Levels of outdoor air pollution in China are among the highest in the world (37
). Increases in fossil fuel use in China’s industry, transport and residential sectors have resulted in a steep rise in emissions. The Yangtze River Delta region, which includes Qidong in eastern Jiangsu Province, is the fastest growing economic development area in China (38
). This region, which constitutes only 2% of the area of China, contributes upwards of 15% of countrywide emissions of greenhouse gases (38
). In rural areas, such as He Zuo Township where this study was conducted, biomass fuel and coal are burned for cooking and heating, resulting in elevated indoor air pollution as well. Visible impact of air pollution is manifest as the Asian Brown Cloud, a haze composed of pollutants and particulates that resides over the sub-Asian continent as well as China for months on end, especially in the winter. Outdoor air pollution has been found to be associated with a wide range of adverse health outcomes, including increased mortality, increased rates of hospital admissions and exacerbation of chronic respiratory conditions along with decreased lung function (39
There is a relative scarcity of air monitoring data in this region and precious little human biomonitoring for exposures. Biomonitoring in China has centered on PAHs and benzene, the latter an environmental carcinogen with especially high exposures in some workplace settings. PAHs, such as phenanthrene, are ubiquitous in the general environment and are released into the ambient air by tobacco smoke, vehicle exhausts and other incomplete combustion sources such as cooking stoves. Analysis of the organic extract of indoor air particles from homes in Yunnan Province, China, indicated phenanthrene to be the most abundant PAH (36
). Hecht et al.
) previously reported mean PheT levels of 1.2 pmol/mg creatinine () in non-smokers in the USA, whereas smokers had levels of 3.8 pmol/mg. In a study conducted in He Zuo Township in the winter of 2003, we observed a modest increase in PheT levels in the 12 smokers compared with the 87 non-smokers who were in the placebo arm (11.6 versus 8.0). Although the magnitude of the difference between smokers and non-smokers is lower in Qidong than in the US study, the baseline values of the PAH biomarker are substantially higher in the non-smoking residents of Qidong compared with the US non-smokers. Thus, non-tobacco-derived sources, such as industrial, automotive and cooking emissions, seem to account for the bulk of PAH airborne exposures in this rural area adjacent to and downwind of Shanghai. Consumption of vegetables contaminated with PAHs in the fields as well as those produced by broiling meat may provide additional vectors for exposure. Lower levels of PheT (6.5 pmol/mg creatinine) were detected in the urine of non-smokers in this study conducted in the fall of 2009. Seasonal differences in levels of air pollution—highest in winter, previously observed in Shanghai (40
), may drive the lower levels seen in the current study. We also find in this study that the levels of mercapturic acids of acrolein, crotonaldehyde and benzene were 25, 290 and 540% higher in non-smoking, rural Qidong residents than found in non-smoking residents of Singapore (29
). The levels of the benzene biomarker, SPMA, were in accord with earlier findings by Kim et al.
) in a control population in Tianjin, China. No differences were seen in levels of the urinary ethylene oxide mercapturic acid. Reference values for these biomarkers in USA or European populations are limited (42
). In addition, health effects of these ambient levels of exposure, if any, are unknown.
Effective low cost means (i.e. functional foods) to empower individuals to reduce body burden of genotoxic air pollutants may exert benefits for public health but are not to be considered as substitutes for regional and countrywide efforts to control pollution. In this study, consumption of broccoli sprout beverage led to reasonably consistent increases (~20 to 50%) in excretion of mercapturides of acrolein, crotonaldehyde, ethylene oxide and benzene. Interestingly, increases were seen in both smokers and non-smokers indicating that level of exposure upon entry into the intervention did not have much influence on the inductive response. However, broccoli sprout related increases in mercapturic acid excretion of the four air pollutants did not track among individual participants. That is, elevated excretion of one mercapturic acid was not linked to elevations in excretion rates for the others. This heterogeneous outcome may relate to the involvement of distinct isoforms of GSTs in the conjugation of these different substrates. Overlaid on that effect is the knowledge that 34% of the participants were null for GSTM1
alleles, whereas 51% were null for GSTT1
; 29% were null for both (20
). The induction of different isoforms of human GSTs in response to sulforaphane and their proportional contributions to the conjugations of the four substrates in this study are not well characterized. Considerable evidence suggests that polymorphisms in GSTM1
profoundly affect the formation of SPMA from benzene (44
), and to a lesser extent ethylene oxide (45
). In contrast, no effects of these isoforms on acrolein conjugation have been observed (Hecht S., unpublished results).
