Mucus Proteins Make Hydrophobic Pollutants More Toxic

A slippery gel coats the surfaces of animals' respiratory, digestive, and reproductive tracts. Scientists thought that this mucus layer, made of glycoproteins called mucins, helped shield organisms from pathogens, particulates, and chemicals that they breathed or ingested. But a new study has revealed a possible chink in this armor: Mucins could actually help certain pollutants enter organisms' cells (Chem. Res. Toxicol., DOI: 10.1021/tx100426s).

The pollutants in question are powerful mutagens and carcinogens that come from burning wood, fossil fuels, or waste. They include polycyclic aromatic hydrocarbons like benzo[a]pyrene (BaP), anthracene, and coronene. Until now scientists assumed that the pollutants' hydrophobicity limited their transport across the mucus layer and into cells.

Chemist Michael Gozin, biologist Dan Peer, and their colleagues at Tel Aviv University, in Israel, started to question that assumption last year after they observed that hydrophobic nanomaterials could dissolve in solutions containing mucins (Small, DOI: 10.1002/smll.200900637). Mucins are large extracellular proteins shaped like a bottle brush with a polypeptide core and oligosaccharide "bristles." The researchers wondered if the glycoproteins could interact with small hydrophobic molecules, such as polycyclic aromatic hydrocarbons.

When they mixed various polycyclic aromatic hydrocarbons with an aqueous solution of a purified mucin called bovine submaxillary mucin, the protein "basically performed some kind of miracle," says Gozin: It dissolved the normally water-insoluble hydrocarbons. The researchers showed that each mucin protein bound about 40 molecules of BaP, anthracene, or coronene. Because a deglycosylated form of the mucin didn't bind the hydrocarbons, the researchers concluded that the oligosaccharides were critical.

The researchers next bathed a model organism, the single-celled protozoan Paramecium caudatum, in a solution of the BaP–mucin complex. The median lethal dose of free BaP was five-fold greater than that of the BaP–mucin complex, suggesting to the researchers that mucins increased BaP's toxicity. Human-cell experiments produced similar results: Free BaP didn't damage colorectal cancer cells, whereas the same concentration of BaP complexed with mucin killed approximately 30% of the cells.

The researchers suspected that the mucins made BaP more toxic by helping it slip inside cells, thus increasing its bioavailability. To confirm their hypothesis, they measured how much BaP entered the human cells. Since BaP naturally fluoresces, the researchers could watch the cells take up the pollutant. The inside of the cells glowed more intensely when the scientists treated the cells with the BaP-mucin complex than when they exposed the cells to BaP alone. "The mechanism of the complex's entry into cells is not yet clear, but the speed of exposure indicates that it's not just simple diffusion," Gozin says.

Samuel Lai, a biomedical engineer at the University of North Carolina, Chapel Hill, says the idea that mucins can increase hydrophobic pollutants' bioavailability is "an interesting hypothesis that warrants further investigation." However, he notes that the purified mucins used in the study may behave differently than those found in the dense meshwork of the mucus layer.

Gozin hopes to synthesize new molecules that mimic mucins' structures to help sneak hydrophobic drugs into cells. "We can definitely learn a lot of lessons from mucins," he says.