Defining the Role of Adipokines in the Development of Chronic Obstructive Pulmonary Disease and the Related Metabolic Comorbidities

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death globally. Cigarette smoking is the primary risk factor, accounting for over 95% of cases in industrialized countries, including Australia. COPD is a chronic, irreversible disease with systemic effects, leading to comorbidities such as cardiovascular disease, skeletal muscle wasting, and metabolic disorders, which significantly impact hospitalization rates, mortality, and quality of life.

Adipose tissue, once considered merely an energy storage depot, is now recognized as a critical endocrine organ involved in energy metabolism and inflammation. Adipose tissue secretes adipokines that regulate homeostasis in healthy states and contribute to pathophysiology in diseases such as COPD. Clinical evidence shows adipose dysfunction and adipokine dysregulation in COPD patients, though the mechanisms underlying this relationship remain unclear. This thesis investigates adipose tissue function and adipokine regulation under cigarette smoking conditions and explores the role of adipokines in COPD and related comorbidities.

Cigarette smoke (CS) contains numerous toxic chemicals that induce oxidative stress and inflammation in the lungs, with potential effects on other organs such as the cardiovascular system, skeletal muscle, and adipose tissue. Previous studies have shown that CS induces insulin resistance, adipose dysfunction, and body weight loss by increasing lipolysis and energy expenditure in mice. However, the interaction between COPD and adipose dysfunction is not fully understood. Therefore, this thesis explores the impact of CS on adipose tissue and adipokine regulation in COPD.

The first aim of this thesis was to examine the effects of CS exposure on adipose tissue function and metabolic parameters in a pre-clinical model of COPD. The hypothesis was that CS exposure disrupts metabolic parameters and adipokine production within adipose tissue. Results showed that CS exposure caused significant lung inflammation, with increased total cells and neutrophils, as well as oxidative stress in both lung and adipose tissues. Smoking also led to body weight loss, glucose intolerance, and metabolic disturbances. Adipokine expression was dysregulated in both 2-month and 6-month smoking groups, and elevated levels of non-esterified fatty acids (NEFA) and triglycerides (TG) were observed in serum. Lipolysis-related proteins were also upregulated in adipose tissue under smoking conditions, indicating adipose dysfunction.

The second aim was to investigate the role of oxidative stress in adipose dysfunction and metabolic disturbances in COPD. Using the antioxidant apocynin, smoking-induced lung inflammation and oxidative stress were significantly reduced. However, apocynin treatment did not restore body weight or metabolism. Oxidative stress inhibition also reduced lipolysis, with decreased expression of adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) in adipose tissue and reduced NEFA and TG levels in serum. Additionally, oxidative stress reduction improved insulin sensitivity in adipose tissue and alleviated glucose intolerance.

The third aim was to identify adipokine-associated signalling pathways in lung tissue using a phospho-proteomics array. Analysis revealed that CS exposure activated several proteins in lung tissues compared to controls. Apelin, which was consistently altered in smoking-exposed adipose tissues, was identified as a key player in COPD progression. KEGG analysis indicated that the Apelin-related signalling pathway, involving the phosphorylation of AKT1 and RAF1, was activated in COPD. Additionally, apocynin treatment modulated phosphoproteins related to energy regulation and transcription factors, and Tnfα was implicated in COPD through alterations in MAPK1 and RPS6KA5 phosphorylation. These findings suggest the activation of signalling pathways like MAPK in COPD progression.

The fourth aim was to explore the interaction between adipocytes and lung epithelial cells in COPD. This was studied by treating differentiated 3T3-L1 cells with conditioned media (CM) from cigarette smoke extract (CSE)-treated lung cells. CM_CSE treatment led to significant alterations in adipokine expression, particularly Apelin, Adiponectin, and Leptin, indicating that lung epithelial cells influence adipocyte function through secreted factors. In contrast, direct exposure to CSE had minimal effect on adipokine dysregulation. Inflammatory markers, such as Tnfα and IL-6, were upregulated in CM_CSE-treated adipocytes, while only Tnfα increased in direct CSE-treated cells. CM_CSE treatment also impaired mitochondrial biogenesis markers and promoted lipolysis in adipocytes, with increased expression of HSL and ATGL, suggesting a pathological interaction between lung epithelial cells and adipocytes in COPD.

The final aim investigated the impact of adipocyte-released factors on lung cells. Factors secreted from adipocytes triggered oxidative stress in lung cells, leading to increased inflammation and apoptosis-related gene expression. Protein analysis showed increased levels of 19S, 20S, and HSP70, indicating a link between adipocyte-derived factors and lung cell dysfunction.

In conclusion, this thesis provides evidence for active communication between lung and adipose tissue in COPD. The findings suggest that adipokine dysregulation and oxidative stress play pivotal roles in the development of COPD and its comorbidities, including metabolic disturbances and systemic inflammation. By elucidating the lung-adipose tissue axis, this study contributes to a deeper understanding of COPD pathophysiology and offers potential therapeutic avenues targeting both local and systemic aspects of the disease.