LRP1 Regulates Architecture of the Vascular Wall by Controlling PDGFRβ-Dependent Phosphatidylinositol 3-Kinase Activation

Low density lipoprotein receptor-related protein 1 (LRP1) protects against atherosclerosis by regulating the activation of platelet-derived growth factor receptor beta (PDGFRbeta) in vascular smooth muscle cells (SMCs). Activated PDGFRbeta undergoes tyrosine phosphorylation and subsequently interacts with various signaling molecules, including phosphatidylinositol 3-kinase (PI3K), which binds to the phosphorylated tyrosine 739/750 residues in mice, and thus regulates actin polymerization and cell movement.In this study, we found disorganized actin in the form of membrane ruffling and enhanced cell migration in LRP1-deficient (LRP1-/-) SMCs. Marfan syndrome-like phenotypes such as tortuous aortas, disrupted elastic layers and abnormally activated transforming growth factor beta (TGFbeta) signaling are present in smooth muscle-specific LRP1 knockout (smLRP1-/-) mice. To investigate the role of LRP1-regulated PI3K activation by PDGFRbeta in atherogenesis, we generated a strain of smLRP1-/- mice in which tyrosine 739/750 of the PDGFRbeta had been mutated to phenylalanines (PDGFRbeta F2/F2). Spontaneous atherosclerosis was significantly reduced in the absence of hypercholesterolemia in these mice compared to smLRP1-/- animals that express wild type PDGFR. Normal actin organization was restored and spontaneous SMC migration as well as PDGF-BB-induced chemotaxis was dramatically reduced, despite continued overactivation of TGFbeta signaling, as indicated by high levels of nuclear phospho-Smad2.Our data suggest that LRP1 regulates actin organization and cell migration by controlling PDGFRbeta-dependent activation of PI3K. TGFbeta activation alone is not sufficient for the expression of the Marfan-like vascular phenotype. Thus, regulation of PI3 Kinase by PDGFRbeta is essential for maintaining vascular integrity, and for the prevention of atherosclerosis as well as Marfan syndrome

Mouse apolipoprotein J: characterization of a gene implicated in atherosclerosis.

Apolipoprotein J (apoJ), a glycoprotein associated with subclasses of plasma high density lipoproteins (HDL), was found to accumulate in aortic lesions in a human subject with transplantation-associated arteriosclerosis and in mice fed a high-fat atherogenic diet. Foam cells present in mouse aortic valve lesions expressed apoJ mRNA, suggesting local synthesis contributes to apoJ\u27s localization in atherosclerotic plaque. As a prerequisite for elucidating the physiological function of apoJ by using a mouse model, cDNA clones representing the mouse homolog of apoJ were isolated, characterized, and sequenced. The nucleotide sequence predicts a 448 amino acid, 50,260 dalton protein. There was 81% nucleotide sequence similarity between mouse and human apoJ, and 75% similarity at the amino acid level. Mouse apoJ contains six potential N-glycosylation sites, a potential Arg-Ser cleavage site to generate alpha and beta subunits, a cluster of five cysteine residues in each subunit, three putative amphipathic helices, and four potential heparin-binding domains. Southern blot analysis indicates that the gene encompasses approximately 23 kb of DNA. Recombinant inbred strains were used to map apoJ to mouse chromosome 14, tightly linked to Mtv-11. All of the transcribed portions of the gene were cloned and analyzed, and all intron-exon boundaries were defined. The first of the 9 exons is untranslated. Single exons encode the signal peptide, the cysteine-rich domain in the alpha subunit, two potential amphipathic helices flanking a heparin-binding consensus sequence, and a potential amphipathic helix overlapping a heparin-binding domain, supporting their potential functional significance in apoJ. A variety of mouse tissues constitutively express a 1.9 kb apoJ mRNA, with apparently identical transcriptional start sites utilized in all tissues tested. ApoJ mRNA was most abundant in stomach, liver, brain, and testis, with intermediate levels in heart, ovary, and kidney. The high degree of similarity between mouse and human apoJ, in structure and distribution of the gene product, gene structure, and deposition in atherosclerotic plaques, suggests that the mouse is an ideal model with which to elucidate the role of apoJ in HDL metabolism and atherogenesis