Gestational Exposure to Phenanthrene Induces Superfluous Fibrosis and Calcification and Metabolic Imbalance of the Placenta in Mice

The placenta is a vital organ that facilitates maternal-fetal circulation, ensuring proper fetal development. Phenanthrene (Phe), a typical low-molecular-weight polycyclic aromatic hydrocarbon, is widely present in the environment and food. In this study, pregnant mice were exposed to Phe (0, 0.6, 6, and 60 μg/kg of body weight) via gavage every 3 days from pregnancy day 0.5 (PGD 0.5) for a total of six exposures during pregnancy. Placentas were collected on PGD of 18.5 for analysis. The results showed that Phe exposure altered placental structure and function, inducing trophoblast thickening at low doses (0.6 μg/kg) but thinning at higher doses (6 and 60 μg/kg), reducing blood cell density in the placental labyrinth, disrupting metabolite composition, causing oxidative damage, and leading to excessive fibrosis and calcification. Molecular analysis revealed that PCNA was significantly upregulated in the 0.6 μg/kg group and downregulated in 6 and 60 μg/kg groups, indicating an initial compensatory proliferative response at low doses and impaired placental proliferation at higher doses, while Bad was abnormally accumulated in trophoblasts and dose-dependently upregulated, along with decline in antioxidant capacity. Meanwhile, increases in protein levels of TGF-β1, Smad2, p-Smad2, Smad1/5/9, p-Smad1/5/9, BMP2, TIMP1, Runx2, Collagen I, and SMA, and a decrease in MMP1 level was observed. These findings suggested that Phe exposure during pregnancy induced activation of the TGF-β/Smad2 pathway and BMP2/Smad1/5/9/Runx2 pathway, which might further lead to excessive fibrosis and calcification. The abnormally increased fibrosis and calcification, together with the oxidative damage, further elevated cellular apoptosis, destroyed the structure of the placenta, and reduced blood cell counts, impairing placental exchange efficiency and leading to systemic metabolic imbalances, which might further impair the health of their offspring.