CEACAM6: A Novel Marker of Chronic Obstructive Pulmonary Disease Susceptibility?

Chronic obstructive pulmonary disease (COPD) is a progressive disease typified by airflow obstruction, lung inflammation, and airway remodeling. COPD is a major global health burden and is predicted to be the leading cause of death worldwide by the year 2030. Current therapeutic strategies are sparse and largely ineffective (1, 2). Although cigarette smoke (CS) exposure is the primary risk factor for COPD development, only 25% of people who smoke develop COPD, highlighting the importance of predisposing genetic and epigenetic factors and of environmental exposures as key contributors to COPD pathogenesis (3–5). Long-term imbalances in oxidative stress and inflammation are believed to drive the airway remodeling and alveolar destruction observed in chronic bronchitis and emphysema (1). Likewise, extrapulmonary manifestations such as cardiovascular disease, osteoporosis, and skeletal muscle wasting are also linked to oxidative stress and inflammation. The uncovering of novel therapeutic strategies to treat patients with COPD depends on understanding how these dysfunctions mechanistically support the pathogenesis of COPD.

Increases in cellular nitric oxide, induced by inflammation, result in a type of oxidative stress known as nitrosative stress, which is marked by increases in 3-nitrotyrosine (3-NT) and nitrogen dioxide. 3-NT immunostaining and amounts of inducible nitric oxide synthase (iNOS) are abnormally increased in inflammatory cells present in sputum from patients with COPD and directly correlate with disease severity (1, 6, 7). These findings, which persist well after cessation of smoking, result in vascular damage and remodeling as a long-term consequence (8). Notably, studies in the murine chronic smoke exposure model show that blocking of nitrosative stress via inhibition of iNOS protects against development of emphysema (9, 10).

Heme oxygenase-1 (HO-1) is a key enzyme in heme biosynthesis and is known to be regulated by increases in various types of oxidative stress, including nitric oxide accumulation. Lung protein levels of HO-1 increase in the lung in response to oxidative stress and have been linked to disease states of asthma and COPD (11–16). In the smoke-induced rat emphysema model, augmenting HO-1 levels by delivery of hemin blunted the oxidative stress response and protected the development of emphysema (12). As such, the identification of HO-1 regulators represents an area of investigation with great potential to uncover new therapies in COPD.

In this issue of the Journal (pp. 1576–1590) Wu and colleagues report a series of studies that shed light on the contribution of nitrosative stress to COPD pathogenesis and the potential to leverage inherent stress defenses as novel therapies (17). They used human tissue and sputum samples to assess the level of 3-NT, as a marker of nitrosative stress across nonsmokers, smokers without COPD, and smokers with COPD. Disease severity by Global Initiative for Chronic Obstructive Lung Disease stage criteria positively correlated with 3-NT levels in sputum, lung tissue homogenates, and isolated human alveolar epithelial cells. Notably, they found that elevated 3-NT levels specifically in the alveolar septa predicted the degree of emphysema observed histologically. Because oxidative stress is a known consequence of CS exposure, the investigators took their data to suggest that smokers with no to mild COPD have protective mechanisms to blunt the long-term accumulation of reactive nitrogen species and the development of emphysema.

To determine the molecular mechanisms mitigating the development of COPD in a subset of smokers, they developed an in vitro system to generate CS extract–resistant cells. They used the A549 cell line, which derives from alveolar basal epithelial cells to generate the resistant phenotype. After treatment with CS extract, cellular morphology and proliferation were distinct between resistant and control cells. Notably, 3-NT, nitric oxide, and superoxide levels remained low in the resistant cells, supporting the hypothesis that they are primed for antioxidant functions. The resistant phenotype was not explained by alterations in either NOXO1 or iNOS, key regulators of the oxidative stress response.

Exploration of the antioxidant activity of the resistant cells revealed that HO-1 was upregulated, as both mRNA and protein within the A549 cells were resistant to CS extract. Further studies showed that the resistant state depended on the presence of HO-1, as loss of HO-1 increased cellular susceptibility to CS extract–induced nitrosative stress. Transcriptomics analysis of control and resistant cells identified several genes that were downregulated in the resistant state. Importantly, in resistant cells, increased HO-1 protein levels correlated inversely with CEACAM6, a molecule previously shown to support the development of multiple lung pathologies (18). Overexpression of CEACAM6 in resistant cells obliterated the protective gain of HO-1 protein, and 3-NT levels climbed. These data position CEACAM6 as a critical determinant of the antioxidant response in the alveolar epithelial cells in vitro.

Supporting the translational relevance of these findings, Wu and colleagues noted elevated CEACAM6 expression and a reduction in HO-1 levels in the alveolar septa of patients with COPD compared with smokers without COPD (17). Using an ex vivo model of CS extract exposure that leveraged human precision-cut lung slice technology, overexpression of CEACAM6 in healthy donor tissue increased both 3-NT levels and cell death in response to CS extract exposure. In addition, in peripheral blood mononuclear cells from patients with severe COPD, 3-NT levels correlated positively with CECAM6 and inversely with HO-1.

These highly compelling data from Wu and colleagues highlight the importance of nitrosative stress in the pathogenesis of COPD (17). They also identify a novel link between CEACAM6 and HO-1, whereby CEACAM6 post-transcriptionally regulates HO-1. Their findings emphasize the importance of HO-1 to stabilize oxidative stress in response to CS and potentially to prevent the development of the severe COPD phenotype. The confirmation of their in vitro findings in both lung tissue and peripheral blood mononuclear cells from patients with COPD supports the notion that CEACAM6 may serve as a biomarker of COPD severity. How the presence of alveolar CEACAM6 impacts the nitrosative stress in adjacent cell types (e.g., endothelial cells) is critical to understanding whether CEACAM6 could also be a potential therapeutic target. Notably, humanized CEACAM6 blocking antibodies were previously generated as a potential therapy in cancers originating in the epithelia (19). Further studies examining the utility of these antibodies in the in vitro and ex vivo COPD models and as a potential therapy for the treatment of COPD are of great interest.