TRIB3 regulates adipose tissue expansion and macrophage function affecting atherosclerosis development

Chronic inflammation is one of the key processes in the development of metabolic dysfunction under obesity which represents a major risk for cardiovascular disease. The metabolic and immune responses to obesity affect many organs and particularly the adipose tissue where macrophages are key effector cells modulating inflammation through the signals received from adipocytes. The Tribbles (Trib) family of proteins has been described as major regulators of inflammation and energy metabolism. Specifically, Trib3 participates in a wide range of inflammatory and metabolic signalling pathways and plays a key role in the regulation of adipose homeostasis, lipid and glucose metabolism and macrophage lipid uptake. I propose that Trib3 regulates metabolic and immune responses to obesity modulating adipose tissue expansion and affecting atherosclerosis development. The aims of this study were to (1) investigate morphological changes in the adipose tissue of Trib3 full body knock out (Trib3KO) mice; (2) study the role of Trib3 regulating macrophage function and polarisation; (3) assess Trib3-mediated changes in the development of experimental atherosclerosis. To investigate these aims in vivo, the phenotype of Trib3 full body knock out (Trib3KO) mice was assessed. To analyse the molecular mechanisms by which Trib3 affects adipose tissue function, histological analysis of the tissue and gene expression analysis of pre-adipocytes isolated from the stromal vascular fraction were carried out. Bone marrow derived macrophages (BMDMs) from Trib3WT and Trib3KO mice were isolated and polarised in vitro to further characterise their morphology, metabolism and pro- or anti- inflammatory properties. Metabolic tissues, circulating lipid levels and atherosclerotic plaque formation were analysed in Trib3WT and Trib3KO mice, injected with rAAV8/mPCSK9 and fed with western diet for 12 weeks. In addition, BMDMs were treated with oxLDL to mimic the plaque microenvironment and characterise the effects of Trib3 deletion in extracellular matrix remodelling function, affecting plaque stability. Phenotypic analysis of Trib3KO mice showed that loss of Trib3 increases body weight via increased adiposity. The role of Trib3 in adipose expansion is depot dependent, significantly affecting subcutaneous adipocyte proliferation. Genetic deletion of Trib3 modulates macrophage function, promoting changes in morphology, metabolism and polarisation linked to a pro-inflammatory macrophage phenotype. The analysis of Trib3 deletion in experimental atherosclerosis showed a reduced advanced plaque development and an increase in plaque stability. The fibrous cap was thicker in plaques from Trib3KO mice with an increased collagen content paired with an increased macrophage content within the plaque. Trib3KO foam cells showed a downregulated expression of genes involved in extracellular matrix remodelling. 15 In conclusion, genetic deletion of Trib3 appears to promote a metabolic “healthy obese” phenotype with increased body weight and subcutaneous adipose expansion via adipocyte hyperplasia and altered adipose inflammation by a decreased macrophage content. In addition, TRIB3 regulates macrophage gene expression ultimately affecting atherosclerosis development in vivo. Mechanistically, TRIB3 controls plaque stability via metalloproteinase expression in macrophages, thus inhibiting collagen and elastic fibre degradation, ultimately increasing fibrous cap thickness.