Conformational changes of the alpha 1-proteinase inhibitor affecting its cholesterol binding ability

S-Nitrosylation of α1-Antitrypsin Triggers Macrophages Toward Inflammatory Phenotype and Enhances Intra-Cellular Bacteria Elimination

Background: Human α1-antitrypsin (hAAT) is a circulating anti-inflammatory serine-protease inhibitor that rises during acute phase responses. in vivo, hAAT reduces bacterial load, without directly inhibiting bacterial growth. In conditions of excess nitric-oxide (NO), hAAT undergoes S-nitrosylation (S-NO-hAAT) and gains antibacterial capacity. The impact of S-NO-hAAT on immune cells has yet to be explored.

Aim: Study the effects of S-NO-hAAT on immune cells during bacterial infection.

Methods: Clinical-grade hAAT was S-nitrosylated and then compared to unmodified hAAT, functionally, and structurally. Intracellular bacterial clearance by THP-1 macrophages was assessed using live Salmonella typhi. Murine peritoneal macrophages were examined, and signaling pathways were evaluated. S-NO-hAAT was also investigated after blocking free mambranal cysteine residues on cells.

Results: S-NO-hAAT (27.5 uM) enhances intracellular bacteria elimination by immunocytes (up to 1-log reduction). S-NO-hAAT causes resting macrophages to exhibit a pro-inflammatory and antibacterial phenotype, including release of inflammatory cytokines and induction of inducible nitric oxide synthase (iNOS) and TLR2. These pro-inflammatory effects are dependent upon cell surface thiols and activation of MAPK pathways.

Conclusions: hAAT duality appears to be context-specific, involving S-nitrosylation in a nitric oxide rich environment. Our results suggest that S-nitrosylation facilitates the antibacterial activity of hAAT by promoting its ability to activate innate immune cells. This pro-inflammatory effect may involve transferring of nitric oxide from S-NO-hAAT to a free cysteine residue on cellular targets.

In vitro amyloid fibril formation from alpha 1-antitrypsin

We have previously shown that the interaction between alpha 1-antitrypsin (AAT) and lithocholic acid (LA) results in changes of AAT properties leading to its polymerization and inactivation. To define the structural rearrangements of AAT induced by such interaction, we studied the in vitro binding between AAT and LA at molar ratio 1:5 for varying time intervals at a physiological pH. Complex formation was shown by electrophoretic techniques and autoradiography. Studies of the AAT in complex with LA by using far-UV spectra circular dichroism and fluorescence measurements indicated an increase of beta-structure of AAT and pronounced changes in surroundings of the chromophores. In addition, complexed AAT showed increase in thermal stability, compatible with that after proteolytic cleavage. Characterization of the AAT-LA complexes by Congo red binding, polarization and negative staining electron microscopy provided clear evidence that AAT, under chosen experimental conditions, can self-assemble into amyloid fibrils, compatible with accepted models of fibrillar structures. This propensity of AAT to form stable beta-structures in a hydrophobic surrounding may contribute to improved characterization of various amyloid deposits occurring in vivo and be a guide for understanding details of structure-function relationships in the intact AAT-molecule.

An interaction between Gemfibrozil and alpha 1-antitrypsin

Previous studies have demonstrated such hydrophobic compounds as cholesterol and lithocholic acid, to interact with alpha 1-antitrypsin (alpha 1-AT). We observed a similar interaction between the anionic, hydrophobic lipid-lowering drug Gemfibrozil and alpha 1-AT in a deficient (PiZZ) patient, affecting the physicochemical and antiproteinase properties of the inhibitor protein. Gemfibrozil treatment in non-PiZ patients also resulted in a reduction of alpha 1-AT activity which could be reproduced in vitro.

The interaction of hydrophobic bile acids with the alpha 1-proteinase inhibitor

An in vitro complex formation between cholesterol and human alpha 1-proteinase inhibitor (alpha 1-antitrypsin, alpha 1-Pi) has been described. Hydrophobic bile acids were studied for a similar interaction using lithocholic acid (LC) as a prototype of a hydrophobic acid. At a molar ratio of 5:1, LC induced conformational changes of alpha 1-Pi reflected in an abnormal gel-electrophoretic appearance, loss of anodal immunoreactivity on crossed immunoelectrophoresis, exposition of new antigenic determinant(s) on immunodiffusion, and loss of antiproteinase activity. After 6 h incubation, LC and alpha 1-Pi form a complex of approximately 200 kDa molecular mass seen following gel-filtration. After prolonged (24 h) interaction a series of large alpha 1-Pi polymers were seen on SDS-PAGE under reducing conditions followed by Western blotting. Glycolitho-, sulfolitho-, deoxycholic and 3-beta-hydroxy-5-cholenoic acids induced similar but less pronounced changes of alpha 1-Pi, whereas transferrin remained unaffected. Hydrophilic acids lacked effect on alpha 1-Pi. The results are compatible with a specific, irreversible interaction of alpha 1-Pi with hydrophobic bile acids affecting its physical and proteinase inhibitory properties. The cholestatic potency of the hydrophobic acids studied and their ability to induce alpha 1-Pi polymerization may be important in cholestatic conditions.