ABSTRACT
OBJECTIVE: Low-density lipoprotein receptor-related protein 1 (LRP1) is a large endocytic and signaling receptor that is abundant in vascular smooth muscle cells. Mice in which the lrp1 gene is deleted in smooth muscle cells (smLRP1(-/-)) on a low-density lipoprotein receptor-deficient background display excessive platelet derived growth factor-signaling, smooth muscle cell proliferation, aneurysm formation, and increased susceptibility to atherosclerosis. The objectives of the current study were to examine the potential of LRP1 to modulate vascular physiology under nonatherogenic conditions. APPROACH AND RESULTS: We found smLRP1(-/-) mice to have extensive in vivo aortic dilatation accompanied by disorganized and degraded elastic lamina along with medial thickening of the arterial vessels resulting from excess matrix deposition. Surprisingly, this was not attributable to excessive platelet derived growth factor-signaling. Rather, quantitative differential proteomic analysis revealed that smLRP1(-/-) vessels contain a 4-fold increase in protein levels of high-temperature requirement factor A1 (HtrA1), which is a secreted serine protease that is known to degrade matrix components and to impair elastogenesis, resulting in fragmentation of elastic fibers. Importantly, our study discovered that HtrA1 is a novel LRP1 ligand. Proteomics analysis also identified excessive accumulation of connective tissue growth factor, an LRP1 ligand and a key mediator of fibrosis. CONCLUSIONS: Our findings suggest a critical role for LRP1 in maintaining the integrity of vessels by regulating protease activity as well as matrix deposition by modulating HtrA1 and connective tissue growth factor protein levels. This study highlights 2 new molecules, connective tissue growth factor and HtrA1, which contribute to detrimental changes in the vasculature and, therefore, represent new target molecules for potential therapeutic intervention to maintain vessel wall homeostasis.
Subject(s)
Aorta/enzymology , Aortitis/enzymology , Connective Tissue Growth Factor/metabolism , Myocytes, Smooth Muscle/enzymology , Receptors, LDL/metabolism , Serine Endopeptidases/metabolism , Tumor Suppressor Proteins/metabolism , Age Factors , Aging , Animals , Aorta/physiopathology , Aorta/ultrastructure , Aortitis/genetics , Aortitis/pathology , Aortitis/physiopathology , Blood Pressure , Cells, Cultured , Dilatation, Pathologic , Elastic Tissue/metabolism , Endocytosis , Enzyme Activation , Extracellular Matrix/metabolism , Extracellular Signal-Regulated MAP Kinases/metabolism , Fibrosis , High-Temperature Requirement A Serine Peptidase 1 , Ligands , Low Density Lipoprotein Receptor-Related Protein-1 , Male , Mice , Mice, Knockout , Proteomics/methods , Receptors, LDL/deficiency , Receptors, LDL/genetics , Tumor Suppressor Proteins/deficiency , Tumor Suppressor Proteins/geneticsABSTRACT
BACKGROUND: Vascular remodeling in response to alterations in blood flow has been shown to modulate the formation of neo-intima. This process results from a proliferative response of vascular smooth muscle cells and is influenced by macrophages, which potentiate the development of the intima. The LDL receptor-related protein 1 (LRP1) is a large endocytic and signaling receptor that recognizes a number of ligands including apoE-containing lipoproteins, proteases and protease-inhibitor complexes. Macrophage LRP1 is known to influence the development of atherosclerosis, but its role in vascular remodeling has not been investigated. METHODOLOGY/PRINCIPAL FINDINGS: To define the contribution of macrophage LRP1 to vascular remodeling, we generated macrophage specific LRP1-deficient mice (macLRP1-/-) on an LDL receptor (LDLr) knock-out background. Using a carotid ligation model, we detected a 2-fold increase in neointimal thickening and a 2-fold increase in the intima/media ratio in macLRP1-/- mice. Quantitative RT-PCR arrays of the remodeled vessel wall identified increases in mRNA levels of the TGF-ß2 gene as well as the Pdgfa gene in macLRP1-/- mice which could account for the alterations in vascular remodeling. Immunohistochemistry analysis revealed increased activation of the TGF-ß signaling pathway in macLRP1-/- mice. Further, we observed that LRP1 binds TGF-ß2 and macrophages lacking LRP1 accumulate twice as much TGF-ß2 in conditioned media. Finally, TNF-α modulation of the TGF-ß2 gene in macrophages is attenuated when LRP1 is expressed. Together, the data reveal that LRP1 modulates both the expression and protein levels of TGF-ß2 in macrophages. CONCLUSIONS/SIGNIFICANCE: Our data demonstrate that macrophage LRP1 protects the vasculature by limiting remodeling events associated with flow. This appears to occur by the ability of macrophage LRP1 to reduce TGF-ß2 protein levels and to attenuate expression of the TGF-ß2 gene resulting in suppression of the TGF-ß signaling pathway.
