My laboratory focuses on the response of the lung to occupational and environmental exposure to toxic substances and the mechanisms of increased susceptibility to lung diseases in aging. We use a combination of molecular biology and biochemistry approaches to investigate the signal transduction and cellular adaptation in response to the production of reactive oxygen species and other electrophiles that underlie the effect of toxic substances. Our projects involve gene responses to xenobiotic compounds and particulates that can cause either pulmonary inflammation or adaptive responses.

Glutathione (GSH), the major antioxidant produced in human cells, is key in both resistance and adaptation to oxidative stress, but its concentration in tissues declines with age. Our focus is primarily on lung diseases and inflammation caused by air pollution. Interestingly, several lung diseases, including interstitial pulmonary fibrosis, and emphysema, increase with age and have been associated with a decrease in GSH. Similarly, epidemiological evidence suggests that susceptibility to the toxicity of air pollutants that cause oxidative stress in the lung, also increases with age. GSH synthesis can be induced by mild oxidative stress as an adaptive response. Our principle specific hypothesis is that the signal transduction regulating synthesis of GSH decreases in all tissues with age and underlies both the development of lung diseases as well as the increased susceptibility to environmental pulmonary diseases with age.

The rate limiting enzyme in GSH synthesis is glutamate cysteine ligase (GCL), a two subunit enzyme. Increased transcription, mRNA stability, and protein of both GCS subunits, along with increased enzymatic activity and GSH concentration have been demonstrated in many tissues in response to non-lethal exposure to a large variety of electrophiles including reactive oxygen species. Another enzyme regulating GSH synthesis is g-glutamyl transpeptidase (GGT), located on the outer surface of cells.  GGT provides substrates for de novo GSH synthesis by breaking down extracellular GSH. GGT mRNA, protein and enzymatic activity also increase during adaptation to stress from oxidants and electrophilic xenobiotics.  The signaling pathways regulating GSH biosynthesis are being studied at the transcriptional and post-transcriptional levels. It is likely that the signal transduction pathways that cause GSH to increase in response to oxidative stress become less responsive with age.

One of the major challenges to public health is environmental and occupational exposure to airborne toxicants. Most of the predominant components of air pollution, ozone, nitrogen oxides and particulates produce oxidative stress in the lung. Particles, both ambient and manufactured, can produce oxidative stress from reactions, particularly if there are transition metals or polyaromatic hydrocarbons present on their surface. Another mechanism through which particles can produce oxidative stress is through stimulation of the respiratory burst of alveolar macrophages, which generates superoxide and hydrogen peroxide (H₂O₂). These reactive oxygen species can initiate inflammation through chemical oxidation of cellular components, but can also signal for processes in the macrophage that propagate inflammation. Generation of leukotrienes and prostaglandins, and activation of NF‑kB and AP-1, which regulate cytokine production, are among those processes. Our hypothesis is that the underlying mechanisms for particulate-induced inflammation involve H₂O₂ acting as a second messenger in multiple signaling pathways in which activation of protein kinases, reversible inactivation of protein tyrosine phosphatases, transient elevation of intracellular free calcium concentration, and activation of phosphocholine-specific phospholipase C play key roles. All of these signaling processes in which H₂O₂ are involved are modulated by the intracellular concentration of GSH. Thus, our second hypothesis is that a decrease in GSH, such as occurs in aging, underlies an increase in susceptibility to particulates.