Previous studies revealed that exposure of mesangial cells to high glucose concentration induces the production of matrix proteins mediated by TGF-beta1. We tested if structural analogues of D-glucose may mimic the high glucose effect and found that D-glucosamine was strikingly more potent than D-glucose itself in enhancing the production of TGF-beta protein and subsequent production of the matrix components heparan sulfate proteoglycan and fibronectin in a time- and dose-dependent manner. D-Glucosamine also promoted conversion of latent TGF-beta to the active form. Therefore, we suggested that the hexosamine biosynthetic pathway (the key enzyme of which is glutamine:fructose-6-phosphate amidotransferase [GFAT]) contributes to the high glucose-induced TGF-beta1 production. Inhibition of GFAT by the substrate analogue azaserine or by inhibition of GFAT protein synthesis with antisense oligonucleotide prevented the high glucose-induced increase in cellular glucosamine metabolites and TGF-beta1 expression and bioactivity and subsequent effects on mesangial cell proliferation and matrix production. Overall, our study indicates that the flux of glucose metabolism through the GFAT catalyzed hexosamine biosynthetic pathway is involved in the glucose-induced mesangial production of TGF-beta leading to increased matrix production.

Protein tyrosine phosphatase-like IA-2 autoantigen is one of the major targets of humoral autoimmunity in patients with insulin-dependant diabetes mellitus (IDDM). In an effort to define the epitopes recognized by autoantibodies against IA-2, we generated five human mAbs (hAbs) from peripheral B lymphocytes isolated from patients most of whom had been recently diagnosed for IDDM. Determination and fine mapping of the critical regions for autoantibody binding was performed by RIA using mutant and chimeric constructs of IA-2- and IA-2β-regions. Four of the five IgG autoantibodies recognized distinct epitopes within the protein tyrosine phosphatase (PTP)-like domain of IA-2. The minimal region required for binding by three of the PTP-like domain-specific hAbs could be located to aa 777–979. Two of these hAbs cross-reacted with the related IA-2β PTP-like domain (IA-2β aa 741-1033). A further PTP-like domain specific hAb required the entire PTP-like domain (aa 687–979) for binding, but critical amino acids clustered in the N-terminal region 687–777. An additional epitope could be localized within the juxtamembrane domain (aa 603–779). In competition experiments, the epitope recognized by one of the hAbs was shown to be targeted by 10 of 14 anti-IA-2-positive sera. Nucleotide sequence analysis of this hAb revealed that it used a VH germline gene (DP-71) preferably expressed in autoantibodies associated with IDDM. The presence of somatic mutations in both heavy and light chain genes and the high affinity or this Ab suggest that the immune response to IA-2 is Ag driven.

Recent in vitro and in vivo studies suggested that the increased flux of glucose through the hexosamine biosynthetic pathway may contribute to glucose-induced insulin resistance and to the induction of the synthesis of growth factors. Because glutamine:fructose-6-phosphate amidotransferase (GFAT) catalyzes the first and rate-limiting step in the formation of hexosamine products, this enzyme is the key regulator in this pathway and is therefore possibly also involved in the alterations occurring in preclinical or manifest diabetic patients. To study the expression of GFAT in human tissues, we produced and characterized a peptic antiserum specifically recognizing GFAT protein and a riboprobe for the detection of GFAT mRNA. Immunohistochemical and nonradioactive in situ hybridization analysis revealed high levels of expression of GFAT protein and mRNA in adipocytes and skeletal muscle. Furthermore, a marked GFAT expression was found in vascular smooth muscle cells with unexpectedly high variability and lower levels in other cells, e.g., peripheral nerve sheath cells or endocrine-active cells, including the pancreatic islet cell. GFAT protein expression was below detection level in endothelium, osteocytes, lymphocytes, granulocytes, and in most quiescent fibroblasts. In renal tissue, GFAT was expressed in tubular epithelial cells, while glomerular cells remained essentially unstained. Renal sections obtained from patients with diabetic nephropathy showed significant GFAT expression in some glomerular epithelial and mesangial cells, indicating that GFAT expression may be induced by manifest diabetes. Our data indicate that GFAT is expressed in most tissues involved in the development of diabetic late complications. Furthermore, the results suggest that GFAT gene expression is highly regulated.

Activation of protein kinase C (PKC) has been implicated in the high glucose-induced stimulation of matrix protein production in mesangial cells. Since we have found (Kolm-Litty et al., 1998) that glucosamine, similar to the PKC activator phorbol myristate acetate (PMA), mimicks high glucose-induced TGF-beta1 overexpression and subsequent matrix overproduction, the action of these agents on the translocation of PKC isoenzymes was studied in cultured mesangial cells. Exposure to 12 mM glucosamine resulted in rapid and specific translocation of PKC-isoenzymes in mesangial cells i.e. glucosamine caused an increased and sustained translocation of PKC-alpha, -beta and -epsilon while PKC-zeta was essentially unaffected. Comparison with PMA-induced translocation exhibited distinct differences. Exposure to high glucose concentrations of mesangial cells induced translocation of PKC-beta and down-regulation of PKC-epsilon while PKC-alpha and -zeta were essentially unaltered. Presence of azaserine an inhibitor of glutamine: fructose-6-phosphate amidotransferase, the key enzyme of the hexosamine pathway, attenuated the high glucose-induced effects on the membrane fraction of PKC-beta. Our results indicate that i) glucosamine is a potent stimulator of PKC-translocation exhibiting an isoenzyme specific translocation kinetic which is different from PMA-induced PKC-isoenzyme translocation ii) the hexosamine pathway may be possibly involved in the high glucose-induced activation of PKC.