Young-Joon Hong

Up‐regulation of Bak and Bim via JNK downstream pathway in the response to nitric oxide in human glioblastoma cells

Nitric oxide (NO) is a chemical messenger implicated in neuronal damage associated with ischemia neurodegenerative disease and excitotoxicity. In the present study, we examined the biological effects of NO and its mechanisms in human malignant glioblastoma cells. Addition of a NO donor, S‐nitroso‐N‐acetyl‐penicillamine (SNAP), induced apoptosis in U87MG human glioblastoma cells, accompanied by opening mitochondrial permeability transition pores, release of cytochrome c and AIF, and subsequently by caspase activation. NO‐induced apoptosis occurred concurrently with significantly increased levels of the Bak and Bim. Treatment with SNAP resulted in sustained activation of JNK and its downstream pathway, c‐Jun/AP‐1. The expression of dominant‐negative (DN)‐JNK1 and DN‐c‐Jun suppressed the activation of AP‐1, the induction of Bak and Bim, and the SNAP‐induced apoptosis. In addition, de novo protein synthesis was required for the initiation of apoptosis in that the protein synthesis inhibitor, cycloheximide (CHX), inhibited NO‐induced apoptotic cell death as well as up‐regulation of Bak and Bim. These results suggest that NO activates an apoptotic cascade, involving sustained JNK activation, AP‐1 DNA binding activity, and subsequent Bak and Bim induction, followed by cytochrome c and AIF releases and caspases cascade activation, resulting in human malignant brain tumor cell death. J. Cell. Physiol. 206: 477–486, 2006.

Arsenic trioxide sensitizes CD95/Fas‐induced apoptosis through ROS‐mediated upregulation of CD95/Fas by NF‐κB activation

CD95/Fas is a cell surface protein that belongs to the tumor necrosis factor receptor family. Signals through CD95/Fas are able to induce apoptosis in sensitive cells. Therefore, modalities to regulate the CD95/Fas expression level in tumor cells are called for. In the present study, we show that sublethal doses of arsenic trioxide (As2O3) sensitized CD95/Fas‐induced apoptosis in human cervical cancer cells, and the sensitizing effects resulted from As2O3‐mediated increase in the expression of the CD95/Fas. N‐acetyl‐L‐cysteine, a specific scavenger of reactive oxygen species, abrogated As2O3‐induced upregulation of CD95/Fas and enhancement of CD95/Fas‐mediated apoptosis. Furthermore, inhibition of NF‐κB by transient transfection of IκBα supersuppressor blocked the increase of CD95/Fas expression following As2O3 treatment. Antisense oligonucleotide of CD95/Fas and ZB4, an antibody that blocks the binding of CD95/Fas ligand to CD95/Fas, reduced the amount of As2O3‐sensitized CD95/Fas‐induced apoptosis, demonstrating the specificity of CD95/Fas‐binding ligands in the As2O3‐sensitized CD95/Fas‐induced apoptosis. These findings demonstrate that sensitization of human cervical cancer cells to CD95/Fas‐mediated apoptosis by As2O3 can be partly due to induction of ROS and subsequent upregulation of CD95/Fas gene expression by NF‐κB activation.

Influence of Glutathione S-Transferase M1 and T1 Genotypes on Larynx Cancer Risk among Korean Smokers

Abstract Glutathione S-transferase (GST) isoenzymes are involved in the detoxification of major carcinogens present in tobacco smoke. It is thus conceivable that deficiency in GST activity due to homozygous deletions of the GSTM1 and GSTT1 genes (the null genotypes) may modulate susceptibility to smoking-induced cancers. The influence of the GSTM1 and GSTT1 null genotypes on larynx cancer risk among the Korean population were evaluated using peripheral blood DNA from 82 larynx cancer patients and 63 healthy controls, all of whom were male current smokers. Increased larynx cancer risk was related to the GSTM1 null genotype (odds ratio (OR)=3.53, 95% confidence interval (CI)=1.27−9.83). The OR associated with the GSTT1 null genotype was also increased, but did not reach statistical significance (OR=1.83, 95% CI=0.70−4.79). Individuals lacking both the GSTM1 and GSTT1 genes were at a significantly higher risk for larynx cancer than individuals with both genes present (OR=4.04, 95% CI=1.33−12.30). These data confirm that the GSTM1 null genotype is an important risk modifier for larynx cancer among Korean smokers and combined GSTM1 and GSTT1 null genotypes could be a useful predictor of genetic susceptibility to larynx cancer.

Activation of Epidermal Growth Factor Receptor and Its Downstream Signaling Pathway by Nitric Oxide in Response to Ionizing Radiation

Epidermal growth factor receptor (EGFR) is activated by ionizing radiation (IR), but the molecular mechanism for this effect is unknown. We have found that intracellular generation of nitric oxide (NO) by NO synthase (NOS) is required for the rapid activation of EGFR phosphorylation by IR. Treatment of A549 lung cancer cells with IR increased NOS activity within minutes, accompanied by an increase of NO. 2-Phenyl-4,4,5,5,-tetramethylimidazolline-1-oxyl-3-oxide, an NO scavenger, and NG-monomethyl-l-arginine, an NOS inhibitor, abolished the increase in intracellular NO and activation of EGFR by IR. In addition, an NO donor alone induced EGFR phosphorylation. Transient transfection with small interfering RNA for endothelial NOS reduced IR-induced NO production and suppressed IR-induced EGFR activation. Overexpression of endothelial NOS increased IR-induced NO generation and EGFR activation. These results indicate a novel molecular mechanism for EGFR activation by IR-induced NO production via NOS. (Mol Cancer Res 2008;6(6):996–1002)

Blockage of Stat3 enhances the sensitivity of NSCLC cells to PI3K/mTOR inhibition

The PI3K/Akt/mTOR axis in lung cancer is frequently activated and implicated in tumorigenesis. Specific targeting of this pathway is therefore an attractive therapeutic approach for lung cancer. However, non-small cell lung cancer cells are resistant to BEZ235, a dual inhibitor of PI3K and mTOR. Interestingly, blockage of Stat3 with a selective inhibitor, S3I-201, or siRNA dramatically sensitized the BEZ235-induced cell death, as evident from increased PARP cleavage. Furthermore, inhibition of Stat3 led to enhancement of cell death induced by LY294002, a PI3K inhibitor. Treatment of cells with a combination of BEZ235 and S3I-201 significantly induced the proapoptotic transcription factor, CHOP, and its targets, Bim and DR4. Knockdown of CHOP or Bim suppressed cell death stimulated by the combination treatment, implicating the involvement of these BEZ235/S3I-201-induced factors in pronounced cell death. Moreover, the BEZ235/S3I-201 combination enhanced TRAIL-induced cell death. Our results collectively suggest that blockage of Stat3 presents an effective strategy to overcome resistance to PI3K/Akt/mTOR inhibition.

Sustained overexpression of Redd1 leads to Akt activation involved in cell survival

Herein, we show that the constitutive overexpression of Redd1, a negative regulator of mTORC1, induces Akt activation in lung cancer cells. Akt phosphorylation was reduced to basal levels by Rictor siRNA, suggesting the involvement of mTORC2 in this process. Perifosine and PP242, selective inhibitors of Akt and mTORC1/2, respectively, efficiently suppressed the Akt phosphorylation that was induced by the sustained overexpression of Redd1 and increased the sensitivity of the cells to cisplatin. Therefore, the sustained overexpression of Redd1 leads to mTORC1 inhibition and to consequent Akt activation that is involved in cell survival. This finding highlights the importance of Akt activation as a therapeutic target to overcome resistance to chemotherapy.