Hee Yun

Kyung Hee University, Sŏul, Seoul, South Korea

Are you Hee Yun?

Claim your profile

Publications (12)47.26 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Numerous physiological functions are controlled by redox-responsive signaling pathways. Disruption of redox balance by oxidative stress is recognized as a major cause of many pathological conditions, including aging, highlighting the importance of investigating how antioxidants maintain redox homeostasis. AMP-activated protein kinase (AMPK) is activated in response to cellular conditions that accompany energy depletion and plays a central role in regulation of energy homeostasis, tumorigenesis, and longevity. Recently, several antioxidants have been reported to activate AMPK, but the mechanisms by which AMPK acts to adjust the levels of cellular reactive oxygen species are not fully characterized. In this study, we investigated the role of AMPK in mediating resveratrol-induced antioxidant effects and the molecular mechanisms underlying its actions. We demonstrate that AMPK activity plays an indispensable role in the operation of the ROS defense system by inducing expression of the antioxidant enzymes, manganese superoxide dismutase (MnSOD) and catalase, in response to resveratrol or the AMPK agonist 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleotide (AICAR). In addition, we identified the mechanism involved in the antioxidant function of AMPK, demonstrating that AMPK directly phosphorylates human FoxO1 (forkhead box O1) at Thr649in vitro and increases FoxO1-dependent transcription of MnSOD and catalase. Mutagenesis studies showed that this AMPK-mediated phosphorylation of FoxO1 is critical for FoxO1 stability and nuclear localization, establishing the molecular basis for the induction of FoxO1 transcriptional activity. Our results reveal a novel FoxO1-dependent mechanism by which AMPK controls expression of antioxidant enzymes and suggest that AMPK has an important role in maintaining redox homeostasis.This article is protected by copyright. All rights reserved.
    FEBS Journal 07/2014; · 4.25 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Doxorubicin is one of the most widely used anti-cancer drugs, but its clinical application is compromised by severe adverse effects in different organs including cardiotoxicity. In the present study, we explored mechanisms of doxorubicin-induced cytotoxicity by revealing a novel role for the AMP-activated protein kinase (AMPK) α2 in mouse embryonic fibroblasts (MEF). Doxorubicin robustly induced the expression of AMPKα2 in MEFs, but slightly reduced AMPKα1 expression. Our data support the previous notion that AMPKα1 harbors survival properties under doxorubicin treatment. In contrast, analyses of Ampkα2-/- MEFs, gene knock-down of AMPKα2 by shRNA, and inhibition of AMPKα2 activity with an AMPK inhibitor indicated that AMPKα2 functions as a pro-apoptotic molecule under doxorubicin treatment. Doxorubicin induced AMPKα2 at the transcription level via E2F1, a transcription factor that regulates apoptosis in response to DNA damage. E2F1 directly transactivated the Ampkα2 gene promoter. In turn, AMPKα2 significantly contributed to stabilization and activation of E2F1 by doxorubicin, forming a positive signal amplification loop. AMPKα2 directly interacted with and phosphorylated E2F1. This signal loop was also detected in H9c2, C2C12, and ECV cells, as well as mouse liver under doxorubicin treatment. Resveratrol, which has been suggested to attenuate doxorubicin-induced cytotoxicity, significantly blocked induction of AMPKα2 and E2F1 by doxorubicin, leading to protection of these cells. This signal loop appears to be non-carcinoma specific because AMPKα2 was not induced by doxorubicin in five different tested cancer cell lines. These results suggest that AMPKα2 may serve as a novel target for alleviating the cytotoxicity of doxorubicin.
    Journal of Biological Chemistry 01/2014; · 4.65 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ginseng saponins exert various important pharmacological effects with regard to the control of many diseases, including cancer. In this study, the anticancer effect of ginsenosides on human cancer cells was investigated and compared. Among the tested compounds, ginsenoside-Rh2 displays the highest inhibitory effect on cell viability in HepG2 cells. Ginsenoside-Rh2, a ginseng saponin isolated from the root of Panax ginseng, has been suggested to have potential as an anticancer agent, but the underlying mechanisms remain elusive. In the present study, we have shown that cancer cells have differential sensitivity to ginsenoside-Rh2-induced apoptosis, raising questions regarding the specific mechanisms responsible for the discrepant sensitivity to ginsenoside-Rh2. In this study, we demonstrate that AMP-activated protein kinase (AMPK) is a survival factor under ginsenoside-Rh2 treatment in cancer cells. Cancer cells with acute responsiveness of AMPK display a relative resistance to ginsenoside-Rh2, but cotreatment with AMPK inhibitor resulted in a marked increase of ginsenoside-Rh2-induced apoptosis. We also observed that p38 MAPK (mitogen-activated protein kinase) acts as another survival factor under ginsenoside-Rh2 treatment, but there was no signaling crosstalk between AMPK and p38 MAPK, suggesting that combination with inhibitor of AMPK or p38 MAPK can augment the anticancer potential of ginsenoside Rh2.
