TOXICOLOGICAL HIGHLIGHT |
Highlight for Phenols, Quinolines, Indoles, Benzene and 2-Cyclopenten-1-ones Are Oviduct Toxicants in Cigarette Smoke, by Prue Talbot, Karen Riveles, and Ryan Rosa: List of Tobacco-Smoke Constituents That Are Harmful for Reproduction Grows—Passive Smokers May Be at Risk
University of California San Francisco, Department of Cell and Tissue Biology, San Francisco, California 94143
1 To whom correspondence should be addressed. E-mail: genbacev{at}comcast.net.
Received May 1, 2005; accepted May 9, 2005
Epidemiological studies provide compelling evidence that smoking has serious negative consequences on fertility and pregnancy outcome. Numerous epidemiological studies have linked smoking with higher frequency of ectopic pregnancies and problems related to the oviduct function (Castles et al., 1999
). Smoking decreases the success rate of in vitro fertilization and, more specifically, reduces the rates of oocyte production (reviewed by Shiverick and Salafia, 1999). Clearly, we do not understand the exact mechanisms by which smoking decreases fertility and negatively affects pregnancy outcome. One of the reasons is the complex composition of the cigarette smoke, which contains over 4000 chemicals, and only few of them have been identified and tested toxicologically (nicotine, benzopyrenes, PAHs, carbon monoxide, heavy metals such as cadmium and lead.). The other reason is the lack of experimental models that provide specific, quantitative, and reproducible tests to assess the effect of defined tobacco constituents on specific biological processes of the reproductive cycle.
The article by Riveles et al. in this issue of Toxicological Sciences is the most recent in the series of studies published by the same group of authors (DiCarlantonio and Talbot, 1999; Huang et al., 1997; Ji et al., 2002; Knoll and Talbot, 1998; Lam et al., 2000; Magers et al., 1995; Melkonian et al., 2003; Riveles et al., 2003, 2004; Talbot et al., 1999). This study confirms how important it is to have a well-characterized biological system in order to identify the effects and specific targets of different toxicants. The approach used by these authors to assay oviduct function is based on an in vitro culture of the hamster infundibulum (region of the oviduct closest to the ovary). The main functions of the oviduct are to pick up the oocyte cumulus complex after follicular rupture through the activity of the ciliary epithelium and to provide the entry into the oviduct lumen. Exposure to inappropriate conditions during this transition between peritoneal cavity and the tubal lumen can disrupt fertilization and early development. The oviduct assays used by these authors include measurements of ciliary beat frequency (CBC), oocyte cumulus complex pick-up rate (OPR) (Huang et al., 1997; Riveles et al., 2003), and smooth muscle contraction (SMC). Oocyte cumulus complex (OCC) pick-up rate depends on beating of cilia on the outer surface of the infundibulum and transient adhesion of the OCC to the tips of the cilia (Talbot et al., 1999). Movement of the zygote and preimplantation embryos through the oviduct depends on muscle contraction (Harper, 1994). To test the effect of different toxic compounds, explants of the infundibular portion of the oviduct were placed in culture medium in a holding pipette, and sequential measurements of CBC, OPR, and SMC were made before, during, and after exposure to test chemicals. These assays, thus, allow various cellular processes to be studied in a single explant and enable reversal of effects to be monitored. The results obtained using this system suggest that similar adverse effects can be potentially produced by the same chemicals in the other organs such as lungs (cilial beating) or smooth muscles (contractions).
Prior work done by this group showed that a solution of mainstream cigarette smoke inhibits both CBF and OCC pick-up in the oviduct bioassay (Knoll and Talbot, 1998). To determine which chemicals of the over 4000 present in smoke solution could produce these effects, the authors passed mainstream cigarette smoke through various solid-phase extraction cartridges and screened the eluates for inhibitory activity using CBS, OPR, and SMC assays. Eluates that induced inhibition of oviduct activity by 80% or more were then analyzed using gas chromatography and mass spectrometry to identify the chemicals in the inhibitory eluates. Standards of the identified chemicals were purchased and used in the oviduct assays to determine the potency and efficacy of each chemical in the three oviduct assays. The main groups of inhibitory chemicals identified using the above strategy included derivatives of pyrimidine, pyrazine, phenol, indole, quinolines, and 2-cyclopenten-1-ones. The authors have presented evidences that all ten phenolic derivatives tested were more inhibitory in the three oviduct assays than the parent compound phenol. Ethyl and methyl substitutions at positions 2 and 4 produced the strongest inhibition. Indole was the most inhibitory of any of the chemicals tested in all three studies. Most importantly, seven of the tested compounds produced effects at very low doses—in the femto or picomolar range. For the first time, it is suggested that quinoline and isoquinoline have adverse effects on reproduction.
The very important contribution of this highlighted study is that it identifies some chemicals, previously considered safe, to be toxic. Some of the chemicals identified in this (methoxyphenol, 4-ethylphenol, 2,6-dimethylphenol) and the previous studies (3-ethylpyridine, pyrazine) are on the Flavor and Extract Manufactures Association (FEMA) list of chemicals that are Generally Regarded as Safe (GRAS) as well as the FDA EAFUS list (Everything Added to Food in the United States). Some of the oviduct toxicants (3-ethylpyridine, methoxyphenol) are added to cigarettes to enhance flavor.
Talbot et al. (this issue) raise many important questions, including how relevant are these results for human smokers? The authors are aware of the limitations of their study. The message is that the toxicological testing and the follow-up of women who smoke are needed to clarify whether these compounds are hazardous for humans. The fact that some of the chemicals were effective at such low doses suggests that they may be toxic for human smokers who are exposed to doses within the effective range. In addition, this paper has provided the very important message that the concentrations of the most toxic chemicals are higher in side than in main stream smoke, suggesting that passive smokers can be at risk.
ACKNOWLEDGMENTS
Conflict of interest: none declared.
REFERENCES
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DiCarlantonio, G., and Talbot, P. (1999). Inhalation of mainstream and sidestream cigarette smoke retards embryo transport and slows muscle contraction in oviducts of hamsters (Mesocricetus auratus). Biol. Reprod. 61, 651–656.
Harper, J. K. (1994). Gamete and zygote transport. In The Physiology of Reproduction (E. Knobil and J. D. Neill, Eds.), pp. 123–185. Raven Press, New York.
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Riveles, K., Talbot, P., and Rosa, R. (2005). Phenols, quinolines, indoles, benzene and 2-cyclopenten-1-ones are oviduct toxicants in cigarette smoke. Toxicol. Sci. 86, 132–140.
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