An investigation into the interrelationship between HDL particle composition and anti-atherosclerotic functions inpatients with cardiometabolic disease

Diabetes (DM) is one of the fastest growing global health emergencies of the 21st century. People with type 1 diabetes mellitus (T1DM) and people with type 2 diabetes mellitus (T2DM) have an increased risk of cardiovascular disease (CVD), and furthermore, women with T1DM/T2DM have an increased risk relative to men with T1DM/T2DM. High-density lipoprotein particles (HDL-P) exert numerous atheroprotective functions including the ability to promote cholesterol efflux from peripheral cells (cholesterol efflux capacity (CEC)), antioxidant and anti-inflammatory functions. HDL-CEC, the first step of the reverse cholesterol transport pathway, is widely considered the dominant mechanism underpinning the cardioprotective properties of HDL. Small HDL-P (S-HDL-P) facilitate cellular efflux via the ATP-Binding Cassette Subfamily A Member 1 (ABCA1) cholesterol transporter (ABCA1-dependent efflux), while larger HDL-P (L-HDL-P) mediate efflux via ATP-binding cassette subfamily G, member 1 (ABCG1) and Scavenger Receptor Class B Type 1 (SR-B1) pathways (ABCA1-independent efflux). HDL-P carry approximately 250 different proteins known as the HDL proteome. The functions of the proteins associated with HDL align strongly with the assigned functions of the particles indicative that measuring HDL associated proteins may serve as novel biomarkers of HDL function. In this thesis, we hypothesised that the HDL proteome is impacted in T1DM/T2DM resulting in the generation of dysfunctional HDL-P contributing to enhanced CVD risk within these populations. We further hypothesised that the HDL proteome may serve as a surrogate biomarker for HDL-CEC and sought to identify key proteins governing HDL-CEC in people with/without metabolic disease. In Chapter 3, we investigated the relationship between HDL-associated proteins and HDL-CEC, and furthermore developed novel biomarker panels and a scoring algorithm for total CEC, ABCA1-dependent CEC and ABCA1-independent CEC. Our Receiver Operating Characteristic area under the curve was 0.859, P=0.0001 for total efflux score, 0.862, P=0.0001 for ABCA1-dependent efflux score and 0.781, P=0.0002 for ABCA1-independent efflux score, highlighting the ability of the HDL proteome to accurately predict HDL-CEC. In Chapter 4, we investigated the impact of T1DM, T2DM and obesity on HDL efflux capacity, particle size and HDL sub-fraction proteomic composition, to gain greater understanding of the residual CVD risk present in these high-risk populations that is not attributable to traditional risk factors. We found that different proteins are associated with HDL-CEC conditional on T1DM/T2DM status e.g. HDL-associated ApoA-I and ApoA-II lost their association with HDL-CEC in the setting of DM. We also demonstrated that separation of HDL into L-HDL-P and S-HDL-P provided greater insight into the association of HDL proteins with ABCA1-independent and ABCA1-dependent CEC. Lastly, in Chapter 5, we examined sex-specific differences in HDL protein composition and function in people with/without T1DM, T2DM and obesity to gain greater insight into potential mechanisms driving loss of the female 'advantage’ against CVD in DM. We report that women with both T1DM and T2DM are disadvantaged in terms of HDL-CEC relative to male counterparts. We also report sex-specific differences in the proteins governing HDL-CEC in women versus men that may have important implications for their respective CVD risk profile. Indeed, our findings indicate the use of separate biomarkers to predict HDL function in men and women. Our findings have collectively demonstrated that the HDL proteome tightly aligns with HDL-CEC, but proteins governing HDL-CEC differ in those with/without DM, and also differs between men and women. We have further demonstrated that measuring the proteomic composition of L-HDL-P vs. S-HDL-P, and in turn the function of L-HDL-P and S-HDL-P was critical to truly untangle the complex relationship between HDL structure and function.