Sphingolipids are a class of lipid molecules that function both as structural membrane components and as bioactive signaling molecules. Sphingolipids can be produced de novo or salvaged and recycled. Despite the established roles of sphingolipids such as sphingosine 1-phosphate and ceramides in regulating signaling involved in pro- and anti-tumorigenic cellular processes, the role of the de novo sphingolipid biosynthesis pathway in cancer is unclear. The main objective of this thesis study was to determine whether there is an essential role for this pathway in cancer and whether its disruption can be a cancer-specific metabolic vulnerability. Here, we find that de novo sphingolipid synthesis through the rate-limiting enzyme serine palmitoyltransferase (SPT) is not required in cancer cells due to their salvage capacity. However, upregulation of SPT in cancer cells creates a requirement to detoxify its product, 3-ketodihydrosphingosine (3KDS), via the downstream enzyme 3-ketodihydrosphingosine reductase (KDSR). We demonstrate that KDSR is essential in cancer cells both in vitro and in vivo to restrain the levels of its substrate 3KDS, the accumulation of which can disrupt ER structure and function, resulting in proteotoxic stress and cell death. Our findings also reveal that KDSR is essential specifically in cancer cells and not normal cells and that upregulation of SPT in cancer may act as a biomarker for sensitivity to targeting KDSR. Altogether, this thesis study provides new insights into the role of KDSR in the de novo sphingolipid biosynthesis pathway in both cancer and ER homeostasis and demonstrates the potential to exploit this for therapeutic purposes in a cancer-specific manner.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The sudden death syndrome (SDS), caused by species of Fusarium, is a disease difficult to control and there are no management techniques and resistant cultivars that give long-term positive results. This study aimed to assess populations of Fusarium spp. and Trichoderma spp. associated to eight soybean genotypes, cultivated under no tillage; to determine if the control of Fusarium varies among isolates of Trichoderma; to identify isolates of Fusarium spp. obtained from different genotypes, and to determine their toxicity and pathogenicity to two genotypes of soybean. Soil and roots were collected from experimental plots located at the Fundação Centro de Experimentação e Pesquisa (FUNDACEP), obtaining isolates of Fusarium spp. and Trichoderma spp. There were no differences in the population of Fusarium spp. in plants with symptoms of SDS among the different genotypes. For Trichoderma spp. there was a significant difference among genotypes from plants with and without symptoms of SDS and the genotype CEPS 06006 RR had the biggest population of the antagonist. In the direct confrontation test, eight isolates of Trichoderma spp. obtained the highest score (1) in relation to the control of Fusarium spp. and isolates of Trichoderma spp. from soil of areas with the SDS symptom were more aggressive. Nine species of Fusarium: F. avenaceum, F. solani, F. equiseti, F. Acuminatum, F. kyushuense, F. graminum, F. subglutinans, F. verticillioides and F. lateritium were identified. All isolates promoted internerval chlorosis, necrosis and death of seedlings in the toxicity test. In the pathogenicity test in plants inoculated with the pathogen there was an increase in leaves and roots dry weight and in the number of pods, indicating that there may be species specificity of the species studied in relation to genotype. / A Podridão Vermelha da Raiz da soja (PVR), causada por espécies de Fusarium, é uma doença de difícil controle, não existindo técnicas de manejo e cultivares resistentes que tenham resultados positivos a longo prazo. Assim, este trabalho objetivou quantificar populações de Fusarium spp. e Trichoderma spp. associadas à oito genótipos de soja, cultivados em sistema plantio direto, observar se o biocontrole de Fusarium spp. difere entre isolados de Trichoderma spp., identificar isolados de Fusarium spp. obtidos de diferentes genótipos e verificar sua toxicidade e patogenicidade a dois genótipos de soja. Amostras de solo e raízes foram coletadas em parcelas experimentais na Fundação Centro de Experimentação e Pesquisa (FUNDACEP), obtendo-se isolados de Fusarium spp. e Trichoderma spp. Não houve diferença na população de Fusarium spp. em plantas com sintomas de PVR entre os diferentes genótipos. Para Trichoderma spp., houve diferença entre as populações nos genótipos de plantas com e sem sintomas de PVR, tendo o genótipo CEPS 06006 RR apresentado maior população do antagonista. No teste de confronto direto, oito isolados de Trichoderma spp. obtiveram a melhor nota (um), em relação a Fusarium spp. e isolados de Trichoderma spp. oriundos de solo em áreas com o sintoma de PVR foram mais eficientes. Foram identificadas nove espécies de Fusarium: F. avenaceum, F. solani, F. equiseti, F. acuminatum, F. kyushuense, F. graminum, F. subglutinans, F. verticillioides e F. lateritium. Todos os isolados de Fusarium spp. provocaram clorose internerval, necrose e morte de plântulas no teste de toxicidade. No teste de patogenicidade, em plantas inoculadas com o patógeno houve aumento na massa seca de parte aérea e de raízes, no número de vagens, indicando que pode haver especificidade das espécies estudadas em relação ao genótipo.

Selenium is a micronutrient that is used by the selenocysteine biosynthesis pathway to produce the amino acid selenocysteine, which is required in selenoproteins. Many of the 25 human selenoproteins, such as glutathione peroxidases and thioredoxin reductases, play important roles in maintaining cellular redox homeostasis. In this study we characterize how this metabolic pathway is upregulated in cancer cells and how this increase in activity creates a unique vulnerability. We have outlined the evidence and underlying mechanisms for how many metabolites normally produced in cells are highly toxic, and we describe this concept as illustrated in selenocysteine metabolism. My thesis explores how SEPHS2, an enzyme in the selenocysteine biosynthesis pathway, is essential for survival of cancer, but not normal cells. SEPHS2 is required in cancer cells to detoxify selenide, an intermediate that is formed during selenocysteine biosynthesis. Breast and other cancer cells are selenophilic, owing to a secondary function of the cystine/glutamate antiporter SLC7A11 that promotes selenium uptake and selenocysteine biosynthesis, which, by allowing production of selenoproteins such as GPX4, protects cells against ferroptosis. However, this activity also becomes a liability for cancer cells because selenide is poisonous and must be processed by SEPHS2. These results show that SEPHS2 is a cancer specific target and indicates the therapeutic potential of SEPHS2 inhibition in the treatment of cancer. Collectively, this thesis identifies SEPHS2 as a targetable vulnerability of cancer cells, defines the role of selenium metabolism in cancer, and outlines a roadmap for future studies regarding toxic metabolites and cancer.