Ju-Hong Jeon

Suppression of transient receptor potential melastatin 7 channel induces cell death in gastric cancer

Ca2+ and Mg2+ have a fundamental role in many cellular processes and ion channels are involved in normal physiologic processes and in the pathology of various diseases. The aim here was to show that the presence and potential role of transient receptor potential melastatin 7 (TRPM7) channels in the growth and survival of AGS cells, the most common human gastric adenocarcinoma cell line. The patch‐clamp technique for whole‐cell recording was used in AGS cells. TRPM7‐specific small interfering RNAs were used for specific inhibition of TRPM7. Whole‐cell voltage‐clamp recordings revealed the TRPM7‐like currents that activated spontaneously following loss of intracellular Mg2+. The current had a non‐linear current–voltage relationship with the characteristic steep outward rectification associated with TRPM7 channels. Reverse transcription–polymerase chain reaction, western blotting, and immunoreactivity all showed abundant expression of TRPM7 messenger RNA and protein in AGS cells. Transfection of AGS cells with TRPM7 siRNA significantly reduced the expression of TRPM7 mRNA and protein as well as the amplitude of the TRPM7‐like currents. Furthermore, we found that Mg2+ is critical for the growth and survival in AGS cells. Blockade of TRPM7 channels by La3+ and 2‐APB or suppression of TRPM7 expression by siRNA inhibited the growth and survival of these cells. Human gastric adenocarcinoma cells express TRPM7 channel whose presence is essential for cell survival. The protein is a likely potential target for the pharmacological treatment of gastric cancer. (Cancer Sci 2008; 99: 2502–2509)

Ethyl pyruvate has an anti-inflammatory effect by inhibiting ROS-dependent STAT signaling in activated microglia

Ethyl pyruvate (EP) has been demonstrated to have an anti-inflammatory function. However, the molecular mechanisms underlying the anti-inflammatory action of EP are largely unknown. We here show that EP exerts its anti-inflammatory effect by inhibiting ROS-dependent STAT signaling through its antioxidant activity, like vitamin C or N-acetyl-L-cysteine. The inhibition of STAT1 and STAT3 by EP prevented their translocation to the nucleus and consequently inhibited expression of iNOS and COX-2 by inhibiting STAT1- and STAT3-mediated transcriptional activity, followed by changes in chromatin conformation via deacetylation of histones H3 and H4 in both gene promoters. EP also suppressed transcripts of other STAT-responsive inflammatory genes such as IL-1beta, IL-6, TNF-alpha, and MCP-1. We further found that the mechanism of inhibition of STAT1 and STAT3 by EP is due to inhibition of JAK2 through Rac1 inactivation and SOCS1 induction. These findings offer new therapeutic possibilities for EP based on a better understanding of the mechanism underlying the action of EP.

Glucose Deprivation Regulates KATP Channel Trafficking via AMP-Activated Protein Kinase in Pancreatic β-Cells

OBJECTIVE AMP-activated protein kinase (AMPK) and the ATP-sensitive K+ (KATP) channel are metabolic sensors that become activated during metabolic stress. AMPK is an important regulator of metabolism, whereas the KATP channel is a regulator of cellular excitability. Cross talk between these systems is poorly understood. RESEARCH DESIGN AND METHODS Rat pancreatic β-cells or INS-1 cells were pretreated for 2 h at various concentrations of glucose. Maximum KATP conductance (Gmax) was monitored by whole-cell measurements after intracellular ATP washout using ATP-free internal solutions. KATP channel activity (NPo) was monitored by inside-out patch recordings in the presence of diazoxide. Distributions of KATP channel proteins (Kir6.2 and SUR1) were examined using immunofluorescence imaging and surface biotinylation studies. Insulin secretion from rat pancreatic islets was measured using an enzyme immunoassay. RESULTS Gmax and NPo in cells pretreated with glucose-free or 3 mmol/l glucose solutions were significantly higher than in cells pretreated in 11.1 mmol/l glucose solutions. Immunofluorescence imaging and biotinylation studies revealed that glucose deprivation induced an increase in the surface level of Kir6.2 without affecting the total cellular amount. Increases in Gmax and the surface level of Kir6.2 were inhibited by compound C, an AMPK inhibitor, and siAMPK transfection. The effects of glucose deprivation on KATP channels were mimicked by an AMPK activator. Glucose deprivation reduced insulin secretion, but this response was attenuated by compound C. CONCLUSIONS KATP channel trafficking is regulated by energy status via AMPK, and this mechanism may play a key role in inhibiting insulin secretion under low energy status.

