M2 isoform of pyruvate kinase rewires glucose metabolism during radiation therapy to promote an antioxidant response and glioblastoma radioresistance

BACKGROUND

Resistance to existing therapies is a significant challenge in improving outcomes for glioblastoma (GBM) patients. Metabolic plasticity has emerged as an important contributor to therapy resistance, including radiation therapy (RT). Here, we investigated how GBM cells reprogram their glucose metabolism in response to RT to promote radiation resistance.

METHODS

Effects of radiation on glucose metabolism of human GBM specimens were examined in vitro and in vivo with the use of metabolic and enzymatic assays, targeted metabolomics, and FDG-PET. Radiosensitization potential of interfering with M2 isoform of pyruvate kinase (PKM2) activity was tested via gliomasphere formation assays and in vivo human GBM models.

RESULTS

Here, we show that RT induces increased glucose utilization by GBM cells, and this is accompanied with translocation of GLUT3 transporters to the cell membrane. Irradiated GBM cells route glucose carbons through the pentose phosphate pathway (PPP) to harness the antioxidant power of the PPP and support survival after radiation. This response is regulated in part by the PKM2. Activators of PKM2 can antagonize the radiation-induced rewiring of glucose metabolism and radiosensitize GBM cells in vitro and in vivo.

CONCLUSIONS

These findings open the possibility that interventions designed to target cancer-specific regulators of metabolic plasticity, such as PKM2, rather than specific metabolic pathways, have the potential to improve the radiotherapeutic outcomes in GBM patients.

Glioblastomas: Hijacking Metabolism to Build a Flexible Shield for Therapy Resistance

Significance: Glioblastomas (GBMs) are among the most lethal tumors despite the almost exclusive localization to the brain. This is largely due to therapeutic resistance. Radiation and chemotherapy significantly increase the survival for GBM patients, however, GBMs always recur, and the median overall survival is just over a year. Proposed reasons for such intractable resistance to therapy are numerous and include tumor metabolism, in particular, the ability of tumor cells to reconfigure metabolic fluxes on demand (metabolic plasticity). Understanding how the hard-wired, oncogene-driven metabolic tendencies of GBMs intersect with flexible, context-induced metabolic rewiring promises to reveal novel approaches for combating therapy resistance. Recent Advances: Personalized genome-scale metabolic flux models have recently provided evidence that metabolic flexibility promotes radiation resistance in cancer and identified tumor redox metabolism as a major predictor for resistance to radiation therapy (RT). It was demonstrated that radioresistant tumors, including GBM, reroute metabolic fluxes to boost the levels of reducing factors of the cell, thus enhancing clearance of reactive oxygen species that are generated during RT and promoting survival. Critical Issues: The current body of knowledge from published studies strongly supports the notion that robust metabolic plasticity can act as a (flexible) shield against the cytotoxic effects of standard GBM therapies, thus driving therapy resistance. The limited understanding of the critical drivers of such metabolic plasticity hampers the rational design of effective combination therapies. Future Directions: Identifying and targeting regulators of metabolic plasticity, rather than specific metabolic pathways, in combination with standard-of-care treatments have the potential to improve therapeutic outcomes in GBM. Antioxid. Redox Signal. 39, 957-979.

Abstract 6058: The serine synthesis pathway contributes to the radiation-induced metabolic plasticity in glioblastoma multiforme

Purpose: Glioblastoma multiforme (GBM) remains a fatal disease despite aggressive treatment approaches. Post-surgical radiation therapy (RT) is the only treatment that significantly improves survival of GBM patients but recurrence is inevitable. Elucidating the mechanisms of resistance to RT could transform GBM patient outcomes. The purpose of this study was to elucidate the effect of radiation on the serine synthesis pathway (SSP) and the role of SSP in boosting antioxidant defenses in irradiated GBM cells and promoting radioresistance.

