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The proteasome activator PA200 regulates tumor cell responsiveness to glutamine and resistance to ionizing radiation.

ABSTRACT: The cellular response to ionizing radiation (IR) involves a variety of mechanisms to repair damage and maintain cell survival. We previously reported that the proteasome activator PA200 promotes long-term cell survival after IR exposure. The molecular function of PA200 is to enhance proteasome-mediated cleavage after glutamate; however, it is not known how this molecular function promotes survival after IR exposure. Here, we report that upon IR exposure, cellular demand for exogenous glutamine is increased. Cells containing PA200 are capable of surviving this IR-induced glutamine demand, whereas PA200-deficient cells show impaired long-term survival. Additional glutamine supplementation reverses the radiosensitivity of PA200-knockdown cells suggesting impaired glutamine homeostasis in these cells. Indeed, PA200-knockdown cells are unable to maintain intracellular glutamine levels. Furthermore, when extracellular glutamine is limiting, cells that contain PA200 respond by slowing growth, but PA200-knockdown cells and cells in which post-glutamyl proteasome activity is inhibited are nonresponsive and continue rapid growth. This cellular unresponsiveness to nutrient depletion is also reflected at the level of the mTOR substrate ribosomal S6 kinase (S6K). Thus, inability to restrict growth causes PA200-deficient cells to continue growing and eventually die due to lack of available glutamine. Together, these data indicate an important role for PA200 and post-glutamyl proteasome activity in maintaining glutamine homeostasis, which appears to be especially important for long-term survival of tumor cells after radiation exposure.

SUBMITTER: Blickwedehl J

PROVIDER: S-EPMC5261820 | biostudies-literature | 2012 Jul

REPOSITORIES: biostudies-literature

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The proteasome activator PA200 regulates tumor cell responsiveness to glutamine and resistance to ionizing radiation.

Blickwedehl Jennifer J Olejniczak Scott S Cummings Ryan R Sarvaiya Nilofar N Mantilla Ana A Chanan-Khan Asher A Pandita Tej K TK Schmidt Marion M Thompson Craig B CB Bangia Naveen N

Molecular cancer research : MCR 20120501 7

The cellular response to ionizing radiation (IR) involves a variety of mechanisms to repair damage and maintain cell survival. We previously reported that the proteasome activator PA200 promotes long-term cell survival after IR exposure. The molecular function of PA200 is to enhance proteasome-mediated cleavage after glutamate; however, it is not known how this molecular function promotes survival after IR exposure. Here, we report that upon IR exposure, cellular demand for exogenous glutamine i ...[more]

PMID: 22550082

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Project description:BackgroundIonizing radiation (IR) is a mainstay of cancer therapy, but irradiation can at times also lead to stress responses, which counteract IR-induced cytotoxicity. IR also triggers cellular secretion of vascular endothelial growth factor, transforming growth factor ? and matrix metalloproteinases, among others, to promote tumor progression. Lysyl oxidase is known to play an important role in hypoxia-dependent cancer cell dissemination and metastasis. Here, we investigated the effects of IR on the expression and secretion of lysyl oxidase (LOX) from tumor cells.MethodsLOX-secretion along with enzymatic activity was investigated in multiple tumor cell lines in response to irradiation. Transwell migration assays were performed to evaluate invasive capacity of naïve tumor cells in response to IR-induced LOX. In vivo studies for confirming IR-enhanced LOX were performed employing immunohistochemistry of tumor tissues and ex vivo analysis of murine blood serum derived from locally irradiated A549-derived tumor xenografts.ResultsLOX was secreted in a dose dependent way from several tumor cell lines in response to irradiation. IR did not increase LOX-transcription but induced LOX-secretion. LOX-secretion could not be prevented by the microtubule stabilizing agent patupilone. In contrast, hypoxia induced LOX-transcription, and interestingly, hypoxia-dependent LOX-secretion could be counteracted by patupilone. Conditioned media from irradiated tumor cells promoted invasiveness of naïve tumor cells, while conditioned media from irradiated, LOX- siRNA-silenced cells did not stimulate their invasive capacity. Locally applied irradiation to tumor xenografts also increased LOX-secretion in vivo and resulted in enhanced LOX-levels in the murine blood serum.ConclusionsThese results indicate a differential regulation of LOX-expression and secretion in response to IR and hypoxia, and suggest that LOX may contribute towards an IR-induced migratory phenotype in sublethally-irradiated tumor cells and tumor progression.

Project description:Glutamine metabolism provides energy to tumor cells and also produces reactive oxygen species (ROS). Excessive accumulation of ROS can damage mitochondria and eventually lead to cell death. xCT (SLC7A11) is responsible for the synthesis of glutathione in order to neutralize ROS. In addition, mitophagy can remove damaged mitochondria to keep the cell alive. Ionizing radiation kills tumor cells by causing the accumulation of ROS, which subsequently induces nuclear DNA damage. With this in mind, we explored the mechanism of intracellular ROS accumulation induced by ionizing radiation and hypothesized new methods to enhance the effect of radiotherapy. We used MCF-7 breast cancer cells and HCT116 colorectal cancer cells in our study. The above-mentioned cells were irradiated with different doses of X-rays or carbon ions. Clone formation assays were used to detect cell proliferation, enzyme-linked immunosorbent assay (ELISA) detected ATP, and glutathione (GSH) production, while the expression of proteins was detected by Western blot and quantitative real-time PCR analysis. The production of ROS was detected by flow cytometry, and immunofluorescence was used to track mitophagy-related processes. Finally, BALB/C tumor-bearing nude mice were irradiated with X-rays in order to further explore the protein expression found in tumors with the use of immunohistochemistry. Ionizing radiation increased the protein expressions of ASCT2, GLS, and GLUD in order to upregulate the glutamine metabolic flux in tumor cells. This caused an increase in ATP secretion. Meanwhile, ionizing radiation inhibited the expression of the xCT (SLC7A11) protein and reduced the generation of glutathione, leading to excessive accumulation of intracellular ROS. The mitophagy inhibitor, or knockdown Parkin gene, is able to enhance the ionizing radiation-induced ROS production and increase nucleus DNA damage. This combined treatment can significantly improve the killing effect of radiation on tumor cells. We concluded that ionizing radiation could upregulate the glutamine metabolic flux and enhance ROS accumulation in mitochondria. Ionizing radiation also decreased the SLC7A11 expression, resulting in reduced GSH generation. Therefore, inhibition of mitophagy can increase ionizing radiation-induced cell death.

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The proteasome activator PA200 regulates tumor cell responsiveness to glutamine and resistance to ionizing radiation.

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The proteasome activator PA200 regulates tumor cell responsiveness to glutamine and resistance to ionizing radiation.

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