Induction of cystine and glutamate transport activity in human fibroblasts by diethyl maleate and other electrophilic agents.

The transport activity for cystine and glutamate in cultured human diploid fibroblasts is enhanced in response to diethyl maleate treatment. The enhancement is time- and dose-related, with a lag of about 3 h, and maximum enhancement (approximately 3-fold increase in the rate of uptake) is attained after 1 to 2 days of incubation of the cells with 0.1 mM diethyl maleate. The enhancement of the transport activity is accompanied by an increase in the Vmax and little change in the Km, and it requires RNA and protein synthesis. Other electrophilic agents, such as cyclohex-2-en-1-one, ethacrynic acid, 1,2-epoxy-3-(p-nitro-phenoxy)propane, and sulfobromophthalein, similarly enhance the transport activity. These electrophiles are known as agents that interact with glutathione. For example, diethyl maleate at high concentrations, i.e. 1 mM, depletes intracellular glutathione and injures the cells. However, at relatively low concentrations diethyl maleate and other electrophilic compounds do cause increases in the intracellular levels of glutathione which we attribute to the enhanced uptake of cystine. It is suggested that the transport system for cystine and glutamate is involved in a protective mechanism of cells against an electrophilic attack.