Exposure to cigarette smoke is associated with the development and severity of chronic obstructive pulmonary disease (COPD), which is the third leading cause of death worldwide.
Cigarette smoke contains 2 to 3 micrograms of cadmium, a highly toxic metal and environmental pollutant, per cigarette. When tobacco is burned, it releases cadmium oxide, which can be adsorbed onto micro-particles in the smoke that can penetrate deep into the lungs. Additionally, the body is unable to eliminate cadmium that accumulates in long-term smokers.
In a Scientific Reports study conducted at the University of Alabama in Birmingham, researchers demonstrate how low doses of cadmium induce harmful stress in lung epithelial cells, highlighting potential therapeutic targets for cadmium exposure and resulting lung damage that need further investigation.
The research, led by Veena Antony, MD, a professor at the UAB School of Medicine, focuses on microRNA-381 and the expression of a chloride channel gene called ANO1 in lung tissue samples and airway epithelial cells. ANO1 assists in mucus production in the airways; however, overproduction of mucus in chronic lung diseases can lead to airway thickening and mucus blockage, increasing the severity of the disease. Therefore, overexpression of ANO1 can worsen COPD.
UAB researchers compared lung tissue samples from nine „never“ smokers, who had not smoked cigarettes in the past, with lung tissue samples from 13 „ever“ smokers with COPD, who had a history of smoking between 15 and 25 pack-years per person in the past. A pack year is generally defined as smoking a pack of cigarettes per day over a period of one year. The researchers found that „ever“ smokers, as opposed to „never“ smokers, exhibited highly upregulated ANO1 expression in airway epithelial cells.
Similarly, airway epithelial cells in bronchoalveolar lavage fluid from a non-COPD patient and a COPD smoker showed increased ANO1 expression in the COPD patient’s cells.
Next, the researchers tested the direct effect of very low doses of cadmium on normal human airway epithelial cells. These cells were cultured at an air-liquid interface, allowing for normal differentiation of airway cells. A two-week exposure to 0.5 or 1.0 micromolar cadmium chloride in the fluid layer increased the expression of ANO1 by 12 to 14 times.
MicroRNAs have the ability to downregulate the expression of a gene by directly interacting with the mRNA sequence of that gene. Using computer software analysis, the UAB team identified microRNA-381 as the microRNA with the greatest interaction with ANO1 mRNAs, suggesting that microRNA-381 is a negative regulator of ANO1. It is known that some heavy metals negatively regulate microRNAs.
Antony and colleagues used a synthetic inhibitor for microRNA-381 to inhibit the expression of microRNA-381 in primary human airway epithelial cells from COPD patients, and found that ANO1 expression was significantly increased. In contrast, adding a microRNA-381 mimic; a synthetic RNA that acts like microRNA-381 to increase the extent of negative regulation –; to these cells reduced ANO1 expression. These results reinforced the UAB researchers‘ hypothesis that cadmium negatively regulates microRNA-381 expression, thus upregulating ANO1 expression in airway epithelial cells.
Finally, the researchers found that the microRNA-381 inhibitor still upregulated ANO1 and the mimic still downregulated ANO1, even when primary human airway epithelial cells from COPD patients were exposed to 1 micromolar cadmium chloride.
Our findings from experiments with low-dose cadmium exposure of epithelial cells suggest that ANO1 is a direct target of miR-381, which is downregulated with low-dose cadmium exposure. Therefore, cigarette-induced cadmium toxicity may alter mechanisms of cellular homeostasis at very low concentrations, and cadmium exposure in a person with an existing lung disease may have an additional or adverse effect, increasing susceptibility to infections and environmental allergens.
This interaction of cadmium, microRNA-381, and ANO1 suggests that microRNAs could serve as potential therapeutic targets that need further exploration in cadmium exposure and subsequent lung damage.“
Veena Antony, MD, Professor, UAB Department of Medicine
At UAB, Antony holds the Endowed Chair for Environmental Health Sciences, leads the Superfund Research Center, and is a member of the Department of Pulmonary, Allergy, and Critical Care Medicine.
Co-authors with Antony in the study „Low-dose Cadmium Exposure Regulates miR-381-ANO1 Interaction in Airway Epithelial Cells“ include Pooja Singh, Fu Jun Li, Kevin Dsouza, Crystal T. Stephens, Huaxiu Zheng, and Mark T. Dransfield, from the UAB Department of Medicine, Department of Pulmonary, Allergy, and Critical Care Medicine; and Abhishek Kumar, UAB Superfund Center Advisory Board, Gainesville, Florida.
The study was supported by National Institute of Environmental Health Sciences grant ES027723.
University of Alabama at Birmingham
Singh, P., et al. (2024). Exposure to low-dose cadmium regulates miR-381-ANO1 interaction in airway epithelial cells. Scientific Reports. doi.org/10.1038/s41598-023-50471-z.