Cytokine and growth factor regulation of murine macrophage scavenger receptor expression and function
The macrophage (Mφ) foam cell in the atherosclerotic plaque microenvironment is subjected to cytokines and growth factors secreted by smooth muscle cells (SMCs), endothelial cells (ECs), platelets and lesional Mφ themselves. This thesis examines the effects of some of these soluble factors on a murine macrophage membrane molecule, the macrophage scavenger receptor (MSR), considered pivotal in foam cell formation and atherogenesis. Macrophage Colony-stimulating factor (MCSF) enhances MSR expression and functional activity (including MSR-dependent adhesion) in elicited peritoneal Mφ markedly and selectively. The T lymphocyte products, Th1 (interferon-gamma(IFN-γ)) and Th2 (chiefly interleukin-4 (IL-4)) cytokines, have divergent effects with IL-4 upregulating and IFN-γ either maintaining or downregulating MSR status. IL-4 induced MSR microheterogeneity is due to changes in N-linked glycosylation, specifically sialylation and may be physiologically significant. In contrast to interleukin-10 (IL-10), transforming growth factor-beta (TGF-β) inhibits MSR upregulation, also when produced endogenously. TGF-β is as potent an inhibitor of MSR function as M-CSF is a stimulator. A cleaved truncated soluble form of MSR which lacks the cytoplasmic domain is present, by immunochemical assays, in culture supernatants. M-CSF increases soluble MSR release in vitro which is functionally active. Following upregulation of MSR surface expression in transfected CHO cells by prolonged culture, a modest MSR-dependent contribution to adhesion becomes apparent. To determine a possible adhesionpromoting region in the MSR, the binding site of mAb 2F8 was mapped using a series of MSR truncation mutants, and localized to residues 183 to 197 in the proximal cchelical coiled-coil domain. Morphological evidence, obtained by confocal and electron microscopy, supports an adhesion role for the MSR in primary Mφ and transfected CHO cells. MSR expression is prominently directed to the adherent surface and its distribution is restricted to cellular contact areas with the substratum. Organs and atherosclerotic lesions from mice deficient in M-CSF (osteopetrotic) and apolipoprotein E were examined to determine the effects of M-CSF on Mφ phenotype (including MSR expression) and lesion development in vivo. Though severely hypercholesterolemic, doubly deficient mice are protected against atherosclerosis and exhibit fewer Mφ and low MSR expression on remaining M-CSF independent populations. Prominent hepatic lipid accumulation suggests a crucial M-CSF dependent role for Kupffer cells in lipoprotein uptake, transfer to hepatocytes and biliary excretion of cholesterol. Regulation of MSR activity may therefore be important for the recruitment of Mφ into the arterial wall and, at the post-endothelial stage, to anchor Mφ at specific locations, thus favouring foam cell formation.