Green tea polyphenol EGCG induces lipid-raft clustering and apoptotic cell death by activating protein kinase Cδ and acid sphingomyelinase through a 67 kDa laminin receptor in multiple myeloma cells

EGCG [(-)-epigallocatechin-3-O-gallate], the major polyphenol of green tea, has cancer chemopreventive and chemotherapeutic activities. EGCG selectively inhibits cell growth and induces apoptosis in cancer cells without adversely affecting normal cells; however, the underlying molecular mechanism in vivo is unclear. In the present study, we show that EGCG-induced apoptotic activity is attributed to a lipid-raft clustering mediated through 67LR (67 kDa laminin receptor) that is significantly elevated in MM (multiple myeloma) cells relative to normal peripheral blood mononuclear cells, and that aSMase (acid sphingomyelinase) is critical for the lipid-raft clustering and the apoptotic cell death induced by EGCG. We also found that EGCG induces aSMase translocation to the plasma membrane and PKCδ (protein kinase Cδ) phosphorylation at Ser664, which was necessary for aSMase/ceramide signalling via 67LR. Additionally, orally administered EGCG activated PKCδ and aSMase in a murine MM xenograft model. These results elucidate a novel cell-death pathway triggered by EGCG for the specific killing of MM cells.     

Oxygen partial pressure modulates 67-kDa laminin receptor expression, leading to altered activity of the green tea polyphenol, EGCG

(−)-Epigallocatechin-3-O-gallate (EGCG) exhibits anti-tumor activity mediated via the 67-kDa laminin receptor (67LR). In this study, we found that 67LR protein levels are reduced by exposure to low O₂ levels (5%), without affecting the expression of HIF-1α. We also found that EGCG-induced anti-cancer activity is abrogated under low O₂ levels (5%) in various cancer cells. Notably, treatment with the proteasome inhibitor, prevented down-regulation of 67LR and restored sensitivity to EGCG under 5% O₂. In summary, 67LR expression is highly sensitive to O₂ partial pressure, and the activity of EGCG can be regulated in cancer cells by O₂ partial pressure.     

The involvement of the 67 kDa laminin receptor-mediated modulation of cytoskeleton in the degranulation inhibition induced by epigallocatechin-3-O-gallate

Recently, we have reported that (-)-epigallocatechin-3-O-gallate (EGCG) acts as an inhibitor of degranulation. However, the inhibitory mechanism for degranulation is still poorly understood. Here we show that suppression of exocytosis-related myosin II regulatory light chain phosphorylation and alteration of actin remodeling are involved in the inhibitory effect of EGCG on the calcium ionophore-induced degranulation from human basophilic KU812 cells. Surface plasmon resonance assay also revealed that EGCG binds to the cell surface, and the disruption of lipid rafts resulted in reduction of EGCG's ability. We have previously identified the raft-associated 67kDa laminin receptor (67LR) as an EGCG receptor on the cell surface. Treatment of the cells with anti-67LR antibody or RNA interference-mediated downregulation of 67LR expression abolished the effects of EGCG. These findings suggest that EGCG-induced inhibition of the degranulation includes the primary binding of EGCG to the cell surface 67LR and subsequent modulation of cytoskeleton.     

Green tea polyphenol epigallocatechin-3-gallate signaling pathway through 67-kDa laminin receptor

(-)-Epigallocatechin-3-gallate (EGCG), the principal polyphenol in green tea, has been shown to be a potent chemopreventive agent. Recently, 67-kDa laminin receptor (67LR) has been identified as a cell surface receptor for EGCG that mediates the anticancer activity of EGCG. Indeed, expression of 67LR confers EGCG responsiveness to tumor cells; however, the molecular basis for the anticancer activity of EGCG in vivo is not entirely understood. Here we show that (i) using a direct genetic screen, eukaryotic translation elongation factor 1A (eEF1A) is identified as a component responsible for the anticancer activity of EGCG; (ii) through both eEF1A and 67LR, EGCG induces the dephosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) at Thr-696 and activates myosin phosphatase; and (iii) silencing of 67LR, eEF1A, or MYPT1 in tumor cells results in abrogation of EGCG-induced tumor growth inhibition in vivo. Additionally, we found that eEF1A is up-regulated by EGCG through 67LR. Overall, these findings implicate both eEF1A and MYPT1 in EGCG signaling for cancer prevention through 67LR.     

