Macrophage Proliferation In Atherosclerotic Lesions Research Articles

Deficiency in lymphotoxin β receptor protects from atherosclerosis in apoE-deficient mice.

Lymphotoxin β receptor (LTbR) regulates immune cell trafficking and communication in inflammatory diseases. However, the role of LTbR in atherosclerosis is still unclear. The aim of this study was to elucidate the role of LTbR in atherosclerosis. After 15 weeks of feeding a Western-type diet, mice double-deficient in apolipoprotein E and LTbR (apoE(-/-)/LTbR(-/-)) exhibited lower aortic plaque burden than did apoE(-/-) littermates. Macrophage content at the aortic root and in the aorta was reduced, as determined by immunohistochemistry and flow cytometry. In line with a decrease in plaque inflammation, chemokine (C-C motif) ligand 5 (Ccl5) and other chemokines were transcriptionally downregulated in aortic tissue from apoE(-/-)/LTbR(-/-) mice. Moreover, bone marrow chimeras demonstrated that LTbR deficiency in hematopoietic cells mediated the atheroprotection. Furthermore, during atheroprogression, apoE(-/-) mice exhibited increased concentrations of cytokines, for example, Ccl5, whereas apoE(-/-)/LTbR(-/-) mice did not. Despite this decreased plaque macrophage content, flow cytometric analysis showed that the numbers of circulating lymphocyte antigen 6C (Ly6C)(low) monocytes were markedly elevated in apoE(-/-)/LTbR(-/-) mice. The influx of these cells into atherosclerotic lesions was significantly reduced, whereas apoptosis and macrophage proliferation in atherosclerotic lesions were unaffected. Gene array analysis pointed to chemokine (C-C motif) receptor 5 as the most regulated pathway in isolated CD115(+) cells in apoE(-/-)/LTbR(-/-) mice. Furthermore, stimulating monocytes from apoE(-/-) mice with agonistic anti-LTbR antibody or the natural ligand lymphotoxin-α1β2, increased Ccl5 mRNA expression. These findings suggest that LTbR plays a role in macrophage-driven inflammation in atherosclerotic lesions, probably by augmenting the Ccl5-mediated recruitment of monocytes.

Hyperlipidemia in Concert With Hyperglycemia Stimulates the Proliferation of Macrophages in Atherosclerotic Lesions

Hyperglycemia and hyperlipidemia are important risk factors for diabetes-accelerated atherosclerosis. Macrophage proliferation has been implicated in the progression of atherosclerosis. We therefore investigated the effects of hyperglycemia and hyperlipidemia on macrophage proliferation in murine atherosclerotic lesions and isolated primary macrophages. Hyperglycemic LDL receptor-deficient mice that were fed a cholesterol-free diet for 12 weeks did not have elevated cholesterol levels compared with nondiabetic mice, and there was no evidence of increased macrophage proliferation in atherosclerotic lesions. Moreover, elevated glucose levels did not increase proliferation of isolated mouse peritoneal macrophages. In contrast, hyperglycemic LDL receptor-deficient mice that were fed a cholesterol-rich diet showed increased cholesterol levels concomitant with macrophage proliferation in atherosclerotic lesions. Glucose promoted lipid and protein oxidation of LDL in vitro. Glucose-oxidized LDL resulted in phosphorylation of extracellular signal-regulated kinase and protein kinase B/Akt and stimulated proliferation of isolated macrophages. The mitogenic effect of glucose-oxidized LDL was mediated by CD36 and by extracellular signal-regulated kinase activation induced by protein kinase C-dependent and phosphatidylinositol 3-kinase-dependent pathways. Thus, hyperglycemia is not sufficient to stimulate macrophage proliferation in lesions of atherosclerosis or in isolated macrophages. A combination of hyperglycemia and hyperlipidemia, however, stimulates macrophage proliferation by a pathway that may involve the glucose-dependent oxidation of LDL.

The effects of interleukin (IL)-4 and IL-10 on macrophage growth-stimulating activities of oxidized low density lipoprotein (LDL), acetylated LDL and macrophage colony-stimulating factor: the activity of oxidized LDL is refractory to the inhibitory cytokines.

We have previously reported that ligands of scavenger receptor such as acetylated low density lipoprotein (acetyl-LDL) and oxidized LDL induced growth of peripheral macrophages in vitro. This suggests the possibility that in addition to foam cell formation, modified or oxidized LDLs induce macrophage proliferation in atherosclerotic lesions. To learn further the physiological regulation of macrophage growth, we comparatively examined the effect of interleukin (IL)-4 and IL-10 which have been reported to be suppressive to various macrophage functions on macrophage growth-stimulating activities of the acetyl-LDL, oxidized LDL and macrophage colony-stimulating factor (M-CSF). An in vitro study showed that the activity of M-CSF-containing L-cell-conditioned medium was the most sensitive to the suppressive effects of these cytokines. The growth-inducing activity of acetyl-LDL was significantly inhibited by both IL-4 and IL-10. On the other hand, the activity of oxidized LDL was not attenuated by IL-4 or IL-10. These data indicate that macrophage growth-stimulating activity of oxidized LDL, in contrast to that of M-CSF or acetyl-LDL, is refractory to these suppressive cytokines. Oxidized LDL may act as a potent macrophage growth-stimulating factor in atherosclerotic or other inflammatory sites, even when these cytokines are produced by inflammatory and immunological reactions in situ.