As one of the most important proteins in human lipid metabolism, apolipoprotein E (apoE) is also one of the most well studied apolipoproteins. ApoE has an undisputable role in the plasma clearance of atherogenic triglyceride-rich lipoproteins via receptor-mediated hepatic uptake. For years, apoE has been touted to play a protective role in atherosclerosis. Both quantitative and qualitative changes of apoE are well-known to cause a number of rare dyslipidaemia syndromes.
Until recent times, research efforts have mainly focused on the discovery of the differential risk of atherosclerotic vascular disease and Alzheimer’s dementia associated with specific apoE isoforms. Despite contemporary knowledge on the effect of common apoE alleles on cardiovascular risk, little is known if the plasma levels of the gene product, i.e. apoE, is also independently associated with cardiovascular risk. There have also been studies that demonstrated possible links between apoE and renal diseases. Recent population-based studies on healthy, mostly non-diabetic subjects have yielded conflicting results. To this day, no consensus has yet been reached regarding the role of plasma apoE concentration in the prediction of atherosclerotic and renal disease outcomes. Our study, based on the subjects of the FIELD randomised control trial cohort, is the largest one to-date on plasma apoE levels on a purely diabetic population. We were the first to characterise and describe, in detail, the apoE concentration profile in type 2 diabetic subjects, and its relationship with other biochemical markers and clinical measures. We also confirmed that some of these variables are also, in fact, independent predictors of the apoE levels in diabetes.
In Chapter 3, we described in detail the statistical distribution of plasma apoE concentration in the FIELD cohort. We obtained the reference range for type 2 diabetic subjects and compared this with those of the healthy non-diabetic populations from previous studies. We assessed the statistical correlation between diabetic plasma apoE concentration with other important (apo)lipoproteins, biochemical markers and clinical parameters. We compared the clinical and biochemical differences between individuals in the highest and lowest quintiles of apoE and described the metabolic phenotypes associated with both high and low plasma ApoE levels. We found that diabetic apoE concentration is on average higher than non-diabetics, and that apoE concentration in diabetics is quite strongly correlated with female gender, systolic and diastolic blood pressures, smoking, atherogenic lipoproteins, body-mass index as well as C-peptide and hepatic transaminases. These parameters were also present, or higher, in the highest apoE quintile subjects compared to the lowest. Thus, we were able to deduce that apoE levels were highest in those who smoked and were obese, and who had hypertension, dyslipidaemia, hepatic steatosis, increased insulin resistance and those with features of metabolic syndrome – these are clearly adverse cardiometabolic features.
In Chapter 4, we evaluated the influence of various important clinical and biochemical variables in the prediction of diabetic plasma apoE concentration, using multi-variable linear regression analysis, also taking into account the complex interactions amongst the covariates themselves. We identified the strongest independent predictors of diabetic apoE plasma levels and compared the influence of each predictor in relation to one another. We once again confirmed clinical and biochemical parameters most strongly correlated statistically with apoE plasma levels were also independent predictors of apoE concentration in type 2 diabetes, and that these predictors accounted for up to 29% of a diabetic’s circulating apoE level. Importantly, age was found to be a significant independent predictor in multi-variable regression analysis, albeit not a univariate predictor, with this finding being consistent with other studies on non- diabetic populations. Furthermore, we detected significant statistical interactions between at least some of the covariates, confirming that complex interplay exists between the predictors to influence circulating apoE levels.
In Chapter 5 and 6, we assessed the risk of adverse cardiovascular outcomes and diabetic nephropathy in the FIELD cohort in relation to their baseline apoE plasma concentration. We confirmed that circulating apoE concentration was not a direct predictor of adverse cardiovascular outcomes or diabetic renal disease. We proposed that apoE is, rather, a composite surrogate biomarker for metabolic syndrome and an adverse cardiovascular body- type.
Finally, in Chapter 7, we compared the plasma apoE concentration profiles in male and female diabetics of the FIELD cohort, based on earlier finding that gender was a strong independent predictor. Female diabetics had higher mean apoE levels than diabetic man. We identified a number of other clinical and biochemical parameters that interacted significantly with gender to influence apoE levels in both genders, which included age, smoking, diabetic nephropathy, ApoCIII, triglyceride, total bilirubin and a number of medications commonly prescribed in type 2 diabetes.
The aim of this thesis is to provide further insight to the controversial field of apoE and cardiovascular disease. We sought to add to the current evidence our findings in a population which has arguably the highest risk of atherosclerosis. In the context of recent conflicting observations from large population-based study, we demonstrated comprehensively that circulating apoE concentration is not a direct biomarker for cardiovascular diseases or diabetic nephropathy. On the other hand, we established that plasma ApoE is a readily measurable surrogate marker of metabolic syndrome and adverse cardiovascular phenotype.
Traditionally, metabolic syndrome is a clinical diagnosis based on a number of diagnostic features that may not be easily quantified. Plasma apoE level thus serve, potentially, as an objective composite score of metabolic dysfunction. Early identification of at-risk individuals enables aggressive preventative therapy against cardiovascular complications. As such, we hope that this thesis will provide impetus for further research into the utility of apoE plasma concentration as a screening and diagnostic tool for metabolic syndrome and also, potentially, an accessible therapeutic target for this condition.