Involvement of 67-kDa laminin receptor-mediated myosin phosphatase activation in antiproliferative effect of epigallocatechin-3-O-gallate at a physiological concentration on Caco-2 colon cancer cells

Previously we reported that 67-kDa laminin receptor (67LR) mediates epigallocatechin-3-O-gallate (EGCG)-induced cell growth inhibition and reduction of myosin regulatory light chain (MRLC) phosphorylation at Thr-18/Ser-19, which is important for cytokinesis. Here, we found that human colon adenocarcinoma Caco-2 cells exhibited higher expression level of 67LR and EGCG at a physiologically achievable concentration (1 microM) significantly accumulated the cells in G(2)/M phase without affecting expression of Wnt-signaling components. We also found that myosin phosphatase targeting subunit 1 (MYPT1) phosphorylation at Thr-696, which inhibits myosin phosphatase and promotes MRLC phosphorylation, was reduced in response to 1 microM EGCG. 67LR knockdown by RNA interference abolished the inhibitory effects of 1 microM EGCG on cell cycle progression and the phosphorylation of MRLC and MYPT1. These results suggest that through 67LR, EGCG at a physiological concentration can activate myosin phosphatase by reducing MYPT1 phosphorylation and that may be involved in EGCG-induced cell growth inhibition.     

Vitamin A Enhances Antitumor Effect of a Green Tea Polyphenol on Melanoma by Upregulating the Polyphenol Sensing Molecule 67-kDa Laminin Receptor

Background

Green tea consumption has been shown to have cancer preventive qualities. Among the constituents of green tea, (-)-Epigallocatechin-3-O-gallate (EGCG) is the most effective at inhibiting carcinogenesis. However, the concentrations of EGCG that are required to elicit the anticancer effects in a variety of cancer cell types are much higher than the peak plasma concentration that occurs after drinking an equivalent of 2–3 cups of green tea. To obtain the anticancer effects of EGCG when consumed at a reasonable concentration in daily life, we investigated the combination effect of EGCG and food ingredient that may enhance the anticancer activity of EGCG on subcutaneous tumor growth in C57BL/6N mice challenged with B16 melanoma cells.

Methodology/Principal Findings

All-trans-retinoic acid (ATRA) enhanced the expression of the 67-kDa laminin receptor (67LR) and increased EGCG-induced cell growth inhibition in B16 melanoma cells. The cell growth inhibition seen with the combined EGCG and ATRA treatment was abolished by treatment with an anti-67LR antibody. In addition, the combined EGCG and ATRA treatment significantly suppressed the melanoma tumor growth in mice. Expression of 67LR in the tumor increased upon oral administration of ATRA or a combined treatment of EGCG and ATRA treatment. Furthermore, RNAi-mediated silencing of the retinoic acid receptor (RAR) α attenuated the ATRA-induced enhancement of 67LR expression in the melanoma cells. An RAR agonist enhanced the expression levels of 67LR and increased EGCG-induced cell growth inhibition.

Conclusions/Significance

Our findings provide a molecular basis for the combination effect seen with dietary components, and indicate that ATRA may be a beneficial food component for cancer prevention when combined with EGCG.     

Phosphodiesterase 5 inhibitor acts as a potent agent sensitizing acute myeloid leukemia cells to 67-kDa laminin receptor-dependent apoptosis

(−)-Epigallocatechin-3-O-gallate (EGCG), a polyphenol in green tea, induces apoptosis in acute myeloid leukemia (AML) cells without affecting normal cells. In this study, we observed that cGMP acts as a cell death mediator of the EGCG-induced anti-AML effect through acid sphingomyelinase activation. EGCG activated the Akt/eNOS axis, a well-known mechanism in vascular cGMP upregulation. We also observed that a major cGMP negative regulator, phosphodiesterase 5, was overexpressed in AML cells, and PDE5 inhibitor, an anti-erectile dysfunction drug, synergistically enhanced the anti-AML effect of EGCG. This combination regimen killed AML cells via overexpressed 67-kDa laminin receptors.     

