Cellular responses to environmental contaminants in amoebic cells of the slime mould Dictyostelium discoideum

Amoebic Dictyostelium discoideum cells were employed in a bioassay to evaluate stress responses after exposures to the polyaromatic hydrocarbon benzo[a]pyrene (B[a]P) and two heavy metals (copper and mercury). Furthermore, we developed a recombinant cell line expressing a labile Green Fluorescent Protein (GFP) variant expressed under the control of an actin promoter to monitor stress-related protein degradation. Finally, cell viability was monitored to discriminate lethal exposure concentrations. The results demonstrated that exposure to sub-micromolar concentrations of mercury rendered significant changes in all studied physiological parameters, whereas B[a]P became toxic at low micromolar, and copper at high micromolar concentrations. Exposure to 0.5 mu M mercury significantly reduced lysosomal membrane stability (LMS), endocytosis rate, GFP expression, and further resulted in the elevation of cytosolic free Ca2+ ([Ca2+](i)). LMS in mercury-treated cells that had been pre-incubated with a specific Ca2+-dependent phospholipase A2 blocking agent was however not affected by the exposure, indicating that the toxic action of mercury is linked to the activation of phospholipase A2 via a Ca2+-signaling pathway. Exposure to 20 mu M B[a]P significantly reduced LMS, endocytosis rate, and GFP expression, however without affecting [Ca2+](i), suggesting a calcium-independent route of toxicity for this compound. None of the physiological parameters were significantly affected by copper exposure at concentrations < 400 mu M, demonstrating a high resistance to this metal. Our results further showed that neither cell growth nor viability was affected by concentrations altering the studied physiological parameters. LMS, endocytosis rate, and [Ca2+](i), therefore, appear sensitive biomarkers of pollutant-related stress in amoebic cells. (c) 2006 Elsevier Inc. All rights reserved.