The Anti-Apoptotic Activity of Albumin for Endothelium Is Mediated by a Partially Cryptic Protein Domain and Reduced by Inhibitors of G-Coupled Protein and PI-3 Kinase, but Is Independent of Radical Scavenging or Bound Lipid

Abstract

Increased vascular disease occurs with low albumin (human serum albumin, HSA), possibly reflecting specific inhibition of endothelial apoptosis reported for tissue culture. Despite the reported specificity for endothelial protection by HSA, the high but physiological concentrations needed appear more consistent with non-specific low-affinity interactions. We reconcile this contradiction by demonstrating protection is mediated by a partially cryptic HSA protein domain, which becomes more exposed and active following cyanogen bromide fragmentation (p < 0.001). Also, although others reported HSA radical scavenging and bound lipids as important for inhibiting apoptosis in non-endothelial cell types, we demonstrate the protective effect for endothelium is unaffected when HSA radical scavenging is blocked by alkylation, or following delipidation. Further probing the mechanism responsible, we found that the G-coupled protein inhibitors pertussis toxin and suramin reduced protection of endothelium by HSA (p < 0.005), while the tyrosine kinase inhibitor genistein had no effect. Consistent with a role for phosphoinositide 3 kinase (PI3K) was inhibition by both wortmannin and LY294002 (p < 0.05), as well as phosphorylation of Akt, while MAP kinase inhibitors had no effect. We conclude the active site in HSA inhibiting endothelial apoptosis is partially cryptic, and acts via a G-coupled protein PI3K-dependent mechanism.

The anti-apoptotic activity of albumin for endothelium is inhibited by advanced glycation end products restricting intramolecular movement

“…Further reports that HSA displays vasculoprotective activity also support an in vivo role for its antiapoptotic activity [5,6]. Importantly for this study, CNBr-mediated fragmentation of HSA increases its anti-apoptotic activity for endothelium up to 100 fold [4]. This suggests that the HSA protein domain that binds the anti-apoptotic endothelial receptor is partially cryptic, and is only exposed in a transient conformational subpopulation of molecules (Fig.…”

“…This suggests that the HSA protein domain that binds the anti-apoptotic endothelial receptor is partially cryptic, and is only exposed in a transient conformational subpopulation of molecules (Fig. 1A, B) [4]. This model for the transient exposure of the receptor-binding domain is consistent with the unusually high degree of intra-molecular movement of HSA relative to other proteins [7].…”

“…We earlier reported that human serum albumin (HSA) strongly inhibits this process. This protective activity of HSA is dependent upon the native protein configuration and is independent of possible serum contaminants, bound lipids, or radical scavenging activity [2][3][4]. Although the precise identity of the endothelial receptor responsible for the antiapoptotic activity of HSA is unknown, it appears to be a G-coupled protein receptor acting via a PI-3 kinase-dependent mechanism [4].…”

“…This protective activity of HSA is dependent upon the native protein configuration and is independent of possible serum contaminants, bound lipids, or radical scavenging activity [2][3][4]. Although the precise identity of the endothelial receptor responsible for the antiapoptotic activity of HSA is unknown, it appears to be a G-coupled protein receptor acting via a PI-3 kinase-dependent mechanism [4]. Further reports that HSA displays vasculoprotective activity also support an in vivo role for its antiapoptotic activity [5,6].…”

Detection of VipAlbumin® Effect in CD34 and SDF-1 Mobilization, in Streptozotocin-induced Diabetes Mellitus Mice

“…Albumin functioned as the antioxidant to obstruct the free radicals ROS/NOS movement, maintaining the extracellular redox equilibrium, and playing role in the transportation process of various molecules such as fatty acid, nitric oxide, hemin, and drugs [37,38]. Some other researches show that in culture condition, albumin can be used as the inhibitor of apoptosis process for macrophages, neutrophil, lymphocyte, and endhotelial cells [39][40][41][42]. In primary structure form, albumin contains 34 cysteine residues contributing with 17 disulfide bridges to form whole tertiary structure with one free cysteine residue (Cys34) which plays important role in various functions of albumin mentioned before.…”