Methods: We compared copy number alterations (CNA) and amplification of SSP enzymes (PHGDH, PSAT1, PSPH, SHMT1/2) in low-grade gliomas (LGG, n = 511) and GBM (n = 575) via TCGA pan-cancer analysis. In vitro HPLC/MS-based metabolomics assays were used to study the changes in overall metabolite levels induced by radiation in patient-derived GBM specimens. Targeted metabolomics with 13C-labeled glucose was used for tracking the fate of glucose towards SSP intermediates and antioxidant species. Gene expression levels after radiation were measured via molecular biology approaches. De novo SSP was targeted via pharmacological inhibition of PHGDH. Modified clonogenic, sphere forming assays (SFA) on gliomasphere cultures were used to determine radiosensitivity. Dependence on extracellular serine/glycine was determined via serine/glycine depletion experiments.

Results: The TCGA analysis revealed increased CNAs in all the de novo SSP enzymes in GBM tumors relative to the LGGs. 82% of GBM tumors had CNAs or amplification of the PSPH gene, which codes for the ultimate enzyme in serine synthesis. In vitro metabolomics assays revealed that radiation increases the levels of serine, glycine and cystine in gliomaspheres, and this is accompanied by elevated levels of reduced glutathione (GSH). Targeted glucose metabolomics revealed radiation-induced upregulation of the de novoSSP, with a concomitant increase in gene expression of SSP enzymes. Downstream of the SSP, metabolomics data show that irradiated GBM cells elevate nucleotide and precursor levels, likely to support DNA damage repair mechanisms after radiation. The inhibition of PHGDH, the first rate-limiting enzyme in de novo SSP, significantly reduced GBM cell survival in vitro. Extracellular serine/glycine appeared to also be important contributors to radiation survival. Gene expression of SLC1A4, an amino acid transporter, was upregulated by radiation, while depletion of extracellular serine/glycine led to significant decreased survival in in vitro SFAs.

Conclusion: Here we provide evidence that the de novo serine synthesis pathway is an important contributor to the metabolic reprogramming of GBM cells after radiation that promotes radiation resistance. These data call for further investigations to evaluate the radiosensitizing potential of targeting de novo SSP in GBM.

Citation Format: Justine Bailleul, Yangjingyi Ruan, Erina Vlashi. The serine synthesis pathway contributes to the radiation-induced metabolic plasticity in glioblastoma multiforme [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6058.

Metabolic response to radiation therapy in cancer

Tumor metabolism has emerged as a hallmark of cancer and is involved in carcinogenesis and tumor growth. Reprogramming of tumor metabolism is necessary for cancer cells to sustain high proliferation rates and enhanced demands for nutrients. Recent studies suggest that metabolic plasticity in cancer cells can decrease the efficacy of anticancer therapies by enhancing antioxidant defenses and DNA repair mechanisms. Studying radiation-induced metabolic changes will lead to a better understanding of radiation response mechanisms as well as the identification of new therapeutic targets, but there are few robust studies characterizing the metabolic changes induced by radiation therapy in cancer. In this review, we will highlight studies that provide information on the metabolic changes induced by radiation and oxidative stress in cancer cells and the associated underlying mechanisms.

Radiosensitizing Pancreatic Cancer via Effective Autophagy Inhibition

Despite aggressive treatments, pancreatic ductal adenocarcinoma (PDAC) remains an intractable disease, largely because it is refractory to therapeutic interventions. To overcome its nutrient-poor microenvironment, PDAC heavily relies on autophagy for metabolic needs to promote tumor growth and survival. Here, we explore autophagy inhibition as a method to enhance the effects of radiotherapy on PDAC tumors. Hydroxychloroquine is an autophagy inhibitor at the focus of many PDAC clinical trials, including in combination with radiotherapy. However, its acid-labile properties likely reduce its intratumoral efficacy. Here, we demonstrate that EAD1, a synthesized analogue of HCQ, is a more effective therapeutic for sensitizing PDAC tumors of various KRAS mutations to radiotherapy. Specifically, in vitro models show that EAD1 is an effective inhibitor of autophagic flux in PDAC cells, accompanied by a potent inhibition of proliferation. When combined with radiotherapy, EAD1 is consistently superior to HCQ not only as a single agent, but also in radiosensitizing PDAC cells, and perhaps most importantly, in decreasing the self-renewal capacity of PDAC cancer stem cells (PCSC). The more pronounced sensitizing effects of autophagy inhibitors on pancreatic stem over differentiated cells points to a new understanding that PCSCs may be more dependent on autophagy to counter the effects of radiation toxicity, a potential mechanism explaining the resistance of PCSCs to radiotherapy. Finally, in vivo subcutaneous tumor models demonstrate that combination of radiotherapy and EAD1 is the most successful at controlling tumor growth. The models also confirmed a similar toxicity profile between EAD1 and Hydroxychloroquine.

