Copper depletion modulates mitochondrial oxidative phosphorylation to impair triple negative breast cancer metastasis

Copper serves as a co-factor for a host of metalloenzymes that contribute to malignant progression. The orally bioavailable copper chelating agent tetrathiomolybdate (TM) has been associated with a significant survival benefit in high-risk triple negative breast cancer (TNBC) patients. Despite these promising data, the mechanisms by which copper depletion impacts metastasis are poorly understood and this remains a major barrier to advancing TM to a randomized phase II trial. Here, using two independent TNBC models, we report a discrete subpopulation of highly metastatic SOX2/OCT4+ cells within primary tumors that exhibit elevated intracellular copper levels and a marked sensitivity to TM. Global proteomic and metabolomic profiling identifies TM-mediated inactivation of Complex IV as the primary metabolic defect in the SOX2/OCT4+ cell population. We also identify AMPK/mTORC1 energy sensor as an important downstream pathway and show that AMPK inhibition rescues TM-mediated loss of invasion. Furthermore, loss of the mitochondria-specific copper chaperone, COX17, restricts copper deficiency to mitochondria and phenocopies TM-mediated alterations. These findings identify a copper-metabolism-metastasis axis with potential to enrich patient populations in next-generation therapeutic trials.

Mitochondrial NADP(H) generation is essential for proline biosynthesis

The coenzyme nicotinamide adenine dinucleotide phosphate (NADP⁺) and its reduced form (NADPH) regulate reductive metabolism in a subcellularly compartmentalized manner. Mitochondrial NADP(H) production depends on the phosphorylation of NAD(H) by NAD kinase 2 (NADK2). Deletion of NADK2 in human cell lines did not alter mitochondrial folate pathway activity, tricarboxylic acid cycle activity, or mitochondrial oxidative stress, but rather led to impaired cell proliferation in minimal medium. This growth defect was rescued by proline supplementation. NADK2-mediated mitochondrial NADP(H) generation was required for the reduction of glutamate and hence proline biosynthesis. Furthermore, mitochondrial NADP(H) availability determined the production of collagen proteins by cells of mesenchymal lineage. Thus, a primary function of the mitochondrial NADP(H) pool is to support proline biosynthesis for use in cytosolic protein synthesis.

Proline biosynthesis is a vent for TGFβ-induced mitochondrial redox stress

The production and secretion of matrix proteins upon stimulation of fibroblasts by transforming growth factor-beta (TGFβ) play a critical role in wound healing. How TGFβ supports the bioenergetic cost of matrix protein synthesis is not fully understood. Here, we show that TGFβ promotes protein translation at least in part by increasing the mitochondrial oxidation of glucose and glutamine carbons to support the bioenergetic demand of translation. Surprisingly, we found that in addition to stimulating the entry of glucose and glutamine carbon into the TCA cycle, TGFβ induced the biosynthesis of proline from glutamine in a Smad4-dependent fashion. Metabolic manipulations that increased mitochondrial redox generation promoted proline biosynthesis, while reducing mitochondrial redox potential and/or ATP synthesis impaired proline biosynthesis. Thus, proline biosynthesis acts as a redox vent, preventing the TGFβ-induced increase in mitochondrial glucose and glutamine catabolism from generating damaging reactive oxygen species (ROS) when TCA cycle activity exceeds the ability of oxidative phosphorylation to convert mitochondrial redox potential into ATP. In turn, the enhanced synthesis of proline supports TGFβ-induced production of matrix proteins.

Transsulfuration Activity Can Support Cell Growth upon Extracellular Cysteine Limitation

Cysteine acts both as a building unit for protein translation and as the limiting substrate for glutathione synthesis to support the cellular antioxidant system. In addition to transporter-mediated uptake, cellular cysteine can also be synthesized from methionine through the transsulfuration pathway. Here, we investigate the regulation of transsulfuration and its role in sustaining cell proliferation upon extracellular cysteine limitation, a condition reported to occur in human tumors as they grow in size. We observed constitutive expression of transsulfuration enzymes in a subset of cancer cell lines, while in other cells, these enzymes are induced following cysteine deprivation. We show that both constitutive and inducible transsulfuration activities contribute to the cellular cysteine pool and redox homeostasis. The rate of transsulfuration is determined by the cellular capacity to conduct methylation reactions that convert S-adenosylmethionine to S-adenosylhomocysteine. Finally, our results demonstrate that transsulfuration-mediated cysteine synthesis is critical in promoting tumor growth in vivo.

