Effect of Recombinant α1-Antitrypsin Fc-Fused (AAT-Fc) Protein on the Inhibition of Inflammatory Cytokine Production and Streptozotocin-Induced Diabetes
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α1-Antitrypsin (AAT) is a member of the serine proteinase inhibitor family that impedes the enzymatic activity of serine proteinases, including human neutrophil elastase, cathepsin G and neutrophil proteinase 3. Here, we expressed recombinant AAT by fusing the intact AAT gene to the constant region of IgG1 to generate soluble recombinant AAT-Fc protein. The recombinant AAT-Fc protein was produced in Chinese hamster ovary (CHO) cells and purified using mini-protein A affinity chromatography. Recombinant AAT-Fc protein was tested for antiinflammatory function and AAT-Fc sufficiently suppressed tumor necrosis factor (TNF)-α-induced interleukin (IL)-6 in human peripheral blood mononuclear cells (PBMCs) and inhibited cytokine-induced TNFα by different cytokines in mouse macrophage Raw 264.7 cells. However, AAT-Fc failed to suppress lipopolysaccharide-induced cytokine production in both PBMCs and macrophages. In addition, our data showed that AAT-Fc blocks the development of hyperglycemia in a streptozotocin-induced mouse model of diabetes. Interestingly, we also found that plasma-derived AAT specifically inhibited the enzymatic activity of elastase but that AAT-Fc had no inhibitory effect on elastase activity.
α1-Antitrypsin (AAT) is a serine protease inhibitor and treatment with AAT prolongs islet allograft survival in mice (1). Moreover, AAT gene therapy modulates cellular immunity and efficiently prevents the development of type 1 diabetes in nonobese diabetic mice (2, 3, 4). AAT significantly reduces cytokine- and streptozotocin (STZ)-induced β-cell apoptosis by abolishing caspase-3 activity (5). AAT treatment also induces immune tolerance (6) and dampens inflammation, resulting in the expansion of the functional mass of β-cells in nonobese diabetic (NOD) mice (7), whereas this effect is not observed in a nonautoimmune mouse strain (8).
The association of AAT with diabetes was initially investigated by analyzing the meconium of infants delivered via cesarean section to diabetic mothers (9). The highest AAT concentration was observed in the third trimester of the diabetic pregnancy. However, the increased concentration of AAT in the sera of pregnant diabetic women did not correlate with the glycemic control value (9). A study reported that the urine sample of diabetic patients had markedly increased AAT concentrations (10). The presence of increased AAT levels in patients with diabetes mellitus was suggested by the reduction of inhibitory capacity on serum proteinases (11, 12, 13, 14); however, the precise molecular mechanism underlying the loss of AAT-mediated inhibitory functions is not clear.
The hereditary disorder of AAT deficiency is caused by mutations in the α1-antitrypsin (SERPINA1) gene (15,16), and the most common form of AAT deficiency occurs because of the Z mutation (17,18), resulting in aberrant folding and accumulation of the protein in the endoplasmic reticulum (ER). This step leads to ER stress and contributes significantly to liver disease. AAT is also synthesized by monocytes, neutrophils and epithelial cells (19,20). The unfolded AAT protein is activated in quiescent monocytes and contributes to an inflammatory phenotype, with Z mutation (ZZ) monocytes exhibiting enhanced cytokine production and activation of the nuclear factor (NF)-κB pathway when compared with normal M variant (MM) monocytes (19). These findings changed the previous paradigm of lung inflammation in AAT deficiency due to defects in AAT-mediated inhibition of neutrophil proteinase.
AAT-mediated suppression of formylmet-leu-phe (fMLP)-stimulated and non-stimulated neutrophil adhesion to fibronectin, as well as the inhibition of lipopolysaccharide (LPS)-induced interleukin (IL)-8 release and delayed neutrophil apoptosis, is independent of the inhibition of neutrophil proteinase activity (21). The effect of AAT on TNFα-induced self-expression is inhibited by the oxidation and modification of AAT, which abolishes the serine protease-inhibitor activity of AAT (22). These data further support the idea that the antiinflammatory function of AAT is independent of its inhibitory effect on elastase. In general, the patterns of gene expression regulated by native and oxidized AAT are similar, with neither of them stimulating proinflammatory genes or cytokine expression (23).
