Generation of type VIII collagen-specific antibodies

Oxidized phospholipids induce type VIII collagen expression and vascular smooth muscle cell migration

Phenotypic switching of vascular smooth muscle cells (VSMCs) is known to play a critical role in the development of atherosclerosis. However, the factors present within lesions that mediate VSMC phenotypic switching are unclear. Oxidized phospholipids (OxPLs), including 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC), are active components of minimally modified low density lipoprotein and have been previously shown to induce multiple proatherogenic events in endothelial cells and macrophages, but their effects on VSMCs have been largely unexplored until recently. We previously showed that OxPLs induced phenotypic switching of VSMCs, including suppression of SMC differentiation marker genes. The goal of the present studies was to test the hypothesis that OxPLs alter extracellular matrix production and VSMC migration. Results showed that POVPC activated expression of several extracellular matrix proteins in VSMC. POVPC increased expression of type VIII collagen alpha1 chain (Col8a1) mRNA in cultured VSMCs and in vivo in rat carotid arteries by 9-fold and 4-fold, respectively. POVPC-induced activation of Col8a1 gene expression was reduced by small interfering RNA-mediated suppression of Krüppel-like factor 4 (Klf4) and Sp1, and was abolished in Klf4-knockout VSMCs. POVPC increased Klf4 binding to the Col8a1 gene promoter both in vivo in rat carotid arteries and in cultured VSMCs based on chromatin immunoprecipitation assays. Moreover, POVPC-induced VSMC migration was markedly reduced in Klf4- or type VIII collagen-knockout VSMCs. Given evidence that OxPLs are present within atherosclerotic lesions, it is interesting to suggest that OxPL-induced changes in VSMC phenotype may contribute to the pathogenesis of atherosclerosis at least in part through changes in extracellular matrix composition.

The alpha1(VIII) and alpha2(VIII) collagen chains form two distinct homotrimeric proteins in vivo

The short chain collagen variant, type VIII, is considered to be comprised of two distinct gene products, the alpha1 and alpha2 polypeptide chains. However, recent in vitro translation studies suggest that these chains can form homotrimers. We report here data from biochemical, immunohistochemical and molecular biological experiments, which together provide evidence that alpha1 and alpha2 polypeptides of type VIII collagen exist as homotrimers in cells and tissues. High-performance liquid chromatographic separation of type VIII collagen isolated from Descemet's membrane consistently demonstrated equimolar quantities of the two chains (alpha1:alpha2 1. 03+/-0.02 (S.E.M.); n=41). The availability of highly specific antibodies for the two polypeptides has assisted the in vivo characterisation of type VIII collagen. Immunoprecipitation of trimeric type VIII collagen from Descemet's membrane with purified anti-alpha1(VIII) and anti-alpha2(VIII) yielded fractions that contained only the alpha1(VIII) and alpha2(VIII) chains, respectively. Cultured human mesangial cells synthesised both polypeptides, but the alpha1(VIII) chain was found exclusively in the cell pellet, while the media contained only the alpha2(VIII) chain. The RNA from human mesangial cells and cornea showed message for both chains. However, in peritoneal fibroblast and mesothelial cell RNA, only alpha1(VIII) mRNA was detectable, demonstrating that the transcription of these two genes was not always co-ordinated. Immunohistochemistry showed that both polypeptides were present in cornea, optic nerve, aorta and umbilical cord but did not always co-localise. These results indicate the alpha1(VIII) and alpha2(VIII) chains preferentially form pepsin-resistant, homotrimeric molecules and so can exist as two distinct proteins.