Previous studies have shown that hypertension was induced by the intraperitoneal administration of 1 mg/kg/day Cd for 15 days in non-pregnant rats [
18,
19]. Hypothesizing that pregnant rats are more susceptible to Cd toxicity, we exposed pregnant rats to relatively low dosages of Cd and evaluated them for maternal-placental-fetal parameters linked to preeclampsia. We demonstrated that the hallmarks of preeclampsia, including acute maternal hypertension, hyperalbuminuria and IUGR, were found in SD rats exposed to 0.5 mg/kg Cd during gestation (GD5-19). The typical changes in the maternal endothelium in preeclampsia ere also observed in the rat kidney after Cd treatment. The reduction in fetal weight independent of placental weight in the 0.5 mg/kg Cd-treated group has been postulated to result from an abnormal placenta with impaired function and reduced perfusion after fetal and/or placental exposure to Cd.
To explore the biochemistry underlying these physiological changes, we used a very sensitive ELISA to detect placental corticosterone. A marked increase in the concentration of placental corticosterone was observed in rats treated with Cd. High corticosteroid concentrations in extra-adrenal organs, sometimes much higher than circulating concentrations, is evidence of local synthesis. Despite the presence of
11-HSD, which is involved in the upregulation of glucocorticoids in the placenta [
20], in our opinion it is also possible that a classical glucocorticoid biosynthesis pathway may be involved in these processes. To date, few studies have examined the levels of steroidogenic enzymes necessary for
de novo synthesis of corticosterone from cholesterol in the placenta [
21].
The placenta utilizes maternal cholesterol as the initial substrate to produce progesterone for the duration of the pregnancy, and most of the produced progesterone enters the maternal circulation [
22]. Progesterone is also a source of corticosteroid precursors.
StAR is a protein involved in the shuttling of cholesterol to the inner mitochondrial membrane for enzymatic conversion. In the placenta, cytochrome
P450scc (
CYP11A1) is the most important and rate-limiting enzyme for the conversion of cholesterol to pregnenolone [
23]. 21-hydroxylase (
CYP21) and 11-beta-hydroxylase (
CYP11B1) are required for corticosterone synthesis; they convert progesterone to corticosterone. According to our research, although there was a slight increase in
CYP11A1 mRNA, there was no significant change in
StAR and
3β-HSD mRNA after exposure to Cd, indicating that 0.5 mg/kg Cd does not affect placental gonadal steroidogenesis, especially progesterone biosynthesis, in rats.
CYP11B1 is required for the final steps of glucocorticoids synthesis and is followed by the activity of the
CYP21 enzyme [
24]. In our studies, the expression of
CYP21A and
CYP11B1 mRNA in normal rat placenta tissue was too low to be detected consistently and reproducibly, so the ability of normal placenta to produce corticosterone is very limited, which is in agreement with the data reported by Sybulski [
25]. After Cd injection,
CYP21A and
CYP11B1 mRNAs in the placenta increased, although the levels were still lower levels than those in the adrenal gland. However, it should be remembered that because the placenta is much larger than the adrenal gland, the total mRNA for glucocorticoid-synthesizing enzymes in the entire placenta could not be underestimated. The enzyme
CYP11B1 was detected rather easily using immunohistochemistry or western blot, suggesting that the protein is stable in the placenta. The expression of either of these enzymes would enable the placenta to use circulating steroid precursors or local progesterone to increase the local levels of active glucocorticoids, which influence placental structure and function. The mechanisms by which Cd affect gene expression may include mimicry of the effects of Ca
2+ at low concentrations of Cd
2+ due to the similarity of the ionic radii [
26], resulting in stimulation of the steroidogenic pathway and synthesis of corticosteroids. We proved that local corticosteroid synthesis may be activated under pathological conditions in placenta.
In normotensive pregnancy, glucocorticoids are inactivated but are frequently present after downregulation of
11β-HSD2 by adverse prenatal experiences [
27,
28]. We found that the placenta of rats with preeclampsia during pregnancy is characterized by decreased
11β-HSD2 with unchanged
11β-HSD1. Cd can directly inhibit the transcription of the
11β-HSD2 gene [
29]. This local increase in the utility of glucocorticoids is very important because it allows increased ability of active glucocorticoids to cross the placenta barrier, which accounts for the elevated fetal corticosterone concentrations associated with low birth weight.
In conclusion, this study demonstrates for the first time that the placenta may be an extra-adrenal site of glucocorticoid synthesis due to prenatal Cd exposure in rats. Most of the reviewed research findings suggest that in micromolar concentrations Cd causes upregulation of the mediators and markers of inflammation and appears to have proinflammatory properties [
30]. Increasing the local concentration of glucocorticoids is a protective mechanism to control the deleterious effects of excessive production of inflammatory mediators. Although we feel that the present results are encouraging, further studies are needed to elucidate the clinical implications of extra-adrenal corticosteroidogenesis in the preeclamptic placentas of Cd-exposed pregnant rats.