In the present study, we generated a recombinant form of AAT as a chimeric protein fused to Fc and successfully produced much recombinant AAT-Fc protein from stable clones of Chinese hamster ovary (CHO) cells. We evaluated the antiinflammatory properties of the purified AAT-Fc protein in vitro and in vivo. Our results demonstrate that the biological properties of AAT-Fc are very similar to those of plasma-derived AAT and that the antiinflammatory activity of AAT-Fc is more potent than that of plasma-derived AAT.
Materials and Methods
Recombinant AAT-Fc Protein Expression
Human pCAGGs-IgG1/AAT plasmids were cotransfected with the pSV-dihydrolfolate reductase (DHFR) vector (ATCC, Manassas, VA, USA) into the DHFR-deficient CHO cell line (DG44). Stable clones were selected in medium containing G418 (500 µ/mL) and subsequently subjected to methotrexate selection for gene amplification as previously described (24). The CHO stable clones that secrete soluble recombinant AAT-Fc were grown in serum-free medium (CHOSSFMII; Life Technologies, Carlsbad, CA, USA), as previously described (24).
Purification of AAT-Fc
The single cell clone selected was cultured in a 2-liter flask. The culture medium was first filtered through membranes with a pore size of 0.2 µm to remove debris. The flow-through result was then purified with protein A-conjugated beads that had been equilibrated with phosphate-buffered saline (PBS) containing 0.5 mol/L NaCl. The filtered cell culture medium was passed through a column containing the protein A resin, and 1-mL fractions of purified protein were collected. Glycine (pH 2.7) was used as the elution buffer, and the fractions were neutralized immediately with 100 µL of 1 mol/L Tris base.
For the detection of recombinant AAT-Fc protein, aliquots of the purified protein fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) on 10% acrylamide gels. The resolved proteins were transferred to a nitrocellulose membrane and Western blotting analysis was performed using rabbit anti-human AAT (YbdY, Seoul Korea) and horse radish peroxidase-conjugated goat anti-human IgG Fc antibodies (Sigma-Aldrich, St. Louis, MO, USA). The protein-antibody complexes were detected by using Supex (Neuronex, Seoul, Korea) and an LAS-4000 imaging device (Fujifilm, Japan).
Inhibitory Effect of AAT-Fc on the Enzymatic Activity of Elastase
Elastase (20 ng; Athens Research and Technology, Athens, GA, USA) and IL-32γ (20 ng; YbdY) were incubated in 20 µL PBS (pH 7.4) at room temperature for 10 min in the presence or absence of AAT-Fc. After completion of the reaction, the samples were subjected to SDS-PAGE and Western blotting analysis. A human IL-32-specific polyclonal antibody (YbdY) was used for detecting cleaved IL-32γ. A similar experiment was performed, and the reactions were used to assess the effect of AAT-Fc on the elastase-mediated cleavage of IL-32γ by using an IL-32γ-specific enzyme-linked immunosorbent assay (ELISA), instead of Western blotting analysis.
To examine the effect of AAT-Fc on TNFα- or LPS-induced cytokine production, peripheral blood mononuclear cells (PBMCs) were preincubated with plasma-derived AAT or AAT-Fc for 1 h and then simulated with fresh medium (0.2 mL) containing human TNFα (YbdY) or LPS (Sigma-Aldrich). Raw 264.7 cells were seeded in 96-well plates at a density of 1 ×105 cells per well 1 d before stimulation. The plated cells were preincubated for 1 h with plasma-derived AAT or AAT-Fc and then stimulated with LPS, human IL-32, human IL-33 or mouse IL-33 overnight. The cell culture supernatant of human PBMCs and mouse Raw 264.7 cells was harvested on the next day for the cytokine assay. Human IL-6 and mouse TNFα levels were measured in the supernatant by using sandwich ELISA kits (R&D Systems, Minneapolis, MN, USA) according to the manufacturer’s instructions. For measuring tissue cytokine levels, liver and pancreas tissues of STZ-treated mice (see below) were homogenized in cell lysis buffer containing 1% Triton X-100, and the IL-6 levels were measured as described above.
