The inducible ß5i proteasome subunit contributes to proinsulin degradation in GRP94-deficient ß-cells and is overexpressed in type 2 diabetes pancreatic islets.

Proinsulin is a misfolding-prone protein, and its efficient breakdown is critical when ß-cells are confronted with high-insulin biosynthetic demands, to prevent endoplasmic reticulum stress, a key trigger of secretory dysfunction and, if uncompensated, apoptosis. Proinsulin degradation is thought to be performed by the constitutively expressed standard proteasome, while the roles of other proteasomes are unknown. We recently demonstrated that deficiency of the proinsulin chaperone glucose-regulated protein 94 (GRP94) causes impaired proinsulin handling and defective insulin secretion associated with a compensated endoplasmic reticulum stress response. Taking advantage of this model of restricted folding capacity, we investigated the role of different proteasomes in proinsulin degradation, reasoning that insulin secretory dynamics require an inducible protein degradation system. We show that the expression of only one enzymatically active proteasome subunit, namely, the inducible ß5i-subunit, was increased in GRP94 CRISPR/Cas9 knockout (KO) cells. Additionally, the level of ß5i-containing intermediate proteasomes was significantly increased in these cells, as was ß5i-related chymotrypsin-like activity. Moreover, proinsulin levels were restored in GRP94 KO upon ß5i small interfering RNA-mediated knockdown. Finally, the fraction of ß-cells expressing the ß5i-subunit is increased in human islets from type 2 diabetes patients. We conclude that ß5i is an inducible proteasome subunit dedicated to the degradation of mishandled proinsulin.

Proinflammatory Cytokines Perturb Mouse and Human Pancreatic Islet Circadian Rhythmicity and Induce Uncoordinated ß-Cell Clock Gene Expression via Nitric Oxide, Lysine Deacetylases, and Immunoproteasomal Activity.

Pancreatic ß-cell-specific clock knockout mice develop ß-cell oxidative-stress and failure, as well as glucose-intolerance. How inflammatory stress affects the cellular clock is under-investigated. Real-time recording of Per2:luciferase reporter activity in murine and human pancreatic islets demonstrated that the proinflammatory cytokine interleukin-1ß (IL-1ß) lengthened the circadian period. qPCR-profiling of core clock gene expression in insulin-producing cells suggested that the combination of the proinflammatory cytokines IL-1ß and interferon-γ (IFN-γ) caused pronounced but uncoordinated increases in mRNA levels of multiple core clock genes, in particular of reverse-erythroblastosis virus α (Rev-erbα), in a dose- and time-dependent manner. The REV-ERBα/ß agonist SR9009, used to mimic cytokine-mediated Rev-erbα induction, reduced constitutive and cytokine-induced brain and muscle arnt-like 1 (Bmal1) mRNA levels in INS-1 cells as expected. SR9009 induced reactive oxygen species (ROS), reduced insulin-1/2 (Ins-1/2) mRNA and accumulated- and glucose-stimulated insulin secretion, reduced cell viability, and increased apoptosis levels, reminiscent of cytokine toxicity. In contrast, low (<5,0 µM) concentrations of SR9009 increased Ins-1 mRNA and accumulated insulin-secretion without affecting INS-1 cell viability, mirroring low-concentration IL-1ß mediated ß-cell stimulation. Inhibiting nitric oxide (NO) synthesis, the lysine deacetylase HDAC3 and the immunoproteasome reduced cytokine-mediated increases in clock gene expression. In conclusion, the cytokine-combination perturbed the intrinsic clocks operative in mouse and human pancreatic islets and induced uncoordinated clock gene expression in INS-1 cells, the latter effect associated with NO, HDAC3, and immunoproteasome activity.

Enhancer of Zeste Homolog 2 (EZH2) Mediates Glucolipotoxicity-Induced Apoptosis in ß-Cells.