    Journal of ginseng research 01/2014; 38(1):16-21. · 2.26 Impact Factor
  • Hee Yun, Joohun Ha
    [Show abstract] [Hide abstract]
    ABSTRACT: INTRODUCTION: AMPK is a key player in the regulation of energy balance at both the cellular and whole-body levels, placing it at the center stage in studies of metabolic disorders. Recently, AMPK has also been identified as a potential target for either therapy or prevention of some types of cancer. Thus, identification of AMPK modulators for possible use as novel therapeutic drugs, both for treatment of metabolic disorders and cancer, will have a high commercial potential. AREAS COVERED: This review covers the structures and activities of AMPK modulators described in the patent literature since 2006. The patents reviewed include those for direct and/or indirect activators of AMPK, and novel pharmaceutical compounds with potential for use in the prevention and/or treatment of metabolic disorders, and cancer targeting AMPK. EXPERT OPINION: Targeting of AMPK appears to be an attractive strategy in the treatment of metabolic disorders. However, some detrimental effects of AMPK have also been reported, including a possible tumor-promoting effect in some settings and a heart disease-causing effect. Moreover, activation of AMPK in the hypothalamus may cause undesired consequences, such as an increase in feeding and body weight gain. These effects, therefore, must be carefully assessed for the development of therapeutic drugs targeting AMPK.
    Expert Opinion on Therapeutic Patents 07/2011; 21(7):983-1005. · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Epidemiological evidence suggests a lower incidence of prostate cancer in Asian countries, where soy products are more frequently consumed than in Western countries, indicating that isoflavones from soy have chemopreventive activities in prostate cells. Here, we tested the effects of the soy isoflavone genistein on antioxidant enzymes in DU145 prostate cancer cells. Genistein significantly decreased reactive oxygen species levels and induced the expression of the antioxidant enzymes manganese (Mn) superoxide dismutase (SOD) and catalase, which were associated with AMP-activated protein kinase (AMPK) and phosphatase and tensin homolog deleted from chromosome 10 (PTEN) pathways. The induced expression of catalase, MnSOD, and PTEN were attenuated by pretreatment with a pharmacological inhibitor for AMPK, indicating the effects of genistein primarily depend on AMPK. Furthermore, PTEN is essential for genistein activity, as shown by PTEN transfection in PTEN-deficient PC3 cells. Thus, genistein induces antioxidant enzymes through AMPK activation and increased PTEN expression.
    Journal of medicinal food 08/2010; 13(4):815-20. · 1.39 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Mitogen-activated protein kinase (MAPK) pathways are involved in the regulation of cellular responses, including cell proliferation, differentiation, cell growth, and apoptosis. Because these responses are tightly related to cellular energy level, AMP-activated protein kinase (AMPK), which plays an essential role in energy homeostasis, has emerged as another key regulator. In the present study, we demonstrate a novel signal network between AMPK and MAPK in HCT116 human colon carcinoma. Glucose deprivation activated AMPK and three MAPK subfamilies, extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 MAPK. Under these conditions, inhibition of endogenous AMPK by expressing a dominant-negative form significantly potentiated ERK activation, indicating that glucose deprivation-induced AMPK is specifically antagonizing ERK activity in HCT116 cells. Moreover, we provide novel evidence that AMPK activity is critical for p53-dependent expression of dual-specificity phosphatase (DUSP) 1 & 2, which are negative regulators of ERK. Notably, ERK exhibits pro-apoptotic effects in HCT116 cells under glucose deprivation. Collectively, our data suggest that AMPK protects HCT116 cancer cells from glucose deprivation, in part, via inducing DUSPs, which suppresses pro-apoptotic ERK, further implying that a signal network between AMPK and ERK is a critical regulatory point in coupling the energy status of the cell to the regulation of cell survival.
