Cellular thiol status-dependent inhibition of tumor cell growth via modulation of retinoblastoma protein phosphorylation by (-)-epigallocatechin

Evasion of anti-growth signaling: A key step in tumorigenesis and potential target for treatment and prophylaxis by natural compounds

The evasion of anti-growth signaling is an important characteristic of cancer cells. In order to continue to proliferate, cancer cells must somehow uncouple themselves from the many signals that exist to slow down cell growth. Here, we define the anti-growth signaling process, and review several important pathways involved in growth signaling: p53, phosphatase and tensin homolog (PTEN), retinoblastoma protein (Rb), Hippo, growth differentiation factor 15 (GDF15), AT-rich interactive domain 1A (ARID1A), Notch, insulin-like growth factor (IGF), and Krüppel-like factor 5 (KLF5) pathways. Aberrations in these processes in cancer cells involve mutations and thus the suppression of genes that prevent growth, as well as mutation and activation of genes involved in driving cell growth. Using these pathways as examples, we prioritize molecular targets that might be leveraged to promote anti-growth signaling in cancer cells. Interestingly, naturally occurring phytochemicals found in human diets (either singly or as mixtures) may promote anti-growth signaling, and do so without the potentially adverse effects associated with synthetic chemicals. We review examples of naturally occurring phytochemicals that may be applied to prevent cancer by antagonizing growth signaling, and propose one phytochemical for each pathway. These are: epigallocatechin-3-gallate (EGCG) for the Rb pathway, luteolin for p53, curcumin for PTEN, porphyrins for Hippo, genistein for GDF15, resveratrol for ARID1A, withaferin A for Notch and diguelin for the IGF1-receptor pathway. The coordination of anti-growth signaling and natural compound studies will provide insight into the future application of these compounds in the clinical setting.

Cytotoxic, genotoxic and antigenotoxic potencies of oligorutins

Rutin has been enzymatically oligomerized by laccase from Trametes versicolor. Five fractions of oligomers were obtained from the monomers having high solubility in water, which can reach 351-times that of rutin. Cytotoxicity of rutin and oligorutin fractions was evaluated towards K562 cells. Oligorutin fractions showed a lower antiproliferative effect compared with its monomer. The genotoxic potential of rutin and oligorutin fractions was assessed, at the limit of the solubility of each molecule, using the comet test. None of the tested concentrations of either rutin or oligorutin fractions has showed a genotoxic effect. Similarly, the antigenotoxic effect of these flavonoids was tested using the same assay. The obtained results showed a higher ability of oligorutin fractions to reduce the genotoxicity induced by hydrogen peroxide compared with monomeric rutin.

Anticancer Activities and Mechanisms of Blumea balsamifera Extract in Hepatocellular Carcinoma Cells

Blumea balsamifera (also known as sambong), a medicinal plant, is known to improve physiological disorders such as rheumatism and hypertension. However, its anticancer activity has not been well elucidated. In this study, we found that Blumea balsamifera MeOH extract (BME) induced growth inhibitory activity in rat and human hepatocellular carcinoma cells (McA-RH7777, HepG2, respectively) without cytotoxicity as in with rat hepatocytes used as a normal cell model. BME induced cell cycle arrest at G1 phase via decreases in expression of cyclin-E and phosphorylation of retinoblastoma (Rb) protein in both dose- and time-dependent manners. Furthermore, BME reduced the level of a proliferation related ligand (APRIL) which stimulates tumor cell growth. The anti-proliferative effect of BME was improved slightly but significantly by the treatment with recombinant human APRIL. These findings suggest that BME may have a possible therapeutic potential in hepatoma cancer patients and the depletion of cellular APRIL may be one of the important mechanisms on the growth inhibitory effect of BME.