Induction of apoptosis by epigallocatechin-3-gallate in human lymphoblastoid B cells

(-)-Epigallocatechin-3-gallate (EGCG), a major constituent of green tea polyphenols, has been shown to suppress cancer cell proliferation and induce apoptosis. In this study we investigated its efficacy and the mechanism underlying its effect using human B lymphoblastoid cell line Ramos, and effect of co-treatment with EGCG and a chemotherapeutic agent on apoptotic cell death. EGCG induced dose- and time-dependent apoptotic cell death accompanied by loss of mitochondrial transmembrane potential, release of cytochrome c into the cytosol, and cleavage of pro-caspase-9 to its active form. EGCG also enhanced production of intracellular reactive oxygen species (ROS). Pretreatment with diphenylene iodonium chloride, an inhibitor of NAD(P)H oxidase and an antioxidant, partially suppressed both EGCG-induced apoptosis and production of ROS, implying that oxidative stress is involved in the apoptotic response. Furthermore, we showed that combined-treatment with EGCG and a chemotherapeutic agent, etoposide, synergistically induced apoptosis in Ramos cells.     

Epigallocatechin-3-O-gallate disrupts stress fibers and the contractile ring by reducing myosin regulatory light chain phosphorylation mediated through the target molecule 67 kDa laminin receptor

Epigallocatechin-3-O-gallate (EGCG), a major polyphenol of green tea, has been shown to inhibit the growth of various cancer cell lines. We show here that EGCG induced the disruption of stress fibers and decreased the phosphorylation of the myosin II regulatory light chain (MRLC) at Thr18/Ser19, which is necessary for both contractile ring formation and cell division. Indirect immunofluorescence analysis revealed that EGCG inhibited the concentration of both F-actin and the phosphorylated MRLC in the cleavage furrow at the equator of dividing cells. In addition, EGCG increased the percentages of cells in the G(2)/M phase and inhibited cell growth. Recently, we have demonstrated that the anticancer activity of EGCG is mediated by the metastasis-associated 67kDa laminin receptor (67LR). To explore whether the effect of EGCG is mediated by the 67LR, we transfected cells with short hairpin RNA (shRNA) expression vector to downregulate 67LR expression. When the 67LR was silenced, the suppressive effect of EGCG on the MRLC phosphorylation was significantly attenuated. These results suggest that EGCG inhibits the cell growth by reducing the MRLC phosphorylation and this effect is mediated by the 67LR.     

(?)-Epigallocatechin-3-Gallate Induces Non-Apoptotic Cell Death in Human Cancer Cells via ROS-Mediated Lysosomal Membrane Permeabilization

(−)-Epigallocatechin-3-gallate (EGCG) is the most extensive studied tea polyphenol for its anti-cancer function. In this study, we report a novel mechanism of action for EGCG-mediated cell death by identifying the critical role of lysosomal membrane permeabilization (LMP). First, EGCG-induced cell death in human cancer cells (both HepG2 and HeLa) was found to be caspase-independent and accompanied by evident cytosolic vacuolization, only observable when cells were treated in serum-free medium. The cytosolic vacuolization observed in EGCG-treated cells was most probably caused by lysosomal dilation. Interestingly, EGCG was able to disrupt autophagic flux at the degradation stage by impairment of lysosomal function, and EGCG-induced cell death was independent of Atg5 or autophagy. The key finding of this study is that EGCG is able to trigger LMP, as evidenced by Lyso-Tracker Red staining, cathepsin D cytosolic translocation and cytosolic acidification. Consistently, a lysosomotropic agent, chloroquine, effectively rescues the cell death via suppressing LMP-caused cytosolic acidification. Lastly, we found that EGCG promotes production of intracellular ROS upstream of LMP and cell death, as evidenced by increased level of ROS in cells treated with EGCG and the protective effects of antioxidant N-acetylcysteine (NAC) against EGCG-mediated LMP and cell death. Taken together, data from our study reveal a novel mechanism underlying EGCG-induced cell death involving ROS and LMP. Therefore, understanding this lysosome-associated cell death pathway shed new lights on the anti-cancer effects of EGCG.     

