Cadmium-induced aggregation of iron regulatory protein-1

Abstract

Iron regulatory protein-1 (IRP-1) is central to regulation of iron homeostasis, and has been shown to be sensitive to Cd(2+) in vitro. Although Cd(2+) induces disulfide-bond formation in many proteins, the critical cysteine residues for iron binding in IRP-1 were shown not to be involved in Cd-induced IRP-1 aggregation in vitro. Here we show that Cd(2+) causes polymerization and aggregation of IRP-1 in vitro and in vivo, and decreases in a dose-dependent manner both its RNA-binding and aconitase enzymatic activities, as well as its cytosolic expression. We have used two-dimensional electrophoresis to demonstrate thiol-dependent self-association of purified recombinant IRP-1 treated with Cd(2+), as well as self-association in Cd(2+)-exposed mesangial cells. Circular dichroism spectra confirm significant conformational changes in the purified protein upon Cd(2+) exposure. Following Cd(2+) treatment, there is increased translocation of inactive IRP-1 to the actin cytoskeletal fraction, and this translocation is diminished by both antioxidant (BHA) treatment and inhibition of CaMK-II. These changes differ from those elicited by manipulation of iron levels. Cadmium-induced translocation of proteins to cellular compartments, and particularly to the cytoskeleton, is becoming a recognized event in Cd(2+) toxicity. Polymer-dependent translocation of IRP-1 in Cd(2+)-exposed cells may underlie effects of Cd(2+) on iron homeostasis.

Keywords: Cadmium ion; Calcium/calmodulin-dependent protein kinase-II; Cytoskeleton; Iron regulatory protein-1; Mesangial cell.