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      Upregulation of pERK and c-JUN by γ-tocotrienol and not α-tocopherol are essential to the differential effect on apoptosis in prostate cancer cells

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          Abstract

          Background

          α-tocopherol (AT) and γ-tocotrienol (GT3) are vitamin E isoforms considered to have potential chemopreventive properties. AT has been widely studied in vitro and in clinical trials with mixed results. The latest clinical study (SELECT trial) tested AT in prostate cancer patients, determined that AT provided no benefit, and could promote cancer. Conversely, GT3 has shown antineoplastic properties in several in vitro studies, with no clinical studies published to date. GT3 causes apoptosis via upregulation of the JNK pathway; however, inhibition results in a partial block of cell death. We compared side by side the mechanistic differences in these cells in response to AT and GT3.

          Methods

          The effects of GT3 and AT were studied on androgen sensitive LNCaP and androgen independent PC-3 prostate cancer cells. Their cytotoxic effects were analyzed via MTT and confirmed by metabolic assays measuring ATP. Cellular pathways were studied by immunoblot. Quantitative analysis and the determination of relationships between cell signaling events were analyzed for both agents tested. Non-cancerous prostate RWPE-1 cells were also included as a control.

          Results

          The RAF/RAS/ERK pathway was significantly activated by GT3 in LNCaP and PC-3 cells but not by AT. This activation is essential for the apoptotic affect by GT3 as demonstrated the complete inhibition of apoptosis by MEK1 inhibitor U0126. Phospho-c-JUN was upregulated by GT3 but not AT. No changes were observed on AKT for either agent, and no release of cytochrome c into the cytoplasm was detected. Caspases 9 and 3 were efficiently activated by GT3 on both cell lines irrespective of androgen sensitivity, but not in cells dosed with AT. Cell viability of non-cancerous RWPE-1 cells was affected neither by GT3 nor AT.

          Conclusions

          c-JUN is a recognized master regulator of apoptosis as shown previously in prostate cancer. However, the mechanism of action of GT3 in these cells also include a significant activation of ERK which is essential for the apoptotic effect of GT3. The activation of both, ERK and c-JUN, is required for apoptosis and may suggest a relevant step in ensuring circumvention of mechanisms of resistance related to the constitutive activation of MEK1.

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          Most cited references 21

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          Bad, a heterodimeric partner for Bcl-XL and Bcl-2, displaces Bax and promotes cell death.

          To extend the mammalian cell death pathway, we screened for further Bcl-2 interacting proteins. Both yeast two-hybrid screening and lambda expression cloning identified a novel interacting protein, Bad, whose homology to Bcl-2 is limited to the BH1 and BH2 domains. Bad selectively dimerized with Bcl-xL as well as Bcl-2, but not with Bax, Bcl-xs, Mcl-1, A1, or itself. Bad binds more strongly to Bcl-xL than Bcl-2 in mammalian cells, and it reversed the death repressor activity of Bcl-xL, but not that of Bcl-2. When Bad dimerized with Bcl-xL, Bax was displaced and apoptosis was restored. When approximately half of Bax was heterodimerized, death was inhibited. The susceptibility of a cell to a death signal is determined by these competing dimerizations in which levels of Bad influence the effectiveness of Bcl-2 versus Bcl-xL in repressing death.
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            Identification of the JNK signaling pathway as a functional target of the tumor suppressor PTEN.

            Although most oncogenic phenotypes of PTEN loss are attributed to AKT activation, AKT alone is not sufficient to induce all of the biological activities associated with PTEN inactivation. We searched for additional PTEN-regulated pathways through gene set enrichment analysis (GSEA) and identified genes associated with JNK activation. PTEN null cells exhibit higher JNK activity, and genetic studies demonstrate that JNK functions parallel to and independently of AKT. Furthermore, PTEN deficiency sensitizes cells to JNK inhibition and negative feedback regulation of PI3K was impaired in PTEN null cells. Akt and JNK activation are highly correlated in human prostate cancer. These findings implicate JNK in PI3K-driven cancers and demonstrate the utility of GSEA to identify functional pathways using genetically defined systems.
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              Molecular mechanisms mediating mammalian mitogen-activated protein kinase (MAPK) kinase (MEK)-MAPK cell survival signals.

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                Author and article information

                Contributors
                krishnak@etsu.edu
                Journal
                BMC Cancer
                BMC Cancer
                BMC Cancer
                BioMed Central (London )
                1471-2407
                15 May 2020
                15 May 2020
                2020
                : 20
                Affiliations
                [1 ]GRID grid.255381.8, ISNI 0000 0001 2180 1673, Division of Hematology-Oncology Department of Internal Medicine, , James H. Quillen College of Medicine, East Tennessee State University, ; Dogwood Avenue, Building 119, Johnson City, USA
                [2 ]GRID grid.255381.8, ISNI 0000 0001 2180 1673, Department of Pharmaceutical Sciences, , Bill Gatton College of Pharmacy, East Tennessee State University, ; Johnson City, TN 37614 USA
                [3 ]Wellmont Hospitalists at Kingsport, Kingsport, TN 37660 USA
                [4 ]GRID grid.255381.8, ISNI 0000 0001 2180 1673, Department of Pediatrics, , James H. Quillen College of Medicine, East Tennessee State University, ; Johnson City, TN 37614 USA
                Article
                6947
                10.1186/s12885-020-06947-6
                7227069
                32414345
                © The Author(s) 2020

                Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

                Funding
                Funded by: FundRef http://dx.doi.org/10.13039/100005332, Alpha Omega Alpha Honor Medical Society;
                Award ID: Postgraduate award
                Award Recipient :
                Categories
                Research Article
                Custom metadata
                © The Author(s) 2020

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