SEATTLELong-term, high-level exposure to bacterial endotoxins in raw cotton fiber and cotton dust was associated with a 40% decrease in lung cancer risk among female Chinese textile workers. The risk of developing lung cancer decreased for workers exposed to greater amounts of endotoxins over many years.
The new research, conducted by George Astrakianakis, PhD, of the Occupational Health and Safety Agency for Healthcare, Vancouver, BC, and the University of Washington, Seattle, and his colleagues, involved 267,000 female textile workers in Shanghai. The researchers compared the cumulative exposure histories of 628 patients diagnosed with incident lung cancer from 1989 through 1998, with the experiences of a lung-cancer-free reference subcohort of 3,184 workers who were matched by 5-year age groups to the cancer patients.
They found that 20 years of exposure to endotoxins reduced the incidence of lung cancer to approximately 7.6 per 100,000 women, compared with 19.1 per 100,000 for the average Shanghai woman.
Furthermore, the risk of lung cancer proved lowest for women whose endotoxin exposure occurred early in their working life. The study also showed similar inverse dose-response trends for the risk of lung cancer with cumulative exposure to endotoxin among women who ever smoked and women who never smoked. "Cumulative exposure to endotoxin was strongly, statistically significantly, and inversely associated with lung cancer risk," the researchers concluded.
Endotoxins are a contaminant found in raw cotton fiber and cotton dust. They are complex, heat-stable lipopolysaccharide constituents of the outer membranes of Gram-negative bacterial cell walls. They consist of O-specific polysaccharide, the core polysaccharide, and lipid A, which is the least variable but most biologically active component. "Potential anticarcinogenic effects of endotoxin are probably mediated by the innate and acquired immune systems, although specific mechanisms have yet to be elucidated," the researchers wrote (J Natl Cancer Inst 99:357-364, 2007).
Investigators in the United States and elsewhere have reported a lower risk of lung cancer for textile workers since the 1970s. Additional studies have found that workers in other occupations that expose them to heavy doses of endotoxins, including dairy farming, also have reduced lung cancer risks. Until now, however, no one had quantified the relationship between endotoxin exposure and lung cancer risk. Although they do not yet understand the mechanism, the researchers postulated a potentially anticarcinogenic effect of endotoxins "specific to lipid A, as shown by the increased survival of tumor-bearing animals or the reduced growth rate of established tumors inoculated with this component of the lipopolysaccharide."
In an editorial, Paolo Boffetta, MD, of the International Agency for Research on Cancer, Lyon, France, acknowledged the importance of the findings to lung cancer research, but cautioned that the study's limitations make it too early to consider using endotoxins for lung cancer prevention.