Les résultats des études cliniques récentes évaluant l'impact de l'augmentation du HDL-C sur la survenue des maladies cardiovasculaires n'ont pas permis de démontrer une efficacité significative de cette stratégie thérapeutique ciblée sur le HDL-C. L'évaluation de la fonctionnalité des particules HDL dans sa capacité à assurer le retour du cholestérol des tissus périphériques au foie s'avère plus importante que la simple mesure du HDL-C. L'objectif de mes travaux de recherche a été d'évaluer la fonctionnalité des particules HDL en fonction du contexte métabolique, inflammatoire et génétique de chaque individu. Mes travaux de recherche ont démontré que la niacine réduit l'athérogénicité des lipoprotéines durant la période postprandiale modulant par conséquent la fonctionnalité des particules HDL dans la voie du transport inverse de cholestérol. J'ai également montré que certaines pathologies infectieuses associées à des taux bas de HDL-C, entrainent une baisse de la capacité des particules HDL à stimuler l'efflux de cholestérol à travers le macrophage humain. En revanche, j'ai montré que des variations génétiques entrainant des taux élevés de HDL-C ne sont pas associées à une dysfonction des particules HDL dans la voie du transport inverse de cholestérol. Mes travaux soulignent l'importance de considérer que la fonctionnalité des particules HDL est modulée in vivo par des contraintes métaboliques inflammatoires et génétiques d'où il est nécessaire que les thérapies en cours de développement visant à augmenter la fonction des HDL soient résistantes aux modifications métaboliques et inflammatoires afin d'assurer une athéroprotection efficace. / HDL cholesterol as a therapeutic target has been the focus over the past 20 years mainly because its plasma concentrations are inversely correlated with the risk of coronary events. However, recent clinical trials involved the addition of niacin or CETP inhibitors to a statin in patients with CHD who had well controlled LDL-C levels have failed to show evidence of a significant reduction in cardiovascular events. Studies on the capacity of HDL to promote reverse cholesterol transport pathway have suggested that this is a better predictor of prevalent atherosclerotic disease than HDL-C itself. The main purpose of my project was to evaluate the impact of metabolic context, inflammation and genetics on modulation of HDL functionality. I have shown that extended release niacin/laropiprant treatment reduces atherogenic postprandial lipoproteins in patients with metabolic disorders stabilized by statins, having consequences on reverse cholesterol transport pathway and HDL functionality. Moreover, I have shown that infectious diseases such as HIV, induce an alteration in the capacity of HDL particles to stimulate cholesterol efflux from human macrophages. I have also shown that mutations causing high levels of HDL-C, are characterized by the presence of functional HDL in their capacity to stimulate reverse cholesterol pathway. However such patients are not systematically protected against cardiovascular disease highlighting the complex relationship between cholesterol efflux and the prediction of cardiovascular events. Those results point out the relevance of developing therapeutic approaches targeting HDL function and reverse cholesterol transport pathway.

Nicotinic acid (NA) has been clinically used for over 50 years to regulate lipid plasma levels. It is the only drug in current clinical use that significantly raises HDL cholesterol and reduces inflammatory markers. However, mechanistic understanding into its wide range of actions remains unclear. The recent identification of the Gi-coupled protein receptor HCAR2, for which NA is a potent agonist, provides intriguing insight due to its anti-lipolytic action and restricted, yet specific, expression in adipose tissue and immune cells. The HCAR2 gene is 96% homologous to HCAR3, but the HCAR3 receptor shares neither the specificity for NA, nor the range of functional effects. Moreover, the close homology makes it difficult to separate the genetic variability and regulation of the two genes. To this end, I resequenced HCAR2 and HCAR3 in a selected population to characterize the variability of the two genes and to inform the subsequent design of specific genotyping assays. The Oxford Biobank, which is a random population-based collection of 30-50 year old men and women in Oxfordshire with a wide range of collected phenotypes, was used to explore genetic associations. A preliminary trend with HDL and rs7314976 in HCAR2 motivated the further search associations. However after increasing the sample size, the HDL association did not reach significance. When looking at inflammatory phenotypes, a 20% lower level of systemic hsCRP was found in males with a promoter region variant in HCAR3 (N=1808, p=0.007 for rs55718746). Replication of this finding in two relevant cohorts (NPHS-II, N=2185 and Whitehall, N=4228) resulted in conflicting findings. After optimising the specific detection of both HCAR2 and HCAR3 transcripts, I characterized gene expression in human AT biopsies. This revealed an 18% increase in HCAR2 expression in the female abdominal depot (N=106, p<0.0001) and a reduction in abdominal HCAR2 in both males (β=-0.37, p<0.001, N=107) and females (β=-0.251, p=0.005, N=106) with increasing adiposity. The rs55718746 variant in HCAR3 was also seen to influence expression of both HCAR2 (N=182, p=0.018 in the abdominal depot) and HCAR3 (N=198, p=0.005) but surprisingly in opposite directions, establishing it as the first cis-eQTL for this genomic region. Finally, I used human adipocyte in vitro culture systems to setup a pilot to study the anti-inflammatory effects of NA. The gene expression of HCAR2 and HCAR3 increased significantly with adipocyte differentiation in vitro. NA led to a drop in IL-6 transcript abundance in two out of three of the in vitro differentiated human adipocytes. In conclusion, genetic variability in HCAR2 and HCAR3 shows weak associations with cardiovascular disease risk phenotypes relating to their respective pathways. The relevance of HCAR2 and HCAR3 gene expression and the role of the receptor in the control of inflammation will require further studies.

