Ethanol extract of paeonia suffruticosa Andrews (PSE) induced AGS human gastric cancer cell apoptosis via fas-dependent apoptosis and MDM2-p53 pathways

ABSTRACT: BACKGROUND: The root bark of Paeonia suffruticosa Andrews (PSE), also known as Moutan Cortex, has been widely used in Asia to treat various diseases. The molecular mechanisms by which PSE exerts its anti-oxidant and anti-inflammatory activities are well known, but its anti-cancer activity is not yet well understood. Here, we present evidence demonstrating that PSE can be used as a potent anti-cancer agent to treat gastric cancer. METHODS: The effects of the ethanol extract of PSE on cell proliferation were determined using an MTT (1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan) assay. Cell cytotoxicity induced by the PSE extact is measured using an LDH leakage assay. Flow cytometry was used to analyze the cell cycle and to measure the subG0/G1 apoptotic cell fraction. Apoptosis induced by the PSE extact is also examined using a DNA fragmentation assay. Western blot analysis is used to measure the levels of apoptotic proteins such as Fas receptor, caspase-8, caspase-3, PARP, Bax, Bcl-2, MDM2, and p53. RESULTS: This study demonstrated that treating AGS cells with the PSE extact significantly inhibited cell proliferation and induced cytotoxicity in a dose- and time-dependent manner. The PSE extract also induced apoptosis in AGS cells, as measured by flow cytometry and a DNA fragmentation assay. We found that the PSE extract induced apoptosis via the extrinsic Fas-mediated apoptosis pathway, which was concurrent with the activation of caspases, including caspase-8 and caspase-3, and cleavage of PARP. The MDM2-p53 pathway also played a role in the apoptosis of AGS cells that was induced by the PSE extract. CONCLUSIONS: These results clearly demonstrate that the PSE extact displays growth-suppressive activity and induces apoptosis in AGS cells. Our data suggest that the PSE extact might be a potential anti-cancer agent for gastric cancer.     

AKT-dependent phosphorylation of Niban regulates nucleophosmin- and MDM2-mediated p53 stability and cell apoptosis

Although Niban is highly expressed in human cancer cells, the cellular functions of Niban remain largely unknown. We demonstrate here that ultraviolet irradiation induces phosphorylation of Niban at S602 by AKT, which increases the association of Niban with nucleophosmin and disassociation of nucleophosmin from the MDM2 complex. This leads to the promotion of MDM2-p53 interaction and subsequent p53 degradation, thereby providing an antiapoptotic effect. Conversely, depletion of or deficiency in Niban expression promotes stabilization of p53 with increased cell apoptosis. Our findings illustrate a pivotal role for AKT-mediated phosphorylation of Niban in protecting cells from genotoxic stress-induced cell apoptosis.     

Cyclin G1 has growth inhibitory activity linked to the ARF-Mdm2-p53 and pRb tumor suppressor pathways

Cyclin G1 is a p53-responsive gene that is induced in alternative reading frame (ARF)-arrested cells, yet its role in growth control is unclear. We tested its effects on growth and involvement in the ARF-Mdm2-p53 tumor suppressor pathway. We show that cyclin G1 interacts with ARF, Mdm2, and p53 in vitro and in vivo. At high levels, cyclin G1 induces a G(1)-phase arrest in mammalian cells that coincides with p53 activation. Conversely, lower levels of cyclin G1 lack intrinsic growth inhibitory effects yet potentiate ARF-mediated growth arrest. Notably, cyclin G1 is down-regulated by Mdm2 through proteasome-mediated degradation. These data suggest that cyclin G1 is a positive feedback regulator of p53 whose expression is restrained by Mdm2. Interestingly, growth inhibition by cyclin G1 does not require p53 but instead exhibits partial retinoblastoma protein (pRb) dependence. These findings reveal that cyclin G1 has growth inhibitory activity that is mechanistically linked to ARF-p53 and pRb tumor suppressor pathways.     

