Sensitization of cancer cells to ferroptosis coincident with cell cycle arrest

Ferroptosis is a non-apoptotic form of cell death that can be triggered by inhibiting the system xc- cystine/glutamate antiporter or the phospholipid hydroperoxidase glutathione peroxidase 4 (GPX4). We have investigated how cell cycle arrest caused by stabilization of p53 or inhibition of cyclin-dependent kinase 4/6 (CDK4/6) impacts ferroptosis sensitivity. Here, we show that cell cycle arrest can enhance sensitivity to ferroptosis induced by covalent GPX4 inhibitors (GPX4i) but not system xc- inhibitors. Greater sensitivity to GPX4i is associated with increased levels of oxidizable polyunsaturated fatty acid-containing phospholipids (PUFA-PLs). Higher PUFA-PL abundance upon cell cycle arrest involves reduced expression of membrane-bound O-acyltransferase domain-containing 1 (MBOAT1) and epithelial membrane protein 2 (EMP2). A candidate orally bioavailable GPX4 inhibitor increases lipid peroxidation and shrinks tumor volumes when combined with a CDK4/6 inhibitor. Thus, cell cycle arrest may make certain cancer cells more susceptible to ferroptosis in vivo.

A Cell Cycle-Dependent Ferroptosis Sensitivity Switch Governed by EMP2

Ferroptosis is a non-apoptotic form of cell death characterized by iron-dependent lipid peroxidation. Ferroptosis can be induced by system xc- cystine/glutamate antiporter inhibition or by direct inhibition of the phospholipid hydroperoxidase glutathione peroxidase 4 (GPX4). The regulation of ferroptosis in response to system xc- inhibition versus direct GPX4 inhibition may be distinct. Here, we show that cell cycle arrest enhances sensitivity to ferroptosis triggered by GPX4 inhibition but not system xc- inhibition. Arrested cells have increased levels of oxidizable polyunsaturated fatty acid-containing phospholipids, which drives sensitivity to GPX4 inhibition. Epithelial membrane protein 2 (EMP2) expression is reduced upon cell cycle arrest and is sufficient to enhance ferroptosis in response to direct GPX4 inhibition. An orally bioavailable GPX4 inhibitor increased markers of ferroptotic lipid peroxidation in vivo in combination with a cell cycle arresting agent. Thus, responses to different ferroptosis-inducing stimuli can be regulated by cell cycle state.

In vivo characterization of glutamine metabolism identifies therapeutic targets in clear cell renal cell carcinoma

Targeting metabolic vulnerabilities has been proposed as a therapeutic strategy in renal cell carcinoma (RCC). Here, we analyzed the metabolism of patient-derived xenografts (tumorgrafts) from diverse subtypes of RCC. Tumorgrafts from VHL-mutant clear cell RCC (ccRCC) retained metabolic features of human ccRCC and engaged in oxidative and reductive glutamine metabolism. Genetic silencing of isocitrate dehydrogenase-1 or isocitrate dehydrogenase-2 impaired reductive labeling of tricarboxylic acid (TCA) cycle intermediates in vivo and suppressed growth of tumors generated from tumorgraft-derived cells. Glutaminase inhibition reduced the contribution of glutamine to the TCA cycle and resulted in modest suppression of tumorgraft growth. Infusions with [amide-15N]glutamine revealed persistent amidotransferase activity during glutaminase inhibition, and blocking these activities with the amidotransferase inhibitor JHU-083 also reduced tumor growth in both immunocompromised and immunocompetent mice. We conclude that ccRCC tumorgrafts catabolize glutamine via multiple pathways, perhaps explaining why it has been challenging to achieve therapeutic responses in patients by inhibiting glutaminase.

Compartmentalized metabolism supports midgestation mammalian development

Mammalian embryogenesis requires rapid growth and proper metabolic regulation¹. Midgestation features increasing oxygen and nutrient availability concomitant with fetal organ development^2,3. Understanding how metabolism supports development requires approaches to observe metabolism directly in model organisms in utero. Here we used isotope tracing and metabolomics to identify evolving metabolic programmes in the placenta and embryo during midgestation in mice. These tissues differ metabolically throughout midgestation, but we pinpointed gestational days (GD) 10.5-11.5 as a transition period for both placenta and embryo. Isotope tracing revealed differences in carbohydrate metabolism between the tissues and rapid glucose-dependent purine synthesis, especially in the embryo. Glucose's contribution to the tricarboxylic acid (TCA) cycle rises throughout midgestation in the embryo but not in the placenta. By GD12.5, compartmentalized metabolic programmes are apparent within the embryo, including different nutrient contributions to the TCA cycle in different organs. To contextualize developmental anomalies associated with Mendelian metabolic defects, we analysed mice deficient in LIPT1, the enzyme that activates 2-ketoacid dehydrogenases related to the TCA cycle^4,5. LIPT1 deficiency suppresses TCA cycle metabolism during the GD10.5-GD11.5 transition, perturbs brain, heart and erythrocyte development and leads to embryonic demise by GD11.5. These data document individualized metabolic programmes in developing organs in utero.