Selective inhibition of histone deacetylase 3 (HDAC3) prevents glucolipotoxicity-induced ß-cell dysfunction and apoptosis by alleviation of proapoptotic endoplasmic reticulum (ER) stress-signaling, but the precise molecular mechanisms of alleviation are unexplored. By unbiased microarray analysis of the ß-cell gene expression profile of insulin-producing cells exposed to glucolipotoxicity in the presence or absence of a selective HDAC3 inhibitor, we identified Enhancer of zeste homolog 2 (EZH2) as the sole target candidate. ß-Cells were protected against glucolipotoxicity-induced ER stress and apoptosis by EZH2 attenuation. Small molecule inhibitors of EZH2 histone methyltransferase activity rescued human islets from glucolipotoxicity-induced apoptosis. Moreover, EZH2 knockdown cells were protected against glucolipotoxicity-induced downregulation of the protective non-canonical Nuclear factor of kappa light polypeptide gene enhancer in B-cells (NFκB) pathway. We conclude that EZH2 deficiency protects from glucolipotoxicity-induced ER stress, apoptosis and downregulation of the non-canonical NFκB pathway, but not from insulin secretory dysfunction. The mechanism likely involves transcriptional regulation via EZH2 functioning as a methyltransferase and/or as a methylation-dependent transcription factor.

Possible predisposition for colorectal carcinogenesis due to altered gene expressions in normal appearing mucosa from patients with colorectal neoplasia.

BACKGROUND: Investigations of colorectal carcinogenesis have mainly focused on examining neoplastic tissue. With our aim of identifying potentially cancer-predisposing molecular compositions, we chose a different approach by examining endoscopically normal appearing colonic mucosa of patients with and without colorectal neoplasia (CRN). Directed by this focus, we selected 18 genes that were previously found with altered expression in colorectal cancer affected mucosa. METHODS: Biopsies of colonic mucosa were sampled from 27 patients referred for colonoscopy on suspicion of colorectal disease. Of these, 14 patients had present or previous CRN and the remaining 13 patients served as controls. Using qPCR and Western blot technique, we investigated mRNA and protein expressions. Expressions were investigated for selected kinases in the extracellular signal-regulated kinase/mitogen activated protein kinase (ERK/MAPK), the phosphoinositide 3-kinase/Akt, and the Wnt/ß-catenin pathways as well as for selected phosphatases and several entities associated with prostaglandin E2 (PGE2) signaling. Colonic mucosal contents of PGE2 and PGE2 metabolites were determined by use of ELISA. RESULTS: We found up-regulation of ERK1, ERK2, Akt1, Akt2, PLA2G4A, prostanoid receptor EP3 and phosphatase scaffold subunit PPP2R1B mRNA expression in normal appearing colonic mucosa of CRN patients compared to controls. CONCLUSION: Present study supports that even normal appearing mucosa of CRN patients differs from that of non-CRN patients at a molecular level. Especially expression of ERK1 mRNA was increased (p = 0.007) in CRN group. ERK1 may therefore be considered a potential candidate gene as predictive biomarker for developing CRN. Further validation in larger cohorts are required to determine such predictive use in translational medicine and clinics.

Defective Proinsulin Handling Modulates the MHC I Bound Peptidome and Activates the Inflammasome in ß-Cells.