Aromatic-aromatic interactions in the formation of the MDM2-p53 complex

Although the molecular interaction of MDM2 with the transactivation domain of p53 has been thoroughly studied, there is very limited information regarding the steps involved in the recognition mechanism between these proteins. On this basis, we performed four high-temperature molecular dynamics simulations in explicit solvent to gain insight into the interactions involved in the fist contact toward the formation of the complex. We found that the presence of specific intermolecular aromatic pairs at the interface of the complex, around the native-like state of MDM2, is consistent among independent molecular dynamics runs. This observation suggests that aromatic-aromatic interactions are closely related to the first contact between MDM2 and p53. Thus, we propose that aromatic-aromatic interactions are an important, and probably essential, requirement for the formation and stabilization of the MDM2-p53 complex.     

Transient Protein States in Designing Inhibitors of the MDM2-p53 Interaction

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Crosstalk between hepatocyte growth factor and integrin signaling pathways

Summary Most types of normal cells require integrin-mediated attachment to extracellular matrix to be able to respond to growth factor stimulation for proliferation and survival. Therefore, a consensus that integrins are close collaborators with growth factors in signal transduction has gradually emerged. Some integrins and growth factor receptors appear to be normally in relatively close proximity, which can be induced to form complexes upon cell adhesion or growth factor stimulation. Moreover, since integrins and growth factor receptors share many common elements in their signaling pathways, it is clear tzhat there are many opportunities for integrin signals to modulate growth factor signals and vice versa. Increasing evidence indicates that integrins can crosstalk with receptor tyrosine kinases in a cell- and integrin-type-dependent manner through a variety of specific mechanisms. This review is intended specifically for summarizing recent progress uncovering how the hepatocyte growth factor receptor c-Met coordinates with integrins to transmit signals.     

MYCN silencing induces differentiation and apoptosis in human neuroblastoma cells

MYCN amplification strongly correlates with unfavorable outcomes in patients with neuroblastoma. The aim of this study was to investigate the role of MYCN in neuroblastoma cell differentiation and apoptosis. We used the technique of RNA interference to inhibit MYCN gene expression in neuroblastoma cells with variable expression of MYCN. Our results showed that inhibition of MYCN gene expression in MYCN amplified cells induced apoptosis and suppressed cell growth; neuronal differentiation also occurred after MYCN gene silencing. Moreover, N-myc downregulation was associated with decreased Bcl-xL protein levels and caspase-3 activation. These data show that small interfering RNA directed to MYCN, which plays a crucial role in neuroblastoma cell survival, may provide a potential novel therapeutic option for aggressive neuroblastomas.     

Both p53-PUMA/NOXA-Bax-mitochondrion and p53-p21cip1 pathways are involved in the CDglyTK-mediated tumor cell suppression

CDglyTK fusion suicide gene has been well characterized to effectively kill tumor cells. However, the exact mechanism and downstream target genes are not fully understood. In our study, we found that CDglyTK/prodrug treatment works more efficiently in p53 wild-type (HONE1) cells than in p53 mutant (CNE1) cells. We then used adenovirus-mediated gene delivery system to either knockdown or overexpress p53 and its target genes in these cells. Consistent results showed that both p53-PUMA/NOXA/Bcl2-Bax and p53-p21 pathways contribute to the CDglyTK induced tumor cell suppression. Our work for the first time addressed the role of p53 related genes in the CDglyTK/prodrug system.     

Crosstalk between VEGFR2 and muscarinic receptors regulates the mTOR pathway in serum starved SK-N-SH human neuroblastoma cells

Muscarinic acetylcholine receptors (mAchRs) are guanosine nucleotide-binding protein (G protein) coupled receptors that crosstalk with receptor tyrosine kinases (RTKs) to signal mitogenic pathways. In particular, mAchRs are known to couple with RTKs for several growth factors to activate the mammalian target of rapamycin (mTOR)/Akt pathway, a regulator of protein synthesis. The RTK for the vascular endothelial growth factor (VEGF), VEGFR2, can signal protein synthesis but whether it cooperates with mAchRs to mediate mTOR activation has not been demonstrated. Using serum starved SK-N-SH neuroblastoma cells, we show that the muscarinic receptor agonists carbachol and pilocarpine enhance the activation of the mTOR substrate p70 S6 Kinase (S6K) and its target ribosomal protein S6 (S6) in a VEGFR2 dependent manner. Treatments with carbachol increased VEGFR2 phosphorylation, suggesting that mAchRs stimulate VEGFR2 transactivation to enhance mTOR signaling. Inhibitor studies revealed that phosphatidylinositol 3 kinase resides upstream from S6K, S6 and Akt phosphorylation while protein kinase C (PKC) functions in an opposing fashion by positively regulating S6K and S6 phosphorylation and suppressing Akt activation. Treatments with the phosphatase inhibitors sodium orthovanadate and okadaic acid increase S6, Akt and to a lesser extent S6K phosphorylation, indicating that tyrosine and serine/threonine dephosphorylation also regulates their activity. However, okadaic acid elicited a far greater increase in phosphorylation, implicating phosphatase 2A as a critical determinant of their function. Finally, pilocarpine but not carbachol induced a time and dose dependent cell death that was associated with caspase activation and oxidative stress but independent of S6K and S6 activation through VEGFR2. Accordingly, our findings suggest that mAchRs crosstalk with VEGFR2 to enhance mTOR activity but signal divergent effects on survival through alternate mechanisms.     