Investigation of cytotoxic, apoptosis-inducing, genotoxic and protective effects of the flavonoid rutin in HTC hepatic cells

Rutin is a flavonoid with antioxidant, vasodilatory, anti-inflammatory and immune-stimulating activities. To study the toxicity of rutin and its protective effect, this work investigated the cytotoxic, apoptosis-inducing, genotoxic and protective effects of rutin in HTC cells. In the MTT assay, the highest concentration tested (810 μM) showed cytotoxicity after 72 h of treatment, where cell viability and cell proliferation was diminished. None of the concentrations of rutin tested induced apoptosis after 24h treatment. The highest concentration of rutin after 24h treatment induced DNA damage, shown in the comet assay, but did have a genotoxic effect in the micronucleus test. Rutin was tested against the pro-carcinogenic agent benzo(a)pyrene, at concentrations of 90, 270 and 810 μM, and was found to reduce induced DNA damage significantly. This protective effect of rutin against a pro-carcinogen, suggests an important biological activity for this compound, which can contribute to human health through the diet.

Combination of Zizyphus jujuba and green tea extracts exerts excellent cytotoxic activity in HepG2 cells via reducing the expression of APRIL

Hepatocellular carcinoma is a type of tumor highly resistant to available chemotherapeutic agents. The treatment of hepatocellular carcinoma remains a challenge that needs new approaches in the future. In a previous study, we demonstrated that the chloroform fraction (CHCl(3)-F) from Z. jujuba has anticancer activity in human liver cancer cells (HepG2), and that combining CHCl(3)-F with green tea extracts (GTE) results in enhanced effects of anticancer activity in the cells. To further understand the mechanism of the anticancer activity of combining CHCl(3)-F and GTE in HepG2 cells, we investigated whether the addition of a mixture of CHCl(3)-F and GTE would affect the expression of APRIL (a proliferation-inducing ligand), which was expressed in HepG2 cells from 4 hours of incubation in vitro. We have shown that CHCl(3)-F and GTE enhanced anti-cancer activity by reducing the expression of APRIL. We speculate that the CHCl(3)-F and GTE mixture might provide a lead to a new drug design to treat hepatocellular carcinoma in the future.

Green tea extract enhances the selective cytotoxic activity of Zizyphus jujuba extracts in HepG2 cells

Anticarcinogenic effects attributed to phytochemicals may be based on synergistic, additive, or antagonistic interactions of many compounds. In our previous study, we demonstrated that the chloroform fraction (CHCl(3)-F) from Z. jujuba has anticancer activity in HepG2 cells. In China, many people drink jujuba tea and believe in the synergic effects of jujuba and tea for better health. We therefore investigated the effects of CHCl(3)-F and green tea extract (GTE), and their underlying mechanisms of action in HepG2 cells. Our results showed that GTE enhanced the effect of CHCl(3)-F on cell viability in HepG2 cells, without cytotoxicity in rat hepatocytes, which was used as a normal cell model. Furthermore, combination of CHCl(3)-F and GTE caused an effect on G1 phase arrest but not on apoptosis. Interestingly, the mechanism of the G1 arrest was associated, not with an increase in p27(Kip1) levels and the hypohosphorylation of Rb, which are pathways used by CHCl(3)-F on G1 arrest in HepG2 cells, but with increases in p53 and p21(Waf1/Cip1) levels, and a decrease in cyclin E levels. Collectively, our findings suggest that combination of CHCl(3)-F and GTE produces an enhanced cell growth inhibition effect, and that the resultant G1 arrest was caused via a different mechanism as that of CHCl(3)-F treatment alone.

Mechanism of growth inhibitory effect of Blumea balsamifera extract in hepatocellular carcinoma

Blumea balsamifera is known to improve physiological disorders such as rheumatism and hypertension, but its anticancer activity has not been well elucidated. In this study, we found that Blumea balsamifera MeOH extract (BME) induced growth-inhibitory activity in rat and human hepatocellular carcinoma cells without cytotoxicity in rat hepatocytes which were used as a normal cell model. BME induced cell cycle arrest at the G1 phase via decreases in the expression of cyclin-E and phosphorylation of retinoblastoma protein. Furthermore, BME reduced the level of a proliferation-inducing ligand, that stimulates tumor cell growth. These findings suggest that BME has possible therapeutic potential in hepatoma cancer patients and that depletion of cellular APRIL is an important mechanism in the growth-inhibitory effect of BME.

