Worldwide, cardiovascular disease is the leading cause of death, with air pollution and smoking being major contributors. Epidemiological studies on air pollution have demonstrated that people living in areas with poorer air quality are at greater risk for hospitalizations, morbidities, and mortality due to cardiovascular and respiratory events. Tobacco cigarette smokers are also at risk for similar events, while users of the increasingly popular electronic cigarette (e-cig) are known to experience symptoms such as coughing and reduced lung function.
While much is known about the cardiovascular health effects of air pollution and tobacco cigarette smoking, though, there remain important knowledge gaps – (1) what are the molecular mechanisms linking air pollution with disease that extends beyond the lungs, and (2) are e-cig users still at risk for developing cardiovascular disease like tobacco smokers?
Our research with a model air pollutant, disease exhaust particles (DEP), therefore focused on the role of a particular cell type, the macrophage, in the mechanism behind the toxicity of air pollution. This portion of the work utilized experiments conducted on cells and mice to dissect gene-environment interactions. For our research regarding e-cigs, this focused on analyzing heart rate and oxidative status to look at cardiovascular function. For the work here, two groups of subjects, habitual e-cig smokers and non-smokers, were compared.
In this dissertation, Chapter 1 will lay out the foundation for the issues of air pollution and smoking, particularly e-cig smoking, and explain the need to study the toxicological mechanisms behind their health effects. Chapter 2 describes an in vitro study that takes advantage of two powerful tools for studying the role of genes in responses – the Hybrid Mouse Diversity Panel (HMDP) and Affymetrix microarrays. Peritoneal macrophages isolated from HMDP mice were treated with an extract of DEP, and their gene expressions profiles were analyzed and compared in order to determine key pathways. Following that discussion, Chapter 3 describes a study using myeloid-specific Nrf2 (nuclear factor, erythroid derived 2, like 2) knockout (mNrf2 KO) mice, which were developed using the Cre-Lox technique, in order to determine the role of this antioxidant transcription factor in hyperacute in vivo responses. To characterize the physiological responses, echocardiographies and cardiac catheterizations were performed. Additionally, to characterize the genetic responses in the lungs, we used a recently established tool called Drop-seq to perform single cell RNA sequencing (scRNA-seq). And lastly, Chapter 4 focuses on the cardiovascular effects of e-cig smoking in human subjects. The work here is the result of a collaborative effort between our lab and a clinical team led by Dr. Holly Middlekauff.
Two supplementary video files are also a part of this dissertation. Both videos relate to Chapter 3 and show the behavior of mice exposed to DEP, with one of the mice being a genotypic control mouse (DEP-Treated-Control-Mouse.MOV) and the other being an mNrf2 KO mouse (DEP-Treated-KO-Mouse.MOV).
Overall, our results demonstrated that DEP treatment elicited in vitro responses in macrophages that were driven by the NRF2 antioxidant transcription factor and conducive to polarization into a Mox or Mox-like macrophage subtype. Additionally, mice that were knocked out for Nrf2 in macrophages and other myeloid cells were highly susceptible to DEP exposure, showing signs of diastolic dysfunction and a dysregulated inflammatory response resulting in pro-inflammatory macrophages. Our results also showed that e-cig use led to harmful effects on the cardiovascular system, including reduced heart rate variability and increased systemic oxidative stress in the form of increased oxidizability of low-density lipoproteins (LDL).
The research work carried out here offers insight into a potential mechanism explaining DEP-induced cardiovascular effects and why certain people could be more susceptible to air pollution exposures. Our work also provides a greater understanding of the little-known chronic consequences of using e-cigs and could be a warning to those considering using these devices.