Local M-CSF (Macrophage Colony-Stimulating Factor) Expression Regulates Macrophage Proliferation and Apoptosis in Atherosclerosis

Abstract

Objective: Previous studies have shown that deficiency of M-CSF (macrophage colony-stimulating factor; or CSF1 [colony stimulating factor 1]) dramatically reduces atherosclerosis in hyperlipidemic mice. We characterize the underlying mechanism and investigate the relevant sources of CSF1 in lesions. Approach and Results: We quantitatively assessed the effects of CSF1 deficiency on macrophage proliferation and apoptosis in atherosclerotic lesions. Staining of aortic lesions with markers of proliferation, Ki-67 and bromodeoxyuridine, revealed around 40% reduction in CSF1 heterozygous (Csf1±) as compared with WT (wild type; Csf1 +/+ ) mice. Similarly, staining with a marker of apoptosis, activated caspase-3, revealed a 3-fold increase in apoptotic cells in Csf1± mice. Next, we determined the cellular sources of CSF1 contributing to lesion development. Cell-specific deletions of Csf1 in smooth muscle cells using SM22α-Cre (smooth muscle protein 22-alpha-Cre) reduced lesions by about 40%, and in endothelial cells, deletions with Cdh5-Cre (VE-cadherin-Cre) reduced lesions by about 30%. Macrophage-specific deletion with LysM-Cre (lysozyme M-Cre), on the other hand, did not significantly reduce lesions size. Transplantation of Csf1 null (Csf1 −/ − ) mice bone marrow into Csf1 +/+ mice reduced lesions by about 35%, suggesting that CSF1 from hematopoietic cells other than macrophages contributes to atherosclerosis. None of the cell-specific knockouts affected circulating CSF1 levels, and only the smooth muscle cell deletions had any effect on the percentage monocytes in the circulation. Also, Csf1± mice did not exhibit significant differences in Ly6C high /Ly6C low monocytes as compared with Csf1 +/+ . Conclusions: CSF1 contributes to both macrophage proliferation and survival in lesions. Local CSF1 production by smooth muscle cell and endothelial cell rather than circulating CSF1 is the primary driver of macrophage expansion in atherosclerosis.

Inhibition of macrophage proliferation dominates plaque regression in response to cholesterol lowering

“…Oxidized LDL provokes colonystimulating factor 1 (Csf1) secretion by endothelial cells, for example [44]. A recent study described increased macrophage survival and proliferation in the plaque in response to Csf1 production by endothelial cells and vascular smooth muscle cells, in particular [57]. This may represent an additional, indirect mechanism by which statins, via systemic reduction in cholesterol-rich LDL particle numbers, inhibit lesional macrophage proliferation.…”

Regulation of Macrophage Activation and Differentiation in Atherosclerosis

“… 71 72 Recently, it has been shown that CSF-1 derived from local smooth muscle and endothelial cells supports plaque macrophage survival. 73 In addition, rapid adaption and differentiation of monocytes in plaques requires CSF-1 receptor-mediated signaling in aortic stromal cells and macrophages. 71 72 Among the various subsets of monocytes, CD14 + CD16 ++ monocytes are associated with maintaining the vasculature, cell survival, and localization within plaques through CX3CR1/CX3CL1 interactions in atherosclerosis.…”

Monocyte Recruitment, Specification, and Function in Atherosclerosis

“…Once monocytes access the peripheral tissues, they differentiate to macrophage populations and require CSF-1 for their survival and maintenance [ 51 , 52 ]. It was recently found that production of CSF-1 that supports plaque macrophage survival comes from local smooth muscle and endothelial cells, not from systemic production [ 53 ]. The topics of transition of Ly6C hi monocytes to macrophages in pathological conditions (e.g., atherosclerosis) are discussed below.…”