Cigarette Smoke Toxins Deposited on Surfaces: Implications for Human Health

Cigarette smoking remains a significant health threat for smokers and nonsmokers alike. Secondhand smoke (SHS) is intrinsically more toxic than directly inhaled smoke. Recently, a new threat has been discovered – Thirdhand smoke (THS) – the accumulation of SHS on surfaces that ages with time, becoming progressively more toxic. THS is a potential health threat to children, spouses of smokers and workers in environments where smoking is or has been allowed. The goal of this study is to investigate the effects of THS on liver, lung, skin healing, and behavior, using an animal model exposed to THS under conditions that mimic exposure of humans. THS-exposed mice show alterations in multiple organ systems and excrete levels of NNAL (a tobacco-specific carcinogen biomarker) similar to those found in children exposed to SHS (and consequently to THS). In liver, THS leads to increased lipid levels and non-alcoholic fatty liver disease, a precursor to cirrhosis and cancer and a potential contributor to cardiovascular disease. In lung, THS stimulates excess collagen production and high levels of inflammatory cytokines, suggesting propensity for fibrosis with implications for inflammation-induced diseases such as chronic obstructive pulmonary disease and asthma. In wounded skin, healing in THS-exposed mice has many characteristics of the poor healing of surgical incisions observed in human smokers. Lastly, behavioral tests show that THS-exposed mice become hyperactive. The latter data, combined with emerging associated behavioral problems in children exposed to SHS/THS, suggest that, with prolonged exposure, they may be at significant risk for developing more severe neurological disorders. These results provide a basis for studies on the toxic effects of THS in humans and inform potential regulatory policies to prevent involuntary exposure to THS

Effects of THS-exposure on hyperactive behavior.

<p>(<b>A</b>) Distance and (<b>B</b>) velocity of travel during the first and last 10 minutes of the 1 hr test. (<b>C</b>) Time spent in center of the field vs. the periphery during the first 10 minutes. (<b>D</b>) Raster plots showing the integrated paths that a control mouse (top) and a THS-exposed mouse (bottom) traveled in the first 10 mins and last 10 mins of the hour. The plots were chosen based on the average distance traveled by each cohort of control and THS exposed mice. The Ethnovision software produced the raster images for each individual mouse along with an Excel sheet containing the distance traveled by each mouse as represented by the raster image. The mean distance traveled for each cohort was then calculated and, based on that mean, the raster image representing the path traveled by the single mouse closest to the cohort mean distance traveled is shown. See also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086391#pone.0086391.s003" target="_blank">video s1</a> showing the behavior of the control and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086391#pone.0086391.s004" target="_blank">video s2</a> showing the behavior of the THS-exposed mice. n = 12 control and 12 THS-exposed mice. * p<0.05, ** p<0.01.</p

THS exposure delays closure of and weakens cutaneous wounds.

<p>(<b>A</b>) Representative excision wounds performed on the backs of mice after hair removal show that THS exposure results in keratinization of the epithelium (crusty appearance) and delayed wound closure. (<b>B</b>) At day 14, when control wounds are closed, gene expression evaluated by Affirmetrix Arrays shows that THS exposure upregulates keratin genes and genes involved in epithelial migration and contractile function of wound tissue and downregulates genes involved in inflammatory and immune responses. (<b>C</b>) The upregulated keratin genes are primarily associated with hair and nail production, resulting in stiffening of the wound, making it crusty and potentially more fragile. “Ker.-Assoc. Prot.” = Keratin-Associated Protein. (<b>D–G</b>) In cross sections through the healing tissue, Masson-trichrome staining shows great decrease of interstitial collagen (blue) in THS-exposed tissue; second-harmonic imaging microscopy (SHIM) shows that the collagen is strongly fibrillar in the control (arrow) but not at all fibrillar in THS-exposed mice. For <b>A</b>; for <b>B,C</b>, for <b>D–G</b>. Scale bar for <b>D,E</b> = 100 µm and for <b>F,G</b> = 20 µm. White * labels a hair follicle.</p

Levels of NNAL (metabolite of NNK) in the urine of THS-exposed mice, humans exposed to SHS and in smokers.

