The mitochondrial NADH shuttle system is a targetable vulnerability for Group 3 medulloblastoma in a hypoxic microenvironment.

Medulloblastoma is a cancerous brain tumor that affects mostly children. Among the four groups defined by molecular characteristics, Group 3, the least well characterized, is also the least favorable, with a survival rate of 50%. Current treatments, based on surgery, radiotherapy, and chemotherapy, are not adequate and the lack of understanding of the different molecular features of Group 3 tumor cells makes the development of effective therapies challenging. In this study, the problem of medulloblastoma is approached from a metabolic standpoint in a low oxygen microenvironment. We establish that Group 3 cells use both the mitochondrial glycerol-3 phosphate (G3PS) and malate-aspartate shuttles (MAS) to produce NADH. Small molecules that target G3PS and MAS show a greater ability to decrease cell proliferation and induce apoptosis specifically of Group 3 cells. In addition, as Group 3 cells show improved respiration in hypoxia, the use of Phenformin, a mitochondrial complex 1 inhibitor, alone or in combination, induced significant cell death. Furthermore, inhibition of the cytosolic NAD+ recycling enzyme lactate dehydrogenase A (LDHA), enhanced the effects of the NADH shuttle inhibitors. In a 3D model using Group 3 human cerebellar organoids, tumor cells also underwent apoptosis upon treatment with NADH shuttle inhibitors. Our study demonstrates metabolic heterogeneity depending on oxygen concentrations and provides potential therapeutic solutions for patients in Group 3 whose tumors are the most aggressive.

HIF-1 inactivation empowers HIF-2 to drive hypoxia adaptation in aggressive forms of medulloblastoma.

Medulloblastoma (MB) is the most prevalent brain cancer in children. Four subgroups of MB have been identified; of these, Group 3 is the most metastatic. Its genetics and biology remain less clear than the other groups, and it has a poor prognosis and few effective treatments available. Tumor hypoxia and the resulting metabolism are known to be important in the growth and survival of tumors but, to date, have been only minimally explored in MB. Here we show that Group 3 MB tumors do not depend on the canonical transcription factor hypoxia-inducible factor-1α (HIF-1α) to mount an adaptive response to hypoxia. We discovered that HIF-1α is rendered inactive either through post-translational methylation, preventing its nuclear localization specifically in Group 3 MB, or by a low expression that prevents modulation of HIF-target genes. Strikingly, we found that HIF-2 takes over the role of HIF-1 in the nucleus and promotes the activation of hypoxia-dependent anabolic pathways. The exclusion of HIF-1 from the nucleus in Group 3 MB cells enhances the reliance on HIF-2's transcriptional role, making it a viable target for potential anticancer strategies. By combining pharmacological inhibition of HIF-2α with the use of metformin, a mitochondrial complex I inhibitor to block respiration, we effectively induced Group 3 MB cell death, surpassing the effectiveness observed in Non-Group 3 MB cells. Overall, the unique dependence of MB cells, but not normal cells, on HIF-2-mediated anabolic metabolism presents an appealing therapeutic opportunity for treating Group 3 MB patients with minimal toxicity.

Oncogenic PI3K and K-Ras stimulate de novo lipid synthesis through mTORC1 and SREBP.

An enhanced capacity for de novo lipid synthesis is a metabolic feature of most cancer cells that distinguishes them from their cells of origin. However, the mechanisms through which oncogenes alter lipid metabolism are poorly understood. We find that expression of oncogenic PI3K (H1047R) or K-Ras (G12V) in breast epithelial cells is sufficient to induce de novo lipogenesis, and this occurs through the convergent activation of the mechanistic target of rapamycin complex 1 (mTORC1) downstream of these common oncogenes. Oncogenic stimulation of mTORC1 signaling in this isogenic setting or a panel of eight breast cancer cell lines leads to activation of the sterol regulatory element-binding proteins (SREBP1 and SREBP2) that are required for oncogene-induced lipid synthesis. The SREBPs are also required for the growth factor-independent growth and proliferation of oncogene-expressing cells. Finally, we find that elevated mTORC1 signaling is associated with increased mRNA and protein levels of canonical SREBP targets in primary human breast cancer samples. These data suggest that the mTORC1/SREBP pathway is a major mechanism through which common oncogenic signaling events induce de novo lipid synthesis to promote aberrant growth and proliferation of cancer cells.