How immune tolerance is lost to pancreatic ß-cell peptides triggering autoimmune type 1 diabetes is enigmatic. We have shown that loss of the proinsulin chaperone glucose-regulated protein (GRP) 94 from the endoplasmic reticulum (ER) leads to mishandling of proinsulin, ER stress, and activation of the immunoproteasome. We hypothesize that inadequate ER proinsulin folding capacity relative to biosynthetic need may lead to an altered ß-cell major histocompatibility complex (MHC) class-I bound peptidome and inflammasome activation, sensitizing ß-cells to immune attack. We used INS-1E cells with or without GRP94 knockout (KO), or in the presence or absence of GRP94 inhibitor PU-WS13 (GRP94i, 20 µM), or exposed to proinflammatory cytokines interleukin (IL)-1ß or interferon gamma (IFNγ) (15 pg/mL and 10 ng/mL, respectively) for 24 h. RT1.A (rat MHC I) expression was evaluated using flow cytometry. The total RT1.A-bound peptidome analysis was performed on cell lysates fractionated by reverse-phase high-performance liquid chromatography (RP-HPLC), followed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). The nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain containing protein (NLRP1), nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha (IκBα), and (pro) IL-1ß expression and secretion were investigated by Western blotting. GRP94 KO increased RT1.A expression in ß-cells, as did cytokine exposure compared to relevant controls. Immunopeptidome analysis showed increased RT1.A-bound peptide repertoire in GRP94 KO/i cells as well as in the cells exposed to cytokines. The GRP94 KO/cytokine exposure groups showed partial overlap in their peptide repertoire. Notably, proinsulin-derived peptide diversity increased among the total RT1.A peptidome in GRP94 KO/i along with cytokines exposure. NLRP1 expression was upregulated in GRP94 deficient cells along with decreased IκBα content while proIL-1ß cellular levels declined, coupled with increased secretion of mature IL-1ß. Our results suggest that limiting ß-cell proinsulin chaperoning enhances RT1.A expression alters the MHC-I peptidome including proinsulin peptides and activates inflammatory pathways, suggesting that stress associated with impeding proinsulin handling may sensitize ß-cells to immune-attack.

The intermediate proteasome is constitutively expressed in pancreatic beta cells and upregulated by stimulatory, low concentrations of interleukin 1 ß.

A central and still open question regarding the pathogenesis of autoimmune diseases, such as type 1 diabetes, concerns the processes that underlie the generation of MHC-presented autoantigenic epitopes that become targets of autoimmune attack. Proteasomal degradation is a key step in processing of proteins for MHC class I presentation. Different types of proteasomes can be expressed in cells dictating the repertoire of peptides presented by the MHC class I complex. Of particular interest for type 1 diabetes is the proteasomal configuration of pancreatic ß cells, as this might facilitate autoantigen presentation by ß cells and thereby their T-cell mediated destruction. Here we investigated whether so-called inducible subunits of the proteasome are constitutively expressed in ß cells, regulated by inflammatory signals and participate in the formation of active intermediate or immuno-proteasomes. We show that inducible proteasomal subunits are constitutively expressed in human and rodent islets and an insulin-secreting cell-line. Moreover, the ß5i subunit is incorporated into active intermediate proteasomes that are bound to 19S or 11S regulatory particles. Finally, inducible subunit expression along with increase in total proteasome activities are further upregulated by low concentrations of IL-1ß stimulating proinsulin biosynthesis. These findings suggest that the ß cell proteasomal repertoire is more diverse than assumed previously and may be highly responsive to a local inflammatory islet environment.

Endoplasmic Reticulum Chaperone Glucose-Regulated Protein 94 Is Essential for Proinsulin Handling.

Although endoplasmic reticulum (ER) chaperone binding to mutant proinsulin has been reported, the role of protein chaperones in the handling of wild-type proinsulin is underinvestigated. Here, we have explored the importance of glucose-regulated protein 94 (GRP94), a prominent ER chaperone known to fold insulin-like growth factors, in proinsulin handling within ß-cells. We found that GRP94 coimmunoprecipitated with proinsulin and that inhibition of GRP94 function and/or expression reduced glucose-dependent insulin secretion, shortened proinsulin half-life, and lowered intracellular proinsulin and insulin levels. This phenotype was accompanied by post-ER proinsulin misprocessing and higher numbers of enlarged insulin granules that contained amorphic material with reduced immunogold staining for mature insulin. Insulin granule exocytosis was accelerated twofold, but the secreted insulin had diminished bioactivity. Moreover, GRP94 knockdown or knockout in ß-cells selectively activated protein kinase R-like endoplasmic reticulum kinase (PERK), without increasing apoptosis levels. Finally, GRP94 mRNA was overexpressed in islets from patients with type 2 diabetes. We conclude that GRP94 is a chaperone crucial for proinsulin handling and insulin secretion.