(1'S)-Acetoxychavicol acetate and its enantiomer inhibit tumor cells proliferation via different mechanisms

Elucidation of the mechanisms underlying potential anticancer drugs continues and unraveling these mechanisms would not only provide a conceptual framework for drug design but also promote use of natural products for chemotherapy. The biological effects of (1'S)-acetoxychavicol acetate ((S)-ACA) have been widely investigated. However, in most cases, a natural product or synthetic racemic compound was used in the study. In this study, we prepared (S)-ACA and its enantiomer (R)-ACA by a lipase-catalyzed esterification method and sought to determine the mechanisms of action of (S)-ACA and (R)-ACA in the growth inhibitory effect in Ehrlich ascites tumor cells (EATC). (S)-ACA caused an accumulation of tumor cells in the G1 phase of the cell cycle, which was accompanied by a decrease in phosphorylated retinoblastoma protein (Rb), an increase in Rb and a decrease in the phosphorylation of p27kip1. However, (R)-ACA caused an accumulation of tumor cells in the G2 phase of the cell cycle, an increase in hyperphosphorylated Rb and an increase in the phosphorylation of p27kip1. The results obtained in the present study demonstrate for the first time, to the best of our knowledge, that both (S)-ACA and (R)-ACA caused the inhibition of tumor cells growth but the inhibition was caused via different mechanisms.

Mechanism of the anti-cancer activity of Zizyphus jujuba in HepG2 cells

The Zizyphus jujuba fruit has been used as a traditional Chinese medicinal herb and considered to affect various physiological functions in the body for thousands of years. However, its anti-cancer activity and mechanism of action remain to be elucidated. We investigated the anti-cancer activity of Zizyphus jujuba Mill and its underlining mechanisms of action in human hepatoma cells (HepG2) and found that the extract of Z. jujuba decreased the viability of the cells. Further extraction of the initial Z. jujuba extract with organic solvents revealed that the chloroform fraction (CHCl(3)-F) was the most effective. Interestingly, the CHCl(3)-F induced not only apoptosis but also G1 arrest at a low concentration (100 mug/ml) and G2/M arrest at a higher concentration (200 mug/ml) by cell cycle assay. Apoptosis, an increase in intracellular ROS (reactive oxygen species) level, a decline of mitochondrial membrane potential at low Z. jujuba concentrations, and a ROS-independent mitochondrial dysfunction pathway at high concentrations were all observed. CHCl(3)-F-induced G1 arrest in HepG2 cells was associated with an increase in hypohosphorylation of Rb and p27(Kip1), and a decrease of phosphorylated Rb. However, CHCl(3)-F-induced G2/M arrest in HepG2 cells correlated with a decrease of the p27(Kip1) levels and generation of the phosphorylation of p27(Kip1), however the hypohosphorylation of Rb protein remained. Collectively, our findings suggest that the CHCl(3)-F extract of Z. jujuba extract induced a concentration dependent effect on apoptosis and a differential cell cycle arrest in HepG2 cells.

Evaluation of the effect of paclitaxel, epirubicin and tamoxifen by cell kinetics parameters in estrogen-receptor-positive ehrlich ascites tumor (eat) cells growing in vitro

In this study the antiproliferative effects of Paclitaxel (PAC), Epirubicin (EPI) and Tamoxifen (TAM) on growth kinetics of Ehrlich Ascites Tumor (EAT) cells were examined in culture. An estrogen-receptor-positive ER (+) hyperdiploid EAT cell line growing in vitro was also analysed in the present study. IC50 doses of PAC, EPI and TAM (12 microg/ml, 12 microg/ml and 2 microg/ml, respectively) were used. Cells were treated with the above doses for 0, 4, 8, 16, 24 and 32 hrs. At the end of these periods, living cell numbers were determined by collecting EAT cells in every group for growth study rate and for MTT assay. Therefore, the mitotic index was determined in the same experimental groups. The proliferation of EAT cells, inhibited by PAC, EPI and TAM concentrations was compared to control with increasing treatment time (4-32 hrs). Treatment of PAC, EPI and TAM alone for 24 hrs decreased the proliferation rate of EAT cells by 50% with respect to control. The inhibition of proliferation rate was higher in double drug treatment than that in single drug treatment with increased treatment time. In the treatment of three drugs applied for 32 hrs, this effect reached a maximum and proliferation rate decreased by 12% as compared to the (100%) control. In our studies, when the mitotic index parameter data were evaluated to determine which phase of the cell cycle was affected by PAC to cause the repression of cell reproduction, it was found that PAC exerted of its cytotoxic effect by causing cell accumulation at mitosis. The accumulation of the cells resulted in an increase in mitotic index values, which was an expected consequence of PAC treatment. It was observed that depending on the drug treatments, inhibition of proliferation rate and mitotic index in EAT cells were increased with respect to control, being with statistically significant occurrence (p < 0.01 - p < 0.001). As a result, concomitant treatment combined with hormonal therapy has given improved results compared with single treatment and PAC + EPI + TAM treatments had a maximum synergistic effect for 32 hrs (p < 0.001).

