Bcl-xL prevents staurosporine-induced hepatocyte apoptosis by restoring protein kinase B/mitogen-activated protein kinase activity and mitochondria integrity

Cytotoxicity of Air Pollutant 9,10-Phenanthrenequinone: Role of Reactive Oxygen Species and Redox Signaling

Atmospheric pollution has been a principal topic recently in the scientific and political community due to its role and impact on human and ecological health. 9,10-phenanthrenequinone (9,10-PQ) is a quinone molecule found in air pollution abundantly in the diesel exhaust particles (DEP). This compound has studied extensively and has been shown to develop cytotoxic effects both in vitro and in vivo. 9, 10-PQ has been proposed to play a critical role in the development of cytotoxicity via generation of reactive oxygen species (ROS) through redox cycling. This compound also reduces expression of glutathione (GSH), which is critical in Phase II detoxification reactions. Understanding the underlying cellular mechanisms involved in cytotoxicity can allow for the development of therapeutics designed to target specific molecules significantly involved in the 9,10-PQ-induced ROS toxicity. This review highlights the developments in the understanding of the cytotoxic effects of 9, 10-PQ with special emphasis on the possible mechanisms involved.

Activation of PPARβ/δ protects pancreatic β cells from palmitate-induced apoptosis by upregulating the expression of GLP-1 receptor

We previously showed that activated peroxisome proliferator-activated receptor (PPAR)β/δ can protect pancreatic β cells against lipotoxic apoptosis. However, the molecular mechanism remained unclear. Glucagon-like peptide-1 receptor (GLP-1R) has been reported to exhibit a protective effect against lipotoxic apoptosis in pancreatic β cells. In the present study, we aimed to investigate the underlying molecular mechanisms that PPARβ/δ activation suppressed apoptosis and improved β cell function impaired by fatty acids, focusing on contribution of GLP-1R. Isolated rat islets and rat insulin-secreting INS-1 cells were treated with the PPARβ/δ agonist GW501516 (GW) in the presence or absence of palmitate (PA) and transfected with siRNA for PPARβ/δ or treated with the PPARβ/δ antagonist GSK0660. Apoptosis was assessed by DNA fragmentation, Hoechst 33342 staining and flow cytometry. GLP-1R expression in INS-1 cells and islets was assayed by immunoblotting, quantitative PCR (qPCR) and immunofluorescence staining. SREBP-1c, Caveolin-1, Akt, Bcl-2, Bcl-xl and caspase-3 expression was measured using immunoblotting and qPCR. Our results showed that PPARβ/δ activation decreased apoptosis in β cells and robustly stimulated GLP-1R expression under lipotoxic conditions. GW enhanced glucose-stimulated insulin secretion (GSIS) impaired by PA through stimulation of GLP-1R expression in β cells. Moreover, SREBP-1c/Caveolin-1 signaling was involved in PPARβ/δ-regulated GLP-1R expression. Finally, GW exerted anti-apoptotic effects via interfering with GLP-1R-dependent Akt/Bcl-2 and Bcl-xl/caspase-3 signaling pathways. Our study suggested that the anti-apoptotic action of GW may involve its transcriptional regulation of GLP-1R, and PPARβ/δ activation may represent a new therapeutic method for protecting pancreatic β cells from lipotoxicity.

Role of naofen in apoptosis of hepatocytes induced by lipopolysaccharide through mitochondrial signaling in rats

AIM

  Lipopolysaccharide (LPS) causes apoptosis of hepatocytes, which is probably mediated by inflammatory substances released from Kupffer cells (KCs). Recently, we have reported that naofen, a newly found intracellular WD40-repeat protein, has a role in inducing the apoptosis in HEK293 cells. Hence, the present study was undertaken to investigate a role of naofen in the LPS-induced apoptosis of rat hepatocytes.

METHODS

  Rats were treated with i.v. injections of LPS, and livers were extirpated to evaluate expression of naofen and apoptosis. In in vitro experiments, hepatocytes and KCs were separately isolated from rat livers. The incubation medium for KCs treated with LPS (KC-CM) was used for hepatocyte culture.

RESULTS

  Intravenous injections of LPS enhanced the expression of naofen in the livers. Livers showed terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive staining, and elevated caspase-3 activity. In isolated KCs or hepatocytes, LPS hardly affected naofen expression and caspase-3 activity, whereas incubation of hepatocytes with KC-CM enhanced both naofen expression and caspase-3 activation. Transfection of hepatocyte with naofen siRNA prevented such effects of KC-CM, and clearly eliminated KC-CM-induced reduction of Bcl-2 and Bcl-xL. In contrast, overexpression of naofen in hepatocytes downregulated Bcl-2 and Bcl-xL, released cytochrome c from mitochondria, and activated caspase-3.

