Cellular cadmium responses in subpopulations T20 and T27 of human lung carcinoma A549 cells

Abstract

Subpopulations T20 and T27, cloned from the human lung carcinoma line A549, differ significantly in their Cd 2+ cytotoxic response. T27 has an LC 50 of 31 μM Cd 2+ and a cytotoxic response threshold of 5 μM Cd 2+, whereas the T20s LC 50 is 15 μM Cd 2+ and there is no observed threshold for cytotoxicity. Cadmium-induced metallothionein (MT) synthesis, cadmium accumulation, glutathione (GSH) content, and Cd 2+-induced changes in GSH content were studied in T20 and T27 in an attempt to determine the mechanism(s) causing differential cytotoxic response. MT synthesis measured by following Cd 2+-induced [ 35S] incorporation into MT was found not to differ between T20 and T27. There is, however, a difference in Cd 2+ accumulation between the two subclones. T20 and T27 cells were exposed to 5 μM Cd 2+ for different times or to different concentrations of Cd 2+ for 8 h. The T27 subline, which is the more Cd 2+ resistant, was found to accumulate significantly more Cd 2+-both as a function of time exposed to Cd 2+ and as a function of Cd 2+ concentration. The two subpopulations were found to have comparable initial GSH contents, but showed different Cd 2+-induced changes in [GSH] when the cells were exposed to 5 μM Cd 2+. T27 cells maintained their GSH content following Cd 2+ exposure but T20 cells showed a Cd 2+-induced decrease in GSH content. The results indicate that the difference in Cd 2+ cytotoxic response between A549-T20 and A549-T27 cells is not attributable to alterations in MT synthesis nor to a difference in initial GSH content. Relative Cd 2+ cytotoxicity also does not in these cells correlate with relative Cd 2+ accumulation. The fact that T27 cells accumulate more Cd 2+ and yet are more Cd 2+ resistant than T20 cells suggests that T27 cells have a much more effective non-MT mechanism to handle intracellular Cd 2+. This may involve different GSH metabolism and/or yet undefined molecular factors.