Purification of proteinase-like and Na+/K(+)-ATPase stimulating substance from plasma of insulin-dependent diabetics and its identification as alpha 1-antitrypsin

A mutant α1antitrypsin in complex with heat shock proteins as the primary antigen in type 1 diabetes in silico investigation

Based on previous results demonstrating that complexes of a mutant α1-antitrypsin with the heat shock proteins (HSP)70 and glucose-regulated protein94 (Grp94) circulate in the blood of patients with type 1 diabetes, we raised the hypothesis that these complexes could represent the primary antigen capable of triggering the autoimmune reactions leading to overt diabetes. As a first approach to this issue, we searched whether A1AT and HSPs had a sequence similarity to major islet antigen proteins so as to identify among the similar sequences those with potential relevance for the pathogenesis of diabetes. A thorough in silico analysis was performed to establish the score of similarity of the human proteins: A1AT, pro-insulin (INS), GAD65, IAPP, IA-2, ICA69, Grp94, HSP70 and HSP60. The sequences of A1AT and HSPs with the highest score of similarity to the islet peptides reported in the literature as the main autoantigens in human diabetes were recorded. At variance with other HSPs, also including HSP90 and Grp78, Grp94 contained the highest number and the longest sequences with structural similarity to A1AT and to well-known immunogenic peptides/epitopes of INS, GAD65, and IA-2. The similarity of A1AT with Grp94 and that of Grp94 with INS also suggested a functional relationship among the proteins. Specific sequences were identified in A1AT, Grp94 and HSP70, with the highest score of cross-similarity to a pattern of eight different islet protein epitopes. The similarity also involved recently discovered autoantigens in type 1 diabetes such as a hybrid peptides of insulin and the defective ribosomal insulin gene product. The significant similarity displayed by specific sequences of Grp94 and A1AT to the islet peptides considered main antigens in human diabetes, is a strong indication for testing these sequences as new peptides of immunogenic relevance in diabetes.

Identification and purification from the plasma of Type 1 diabetic subjects of a proteolytically active Grp94Evidence that Grp94 is entirely responsible for plasma proteolytic activity

AIMS/HYPOTHESIS

The overall increase in proteolytic activity in diabetes is known to be associated with the development and progression of vascular complications. Our aim was to investigate in detail the molecular nature of this activity in the plasma of Type 1 diabetic subjects.

METHODS

Plasma of both diabetic and control subjects was subjected to various purification procedures (ion exchange and affinity chromatography, HPLC, immunoprecipitation, electrophoresis, immunoblot and mass analyses) to identify the proteins of interest. Biological activities were measured on specific substrates.

RESULTS

In diabetic but not normal plasma we identified the presence of two heat shock proteins, Grp94 (Glucose-regulated protein94) and HSP70. The higher-than-normal proteolytic activity of Grp94 was: (i) directed against casein, but not against endogenous plasma proteins; (ii) fully and specifically inhibited only by anti-Grp94 polyclonal antibodies; and (iii) coupled with low-level ATPase activity. In addition, ATP binding to Grp94 was able to modulate proteolytic activity. We found that Grp94 in plasma circulates only as high molecular mass homo- and hetero-complexes, the latter mostly formed with IgG to which Grp94 is also linked by tenacious binding. Proteolytically-active Grp94 was purified by immunoprecipitation, which co-immunoprecipitated alpha(1)antitrypsin.

CONCLUSION/INTERPRETATION

Our results show the unexpected extracellular location and characteristic biological function of Grp94 even at a late stage of disease. These findings have physiopathological relevance for predicting activation of both autoimmune and inflammatory processes potentially associated with vascular complications.

