Under-expression of α8 integrin aggravates experimental atherosclerosis.

Integrins play an important role in vascular biology. The α8 integrin chain attenuates smooth muscle cell migration but its functional role in the development of atherosclerosis is unclear. Therefore, we studied the contribution of α8 integrin to atherosclerosis and vascular remodelling. We hypothesized that α8 integrin expression is reduced in atherosclerotic lesions, and that its under-expression leads to a more severe course of atherosclerosis. α8 Integrin was detected by immunohistochemistry and qPCR and α8 integrin-deficient mice were used to induce two models of atherosclerotic lesions. First, ligation of the carotid artery led to medial thickening and neointima formation, which was quantified in carotid cross-sections. Second, after crossing into ApoE-deficient mice, the formation of advanced vascular lesions with atherosclerotic plaques was quantified in aortic en face preparations stained with Sudan IV. Parameters of renal physiology and histopathology were assessed: α8 integrin was detected in the media of human and murine vascular tissue and was down-regulated in arteries with advanced atherosclerotic lesions. In α8 integrin-deficient mice (α8(-/-) ) as well as α8(+/-) and α8(+/+) littermates, carotid artery ligation increased media:lumen ratios in all genotypes, with higher values in ligated α8(-/-) and α8(+/-) compared to ligated α8(+/+) animals. Carotid artery ligation increased smooth muscle cell number in the media of α8(+/+) mice and, more prominently, of α8(-/-) or α8(+/-) mice. On an ApoE(-/-) background, α8(+/-) and α8(-/-) mice developed more atherosclerotic plaques than α8(+/+) mice. α8 Integrin expression was reduced in α8(+/-) animals. Renal damage with increased serum creatinine and glomerulosclerosis was detected in α8(-/-) mice only. Thus, under-expression of α8 integrin aggravates vascular lesions, while a complete loss of α8 integrin results in reduced renal mass and additional renal disease in the presence of generalized atherosclerosis. Our data support the hypothesis that integrin α8ß1 has a protective role in arterial remodelling and atherosclerosis.

Contribution of the α8 integrin chain to the expression of extracellular matrix components.

In the kidney, the α8 integrin chain (itga8) is expressed in mesenchymal cells and is upregulated in fibrotic disease. We hypothesized that itga8 mediates a profibrotic phenotype of renal cells by promoting extracellular matrix and cytokine expression. Genetic itga8 deficiency caused complex changes in matrix expression patterns in mesangial and smooth-muscle cells, with the only concordant effect in both cell types being a reduction of collagen III expression. Silencing of itga8 with siRNA led to a decline of matrix turnover with repression of matrix metalloproteinases and reduction of matrix production. In contrast, de novo expression of itga8 in tubular epithelial cells resulted in reduced collagen synthesis. Overexpression of itga8 in fibroblasts did not change the expression of matrix molecules or regulators of matrix turnover. Thus, the influence of itga8 on the expression of matrix components was not uniform and celltype dependent. Itga8 seems unlikely to exert overall profibrotic effects in renal cells.

Lack of α8 integrin leads to morphological changes in renal mesangial cells, but not in vascular smooth muscle cells.

BACKGROUND: Extracellular matrix receptors of the integrin family are known to regulate cell adhesion, shape and functions. The α8 integrin chain is expressed in glomerular mesangial cells and in vascular smooth muscle cells. Mice deficient for α8 integrin have structural alterations in glomeruli but not in renal arteries. For this reason we hypothesized that mesangial cells and vascular smooth muscle cells differ in their respective capacity to compensate for the lack of α8 integrin. RESULTS: Wild type and α8 integrin-deficient mesangial cells varied markedly in cell morphology and expression or localization of cytoskeletal molecules. In α8 integrin-deficient mesangial cells α-smooth muscle actin and CTGF were downregulated. In contrast, there were no comparable differences between α8 integrin-deficient and wild type vascular smooth muscle cells. Expression patterns of integrins were altered in α8 integrin-deficient mesangial cells compared to wild type mesangial cells, displaying a prominent overexpression of α2 and α6 integrins, while expression patterns of the these integrins were not different between wild type and α8 integrin-deficient vascular smooth muscle cells, respectively. Cell proliferation was augmented in α8 integrin-deficient mesangial cells, but not in vascular smooth muscle cells, compared to wild type cells. CONCLUSIONS: Our findings suggest that α8 integrin deficiency has differential effects in mesangial cells and vascular smooth muscle cells. While the phenotype of vascular smooth muscle cells lacking α8 integrin is not altered, mesangial cells lacking α8 integrin differ considerably from wild type mesangial cells which might be a consequence of compensatory changes in the expression patterns of other integrins. This could result in glomerular changes in α8 integrin-deficient mice, while the vasculature is not affected in these mice.

Role of alpha8 integrin in mesangial cell adhesion, migration, and proliferation.

BACKGROUND: Extracellular matrix receptors of the integrin family are known to regulate cell adhesion, migration, and proliferation. The alpha8 integrin chain is expressed in the glomerulus exclusively by mesangial cells. The contribution of alpha8 to mesangial cell function, however, has not yet been studied. METHODS: Mesangial cells from wild-type and alpha8-deficient mice were isolated and characterized. Integrin expression was assessed by real-time polymerase chain reaction (PCR), Western blot, or fluorescence-activated cell sorter (FACS) analysis. Cell adhesion was determined by conventional attachment assay and a centrifugal assay for cell adhesion. Cell migration was determined by a fluorescence-based transmigration assay and a chemotaxis assay. Proliferation rates were determined by BrdU and [3H]-thymidine assays. RESULTS: On the alpha8 ligands fibronectin and vitronectin, but not on collagens, attachment of alpha8-deficient mesangial cells was reduced compared to wild-type cells. In contrast, alpha8-deficient mesangial cells migrated more easily and displayed an increased proliferative response on fibronectin or vitronectin, but not on collagens, compared to wild-type cells. These effects were not due to an up-regulation of the fibronectin or vitronectin receptors alpha5 or alphav in alpha8-deficient mesangial cells, as the cell surface expression of integrins alpha5 and alphav was comparable in wild-type and alpha8-deficient mesangial cells. CONCLUSION: These findings confirm a role for alpha8 integrin in the regulation of the mesangial cell phenotype. alpha8 integrin seems to promote adhesion, but inhibit migration and proliferation of mesangial cells. Thus, the data support the hypothesis that alpha8 integrin could play an important role for maintaining tissue integrity in the glomerulus during glomerular injury.