Notch signaling: a mediator of beta-cell de-differentiation in diabetes?

Cytokines are mediators of pancreatic beta-cell dysfunction and death in type 1 diabetes mellitus. Microarray analyses of insulin-producing cells exposed to interleukin-1beta+interferon-gamma showed decreased expression of genes related to beta-cell-differentiated functions and increased expression of members of the Notch signaling pathway. Re-expression of this developmental pathway may contribute for loss-of-function of beta-cells exposed to an autoimmune attack. In this study, we show that rat primary beta-cells exposed to cytokines up-regulate several Notch receptors and ligands, and the target gene Hes1. Transfection of insulin-producing INS-1E cells and primary rat beta-cells with a constitutively active form of the Notch receptor down-regulated Pdx1 and insulin expression in INS-1E cells but not in primary beta-cells. Thus, activation of the Notch pathway inhibits differentiated functions in dividing but not in terminally differentiated beta-cells.

Macrophages enhance the radiosensitizing activity of lipid A: a novel role for immune cells in tumor cell radioresponse.

PURPOSE: This study examines whether activated macrophages may radiosensitize tumor cells through the release of proinflammatory mediators. METHODS AND MATERIALS: RAW 264.7 macrophages were activated by lipid A, and the conditioned medium (CM) was analyzed for the secretion of cytokines and the production of nitric oxide (NO) through inducible nitric oxide synthase (iNOS). EMT-6 tumor cells were exposed to CM and analyzed for hypoxic cell radiosensitivity. The role of nuclear factor (NF)-kappaB in the transcriptional activation of iNOS was examined by luciferase reporter gene assay. RESULTS: Clinical immunomodulator lipid A, at a plasma-relevant concentration of 3 microg/mL, stimulated RAW 264.7 macrophages to release NO, tumor necrosis factor (TNF)-alpha, and other cytokines. This in turn activated iNOS-mediated NO production in EMT-6 tumor cells and drastically enhanced their radiosensitivity. Radiosensitization was abrogated by the iNOS inhibitor aminoguanidine but not by a neutralizing anti-TNF-alpha antibody. The mechanism of iNOS induction was linked to NF-kappaB but not to JAK/STAT signaling. Interferon-gamma further increased the NO production by macrophages to a level that caused radiosensitization of EMT-6 cells through the bystanding effect of diffused NO. CONCLUSIONS: We demonstrate for the first time that activated macrophages may radiosensitize tumor cells through the induction of NO synthesis, which occurs in both tumor and immune cells.

An octamer motif is required for activation of the inducible nitric oxide synthase promoter in pancreatic beta-cells.

Nitric oxide, generated by the inducible form of nitric oxide synthase (iNOS), is a potential mediator of cytokine-induced beta-cell dysfunction in type 1 diabetes mellitus. We have previously shown that cytokine-induced iNOS expression is cycloheximide (CHX) sensitive and requires nuclear factor-kappa B (NF-kappa B) activation. In the present study, we show that an octamer motif located 20 bp downstream of the proximal NF-kappa B binding site in the rat iNOS promoter is critical for IL-1 beta and interferon-gamma induction of promoter activity in rat primary beta-cells and insulin-producing RINm5F cells. In gel shift assays, the octamer motif bound constitutively the transcription factor Oct1. Neither Oct1 nor NF-kappa B binding activities were blocked by CHX, suggesting that other factor(s) synthesized in response to IL-1 beta contribute to iNOS promoter induction. The high mobility group (HMG)-I(Y) protein also bound the proximal iNOS promoter region. HMG-I(Y) binding was decreased in cells treated with CHX and HMG-I(Y) silencing by RNA interference reduced IL-1 beta-induced iNOS promoter activity. These results suggest that Oct1, NF-kappa B, and HMG-I(Y) cooperate for transactivation of the iNOS promoter in pancreatic beta-cells.

Discovery of gene networks regulating cytokine-induced dysfunction and apoptosis in insulin-producing INS-1 cells.

Locally released cytokines contribute to beta-cell dysfunction and apoptosis in type 1 diabetes. In vitro exposure of insulin-producing INS-1E cells to the cytokines interleukin (IL)-1beta + interferon (IFN)-gamma leads to a significant increase in apoptosis. To characterize the genetic networks implicated in beta-cell dysfunction and apoptosis and its dependence on nitric oxide (NO) production, we performed a time-course microarray analysis of cytokine-induced genes in insulin-producing INS-1E cells. INS-1E cells were exposed in duplicate to IL-1beta + IFN-gamma for six different time points (1, 2, 4, 8, 12, and 24 h) with or without the inducible NO synthase (iNOS) blocker N(G)-monomethyl-L-arginine (NMA). The microarray analysis identified 698 genes as cytokine modified (>or=2.5-fold change compared with control) in at least one time point. Based on their temporal pattern of variation, the cytokine-regulated genes were classified into 15 clusters by the k-means method. These genes were further classified into 14 different groups according to their putative function. Changes in the expression of genes related to metabolism, signal transduction, and transcription factors at all time points studied indicate beta-cell attempts to adapt to the effects of continuous cytokine exposure. Notably, several apoptosis-related genes were modified at early time points (2-4 h) preceding iNOS expression. On the other hand, 46% of the genes modified by cytokines after 8-24 h were NO dependent, indicating the important role of this radical for the late effects of cytokines. The present results increase by more than twofold the number of known cytokine-modified genes in insulin-producing cells and yield comprehensive information on the role of NO for these modifications in gene expression. These data provide novel and detailed insights into the gene networks activated in beta-cells facing a prolonged immune assault.

