The stress protein BiP is overexpressed and is a major B and T cell target in rheumatoid arthritis

OBJECTIVE

The ubiquitously expressed intracellular protein formerly designated p68 has been identified as autoantigen at both the antibody and the T cell level in rheumatoid arthritis (RA).

METHODS

We used 2 independent approaches, Edman degradation and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry, to characterize p68, and we compared its features with those of the endoplasmic reticulum stress protein BiP.

RESULTS

In synovial sections from RA patients, BiP was highly overexpressed as compared with control sections. Under in vitro stress conditions, BiP was found to translocate to the nucleus and the cell surface. BiP-specific autoantibodies were present in 63% of 400 RA patients, in 7% of 200 patients with other rheumatic diseases, and in none of the healthy subjects. Thus, BiP-specific autoantibodies represent a new diagnostic marker in RA. Furthermore, we found that BiP-specific T cell reactivity was altered in RA. In healthy individuals and patients with other rheumatic diseases, BiP-reactive T cells were undetectable. In RA, overt T cell reactivity to BiP was observed or could be induced by specifically blocking antigen presentation to potentially regulatory T cells.

CONCLUSION

Since overexpression of BiP has been shown to decrease the sensitivity of cells to killing by cytotoxic T cells, BiP overexpression and BiP-specific autoimmunity may be involved in the pathogenesis of RA.

The monoclonal antibodies 18d7/91f2 recognize a receptor regulatory protein on mouse bone marrow stromal cells

Two monoclonal antibodies 18D7 and 91F2 were developed by immunizing rats with the mouse bone marrow-derived osteogenic cell line MN7. Hybridomas secreting rat antibodies against MN7 cell surface markers were selected by flow cytometry analysis. Both the monoclonal antibody 18D7 and the monoclonal antibody 91F2 are directed against the same cell surface antigen present on MN7 cells. Here, we report on the immunopurification of the 18D7/91F2 antigen and its identification as the prostaglandin F2 alpha receptor regulatory protein (FPRP). FPRP is expressed as a single messenger RNA (mRNA) of approximately 6 kilobases (kb) in MN7 cells and is differentially expressed in developing osteogenic cultures of bone marrow cells of the mouse. However, addition of the monoclonal antibodies 18D7 and 91F2 to these cultures did not inhibit bone formation in vitro. Both monoclonal antibodies reacted with mouse stromal cell lines established from bone marrow, thymus, spleen, and mandibular condyles. Immunohistochemical analysis of mature tibia of mice using the monoclonal antibody 18D7 revealed the presence of a distinct population of bone marrow cells close to trabecular and endosteal bone surfaces. In the central bone marrow, hardly any positive cells were found. In 17-day-old fetal mouse radius 18D7 immunoreactivity was restricted to cells in the periosteum in close vicinity to the bone collar. Mature osteoblasts, osteoclasts, osteocytes, growth plate chondrocytes, and mature macrophages were all negative. Taken together, these results suggest that FPRP plays a role in the osteogenic differentiation process.

The C-terminal RG dipeptide repeats of the spliceosomal Sm proteins D1 and D3 contain symmetrical dimethylarginines, which form a major B-cell epitope for anti-Sm autoantibodies

The Sm proteins B/B', D1, D2, D3, E, F, and G are components of the small nuclear ribonucleoproteins U1, U2, U4/U6, and U5 that are essential for the splicing of pre-mRNAs in eukaryotes. D1 and D3 are among the most common antigens recognized by anti-Sm autoantibodies, an autoantibody population found exclusively in patients afflicted with systemic lupus erythematosus. Here we demonstrate by protein sequencing and mass spectrometry that all arginines in the C-terminal arginine-glycine (RG) dipeptide repeats of the human Sm proteins D1 and D3, isolated from HeLa small nuclear ribonucleoproteins, contain symmetrical dimethylarginines (sDMAs), a posttranslational modification thus far only identified in the myelin basic protein. The further finding that human D1 individually overexpressed in baculovirus-infected insect cells contains asymmetrical dimethylarginines suggests that the symmetrical dimethylation of the RG repeats in D1 and D3 is dependent on the assembly status of D1 and D3. In antibody binding studies, 10 of 11 anti-Sm patient sera tested, as well as the monoclonal antibody Y12, reacted with a chemically synthesized C-terminal peptide of D1 containing sDMA, but not with peptides containing asymmetrically modified or nonmodified arginines. These results thus demonstrate that the sDMA-modified C terminus of D1 forms a major linear epitope for anti-Sm autoantibodies and Y12 and further suggest that posttranslational modifications of Sm proteins play a role in the etiology of systemic lupus erythematosus.

