Resident Macrophages Mediate Islet Amyloid Polypeptide–Induced Islet IL-1β Production and β-Cell Dysfunction

Abstract

Islet amyloid polypeptide (IAPP) aggregates to form amyloid fibrils in patients with type 2 diabetes and acts as a potent stimulus for interleukin (IL)-1b secretion by bone marrow-derived macrophages. We sought to determine the contribution of resident islet macrophages to IAPP-induced inflammation and b-cell dysfunction. In cultured islets, macrophages (F4/80+ cells) were required for IAPP-induced mRNA expression of the proinflammatory cytokines IL-1b, tumor necrosis factor-a, and IL-6 and the anti-inflammatory cytokines IL-10 and IL-1 receptor antagonist. Moreover, IAPP-induced IL-1b synthesis and caspase-1 activation were detected in macrophages but not other islet cell types. Transgenic mice with b-cell human IAPP (hIAPP) expression had impaired glucose tolerance, elevated islet Il1b mRNA, and decreased Il10 and Il1rn expression following highfat feeding. Islet macrophages were the major source of these transcripts and expressed increased cell surface Ly6C and CD11c in hIAPP transgenic mice. Clodronate liposome-mediated depletion of islet macrophages improved glucose tolerance and blocked proinflammatory gene expression in hIAPPexpressing mice, despite increasing the amount of islet amyloid. These data provide the first evidence that IAPP aggregates skew resident islet macrophages toward a proinflammatory phenotype and suggest a mechanism by which antiinflammatory therapies may protect b-cells from IAPP-induced islet dysfunction.

Differential Activation of Innate Immune Pathways by Distinct Islet Amyloid Polypeptide (IAPP) Aggregates

“…Islet and Cell Culture-Mouse islets were isolated by ductal collagenase injection and filtration (19) and cultured as described previously (7). For macrophage depletion, islets were treated immediately following isolation for 48 h with 1 mg/ml clodronate delivered in liposomes (20) (ClodLip BV, Amsterdam, The Netherlands).…”

“…Western Blot-WT and Nlrp3 Ϫ/Ϫ BMDMs were lysed, protein was quantified, and lysates were separated on 4 -20% polyacrylamide gels along with recombinant mature mouse IL-1␤ (eBioscience, San Diego, CA) and transferred to nitrocellulose membranes as previously described (7). Membranes were incubated in anti-mouse ␤-actin monoclonal antibody (1:5000 overnight at 4°C; #NB600 -501, Novus Biologicals, Littleton, CO) followed by infrared-dye-conjugated goat anti-mouse IgG (1:10,000 for 45 min at room temperature; #926 -68070 LI-COR Biotechnology, Lincoln, NE) and imaged on a LI-COR Odyssey (Lincoln, NE).…”

“…Resident macrophages are the major source of IAPP-induced Il1b gene expression in mouse islets (7,8), but it is unclear how IAPP is sensed by islet macrophages to provide an initial stimulus for proIL-1␤ synthesis. Other amyloidogenic peptides of both mammalian and eukaryotic origin activate the transcription factor NF-B by interacting with cell-surface TLRs, in particular TLR2 (10 -12).…”

“…Recent evidence suggests that IAPP aggregates trigger islet secretion of IL-1, which acts alone or in combination with other pro-inflammatory cytokines to impair beta cell insulin secretion (5,6). Human but not non-amyloidogenic rodent IAPP induces IL-1␤ synthesis in bone marrow-derived and intra-islet macrophages (7,8). IAPP also activates the NACHT, LRR and PYD domaincontaining protein 3 (NLRP3) inflammasome, required for processing of proIL-1␤ into its mature form (9).…”

Loss of prohormone convertase 2 promotes beta cell dysfunction in a rodent transplant model expressing human pro-islet amyloid polypeptide

“…Misfolded IAPP monomers assemble into oligomeric intermediates, which further generate amyloid fibres containing heparan sulphate proteoglycans, serum amyloid P and apolipoprotein E [4][5][6]. Prefibrillar IAPP species that arise during the transition from monomer to amyloid fibril elicit beta cell dysfunction and apoptosis [7][8][9], and provoke islet inflammation [9][10][11]. Cytotoxic IAPP aggregates mediate beta cell death and dysfunction in type 2 diabetes and in transplanted human islets [10,[12][13][14], although the mechanisms leading to cytotoxicity are poorly understood.…”