Fatty acid transport proteins : candidate genes for the insulin resistance syndrome

Abstract

The insulin resistance syndrome is characterised by insulin resistance, obesity, hypertension, and a typical dyslipidaemia, predisposing to diabetes and cardiovascular disease. Metabolic disturbances of the insulin resistance syndrome include dysregulation of fatty acid (FA) homeostasis, for which adipose tissue serves a critical function. The fatty acid transport proteins (FATP1-6), of which FATP1 and FATP4 are proposed to be predominant in adipose tissue, are implicated in cellular FA uptake and acylCoA synthase (ACS) activity, thus potentially being regulators of FA homeostasis of relevance in relation to the insulin resistance syndrome. In this thesis, genetic variation within the FATP1 and FATP4 genes and their expression in adipose tissue have been investigated in relation to markers of the insulin resistance syndrome, in order to evaluate FATP1 and FATP4 as candidate genes for this common syndrome.

Among healthy middle-aged men, homozygotes for the rare A allele of an intron 8 G/A polymorphism in FATP1 showed elevated post-prandial lipaemia compared with G/A and G/G individuals, and carriers of the intron 8 A allele exhibited a smaller, more dense LDL particle phenotype. In vitro studies indicated that binding of nuclear factors and influence on transcriptional activity differed between the intron 8 alleles. These observations suggest that FATP l might play a role in post-prandial lipid metabolism of relevance for the development of the dyslipidaemia of the insulin resistance syndrome.

Heterozygotes for a rare Gly209Ser polymorphism in FATP4 had lower body-mass index (BMI), and correcting for BMI, lower homeostasis model assessment index, systolic blood pressure, and insulin and triglyceride concentrations compared with common homozygotes, among healthy middle-aged men. A three-dimensional model of the FATP4 protein revealed that the variable residue 209 is exposed in a region potentially involved in protein-protein interactions, and indicated functional FATP4 regions with respect to substrate binding, ACS activity, and membrane association. These findings propose FATP4 as a candidate gene for the insulin resistance syndrome, and structurally classifies FATP4 as an adenylateforming enzyme in accordance with its suggested ACS activity. In order to study the involvement of FATP1, FATP4 and other FA handling proteins in genetic and acquired obesity and insulin resistance, their adipose tissue mRNA expression levels were quantified in young monozygotic twins with a range of intra-pair differences in measures of obesity and insulin resistance, allowing discrimination between influences of genetic and non-genetic factors on the studied variables. FATP4 expression was found to be up-regulated in acquired obesity independent of genetic background. Moreover, mRNA levels of FATP4 and the FA transporter CD36 correlated with measures of obesity and insulin resistance influenced by both genetic and non-genetic factors. These findings indicate specific FA handling protein expression related to obesity and insulin resistance, in particular suggesting a role of FATP4 in acquired obesity.

To further investigate FATP1, FATP4 and other FA handling proteins in relation to obesity, insulin resistance and FA metabolism, their adipose tissue mRNA expression levels were analysed in healthy 63year-old men with detailed measures of FA composition in the serum non-esterified FA, serum phospholipid, and adipose tissue compartments. Acyl-CoA synthase-1 (ACS1) expression was negatively correlated with measures of obesity and insulin resistance, whereas an opposite trend was observed for FATP4. Correlations of ACS1 and FATP4 expression levels with FA composition variables of proposed significance in relation to insulin sensitivity were observed. These findings suggest a role of ACS1 and FATP4 with respect to obesity and insulin resistance, via their involvement in FA trafficking and metabolism.

Taken together, these studies indicate important roles of FATP1 and FATP4 in relation to obesity, insulin resistance and lipid metabolism, thereby proposing FATP1 and FATP4 as candidate genes for features of the insulin resistance syndrome.