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Title: The nicotinic acid receptor in human adipose tissue
Author: Chamas, Liliane
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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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.
Supervisor: Karpe, Fredrik; Neville, Matthew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Genetics (life sciences) ; Metabolism ; Medical Sciences ; Cardiovascular disease ; Genetics (medical sciences) ; niacin ; nicotinic acid receptor ; genetics ; polymorphisms ; human ; adipose tissue