Yi-Rong Chen

Inhibition of the c-jun N-terminal kinase (JNK) signaling pathway by curcumin

Curcumin, a dietary pigment in curry, suppresses tumor initiation and tumor promotion. Curcumin is also a potent inhibitor for AP-1 and NF-κB activation. In this report, we show that curcumin inhibits JNK activation by various agonists including PMA plus ionomycin, anisomycin, UV-C, γu2009u2009radiation, TNF-α, and sodium orthovanadate. Although both JNK and ERK activation by phorbol 12-myristate 13-acetate (PMA) plus ionomycin were suppressed by curcumin, the JNK pathway was more sensitive. The IC50 (50% inhibition concentration) of curcumin was between 5–10u2009μM for JNK activation and was 20u2009μM for ERK activation. In transfection assays, curcumin moderately suppressed MEKK1-induced JNK activation; however, it effectively blocked JNK activation caused by co-transfection of TAK1, GCK, or HPK1. Curcumin did not directly inhibit JNK, SEK1, MEKK1 or HPK1 activity. Although curcumin suppressed TAK1 and GCK activities at high concentrations, this inhibition cannot fully account for the JNK inhibition by curcumin in vivo. Our data suggest that curcumin may affect the JNK pathway by interfering with the signaling molecule(s) at the same level or proximally upstream of the MAPKKK level. Taken together, the inhibition of the MEKK1-JNK pathway reveals a possible mechanism of suppression of AP-1 and NF-κB signaling by curcumin, and may explain the potent anti-inflammatory and anti-carcinogenic effects of this chemical.

Down-regulation of the c-Jun N-terminal kinase (JNK) phosphatase M3/6 and activation of JNK by hydrogen peroxide and pyrrolidine dithiocarbamate

Oxidative stress activates the c-Jun N-terminal kinase (JNK) pathway. However, the exact mechanisms by which reactive oxygen species (ROS) activate JNK are unclear. We found that the ability of hydrogen peroxide (H2O2) to induce JNK activation varied in different cell types. Pyrrolidine dithiocarbamate (PDTC), a presumed antioxidant, induced JNK activation on its own and enhanced JNK activation by H2O2 in many cell types, including Jurkat, HEK293, and LNCaP and Tsu-Pr1 prostate cancer cells. The activation of JNK by PDTC, in the presence or absence of exogenous H2O2, was dependent on its chelating ability to metal ions, most likely copper ions. Despite the strong JNK-activating ability, H2O2 plus PDTC did not induce significant activation of the upstream kinases, SEK1/MKK4 and MKK7. However, the JNK inactivation rate was slower in cells treated with H2O2 plus PDTC compared with the rate in cells treated with ultraviolet C (UV-C). Treatment of H2O2 plus PDTC significantly decreased the expression levels of a JNK phosphatase, M3/6 (also named hVH-5), but not the levels of other phosphatases (PP2A and PP4). In contrast, UV-C irradiation did not cause the down-regulation of M3/6. These results suggest that JNK activation by H2O2 plus PDTC resulted from the down-regulation of JNK phosphatases. Our data also reveal a necessity to carefully evaluate the pharmacological and biochemical properties of PDTC.

Vaccinia H1-related phosphatase is a phosphatase of ErbB receptors and is down-regulated in non-small cell lung cancer.

Vaccinia H1-related phosphatase (VHR) is classified as a dual specificity phosphatase. Unlike typical dual specificity phosphatases, VHR lacks the MAPK-binding domain and shows poor activity against MAPKs. We found that EGF receptor (EGFR) was a direct substrate of VHR and that overexpression of VHR down-regulated EGFR phosphorylation, particularly at Tyr-992 residue. Expression of VHR inhibited the activation of phospholipase Cγ and protein kinase C, both downstream effectors of Tyr-992 phosphorylation of EGFR. Decreasing VHR expression by RNA interference caused higher EGFR phosphorylation at Tyr-992. In addition to EGFR, VHR also directly dephosphorylated ErbB2. Consistent with these results, suppression of VHR augmented the foci formation ability of H1299 non-small cell lung cancer (NSCLC) cells, whereas overexpression of VHR suppressed cell growth in both two- and three-dimensional cultures. Expression of VHR also suppressed tumor formation in a mouse xenograft model. Furthermore, VHR expression was significantly lower in NSCLC tissues in comparison to that in normal lung tissues. Collectively, this study shows that down-regulation of VHR expression enhances the signaling of ErbB receptors and may be involved in NSCLC pathogenesis.

