The binding of apolipoprotein H (beta2-Glycoprotein I) to lipoproteins

Elevated serum β2-glycoprotein-I-lipoprotein(a) complexes levels are associated with the presence and complications in type 2 diabetes mellitus

AIMS

To examine the serum levels of β2-glycoprotein I-lipoprotein(a) complexes [β2-GPI-Lp(a)] in type 2 diabetes mellitus (T2DM) patients and evaluate the association of the complexes with complications in T2DM.

METHODS

Fifty two T2DM patients (22 with complications and 30 free of complications) and 52 age/gender-matched healthy controls were studied. Serum concentrations of β2-GPI-Lp(a) and ox-Lp(a) were measured by "sandwich" ELISAs and their associations with complications were examined using multiple linear regression.

RESULTS

Mean serum β2-GPI-Lp(a) (1.19 ± 0.30 U/mL vs. 0.89 ± 0.20 U/mL, p<0.001) and ox-Lp(a) concentrations (13.34 ± 11.73 mg/L vs. 5.26 ± 3.34 mg/L, p<0.001) were both significantly higher in T2DM than in controls. The area under the ROC curve (AUC) for β2-GPI-Lp(a) and ox-Lp(a) was 0.725 and 0.738, respectively. β2-GPI-Lp(a) levels were markedly higher in patients with complications than those without complication (1.39 ± 0.28 U/mL vs. 1.04 ± 0.31 U/mL, p<0.01), whereas no marked difference was found in ox-Lp(a). In multivariate regression analysis, the association between β2-GPI-Lp(a) and complications remained significant (β=0.249, p<0.05, respectively) after adjustments were made for other traits.

CONCLUSIONS

Elevated β2-GPI-Lp(a) may reflect chronic underlying pathophysiological processes involved in development of complications of T2DM.

β2-Glycoprotein I inhibits endothelial cell migration through the nuclear factor κB signalling pathway and endothelial nitric oxide synthase activation

β2-GPI (β2-glycoprotein I) is a plasma glycoprotein ascribed with an anti-angiogenic function; however, the biological role and molecular basis of its action in cell migration remain unknown. The aim of the present study was to assess the contribution of β2-GPI to HAEC (human aortic endothelial cell) migration and the details of its underlying mechanism. Using wound healing and Boyden chamber assays, we found that β2-GPI inhibited endothelial cell migration, which was restored by its neutralizing antibody. NF-κB (nuclear factor κB) inhibitors and lentiviral siRNA (small interfering RNA) silencing of NF-κB significantly attenuated the inhibitory effect of β2-GPI on cell migration. Moreover, β2-GPI was found to induce IκBα (inhibitor of NF-κB) phosphorylation and translocation of p65 and p50. We further demonstrated that mRNA and protein levels of eNOS [endothelial NO (nitric oxide) synthase] and NO production were all increased by β2-GPI and these effects were remarkably inhibited by NF-κB inhibitors and siRNAs of p65 and p50. Furthermore, β2-GPI-mediated inhibition of cell migration was reversed by eNOS inhibitors and eNOS siRNAs. The findings of the present study provide novel insight into the ability of β2-GPI to inhibit endothelial cell migration predominantly through the NF-κB/eNOS/NO signalling pathway, which indicates a potential direction for clinical therapy in vascular diseases.

Functional and genetic characterization of the promoter region of apolipoprotein H (beta2-glycoprotein I)

This study characterized the human apolipoprotein H [APOH; beta(2)-glycoprotein I (beta(2)GPI)] promoter and its variants by in vitro functional experiments and investigated their relationship with human plasma beta(2)GPI levels. We examined the individual effects of 12 APOH promoter single nucleotide polymorphisms in the 5' flanking region of APOH (approximately 1.4 kb) on luciferase activity in COS-1 cells and HepG2 cells and their impact on plasma beta(2)GPI levels in 799 American White people, the DNA binding properties of the APOH promoter using an electrophoretic mobility shift assay in HepG2 cells, the effects of serial deletion analysis of the APOH 5' flanking region in COS-1 and HepG2 cells and cross-species conservation of the APOH promoter sequence. The variant alleles of three single nucleotide polymorphisms (-1219G>A, -643T>C and -32C>A) showed significantly lower luciferase expression (51, 40 and 37%, respectively) as compared with the wild-type allele. The electrophoretic mobility shift assay demonstrated that these three variants specifically bind with protein(s) from HepG2 cell nuclear extracts. Three-site haplotype analysis (-1219G>A, -643T>C and -32C>A) revealed one haplotype carrying -32A (allele frequency = 0.075) to be significantly associated with decreased plasma beta(2)GPI levels (P < 0.001). Deletion analysis localized the core APOH promoter to approximately 160 bp upstream of ATG codon with the presence of critical cis-acting elements between -166 and -65. Cross-species conservation analysis of the APOH promoters of seven species indicated that basic promoter elements are highly conserved across species. In conclusion, we have characterized the functional promoter of APOH and identified functional variants that affect the transcriptional activity of the APOH promoter.

Update on antiphospholipid antibodies

The association of antibodies with an apparent specificity for anionic phospholipids with thrombosis, fetal loss, thrombocytopenia, and certain other clinical manifestations is now well-recognized as the antiphospholipid syndrome (APS). Recent advances in our understanding of the antibodies and antigens involved include discovery of the crystal structure of beta2-glycoprotein I, (beta2GPI), genetic studies of beta2GPI polymorphisms, and the development of anti-beta2GPI and antiprothrombin immunoassays as clinical laboratory tests. The identification of antigen-specific T cells in APS patients has stimulated interest in the role of the cellular immune response in the syndrome. Clinical research in APS will also benefit from the development of preliminary classification criteria.