rate and expression of c-fos gene, but reduce the expression of c-myc and bcl-2 genes also in a dose-dependent manner.

CONCLUSION: The therapeutic mechanism of GBEP on human gastric cancer may relate to its effects on the expression of c-myc, bcl-2 and c-fos genes, which can inhibit proliferation and induce apoptosis and differentiation of tumor cells.

Oxidant-induced hepatocyte injury from menadione is regulated by ERK and AP-1 signaling

Oxidative stress has been implicated as a mechanism for a variety of forms of liver injury. Although reactive oxygen species (ROS) may damage cellular macromolecules directly, oxidant-induced cell death may result from redox effects on signal transduction pathways. To understand the mechanisms of hepatocyte death from oxidative stress, the functions of the mitogen-activated protein kinases (MAPKs) were determined during oxidant-induced hepatocyte injury from menadione. Low, nontoxic, and high toxic concentrations of the superoxide generator menadione were established in the RALA255-10G rat hepatocyte cell line. Death from menadione was blocked by catalase and ebselen, indicating that death was secondary to oxidant generation and not arylation. Treatment with a nontoxic menadione concentration resulted in a brief activation of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). In contrast, treatment with a toxic menadione concentration induced a prolonged activation of both ERK and JNK. Chemical inhibition of ERK function sensitized RALA hepatocytes to death from previously nontoxic menadione concentrations in association with sustained JNK activation. Adenoviral expression of a dominant-negative protein for c-Jun, a downstream substrate for JNK, blocked death from menadione. The pro-apoptotic effect of c-Jun was not mediated through the mitochondrial death pathway. In conclusion, RALA hepatocyte resistance to oxidant-induced death from menadione is dependent on ERK, whereas cell death is mediated by AP-1 activation. These findings identify signaling pathways that may be therapeutic targets in the prevention or treatment of oxidant-induced liver injury.

Effect of apoptosis on gastric adenocarcinoma cell line SGC-7901 induced by cis-9, trans-11-conjugated linoleic acid

AIM: To determine the effect of apoptosis on gastric cancer cells (SGC-7901) induced by cis-9, trans-11-conjugated linoleic acid (c9, t11-CLA) and its possible mechanism in the inhibition of cancer cells growth.

METHODS: Using cell culture, flow cytometery and immunocytochemical techniques, we examined the cell growth, frequency of apoptosis and distribution of cell cycle, expression of ki67, bcl-2, Fas, and c-myc of SGC-7901 cells which were treated with various c9, t11-CLA concentrations (25, 50, 100 and 200 μmol·L^-1) of c9, t11-CLA for 24 h and 48 h, with a negative control (0.1% ethanol).

RESULTS: The growth of SGC-7901 cells was inhibited by c9,t11-CLA. Eight days after treatment with various concentrations of c9,t11-CLA, as mentioned above, the inhibition rates were 5.9%, 20.2%, 75.6% and 82.4%, respectively. The frequency of apoptosis on SGC-7901 cells induced by different concentrations of c9, t11-CLA (except for 25 μmol·L^-1, 24 h) was significantly greater than that in the negative control (P < 0.01). To further investigate the influence of the cell cycle progression, we found that apoptosis induced by c9, t11-CLA may be involved in blocking the cell cycle of SGC-7901 cells. Immunocytochemical staining demonstrated that SGC-7901 cells preincubated in media supplemented with different c9, t11-CLA concentrations for various time periods significantly decreased the expressions of ki67 (the expression rates were 18.70%-3.20%, at 24 h and 8.10%-0.20% at 48 h, respectively), bcl-2 (4.30%-0.15% at 24 h and 8.05%-0% at 48 h),and c-myc (4.85%-2.20% at 24 h and 4.75%-0.30% at 48 h) as compared with those in the controls (the expressions of ki67, bcl-2, and c-myc were 15.1% at 24 h and 13.5% at 48 h, 6.80% at 24 h and 8.00% at 48 h, 5.50% at 24 h and 5.30% at 48 h, respectively) (P < 0.01), whereas the expressions of Fas were increased (0.60%-2.75%, 24 h and 0.45%-5.95%, 48 h).

CONCLUSION: The growth and proliferation of SGC-7901 cells are inhibited by c9, t11-CLA via blocking the cell cycle, pathways of bcl-2-associated mitochondria with reduced expression of bcl-2 and Fas-associated death domain protein (FADD) with enhanced expression of Fas. But expression of c-myc on SGC-7901 cells is lower than that in negative control, which needs to be studied further.

Induction and regulation of hepatocyte apoptosis by oxidative stress

Reactive oxygen intermediates (ROI) have been implicated in the induction of hepatocyte apoptosis that results from a variety of forms of liver injury. Exogenous oxidants induce hepatocyte apoptosis and may mediate death during inflammatory liver injury. Lethal levels of intracellularly generated ROI resulting from hepatotoxin metabolism, or the induction of enzymes in the cytochrome P450 family, are also important inducers of apoptosis. In addition, ROI production may mediate death from a number of diverse factors, including tumor necrosis factor-alpha, bile acids, ischemia, and transforming growth factor-beta1. Oxidants alter many redox-sensitive cellular signaling pathways, including mitogen-activated protein kinases and transcription factors such as activator protein-1 and nuclear factor-kappaB. The mechanisms of oxidant-induced hepatocyte apoptosis remain unclear, but probably involve effects on cell signaling, as well as direct chemical interactions. The delineation of stimulus-specific mechanisms of oxidant-dependent hepatocyte apoptosis is important to the design of effective therapies for a number of forms of liver injury.