TGFβ mediates activation of transglutaminase 2 in response to oxidative stress that leads to protein aggregation

Transglutaminase 2 (TGase2) is a ubiquitously expressed enzyme that catalyzes irreversible post‐translational modification of protein, forming cross‐linked protein aggregates. We previously reported that intracellular TGase2 is activated by oxidative stress. To elucidate the functional role of TGase2 activation in cells under the oxidatively stressed condition, we identified the mediator that activates TGase2. In this study, we showed that low levels of oxidative stress trigger the release of TGFβ, which subsequently activates TGase2 through the nuclear translocation of Smad3. Analysis of substrate proteins reveals that TGase2‐mediated protein modification results in a decrease of protein solubility and a collapse of intermediate filament network, which leads to aggregation of proteins. We confirm these results using lens tissues from TGase2‐deficient mice. Among several antioxidants tried, only N‐acetylcysteine effectively inhibits TGFβ‐mediated activation of TGase2. These results indicate that TGFβ mediates oxidative stress‐induced protein aggregation through activation of TGase2 and suggest that the formation of protein aggregation may not be a passive process of self‐assembly of oxidatively damaged proteins but may be an active cellular response to oxidative stress. Therefore, TGFP‐TGase2 pathway may have implications for both the pathogenesis of age‐related degenerative diseases and the development of pharmaceutics.—Shin, D.‐M., Jeon, J.‐H., Kim, C.‐W., Cho, S.‐Y., Lee, H.‐J., Jang, G.‐Y., Jeong, E. M., Lee, D.‐S., Kang, J.‐H., Melino, G., Park, S.‐C., Kim, I.‐G. TGFβ mediates activation of transglutaminase 2 in response to oxidative stress that leads to protein aggregation. FASEB J. 22, 2498–2507 (2008)

Selective Gαi Subunits as Novel Direct Activators of Transient Receptor Potential Canonical (TRPC)4 and TRPC5 Channels

Background: Activation of TRPC4/5 channels is mediated by GPCR activation. Results: TRPC4/5 was activated by the Gαi/o-coupled receptor and the Gαi protein, which interacted directly with each other. Conclusion: Gαi proteins play an essential role as novel activators of TRPC4/5. Significance: Our findings provide new insights into the activation mechanism of inhibitory Gα proteins. The ubiquitous transient receptor potential canonical (TRPC) channels function as non-selective, Ca2+-permeable channels and mediate numerous cellular functions. It is commonly assumed that TRPC channels are activated by stimulation of Gαq-PLC-coupled receptors. However, whether the Gαq-PLC pathway is the main regulator of TRPC4/5 channels and how other Gα proteins may regulate these channels are poorly understood. We previously reported that TRPC4/TRPC5 can be activated by Gαi. In the current work, we found that Gαi subunits, rather than Gαq, are the primary and direct activators of TRPC4 and TRPC5. We report a novel molecular mechanism in which TRPC4 is activated by several Gαi subunits, most prominently by Gαi2, and TRPC5 is activated primarily by Gαi3. Activation of Gαi by the muscarinic M2 receptors or expression of the constitutively active Gαi mutants equally and fully activates the channels. Moreover, both TRPC4 and TRPC5 are activated by direct interaction of their conserved C-terminal SESTD (SEC14-like and spectrin-type domains) with the Gαi subunits. Two amino acids (lysine 715 and arginine 716) of the TRPC4 C terminus were identified by structural modeling as mediating the interaction with Gαi2. These findings indicate an essential role of Gαi proteins as novel activators for TRPC4/5 and reveal the molecular mechanism by which G-proteins activate the channels.

Cancer vaccination drives Nanog-dependent evolution of tumor cells toward an immune-resistant and stem-like phenotype.

Due to the exquisite specificity and potency of the immune system, vaccination is in theory the most precise and powerful approach for controlling cancer. However, current data from clinical trials indicate that vaccination rarely yields significant benefits for cancer patients in terms of tumor progression and long-term survival. The poor clinical outcomes of vaccination are primarily caused by mechanisms of immune tolerance, especially within the tumor microenvironment. Here, we report that vaccination drives the evolution of tumor cells toward an immune-resistant and stem-like phenotype that promotes tumor growth and nullifies the CTL response. The emergence of this phenotype required the transcription factor Nanog, which is induced as a consequence of immune selection. Nanog expression enhanced the stem-like features of tumor cells and protected them from killing by tumor-reactive CTLs. Delivery of siNanog into tumor-bearing mice rendered the tumor vulnerable to immune surveillance and strongly suppressed its growth. Together, our findings show tumor adaptation to vaccination through gain of an immune-resistant, stem-like phenotype and identify Nanog as a central molecular target in this process. Future vaccination technology should consider Nanog an important target to enhance the immunotherapeutic response.

