Maternal Cadmium Exposure Induces Fetal Growth Restriction Partially Through Down-regulating Placental Zinc Transport

Objective The present study was to investigate the effects of maternal cadmium exposure during pregnancy on fetal growth and placental zinc transport, and its mechanism.Methods The present study consisted of three independent experiments. Experiment 1 To investigate the effects of maternal cadmium(Cd) exposure on fetal growth and development. The present study established mouse model of Cd-induced FGR through two different exposure methods. Thirty-two pregnant mice were randomly divided into four groups. All pregnant mice were administered with different concentrations of Cd Cl2(0, 5, 50 and 250 ppm, dissolved in ultrapure water) through drinking water throughout pregnancy. The remaining pregnant mice were randomly divided into two groups. In the Cd-treated group, pregnant mice were intraperitoneally(i.p.) injected with Cd Cl2(4.5 mg/kg) on GD 9. Normal saline-treated pregnant mice served as controls. All dams were sacrificed on GD18. The uterine horns were incised and weighed. Live fetuses were counted. The gender of fetal mice was determined by anogenital distance(AGD). Male and female fetuses per litter were weighed. Crown-rump length was measured. Experiment 2. To investigate the short-term effects of maternal Cd exposure on Cd and Zn distribution in mother, placenta and fetus. Pregnant mice were randomly divided into two groups. In the Cd-treated group, pregnant mice were intraperitoneally(i.p.) injected with Cd Cl2(4.5 mg/kg) on GD 9. Normal saline-treated pregnant mice served as controls. All dams were sacrificed at different time points(0, 2, 12 and 24 h) after Cd injection. Maternal serum, maternal liver, maternal kidney, placenta and embryo were collected and stored at-80 ℃for measurement of Cd and Zn. Maternal liver and placenta were collected for real-timeRT-PCR. Experiment 3. To investigate the long-term effects of maternal Cd exposure on Cd and Zn distribution in mothers, placentas and fetuses. Pregnant mice were randomly divided into two groups. In the Cd-treated group, pregnant mice were intraperitoneally(i.p.) injected with Cd Cl2(4.5 mg/kg) on GD 9. Normal saline-treated pregnant mice served as controls. All dams were sacrificed on GD18. The uterine horns were exposed and weighted. Live and dead fetuses were counted. Live fetuses were weighed. Maternal serum, maternal liver, maternal kidney, placenta, fetal serum and fetal liver were collected and stored at-80 for measurement of Zn and Cd. Maternal liver and ℃placenta were collected for real-time RT-PCR. Placental cross sections were stained with Hematoxylin & Eosin(H&E).Results Maternal high-Cd(250 ppm) exposure markedly induced the reduction of fetal weight and crown-rump length in a gender-independent manner. Maternal exposure to Cd(4.5 mg/kg, ip) also caused the decrease in fetal weight, crown-rump length, placental weight. Histopathology showed that the labyrinth layer was severely disrupted in placentas of Cd-treated mice, with a reduction in the internal space of fetal and maternal blood vessels. Cd measurement showed that level of Cd in maternal serum, maternal liver and maternal kidney was markedly increased at 24 h after Cd injection. Placental Cd content was elevated by 60 folds. Moreover, embryonic Cd content was increased by more than 11 folds at 24 h after maternal Cd injection. The levels of Cd in maternal serum, maternal liver and maternal kidney remained elevated at 9 days after Cd injection. Placental Cd content was only 2.4% of maternal liver, Cd content in fetal liver was increased by 4.9 folds and there is no change on fetal serum Cd level at 9 days after Cd injection. Expression of metallothionein(MT) showed that Mt1 and Mt2 m RNA in maternal liver were markedly up-regulated 2 h after Cd exposure; Mt1, Mt2 m RNA in maternal liver and placental Mt2 m RNA was slightly up-regulated at 24 h after maternal Cd injection. Mt2 m RNA in maternal liver remained slightly elevated but there was no significant difference on placental Mt1 and Mt2 m RNA between two groups at 9 days after Cd injection. Zn measurement showed that placental Zn level was markedly increased at 24 h after maternal Cd injection. on the contrary, Zn level in maternal serum and embryo was significantly reduced at 24 h after Cd injection. Zn level in maternal liver remained elevated. However, there was no significant difference in Zn level of placenta, fetal serum and fetal liver between two groups at 9 days after Cd injection. Expression of zinc transporters showed that placental Znt1 and Znt2 m RNA was markedly down-regulated at 12 h after maternal Cd injection. Placental Znt2 m RNA level remained reduced at 9 days after Cd injection.Conclusion Maternal Cd exposure during pregnancy induces fetal growth restriction partially through down-regulating the expression of zinc transporter, which reduced placental Zn transportation from maternal circulation to the fetuses.