Improvement of the synthesis and pharmacokinetic properties of chromenotriazolopyrimidine MDM2-p53 protein-protein inhibitors

Human murine double minute 2 (MDM2) is a negative regulator of p53, which plays an important role in cell cycle and apoptosis. We report several optimizations to the synthesis of the chromenotriazolopyrimidine series of MDM2-p53 protein-protein interaction inhibitors. Additionally, the in vitro and in vivo stability, pharmacokinetic properties and solubility were improved through N-substitution.     

Insulin-like growth factor-I regulates glucose-induced mitochondrial depolarization and apoptosis in human neuroblastoma

Neuroblastoma, a pediatric peripheral nervous system tumor, frequently contains alterations in apoptotic pathways, producing chemoresistant disease. Insulin-like growth factor (IGF) system components are highly expressed in neuroblastoma, further protecting these cells from apoptosis. This study investigates IGF-I regulation of apoptosis at the mitochondrial level. Elevated extracellular glucose causes rapid mitochondrial enlargement coupled with an increase in the mitochondrial membrane potential (Delta Psi(M)) followed by mitochondrial membrane depolarization (MMD), uncoupling protein 3 (UCP3) downregulation, caspase-3 activation and decreased Bcl-2. MMD inhibition by Bongkrekic acid prevents high-glucose-induced loss of UCP3 and apoptosis. Glucose exposure induces caspase-9 cleavage within 30 min, and caspase-9 inhibition prevents glucose-mediated apoptosis. IGF-I prevents caspase activation and mitochondrial events leading to apoptosis. These results suggest that elevated glucose produces early initiator caspase activation, followed by Delta Psi(M) changes, in neuroblastoma cells; in turn, IGF-I prevents apoptosis by preventing downstream caspase activation, maintaining Delta Psi(M) and regulating Bcl proteins.     

Linc-ROR regulates apoptosis in esophageal squamous cell carcinoma via modulation of p53 ubiquitination by targeting miR-204-5p/MDM2

The long intergenic noncoding RNA, regulator of reprogramming (linc-ROR) has been reported to participate in tumorigenesis, while its functions and fundamental mechanisms in esophageal squamous cell carcinoma (ESCC) remain unclear. In this study, gain-of-function assays showed that linc-ROR upregulation enhanced cell viability, promoted cell proliferation, and inhibited apoptosis. Mechanistically, the regulatory network of linc-ROR/miR-204-5p/MDM2 was established with bioinformatics analysis and online databases, then validated via dual-luciferase reporter assays, RNA immunoprecipitation assays in ESCC cells. Linc-ROR positively regulates the expression of MDM2 as a molecular sponge of miR-204-5p. Moreover, results of western blot and coimmunoprecipitation indicated that linc-ROR overexpression enhanced the ubiquitination level of p53, and its downstream apoptosis-related genes have showed higher bcl-2 expression, lower bax, and cleaved caspase-3 expressions, while miR-204-5p could counteract with this effect. Finally, small interfering RNAs tailored to linc-ROR were established to further evaluate its effects on ESCC comprehensively. In summary, this study revealed that linc-ROR modulated cell apoptosis and regulated p53 ubiquitination via targeting miR-204-5p/MDM2 axis, which provides a novel therapeutic insight into treatments for ESCC.