Screening for developmental toxicity of tobacco smoke constituents.

Cigarette smoking is unrivaled among developmental toxicants in terms of total adverse impact on the human population. According to the American Lung Association, smoking during pregnancy is estimated to account for 20–30% of low-weight babies, up to 14% of preterm deliveries, and about 10% of all infant deaths (http://www.lungusa.org/tobacco/pregnancy_ factsheet99.html). Both active (Stillman et al., 1986) and passive smokers (Martin and Bracken, 1986) have babies with lower than normal birthweights. The long-term consequences associated with low birthweight are just beginning to come to light, and they are many. Risks of childhood and adult morbidity, including—but not limited to—diabetes, cardiovascular disease, obesity, and cancer, are inversely related to birthweight (for review, see Godfrey and Barker, 2001; Slikker and Schwetz, 2003). It has been estimated that 12–24% of pregnant women smoke, with the lower figure coming from surveys based on self-reporting. Smoking is the single largest preventable risk factor for pregnancy-related morbidity and mortality in the US (Dempsey and Benowitz, 2001), and the Surgeon General’s Report (USDHHS, 2001) states that the known adverse women’s health effects of smoking “compels the Nation to make reducing and preventing smoking one of the highest contemporary priorities for women’s health.” Vasculogenesis and angiogenesis are essential for embryonic development (Drake, 2003), and disruption of these processes can be a powerful mechanism of teratogenesis. In fact, thalidomide, the most notorious of human teratogens, may work at least in part through its known antiangiogenic properties (Ng et al., 2003; Stephens et al., 2000). In this issue, Melkonian and coworkers present another in a series of papers from their group demonstrating that constituents of tobacco smoke are developmentally toxic and antiangiogenic in the chick chorioallantoic membrane (CAM) assay. The use of the CAM to study effects on angiogenesis is well established, if not yet well standardized (Ribatti and Vacca, 1999; Richardson and Singh, 2003), and the signaling pathways controlling vasculogenesis and angiogenesis are highly conserved across species (Drake, 2003). The type of screening done in this paper demonstrates the advantages of screening systems when used appropriately in a mechanism or mode of action framework. Previous papers from the same group presented CAM assay results for pyridines identified in tobacco smoke (Ji et al., 2002) as well as for mainstream and sidestream tobacco smoke solutions (Melkonian et al., 2000, 2002). Pyridine derivatives with single methyl or ethyl substitutions were the most potent, inhibiting CAM growth down to picomolar concentrations. Both mainstream and sidestream cigarette smoke solutions caused abnormal pattern formation of CAM blood vessels and altered the composition of the extracellular matrix in the CAM mesoderm. In the paper highlighted here, the effects of pyrazine and derivatives previously identified in tobacco smoke are examined. Pyrazine itself was found to be a more potent inhibitor of CAM and embryo growth (picomolar concentrations) than any of the six derivatives tested. Effects on growth were apparently due to inhibition of cell proliferation, as DNA synthesis was inhibited by pyrazine. While higher concentrations inhibited angiogenesis and blood vessel pattern formation, these parameters were not affected at the lowest concentration that affected CAM and embryo growth. Since they previously demonstrated that tobacco smoke extracts were antiangiogenic, the authors speculate that there may be other, as yet untested, compounds in cigarette smoke that are more strongly antiangiogenic than is pyrazine. Yet the potency of pyrazine in this developmental assay is important, particularly in light of its inclusion on a list of chemicals that are generally regarded as safe additives for human foods and consumer products (Adams et al., 2002; Smith et al., 2001). Further work on its developmental toxicity is certainly merited. Given the immense deleterious health effects of cigarette 1 To whom correspondence should be addressed at [email protected].