Regulation of macrophage lipoprotein lipase gene expression by interferon-gamma

• Main Author: Evans, Sandra Michelle
• Published: Cardiff University 2005
• Subjects: 572
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583696

Lipoprotein Lipase (LPL) plays a key role in the control of lipid metabolism and transport. It is responsible for the hydrolysis of the triacylglycerol component of circulating chylomicrons and very low density lipoproteins into free fatty acids and 2-monoacylglycerol that can be used by the cells. LPL has also been implicated in the initiation and the progression of atherosclerosis, a major contributor of coronary heart disease. LPL, expressed by macrophages in the atherosclerotic plaque, contributes to the initiation and the progression of foam cells, which represent a critical stage in the onset of the disease. Macrophage LPL therefore represents an excellent target against atherosclerosis and it is important that the processes that regulate its expression are understood. Macrophage LPL can be regulated by several factors present in the atherosclerotic plaque including cytokines. It has been shown that cytokines exert their effects via transcriptional mechanisms and, previous work within the laboratory has identified some of the signal transduction pathways and transcription factors that are involved in the cytokine-mediated regulation of LPL. Of particular interest in the laboratory is the effect of interferon- (IFN-y) on LPL. IFN-y has been shown to decrease the transcription of macrophage LPL via decreased binding of the transcription factors Sp1 and Sp3 to regulatory sequences present in the LPL promoter region. Preliminary studies had identified a role for casein kinase 2 (CK2) in this response. Due to the potential significance of these findings, the aim of the studies presented in this thesis was to investigate the molecular mechanisms underlying the IFN-y-mediated inhibition of macrophage LPL gene transcription in detail. Specifically, it was decided to confirm the role of CK2 and to identify other signalling pathways that were involved in this response. Through the use of pharmacological inhibitors and dominant negative constructs, we confirmed a role of CK2 and identified a role for phosphatidylinositol-3-kinase (PI3K) and janus kinase 2 (JAK2) activation in the IFN-y-mediated suppression of LPL mRNA expression and promoter activity through decreased binding of the transcription factors Sp1 and Sp3. We also identified a novel mechanism for the action of CK2 on macrophage LPL gene transcription. It was shown that IFN-y causes an increase in the activity of the catalytic subunits of CK2, and this leads to an increase in the interaction of CK2 and Sp1/Sp3. This leads to the phosphorylation of at least Sp1, which then triggers a decrease in its binding to the LPL promoter. In addition it was shown that IFN-y phosphorylates PKB, a key downstream target of PI3K, and increases its activity. Indeed, phosphorylation of extracts from untreated cells with recombinant PKB could decrease the* binding of Spl/Sp3 to the LPL promoter to levels seen in IFN-y treated cells. The potential of other key components of the PI3K pathway in the response was also investigated. It was found that glycogen synthase kinase 3J3 (GSK3-p) and the forkhead transcription factor, FRDH were not required. In contrast, a role for mammalian target of rapamycin (mTOR) pathway was identified. These studies provide novel insights into the signalling pathways and molecular mechanisms underlying the IFN-y-mediated inhibition of LPL gene transcription in macrophages. The studies suggest potentially new avenues for further research and therapeutic approaches against atherosclerosis.