Cytokine-induced osteoprotegerin expression protects pancreatic beta cells through p38 mitogen-activated protein kinase signalling against cell death
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Pro-inflammatory cytokines play a crucial role in immune-mediated beta cell destruction, an essential mechanism in the pathogenesis of type 1 diabetes mellitus. Microarray analysis recently identified osteoprotegerin (OPG; now known as tumour necrosis factor receptor superfamily, member 11b [TNFRSF11B]) as a cytokine-induced gene in beta cells. The aim of the present study was to characterise the functional role and signalling pathways of OPG that are involved in cytokine-induced beta cell death.
Materials and methods
As cellular models, the rat beta cell line INS-1E and human primary pancreatic islets were employed. The effects of IL-1β and TNF-α on OPG expression were characterised by northern blot and immunoassay. The effect of OPG on beta cell survival was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Signalling pathways were evaluated by western blot analysis using antibodies against p38 mitogen-activated protein kinases (MAPK), c-Jun N-terminal kinase and extracellular signal-regulated kinase 1/2.
The INS-1E cell line and primary pancreatic islets expressed OPG mRNA and secreted OPG protein, both of which were enhanced by IL-1β and TNF-α. Exposure to IL-1β resulted in sustained phosphorylation of p38 MAPK in INS-1E cells and subsequent cell death. Administration of exogenous OPG prevented both IL-1β-induced beta cell death and sustained p38 MAPK phosphorylation.
Our data indicate that cytokine-induced production of OPG may protect beta cells from further damage. This protective effect is, at least in part, mediated through inhibition of p38 MAPK phosphorylation. Thus OPG is an autocrine or paracrine survival factor for beta cells.
KeywordsApoptosis Beta cell Cytokine Diabetes IL-1β MAPK Osteoprotegerin p38
extracellular signal-regulated kinase
c-Jun N-terminal kinase
mitogen-activated protein kinase
receptor activator of nuclear factor-κB
receptor activator of nuclear factor-κB ligand
TNF-related apoptosis-inducing ligand
T cells and pro-inflammatory cytokines play a crucial role in the pathogenesis of type 1 diabetes mellitus . While islet cell transplantation has emerged as a therapy for selected patients , cellular stress provoked by cytokines may limit the outcome of this procedure . Several studies indicate that cytokines act through phosphorylation of stress kinases, including c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) [1, 4]. Osteoprotegerin (OPG; now known as tumour necrosis factor receptor superfamily, member 11b [TNFRSF11]) is a glycoprotein that neutralises receptor activator of nuclear factor κB ligand (RANKL; now known as tumour necrosis factor [ligand] superfamily, member 11 [TNFSF11]) . RANKL promotes the differentiation and activation of osteoclasts and dendritic cells, and is essential for bone biology and immune functions .
We have shown that OPG is expressed in the pancreas . In addition, a microarray identified OPG as a cytokine-induced gene in beta cells . Here, we tested the hypothesis that cytokines regulate OPG expression in beta cells and evaluated the signalling pathways of OPG in cytokine-induced beta cell death. We found that cytokines enhance OPG production and that exposure to IL-1β induced beta cell death and prolonged p38 MAPK activation, which was prevented by OPG.
Materials and methods
Antibodies were from Cell Signaling (Frankfurt, Germany) or from Santa Cruz (Heidelberg, Germany), electrochemoluminescence (ECL) detection reagents were from Amersham Bioscience (Little Chalfont, UK).
INS-1E cells were maintained in RPMI-1640 medium supplemented with 10% fetal calf serum, ciprofloxacin (20 μg/ml), sodium pyruvate (1 mmol/l) and HEPES (10 mmol/l). Primary human islets were from the islet cell transplantation programme at the University of Giessen ; they were used after Institutional Review Board approval and informed consent had been obtained and were cultured in tissue culture medium-199 and identical supplements.
Northern blot analysis
RNA was isolated with a kit (RNeasy; Qiagen, Hilden, Germany), analysed on a denaturing agarose gel and transferred to nylon membranes. The cDNAs were radiolabelled using random primer DNA labelling .
RANKL and receptor activator of nuclear factor-κB (RANK; now known as tumour necrosis factor receptor superfamily, member 11a [TNFRSF11A]) gene expression by human islets was analysed by RT-PCR using 30 and 35 cycles, respectively, . As positive controls, RNA from SaOS-2 osteosarcoma cells, buffy coat cells and dendritic cells was used.
ELISA measurement of secreted OPG
OPG protein from rat INS-1E cells was measured using a murine OPG antibody pair (R & D Systems, Minneapolis, MN, USA) and a streptavidin-horseradish peroxidase construct (CV: 5.1%, lower limit of detection: 62 pg/ml). Human OPG protein was determined as described . Protein values were normalised for total protein content.
Assessment of cell survival
INS-1E cells were cultured for 24 h in the presence of IL1β with or without recombinant mouse OPG (R & D Systems). Cell survival was measured by adding 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) for 2 h. The supernatant fraction was removed, the tetrazolium dissolved in DMSO and absorbance was measured at 570 nm.