Nucleotide biosynthesis links glutathione metabolism to ferroptosis sensitivity

The tumor suppressor protein p53 inhibits ferroptosis by reducing the consumption of glutathione in nucleotide biosynthesis.

Nucleotide synthesis is a metabolically demanding process essential for DNA replication and other processes in the cell. Several anticancer drugs that inhibit nucleotide metabolism induce apoptosis. How inhibition of nucleotide metabolism impacts non-apoptotic cell death is less clear. Here, we report that inhibition of nucleotide metabolism by the p53 pathway is sufficient to suppress the non-apoptotic cell death process of ferroptosis. Mechanistically, stabilization of wild-type p53 and induction of the p53 target gene CDKN1A (p21) leads to decreased expression of the ribonucleotide reductase (RNR) subunits RRM1 and RRM2. RNR is the rate-limiting enzyme of de novo nucleotide synthesis that reduces ribonucleotides to deoxyribonucleotides in a glutathione-dependent manner. Direct inhibition of RNR results in conservation of intracellular glutathione, limiting the accumulation of toxic lipid peroxides and preventing the onset of ferroptosis in response to cystine deprivation. These results support a mechanism linking p53-dependent regulation of nucleotide metabolism to non-apoptotic cell death.

p53 deficiency triggers dysregulation of diverse cellular processes in physiological oxygen

The mechanisms by which TP53, the most frequently mutated gene in human cancer, suppresses tumorigenesis remain unclear. p53 modulates various cellular processes, such as apoptosis and proliferation, which has led to distinct cellular mechanisms being proposed for p53-mediated tumor suppression in different contexts. Here, we asked whether during tumor suppression p53 might instead regulate a wide range of cellular processes. Analysis of mouse and human oncogene-expressing wild-type and p53-deficient cells in physiological oxygen conditions revealed that p53 loss concurrently impacts numerous distinct cellular processes, including apoptosis, genome stabilization, DNA repair, metabolism, migration, and invasion. Notably, some phenotypes were uncovered only in physiological oxygen. Transcriptomic analysis in this setting highlighted underappreciated functions modulated by p53, including actin dynamics. Collectively, these results suggest that p53 simultaneously governs diverse cellular processes during transformation suppression, an aspect of p53 function that would provide a clear rationale for its frequent inactivation in human cancer.

p53 Suppresses Metabolic Stress-Induced Ferroptosis in Cancer Cells

How cancer cells respond to nutrient deprivation remains poorly understood. In certain cancer cells, deprivation of cystine induces a non-apoptotic, iron-dependent form of cell death termed ferroptosis. Recent evidence suggests that ferroptosis sensitivity may be modulated by the stress-responsive transcription factor and canonical tumor suppressor protein p53. Using CRISPR/Cas9 genome editing, small-molecule probes, and high-resolution, time-lapse imaging, we find that stabilization of wild-type p53 delays the onset of ferroptosis in response to cystine deprivation. This delay requires the p53 transcriptional target CDKN1A (encoding p21) and is associated with both slower depletion of intracellular glutathione and a reduced accumulation of toxic lipid-reactive oxygen species (ROS). Thus, the p53-p21 axis may help cancer cells cope with metabolic stress induced by cystine deprivation by delaying the onset of non-apoptotic cell death.

Exogenous Monounsaturated Fatty Acids Promote a Ferroptosis-Resistant Cell State

The initiation and execution of cell death can be regulated by various lipids. How the levels of environmental (exogenous) lipids impact cell death sensitivity is not well understood. We find that exogenous monounsaturated fatty acids (MUFAs) potently inhibit the non-apoptotic, iron-dependent, oxidative cell death process of ferroptosis. This protective effect is associated with the suppression of lipid reactive oxygen species (ROS) accumulation at the plasma membrane and decreased levels of phospholipids containing oxidizable polyunsaturated fatty acids. Treatment with exogenous MUFAs reduces the sensitivity of plasma membrane lipids to oxidation over several hours. This effect requires MUFA activation by acyl-coenzyme A synthetase long-chain family member 3 (ACSL3) and is independent of lipid droplet formation. Exogenous MUFAs also protect cells from apoptotic lipotoxicity caused by the accumulation of saturated fatty acids, but in an ACSL3-independent manner. Our work demonstrates that ACSL3-dependent MUFA activation promotes a ferroptosis-resistant cell state.

