Exposure to cigarette smoke is associated with the development and severity of chronic obstructive pulmonary disease, or COPD, the third leading cause of death worldwide.
Cigarette smoke contains 2 to 3 micrograms of cadmium per cigarette, a highly toxic metal and environmental pollutant. Burning tobacco releases cadmium oxide that can be absorbed into microparticles in the smoke that travel deep into the lungs. Moreover, the body is not able to remove cadmium, which accumulates in smokers for a long time.
A Scientific report Study researchers from the University of Alabama at Birmingham show how low doses of cadmium create damaging stress on lung epithelial cells, and their findings highlight potential therapeutic targets to explore in cadmium-exposure and subsequent lung injury.
The research, led by UAB Department of Medicine Professor Veena Anthony, MD, focused on microRNA-381 and the expression of a chloride channel gene called ANO1 in lung tissue samples and airway epithelial cells. ANO1 helps produce mucus in the airways; But in chronic lung disease, the overproduction of mucus can cause the airways to thicken and become blocked with mucus, exacerbating the severity of the disease. Thus, overexpression of ANO1 may exacerbate COPD.
UAB researchers compared lung tissue samples from nine “never” smokers, who had zero history of cigarette smoking, and lung tissue samples from 13 “ever” smokers with COPD who had a smoking history that ranged from 15 to 25 pack-years per person. .. A pack year is usually defined as smoking one pack of cigarettes a day for one year. The researchers found that “ever” smokers, as opposed to “never” smokers, up-regulated ANO1 expression in airway epithelial cells.
Similarly, airway epithelial cells in a bronchoalveolar lavage fluid from a non-COPD subject and COPD-subject cells from a smoker with COPD showed greater ANO1 expression.
The researchers next tested the direct effects of very low doses of cadmium on normal human airway epithelial cells. These cells were grown at an air-liquid interface that allowed airway cells to differentiate normally. Two weeks of exposure to 0.5 or 1.0 micromolar cadmium chloride in liquid phase increased ANO1 expression 12- to 14-fold.
MicroRNAs have the ability to downregulate the expression of a gene by directly interacting with the mRNA sequence of that gene. The UAB team used computer software analysis to identify microRNA-381 as the microRNA with the most interactions with ANO1 mRNAs, suggesting that microRNA-381 is a negative regulator of ANO1. Some heavy metals are known to negatively regulate microRNAs.
Anthony and colleagues used a synthetic inhibitor for microRNA-381 to block the expression of microRNA-381 in primary human airway epithelial cells from individuals with COPD, and found that ANO1 expression was significantly upregulated. In contrast, adding a microRNA-381-mimic; A synthetic RNA that acts like microRNA-381 to increase the amount of negative regulation -; ANO1 expression decreased in those cells. These findings reinforce the UAB researchers’ hypothesis that cadmium negatively regulates microRNA-381 expression to upregulate ANO1 expression in airway epithelial cells.
Finally, the researchers found that, even when primary human airway epithelial cells from people with COPD were exposed to 1 micromolar cadmium chloride, the microRNA-381 inhibitor still upregulated ANO1 and the clone still downregulated ANO1.
Our observations from experiments involving low-dose cadmium-exposure of epithelial cells suggest that ANO1 is a direct target for miR-381, which is up-regulated upon low-dose cadmium exposure. Thus, cigarette-induced cadmium-toxicity may alter cellular homeostasis mechanisms at very low concentrations and cadmium-exposure in an individual with existing lung conditions may have an additive or adverse effect with increased susceptibility to infections and environmental allergens.
“This interaction of cadmium, microRNA-381 and ANO1 suggests that microRNAs may serve as potential therapeutic targets to further explore in cadmium-exposure and subsequent lung injury.”
Veena Anthony, MD, Professor, UAB Department of Medicine
At UAB, Anthony holds an endowed professorship in environmental medicine, directs the Superfund Research Center, and is a member of the Department of Pulmonary, Allergy, and Critical Care Medicine.
Co-authors on Anthony’s study, “Low Dose Cadmium Exposure Regulates miR‑381–ANO1 Interaction in Airway Epithelial Cells,” are Pooja Singh, Fu Jun Lee, Kevin D’Souza, Crystal T. Stephens, Huaxiu Zheng and Mark T. Dransfield , UAB Department of Medicine Division of Pulmonary, Allergy and Critical Care Medicine; and Abhishek Kumar, UAB Superfund Center Advisory Board, Gainesville, Florida.
Support came from National Institute of Environmental Health Sciences grant ES027723.