Green tea catechins potentiate the neuritogenic action of brain-derived neurotrophic factor: Role of 67-kDa laminin receptor and hydrogen peroxide

Delivery of optimal amounts of brain-derived neurotrophic factor (BDNF) to regions of the brain affected by neurodegenerative diseases is a daunting task. Using natural products with neuroprotective properties, such as green tea polyphenols, would be a highly useful complementary approach for inexpensive long-term treatment of these diseases. In this study, we used PC12(TrkB) cells which ectopically express TrkB, a high affinity receptor for BDNF. They differentiate and induce neurite outgrowth in response to BDNF. Using this model, we show for the first time that treatment with extremely low concentrations (<0.1 μg/ml) of unfractionated green tea polyphenols (GTPP) and low concentrations (<0.5 μM) of their active ingredient, epigallocatechin-3-gallate (EGCG), potentiated the neuritogenic ability of a low concentration (2 ng/ml) of BDNF. A synergistic interaction was observed between GTPP constituents, where epigallocatechin and epicatechin, both individually lacking this activity, promoted the action of EGCG. GTPP-induced potentiation of BDNF action required the cell-surface associated 67 kDa laminin receptor (67LR) to which EGCG binds with high affinity. A cell-permeable catalase abolished GTPP/EGCG-induced potentiation of BDNF action, suggesting the possible involvement of H₂O₂ in the potentiation. Consistently, exogenous sublethal concentrations of H₂O₂, added as a bolus dose (5 μM) or more effectively through a steady-state generation (1 μM), potentiated BDNF action. Collectively, these results suggest that EGCG, dependent on 67LR and H₂O₂, potentiates the neuritogenic action of BDNF. Intriguingly, this effect requires only submicromolar concentrations of EGCG. This is significant as extremely low concentrations of polyphenols are believed to reach the brain after drinking green tea.     

Attenuated mitochondrial NADP+-dependent isocitrate dehydrogenase activity enhances EGCG-induced apoptosis

(−)-Epigallocatechin-3-gallate (EGCG), a well-known chemopreventive factor, induces cancer cells undergoing apoptosis. Over the last several years, we have shown that the mitochondrial NADP⁺-dependent isocitrate dehydrogenase (IDPm) functions as an antioxidant and anti-apoptotic protein by supplying NADPH to antioxidant systems. Here, we show that EGCG induced the inactivation of IDPm as a purified enzyme and in cultured cancer cells in a dose- and time-dependent manner. Loss of enzyme activity was associated with the depletion of the thiol groups in protein. In addition, transfection of HeLa cells with an IDPm small interfering RNA (siRNA) markedly attenuated the activity of IDPm and substantially enhanced EGCG-induced apoptosis as indicated by the morphological evidence of apoptosis, DNA fragmentation, and the modulation of mitochondrial function and apoptotic marker proteins. Taken together, our results suggest that the suppression of IDPm activity resulted in the disruption of cellular redox balance and subsequently exacerbates EGCG-induced apoptotic cell death in HeLa cells. These results might have implications for developing an effective combination modality in cancer treatment.     

Green Tea Polyphenols Precondition against Cell Death Induced by Oxygen-Glucose Deprivation via Stimulation of Laminin Receptor,Generation of Reactive Oxygen Species,and Activation of Protein Kinase C?