Spontaneous apoptosis in primary cultures of human and rat hepatocytes: molecular mechanisms and regulation by dexamethasone

To elucidate the biochemical pathways leading to spontaneous apoptosis in primary cultures of human and rat hepatocytes, we examined the activation of the caspase cascade, the expression of Bcl-2-related-proteins and heat shock proteins. Comparisons were made before and after dexamethasone (DEX) treatment. We show that DEX inhibited spontaneous apoptosis in a dose-dependent manner. DEX increases the expression of anti-apoptotic Bcl-2 and Bcl-x(L) proteins, decreases the expression of pro-apoptotic Bax and inhibits Bad translocation thereby preventing the release of cytochrome c, the activation of caspases, and cell death. Although, the expression of Hsp27 and Hsp70 proteins remained unchanged, the oncogenic protein c-Myc is upregulated upon DEX-treatment. These results indicate that DEX mediates its survival effect against spontaneous apoptosis by acting upstream of the mitochondrial changes. Thus, the mitochondrial apoptotic pathway plays a major role in regulating spontaneous apoptosis in these cells. Blocking this pathway therefore may assist with organ preservation for transplant, drug screening, and other purposes.

The myc oncogene: MarvelouslY Complex

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

NF-kappaB inhibition sensitizes hepatocytes to TNF-induced apoptosis through a sustained activation of JNK and c-Jun

Hepatocyte resistance to tumor necrosis factor alpha (TNF)-induced apoptosis is dependent on activation of the transcription factor nuclear factor kappaB (NF-kappaB). To determine the mechanism by which NF-kappaB protects against TNF toxicity, the effect of NF-kappaB inactivation on the proapoptotic c-Jun NH(2)-terminal kinase (JNK) signaling pathway was examined in the rat hepatocyte cell line RALA255-10G. Adenovirus-mediated NF-kappaB inactivation led to a prolonged activation of JNK and increased activating protein-1 (AP-1) transcriptional activity in response to TNF treatment. Inhibition of the function of the JNK substrate and AP-1 subunit c-Jun blocked cell death from NF-kappaB inactivation and TNF as determined by measures of cell survival, numbers of apoptotic and necrotic cells, and DNA hypoploidy. Inhibition of c-Jun function blocked mitochondrial cytochrome c release and activation of caspase-3 and -7. NF-kappaB therefore blocks the TNF death pathway through down-regulation of JNK and c-Jun/AP-1. In conclusion, sustained JNK activation that occurs in the absence of NF-kappaB initiates apoptosis through a c-Jun-dependent induction of the mitochondrial death pathway.

Increased cytochrome P-450 2E1 expression sensitizes hepatocytes to c-Jun-mediated cell death from TNF-alpha

The mechanisms underlying hepatocyte sensitization to tumor necrosis factor-alpha (TNF-alpha)-mediated cell death remain unclear. Increases in hepatocellular oxidant stress such as those that occur with hepatic overexpression of cytochrome P-450 2E1 (CYP2E1) may promote TNF-alpha death. TNF-alpha treatment of hepatocyte cell lines with differential CYP2E1 expression demonstrated that overexpression of CYP2E1 converted the hepatocyte TNF-alpha response from proliferation to apoptotic and necrotic cell death. Death occurred despite the presence of increased levels of nuclear factor-kappaB transcriptional activity and was associated with increased lipid peroxidation and GSH depletion. CYP2E1-overexpressing hepatocytes had increased basal and TNF-alpha-induced levels of c-Jun NH(2)-terminal kinase (JNK) activity, as well as prolonged JNK activation after TNF-alpha stimulation. Sensitization to TNF-alpha-induced cell death by CYP2E1 overexpression was inhibited by antioxidants or adenoviral expression of a dominant-negative c-Jun. Increased CYP2E1 expression sensitized hepatocytes to TNF-alpha toxicity mediated by c-Jun and overwhelming oxidative stress. The chronic increase in intracellular oxidant stress created by CYP2E1 overexpression may serve as a mechanism by which hepatocytes are sensitized to TNF-alpha toxicity in liver disease.

Signaling for ethanol-induced apoptosis and repair in vitro

OBJECTIVES

To evaluate whether caspases are involved in ethanol (EtOH)-induced apoptosis and if polyenylphosphatidylcholine (PPC) affects apoptosis, in vitro in Hep G2 cells.

METHODS

Cells were treated with 100 mmol/L EtOH for 24 h and with 2 doses of 100 mmol/L EtOH (1/24 h) in the presence of absence of 20 mmol/L of PPC or 50 micromol/L caspase 3 inhibitor (IDN). Cells were analyzed for apoptosis by transmission electron microscopy (TEM) 6000 cells/treatment, DNA fragmentation by ELISA and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (T dt-mediated d-UTP) nick-end-labeling, TUNEL.

RESULTS

100 mmol/L dose of EtOH resulted in 22 +/- 2.5% (p < 0.001) apoptosis (vs. control). Two consecutive doses of 100 mmol/L EtOH for 24 h each caused 36 +/- 3.0% (p < 0.001 vs. control and p < 0.05 vs. one dose). PPC significantly reduced apoptosis (vs. non exposed to PPC): 100 mmol/L -12 +/- 1.5% (p < 0.05) and 2 x 10(-)(0) mmol/L -20 +/- 2.0% (p < 0.001). Pretreatment with 50 micromol caspase inhibitor reduced EtOH-induced apoptosis in a similar proportion.

CONCLUSIONS

PPC downregulates EtOH-apoptosis by a mechanism similar to caspase inhibition.