In schizophrenia, subtle aberrations in the brain cause functional disturbances like psychotic symptoms and social disability. There are, however, also disturbances outside the CNS indicating a systemic manifestation in the disease. The aim of the present thesis was to gain deeper understanding of the pathophysiological mechanisms underlying schizophrenia with a particular interest in peripheral and systemic manifestations with relevance for the increased risk of obesity and metabolic complications seen in the disease. Therefore, resting energy expenditure (REE), physical capacity, and relevant body composition variables were measured in patients with schizophrenia and in healthy controls. Also niacin skin flush response and electrodermal activity (EDA) were studied. Patients with schizophrenia exhibited significantly lower REE expressed as kJ/kg, and also lower values compared with predicted levels than the controls. The difference could not be attributed to medication or variations in body composition between the two groups. There was a gender difference with the lowest levels found in male patients. Male patients exhibited significantly lower physical capacity in terms of predicted maximal oxygen uptake capacity and faster increase in respiratory quotient than male controls. The oral niacin test revealed a significantly delayed skin flush reaction in patients compared with controls. The patients also exhibited lower EDA response. There was a significant association in response patterns for the niacin and the EDA tests in the patients, but not in controls. In a test-retest study in patients there was acceptable stability for EDA measures but low test-retest stability for niacin variables. The previously found association in responses for the two tests was, however, replicated. The results gain support for the concept of schizophrenia as a disease with systemic manifestations including metabolic dysregulation. The findings add to the understanding of the weight gain and the increased risk for cardiovascular morbidity seen in this condition.

Magister Pharmaceuticae - MPharm / Cardiovascular diseases (CVDs) such as ischaemic heart diseases, heart failure and stroke remain a major cause of death globally. Various deep-rooted factors influence CVD development; these include but are not limited to elevated blood lipids, high blood pressure, obesity and diabetes. A considerable number of proteins are involved directly and indirectly in the transport, maintenance and elimination of plasma lipids, including high and low-density lipoprotein cholesterol (HDL-C and LDL-C). There are several mechanisms involved in the removal of LDL particles from systemic circulation. One such mechanism is associated with the gene that encodes proprotein convertase subtilisin/kexin type 9 (PCSK9), which has become an exciting therapeutic target for the reduction of residual risk of CVDs. Currently, statins are the mainstay treatment to reduce LDL-C, and a need exists to further develop more effective LDL-C-lowering drugs that might supplement statins. This study was aimed at contributing to the generation of knowledge regarding the effect of niacin in reducing LDL levels through PCSK9 interaction. The aims/objectives of this study were achieved by utilizing two approaches, which included animal intervention with niacin followed by genetic screening of five prioritized genes involved in cholesterol synthesis and regulation. For animal intervention, 16 vervet monkeys were divided into two groups of eight animals consisting of a control and an experimental (niacin) group. The control group was given a normal standard diet of pre-cooked maize meal throughout the study, while the experimental group received the same diet supplemented with 100 mg/kg of niacin (SR) for 12 weeks. During the niacin intervention, blood was collected at baseline, every four weeks during the treatment period and the end of the washout period. The collected blood was used for biochemical analysis (total cholesterol, triglycerides, LDL-C, and HDL-C) and downstream genetic applications. The second phase included the screening of PCSK9, LDLR, SREBP-2, CETP and APOB-100 using genotyping and gene expression. Niacin administration produced statistically significant increases in plasma HDL-C at fourtime points (T1, T2, T3 and T4), which resulted in an overall increase in plasma HDL-C. Additionally, niacin administration resulted in a slight reduction in LDL-C and total cholesterol levels. Furthermore, the genotyping analysis revealed 13 sequence variants identified in PCSK9, LDLR, SREBP-2, CETP and APOB-100 genes. Five of these variants were predicted to be disease-causing and correlated with gene expression patterns. Three identified PCSK9 variants (H177N, R148S, G635G) were categorized as LOF mutations, and this was supported by a decline in gene expression in animals harbouring these variants. The LDLR also had LOF variants that were the reason for its decreased mRNA expression. Additionally, SREBP-2 proved to be a key mediator of cholesterol pathways. Therefore, the findings of the study conclusively suggest that niacin does increase HDL-C and decrease LDL-C and total cholesterol. Moreover, an interaction between niacin administration and PCSK9 was observed which resulted in decreased gene expression.

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