Abstract PO-020: The NRF2-redox-metabolism axis protects pancreatic cancer cells from radiation toxicity

More than 90% of pancreatic ductal adenocarcinomas (PDAC) are driven by activating KRAS mutations (mKRAS), which promote a remarkable resistance to all non-surgical therapies, including radiation therapy (RT). Unfortunately, mKRAS remains undruggable for the vast majority of PDAC tumors. This necessitates the discovery of downstream targets, which most likely govern the oxidative stress response and promote the acquisition of radiation resistance. Notably, the master orchestrator of oxidative stress response, the transcription factor NRF2, is upregulated by mKRAS and in some models, NRF2 is activated by irradiation (IR). This suggests that IR may amplify the already heightened, mKRAS-driven activation of NRF2 in PDAC. NRF2 targets include genes involved in restoring redox balance, likely promoting survival during RT. However, the involvement of NRF2 in PDAC radioresistance remains unknown. To fill this knowledge gap, we have systematically investigated the role of the NRF2 pathway in regulating radiation response in PDAC tumors. Our studies show that depletion of NRF2 via CRISPR/Cas9 targeted gene deletion (NRF2-ko) results in the accumulation of significantly higher levels of reactive oxygen species in irradiated PDAC cells. Importantly, this correlates with increased radiosensitivity of NRF2-ko cells in vitro, compared to NRF2-wt controls. Interestingly, irradiation of PDAC cells upregulates the expression of metabolic genes that drive the generation of various antioxidant species, in a NRF2-dependent manner. At the metabolite level, IR induces re-routing of glycolytic metabolites towards the antioxidant pentose phosphate pathway and the depletion of NRF2 abrogates these changes. Together, these findings suggest that part of the mechanism via which NRF2 promotes radiation survival of PDAC cells is by reprogramming cellular metabolism to drive a metabolic antioxidant response. Altogether, these findings support a crucial role for the NRF2-redox-metabolism axis in driving RT resistance in PDAC. This further suggests that inhibition of the NRF2 pathway would be followed by a failure to resist IR-induced oxidative stress, and subsequently sensitize mKRAS pancreatic cancer to RT.

Citation Format: Justine Bailleul, Taha Yazal, David Sung, Juan Nathaniel, Daisy Palomera, Anahita Sehgal, Rachel Ruan, Erina Vlashi. The NRF2-redox-metabolism axis protects pancreatic cancer cells from radiation toxicity [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-020.

Abstract C59: NRF2 drives metabolic reprogramming in irradiated pancreatic cancer cells and promotes radioresistance

