Mitochondria And Radiation Sensitivity Of Cells

Author : Demet Candas
Publisher :
ISBN 13 : 9781267967688
Total Pages : pages
Book Rating : 4.9/5 (676 download)

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Download or read book Regulation of Radioadaptive Response by Mitochondrial Protein Influx in Normal and Cancer Cells written by Demet Candas and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Chronic exposure of cells to ionizing radiation (IR) induces an adaptive response that results in enhanced tolerance to the subsequent cytotoxicity induced by IR. Radioadaptive resistance is a feature of both malignant and non-malignant cells. Accumulating evidence show an extensive communication between the mitochondria and cytoplasmic environment under oxidative stress, namely radiation insult, in both normal and cancer cells. Mitochondria are the major resource generating cellular fuel for the aerobic cells to maintain normal functions. Apart from this critical function in cell respiration, mitochondria are also involved in other fundamental cellular functions such as apoptotic response. To perform and regulate such a wide range of cellular roles, mitochondria are engaged in an extensive communication with its cytoplasmic environment via a constant flux of inorganic ions, metabolites and proteins that traffic between the cytosol and mitochondria. Mitochondrial influx of proteins is implicated in a wide spectrum of mitochondrial functions; and the influx includes proteins that are found in all mitochondrial compartments including the cytoplasm-facing outer surface of the outer membrane. Herein, we have focused on two proteins that are involved in radioadaptive response mediated by mitochondrial protein influx. First of these is the manganese-containing superoxide dismutase (MnSOD), a major antioxidant enzyme resides within the mitochondrial matrix and is responsible for the detoxification of free radicals generated by cellular metabolism and environmental/therapeutic irradiation. We have shown that MnSOD is regulated by cyclin-dependent kinase, Cdk1, in response to low dose (10cGy) of ionizing radiation in normal, non-malignant MCF10A breast epithelial cells. Cdk1, along with its regulatory partner CyclinB1, play important roles in regulation of cell cycle progression as well as in genotoxic stress response. We have identified a minimal Cdk1 phosphorylation consensus sequence ([S/T]106P; Ser106) in human MnSOD, suggesting Cdk1 as a potential upstream regulatory kinase of MnSOD. A substantial amount of CyclinB1/Cdk1 was found to influx into the mitochondrion upon irradiation. The enhanced Cdk1/MnSOD interaction and MnSOD phosphorylation were detected in both the irradiated human cells and mouse tissues. We have reported that CyclinB1/Cdk1 can regulate MnSOD through reversible Ser106 phosphorylation, both in vivo and in vitro. The CyclinB1/Cdk1-mediated MnSOD Ser106 resulted in increased MnSOD activity and stability, along with improved mitochondrial function and cellular resistance to radiation-induced apoptosis. These results demonstrated a unique pro-survival mechanism by which cells enhance survival via CyclinB1/Cdk1-mediated MnSOD activation under genotoxic stress conditions. The second protein that was shown to influx into the mitochondria is a mitogen-activated protein kinase (MAPK) phosphatase, MKP1, which is responsible for dephosphorylation and inactivation of MAPKs depending on the cell type and the nature of the stimuli. MKP1 was previously shown to be linked with chemoresistance in breast cancer cells and was overexpressed in radiation-induced breast cancer MCF-7 cell lines to inhibit cell death. Herein, we show that MKP1 is present in the mitochondria and mitochondrial MKP1 levels are significantly enhanced in irradiated normal and breast cancer cells, suggesting that increased MKP1 translocation into the mitochondria may function to attenuate pro-apoptotic signals from mitochondria-localized MAPK, c-Jun N-terminal kinase (JNK). Increased mitochondrial MKP1 levels correlated with reduced phosphorylated active JNK levels in irradiated cells. Mitochondrial MKP1-mediated dephosphorylation of JNK was also enhanced in radioresistant breast cancer cells and siRNA-mediated inhibition of MKP1 resulted in sensitization of these cells. The absence of a mitochondria targeting sequence in the MKP1 peptide sequence suggested the chaperon-mediated mitochondrial translocation mechanism. However, the loss of 14-3-3 zeta chaperon did not affect MKP1 mitochondrial localization, indicating the involvement of other chaperons in the process. Our results also demonstrated the involvement of extracellular regulated kinase (ERK) as the upstream signal for MKP1 mitochondrial translocation. The identification of cancer stem cells (CSCs) in all human tumors and their involvement in tumor radioresistance encouraged the studies investigating whether MKP1-mediated survival mechanism is present in radioresistant breast cancer cells. Here we showed that MKP1 is overexpressed in irradiated HER2-positive breast CSCs. These data provide critical insights on the aggressive phenotype of breast tumors and define novel diagnostic and therapeutic targets to re-sensitize resistant breast cancer cells. In conclusion, the regulation of radioadaptive response via mitochondrial activity of otherwise cytoplasmic/nuclear enzymes points to the importance of mitochondria not only as the ATP generating powerhouses of cells but also in equipping the cells with enhanced superoxide scavenging capacity and in increasing cell survival. Mitochondrial-translocated Cdk1-mediated MnSOD activation in normal breast epithelial cells suggested a role for this mitotic kinase in the mitochondria in protecting cells against radiation. Further analysis of Cdk1-MnSOD pathway within the regular cell cycle as well as in several pathologies including cancer and aging may improve our understanding of the alternative functions of Cdk1 in addition to its mitosis promoting role. Our study also demonstrated a novel post-translational modification of MnSOD in control of the MnSOD enzymatic activity and protein stability in response to low dose ionizing radiation. Until now, MnSOD has been known to be acetylated, nitrated, methylated and glutathionylated; this study provides the first evidence showing the phosphorylation of MnSOD. This new feature of MnSOD phosphorylation by cell cycle proteins, CyclinB1/Cdk1, represents a tight correlation between cell cycle progression and mitochondrial antioxidant status, which contributes to anti-apoptotic status and survival advantage over genotoxicity of high dose radiation. Similarly, mitochondrial translocation of MKP1 is associated with anti-apoptotic status in irradiated breast cancer cells. MKP1-mediated inhibition of pro-apoptotic signals from JNK in the mitochondria is suggested as a mechanism of radiotherapy resistance in breast cancer cells. MKP1 has been known to be a nuclear localized MAPK phosphatase. In this study, we have shown that MKP1 translocates into the mitochondria upon radiation. The elucidation of the mechanisms of both nuclear and mitochondrial targeting of MKP1 will contribute additional control over this phosphatase. The possibility of sequestering MKP1 in the cytosol as a means to limit its phosphatase activity in the mitochondria, via modifying the molecules involved in its mitochondrial localization, carries a great potential for anti-cancer therapeutics. Additionally, understanding the interaction between mitochondrial MKP1 with JNK along with the elucidation of other potential mitochondrial MKP1 targets may provide new strategies for increasing the efficiency of radiotherapy in resistant breast cancer cells. A combined therapeutic approach involving the sensitization of cancer cells by blocking the MKP1-mediated resistance mechanisms in addition to conventional radiation-based treatment may improve the therapy outcome in breast cancer patients.