Histone modifications have been closely associated with changing levels of gene expression, but their role in determining, or possibly predicting, patterns of expression is uncertain. Here, the link between histone modifications and Hoxb gene expression in mouse embryonic stem (ES) cells was explored. Levels of the “active” modifications H3K9ac and H3K4me3 at Hoxb promoters varied widely from gene to gene, but were closely correlated in ES cells. Contrastingly, the repressive modification H3K27me3 was found at equivalent levels across the cluster. Treatment with the histone deacetylase inhibitor valproate induced a coordinate increase in the levels of H3K9ac and H3K4me3 at all Hoxb promoters, but not other genes, whilst H3K27me3 was unaffected. Such increases were not maintained upon removal of the inhibitor. All Hoxb genes were silent in undifferentiated ES cells, but expression was activated at defined times of differentiation in the expected 3’ to 5’ sequence. The valproate induced increase in active modifications did not induce Hoxb expression from the cluster in undifferentiated cells, nor was there any major shift in the timing of Hoxb expression in cells transiently exposed to valproate (ie. hyperacetylated) during the start of differentiation. Thus, active histone modifications at the Hox genes are uncoupled from transcription.

Placental growth factor (PlGF) is a pro-angiogenic and inflammatory mediator that promotes many pathological conditions including, diabetes, atherosclerosis and cancer. In mouse models, the loss of PlGF or inhibition of vascular endothelial growth factor receptor-1 (VEGFR-1) activity suppresses these disorders. Hyperglycaemia plays a fundamental role in the pathogenesis of type-2 diabetes and associated conditions, resulting in a loss of PI3 kinase (PI3K) signalling and dysfunction in endothelial cells. Using pharmacological inhibitors, siRNA, and adenoviral constructs to modulate the PI3K/Akt signalling activity, I found that the induction of PlGF expression in human umbilical vein endothelial cells (HUVEC) by hyperglycaemia is PI3K/Akt-dependent. Using similar approaches, the FOXO1 transcription factor was identified as the downstream target of Akt involved in the regulation of both PlGF and VEGFR-1 expression. FOXO1 was found to interact directly with the VEGFR-1 gene promoter in vitro, and over-expression of constitutively-active FOXO1 promotes PlGF expression in vivo. Although VEGF activates PI3K/Akt, it stimulates robust PlGF release in endothelial cells. Here I show that this effect is both VEGFR-2 and PKC-dependent, but independent of PI3K/Akt. The PI3K/Akt/FOXO1 axis is an important regulator of vascular homeostasis and stress responses and the identification of its involvement in PlGF expression may provide new therapeutic targets for disorders characterised by endothelial dysfunction.

The studies reported in this thesis aimed to identify the pattern of variation across the CYP11B1/CYP11B2 locus in order to determine the genetic variation responsible for the observed impaired 11ß-hydroxylation and its link with aldosterone levels and hypertension. In Chapter 3, an attempt was made to identify polymorphisms in CYP11B1 that associate with hypertension and a raised aldesterone-to-renin ratio (ARR) and therefore might contribute to the genetic component of these phenotypes. The study in Chapter 4 aimed to analyze the pattern of variation across the CYP11B2/B1 locus in detail. In Chapter 5, the frequency and phenotypic associations of the newly identified CYP11B1 5’UTR polymorphisms -1889 G/T and -1859 A/G were investigated in a hypertensive population. The frequencies were found to be -1889 G-0.53, T- 0.47; -1859 A-0.5, G-0.5. Interestingly, the -1889 G/T and -1859 A/G polymorphisms were found to be associated with impaired 11ß-hydroxylase efficiency in a hypertensive population. The THS/total F ratio (index of 11ß-hydroxylase activity) was significantly higher in -1889 TT homozygotes than in GG homozygotes (p=0.025). A similar pattern was seen with the -1859 SNP, with GG homozygotes tending to have a higher ratio than AA homozygotes (p=0.056). These steroid data were supported by the results from Chapter 6 that examined the effects of these polymorphisms in vitro using luciferase reporter gene constructs transfected into Y1 mouse adrenal cells. In summary, these studies have confirmed that the CYP11B2 and CYP11B1 genes contain a high frequency of genetic variation. Many of the variants identified in CYP11B1 have been shown to alter activity significantly. There is strong evidence to suggest that the impaired 11ß-hydroxylase efficiency previously associated with the -344 C/T and IC variants in CYP11B2 is due to linkage with the -1889 G/T and -1859 A/G polymorphisms upstream of CYP11B1.

No description available.