Characterisation of human adipose tissue : ceramide metabolism, depot differences and evaluation of dysfunctionality

Abstract

Normal adipose tissue function is necessary for maintaining proper energy balance, as both excess and absence of this tissue lead to metabolic disturbances, such as insulin resistance. Adipose tissue can be dysfunctional in many ways, including disturbances in adipocyte size, lipolysis rate, adipokine secretion, inflammation, fibrosis or oxidative stress. Recently ceramides have been proposed as candidate molecules that mediate the development of adipocyte insulin resistance. The aim of this thesis is to evaluate different aspects of dysfunctionality in human adipose depots in relation to their potential contribution to insulin resistance and cardiovascular disease.

Paper I and II focus on the role of ceramides in human adipose tissue. We show that in obese women with high liver fat increased ceramide content in the subcutaneous adipose depot is most probably due to the increased sphingomyelin hydrolysis rather than de novo production. Moreover, sphingomyelinases, that are responsible for this reaction, are present in areas rich in apolipoprotein B, which may suggest that circulating lipoproteins may be a source of sphingomyelin for the local ceramide production within adipose tissue. Additionally, we show increased ceramide levels in the mediastinal as compared to the subcutaneous adipose tissue and show that ceramides in this depot are associated with inflammatory processes. In our unpublished data we demonstrate that ceramide induces inflammatory cytokine expression in both macrophages and adipocytes. Paper III investigates whether the mediastinal depot shows characteristics of brown adipose tissue. A comparison of several markers of brown and white fat between subcutaneous and mediastinal adipose tissue reveals that the mediastinal fat has higher expression levels of some brown (UCP1, PPARGC1A) and lower expression levels of white (SHOX9, HOXC8) markers. Gene ontology analysis indicates that mediastinal depot is enriched in genes related to mitochondrial function. In some sections of mediastinal fat positive UCP1 staining and presence of multilocular cells are observed. In Paper IV we investigate whether adipose tissue in patients with chronic kidney disease is dysfunctional. We report that subcutaneous adipose tissue in patients with kidney failure is characterized by higher numbers of phagocytic cells and smaller adipocytes, but shows no signs of fibrosis as compared to healthy subjects. Additionally, proteomic analysis shows differential expression patterns between the patients and controls. Among the proteins expressed at higher levels in the patients, alpha-1-microglobulin/bikunin precursor is the most significant and among those expressed at lower levels in the patients, the most significant is vimentin – a protein known to be involved in lipid droplet metabolism.

In summary, the work presented in this thesis demonstrates that adipose tissue ceramides can promote local inflammation, a process strongly linked to insulin resistance. Moreover, the mediastinal adipose depot shows some signs of brown fat, however the functional consequences remain to be evaluated. Finally, uremic adipose tissue shows adverse protein composition, which together with an increased number of phagocytic cells and smaller adipocyte size indicates that uremic adipose tissue is dysfunctional, which could lead to increased cardiovascular risk in chronic kidney disease patients.