Identification of two-dimensionally separated human cerebrospinal fluid proteins by N-terminal sequencing, matrix-assisted laser desorption/ionization--mass spectrometry, nanoliquid chromatography-electrospray ionization-time of flight-mass spectrometry, and tandem mass spectrometry

Optimal application of biological mass spectrometry (MS) in combination with two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) of human cerebrospinal fluid (CSF) can lead to the identification of new potential biological markers of neurological disorders. To this end, we analyzed a number of 2-D PAGE protein spots in a human CSF pool using spot co-localization, N-terminal sequencing, matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and nanoliquid chromatography-electrospray ionization-time of flight-mass spectrometry (nanoLC-ESI-TOF-MS) with tandem MS switching. Our constructed CSF master contained 469 spots after image analysis and processing of 2-D gels. Upon visual inspection of our CSF master with the CSF pattern available on the ExPASy server, it was possible to locate and annotate 15 proteins. N-terminal sequence analysis and MALDI-MS peptide mass fingerprint analysis of both silver- and Coomassie Brilliant Blue (CBB) G-250-stained protein spots after in situ trypsin digest not only confirmed nine of the visually annotated spots but additionally resolved the identity of another 13 spots. Six of these proteins were not annotated on the 2-D ExPASy map: complement C3 alpha-chain (1321-1663), complement factor B, cystatin C, calgranulin A, hemoglobin beta-chain, and beta-2-microglobulin. It was clear that MALDI-MS identification from CBB G-250-stained, rather than from silver-stained, spots was more successful. In cases where no N-terminal sequence and/or no clear MALDI-MS result was available, nanoLC-ESI-TOF-MS and tandem MS automated switching was used to clarify and/or identify these protein spots by generating amino acid sequence tags. In addition, enrichment of the concentration of low-abundant proteins on 2-D PAGE was obtained by removal of albumin and immunoglobulins from the CSF pool using affinity chromatography. Subsequent analysis by 2-D PAGE of the fractionated CSF pool showed various new silver-stainable protein spots, of which four were identified by nanoLC-ESI-TOF-MS and tandem MS switching. No significant homology was found in either protein or DNA databases, indicating than these spots were unknown proteins.

Changes in the liver protein pattern of female Wistar rats treated with the hypoglycemic agent SDZ PGU 693

SDZ PGU 693 acts as a hypoglycemic agent by stimulating glucose utilisation in insulin-sensitive peripheral tissues, such as skeletal muscle and fat. In a 28 day toxicity study the compound was found to induce hepatocellular hypertrophy in Wistar rats treated with 300 mg/kg/day. To gain insights into the pathomechanism of these alterations, aliquots of liver samples from control and treated female Wistar rats were separated by two-dimensional protein gel electrophoresis and the digitized images of the protein patterns were searched for protein abundance changes. Significant treatment-related quantitative changes (P < 0.001) were found in 29 liver proteins. Major increases were observed in several microsomal proteins, including NADPH cytochrome P-450 reductase, cytochrome b5 and serine protease inhibitor. The changes in the cytochrome related enzymes, both known co-factors of the P-450 enzyme system, strongly suggest that SDZ PGU 693 induces microsomal proliferation and induction of the P-450 enzyme system. Decreases were observed in a series of mitochondrial proteins, such as F1ATPase-delta subunit and ornithine aminotransferase precursor as well as in several cytosolic proteins such as the liver fatty acid binding protein, arylsulfotransferase and the senescence marker protein-30. The changes in F1ATPase-delta subunit and liver fatty acid binding protein together suggest a down-regulation of the mitochondrial liver fatty acid metabolism, likely reflecting the pharmacological action of the compound. These results show that SDZ PGU 693 produces a complex pattern of gene expression changes which give insights into the molecular mechanisms of both its pharmacological action and a toxic response.

Cholangiocyte-specific rat liver proteins identified by establishment of a two-dimensional gel protein database

The liver is composed of a variety of cells that form a functional unit involved in uptake, synthesis, metabolism, and secretion. Until recently, most studies examining liver function did not analyze the specific proteins expressed or functions performed by the multiple individual cell types that constitute the hepatic mass. In the last decade, novel isolation methods have been developed that allow the purification of liver cell populations highly enriched in one type of liver cell. Here, we present a detailed two-dimensional (2-D) protein map of rat bile duct epithelial cells (i.e., cholangiocytes) using a recently developed isolation procedure. In addition, we identify 27 major cholangiocyte proteins either by comparison to maps of known rat liver proteins (based on pI and Mr) or by tryptic digestion and microsequencing. Finally, we compare the relative abundance of individual proteins present in cholangiocytes to whole liver as well as hepatocyte-specific proteins. Our results show that cholangiocytes express a unique array of individual proteins. The cholangiocyte 2-D protein pattern is markedly different from that of isolated rat hepatocytes or whole rat liver, with high levels of proteins previously known to be expressed by cholangiocytes (e.g., cytokeratins, actins) as well as protein not previously demonstrated to be expressed at high levels (e.g., annexin V, selenium binding protein). We conclude that this cholangiocyte-derived, 2-D protein map will be a crucial resource for studies directed at our understanding of cholangiocyte physiology and pathobiology.