    Journal of Biological Chemistry 03/2010; 285(19):14617-27. · 4.65 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Inulin, a naturally occurring, functional food ingredient found in various edible plants, has been reported to exert potential health benefits, including decreased risk of colonic diseases, non-insulin-dependent diabetes, obesity, osteoporosis, and cancer. However, the mechanism of the antidiabetic activity of inulin has not yet been elucidated. In this study, we showed that inulin increased the uptake of glucose in C2C12 myotubes, which was associated with both AMP-activated protein kinase (AMPK) and phosphatidylinositol 3-kinase (PI3-K) signaling pathways, but both of these pathways appeared to transmit their signals in an independent manner. Moreover, we found that inulin was able to increase the uptake of glucose in C2C12 myotubes in which insulin resistance was induced by exposing cells to high glucose concentrations. The identical effects of inulin were also observed in HepG2 hepatoma cells. Collectively, we report the antidiabetic activity of inulin and further demonstrate for the first time that such activity is associated with AMPK and PI3-K activation.
    Journal of medicinal food 10/2009; 12(5):1023-8. · 1.39 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: As solid tumors outgrow the surrounding vasculature, they encounter microenvironments with a limited supply of nutrients. Therefore, in order to survive, tumor cells need to adapt to glucose-deprived environments. In the present study, we examined the signaling pathways that lead to cancer cell survival in response to glucose deprivation. We primarily focused on the roles of adenosine monophosphate-activated protein kinase (AMPK), its upstream kinase LKB1 and c-Jun N-terminal kinase (JNK). Herein, we showed that in DU145 human prostate carcinomas, glucose deprivation activated JNK with biphasic kinetics. We demonstrated that the early phase of JNK activation promoted cell survival, whereas the late phase of JNK activation induced apoptosis. Our data further showed that AMPK relayed a survival signal transmitted by early activation of JNK and that the sustained AMPK signal in turn inhibited the proapoptotic property of JNK via a negative feedback mechanism involving reactive oxygen species. We induced this negative feedback inhibition by expressing LKB1 ectopically in DU145 cells. In conclusion, our results demonstrated how AMPK controls the molecular mechanism underlying the differential biological functions of JNK, and they also provided a novel explanation for the antiapoptotic role of LKB1.
    Carcinogenesis 12/2008; 30(3):529-37. · 5.64 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hypoxia-inducible factor (HIF-1) plays a central role in the cellular adaptive response to hypoxic conditions, which are closely related to pathophysiological conditions, such as cancer. Although reactive oxygen species (ROS) have been implicated in the regulation of hypoxic and non-hypoxic induction of HIF-1 under various conditions, the role of ROS is quite controversial, and the mechanism underlying the HIF-1 regulation by ROS is not completely understood yet. Here, we investigated the biochemical mechanism for the ROS-induced HIF-1 by revealing a novel role of adenosine monophosphate-activated protein kinase (AMPK) and the upstream signal components. AMPK plays an essential role as energy-sensor under adenosine triphosphate-deprived conditions. Here we report that ROS induced by a direct application of H(2)O(2) and menadione to DU145 human prostate carcinoma resulted in accumulation of HIF-1alpha protein by attenuation of its degradation and activation of its transcriptional activity in an AMPK-dependent manner. By way of contrast, AMPK was required only for the transcriptional activity of HIF-1 under hypoxic condition, revealing a differential role of AMPK in these two stimuli. Furthermore, our data show that inhibition of AMPK enhances HIF-1alpha ubiquitination under ROS condition. Finally, we show that the regulation of HIF-1 by AMPK in response to ROS is under the control of c-Jun N-terminal kinase and Janus kinase 2 pathways. Collectively, our findings identify AMPK as a key determinant of HIF-1 functions in response to ROS and its possible role in the sophisticated HIF-1 regulatory mechanisms.