<p><b>The range of NNAL levels in our THS-exposed mice fall within range we found in SHS- exposed infants/toddlers, with comparable medians.</b></p><p><b>NNAL = 4 (Methylnitrosamino)-1-(3-pyridyl)-1butanol (*40).</b></p

THS exposure results in hyperglycemia and decrease in insulin sensitivity.

<p>(<b>A</b>) Fasting glucose levels of mice exposed to THS were significantly increased in comparison to control. (<b>B</b>) Intraperitoneal Insulin Tolerance Test (IITT) time course and calculated area under the curve reveal that THS-exposed mice have decreased sensitivity to insulin which is highly correlated with both fatty liver disease and smoke exposure. (<b>C</b>) Intraperitoneal Glucose Tolerance Test (IGTT) time course shows impaired glucose clearance following glucose administration. *p<0.05, ** p<0.01, *** p<0.001.</p

Effects of THS-exposure on behavior of mice.

<p>(<b>A,B</b>) <i>Testing for anxiety</i>. Control and THS mice were tested using an Elevated Plus T-maze (<b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086391#pone.0086391.s001" target="_blank">Fig. S1A</a></b>). (<b>A</b>) Total time spent in the open and closed arms was measured. The two groups both spent much more time in closed than in open arms of the maze, indicating normal anxious behavior. (<b>B</b>) The frequency of entries into the closed arms was significantly greater for THS-exposed as compared to control mice, indicating more locomotor activity. n = 6 controls and 6 THS-exposed mice. (<b>C,D</b>) <i>Testing for hyperactivity</i>. The Open Field test (<b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086391#pone.0086391.s001" target="_blank">Fig. S1B</a></b>) was used to perform these experiments. The behavior of control and THS-exposed mice was observed for 1 hr. (<b>C</b>) The THS mice spent much more time walking, much less time stationary, and more time rearing than the controls. (<b>D</b>) The same general pattern was observed for the frequency of transition between these behaviors; n = 12 controls and 12 THS-exposed mice.</p

THS exposure results in excess deposition of collagen in lungs.

<p>Cross-sections through the lungs show that in THS-exposed animals, the alveoli in the region of the alveolar sacs are disrupted in comparison to the control animals (<b>A,B</b>). In the terminal respiratory bronchioles of the lung, however, the walls of the alveoli in the THS-exposed animals are thicker and appear to contain secretions (<b>C,D</b>). (<b>E–F</b>) Masson-trichrome staining for fibrillar collagen (blue) shows that the level of collagen in normal lung is low but THS-exposed animals show higher levels of fibrillar collagen with disrupted structure between alveoli (*). (<b>G,H</b>) Second-harmonic imaging microscopy (SHIM) confirms that collagen between alveoli (bright white) remains fibrillar in THS-exposed animals. (<b>I</b>) Hydroxyproline (an amino acid that is highly present in fibrillar collagen) is much higher in lung tissue of THS-exposed animals than in the control. Alveoli in E–H marked by *. Scale bar in <b>A,B</b> is 100 µm, in <b>C,D</b> is 50 µm, in <b>E–H</b> is 20 µm. In <b>A–H</b> and in <b>I</b>. *** p<0.001.</p

THS exposure results in non-alcohol fatty liver disease (NAFLD) with concomitant dyslipidemia.

<p>(<b>A,B</b>) Livers from mice exposed to THS for 24 weeks are a much paler pink than that of the control, an indication of fatty liver. (<b>C,D</b>) Liver cryosections from mice exposed to THS and stained with Oil-Red-O show increased lipid content and greater lipid aggregation than in the control. (<b>E</b>) Quantification of total hepatic lipid content shows lipid levels ∼2.5 times that of the control, with the majority of this increase being triglyceride accumulation (<b>F</b>). (<b>G</b>) Plasma triglyceride levels of mice exposed to THS were significantly increased. (<b>H,I</b>) Mice exposed to THS showed increased plasma LDL-Cholesterol and decreased HDL-Cholesterol levels. * p<0.05, ** p<0.01, *** p<0.001. Scale bars = 100 µm.</p