As the development of synthetic drugs for the prevention of stroke has proven challenging, utilization of natural products capable of preconditioning neuronal cells against ischemia-induced cell death would be a highly useful complementary approach. In this study using an oxygen-glucose deprivation and reoxygenation (OGD/R) model in PC12 cells, we show that 2-day pretreatment with green tea polyphenols (GTPP) and their active ingredient, epigallocatechin-3-gallate (EGCG), protects cells from subsequent OGD/R-induced cell death. A synergistic interaction was observed between GTPP constituents, with unfractionated GTPP more potently preconditioning cells than EGCG. GTPP-induced preconditioning required the 67-kDa laminin receptor (67LR), to which EGCG binds with high affinity. 67LR also mediated the generation of reactive oxygen species (ROS) via activation of NADPH oxidase. An exogenous ROS-generating system bypassed 67LR to induce preconditioning, suggesting that sublethal levels of ROS are indeed an important mediator in GTPP-induced preconditioning. This role for ROS was further supported by the fact that antioxidants blocked GTPP-induced preconditioning. Additionally, ROS induced an activation and translocation of protein kinase C (PKC), particularly PKCϵ from the cytosol to the membrane/mitochondria, which was also blocked by antioxidants. The crucial role of PKC in GTPP-induced preconditioning was supported by use of its specific inhibitors. Preconditioning was increased by conditional overexpression of PKCϵ and decreased by its knock-out with siRNA. Collectively, these results suggest that GTPP stimulates 67LR and thereby induces NADPH oxidase-dependent generation of ROS, which in turn induces activation of PKC, particularly prosurvival isoenzyme PKCϵ, resulting in preconditioning against cell death induced by OGD/R.     

Epigallocatechin-3-gallate induces cell apoptosis of human chondrosarcoma cells through apoptosis signal-regulating kinase 1 pathway

Chondrosarcoma is a malignant primary bone tumor that responds poorly to both chemotherapy and radiation therapy. (-)-Epigallocatechin-3-gallate (EGCG), the major polyphenol in green tea, has been shown to inhibit tumorigenesis and cancer cell growth in animal models. The aim of this study was to elucidate the mechanism of EGCG-induced apoptosis of human chondrosarcoma cells. EGCG induced cell apoptosis in human chondrosarcoma cell lines but not primary chondrocytes. EGCG induced upregulation of Bax and Bak, downregulation of Bcl-2 and Bcl-XL, and dysfunction of mitochondria in chondrosarcoma. We also found that the accumulation of reactive oxygen species (ROS) is a critical mediator in EGCG-induced cell death. EGCG induced apoptosis signal-regulating kinase 1 (ASK1) dephosphorylation and its dissociation from 14-3-3. Treatment of chondrosarcoma cells with EGCG induced p38 and c-jun-NH2-kinase (JNK) phosphorylation. Transfection with ASK1 siRNA or p38 and JNK mutant antagonized the EGCG-induced cell apoptosis. Therefore, EGCG triggered ROS and activated the ASK1-p38/JNK pathway, resulting chondrosarcoma cell death. Importantly, animal studies revealed a dramatic reduction in tumor volume after 24 days of treatment. Thus, EGCG may be a novel anti-cancer agent for the treatment of chondrosarcoma.     

A lipid raft-associated 67kDa laminin receptor mediates suppressive effect of epigallocatechin-3-O-gallate on FcepsilonRI expression

(-)-Epigallocatechin-3-O-gallate (EGCG), a major green tea polyphenol, has previously exhibited a suppressive effect on the expression of the high-affinity IgE receptor (FcepsilonRI). This effect has been shown to be elicited by interaction with the plasma membrane microdomain lipid rafts. Recently, we have identified the 67 kDa laminin receptor (67LR) as a cell surface EGCG receptor that mediates an anti-cancer action. Here we show that the 67LR is highly associated with lipid rafts on human basophilic KU812 cells. Experiments using 67LR-enhanced and -reduced cells revealed that the EGCG's ability to downregulate FcepsilonRI expression correlated with the amount of 67LR. Thus, these results suggest that the lipid raft-associated 67LR plays an important role in mediating the FcepsilonRI-suppressive action of EGCG.     

