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Thirdhand Smoke (THS) consists of the residual and volatile chemicals that remain in indoor environments after secondhand smoke has cleared. Exposure to THS can occur when an individual touches or ingests (particularly infants) the smoke residue present on surfaces or by breathing the volatile components of THS. Although there is much interest in the impact of THS on human health, there is little scientific data upon which regulatory policy for THS exposure can be based. The purpose of the proposed research is to investigate the adverse effects of THS exposure on normal cell processes such as cell survival/ cell death and the damage caused by THS to DNA. We will use human embryonic stem cells (hESC), which model an early stage of embryogenesis and human pulmonary fibroblasts (hPF), a differentiated adult cell from the lung. The proposed work will fit well with TRDRP Research Priority #1 as it deals with the toxicological evaluation of THS, a tobacco smoke residue. The toxicological data that will emerge from this project will help in gaining an insight into the potential health hazards of THS and will therefore serve as a basis for regulatory policy.
The first aim of this project will assess the cell death and DNA damage caused by water-soluble THS extracts, prepared using an optimized protocol developed in our lab. Evaluation of these endpoints will be done using validated assays that have been adapted in our lab for work with hESC. The assay for cell death is based on cell metabolism, while DNA damage is evaluated based on breaks in DNA. The doses that produce an IC50 (inhibition of 50%), the lowest observable adverse effect, and the no observable adverse effect (non lethal dose) will be determined in these experiments for THS extracts.
The second aim will determine the mode of action of non-lethal doses of THS extracts using different molecular techniques, imaging methods and advanced software. Experiments will be done to evaluate the effect of non-lethal doses of THS extracts on cellular health and processes, including motility (distance traveled, direction), cell division, cellular respiration and chemical modification in DNA, as all these processes are crucial for normal physiology and functions. Follow-up experiments will be performed depending on the mode of action that is identified. Experiments will be done with short-term exposure (24-48 hours) for endpoints such as cell motility and DNA damage and long-term exposure (7-10 days) for endpoints such as chemical modifications to DNA. Our data will provide new information on the cellular processes that are altered by THS exposure and the maximum dose that produces no effects. The doses that affect cell survival and other cellular processes as well as doses that cause DNA damage can be compared to exposure data collected by laboratories working with individuals living or working in THS environments.
These results could help establish maximum acceptable exposure levels when policies for THS exposure are developed. We will also learn the relative sensitivity of embryonic and adult cells to THS, which may influence exposure recommendations for pregnant women, as the embryonic stem cells are likely to be more sensitive to adverse effects of environmental pollutants than adult cells. Our data when combined with future field studies will form a foundation that can be used by government agencies to frame rational policies on indoor smoking, maximum acceptable levels of THS exposure, and criteria for remediation of environments containing THS. Moreover, the standardized extraction protocol can also be applied to remediation procedures to remove THS residues in homes and work places. A very important finding from this study will be the effect of chronic exposure to non-lethal doses on chemical modifications to DNA. The long-term experiments will serve as a model for chronic exposure and can be extrapolated to understand potential effects on humans exposed to THS. Further research to investigate the health hazards of THS can be based on the results obtained from this project. |