STZ-Induced Diabetic Mice
Female Balb/c mice 6 wks of age from Orient Biology (Seoungnam Kyeonggi-do, South Korea) were housed under a 12-h light-dark cycle at 23°C and given restricted access to food and water. Animal experiments were approved by the Institutional Animal Care and Use Committee at Konkuk University. For induction of diabetes, the mice (n = 5 in each group) were fasted for 3 h before intraperitoneal injection with a STZ solution (Sigma-Aldrich; 120 mg/kg body weight) in 0.05 mol/L sodium citrate buffer (pH 4.5); these solutions were prepared immediately before use. After injection, the mice were fasted for another 3 h and were then provided ad libitum access to a diet. To induce sufficient levels of hyperglycemia, the mice were treated with the same dose of STZ after 3 d. The mice were then tested daily for blood glucose levels after the initial injection. Blood glucose levels were determined by testing a drop of blood from an incision in the tip of the tail with a handheld blood glucose meter (Solartek Products, Alameda, CA, USA). For nonfasting blood glucose readings, a blood glucose level >250 mg/dL was set as the lower limit, indicative of the presence of diabetes (%).
The data were expressed as mean ± standard error of the mean (SEM). The statistical significance of differences was analyzed using the unpaired two-tailed Student t test. Values of p < 0.05 were considered statistically significant.
Expression of Recombinant Fc-Human AAT
Currently, several forms of AAT that have been purified from human plasma are available. However, we sought to produce a standardized recombinant form of AAT from CHO cells. The intact human AAT gene, encoding a protein of 418 amino acids, under the control of the chicken β-actin promoter, was transfected into CHO cells and stable transfectants were isolated. Although we were able to confirm the transfection of the human AAT gene by RT-PCR, compared with a mock transfection of CHO cells, recombinant AAT protein was not detectable in the cell culture supernatant (data not shown). We further examined the cell lysate of AAT-transfected CHO cells and found that most of the recombinant AAT protein remained inside the cells, despite the presence of a typical hydrophobic signal peptide at the N-terminus of AAT.
AAT-Fc Suppresses TNFα-Induced IL-6 Production in Human PBMCs
AAT-Fc Inhibits TNFα Production Induced by Various Cytokines
Human AAT-Fc Decreased Blood Glucose Levels in the STZ-Induced Diabetic Mouse Model
Biochemical Analysis of Elastase Activity in the Presence of AAT-Fc
To our knowledge, this study is the first to report the production of much recombinant AAT-Fc and the characterization of its biological activity in vitro and in vivo. We designed a mammalian expression vector to express an Fc-fusion protein containing the constant domain of human IgG1 (CH2 and CH3), including two cysteine residues in the hinge region, fused to the C-terminus of human AAT to facilitate the dimerization of the AAT-Fc molecule (Figure 1). As shown by silver staining and Western blot analyses, the recombinant purified AAT-Fc protein appeared as a dimer under the nonreduced conditions, whereas it migrated as a monomer under the reduced conditions (Figure 1, indicated by 2-mercaptoethanol [ME]).
Plasma-derived AAT augmentation therapy has been approved only for treatment of AAT deficiency in select adults with severe pulmonary emphysema. Recently, augmentation therapy has proven to be highly efficacious in small cohorts of patients with uncommon AAT deficiency-related diseases such as fibromyalgia, systemic vasculitis, relapsing panniculitis and bronchial asthma (25, 26, 27). In addition to these studies, a series of mouse model studies has described the efficacy of AAT under other conditions, including diabetes mellitus, organ transplant rejection and acute myocardial ischemia-reperfusion injury (1,3,4,6,28,29). These results have led to an expanding number of medical applications of AAT, such as allogeneic hematopoietic cell transplantation in patients with acute rejection (30,31).
