Estrogen and liver X receptors in human disease

Abstract

The nuclear hormone receptors (NRs) are a class of transcription factors that has attracted great interest due to their important roles in animal physiology and metabolism. Studies of knockout (KO) mouse models have indicated several pathophysiological conditions in which the NRs are involved. Four NRs, the estrogen receptors (ERs) α and β, and liver X receptors (LXRs) α and β, are studied in this thesis. The ERs have been implicated in several human diseases such as breast cancer, osteoporosis and metabolic disorders such as obesity. ERs exert their regulatory functions upon activation by the steroid hormone 17β-estradiol and structurally related molecules. The LXRs are key players in cholesterol-, lipid-, and carbohydrate metabolism and are activated by certain oxidized cholesterol derivatives. Due to their putative role in human metabolic disease, LXRs are interesting drug targets for pharmacological treatment of metabolic disorders. Single nucleotide polymorphisms (SNPs) are DNA base pair changes that occur at a frequency of more than 1% in a population. SNPs could potentially alter the function of a gene product and are often investigated in relation to human disease. Common SNPs are genotyped in disease cases and healthy controls. If the SNP frequencies differ between cases and controls, the SNP is said to be associated with the investigated disease. The impact of genetic variations in ER and LXR on human disease, in particular the impact of SNPs in ER and LXR in relation to human metabolic and eating disorders, is the focus of this thesis.

As estrogen has been reported to have a role in feeding behaviour, we screened the ERβ gene for variants in bulimic patients and healthy controls (paper I). We report association of two ERβ SNPs and bulimic disease. In addition, we identified a point mutation in the ERβ hinge region in one bulimic patient. Initial characterization of this mutation did not reveal any functional effects compared to the ERβ wild type protein. This study suggests that genetic variations in ERβ could be important for the etiology of eating disorders.

LXRα and LXRβ gene SNPs were identified and genotyped in obese subjects and non-obese controls (paper II). We found association of one LXRα and two LXRβ SNPs with obesity phenotypes. In addition, we observed increased expression of LXRα in obese female subjects. Furthermore, there was evidence of an interaction between LXR alleles in determining body mass index (BMI).

Previous human genetic studies support a role for ERα in obesity. While two investigated ERα SNPs were not associated with female obesity, the expression of ERα was decreased in both adipose tissue and isolated adipocytes in obese compared to healthy control females (paper III). In addition, expression of ERα increased after a modest weight reduction supporting a protective role for ERα in maintaining body weight.

In view of recent reports establishing the LXRs, and particularly LXRβ, in glucose homeostasis, we genotyped three LXRβ SNPs in subjects with impaired glucose tolerance (IGT), type-2 diabetes (T2D) as well as in healthy controls (paper IV). One LXRβ promoter SNP and one haplotype were weakly associated with T2D. Characterization at the molecular level of these SNPs suggested effects on the LXRβ protein and gene promoter, respectively. In summary, our studies suggest a role for NR polymorphisms in the etiology of human disease.