Abstract 5548: Defective α5β1 Integrin Clustering and Impaired Assembly of Fibronectin and Collagen by Vascular Smooth Foam Cells
Vascular smooth muscle cells (SMCs) stabilize the artery wall by elaborating fibrils of type I collagen. A vital and recently discovered step in collagen elaboration by SMCs is integrin-mediated conversion of soluble collagen into polymerized collagen fibrils. However, in atherosclerotic lesions, SMCs can be laden with lipids and the extent to which lipid-laden cells assemble collagen fibrils is unknown. To answer this question, we converted human vascular SMCs to a foam cell state by incubating them with either LDL or VLDL, derived from patients with hyperlipidemia, in the presence of lipoprotein lipase. This yielded cells engorged with cytoplasmic droplets of neutral lipids, evidenced by staining with Oil Red O or Bodipy 493/503. Markers of SMC identity (SM α-actin, h-caldesmon, calponin h1) were preserved. However, dynamic microscopic tracking of Texas Red-labeled solublized collagen revealed a profound perturbation in the ability of SMC foam cells to assemble collagen fibrils on their surface. This dysfunction was mirrored by a striking inability to assemble fibronectin, an integrin-mediated process that has recently been shown to drive collagen assembly. Flow cytometry indicated that cell surface expression of the fibronectin-binding α5β1 integrin was normal in lipid-loaded SMCs. However, there was a pronounced decrease in α5β1 integrin-containing fibrillar adhesion complexes on the apical surface of the cell. The fibrillar adhesion molecule, tensin, also failed to cluster, whereas vinculin-containing focal adhesions and actin microfilament bundles were unaffected. Furthermore, phosphorylation of tensin, a requisite step in fibrillar adhesion formation, was suppressed by lipid-loading as was activation of Src and FAK, both regulators of tensin phosphorylation and fibronectin assembly. In contrast, phosphorylation of Erk1/2 was unaffected.
Conclusions: Uptake of lipids by human SMCs disables the machinery for α5β1 integrin clustering and fibrillar adhesion complex formation, rendering SMCs incapable of assembling a fibronectin and collagen fibril matrix. This newly described perturbation in integrin translocation could underlie atherosclerotic plaque vulnerability and rupture.