Co-exposure to microplastics and cadmium leading to progression from NAFLD to liver tumorigenesis

李宇涵，牟为

This Adverse Outcome Pathway (AOP) describes the pathological transformation of the liver from non-alcoholic fatty liver disease (NAFLD) to cirrhosis and ultimately to hepatocellular carcinoma under the combined exposure of microplastics and cadmium. The Molecular Initiating Events (MIEs) are characterized by COL3A1 signaling pathway, ROS generation, and cytokine aggregation. These triggers lead to a series of Key Events (KEs), including alterations in ECM organization, increased cell proliferation, the occurrence of three distinct types of cell death, and chronic inflammation. This biological cascade eventually results in two primary Adverse Outcomes (AOs): liver cancer and hepatic fibrosis. By delineating these relationships, this AOP provides a novel scientific perspective on the hepatotoxicity induced by the interaction between microplastics and heavy metals.

Microplastics (MPs), fragments and particles ≤5 mm in diameter, persistently accumulate in the environment and have been detected in various human tissues and organs, including infant placental tissues. Once ingested via food or water, MPs can deposit in the liver, leading to significant tissue injury. Polystyrene microplastics (PS-MPs), widely used in disposable food packaging, are frequently employed as models to study these environmental effects. Beyond their inherent toxicity, PS-MPs can enter the human body and serve as carriers for other environmental pollutants. Among these co-exposed contaminants, heavy metals such as cadmium (Cd) are of particular concern due to their high carcinogenicity and shared exposure pathways with MPs through aquatic products, agricultural goods, and water resources.

Both MPs and Cd are ubiquitous pollutants that accumulate in the liver through food chain transfer and biomagnification, with absorbed dietary Cd exhibiting an exceptionally long biological half-life. As a primary target organ for both stressors, the liver undergoes amplified toxicological outcomes during co-exposure. Individual exposure to either Cd or MPs is known to induce oxidative stress, provoke inflammatory cascades, initiate apoptotic pathways, and disrupt hepatic metabolic homeostasis. When combined, these pollutants may result in a synergistic effect that accelerates pathological progression.

External stimuli and a sustained inflammatory microenvironment are key drivers in the development of liver cancer. This tumorigenic process is highly heterogeneous and involves a continuum of metabolic disorders, where Non-Alcoholic Fatty Liver Disease (NAFLD) serves as a significant precursor to liver fibrosis and eventual malignancy. Research indicates that long-term Cd exposure can drive the entire disease progression from NAFLD to fibrosis and liver cancer through the activation of endogenous reactive oxygen species (ROS). Furthermore, higher concentrations of plastic particles have been detected in cirrhotic liver tissues compared to healthy ones, and joint exposure to PS-MPs and Cd has been shown to promote fibrosis by regulating oxidative stress and cell death. Given that the transformation from inflammation to cancer involves complex molecular cascades and extended periods, this AOP is developed as a predictive tool to construct a hazard assessment framework for the synergistic hepatotoxicity of microplastics and heavy metals.

Initial data on chemicals, genes, and disease interactions were collected from the CTD and GeneCards databases, focusing on the progression from NAFLD to liver cirrhosis and liver cancer. These target genes were categorized into three developmental stages and analyzed using GO and KEGG pathways, while protein-protein interaction networks were constructed to establish the molecular foundation. Following this preliminary predictive modeling, in vivo and in vitro experiments were conducted to provide empirical evidence for the proposed biological transitions.

Existing knowledge from the AOP-Wiki regarding cadmium and microplastics was further integrated to build a baseline network of pollutant-induced liver injury. By synthesizing enrichment analysis, interaction networks, and experimental results, the upstream and downstream relationships of key genes and pathways were identified and weighted. This approach, combining computational prediction and experimental verification, led to the final construction of the AOP mapping the progression from NAFLD through liver cirrhosis to liver cancer under combined exposure.

By integrating bioinformatic analysis and findings from in vivo and in vitro experiments with established AOPs of pollutant-induced hepatic injury, a comprehensive AOP network is constructed to delineate the "inflammation-to-cancer" disease progression under the combined exposure to MPs and Cd.

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