TGF-β/SMAD Pathway Mediates Cadmium Poisoning-Induced Chicken Liver Fibrosis and Epithelial-Mesenchymal Transition.

Cadmium(Cd) is a toxic heavy metal widely present in the environment, capable of accumulating in the liver and causing liver damage. In this study, the mechanism of cadmium-induced liver fibrosis in chickens was investigated from the perspective of hepatocyte epithelial-mesenchymal transition (EMT) based on the establishment of a model of chicken cadmium toxicity and a model of cadmium-stained cells in a chicken hepatocellular carcinoma cell line (LMH). The 7-day-old chickens were randomly divided into the regular group (C group) and cadmium poisoning group (Cd group), and the entire test cycle was 60 days. Three sampling time points of 20 days, 40 days, and 60 days were established. By testing the liver coefficient, histopathological and ultrastructural changes in chicken livers were observed. The enzyme activities of liver function and the expression changes of fibrosis markers (COL1A1, Fibronectin), epithelial-mesenchymal transition markers (E-cadherin, Vimentin, and α-SMA), and the critical factors of the TGF-β/SMAD signaling pathway (TGF-β1, SMAD 2, and SMAD 3) were detected in the liver expression changes. The results showed that at the same sampling time point, the chicken liver coefficient in group Cd was significantly higher than that in control group (P < 0.01); the activities of the liver function enzymes ALT and AST in chickens in the Cd group were significantly higher than those in the C group (P < 0.01); liver hepatocytes degenerated and necrotic, the number of erythrocytes in the blood vessels was increased, and inflammatory cells infiltrated in the sinusoidal gap; the perisinusoidal gap of the liver was enlarged, and there was an apparent aggregation of collagen fibers in the intervening period as seen by transmission electron microscopy. The results of Masson staining showed that the percentage of fiber area was significantly higher in the chickens' livers of the Cd group. The fiber area percentage was significantly higher. The results of real-time fluorescence quantitative PCR and Western Blot showed that the expression of E-cadherin in the livers of chickens in the Cd group was significantly lower than that in the C group (P < 0.01). The expression of α-SMA, Vimentin, COL1A1, Fibronectin, TGF-β1, SMAD 2, and SMAD 3 was significantly higher than that in the C group (P < 0.01). The results of in vitro assays showed that in the LMH cell model established by adding trimethylamine N-oxide, an activator of the TGF-β/SMAD signaling pathway, and oxidized picric acid, an inhibitor of the TGF-β/SMAD signaling pathway, the expression of E-cadherin was significantly reduced in cadmium-stained LMH cells (P < 0.01). The expression of α-SMA, Vimentin, COL1A1, Fibronectin, TGF-β, SMAD 2, and SMAD 3 was significantly elevated (P < 0.01). Cadmium and Trimethylamine N-oxide, an activator of the TGF-β/SMAD signaling pathway, promoted the expression of these factors. In contrast, the inhibitor of the TGF-β/SMAD signaling pathway, Oxymatrine, a TGF-β/SMAD signaling pathway inhibitor, significantly slowed down these changes. These results suggest that cadmium induces hepatic epithelial-mesenchymal transition by activating the TGF-β/SMAD signaling pathway in chicken hepatocytes, promoting hepatic fibrosis.