Acute administration of interleukin-6 does not increase secretion of glucagon-like peptide-1 in mice.

Interleukin 6 (IL-6) is a cytokine secreted from skeletal muscle in response to exercise which, based on animal and cell studies, has been suggested to contribute to glucose metabolism by increasing secretion of the incretin hormone glucagon-like peptide-1 (GLP-1) and affecting secretion of insulin and glucagon from the pancreatic islets. We investigated the effect of IL-6 on GLP-1 secretion in GLP-1 producing cells (GLUTag) and using the perfused mouse small intestine (harboring GLP-1 producing cells). Furthermore, the direct effect of IL-6 on insulin and glucagon secretion was studied using isolated perfused mouse pancreas. Incubating GLUTag cells with 1000 ng/mL of IL-6 for 2 h did not significantly increase secretion of GLP-1 whereas 10 mmol/L glucose (positive control) did. Similarly, IL-6 (100 ng/mL) had no effect on GLP-1 secretion from perfused mouse small intestine whereas bombesin (positive control) increased secretion. Finally, administering IL-6 (100 ng/mL) to perfused mouse pancreases did not significantly increase insulin or glucagon secretion regardless of perfusate glucose levels (3.5 vs. 12 mmol/L glucose). Acute effects of IL-6 therefore do not seem to include a stimulatory effect on GLP-1 secretion in mice.

Lysine demethylase inhibition protects pancreatic ß cells from apoptosis and improves ß-cell function.

Transcriptional changes control ß-cell survival in response to inflammatory stress. Posttranslational modifications of histone and non-histone transcriptional regulators activate or repress gene transcription, but the link to cell-fate signaling is unclear. Inhibition of lysine deacetylases (KDACs) protects ß cells from cytokine-induced apoptosis and reduces type 1 diabetes incidence in animals. We hypothesized that also lysine demethylases (KDMs) regulate ß-cell fate in response to inflammatory stress. Expression of the demethylase Kdm6B was upregulated by proinflammatory cytokines suggesting a possible role in inflammation-induced ß-cell destruction. Inhibition of KDM6 demethylases using the selective inhibitor GSK-J4 protected insulin-producing cells and human and mouse islets from cytokine-induced apoptosis by blunting nuclear factor (NF)-κB signaling and endoplasmic reticulum (ER) stress response gene expression. GSK-J4 furthermore increased expression of insulin gene and glucose-stimulated insulin secretion. Expression of genes regulating purinergic and cytokine ligand-receptor interactions was downregulated following GSK-J4 exposure, while expression of genes involved in cell maintenance and survival was upregulated. These data suggest that KDMs are important regulators of inflammation-induced ß-cell dysfunction and death.

The immunoproteasome is induced by cytokines and regulates apoptosis in human islets.

In addition to degrading misfolded and damaged proteins, the proteasome regulates the fate of cells in response to stress. The role of the proteasome in pro-inflammatory cytokine-induced human beta-cell apoptosis is unknown. Using INS-1, INS-1E and human islets exposed to combinations of IFNγ, IL-1ß and TNFα with or without addition of small molecules, we assessed the role of the immunoproteasome in pancreatic beta-cell demise. Here, we show that cytokines induce the expression and activity of the immuno-proteasome in INS-1E cells and human islets. Cytokine-induced expression of immuno-proteasome subunits, but not activity, depended upon histone deacetylase 3 activation. Inhibition of JAK1/STAT1 signaling did not affect proteasomal activity. Inhibition of the immuno-proteasome subunit PSMB8 aggravated cytokine-induced human beta-cell apoptosis while reducing intracellular levels of oxidized proteins in INS-1 cells. While cytokines increased total cellular NFκB subunit P50 and P52 levels and reduced the cytosolic NFκB subunit P65 and IκB levels, these effects were unaffected by PSMB8 inhibition. We conclude that beta cells upregulate immuno-proteasome expression and activity in response to IFNγ, likely as a protective response to confine inflammatory signaling.