Cellular thiol status-dependent inhibition of tumor cell growth via modulation of p27kip1 translocation and retinoblastoma protein phosphorylation by 1′-acetoxychavicol acetate

1'-Acetoxychavicol acetate (ACA) has been shown to inhibit tumor cell growth, but there is limited information on its effects on cell signaling and the cell cycle control pathway. In this study, we sought to determine how ACA alters cell cycle and its related control factors in its growth inhibitory effect in Ehrlich ascites tumor cells (EATC). ACA caused an accumulation of cells in the G1 phase and an inhibition of DNA synthesis, which were reversed by supplementation with N-acetylcysteine (NAC) or glutathione ethyl ester (GEE). Furthermore, ACA decreased hyperphosphorylated Rb levels and increased hypophosphorylated Rb levels. NAC and GEE also abolished the decease in Rb phosphorylation by ACA. As Rb phosphorylation is regulated by G1 cyclin dependent kinase and CDK inhibitor p27(kip1), which is an important regulator of the mammalian cell cycle, we estimated the amount of p27(kip1) levels by western blotting. Treatment with ACA had virtually no effect on the amount of p27(kip1) levels, but caused a decrease in phosphorylated p27(kip1) and an increase in unphosphorylated p27(kip1) as well as an increase in the nuclear localization of p27(kip1). These events were abolished in the presence of NAC or GEE. These results suggest that in EATC, cell growth inhibition elicited by ACA involves decreases in Rb and p27(kip1) phosphorylation and an increase in nuclear localization of p27(kip1), and these events are dependent on the cellular thiol status.

Actions of a histone deacetylase inhibitor NSC3852 (5-nitroso-8-quinolinol) link reactive oxygen species to cell differentiation and apoptosis in MCF-7 human mammary tumor cells

NSC3852 (5-nitroso-8-quinolinol) has cell differentiation and antiproliferative activity in human breast cancer cells in tissue culture and antitumor activity in mice bearing P388 and L1210 leukemic cells. We investigated the mechanism of NSC3852 action in MCF-7 human breast cancer cells using electron spin resonance (ESR). Reactive oxygen species (ROS) were detected in MCF-7 cell suspensions incubated with NSC3852 using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Formation of the DMPO-OH adduct was quenched by the addition of superoxide dismutase but not by catalase, and we concluded that superoxide was generated in the NSC3852-treated cells. The flavoprotein inhibitor diphenylene iodonium suppressed ROS production, providing evidence for the involvement of a flavin-dependent enzyme system in the ROS response to NSC3852. A biologically significant oxidative response to NSC3852 occurred in MCF-7 cells. An early marker of oxidative stress was a decrease in the [glutathione]/[glutathione disulfide] ratio 1 h after NSC3852 addition. Oxidative DNA damage, marked by the presence of 8-oxoguanine, and DNA-strand breakage occurred in cells exposed to NSC3852 for 24 h. Apoptosis peaked 48 h after exposure to NSC3852. Pretreatment with the glutathione precursor N-acetyl-l-cysteine (NAC) prevented DNA-strand breakage and apoptosis. Pretreatment with NAC also reversed NSC3852 decreases in E2F1, Myc, and phosphorylated retinoblastoma protein, indicative of redox-sensitive pathway(s) in MCF-7 cells during G(1) phase of the cell cycle. We conclude that ROS formation is involved in the apoptotic and cell differentiation responses to NSC3852 in MCF-7 cells.

