Alterations in cytoskeletal organization and homeostasis of cellular thiols in cadmium-resistant cells

Abstract

To understand further the mechanisms of cadmium toxicity, cytoskeletal organization and homeostasis of cellular thiols were examined in cadmium-resistant cells isolated from Swiss mouse 3T3 cells by incubation in graded concentrations of CdCl2 (Cd2+) in the culture medium. Cd2+-resistant cells displayed profound alterations in their cytoskeletal organization characterized by the appearance of many elongated, tadpoleshaped cells with a high density of microtubules (MT) and microfilaments (MF), with the former being mainly distributed along the long axis of the cell. Exposure of Cd2+-resistant cells to 50 μM Cd2+ for 16 hr did not cause apparent cytoskeletal perturbations, whereas treatment of parental cells with 5 μM Cd2+ for the same duration produced a severe loss of MT and smeared patches of MF. Thus, the cytoskeleton of Cd2+-resistant cells is markedly more preserved and protected against Cd2+ damage than that of their parental counterparts. Cd2+-resistant cells contained a higher basal level of protein sulfhydryls (PSH) in both the cytoskeletal and cytosolic fractions than the parental cells. Exposure to 50 μM Cd2+ further increased cellular PSH contents, reaching 192 and 215% of the basal levels for the cytoskeletal and cytosolic fractions, respectively. Although 5 μM Cd2+ exposure also elevated the amounts of PSH in parental cells, the 'absolute' values were still below the corresponding basal levels in Cd2+-resistant cells. Furthermore, Cd2+-resistant cells also exhibited enhanced basal levels of metallothionein and cellular glutathione (GSH), amounting to 19- and 2.1-fold of the parental basal levels, respectively. Thus, the Cd2+-resistant cells produced larger quantities of both protein and nonprotein thiol-containing elements than the parental cells. Interestingly, exposure of Cd2+-resistant cells to 50 μM Cd2+ also further increased metallothionein and cellular GSH to 178 and 138% of the basal levels, respectively. Based on the affinity of Cd2+ for sulfhydryls as a mechanism of Cd2+ toxicity, we propose that the coordinately increased levels of metallothionein, GSH, and PSH in Cd2+-resistant cells would provide a mechanistic basis for the homeostasis of cellular thiols which may collectively contribute to the cytoskeletal preservation by protecting the cytoskeleton from Cd2+ insult. © 1994 Academic Press, Inc.