Biochemical and biological characterization of a neuroendocrine-associated phosphatase.

The biochemical and biological properties of a novel neuroendocrine‐associated phosphatase (NEAP) were characterized. NEAP had a sequence characteristic of a dual‐specificity phosphatase (DSP), and was preferentially expressed in neuroendocrine cells/tissues as well as in skeletal muscle and heart. Expression of NEAP was up‐regulated in nerve growth factor (NGF)‐treated, differentiated PC12 cells. NEAP was cytosolic and did not apparently have effects against extracellular signal‐regulated kinase, c‐Jun N‐terminal kinase (JNK) and p38 mitogen‐activated protein kinase activated by various stimuli. Although NEAP and MAPK phosphatase (MPK)‐1 showed similar phosphatase activity towards p‐nitro phenylphosphate (pNPP), in contrast to MKP‐1, NEAP did not dephosphorylate JNK and p38‐MAPK in vitro. Overexpression of NEAP, but not the C152S mutant, in PC12 cells suppressed NGF‐induced phosphorylation of the p85 subunit of phosphatidylinositol 3‐kinase (PI3K) and Akt activation. Overexpression of NEAP also suppressed neurite outgrowth induced by NGF and sensitized PC12 cells to cisplatin‐induced apoptosis. Suppression of NEAP by RNA interference enhanced NGF‐induced neurite outgrowth and Akt activation. Our results indicated that, unlike other DSPs, down‐regulation of conventional MAPKs was not the major function of NEAP. Furthermore, NEAP might be involved in neuronal differentiation via regulation of the PI3K/Akt signaling.

JNK Pathway-associated Phosphatase Dephosphorylates Focal Adhesion Kinase and Suppresses Cell Migration

JNK pathway-associated phosphatase (JKAP, also named DUSP22) is expressed in various tissues, indicating that JKAP may have an important biological function. We showed that JKAP localized in the actin filament-enriched region. Expression of JKAP reduced cell migration, whereas a JKAP mutant lacking catalytic activity promoted cell motility. JKAP efficiently removed tyrosine phosphorylation of several proteins. We have identified focal adhesion kinase (FAK) as a substrate of JKAP. Overexpression of JKAP, but not JKAP mutant lacking catalytic activity, decreased FAK phosphorylation at tyrosines 397, 576, and 577 in H1299 cells. Consistent with these results, decreasing JKAP expression by RNA interference promoted cell migration and Src-induced FAK phosphorylation. Taken together, this study identified a new role for JKAP in the modulation of FAK phosphorylation and cell motility.

Identification of salt-inducible kinase 3 as a novel tumor antigen associated with tumorigenesis of ovarian cancer

Existence of humoral immunity has been previously demonstrated in malignant ascitic fluids. However, only a limited number of immunogenic tumor-associated antigens (TAAs) were identified, and few of which are associated with ovarian cancer. Here, we identified salt-inducible kinase 3 (SIK3) as a TAA through screening of a random peptide library in the phage display system. Overexpression of SIK3 markedly promoted cell proliferation, attenuated p21Waf/Cip1 and p27Kip expressions in low-grade OVCAR3 cells, and permitted the cells to grow in mice. Decrease in SIK3 expression in high-grade SK-OV3 cells consistently demonstrated its tumorigenic potency by modulating the protein levels of cell cycle regulators. When the expressions of SIK3 and CA125 were compared in cancer tissues, immunohistochemical (IHC) studies indicated that cytoplasm-localized SIK3 was highly expressed in 55% of the ovarian cancer samples. In contrast, it was rarely detected in adenomyosis, leiomyoma and normal ovary tissues, showing its higher specificity (97%) to CA125 (65%) in ovarian cancer. Moreover, experiments using pharmacological inhibitors to block SIK3-induced p21Waf/Cip1 expression revealed that activation of c-Src and phosphoinositide-3-kinase were critically required for its biological activity, suggesting that they are the downstream signaling mediators of SIK3. These data were further supported by IHC studies, showing coexpression of c-Src with SIK3 in 85% of the ovarian tumor samples stained positive for SIK3. Collectively, our findings indicate that SIK3 is a novel ovarian TAA. Overexpression of SIK3 promotes G1/S cell cycle progression, bestows survival advantages to cancer cells for growth and correlates the clinicopathological conditions of patients with ovarian cancer.