Molecular regulation of monocyte chemoattractant protein-1 expression in pancreatic beta-cells.

Pancreatic beta-cells are selectively destroyed during the course of type 1 diabetes. In the early stages of the disease, inflammatory infiltrates of mononuclear cells, containing predominantly monocytes and T-cells, are present in the islets (insulitis). Chemokines, such as monocyte chemoattractant protein-1 (MCP-1), play a key role in the recruitment and activation of these immunocytes. We have previously described cytokine-induced MCP-1 gene expression in human and rat pancreatic islets. In the present study, the transcriptional regulation by cytokines of the rat MCP-1 gene in fluorescence-activated cell sorting-purified rat beta-cells, insulin-producing INS-1E cells, and RINm5F cells was investigated. Transient transfections with luciferase-reporter constructs identified an interleukin (IL)-1beta-responsive enhancer region between -2,180 bp and -2,478 bp. Mutation of either of the two nuclear factor (NF)-kappaB sites present in this region abrogated IL-1beta-induced MCP-1 promoter activity. Binding of NF-kappaB to the two sites was shown in vitro by gel shift assays, while supershift assays revealed the presence of p65/p50 heterodimers and p65 homodimers. In vivo binding of NF-kappaB was confirmed by chromatin immunoprecipitation assay. Blocking of NF-kappaB activation in cytokine-exposed primary beta-cells by an adenovirus overexpressing a nondegradable form of IkappaBalpha or by pyrrolidine dithiocarbamate decreased IL-1beta-induced MCP-1 mRNA expression. We conclude that NF-kappaB plays an important role for MCP-1 expression in beta-cells. This transcription factor may be an interesting target for ex vivo gene therapy before islet transplantation.

Double-stranded RNA cooperates with interferon-gamma and IL-1 beta to induce both chemokine expression and nuclear factor-kappa B-dependent apoptosis in pancreatic beta-cells: potential mechanisms for viral-induced insulitis and beta-cell death in type 1 diabetes mellitus.

Viral infections may trigger the autoimmune assault leading to type 1 diabetes mellitus. Double-stranded RNA (dsRNA) is produced by many viruses during their replicative cycle. The dsRNA, tested as synthetic poly(IC) (PIC), in synergism with the proinflammatory cytokines interferon-gamma (IFN-gamma) and/or IL-1 beta, results in nitric oxide production, Fas expression, beta-cell dysfunction, and death. Activation of the transcription nuclear factor-kappa B (NF-kappa B) is required for PIC-induced inducible nitric oxide synthase expression in beta-cells, and we hypothesized that this transcription factor may also participate in PIC-induced Fas expression and beta-cell apoptosis. This hypothesis, and the possibility that PIC induces expression of additional chemokines and cytokines (previously reported as NF-kappa B dependent) in pancreatic beta-cells, was investigated in the present study. We observed that the PIC-responsive region in the Fas promoter is located between nucleotides -223 and -54. Site-directed mutations at the NF-kappa B and CCAAT/enhancer binding protein-binding sites prevented PIC-induced Fas promoter activity. Increased Fas promoter activity was paralleled by enhanced susceptibility of PIC + cytokine-treated beta-cells to apoptosis induced by Fas ligand. beta-Cell infection with the NF-kappa B inhibitor AdI kappa B((SA)2) prevented both necrosis and apoptosis induced by PIC + IL-1 beta or PIC + IFN-gamma. Messenger RNAs for several chemokines and one cytokine were induced by PIC, alone or in combination with IFN-gamma, in pancreatic beta-cells. These included IP-10, interferon-gamma-inducible protein-10, IL-15, macrophage chemoattractant protein-1, fractalkine, and macrophage inflammatory protein-3 alpha. There was not, however, induction of IL-1 beta expression. We propose that dsRNA, generated during a viral infection, may contribute for beta-cell demise by both inducing expression of chemokines and IL-15, putative contributors for the build-up of insulitis, and by synergizing with locally produced cytokines to induce beta-cell apoptosis. Activation of the transcription factor NF-kappa B plays a central role in at least part of the deleterious effects of dsRNA in pancreatic beta-cells.