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.
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.
Bolitho
Christine
C
Cellular and Molecular Pathology Research Unit, Department of Oral Pathology and Oral Medicine, University of Sydney, Westmead Centre for Oral Health, Westmead, Australia.
Bayl
Penelope
P
Hou
Jing Y
JY
Lynch
Garry
G
Hassel
Alexander J
AJ
Wall
Alexandra J
AJ
Zoellner
Hans
H
D000478:Alkylation; D017209:Apoptosis / Q000187:drug effects*; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D003545:Cysteine / Q000737:chemistry; D042783:Endothelial Cells / Q000166:cytology / Q000187:drug effects*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016166:Free Radical Scavengers / Q000378:metabolism; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D050356:Lipid Metabolism / Q000502:physiology; D010446:Peptide Fragments / Q000737:chemistry / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017434:Protein Structure, Tertiary; D043562:Receptors, G-Protein-Coupled / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology*; D013438:Sulfhydryl Compounds / Q000378:metabolism; D014471:Umbilical Veins / Q000166:cytology
D003545:Cysteine; D004791:Enzyme Inhibitors; D016166:Free Radical Scavengers; D010446:Peptide Fragments; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D043562:Receptors, G-Protein-Coupled; D012709:Serum Albumin; D013438:Sulfhydryl Compounds