Stable Isotope Tracers for Metabolic Pathway Analysis

Stable isotope tracing allows a metabolic substrate to be followed through downstream biochemical reactions, thereby providing unparalleled insights into the metabolic wiring of cells. This approach stops short of modeling absolute fluxes but is relatively straightforward and has become increasingly accessible due to the widespread adoption of high-resolution mass spectrometers. Analysis of both dynamic and steady-state labeling patterns in downstream metabolites provides valuable qualitative information as to their origin and relative rates of production. Stable isotope tracing is, therefore, a powerful way to understand the impact of genetic alterations and defined perturbations on metabolism. In this chapter, we describe a liquid chromatography-mass spectrometry (LC-MS) protocol for stable isotope tracing using ¹³C-L-arginine in a macrophage cell line. A similar approach can be used to follow other stable isotope tracers, and notes are provided with advice on how this protocol can be generalized for use in other settings.

Abstract 443: Lymphatic endothelium protects breast cancer cells from death by inducing metabolic adaptations

The lymphatic vasculature is an important pathway for breast cancer dissemination, yet it is not understood whether and how the lymphatic vessel microenvironment influences cancer metastasis. We demonstrate that lymphatic endothelial cells (LECs) promote survival of triple-negative breast cancer cells (TNBCs) under stress by improving mitochondrial function and inducing metabolic shift to promote cellular energy production. LECs protected TNBCs from death in vitro induced by the loss of attachment and nutrient deprivation. Cell death was preceded with a sharp increase in reactive oxygen species (ROS), strong up-regulation of Nrf2-mediated oxidative stress response genes and a rapid decline of mitochondrial activity. LECs lowered ROS levels, decreased mitochondrial superoxide formation and enhanced mitochondrial activity in TNBCs. RNAseq transcriptome analysis identified key regulator of mitochondrial metabolism and cellular bioenergetics, peroxisome proliferator-activated receptor gamma coactivator (PPARGC1A/PGC-1α, to be specifically up-regulated in breast cancer cells by LEC-derived factors. Inhibition studies demonstrated that the TNBC survival was dependent on pentose phosphate pathway (PPP) activity. Notably, LECs induced a metabolic shift from glycolysis to fatty-acid oxidation (FAO) and oxidative phosphorylation to maintain ATP and sustain cell viability. These data demonstrate that lymphatic endothelium promotes survival of breast cancer cells by regulating energy production and maintaining redox homeostasis. Our findings suggest that lymphatic endothelium may facilitate metastasis by promoting survival of breast cancer cells within the lymphatic vasculature.

Citation Format: Mirela Berisa, Simona Podgrabinska, Brandon Nicolay, Raul Mostoslavsky, Jerry Chipuk, Mihaela Skobe. Lymphatic endothelium protects breast cancer cells from death by inducing metabolic adaptations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 443. doi:10.1158/1538-7445.AM2017-443

Abstract LB-333: Lymphatic endothelium increases antioxidant capacity of triple-negative breast cancer cells and protects from cell death

Involvement of lymphatic system with cancer and the extent of lymph node metastases are directly correlated with the poor patient outcome. However, it is not understood whether the presence of lymphatic metastases is only indicative of an aggressive cancer or if the lymphatic vessel microenvironment directly contributes to the metastatic progression. We demonstrate that soluble factors produced by lymphatic endothelial cells (LECs) protect triple negative breast cancer cells from cell death in vitro. Co-culture with LECs or LEC-conditioned medium (LEC-CM) protected cancer cells from death induced by the loss of homotypic cell adhesion, nutrient deprivation, or loss of matrix attachment. High levels of reactive oxygen species (ROS) preceded cell death, and were significantly decreased in tumor cells upon treatment with LEC-CM. Furthermore, LEC-CM protected tumor cells from death induced by exogenous oxidative stress (H2O2), while treatment with the anti-oxidant N-acetyl-cysteine (NAC) recapitulated the cytoprotective effect of LEC-CM. RNA-Seq analysis revealed Nrf2 pathway as the most upregulated stress-pathway induced in tumor cells upon loss of adhesion. Nrf2 and other stress signaling pathways were significantly diminished in the presence of LEC-CM. Pharmacological inhibition of the pentose phosphate pathway (PPP) and the components of thioredoxin and glutathione scavanging systems increased ROS and cell death in LEC-CM indicating that the maintenance of redox homeostasis and cell viability by LEC-CM is dependent on the PPP pathway and in particular thioredoxin system. Furthermore, LEC-CM preserved integrity and function of mitochondria. These results demonstrate that soluble factors produced by lymphatic endothelium promote survival of triple-negative breast cancer cells under stress by regulating tumor cell redox homeostasis and promoting mitochondrial function.

Citation Format: Mirela Berisa, Simona Podgrabinska, Jerry Chipuk, Mihaela Skobe. Lymphatic endothelium increases antioxidant capacity of triple-negative breast cancer cells and protects from cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-333. doi:10.1158/1538-7445.AM2017-LB-333