STUDIES ON THE SULFUR METABOLISM OF TISSUES CULTIVATED IN VITRO

Understanding the role of cysteine in ferroptosis: progress & paradoxes

Cysteine is a conditionally essential amino acid required for the synthesis of proteins and many important intracellular metabolites. Cysteine depletion can trigger iron-dependent nonapoptotic cell death-ferroptosis. Despite this, cysteine itself is normally maintained at relatively low levels within the cell, and many mechanisms that could act to buffer cysteine depletion do not appear to be especially effective or active, at least in cancer cells. How do we reconcile these seemingly paradoxical features? Here, we describe the regulation of cysteine and its contribution to ferroptosis and speculate about how the levels of this amino acid are controlled to govern nonapoptotic cell death.

Formation of sulfhydryl groups in the culture medium by human diploid fibroblasts

When human diploid fibroblasts IMR-90 are cultured in routinely used medium (Eagle's basal medium supplemented with 10% fetal calf serum), sulfhydryl compounds appear in the medium. The major component of these sulfhydryl compounds is cysteine, and it is shown that a part of medium cystine is converted into cysteine by the cells. It is also shown that the sulfhydryl groups of serum albumin, which are masked and barely detectable before the culture, are restored. Probably cysteine formed by the cells reacts with serum albumin to give rise to the protein sulfhydryl groups via sulfhydryl-disulfide exchange reactions. Total sulfhydryl concentrations in the medium are maintained in a considerable level throughout the culture, and a possible physiological function of these sulfhydryl groups is discussed.

Cystinotic and normal fibroblasts: differential susceptibility to cysteine toxicity in vitro

Extracellular cysteine concentrations between 0.5 and 2.5 mM resulted in death of normal but not cystinotic cells grown in Eagle's minimal essential medium containing supplemental fetal bovine serum and antibiotics. Differential cell survival was determined by viable cell counting using Trypan Blue dye exclusion. In cocultivation experiments of [3H]thymidine-labelled cystinotic fibroblasts with nonradioactive normal fibroblasts, autoradiography confirmed the selective survival of cystinotic cells in medium containing 1 mM cysteine. At this concentration of 1 mM cysteine, intracellular cystine content increased slightly in surviving normal cells but not in cystinotic cells, which normally contain a high level of intracellular cystine. This comparative resistance of cystinotic fibroblasts to elevated extracellular cysteine concentrations forms the basis for an in vitro selective system for these mutant human cells. Further exploration of this resistance phenomenon may well expand the understanding of the molecular defect in cystinotic cells.

l -CYSTINE REQUIREMENT FOR PRODUCTION OF COXSACKIE B3 VIRUS IN CULTURED MONKEY HEART CELLS

Tyndall , R. L. (The Pennsylvania State University, University Park) and E. H. Ludwig . l -Cystine requirement for production of Coxsackie B3 virus in cultured monkey heart cells. J. Bacteriol. 85: 1339–1345. 1963.—The requirement for l -cystine in both the production of Coxsackie B3 virus in cultured monkey heart cells and in prolonging the survival of the cultured monkey heart cells is demonstrated. d -Cystine, dl -homocystine, dl -allo-cystathionine, dl -homocysteine thiolactone, 2-mercaptoethlyamine, allylglycine, 3,3′-dithiopropionic acid, and dl -ethionine could not replace l -cystine in either supporting Coxsackie B3 virus production or prolonging monkey heart cell culture survival time. By their ineffectiveness, these compounds suggest the specificity of the l -cystine requirement. Allylglycine, 3,3′-dithiopropionic acid, and dl -ethionine were likewise incapable of inhibiting the l -cystine effect in supporting both virus production and cell survival. Prolonged starvation of cell cultures prior to virus inoculation failed to reveal any additional marked nutritional requirements, but rather tended to accentuate the l -cystine requirement for virus production. Increased cell starvation did, however, lead to the establishment of a latent Coxsackie B3 virus infection of cultured monkey heart cells, the activation of which is l -cystine–dependent.