CONCLUSION

  These results indicate that LPS may induce the hepatic apoptosis in association with enhanced naofen expression, and that naofen may mediate the activation of caspase-3 through downregulating the Bcl-2 and Bcl-xL expression, and releasing cytochrome c from mitochondria to cytoplasm.

Regulation of DNA fragmentation: the role of caspases and phosphorylation

DNA fragmentation is a hallmark of apoptosis that is induced by apoptotic stimuli in various cell types. Apoptotic signal pathways, which eventually cause DNA fragmentation, are largely mediated by the family of cysteinyl aspartate-specific protease caspases. Caspases mediate apoptotic signal transduction by cleavage of apoptosis-implicated proteins and the caspases themselves. In the process of caspase activation, reversible protein phosphorylation plays an important role. The activation of various proteins is regulated by phosphorylation and dephosphorylation, both upstream and downstream of caspase activation. Many kinases/phosphatases are involved in the control of cell survival and death, including the mitogen-activated protein kinase signal transduction pathways. Reversible protein phosphorylation is involved in the widespread regulation of cellular signal transduction and apoptotic processes. Therefore, phosphatase/kinase inhibitors are commonly used as apoptosis inducers/inhibitors. Whether protein phosphorylation induces apoptosis depends on many factors, such as the type of phosphorylated protein, the degree of activation and the influence of other proteins. Phosphorylation signaling pathways are intricately interrelated; it was previously shown that either induction or inhibition of phosphorylation causes cell death. Determination of the relationship between protein and phosphorylation helps to reveal how apoptosis is regulated. Here we discuss DNA fragmentation and protein phosphorylation, focusing on caspase and serine/threonine protein phosphatase activation.

Akt1 intramitochondrial cycling is a crucial step in the redox modulation of cell cycle progression

Akt is a serine/threonine kinase involved in cell proliferation, apoptosis, and glucose metabolism. Akt is differentially activated by growth factors and oxidative stress by sequential phosphorylation of Ser⁴⁷³ by mTORC2 and Thr³⁰⁸ by PDK1. On these bases, we investigated the mechanistic connection of H₂O₂ yield, mitochondrial activation of Akt1 and cell cycle progression in NIH/3T3 cell line with confocal microscopy, in vivo imaging, and directed mutagenesis. We demonstrate that modulation by H₂O₂ entails the entrance of cytosolic P-Akt1 Ser⁴⁷³ to mitochondria, where it is further phosphorylated at Thr³⁰⁸ by constitutive PDK1. Phosphorylation of Thr³⁰⁸ in mitochondria determines Akt1 passage to nuclei and triggers genomic post-translational mechanisms for cell proliferation. At high H₂O₂, Akt1-PDK1 association is disrupted and P-Akt1 Ser⁴⁷³ accumulates in mitochondria in detriment to nuclear translocation; accordingly, Akt1 T308A is retained in mitochondria. Low Akt1 activity increases cytochrome c release to cytosol leading to apoptosis. As assessed by mass spectra, differential H₂O₂ effects on Akt1-PDK interaction depend on the selective oxidation of Cys³¹⁰ to sulfenic or cysteic acids. These results indicate that Akt1 intramitochondrial-cycling is central for redox modulation of cell fate.

Staurosporine induces apoptosis in human papillomavirus positive oral cancer cells at G2/M phase by disrupting mitochondrial membrane potential and modulation of cell cytoskeleton

Our study demonstrates that staurosporine (STS), a protein kinase inhibitor from Streptomyces sp., induces apoptosis in human papillomavirus positive oral carcinoma cells (KB) in a dose dependent manner. Growth inhibition studies revealed an IC(50) value of approximately 100 nM. STS induced marked nuclear fragmentation and inter-nucleosomal cleavage compared to untreated cells. It also caused dose dependent disruption of mitochondrial membrane potential and activation of caspase-3, indicating involvement of mitochondria-mediated cell death signaling in KB cell apoptosis. We found time-dependent arrest of the KB cells at G2/M phase of cell cycle. Using fluorescence microscopy, we have further shown that STS treatment disrupts microtubules and reorganizes F-actin after 6h exposure. Taken together, our results suggest that STS induces mitochondria-mediated KB cell apoptosis at G2/M phase by altering cell cytoskeletal network.