The central question addressed in the clinical trial was to determine the bioavailability and tolerability of two preparations of beverages prepared from 3-day-old broccoli sprouts in order to administer the chemopreventive agent sulforaphane: (i) enterically generated from its cognate glucosinolate (glucoraphanin) by thioglucosidases found in the gut microflora (GRR) and (ii) prereleased when the GRR beverage was treated with myrosinase from daikon to catalyze hydrolysis of glucoraphanin to sulforaphane (SFR). The bioavailability of sulforaphane administered as SFR was far superior to GRR (20
). Estimates of the area under the curves derived from the excretion patterns suggest that 70% of the administered sulforaphane in the SFR beverage was eliminated in 24 h, whereas only 5% of the administered glucoraphanin in GRR beverage was recovered as sulforaphane metabolites (20
). Somewhat surprisingly, despite the higher bioavailability of sulforaphane from the SFR, there was no indication of differential efficacy of urinary mercapturic acid excretion between the two treatment arms (). This result suggests that the 800 μmol dose of glucoraphanin yielded sufficient sulforaphane to provoke a maximal inductive response. Whether an adequate peak concentration (Cmax
) or extended half-live relative to the administration of sulforaphane itself explain the equieffectiveness is not clear at this time. What has clearly been demonstrated, both in in vitro
and animal models, is that the potential of broccoli sprout extract to upregulate cytoprotective enzymes can be completely attributed to the sulforaphane in those extracts and that no other phytochemical components contributed significantly to the induction of these enzymes (7
). Whether other pathways affecting the metabolism of these pollutants are also modulated by the broccoli sprout beverages was not addressed in this study.
The mercapturic acids are measures of internal dose following exposures to air pollutants. As such, it is challenging and perhaps inappropriate to use them as estimates of risk. Excretion rates reflect multiple processes. These rates increase with exposure, such as seen with smoking, but are influenced (up and down) by polymorphisms in metabolizing enzymes. To control for variable exposures, reverse phase 0 studies in which microdoses of heavy isotope-labeled compounds, as has been done with deuterated phenanthrene (46
), could separate effects of exposure from metabolism (47
). Nonetheless, an intriguing question lies in the health impact of 20–50% increases in rates of detoxication of these air pollutants as seen with the broccoli sprout intervention. A few insights can be gleaned from animal experiments where dose–response and time series studies explore the relationships between modulation of expression of conjugating enzymes, metabolic flux of carcinogens, DNA adduct levels and cancer chemopreventive efficacy. In rats treated with the chemopreventive agent ethoxyquin, a 4-fold increase in the biliary excretion of aflatoxin–glutathione conjugate was associated with 75% reductions in DNA adducts and >95% reductions in tumor burden following challenge with the hepatocarcinogen aflatoxin (48
). In rats fed 0.02% oltipraz, hepatic GST activity increased by 50%, DNA adduct burden dropped by 40% and volume percent of preneoplastic lesions were reduced by >95% (49
). Perhaps, one can also extrapolate from the experience in Beijing in 2008 when stringent air quality control measures were implemented before and during the Olympic games. Reductions of 27–66% in atmospheric content of hydrocarbons were reported (50
); comparable reductions were measured for PAHs. Jia et al.
) have estimated that these PAH reductions, if sustained, could contribute to a 46% reduction in cancer risk. Thus, it is reasonable to expect that seemingly modest increases in detoxication capacity, as afforded by broccoli sprouts, can lead to risk reduction in individuals and populations unavoidably exposed to environmental carcinogens.