1  Introduction
The cytokine interferon-γ (IFN-γ) is a key regulator of the immune and inflammatory responses along with many other cellular actions during physiological and pathophysiological conditions [1]. For example, the cytokine plays a crucial role in the pathogenesis of atherosclerosis, an inflammatory disorder, by regulating the function and properties of all the cell types present in the vessel wall [2]. IFN-γ is expressed at high levels in atherosclerotic lesions and modulates foam cell formation, recruitment of leukocytes to the activated endothelium, and plaque stability [2]. The deficiency of either IFN-γ or its receptor limits the development of atherosclerosis in murine models of this disease whereas administration of the cytokine potentiates it [2–4].
The cellular actions of IFN-γ are characterized by dramatic changes in gene expression [1,2]. The mechanisms by which IFN-γ induces the expression of several genes have been extensively investigated, leading to the identification of the classical Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway [2]. However, several studies, including gene expression profiling of STAT1-deficient cells, have revealed the widespread use of additional pathways for IFN-γ action [5], the nature of which remains to be deciphered in detail. In addition, very little is currently understood about the mechanisms by which IFN-γ inhibits gene transcription despite the existence of a large number of genes that are regulated in this manner, several of which have a key role in atherosclerosis. For example, gene expression profiling has identified 660 genes whose expression in macrophages is inhibited by IFN-γ [6]. Among these is the lipoprotein lipase (LPL) gene, which catalyses the hydrolysis of the triacylglycerol component of chylomicrons and very low density lipoproteins, and has been linked to several pathophysiological conditions associated with perturbations in lipid metabolism and transport, including atherosclerosis, diabetes and obesity [7–9]. The IFN-γ-mediated suppression of LPL mRNA expression, protein levels and enzymatic activity has been observed in a range of macrophage sources from different species, including human monocyte-derived macrophages [7–9].
We have previously investigated several aspects of IFN-γ signalling in macrophages [9–11], including the mechanisms underlying transcriptional inhibition using LPL as a model gene [9]. Our studies identified a novel mechanism for transcriptional inhibition by IFN-γ whereby the cytokine decreases the binding of Sp1 and Sp3 to three conserved recognition sequences in the regulatory region of the LPL gene [9]. The purpose of this study was to investigate the signalling pathways underlying inhibition of LPL gene transcription via Sp1 and Sp3.