Modulation of glutathione content and the effect on methionine auxotrophy and cellular distribution of homocysteine and cysteine in mouse cell lines

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Abstract
The inability of cells in culture to grow in medium where methionine is replaced by its metabolic precursor, homocysteine, has been linked to neoplastic transformation and termed ‘methionine dependence’ or ‘methionine auxotrophy’. The present investigation was undertaken to establish the influence of intracellular glutathione level on methionine auxotrophy in different mouse cell lines. A non-transformed, methionine-independent fibroblast cell line with essential normal growth rate in methionine-deficient, homocysteine supplemented medium (Met− Hcy+) showed only a slight initial lag and then the same growth as control when glutathione was reduced to >5% by the glutathione synthesis inhibitor buthionine su (BSO). Increasing cellular glutathione by cystamine in a completely methionine dependent leukemia cell line did not stimulate the cells to proliferate in Met− Hcy+ medium. A partly methionine dependent transformed fibroblast cell line with reduced capacity to proliferate in Met− Hcy+ medium showed increased growth potential when the cells were depleted of glutathione by a non-toxic concentration of BSO. An even higher growth potential of these cells in Met− Hcy+ medium was obtained by addition of a non-toxic concentration of cystamine, while only a transient increase of glutathione content was observed under these conditions. Both BSO and cystainine increased the fraction of protein-bound cysteine and homocysteine in the partly methionine-dependent cells. These metabolic alterations correlated with the increased ability of these cells to utilize homocysteine for growth. Our results suggest that methionine auxotrophy is a metabolic defect that is not related to the cellular glutathione status, but may be related to the intracellular distribution between free and protein-bound forms of other thiols as cysteine and homocysteine.

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