Abstract
Homeostatic mechanisms for the maintenance of glutathione (GSH) are fundamental in the provision of a cellular defense against electrophilic/oxidant challenges. Cyclopentenone prostaglandins (CP-PGs) are powerful antiproliferative endogenous substances that may act as electrophilic regulating compounds, by virtue of the presence of an alpha,beta-unsaturated carbonyl group in the cyclopentane ring. Nevertheless, differential resistance to CP-PG cytotoxic/cytostatic effect has been reported in different cell types. It is reported that the activity/expression of gamma-glutamylcysteine synthetase (gamma-GCS, the rate-limiting enzyme in GSH biosynthesis) can be inducibly activated by electrophiles, including CP-PGs. The response of the human cancer strains HEp-2 (larynx carcinoma) and HL-60 (promyelocytic leukemia) cells to treatment with the CP-PG PGA1 in culture was investigated by evaluating the time-course of GSH synthesis and activity of enzymes of GSH metabolism, other than gamma-GCS, after PGA1 addition. HEp-2 cells, being more resistant to PGA1 cytotoxic and cytostatic effects, have basal GSH levels that were 2.4-fold higher than that of HL-60 cells. The activities of GSH S-transferase (GST), glutathione reductase (GSRd) and glutathione peroxidase (GSPx) are constitutively higher in HL-60 cells than in HEp-2 cells (respectively, 17.0-, 28.5- and 12.3-fold). When challenged with PGA1, both cell types exhibited a dose-dependent rise in GSH content that was maximal 18 h after PGA1 addition and was preceded by a rise in GST and GSRd activities in both cell types (at 12 h). GSPx activity increased only in HEp-2 (PGA1 evoked a 93.4%-inhibition in HL-60 cells). Moreover only HEp-2 cells exhibited early capacity to enhance GSH content (1-2 h just after PGA1 addition). These results and earlier data showing that leukemia cells are sensitive to CP-PG treatment suggest that deficiencies in GSH metabolism may be strategically in therapeutic approaches to the treatment of human leukemias.
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