Differential effects of heparin and glucose on structural conformation of human alpha1 antitrypsin: evidence for a heparin-induced cleaved form of the inhibitor

alpha1 Antitrypsin (alpha1AT) is the archetypal member of the serpin superfamily. Current knowledge of its inhibitory mechanism does not provide for any heparin-induced enhancement of serine proteinase inhibition. Since previous results have shown that an apparently altered alpha1AT form may be purified from the plasma of insulin-dependent diabetics by means of heparin-affinity chromatography, in the present work the possibility was tested that heparin at various concentrations modifies the structural conformation and function of human alpha1AT in the absence and presence of glucose, used at concentrations of 15 mM to mimick mild hyperglycemic conditions. Heparin was observed to bind strongly to alpha1AT, causing maximal enhancement of tryptophan fluorescence emission at 50 microg/ml, mostly in the presence of glucose. Circular dichroism spectra revealed that heparin with glucose caused the most relaxed, ordered structure of the inhibitor with increased heat stability. Modification in conformation was accompanied by loss of inhibitory activity, as demonstrated by the inability of alpha1AT to block bovine trypsin in the specific assay and by alterations of its immunological properties. However, despite inactivation, in the presence of heparin-both with and without glucose-alpha1AT was still able to bind trypsin, as revealed by inhibitor-to-proteinase complexes visible in both SDS- and nondenaturing electrophoreses. These complexes showed the same feature regardless of trypsin concentration and differed from those formed at a molar excess of the inhibitor in the absence of heparin, since they underwent rapid, intense fragmentation accompanied by complete loss of the secondary structure of the inhibitor. Even in the absence of trypsin, cleavage of alpha1AT was also observed to occur at both Val321- and Glu344- in the primary sequence of the inhibitor in the presence of 50 microg/ml heparin, with and without glucose. These results suggest that heparin binding to alpha1AT causes profound structural changes in the molecule, involving both the expulsion of the reactive site out of the molecule plane and a relaxed, heat-stable form of the inhibitor, rendered a substrate for the proteinase. Although glucose apparently does not affect alpha1AT functioning, it does enhance the effects of heparin on the alpha1AT structure. The possibility is discussed that, while heparin and glucose binding occurs at different sites on alpha1AT, glucose favors heparin binding by inducing a partially relaxed form in the inhibitor. Differences in structure and charge between the two substances account for both different individual effects on alpha1AT and the predominance of the effects of heparin.

Heparin-induced structural and functional alterations of bovine trypsin

To investigate the mechanism whereby heparin can modulate the activity of serine proteinases, bovine trypsin was chosen as reference and treated with heparin at 10, 100 and 200 micrograms/ml, in buffer solvents, with and without incubation at 37 degrees C. Heparin caused rapid, buffer- and pH-dependent decrease in trypsin solubility due to the generation of insoluble fragments from proteinase. Desalting treatments variously restored solubility by removing insoluble material. UV absorption and fluorescence emission spectra revealed significant heparin-induced conformational alterations in the trypsin molecule, the maximal effect being apparent at a proteinase-to-heparin molar ratio ranging from 1.6 to 1.0. The involvement of the catalytic sites of trypsin by heparin was further confirmed by the significant reduction in the difference absorption spectra of proflavine. Both proteolytic and esterolytic activities of trypsin were shown to be markedly decreased by heparin, especially after 5 h incubation at 37 degrees C. However, when the proteolytic and esterolytic activities of trypsin were measured on fresh solutions not submitted to desalting treatments, variable activation instead of inhibition of both activities was observed in the presence of heparin, this effect waning spontaneously in time or after desalting treatment. The paradoxical increase in functional activities was not inhibited by soybean trypsin inhibitor and was accompanied by denaturation and fragmentation of the proteinase as demonstrated by spectroscopic analyses and SDS-PAGE of fresh solutions. The results obtained indicated that heparin causes a rapid, time- and temperature-dependent conformational alteration of trypsin with irreversible denaturation and degradation of the proteinase. The underlying mechanism appears to be heparin-catalyzed oxidative degradation of trypsin due to liberation of oxygen radicals which are also responsible for the temporary increase in catalytic functions.