Protective Effect of Green Tea Extract and Tea Polyphenols against FK506-Induced Cytotoxicity in Renal Cells

The nephrotoxicity induced by the immunosuppressive drug FK506 (tacrolimus or fujimycin), limits its usefulness in widespread application, and the underlying mechanism has not been completely understood. The primary targets of FK506 in the kidney are the proximal tubular epithelial cells. In this study, the protection of green tea extract against FK506-induced cell death of LLC-PK1 cells was investigated. FK506 caused a significant decrease in survival of the cells, but the addition of green tea extract reduced this effect in a dose-dependent manner. Treatment of the cells with 50 microM (41.1 microg/ml) FK506 induced a significant increase in annexin V-positive/propidium iodide-negative cells from 2.68 to 14.5%, whereas the addition of 6.25, 12.5, and 25 microg/ml of green tea extract caused a significant protective effect in apoptotic cells from 14.5 to 6.51, 3.20 and 3.02%, respectively. The effect of five different constituent tea polyphenols was also examined. Epigallocatechin-gallate and epigallocatechin significantly reduced FK506-induced cytotoxicity but epicatechin and catechin had no effect on cell viability. Furthermore, changes in cytochrome c release and caspase activation, which characterize apoptosis, were studied. Epigallocatechin-gallate and epigallocatechin suppressed a significant release of cytochrome c and activation of caspase-3 in FK506-treated LLC-PK1 cells.

Reactive oxygen species-independent G1 arrest induced by evening primrose extract in Ehrlich ascites tumor cells

We previously demonstrated that evening primrose extract (EPE) induced apoptosis in Ehrlich ascites tumor cells (EATC), and this effect was specific on tumor cells. Furthermore, our results demonstrated that EPE exposure elicited a rapid increase in the activity of superoxide dismutase and intracellular peroxides levels. These changes caused translocation of Bax to mitochondria and a subsequent release of mitochondrial cytochrome c. However, no activation of caspase-3 was observed in EPE-treated EATC. On the other hand, apoptosis-inducing factor (AIF) was translocated from mitochondria to nuclei. The EPE-induced translocation of AIF was suppressed with the addition of catalase, suggesting that the rapid intracellular peroxide levels after addition of EPE triggers off induction of apoptosis, which is AIF-mediated and caspase-independent. In this study, we have shown that EPE elicited a dose-dependent accumulation of cells in the G1 phase and inhibited DNA synthesis. Our results also demonstrated that cell cycle arrest and inhibition of proliferation in EATC by EPE are associated with decreased Rb phosphorylation. Furthermore, inhibitions of Rb phosphorylation and DNA synthesis by EPE were not suppressed with the addition of catalase. The present study suggests that intracellular peroxides, which trigger off induction of apoptosis, are not the trigger of EPE-induced G1 arrest in cell cycle.

Green tea extract inhibits angiogenesis of human umbilical vein endothelial cells through reduction of expression of VEGF receptors

Epidemiological and animal studies have indicated that consumption of green tea is associated with a reduced risk of developing certain forms of cancer. However, the inhibitory mechanism of green tea in angiogenesis, an important process in tumor growth, has not been well established. In the present study, green tea extract (GTE) was tested for its ability to inhibit cell viability, cell proliferation, cell cycle dynamics, vascular endothelial growth factor (VEGF) and expression of VEGF receptors fms-like tyrosine kinase (Flt-1) and fetal liver kinase-1/Kinase insert domain containing receptor (Flk-1/KDR) in vitro using human umbilical vein endothelial cells (HUVECs). GTE in culture media did not affect cell viability but significantly reduced cell proliferation dose-dependently and caused a dose-dependent accumulation of cells in the G1 phase. The decrease of the expression of Flt-1 and KDR/Flk-1 in HUVEC by GTE was detected with immunohistochemical and Western blotting methods. These results suggest that GTE may have preventive effects on tumor angiogenesis and metastasis through reduction of expression of VEGF receptors.