Role of c-Jun N-terminal kinase in apoptosis

A high-throughput assay for phosphoprotein-specific phosphatase activity in cellular extracts

Protein phosphatases undo the post-translational modifications of kinase-signaling networks, but phosphatase activation in cells is difficult to measure and interpret. Here, we report the design of a quantitative and high-throughput assay platform for monitoring cellular phosphatase activity toward specific phosphoprotein targets. Protein substrates of interest are purified recombinantly, phosphorylated in vitro using the upstream kinase, and adsorbed to 96-well plates. Total phosphatase extracts from cells are then added to trigger a solid-phase dephosphorylation reaction. After stopping the reaction, phosphoprotein levels are quantified by ELISA with a phospho-specific antibody, and the loss of phospho-specific immunoreactivity is used as the readout of phosphatase activity. We illustrate the generality of the method by developing specific phosphatase-activity assays for the three canonical mitogen-activated protein phospho-kinases: ERK, JNK, and p38. The assays capture changes in activity with a dynamic range of 25-100-fold and are sensitive to a limit of detection below 25,000 cells. When applied to cytokine-induced signaling, the assays revealed complex and dynamic regulation of phosphatases suggesting cross-communication and a means for cellular memory. Our assay platform should be beneficial for phosphoproteomic surveys and computational-systems models of signaling, where phosphatases are known to be important but their activities are rarely measured.

Nickel Compounds Render Anti-apoptotic Effect to Human Bronchial Epithelial Beas-2B Cells by Induction of Cyclooxygenase-2 through an IKKβ/p65-dependent and IKKα- and p50-independent Pathway

The carcinogenicity of nickel compounds has been well documented both in vitro and in vivo; however, the molecular mechanisms by which nickel compounds cause cancers are far from understood. Because suppression of apoptosis is thought to contribute to carcinogenesis, we investigated the mechanisms implicated in nickel-induced anti-apoptotic effect in human bronchial epithelial (Beas-2B) cells. We found that exposure of Beas-2B cells to nickel compounds resulted in increased cyclooxygenase-2 (COX-2) expression and that small interfering RNA (siCOX-2) knockdown of COX-2 expression resulted in increased cell sensitivity to nickel-triggered cell apoptosis, demonstrating that COX-2 induction has an anti-apoptotic effect on Beas-2B cells. Overexpression of IKKbeta-KM, a kinase inactive mutant of IKKbeta, blocked NF-kappaB activation and COX-2 induction by nickel compounds, indicating that activated NF-kappaB may be a mediator for COX-2 induction. To further explore the contribution of the NF-kappaB pathway in COX-2 induction and in protection from nickel exposure, mouse embryonic fibroblasts deficient in IKKbeta, IKKalpha, p65, and p50 were analyzed. Loss of IKKbeta impaired COX-2 induction by nickel exposure, whereas knockout of IKKalpha had a marginal effect. Moreover, the NF-kappaB p65, and not the p50 subunit, was critical for nickel-induced COX-2 expression. In addition, a deficiency of IKKbeta or p65 rendered cells more sensitive to nickel-induced apoptosis as compared with those in wild type cells. Finally, it was shown that reactive oxygen species H(2)O(2) were involved in both NF-kappaB activation and COX-2 expression. Collectively, our results demonstrate that COX-2 induction by nickel compounds occurs via an IKKbeta/p65 NF-kappaB-dependent but IKKalpha- and p50-independent pathway and plays a crucial role in antagonizing nickel-induced cell apoptosis in Beas-2B cells.

Cyclooxygenase-2 Induction by Arsenite Is through a Nuclear Factor of Activated T-cell-dependent Pathway and Plays an Antiapoptotic Role in Beas-2B Cells

Arsenite is a well known metalloid human carcinogen, and epidemiological evidence has demonstrated its association with the increased incidence of lung cancer. However, the mechanism involved in its lung carcinogenic effect remains obscure. The current study demonstrated that exposure of human bronchial epithelial cells (Beas-2B) to arsenite resulted in a marked induction of cyclooxygenase (COX)-2, an important mediator for inflammation and tumor promotion. Exposure of the Beas-2B cells to arsenite also led to significant transactivation of nuclear factor of activated T-cells (NFAT), but not activator protein-1 (AP-1) and NFkappaB, suggesting that NFAT, rather than AP-1 or NFkappaB, is implicated in the responses of Beas-2B cells to arsenite exposure. Furthermore, we found that inhibition of the NFAT pathway by either chemical inhibitors, dominant negative mutants of NFAT, or NFAT3 small interference RNA resulted in the impairment of COX-2 induction and caused cell apoptosis in Beas-2B cells exposed to arsenite. Site-directed mutation of two putative NFAT binding sites between-111 to +65 in the COX-2 promoter region eliminated the COX-2 transcriptional activity induced by arsenite, confirming that those two NFAT binding sites in the COX-2 promoter region are critical for COX-2 induction by arsenite. Moreover, knockdown of COX-2 expression by COX-2-specific small interference RNA also led to an increased cell apoptosis in Beas-2B cells upon arsenite exposure. Together, our results demonstrate that COX-2 induction by arsenite is through NFAT3-dependent and AP-1- or NFkappaB-independent pathways and plays a crucial role in antagonizing arsenite-induced cell apoptosis in human bronchial epithelial Beas-2B cells.