    Carcinogenesis 05/2008; 29(4):713-21. · 5.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cisplatin is one of the most effective and widely used chemotherapeutic agents. However, one of the most salient limitations to the clinical application of cisplatin is the acquired or intrinsic drug resistance exhibited by some tumors. In the present study, we have assessed the potential of an intracellular energy balancing system as a target for augmentation of cisplatin sensitivity in tumors. AMP-activated protein kinase (AMPK) regulates the energy balance system by monitoring intracellular energy status. Here we demonstrate that AMPK is rapidly activated by cisplatin in AGS and HCT116 cancer cells. The inhibition of AMPK in those cells and in xenografts of HCT116 resulted in a remarkable increase in cisplatin-induced apoptosis, which was associated with hyper-induction of the tumor suppressor p53. We further showed that ERK, but not ATM (ataxia telangiectasia mutated) and ATR (ATM- and Rad3-related) kinases, was involved in the hyper-induction of p53 by the inhibition of cisplatin-induced AMPK. By way of contrast, cisplatin did not induce AMPK activation in HeLa cells, which appear to have a relatively high sensitivity to cisplatin-induced cytotoxicity, but expression of the constitutive active form of AMPK in HeLa cells resulted in a significant increase of cell viability after cisplatin treatment. Collectively, our data suggest that AMPK performs a pivotal function for protection against the cytotoxic effect of cisplatin, thereby implying that AMPK is one of the cellular factors determining the cellular sensitivity to cisplatin. On the basis of these observations, we propose that a strategy combining cisplatin and AMPK inhibition could be developed into a novel chemotherapeutic modality.
    Journal of Biological Chemistry 03/2008; 283(7):3731-42. · 4.65 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Epidemiological and experimental animal data indicate that exposure to both metals and metalloid species exacerbates the risk of human diseases, particularly cancers. Vascular endothelial growth factor (VEGF), which performs a primary function in both tumor progression and angiogenesis, is up-regulated due to exposure to an array of carcinogenic metals, but the mechanisms responsible for the metal activation remain somewhat poorly understood. Recently, we demonstrated that AMP-activated protein kinase (AMPK), which acts as an energy sensor, providing metabolic adaptation effects under ATP-deprived conditions, is critical for the expression of VEGF under oxygen- and glucose-deprived conditions. As carcinogenic metals are potent VEGF expression inducers, we hypothesized that AMPK would also play a crucial role in metal-induced VEGF expression. Here, we present evidence that carcinogenic metals such as arsenite, vanadate, and cobalt, induce AMPK activation and VEGF expression via several different mechanisms, and that AMPK is able to regulate the expression of VEGF mRNA in a hypoxia-inducible factor-1-dependent or -independent manner, depending on the metal applied. We also attempted to characterize the relevant signal transduction pathways in metal-induced VEGF expression and AMPK activation, as well as the role of reactive oxygen species within this context. Overall, our data suggest that AMPK is a critical regulatory component in metal-induced VEGF expression, which further implies its intrinsic involvement in metal-induced carcinogenesis.
    Biochemical Pharmacology 07/2006; 72(1):91-103. · 4.58 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The induction of proangiogenic cytokines such as vascular endothelial growth factor (VEGF) is a critical feature of tumor angiogenesis. In the present study, we examined the mechanisms of VEGF gene expression induced by glucose deprivation in cancer cells, a role of AMP-activated protein kinase (AMPK) in the process, and the signal transduction pathway. AMPK functions as an energy sensor to provide metabolic adaptation under ATP-depleting conditions such as hypoxia and nutritional deprivation. Here, we show that glucose deprivation leads to a significant increase in the mRNA level of VEGF, GLUT1, and PFKFB3 genes in several cancer cells via a hypoxia-inducible factor-1-independent mechanism, and we demonstrate an essential role of AMPK in these gene expressions. Our data suggest that VEGF mRNA induction by glucose deprivation is due to an increase in mRNA stability, and the AMPK activity is necessary and sufficient to confer the stability to VEGF mRNA. We further show that reactive oxygen species is involved in glucose deprivation-induced AMPK activity in DU145 human prostate carcinomas, and c-Jun amino-terminal kinase acts as an upstream component in AMPK activation cascades under these conditions. LKB1, which was recently identified as a direct upstream kinase of AMPK, was not detected in DU145 cells. In conclusion, our results demonstrate a novel and major role of AMPK in the post-transcriptional regulation of VEGF, further implying its potential role in tumor angiogenesis.
    Journal of Biological Chemistry 04/2005; 280(11):9963-72. · 4.65 Impact Factor