67-kDa Laminin Receptor-dependent Protein Phosphatase 2A (PP2A) Activation Elicits Melanoma-specific Antitumor Activity Overcoming Drug Resistance

The Ras/Raf/MEK/ERK pathway has been identified as a major, druggable regulator of melanoma. Mutational activation of BRAF is the most prevalent genetic alteration in human melanoma, resulting in constitutive melanoma hyperproliferation. A selective BRAF inhibitor showed remarkable clinical activity in patients with mutated BRAF. Unfortunately, most patients acquire resistance to the BRAF inhibitor, highlighting the urgent need for new melanoma treatment strategies. Green tea polyphenol (−)-epigallocatechin-3-O-gallate (EGCG) inhibits cell proliferation independently of BRAF inhibitor sensitivity, suggesting that increased understanding of the anti-melanoma activity of EGCG may provide a novel therapeutic target. Here, by performing functional genetic screening, we identified protein phosphatase 2A (PP2A) as a critical factor in the suppression of melanoma cell proliferation. We demonstrated that tumor-overexpressed 67-kDa laminin receptor (67LR) activates PP2A through adenylate cyclase/cAMP pathway eliciting inhibitions of oncoproteins and activation of tumor suppressor Merlin. Activating 67LR/PP2A pathway leading to melanoma-specific mTOR inhibition shows strong synergy with the BRAF inhibitor PLX4720 in the drug-resistant melanoma. Moreover, SET, a potent inhibitor of PP2A, is overexpressed on malignant melanoma. Silencing of SET enhances 67LR/PP2A signaling. Collectively, activation of 67LR/PP2A signaling may thus be a novel rational strategy for melanoma-specific treatment.     

Apoptosis induction by epigallocatechin gallate involves its binding to Fas

Epigallocatechin gallate (EGCG) is known to induce apoptosis in various types of tumor cells, but the precise mechanism by which EGCG induces apoptosis remains to be elucidated. The Fas-Fas ligand system is one of the major pathways operating in the apoptotic cascade. The aim of this study was to examine the possibility that EGCG-binding to Fas triggers the Fas-mediated apoptosis. The EGCG treatment of human monocytic leukemia U937 cells resulted in elevation of caspase 8 activity and fragmentation of caspase 8. The DNA ladder formation caused by the EGCG treatment was inhibited by the caspase 8 inhibitor. These findings suggested the involvement of the Fas-mediated cascade in the EGCG-induced apoptosis in U937 cells. Affinity chromatography revealed the binding between EGCG and Fas. Thus, the results suggest that EGCG-binding to Fas, presumably on the cell surface, triggers the Fas-mediated apoptosis in U937 cells.     

Molecular bases of thioredoxin and thioredoxin reductase-mediated prooxidant actions of (-)-epigallocatechin-3-gallate

Thioredoxin (Trx) and thioredoxin reductase (TrxR) function as antioxidant and anti-apoptotic proteins, which are often up-regulated in drug-resistant cancer cells. (-)-epigallocatechin-3-gallate (EGCG) is a naturally occurring antioxidant in green tea, but also exhibits prooxidant and apoptosis-inducing properties. We have previously showed a linkage between EGCG-induced inactivation of TrxR and decreased cell survival, revealing TrxR as a new target of EGCG. However, the molecular events underlying the importance of Trx/TrxR in EGCG-induced cytotoxicity remain unclear. Here, we show that the crosstalk between EGCG and Trx/TrxR occurred in a redox-dependent manner, and EGCG induced inactivation of Trx/TrxR in parallel with increased ROS levels in HeLa cells. Moreover, EGCG displayed great reactivity with Cys/Sec residues that have low pK(a) values. The structure of EGCG suggests that its quinone form would readily react with thiolate and selenolate nucleophiles. Using mass spectrometry, we have demonstrated the formation of EGCG-Trx1 (Cys(32)) and EGCG-TrxR (Cys/Sec) conjugates, confirming that EGCG quinone specifically conjugates with active-site Cys(32) in Trx or C-terminal Cys/Selenocysteine (Sec) couple in TrxR under conditions where Trx/TrxR are reduced. Non-reduced form of Trx/TrxR could escape from EGCG inhibition. These data reveal a potential mechanism for enhancing EGCG-induced cancer cell death by the NADPH-dependent reduction of Trx/TrxR.