Cysteine and Methionine Deprivation Sensitizes Cells to N-Acetyl Cysteine

Abstract

Amino acid metabolism is crucial for cells to maintain cell growth, facilitate protein production, and also enables cell signaling. Two amino acids of particular importance are the sulfur containing amino acids, cysteine and methionine. Cysteine is essential to cells as it is the limiting substrate for glutathione (GSH), the primary antioxidant in cells. Cysteine can also be oxidized in proteins and can dictate cell signaling pathways. Methionine is crucial as the starting amino acid in all proteins and is also essential for maintaining adequate cysteine levels through the transsulfuration pathway (TSP). The TSP is governed by two enzymes cystathionine gamma lyase (CTH) and cystathionine beta synthase (CBS) which catalyze the conversion of methionine to cysteine. CTH and CBS can also generate hydrogen sulfide (H2S) utilizing homocysteine and cysteine as substrates. H2S has recently gained attention for its dichotomous role within cells. At low concentrations, H2S can coordinate cell signaling through protein sulfhydration, can stimulate oxidative phosphorylation through donating electrons, and can act as an antioxidant to help cells cope with oxidative stress. At high concentrations, H2S becomes toxic through inhibition of the electron transport chain leading to cell death. Therefore, cells must buffer H2S levels to ensure the toxic threshold is not surpassed. In this dissertation, we found that deprivation of cysteine and methionine leads to a decrease in cell growth and activation of the Integrated Stress Response (ISR) pathway. In addition, cysteine and methionine deprivation leads to an increase in H2S levels, though the levels remain nontoxic. However, the addition of an antioxidant, N-Acetyl Cysteine (NAC) leads to a more pronounced increase in H2S levels and leads to toxicity through an increase of reactive oxygen species (ROS). We show that inhibition of the H2S generating enzyme, cystathionine gamma lyase (CTH), decreases H2S levels and restores cell viability in NAC and cysteine/methionine deprived cells.