NEAP causes down‐regulation of EGFR, subsequently induces the suppression of NGF‐induced differentiation in PC12 cells

Neuroendocrine‐associated phosphatase (NEAP), an atypical dual specificity phosphatase is preferentially expressed in neuroendocrine cells. In this study we found that NEAP, but not NEAP‐(C152S) mutant, evidently reduced epidermal growth factor (EGF) receptor (EGFR) downstream signaling, and impaired cell growth in response to EGF stimulation in PC12 cells. These phenomena were associated with NEAP‐mediated down‐regulation of EGFR mRNA and protein. NEAP had no significant effect on ErbB2/3 expression and phosphorylation levels in response to heregulin, indicating that the negative effect of NEAP on EGFR was selective. We showed that NEAP suppressed EGFR expression via decreasing the EGFR promoter activity and this was mediated through down‐regulations of the Akt pathway and Wilms’ tumor gene product (WT1). Consistent with these results, expression of WT1 reversed the suppressive effect of NEAP on EGFR promoter activity. Additionally, NEAP knockdown by RNA interference enhanced EGFR protein expression and nerve growth factor‐induced differentiation, and an EGFR‐specific inhibitor could reverse the later event. Taken together, our study indicated that NEAP modulates PC12 differentiation via suppression of EGFR expression and signaling.

UTF1 deficiency promotes retinoic acid-induced neuronal differentiation in P19 embryonal carcinoma cells.

UTF1 (undifferentiated embryonic cell transcription factor 1) is a marker for the pluripotency of embryonic stem cells. We found that UTF1-deficient clones, which were isolated from P19 embryonal carcinoma (EC) cells, showed higher neuron-differentiating potentials than the parental cell line. Consistent with this result, suppression of UTF1 expression in P19 cells by RNA interference enhanced retinoic acid (RA)-induced neuronal differentiation. Moreover, reconstitution of UTF1 expression in UTF1-deficient clones decreased their ability to undergo neuronal differentiation. Interestingly, the growth rates of UTF1-deficient P19 cells did not differ from that of parental cells in adherent cultures, but were increased in embryoid bodies during RA-induced differentiation. Furthermore, different from the parental cells, UTF1-deficient P19 clones could proceed to neuronal differentiation without forming embryoid bodies. Based on these results we proposed that endogenous UTF1 prevented P19 EC cells from neuronal differentiation, and that the loss of UTF1 directed EC cells toward the neuronal fate.

NEAP/DUSP26 suppresses receptor tyrosine kinases and regulates neuronal development in zebrafish

Expression of neuroendocrine-associated phosphatase (NEAP, also named as dual specificity phosphatase 26, [DUSP26]) is restricted to neuroendocrine tissues. We found that NEAP, but not its phosphatase-defective mutant, suppressed nerve growth factor (NGF) receptor TrkA and fibroblast growth factor receptor 1 (FGFR1) activation in PC12 cells upon NGF stimulation. Conversely, suppressing NEAP expression by RNA interference enhanced TrkA and FGFR1 phosphorylation. NEAP was capable of de-phosphorylating TrkA and FGFR1 directly in vitro. NEAP-orthologous gene existed in zebrafish. Morpholino (MO) suppression of NEAP in zebrafish resulted in hyper-phosphorylation of TrkA and FGFR1 as well as abnormal body postures and small eyes. Differentiation of retina in zebrafishes with NEAP MO treatment was severely defective, so were cranial motor neurons. Taken together, our data indicated that NEAP/DUSP26 have a critical role in regulating TrkA and FGFR1 signaling as well as proper development of retina and neuronal system in zebrafish.