Low-dose radiation exposure induces a HIF-1-mediated adaptive and protective metabolic response.

Because of insufficient understanding of the molecular effects of low levels of radiation exposure, there is a great uncertainty regarding its health risks. We report here that treatment of normal human cells with low-dose radiation induces a metabolic shift from oxidative phosphorylation to aerobic glycolysis resulting in increased radiation resistance. This metabolic change is highlighted by upregulation of genes encoding glucose transporters and enzymes of glycolysis and the oxidative pentose phosphate pathway, concomitant with downregulation of mitochondrial genes, with corresponding changes in metabolic flux through these pathways. Mechanistically, the metabolic reprogramming depends on HIF1α, which is induced specifically by low-dose irradiation linking the metabolic pathway with cellular radiation dose response. Increased glucose flux and radiation resistance from low-dose irradiation are also observed systemically in mice. This highly sensitive metabolic response to low-dose radiation has important implications in understanding and assessing the health risks of radiation exposure.

Sestrin2 integrates Akt and mTOR signaling to protect cells against energetic stress-induced death.

The phosphoinositide-3 kinase/Akt (PI3K/Akt) pathway has a central role in cancer cell metabolism and proliferation. More importantly, it is one of the cardinal pro-survival pathways mediating resistance to apoptosis. The role of Akt in response to an energetic stress is presently unclear. Here, we show that Sestrin2 (Sesn2), also known as Hi95, a p53 target gene that protects cells against oxidative and genotoxic stresses, participates in the protective role of Akt in response to an energetic stress induced by 2-deoxyglucose (2-DG). Sesn2 is upregulated in response to an energetic stress such as 2-DG and metformin, and mediates the inhibition of mammalian target of rapamycin (mTOR), the major cellular regulator of energy metabolism. The increase of Sesn2 is independent of p53 but requires the anti-apoptotic pathway, PI3K/Akt. Inhibition of Akt, as well as loss of Sesn2, sensitizes cells to 2-DG-induced apoptosis. In addition, the rescue of Sesn2 partially reverses the pro-apoptotic effects of 2-DG. In conclusion, we identify Sesn2 as a new energetic stress sensor, which appears to be protective against energetic stress-induced apoptosis that integrates the pro-survival function of Akt and the negative regulation of mTOR.

The antidiabetic drug metformin exerts an antitumoral effect in vitro and in vivo through a decrease of cyclin D1 level.

Metformin is a widely used antidiabetic agent, which regulates glucose homeostasis through inhibition of liver glucose production and an increase in muscle glucose uptake. Recent studies suggest that metformin may reduce the risk of cancer, but its mode of action in cancer remains not elucidated. We investigated the effect of metformin on human prostate cancer cell proliferation in vitro and in vivo. Metformin inhibited the proliferation of DU145, PC-3 and LNCaP cancer cells with a 50% decrease of cell viability and had a modest effect on normal prostate epithelial cell line P69. Metformin did not induce apoptosis but blocked cell cycle in G(0)/G(1). This blockade was accompanied by a strong decrease of cyclin D1 protein level, pRb phosphorylation and an increase in p27(kip) protein expression. Metformin activated the AMP kinase pathway, a fuel sensor signaling pathway. However, inhibition of the AMPK pathway using siRNA against the two catalytic subunits of AMPK did not prevent the antiproliferative effect of metformin in prostate cancer cells. Importantly, oral and intraperitoneal treatment with metformin led to a 50 and 35% reduction of tumor growth, respectively, in mice bearing xenografts of LNCaP. Similar, to the in vitro study, metformin led to a strong reduction of cyclin D1 protein level in tumors providing evidence for a mechanism that may contribute to the antineoplastic effects of metformin suggested by recent epidemiological studies.