TRAF2 plays a key, nonredundant role in LIGHT-lymphotoxin beta receptor signaling

LIGHT is a member of the tumor necrosis factor (TNF) superfamily, and its function is mediated by at least two receptors, including lymphotoxin beta receptor (LTbetaR) and herpes simplex virus entry mediator. However, the molecular mechanism of LIGHT signaling mediated by LTbetaR has not been clearly defined. In this report, we demonstrate that TRAF2 is critical for LIGHT- and LTbetaR-mediated activation of both the transcription factor NF-kappaB and the mitogen-activated protein kinase JNK. In HeLa cells, LIGHT induces NF-kappaB and JNK activation, which can be blocked by the dominant negative mutant of TRAF2. In these cells, LIGHT causes the recruitment of TRAF2, TRAF3, and IkappaB kinase into the LTbetaR complex. Importantly, while both NF-kappaB and JNK are activated by LIGHT in wild-type mouse embryonic fibroblasts, no activation of either of these two pathways is observed in TRAF2 null fibroblasts. However, LIGHT-induced NF-kappaB and JNK activation can be restored by ectopic expression of TRAF2 in TRAF2-/- cells. Interestingly, in contrast to TNF signaling, the activation of both NF-kappaB and JNK by LIGHT was normal in RIP-/- and TRAF5-/- cells. Taken together, our data demonstrate that TRAF2, an important effector molecule of TNF signaling, plays a critical, nonredundant role in LIGHT-LTbetaR signaling.

The retinoic acid and brain-derived neurotrophic factor differentiated SH-SY5Y cell line as a model for Alzheimer’s disease-like tau phosphorylation

The paired helical filaments of highly phosphorylated tau protein are the main components of neurofibrillary tangles (NFT) in Alzheimer's disease (AD). Protein kinases including glycogen synthase kinase 3 beta (GSK3beta), cyclin-dependent kinase 5 (Cdk5), and c-Jun N-terminal kinase (JNK) have been implicated in NFT formation making the use of selective kinase inhibitors an attractive treatment possibility in AD. When sequentially treated with retinoic acid (RA) and brain-derived neurotrophic factor (BDNF), the human neuroblastoma SH-SY5Y differentiates to neuron-like cells. We found that coincident with morphologically evident neurite outgrowth, both the content and phosphorylation state of tau increased in RA-BDNF differentiated SH-SY5Y cells. Tau phosphorylation increased at all the examined sites ser-199, ser-202, thr-205, ser-396, and ser-404, all of which are hyperphosphorylated in AD brain. We also investigated whether GSK3beta, Cdk5 or JNK was involved in tau phosphorylation in the differentiated SH-SY5Y cells. We found that GSK3beta contributed most and that Cdk5 made a minor contribution. JNK was not involved in tau phosphorylation in this system. The GSK3beta-inhibitor, lithium, inhibited tau phosphorylation in a concentration-dependent manner and with good reproducibility, which enables ranking of substances in this cell model. RA-BDNF differentiated SH-SY5Y cells could serve as a suitable model for studying the mechanisms of tau phosphorylation and for screening potential GSK3beta inhibitors.

Low-dose TNF-alpha protects against hepatic ischemia-reperfusion injury in mice: implications for preconditioning

Tumor necrosis factor alpha (TNF-alpha) is implicated in the pathogenesis of hepatic ischemia reperfusion injury but can also prime hepatocytes to enter the cell cycle. Ischemic preconditioning protects against ischemia-reperfusion (IR) liver injury and is associated with activation of nuclear factor kappaB (NF-kappaB) and cell cycle entry. We examined the pattern of TNF-alpha release during hepatic IR in the presence or absence of ischemic preconditioning, and we tested whether a single low-dose injection of TNF could mimic the biologic effects of ischemic preconditioning. In naïve mice, hepatic and plasma levels of TNF-alpha rose during hepatic ischemia, reaching high levels after 90 minutes; values remained elevated during reperfusion until 44 hours. Following the ischemic preconditioning stimulus, there was an early rise in hepatic and serum TNF-alpha levels, but, during a second prolonged ischemic interval peak, TNF-alpha values were lower than in naïve mice and declined to negligible levels by 2 hours reperfusion. An injection with 1 microg or 5 microg/kg body weight TNF-alpha 30 minutes prior to hepatic IR substantially reduced liver injury determined by liver histology and serum alanine aminotransferase (ALT) levels. As in ischemic preconditioning, TNF-alpha pretreatment activated NF-kappaB DNA binding, STAT3, cyclin D1, cyclin-dependent kinase 4 (cdk4) expression, and cell cycle entry, determined by proliferating cell nuclear antigen (PCNA) staining of hepatocyte nuclei. In conclusion, the hepatoprotective effects of "preconditioning" can be simulated by TNF-alpha injection, which has identical downstream effects on cell cycle entry. We propose that transient increases in TNF-alpha levels may substitute for, as well as, mediate the hepatoprotective effects of ischemic preconditioning against hepatic IR injury.