The current 5-year survival rate of pancreatic ductal adenocarcinoma (PDAC) is ~8%. The dismal prognosis in PDAC reflects in part an exceptional level of resistance to available therapies. Approximately half of PDAC patients present with localized tumors amenable to local therapies, though only a minority are candidates for potentially curative surgery due to unresectable disease, which leaves radiation therapy (RT) as the only other option. The clinical reality is that two thirds of all PDAC patients succumb to local disease burden regardless of clinical stage at diagnosis. Therefore, improving the effectiveness of PDAC RT has the potential to transform overall outcomes for this lethal disease. More than a century after its discovery, RT remains a powerful therapeutic agent against many cancers, but has largely failed to offer much benefit to PDAC patients according to randomized, controlled clinical trials. The biologic reasons for such exceptional radiation resistance remain obscure. However, studies show that activating KRAS mutations in PDAC drive a NRF2-controlled antioxidant program that endows tumor cells with a reduced intracellular environment and lower levels of reactive oxygen species (ROS). Such persistent NRF2 pathway activation would poise PDAC cells to resist oxidative stress induced by radiation and in all likelihood limits the effectiveness of RT in the clinic. Supporting this hypothesis, our study shows that radiation activates the NRF2 pathway, as well as autophagy, and inhibition of either pathway sensitizes PDAC cells to radiation. Moreover, the radiosensitizing effect is enhanced when both the NRF2 and autophagy pathways are inhibited and, perhaps most importantly, NRF2 or autophagy inhibition radiosensitizes the notoriously therapy-resistant PDAC cancer stem cells. Supporting a role for NRF2 in protecting PDAC cells from radiation-induced oxidative stress, irradiated PDAC cells with depleted NRF2 have elevated ROS levels compared to NRF2-wt controls. Interestingly, irradiated PDAC cells reprogram their glucose and glutamine metabolism in an NRF2-dependent manner and seem to reroute glucose through the antioxidant pentose phosphate pathway (PPP), likely for the generation of reducing equivalents in the form of NADPH. This suggests that NRF2 drives metabolic rewiring in irradiated PDAC cells in favor of cellular antioxidant responses. Taken together, our data strongly suggest that NRF2 plays a pivotal role in promoting radiation resistance of KRAS-mutated PDAC tumors by driving powerful antioxidant responses through metabolic reprogramming and cytoprotective autophagy. If this is correct, the NRF2-autophagy-metabolism axis may be targeted therapeutically to reverse the chemo/radioresistant phenotype of PDAC.

Citation Format: Justine Bailleul, Taha Yazal, David Sung, Daisy Palomera, Anahita Sehgal, Amy Dao, Erina Vlashi. NRF2 drives metabolic reprogramming in irradiated pancreatic cancer cells and promotes radioresistance [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C59.

PK-M2-mediated metabolic changes in breast cancer cells induced by ionizing radiation

PURPOSE

Radiotherapy (RT) constitutes an important part of breast cancer treatment. However, triple negative breast cancers (TNBC) exhibit remarkable resistance to most therapies, including RT. Developing new ways to radiosensitize TNBC cells could result in improved patient outcomes. The M2 isoform of pyruvate kinase (PK-M2) is believed to be responsible for the re-wiring of cancer cell metabolism after oxidative stress. The aim of the study was to determine the effect of ionizing radiation (IR) on PK-M2-mediated metabolic changes in TNBC cells, and their survival. In addition, we determine the effect of PK-M2 activators on breast cancer stem cells, a radioresistant subpopulation of breast cancer stem cells.

METHODS

Glucose uptake, lactate production, and glutamine consumption were assessed. The cellular localization of PK-M2 was evaluated by western blot and confocal microscopy. The small molecule activator of PK-M2, TEPP46, was used to promote its pyruvate kinase function. Finally, effects on cancer stem cell were evaluated via sphere forming capacity.

RESULTS

Exposure of TNBC cells to IR increased their glucose uptake and lactate production. As expected, PK-M2 expression levels also increased, especially in the nucleus, although overall pyruvate kinase activity was decreased. PK-M2 nuclear localization was shown to be associated with breast cancer stem cells, and activation of PK-M2 by TEPP46 depleted this population.

CONCLUSIONS

Radiotherapy can induce metabolic changes in TNBC cells, and these changes seem to be mediated, at least in part by PK-M2. Importantly, our results show that activators of PK-M2 can deplete breast cancer stem cells in vitro. This study supports the idea of combining PK-M2 activators with radiation to enhance the effect of radiotherapy in resistant cancers, such as TNBC.

Tomographic diffractive microscopy and multiview profilometry with flexible aberration correction

We have developed a tomographic diffractive microscope in reflection, which permits observation of sample surfaces with an improved lateral resolution, compared to a conventional holographic microscope. From the same set of data, high-precision measurements can be performed on the shape of the reflective surface by reconstructing the phase of the diffracted field. Doing so allows for several advantages compared to classical holographic interferometric measurements: improvement in lateral resolution, easier phase unwrapping, reduction of the coherent noise, combined with the high-longitudinal precision provided by interferometric phase measurements. We demonstrate these capabilities by imaging various test samples.