The effect of statins on cytokine regulated macrophage gene expression in atherosclerosis

Atherosclerosis, a chronic inflammatory disorder of the vasculature, is one of the major causes of cardiovascular disease and is responsible for most deaths in western societies. The disease is characterised by a number of steps that occur during the lifespan of an individual, including fatty streak formation, development of complex lesions containing a fibrous cap, and thinning and rupture of such plaques leading to thrombosis and clinical complications of this disease. Macrophages play key roles during all stages of this disease such as foam cell formation, amplification of the inflammatory response and control of plaque stability. The actions of macrophages during this disease are regulated by cytokines present in atherosclerotic lesions such as interferon-γ (IFN-γ), tumour necrosis factor-like protein 1A (TL1A) and interleukin-17 (IL-17). Statins are widely used for the primary and secondary prevention of atherosclerosis and its complications because of their ability to inhibit cholesterol biosynthesis and, thereby, plasma levels of pro-atherogenic low density lipoprotein. However, statins have actions beyond lowering cholesterol levels, the so-called pleiotropic effects, and includes acting in an anti-inflammatory manner. Unfortunately, the anti-inflammatory effects of statins are not fully understood and therefore formed the focus of studies in this thesis using a combination of human macrophage THP-1 cell line, primary cultures of human monocyte-derived macrophages and mouse RAW264.7 macrophage cell line. Simvastatin generally acted in an anti-inflammatory manner in macrophages in relation to the expression of several pro-atherogenic genes, such as monocyte chemoattractant proein-1 (MCP-1) and intercellular adhesion molecule-1 (ICAM-1),regulated by IFN-γ, TL1A or IL-17. Such an anti-inflammatory action also extended to another statin, Atorvastatin. The inhibitory action of Simvastatin on IFN-γ induced expression of MCP-1 and ICAM-1 was reversed, at least in part, by intermediates of the 3-hydroxy-3-methyl-glutaryl coenzyme A pathway such as farnesyl pyrophosphate and geranylgeranyl pyrophosphate. This suggested a potential role for monomeric G-proteins that require such intermediates for activation. In addition, Simvastatin inhibited the phosphorylation-mediated activation of signal transducer and activator of transcription-1 (STAT1), a key transcription factor in IFN-γ signalling, on tyrosine 701 and serine 727 in response to the cytokine. The effect of Simvastatin on MAP Kinase (MAPK) pathways in macrophages was also analysed. The statin attenuated the IFN-γ induced activation of p38 MAPK and extracellular signal activated kinase (ERK)-1/2. Simvastatin also affected the constituve expression of many components of the MAPK pathways (e.g. ERK-1/2) along with downstream genes involved in atherosclerosis (e.g. ATP-binding cassette transporters-A1 and-G1). The effect of Simvastatin on lipid profile of THP-1 and RAW264.7 macrophages was also investigated. Simvastatin does not affect total polar lipids and triacylglycerol. The statin also had no significant effect on fatty acid distribution into polar lipids and triacylglycerol. The studies presented in this thesis provide insights into the actions, and potential mechanisms, underlying the anti-inflammatory effects of statins on human macrophages along with their effects on lipid profiles. Such studies are essential given the widespread use of statins and a need to gain a deeper understanding of their actions