Cyclosporine A decreases the protein level of the calcium-binding protein calbindin-D 28kDa in rat kidney

Despite the widespread use of cyclosporine A (CsA), its mechanism of action and side effects are not yet completely understood. There exists a large body of evidence suggesting that disturbance of calcium homeostasis is a critical step in the cascade of cellular and molecular events induced by the drug. As recently shown in our laboratory by two-dimensional protein gel electrophoresis (2-DE) analysis of kidney homogenates, CsA induced numerous changes in several kidney proteins. One kidney protein in particular was shown to be strongly down-regulated by the drug. In this work we report the identification of the strongly decreased kidney protein as calbindin-D 28kDa, a vitamin D-dependent calcium-binding protein associated with calcium handling by cells. The assignment of the down-regulated protein spot is based on its internal amino acid sequence analysis and its specific reaction with a monoclonal antibody raised against calbindin-D 28kDa. In kidney homogenates of male Wistar rats treated with 50 mg/kg/d CsA for up to 28 days, calbindin levels were measured by ELISA and were shown to be continuously decreased with prolonged CsA treatment. To our knowledge, this is the first report describing the effect of CsA on kidney calbindin-D 28kDa protein levels. Further studies are needed to elucidate whether the CsA-mediated down-regulation of the calcium-binding protein calbindin-D 28kDa may be a critical factor for the renal adverse effects induced by this drug.

Induction of the adipose differentiation-related protein in liver of etomoxir-treated rats

The effects of etomoxir, an irreversible carnitine palmitoyltransferase I inhibitor, on the liver protein pattern and on liver morphology were examined by two-dimensional gel electrophoresis in female Sprague-Dawley rats treated with 125 mg/kg/day etomoxir for 28 days. In livers of treated animals a protein spot was found which was not present in controls. The spot was identified by internal amino acid sequence analysis as the adipose differentiation-related protein (ADRP). The expression of ADRP in liver is a novel finding as the protein has been described previously as adipocyte-specific. Additionally we found histopathologic evidence of lipid accumulation in the livers of etomoxir rats. The data show that for each treated rat there was a good correlation between ADRP levels and degree of lipid droplet formation. This observation may suggest a potential relationship between drug-induced expression of ADRP in liver and lipid accumulation.

An updated two-dimensional gel database of rat liver proteins useful in gene regulation and drug effect studies

We have improved upon the reference two-dimensional (2-D) electrophoretic map of rat liver proteins originally published in 1991 (N. L. Anderson et al., Electrophoresis 1991, 12, 907-930). A total of 53 proteins (102 spots) are now identified, many by microsequencing. In most cases, spots cut from wet, Coomassie Blue stained 2-D gels were submitted to internal tryptic digestion [2], and individual peptides, separated by high-performance liquid chromatography (HPLC), were sequenced using a Perkin-Elmer 477A sequenator. Additional spots were identified using specific antibodies.

Characterization of a rat C6 glioma-secreted follistatin-related protein (FRP). Cloning and sequence of the human homologue

A protein was isolated from rat C6 glioma-conditioned medium and was biochemically characterized. The heparin-binding protein has a native molecular mass of 55-75,000 Da, a molecular mass of 40-48,000 Da under denaturing conditions, and a pI of 5.0-6.0. Based on the determined partial amino acid sequences, the full lenght cDNA encoding the rat and human proteins were cloned. The cDNA sequences identified the isolated rat and human protein as the homologue of a recently reported mouse osteoblast-transforming-growth-factor-beta 1-inducible protein, encoded by the TSC-36 gene [Shibanuma, M., Mashimo, J., Mita, A., Kuroki, T. & Nose, K. (1993) Eur. J. Biochem. 217, 13-19]. Analysis of the human, rat and mouse amino acid sequences indicates that these proteins are highly conserved (> 92% sequence identity). Sequence similarities with follistatin and the follistatin-like domain of agrin are revealed. The relationship with follistatin and agrin points to possible common functions for the cloned follistatin-related proteins (FRP). The protein has no effect on the inhibitory action of transforming growth factor-beta 1, on CCl-64 cell growth.

