Cadmium-Induced Rat Embryotoxicity in Vitro Is Associated with an Increased Abundance of E-Cadherin Protein in the Yolk-Sac

Abstract

Cadmium, a cytotoxic heavy metal, is embryotoxic. Cadmium interferes with the functions of other cations such as zinc and calcium. Cadherins, calcium-dependent cell adhesion molecules, are expressed and spatiotemporally regulated in embryos and their yolk sacs during organogenesis. Cadmium has been shown to disrupt calcium-dependent cell-cell interactions and to alter the distribution of epithelial or E-cadherin in cultured cells. The purpose of this study was to determine whether the embryotoxicity of cadmium is mediated through an effect on E-cadherin. Day 10 rat embryos were cultured for 1 hr without cadmium and then cultured for another 2, 6, or 20 hr in the presence or absence of 2.5 μM CdCl2. Embryos and yolk sacs were collected and analyzed separately. The growth of embryos and yolk sacs after exposure to cadmium for 2 or 6 hr was not different from that of controls. After 20 hr exposure to cadmium, embryos were growth retarded, with morphological scores 10-15% lower than those of controls; more dramatically, their yolk sacs were thickened and decreased in diameter to only half of that of control yolk sacs. Northern blot analysis revealed no significant differences in the relative amounts of E-cadherin mRNA between control and cadmium-treated embryos or their yolk sacs. There was also no alteration in the abundance of E-cadherin protein in cadmium-treated embryos at any of the times examined. There was a gradual decline in the relative amount of E-cadherin protein in control yolk sacs with time in culture; interestingly, cadmium treatment appeared to prevent this decline, resulting in significantly higher concentrations of E-cadherin protein in the cadmium-exposed yolk sacs after 6 hr (1.7-fold) or 20 hr (2.3-fold) of culture. The relative abundance of other structural proteins such as α- or β-tubulin and actin was unchanged. Exposure of embryos for 20 hr to concentrations of cadmium varying from nonembryotoxic to embryotoxic resulted in concentration-dependent increases in E-cadherin protein in the yolk sac; E-cadherin was only increased in the yolk sac of growth-retarded embryos. Thus, an increase in yolk sac E-cadherin protein is associated with the induction of embryotoxicity by cadmium. To determine the ability of cadmium to interact directly with the cadherins, the binding of radioactive 109Cd to embryo and yolk sac proteins was assessed. 109Cd bound to an unidentified 87-kDa protein; binding to this protein was attenuated by 1 mM zinc, but not by 1 mM calcium. No 109Cd binding was detected in the similar to 120-kDa region, indicating that cadmium did not interact directly with the cadherin molecules. Furthermore, when both 2 mM CaCl2 and 2.5 μM CdCl2 were added to the embryo culture medium, calcium did not prevent the teratogenic effects of cadmium. These data suggest that during the induction of embryo malformations by cadmium, the regulation of E-cadherin protein in the yolk sac is altered. However, cadmium does not appear to directly interact with E-cadherin.