Western blot analysis
Cells were lysed and 20 μg of protein were separated by SDS-PAGE and transferred on to nitrocellulose. Membranes were incubated at 4°C with antibodies to p38 MAPK, JNK, extracellular signal-regulated kinase (ERK)-1/2 and β-actin. Signals were detected using horseradish peroxidase-conjugated anti-rabbit IgG antibody and ECL detection reagent.
Values are expressed as the mean ± SD of triplicate (OPG secretion) or duplicate (MTT assay) measurements. Student’s paired t test and multiple measurement ANOVA corrected by Student–Newman–Keul’s test were employed. A p value < 0.05 was considered statistically significant.
Next we characterised IL-1β, TNF-α and IFN-γ separately in dose–response and time-kinetic studies using INS-1E cells. IL-1β caused a dose- and time-dependent increase of Opg mRNA levels with a maximum effect at 1 ng/ml and after 24 h of exposure (Fig. 1b). At a dose of 10 ng/ml, IL-1β increased OPG secretion in INS-1E cells from 1.1 ± 0.09 ng/ml at baseline to 5.7 ± 0.39 ng/ml after 48 h (p < 0.00001) (Fig. 1c). TNF-α enhanced Opg mRNA levels with a maximum effect at 10 ng/ml and after 6 h (Fig. 1b). Moreover, TNF-α transiently induced an increase of OPG secretion from 0.73 ± 0.02 ng/ml at baseline to 2.72 ± 0.11 ng/ml after 6 h (p < 0.00001) (Fig. 1d). IFN-γ slightly enhanced Opg mRNA levels after 48 h (Fig. 1b), whereas IL6 had no effect. These data suggest that IL-1β and TNF-α were the main inducers of OPG in beta cells.
Our data verify the results of a microarray study  and demonstrate that OPG expression in beta cells is stimulated by IL-1β and TNF-α. Upregulation of OPG expression was detected at the mRNA and protein level, occurred in a dose- and time-dependent fashion and was substantial in magnitude (eightfold). Similar results were obtained in the rat cell line INS-1E and human islets, demonstrating that this regulation is independent of species and disease model. These findings indicate that stimulation of OPG by pro-inflammatory cytokines is a physiologically relevant response. IL-1β was most effective in inducing OPG, while TNF-α only transiently upregulated OPG. Therefore, further cytokine experiments were conducted with IL-1β.
In search of the function of beta-cell-derived OPG, we found that OPG abrogated IL-1β-induced p38 MAPK activation in INS-1E cells and prolonged cell survival. These data are consistent with earlier findings that p38 MAPK activation is required for IL-1β-induced beta cell death . In contrast to RT-PCR findings from the type 1 diabetes base (http://t1dbase.org/), we detected RANKL, but not RANK in beta cells.
RANKL inhibition by OPG is the major mechanism in the maintenance of bone metabolism . However, alternative pathways independent of RANKL binding have been described for OPG. First, OPG may also bind and neutralise TNF-related apoptosis-inducing ligand (TRAIL), thus preventing TRAIL-induced apoptosis in susceptible cells . This mechanism has been demonstrated in tumour cells and beta cells . However, we did not observe TRAIL-induced apoptosis in INS-1E cells (data not shown). Second, OPG binds to cells that express syndecan-1, probably via its heparin-binding domain . Syndecan-1 expression by myeloma cells has been identified as a mechanism to sequester and degrade OPG . Since we did not characterise expression of syndecans in pancreatic beta cells, we cannot exclude this possibility. Third, as yet unidentified receptors for OPG may exist on the cell surface, which confer direct OPG effects. Here, we identified inhibition of IL-1β signalling via p38 MAPK as a novel mechanism of OPG in pancreatic beta cells. IL-1β-induced beta cell death required sustained p38 MAPK activation , which in our study was abrogated by OPG, indicating that OPG is a crucial modulator of beta cell integrity. Our data do not exclude the possibility that other MAPKs contribute to beta cell death.
A potential limitation of this study was that we were unable to perform extensive signalling studies in human islets due to limited cell supply. In addition, the effect of RANKL on MAPK and survival of beta cells was not evaluated, because pancreatic islets did not express RANK mRNA. Because of its autocrine or paracrine mode of action, OPG released upon cytokine-induced stress may act both as a local and systemic safeguard within the pancreas to limit beta cell damage. This potential protective effect should be evaluated to optimise cell survival in the clinical setting of islet cell transplantation.
In conclusion, IL-1β and TNF-α induced OPG production by beta cells, which may limit cell damage, indicating that OPG is an autocrine or paracrine survival factor for beta cells. This protective effect is mediated through inhibition of the p38 MAPK pathway.
Parts of this study were performed during W. Rennekamp’s medical thesis at the Marburg Medical School, Philipps-University. The study was supported by a Heisenberg fellowship from Deutsche Forschungsgemeinschaft (German Research Council; Ho 1875/3-1 and 3-2) to L. C. Hofbauer and a grant from Deutsche Forschungsgemeinschaft (Ho 1875/5-2) to L. C. Hofbauer and M. Schoppet. The human islet cell programme at the University of Giessen was supported by the International Juvenile Diabetes Foundation.
Duality of interest
The authors are not aware of any conflict of interest.