Therefore, it would be useful and timely to implement new strategies for producing AAT. In this study, we produced recombinant AAT-Fc in CHO cells, tested the antiinflammatory effect of AAT-Fc on TNFα- or LPS-induced cytokine production in human PBMCs and compared the functions of plasma-derived AAT and our recombinant AAT-Fc. The recombinant AAT-Fc sufficiently suppressed TNFα-induced IL-6 production but failed to suppress LPS-induced IL-6. However, the same concentration of AAT failed to inhibit the activities of both TNFα and LPS; similar results were obtained in mouse Raw 264.7 cells. Previous studies have used AAT at concentrations that are at least several hundred times higher than that used in this experiment (4,20,21); this difference likely accounts for the lack of inhibition of TNFα and other inflammatory cytokines activities in our assays.
Recent studies on the antiinflammatory effects of AAT have suggested that these effects are independent of the inhibitory activity of serine proteinase (4,19,21) and that the ZZ allele mutation, which causes the formation of polymeric AAT (19), affects innate immunity, including inhibition of the T-cell immunity related to adaptive immunity (1,4). Nevertheless, growing evidence proposes that AAT possesses antiinflammatory functions that are independent of its inhibitory effects on serine proteinases. We also examined the effect of plasma-derived AAT and AAT-Fc on the enzymatic activity of elastase. Interestingly, plasma-derived AAT specifically blocked IL-32γ cleavage by elastase, whereas AAT-Fc could not block it. We cannot exclude AAT-Fc-mediated inhibition of other serine proteinases, since we evaluated the enzymatic activity of elastase on a single substrate (IL-32γ).
Numerous studies on plasma-derived AAT have shown that it protects insulin-producing β-cells in islets under different circumstances such as allogeneic islet transplantation (1,6,8,32,33) and treatment of NOD mice (2,5,7,28). Moreover, clinical studies have suggested that circulating AAT in plasma is associated with type 1 and type 2 diabetes (9,11, 12, 13,34, 35, 36, 37, 38). These studies led us to examine whether AAT-Fc exerts similar effects in the STZ-induced mouse model of diabetes. During earlier time points after induction of diabetes, there was no difference between non-treated control and AAT-Fc-treated mice. However, at 15 d after the first injection of STZ, the AAT-Fc-treated mice had significantly improved blood glucose levels compared with the mice treated with STZ alone; furthermore, these levels were sustained until d 17. These data suggest that AAT-Fc protects insulin-producing β-cells from STZ-induced diabetes. The mechanism underlying AAT-Fc-mediated antiinflammatory effects probably involves the suppression of IL-6 production in the pancreas, subsequently resulting in reduced hyperglycemia in the STZ-induced mouse model of diabetes.
To our knowledge, this study is the first to report a method for the production of much soluble recombinant AAT-Fc protein. The recombinant AAT-Fc protein was synthesized in CHO cells as a chimeric protein fused to the constant region of IgG1 and was purified by using protein A affinity chromatography. The AAT-Fc antiinflammatory activity was evaluated by monitoring the suppression of IL-6 and TNFα by using in vitro assays. In addition, AAT-Fc protein was found to prevent hyperglycemia and inflammatory processes in the STZ-induced mouse model of diabetes. Further studies on AAT-Fc in various autoinflammatory disease models will help identify and develop other clinical applications of AAT-Fc, including the potential replacement of plasma-derived AAT in therapy.
The authors declare that they have no competing interests as defined by Molecular Medicine, or other interests that might be perceived to influence the results and discussion reported in this paper.
This work was supported by National Research Foundation of Korea grant funded by the Korea government (MEST: 2012R1A2A1A01001791 and WCU: R33-2008-000-10022-0). S Kim received support from the Konkuk University research support program.
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