Characterization of the osteogenic stromal cell line MN7: identification of secreted MN7 proteins using two-dimensional polyacrylamide gel electrophoresis, western blotting, and microsequencing

Proteins secreted by the osteogenic stromal cell line MN7 were analyzed using two-dimensional polyacrylamide gel electrophoresis (PAGE), western blotting, immunodetection, and microsequencing. Trichloroacetic acid-precipitated proteins from the conditioned medium of MN7 cell cultures, harvested at different times of growth, were dissolved in denaturing and reducing sample buffer and separated in the first dimension according to isoelectric point and in the second dimension according to molecular weight. Protein patterns were visualized using silver staining. Among the 350 separated protein spots, we identified type I collagen, bone sialoprotein, osteonectin, and cathepsin B by western blotting and immunodetection using polyclonal antibodies. Osteocalcin could not be detected in the conditioned medium of MN7 cells. Furthermore, 15 MN7-specific protein spots were localized after comparison with two-dimensional PAGE patterns from the conditioned medium of the nonosteogenic stromal cell lines MM1 and MV1. Microsequencing of the internal peptides of five selected spots revealed three known proteins, namely the carboxyl-terminal propeptide of the alpha 2 chain of collagen type I, cathepsin L, and the tissue inhibitor of metalloproteinases-2, an 18 kilodalton peptide fragment from osteopontin that has not previously been described, and a novel glycosylated 85 kD protein with an average isoelectric point of 5.7. All identified proteins did not vary in presence between the different time points analyzed by two-dimensional PAGE. The use of two-dimensional PAGE to investigate the secreted proteins of MN7 cells will enable us to establish a complete protein data base of extracellular osteoblast-specific proteins. Furthermore, two-dimensional PAGE in combination with other techniques is a fast and accurate method for the identification of novel proteins that could function as markers in osteoblast differentiation and/or bone formation.

Endonexin II, present on human liver plasma membranes, is a specific binding protein of small hepatitis B virus (HBV) envelope protein

Binding of viral envelope proteins to specific receptors on human hepatocytes is considered to be an important step in HBV infection. In this study, we demonstrate that a 34-kDa human liver plasma membrane protein specifically binds to small HBsAg in a Ca(2+)-dependent manner. By partial amino acid sequence analysis of preparatively isolated 34-kDa protein comigrating with HBsAg-binding protein obtained from binding assay on IEF/SDS-PAGE, we have identified this HBsAg-binding protein as Endonexin II (E-II). Native human liver E-II inhibits binding of HBsAg to intact human hepatocytes and shows specific binding to small HBsAg. This binding can be inhibited by human liver plasma membrane proteins, recombinant E-II, or anti-E-II antibodies. Despite 90% sequence homology, rat liver E-II does not bind to small HBsAg and does not inhibit significantly (less than 20%) binding of HBsAg to intact hepatocytes. Cross-linking of small HBsAg and radiolabeled human liver E-II resulted in a specific additional protein complex on PAGE with an apparent molecular weight of 90 kDa, corresponding to a complex of E-II and small HBsAg with a ratio of 2 to 1 or 1 to 2. These findings indicate that E-II, found in human liver, is a specific HBsAg-binding protein and might play an important role in the initiation of HBV infection.

Purification and sequence of rat extracellular superoxide dismutase B secreted by C6 glioma

An enzyme which converts radical oxygen, produced by phorbol 12-myristate 13-acetate activated neutrophils, into nonluminescent products is secreted by rat C6 glioma. The enzyme was purified from chemically defined conditioned media and identified as an extracellular superoxide dismutase (EC-SOD). The purified enzyme is distinct from human EC-SOD C (Hjalmarsson, K., Marklund, S. L., Engström, A., and Edlund, T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 6340-6344) by its elution from heparin-Sepharose at 300-400 mM NaCl, its pI of 6.1-7.2, and its native M(r) of 85,000 +/- 20,000. The rat EC-SOD is a dimer with a subunit M(r) of 34,000-36,000 and is extensively modified by post-translational processing. Although rat EC-SOD has a high sequence homology with the catalytic center and the polybasic heparin-binding site near the COOH terminus of human EC-SOD C, its NH2-terminal sequence and the sequences flanking the heparin-binding site differ substantially. The sequence of the isolated rat EC-SOD cDNA fully confirms the data obtained from amino acid sequence analysis. The amino acid sequence of the enzyme and its biochemical properties support its identification as the rat EC-SOD B.