Islet Macrophages Shift to a Reparative State following Pancreatic Beta-Cell Death and Are a Major Source of Islet Insulin-like Growth Factor-1.

Macrophages play a dynamic role in tissue repair following injury. Here we found that following streptozotocin (STZ)-induced beta-cell death, mouse islet macrophages had increased Igf1 expression, decreased proinflammatory cytokine expression, and transcriptome changes consistent with macrophages undergoing efferocytosis and having an enhanced state of metabolism. Macrophages were the major, if not sole, contributors to islet insulin-like growth factor-1 (IGF-1) production. Adoptive transfer experiments showed that macrophages can maintain insulin secretion in vivo following beta-cell death with no effects on islet cell turnover. IGF-1 neutralization during STZ treatment decreased insulin secretion without affecting islet cell apoptosis or proliferation. Interestingly, high-fat diet (HFD) combined with STZ further skewed islet macrophages to a reparative state. Finally, islet macrophages from db/db mice also expressed decreased proinflammatory cytokines and increased Igf1 mRNA. These data have important implications for islet biology and pathology and show that islet macrophages preserve their reparative state following beta-cell death even during HFD feeding and severe hyperglycemia.

TLR2/6 and TLR4-activated macrophages contribute to islet inflammation and impair beta cell insulin gene expression via IL-1 and IL-6.

AIMS/HYPOTHESIS: Inflammation contributes to pancreatic beta cell dysfunction in type 2 diabetes. Toll-like receptor (TLR)-2 and -4 ligands are increased systemically in recently diagnosed type 2 diabetes patients, and TLR2- and TLR4-deficient mice are protected from the metabolic consequences of a high-fat diet. Here we investigated the role of macrophages in TLR2/6- and TLR4-mediated effects on islet inflammation and beta cell function. METHODS: Genetic and pharmacological approaches were used to determine the effects of TLR2/6 and TLR4 ligands on mouse islets, human islets and purified rat beta cells. Islet macrophages were depleted and sorted by flow cytometry and the effects of TLR2/6- and TLR4-activated bone-marrow-derived macrophages (BMDMs) on beta cell function were assessed. RESULTS: Macrophages contributed to TLR2/6- and TLR4-induced islet Il1a/IL1A and Il1b/IL1B mRNA expression in mouse and human islets and IL-1ß secretion from human islets. TLR2/6 and TLR4 ligands also reduced insulin gene expression; however, this occurred in a non-beta cell autonomous manner. TLR2/6- and TLR4-activated BMDMs reduced beta cell insulin secretion partly via reducing Ins1, Ins2, and Pdx1 mRNA expression. Antagonism of the IL-1 receptor and neutralisation of IL-6 completely reversed the effects of activated macrophages on beta cell gene expression. CONCLUSIONS/INTERPRETATION: We conclude that islet macrophages are major contributors to islet IL-1ß secretion in response to TLR2/6 and TLR4 ligands. BMDMs stimulated with TLR2/6 and TLR4 ligands reduce insulin secretion from pancreatic beta cells, partly via IL-1ß- and IL-6-mediated decreased insulin gene expression.

Toll-like receptors and NLRP3 as central regulators of pancreatic islet inflammation in type 2 diabetes.

The global health and economic burden of type 2 diabetes (T2D) has reached staggering proportions. Current projections estimate that 592 million people will have diabetes by 2035. T2D-which comprises 90% of cases-is a complex disease, in most cases resulting from a combination of predisposing genes and an unhealthy environment. Clinical onset of the disease occurs when pancreatic ß cells fail in the face of insulin resistance. It has long been appreciated that chronic activation of the innate immune system is associated with T2D, and many organs critical to the regulation of glucose homeostasis show signs of a chronic inflammatory process, including the pancreatic islets of Langerhans. Recent clinical trials using IL-1-targeting agents have confirmed that inflammation contributes to ß-cell failure in humans with T2D. However, little is known about the nature of the pro-inflammatory response within the islet, and there is considerable debate about the triggers for islet inflammation, which may be systemically derived and/or tissue-specific. In this review, we present evidence that Toll-like receptors 2 and 4 and the NLRP3 (Nucleotide-binding oligomerization domain, Leucine-rich Repeat and Pyrin domain containing 3) inflammasome are triggers for islet inflammation in T2D and propose that the activation of macrophages by these triggers mediates islet endocrine cell dysfunction. Therapeutically targeting these receptors may improve hyperglycemia and protect the ß cell in T2D.

Induction and activation of antiviral enzyme 2',5'-oligoadenylate synthetase by in vitro transcribed insulin mRNA and other cellular RNAs.

Double-stranded RNA (dsRNA) can induce antiviral enzyme 2',5'-oligoadenylate synthetase (2'5'AS) expression and activate latent 2'5'AS. Our previous data have shown pancreatic ß cells are sensitive to dsRNA-induced 2'5'AS expression, and constitutive high basal 2'5'AS expression is associated with susceptibility to developing type 1 diabetes (T1D), a disease due to pancreatic ß cell loss. Here we report that in vitro transcribed human insulin mRNA induces the activation of human OAS gene promoter sequences, and specifically and dose-dependently induces 2'5'AS expression in murine pancreatic ßTC3 cells. Over-expression of dsRNA receptor retinoic acid-inducible gene-1 enhances insulin mRNA-induced 2'5'AS expression. In vitro transcribed insulin and other mRNAs, as well as total cellular RNAs, activate latent 2'5'AS in vitro with activation ability likely associated with the sequence and length of individual mRNAs or the sample source of total cellular RNA. Insulin mRNA does not show any specificity to activate 2'5'AS, but total cellular RNA from ßTC3 cells has high activation ability. Constitutive 2'5'AS expression in ßTC3 cells leads to cell proliferation inhibition and apoptosis. Our study suggests the possibility of cellular RNA-regulated 2'5'AS expression and activation, and the potential risk of high insulin gene transcription in pancreatic ß cells, and may help explain genetic predisposition to T1D associated with INS VNTR class I alleles.

A novel pancreatropic coxsackievirus vector expressing glucagon-like peptide 1 reduces hyperglycemia in streptozotocin-treated mice.

A coxsackievirus vector, vCVB(dm) (v stands for vector, CVB stands for group B coxsackievirus, and dm stands for double mutant), has been produced from a unique strain of coxsackievirus B3 (CVB3) containing 2 mutations that confer the property of highly selective pancreatropism. This vector has been tested as a delivery vehicle for glucagon-like peptide 1 (GLP-1), a peptide that enhances pancreatic regeneration following tissue damage. vCVB(dm) is a live vector comprising the entire plus-strand RNA genome with a multiple cloning site (MCS) inserted between the P1 and P2 gene regions. The MCS is flanked by sequences encoding the cleavage site for viral protease 2Apro that processes the polyprotein to release the incorporated gene. Our studies show that this vector selectively delivers GLP-1 to the pancreas where it is expressed in foci scattered throughout the acinar tissue for 4 or 5 days. Moreover, expression is associated with new beta cell clusters in juxtaposition to vector-infected cells. Inoculation of streptozotocin (STZ)-treated mice with vCVB(dm)GLP-1 was found to suppress development of hyperglycemia and increase insulin production relative to mice treated with STZ alone or with empty vector. This vector has the advantage of exclusively targeting pancreas and has potential use for short-term gene delivery to this tissue. The lack of viral integration provides a significant safety feature, making this vector a possible option for use as a therapeutic tool for pancreas-related diseases, including type 1 and 2 diabetes, pancreatitis, and pancreatic cancer.