Vascular smooth muscle cell apoptosis induces interleukin-1-directed inflammation: effects of hyperlipidemia-mediated inhibition of phagocytosis.

RATIONALE: Atherosclerosis is characterized by lipid accumulation in the vessel wall, inflammation, and both macrophage and vascular smooth muscle cell (VSMC) apoptosis. However, whereas VSMC apoptosis in mice with established atherosclerotic plaques or hyperlipidemia increases serum levels of the proatherogenic cytokines monocyte chemotactic protein (MCP)-1, tumor necrosis factor alpha, and interleukin (IL)-6, the link between hyperlipidemia, apoptosis and inflammation, and the mechanisms by which apoptotic cells promote inflammation in atherosclerosis are unknown. OBJECTIVE: To determine whether hyperlipidemia affects apoptotic cell clearance, and identify the molecular pathways downstream of VSMC apoptosis that may promote inflammation. METHODS AND RESULTS: We find that human VSMCs are potent and efficient phagocytes of apoptotic human VSMCs, but phagocytosis is significantly reduced by oxidized low-density lipoprotein in vitro or hyperlipidemia in vivo. Necrotic human aortic VSMCs release IL-1alpha, which induces IL-6 and MCP-1 production from viable human VSMCs in vitro. In contrast, secondary necrotic VSMCs release both IL-1alpha and caspase-activated IL-1beta, augmenting IL-6 and MCP-1 production. Conditionally inducing VSMC apoptosis in situ in hyperlipidemic SM22alpha-hDTR/ApoE(-/-) mice to levels seen in human plaques increases serum MCP-1, tumor necrosis factor alpha, and IL-6, which is prevented by blocking IL-1. CONCLUSIONS: We conclude that VSMC necrosis releases IL-1alpha, whereas secondary necrosis of apoptotic VSMCs releases both IL-1alpha and beta. IL-1 from necrotic VSMCs induces the surrounding viable VSMCs to produce proinflammatory cytokines. Thus, failed clearance of apoptotic VSMCs caused by hyperlipidemia in vivo may promote the increased serum cytokines and chronic inflammation associated with atherosclerosis.

Systemic MCP1/CCR2 blockade and leukocyte specific MCP1/CCR2 inhibition affect aortic aneurysm formation differently.

OBJECTIVE: CCR2, the receptor for monocyte chemoattractant protein 1 (MCP1), is involved in atherosclerosis and abdominal aortic aneurysms (AAAs). Here, we explored the potential beneficial blockade of the MCP1/CCR2 pathway. METHODS: We applied an AAA model in aging apolipoprotein E deficient mice with pre-existing atherosclerotic lesions. These mice were subjected to two therapeutic strategies. First, a dominant negative form of MCP1 was overexpressed in femoral muscles, resulting in circulating levels of MCP1-7ND (7ND), competing with native MCP1. In the second approach, bone marrow transplantation was performed using bone marrow cells that were infected with a lentiviral construct containing siRNA for CCR2, to specifically inhibit only leukocyte CCR2 expression. RESULTS: Both strategies did not influence lesion size of the advanced atherosclerotic plaques. However, 7ND induced a more fibrous plaque phenotype. Yet, surprisingly a trend in increased number and severity of AAA was observed in the 7ND group. Smooth muscle cells in the aneurysm showed decreased phosphorylated signal transducer and activator of transcription five (STAT5, P<0.01) in the 7ND group, which is indicative for a decreased proliferative and migratory (wound healing) response. This presumably resulted in the increased AAA development. In contrast, siRNA-induced inhibition of CCR2 in leukocytes led to a significant inhibition in aneurysm formation. In conclusion, systemic inhibition of the MCP1/CCR2 pathway leads to a fibrous plaque phenotype in the advanced atherosclerotic lesions, but to potential adverse effects on AAA formation, implying that for a beneficial overall therapeutic approach, specific inhibitory targeting of leukocyte CCR2 will be essential.