Rb family proteins enforce the homeostasis of quiescent hematopoietic stem cells by repressing Socs3 expression

The mechanisms regulating the homeostasis of HSCs remain poorly understood. Here, Kim et al. identify the Rb/E2f module as a central molecular hub in the regulation of cell cycle and homeostasis in HSCs. This mechanism drives the enforced differentiation of proliferative HSCs to avoid their unnecessary accumulation.

Prolonged exit from quiescence by hematopoietic stem cells (HSCs) progressively impairs their homeostasis in the bone marrow through an unidentified mechanism. We show that Rb proteins, which are major enforcers of quiescence, maintain HSC homeostasis by positively regulating thrombopoietin (Tpo)-mediated Jak2 signaling. Rb family protein inactivation triggers the progressive E2f-mediated transactivation of Socs3, a potent inhibitor of Jak2 signaling, in cycling HSCs. Aberrant activation of Socs3 impairs Tpo signaling and leads to impaired HSC homeostasis. Therefore, Rb proteins act as a central hub of quiescence and homeostasis by coordinating the regulation of both cell cycle and Jak2 signaling in HSCs.

Abstract PR07: Recruitment of Pontin/Reptin by E2F1 amplifies E2F transcriptional response during cancer progression

Unrestricted E2F activity and increased E2F1 expression are hallmarks of liver cancer, but the consequences of aberrant E2F activity for liver cancer progression remain ill defined. Our data show that aberrant E2f activity, following inactivation of the Rb gene family in a mouse model of liver cancer, initially triggers a robust gene expression program associated with the cell cycle and predominantly driven by E2f2 and E2f3. During liver cancer progression, slowly accumulating E2f1 recruits a Pontin/Reptin complex to insert the H2a.z histone variant and open the chromatin conformation at E2f1-bound target genes. This epigenetic mechanism amplifies the E2f transcriptional response, leading to enhanced transactivation of cell cycle genes and de novo activation of low binding affinity E2f target genes that regulate non-cell cycle oncogenic features, such as the Warburg effect. In addition, extensive mouse genetic approaches indicate that E2f-driven liver cancer initiation and progression is cell type specific, suggesting that a permissive chromatin environment is necessary to enable E2f oncogenic functions. Collectively, these data indicate that sustained E2F activity expands E2F transcriptional response and connects the regulation of cell cycle with multiple non cell cycle functions that drive tumor progression.

This abstract is also being presented as Poster B13.

Citation Format: Amy Tarangelo, Nathanael Lo, Rebecca Teng, Eunsun Kim, Pichai Raman, Patrick Viatour. Recruitment of Pontin/Reptin by E2F1 amplifies E2F transcriptional response during cancer progression. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr PR07.

Nanomedicine: An iron age for cancer therapy

Two reports show FDA-approved nanoparticles can kill cancer cells through iron- and reactive oxygen species-dependent mechanisms, offering new strategies for cancer treatment.

Gastrointestinal microbiota do not significantly contribute to T cell activation or GI inflammation in Ndfip1-cKO mice

The bacteria inhabiting the mammalian gastrointestinal (GI) tract play a vital role in normal digestion and immune function. In a healthy host, the immune system is tolerant to gut bacteria and does not mount an effector response to bacteria-derived antigens. Loss of tolerance to intestinal microflora has been associated with inflammatory bowel disease (IBD) in both mice and humans. Mice lacking Ndfip1, an adaptor protein for E3 ubiquitin ligases of the Nedd4-family, in T cells (Ndfip1-cKO) develop a disease resembling IBD. Inflammation in these mice is characterized by increased activation of peripheral T cells, infiltration of eosinophils into the GI tract, and epithelial hypertrophy in the esophagus. We hypothesized that this intestinal inflammation in Ndfip1-cKO mice is caused by a loss of T-cell tolerance to bacterial antigens. Here, we show that treatment of Ndfip1-cKO mice with broad-spectrum antibiotics drastically reduced bacterial load in stool but had little effect on T-cell activation and did not affect eosinophil infiltration into the GI tract or epithelial hypertrophy in the esophagus. Thus, inflammation in Ndfip1-cKO mice is not caused by a loss of tolerance to intestinal microbiota. Rather, T cell activation and eosinophilia may instead be triggered by other environmental antigens.