title: Calcium-independent effects of cadmium on actin assembly in mesangial and vascular smooth muscle cells.
abstract: Several metal ions are known to cause depolymerization of the actin cytoskeleton under some circumstances. We found that in renal mesangial and vascular smooth muscle cells, micromolar concentrations of Cd2+ result in loss of phalloidinstainable filamentous (F-) actin. The decrease in F-actin was not accompanied by a corresponding increase in G-actin. The decrease in total actin could be accounted for in part by an inhibition by Cd2+ of total protein (and actin) synthesis after 6 to 8 h without an effect on actin degradation, and the equilibrium between F- and G-actin was shifted to maintain near-constant levels of G-actin. However, Cd2+ caused significant decreases in F-actin at earlier times, indicating effects on the polymerization equilibrium independent of those on actin synthesis. Only picomolar concentrations of free intracellular Cd2+ occur in these experiments. However, it is this Cd2+ pool which is responsible for F-actin depolymerization because equal cellular concentrations of cadmium delivered as Cd-metallothionein have no effect. The effect is also very specific for Cd2+ and under the same conditions neither Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, nor Hg2+ result in any loss of F-actin. Addition of Cd2+ to mesangial and vascular smooth muscle cells had no immediate effect on free intracellular calcium concentrations ([Ca2+]i) even though Ca(2+)-signalling pathways were intact as shown with vasopressin and endothelin. Exposure to 10 microM CdCl2 for 8 h nevertheless caused an increase in [Ca2+]i to > 250 nM and increases in [Ca2+]i achieved with ionophores alone were sufficient to decrease F-actin concentrations. However, a rise in [Ca2+]i is not necessary for actin depolymerization. Depletion of cellular Ca2+ by treatment with thapsigargin did not protect F-actin against Cd2+; the effect of Cd2+ was enhanced in cells unable to increase their [Ca2+]i. We conclude that depolymerization of F-actin by Cd2+ in smooth muscle and mesangial cells is metal-specific, Ca(2+)-independent, and accompanied by a depletion of total actin protein.
authors.affiliation: Department of Clinical Biochemistry, University of Toronto, Canada.
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.forename: T A
authors.initials: TA
authors.lastname: Chin
authors.forename: D M
authors.initials: DM
authors.lastname: Templeton
mesh_terms: D000199:Actins / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D002104:Cadmium / Q000737:chemistry / Q000494:pharmacology*; D002118:Calcium / Q000737:chemistry*; D002478:Cells, Cultured; D003571:Cytochalasin B / Q000494:pharmacology; D003599:Cytoskeleton / Q000187:drug effects; D005920:Glomerular Mesangium / Q000187:drug effects* / Q000648:ultrastructure; D008668:Metallothionein / Q000494:pharmacology; D008856:Microscopy, Fluorescence; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000648:ultrastructure; D009994:Osmolar Concentration; D051381:Rats; D015032:Zinc
chemical_list: D000199:Actins; C004203:cadmium-binding protein; D002104:Cadmium; D003571:Cytochalasin B; D008668:Metallothionein; D015032:Zinc; D002118:Calcium