Abstract
Release of pro-inflammatory cytokines from both resident and invading leukocytes within the pancreatic islets impacts the development of Type 1 diabetes mellitus. Synthesis and secretion of the chemokine CCL2 from pancreatic β-cells in response to pro-inflammatory signaling pathways influences immune cell recruitment into the pancreatic islets. Therefore, we investigated the positive and negative regulatory components controlling expression of the CCL2 gene using isolated rat islets and INS-1-derived β-cell lines. We discovered that activation of the CCL2 gene by IL-1β required the p65 subunit of NF-κB and was dependent on genomic response elements located in the −3.6 kb region of the proximal gene promoter. CCL2 gene transcription in response to IL-1β was blocked by pharmacological inhibition of the IKKβ and p38 MAPK pathways. The IL-1β-mediated increase in CCL2 secretion was also impaired by p38 MAPK inhibition and by glucocorticoids. Moreover, multiple synthetic glucocorticoids inhibited the IL-1β-stimulated induction of the CCL2 gene. Induction of the MAP Kinase Phosphatase-1 (MKP-1) gene by glucocorticoids or by adenoviral-mediated overexpression decreased p38 MAPK phosphorylation, which diminished CCL2 gene expression, promoter activity, and release of CCL2 protein. We conclude that glucocorticoid-mediated repression of IL-1β-induced CCL2 gene transcription and protein secretion occurs in part through the upregulation of the MKP-1 gene and subsequent deactivation of the p38 MAPK. Furthermore, the anti-inflammatory actions observed with MKP-1 overexpression were obtained without suppressing glucose-stimulated insulin secretion. Thus, MKP-1 is a possible target for anti-inflammatory therapeutic intervention with preservation of β-cell function.
Highlights
Type 1 diabetes mellitus (T1DM) results from selective elimination of the insulin-producing b-cells within the pancreatic islets via an autoimmune mediated process that requires infiltration of T-lymphocytes and activation of resident macrophages [1,2,3]
While we observed low levels of CCL2 gene expression in the basal state, there was a marked induction in response to IL-1b (cycle threshold (Ct) values are given in Figure S1).we conclude that IL-1b drives the synthesis and secretion of CCL2 in rat pancreatic b-cells
Because GCs do not impair IL-1b-mediated translocation of p65 from cytoplasm to nucleus (Figure S5), and because expression of the CCL2 gene is p38 mitogen-activated protein kinases (MAPKs) dependent (Figure 5), we investigated whether the MAP Kinase Phosphatase-1 (MKP-1) gene was responsive to GCs in 832/13 cells and isolated rat islets
Summary
Type 1 diabetes mellitus (T1DM) results from selective elimination of the insulin-producing b-cells within the pancreatic islets via an autoimmune mediated process that requires infiltration of T-lymphocytes and activation of resident macrophages [1,2,3]. The degradation of IkBs reveals the nuclear localization signals in NF-kB; dimerization and nuclear accumulation of combinations of NF-kB subunit proteins ensues, facilitating signal-mediated regulation of gene transcription within a given cell type, including those that contribute to inflammatory responses [20,22]. Since p38 MAPK integrates cell surface receptor-mediated signaling pathways to inflammatory responses [27], upregulation of MKP-1 in pancreatic b-cells may represent a viable strategy to decrease inflammation-associated pathologies, such as cytokine-mediated increases in chemokine production. We discovered that augmenting MKP-1 levels partially mimicked the inhibition of the IL-1b-mediated increase in CCL2 gene expression and secretion seen with GCs. Importantly, the anti-inflammatory actions associated with MKP-1 overexpression were obtained without suppressing glucose-stimulated insulin secretion, demonstrating that this gene is a possible target for therapeutic intervention with preservation of bcell function
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