Calcium overload is essential for the acceleration of staurosporine-induced cell death following neuronal differentiation in PC12 cells

Differentiation of neuronal cells has been shown to accelerate stress-induced cell death, but the underlying mechanisms are not completely understood. Here, we find that early and sustained increase in cytosolic ([Ca2(+)]c) and mitochondrial Ca2(+) levels ([Ca2(+)]m) is essential for the increased sensitivity to staurosporine- induced cell death following neuronal differentiation in PC12 cells. Consistently, pretreatment of differentiated PC12 cells with the intracellular Ca2(+)-chelator EGTA-AM diminished staurosporine-induced PARP cleavage and cell death. Furthermore, Ca2(+) overload and enhanced vulnerability to staurosporine in differentiated cells were prevented by Bcl-XL overexpression. Our data reveal a new regulatory role for differentiation-dependent alteration of Ca2(+) signaling in cell death in response to staurosporine.

The mRNA-destabilizing protein tristetraprolin is suppressed in many cancers, altering tumorigenic phenotypes and patient prognosis

AU-rich element-binding proteins (ARE-BP) regulate the stability and/or translational efficiency of mRNAs containing cognate binding sites. Many targeted transcripts encode factors that control processes such as cell division, apoptosis, and angiogenesis, suggesting that dysregulated ARE-BP expression could dramatically influence oncogenic phenotypes. Using several approaches, we evaluated the expression of four well-characterized ARE-BPs across a variety of human neoplastic syndromes. AUF1, TIA-1, and HuR mRNAs were not systematically dysregulated in cancers; however, tristetraprolin mRNA levels were significantly decreased across many tumor types, including advanced cancers of the breast and prostate. Restoring tristetraprolin expression in an aggressive tumor cell line suppressed three key tumorgenic phenotypes: cell proliferation, resistance to proapoptotic stimuli, and expression of vascular endothelial growth factor mRNA. However, the cellular consequences of tristetraprolin expression varied across different cell models. Analyses of gene array data sets revealed that suppression of tristetraprolin expression is a negative prognostic indicator in breast cancer, because patients with low tumor tristetraprolin mRNA levels were more likely to present increased pathologic tumor grade, vascular endothelial growth factor expression, and mortality from recurrent disease. Collectively, these data establish that tristetraprolin expression is frequently suppressed in human cancers, which in turn can alter tumorigenic phenotypes that influence patient outcomes.

A novel association between p130Cas and resistance to the chemotherapeutic drug adriamycin in human breast cancer cells

Resistance to chemotherapy remains a major obstacle for the treatment of breast cancer. Understanding the molecular mechanism(s) of resistance is crucial for the development of new effective therapies to treat this disease. This study examines the putative role of p130(Cas) (Cas) in resistance to the cytotoxic agent Adriamycin. High expression of Cas in primary breast tumors is associated with the failure to respond to the antiestrogen tamoxifen and poor prognosis, highlighting the potential clinical importance of this molecule. Here, we show a novel association between Cas and resistance to Adriamycin. We show that Cas overexpression renders MCF-7 breast cancer cells less sensitive to the growth inhibitory and proapoptotic effects of Adriamycin. The catalytic activity of the nonreceptor tyrosine kinase c-Src, but not the epidermal growth factor receptor, is critical for Cas-mediated protection from Adriamycin-induced death. The phosphorylation of Akt and extracellular signal-regulated kinase 1/2 (ERK1/2) is elevated in Cas-overexpressing cells treated with Adriamycin, whereas expression of the proapoptotic protein Bak is decreased. Conversely, Cas depletion in the more resistant T47D and MDA-MB-231 cell lines increases sensitivity to Adriamycin. Based on these data, we propose that Cas activates growth and survival pathways regulated by c-Src, Akt, and ERK1/2 that lead to the inhibition of mitochondrial-mediated apoptosis in the presence of Adriamycin. Because Cas is frequently expressed at high levels in breast cancers, these findings raise the possibility of resensitizing Cas-overexpressing tumors to chemotherapy through perturbation of Cas signaling pathways.

Research progress in anti-apoptosis gene Bcl-xL

Apoptosis is a physiological process that an organism selectively eliminates cells that are no longer needed, or have been damaged, or are dangerous. Bcl-xL, an important member of the Bcl-2 family that plays indispensable roles in regulating cell survival and apoptosis, is frequently over-expressed in various kinds of human cancers. The inhibition of this molecule is associated with decreased tumorigenesis and resistance to conventional chemotherapy. This article briefly reviews some progresses in the study of Bcl-xL in the past few years.