Two chemical flavorings commonly used in electronic cigarettes could impair airway ciliary function, in vitro experiments suggested.
Lab studies showed that diacetyl and 2,3-pentanedione altered gene expression in human bronchial epithelial cells, downregulating genes associated with cilia production and function, according to a team led by Joseph G. Allen, DSc, and Quan Lu, PhD, of the Harvard T.H. Chan School of Public Health in Boston.
Moreover, "the number of ciliated cells was significantly decreased by the flavoring chemicals," they wrote in Scientific Reports.
Previous research led by Allen found that these compounds, commonly used to mimic butter or caramel flavors, were present in most of the commercial flavored e-cigarettes tested.
In an interview with MedPage Today, Allen said flavoring chemicals widely used in e-cigarette liquids have not been shown to be safe for inhalation.
He noted that in the early 2000s, studies linked production of butter-flavor microwave popcorn to an irreversible lung disease among workers, which was dubbed "popcorn lung." Diacetyl was the most widely used chemical in the popcorn butter flavoring, and its effect was found to be dose-dependent in several studies.
"After this, 2,3-pentanedione -- which has a similar flavor profile to diacetyl -- was more widely used," Allen said, adding that 2,3-pentanedione was found in nearly half of the e-cigarettes tested in the earlier study.
He noted that some previous research has shown diacetyl and 2,3-pentanedione inhalation exposure to be associated with epithelium damage in mice and rats, but the reasons for this are not well known.
"We haven't really understood the mechanism," he said. "This study is the first to look at the the impact on human epithelial cells."
His group identified a total of 163 and 568 differentially expressed genes in primary normal human bronchial epithelial (NHBE) cells that were exposed to diacetyl and 2,3-pentanedione, respectively.
"DAVID pathway analysis revealed an enrichment of cellular pathways involved in cytoskeletal and cilia processes among the set of common genes (142 genes) perturbed by both diacetyl and 2,3-pentanedione," Allen and colleagues wrote. "Consistent with this, qRT-PCR confirmed that the expression of multiple genes involved in cilia biogenesis was significantly downregulated by diacetyl and 2,3-pentanedione in NHBE cells."
"What we see in our study is that these flavoring chemicals are impairing the epithelium of the cells that line our lungs. We are seeing a direct impact and change in gene expression," Allen said.
He added that the findings suggest that 2,3-pentanedione may not be a safer alternative to diacetyl as an inhaled flavoring compound.
"They are chemical cousins, and in this study we show that they are actually operating on the same genetic pathway as far as impacting cilia performance," he said.
Ciliated cells constitute between 50% and 80% of the airway epithelial lining. The hairlike structures aid in clearing debris and microbes from the airways; impaired ciliary function is seen in various lung diseases including COPD and asthma.
"Our study has shown that exposure to flavoring chemicals leads to decreased number of ciliated cells with concomitant down-regulation of cilia-related genes in ALI culture system of NHBE cells in vitro, mimicking the in vivo airway characteristics with tight junctions and a differentiation state with ciliated, basal, and secretory cells," Allen and colleagues wrote. "This is consistent with rodent studies showing that diacetyl or 2,3-pentanedione treatment resulted in airway epithelial injury characterized by flattening of cells, loss of microvilli and cilia, and fissure formation."
This study was funded by grants from the NIH and the National Institute of Environmental Health Sciences, Harvard NIEHS Center.
The researchers declared no relevant relationships with industry related to this study.
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