Transglutaminase 2 inhibits Rb binding of human papillomavirus E7 by incorporating polyamine.

Transglutaminase 2 (TGase 2) is one of a family of enzymes that catalyze protein modification through the incorporation of polyamines into substrates or the formation of protein crosslinks. However, the physiological roles of TGase 2 are largely unknown. To elucidate the functions of TGase 2, we have searched for its interacting proteins. Here we show that TGase 2 interacts with E7 oncoprotein of human papillomavirus type 18 (HPV18) in vitro and in vivo. TGase 2 incorporates polyamines into a conserved glutamine residue in the zinc‐binding domain of HPV18 E7 protein. This modification mediates the inhibition of E7s Rb binding ability. In contrast, TGase 2 does not affect HPV16 E7, due to absence of a glutamine residue at this polyamination site. Using E7 mutants, we demonstrate that TGase 2‐dependent inhibition of HPV E7 function correlates with the presence of the polyamination site. Our results indicate that TGase 2 is an important cellular interfering factor and define a novel host–virus interaction, suggesting that the inability of TGase 2 to inactivate HPV16 E7 could explain the high prevalence of HPV16 in cervical cancer.

Leptin promotes KATP channel trafficking by AMPK signaling in pancreatic β-cells

Leptin is a pivotal regulator of energy and glucose homeostasis, and defects in leptin signaling result in obesity and diabetes. The ATP-sensitive potassium (KATP) channels couple glucose metabolism to insulin secretion in pancreatic β-cells. In this study, we provide evidence that leptin modulates pancreatic β-cell functions by promoting KATP channel translocation to the plasma membrane via AMP-activated protein kinase (AMPK) signaling. KATP channels were localized mostly to intracellular compartments of pancreatic β-cells in the fed state and translocated to the plasma membrane in the fasted state. This process was defective in leptin-deficient ob/ob mice, but restored by leptin treatment. We discovered that the molecular mechanism of leptin-induced AMPK activation involves canonical transient receptor potential 4 and calcium/calmodulin-dependent protein kinase kinase β. AMPK activation was dependent on both leptin and glucose concentrations, so at optimal concentrations of leptin, AMPK was activated sufficiently to induce KATP channel trafficking and hyperpolarization of pancreatic β-cells in a physiological range of fasting glucose levels. There was a close correlation between phospho-AMPK levels and β-cell membrane potentials, suggesting that AMPK-dependent KATP channel trafficking is a key mechanism for regulating β-cell membrane potentials. Our results present a signaling pathway whereby leptin regulates glucose homeostasis by modulating β-cell excitability.

Geraniol inhibits prostate cancer growth by targeting cell cycle and apoptosis pathways

The progression of prostate cancer is associated with escape from cell cycle arrest and apoptosis under androgen-depleted conditions. Here, we found that geraniol, a naturally occurring monoterpene, induces cell cycle arrest and apoptosis in cultured cells and tumor grafted mice using PC-3 prostate cancer cells. Geraniol modulated the expression of various cell cycle regulators and Bcl-2 family proteins in PC-3 cells in vitro and in vivo. Furthermore, we showed that the combination of sub-optimal doses of geraniol and docetaxel noticeably suppresses prostate cancer growth in cultured cells and tumor xenograft mice. Therefore, our findings provide insight into unraveling the mechanisms underlying escape from cell cycle arrest and apoptosis and developing therapeutic strategies against prostate cancer.

Menthol regulates TRPM8-independent processes in PC-3 prostate cancer cells

Menthol, a naturally occurring compound from peppermint oil, binds and activates the TRPM8 Ca(2+)-permeable channel that exhibits abnormal expression patterns in prostate cancer, suggesting that TRPM8 links Ca(2+) transport pathways to tumor biology. We thus investigated the cellular responses of prostate cancer cells to menthol. Here we found that menthol increases [Ca(2+)](i) via Ca(2+) influx mechanism(s) independent of TRPM8 in PC-3 cells. We demonstrated that menthol induces cell death at supramillimolar concentrations in PC-3 cells and the cell death is not suppressed by low extracellular Ca(2+) condition which indicates that menthol-induced cell death is not associated with Ca(2+) influx pathways. In addition, we showed that menthol increases a phosphorylated form of c-jun N-terminal kinase (JNK) in PC-3 cells through TRPM8-independent mechanisms. Thus, our data indicate that there is an apparent lack of causality between TRPM8 activation and menthol-induced cell death and that menthol can regulate TRPM8-independent Ca(2+)-transport and cellular processes.