C4b-binding Protein Protects β-Cells from Islet Amyloid Polypeptide-induced Cytotoxicity

Abstract

C4BP (C4b-binding protein) is a polymer of seven identical ␣ chains and one unique ␤ chain synthesized in liver and pancreas. We showed previously that C4BP enhances islet amyloid polypeptide (IAPP) fibril formation in vitro. Now we report that polymeric C4BP strongly inhibited lysis of human erythrocytes incubated with monomeric IAPP, whereas no lysis was observed after incubation with preformed IAPP fibrils. In contrast, incubation with the monomeric ␣-chain of C4BP was less effective. These data indicate that polymeric C4BP with multiple binding sites for IAPP neutralizes lytic activity of IAPP. The present study focuses on C4BP 2 (C4b-binding protein), which is an essential inhibitor of the complement system (1). Complement has important roles in innate immunity and consists of more than 40 proteins involved in initiation and control of the system (2). C4BP is an abundant polymeric plasma protein that consists of seven identical ␣-chains with eight complement control protein (CCP) domains each, as well as one ␤-chain with three CCPs. The ␤-chain always carries anticoagulant PS (protein S) (3). Because of the high affinity of PS for negatively charged phospholipids, the C4BP-PS complex efficiently binds to apoptotic (4) and necrotic cells (5), ensuring controlled complement activation necessary for clearance of these cells. So far, C4BP has been characterized in detail as a complement inhibitor and is often employed by bacterial pathogens as a form of evasion strategy leading to decreased opsonization and phagocytosis (6). C4BP may have other immunomodulatory functions such as the induction of an antiinflammatory state in dendritic cells (7). Complete deficiency of C4BP has not been described, but non-synonymous polymorphisms causing impaired complement inhibitory activity were found in atypical hemolytic uremic syndrome (8) and recurrent, spontaneous pregnancy loss (9). C4BP is mainly secreted by hepatocytes but significant expression is also detected in lung and pancreatic islets. We found previously that C4BP interacts with various types of amyloid (10 -12), which similarly to in dying cells, allows a certain level of complement opsonization for clearance while preventing overt inflammation.In type 2 diabetes amyloid deposits are localized to the islets of Langerhans in the pancreas and mainly formed by islet amyloid polypeptide (IAPP). IAPP deposits are present in 90% of patients with type 2 diabetes and can cause apoptosis of ␤-cells (13). Apart from this cytotoxic effect IAPP also has a physiological role in regulating and stabilizing blood glucose levels. When insulin resistance develops in type 2 diabetes, so does the production and release of insulin and concomitantly IAPP in the ␤-cell (14, 15). Once IAPP reaches a critical concentration, it starts to aggregate. The small aggregates form ␤-sheet-rich proto-filaments that interact with each other and eventually form mature fibrils. Even though the exact cytotoxic form of IAPP (monomers, oligomers, or fibrils) is still discussed, there is a growing co...

Complement C3 and C4, but not their regulators or activated products, are associated with incident metabolic syndrome: the CODAM study

“…Genetic variation in the C4 binding protein (C4BP), which is an inhibitor of the classical and the lectin pathway, has been related to higher blood pressure and higher fasting blood glucose [ 37 ]. C4BP may also have a protective effect on β cell function [ 38 ]. In addition, recent developments showed that C4a, which is generated upon activation of C4, can bind to and activate the G-coupled protein receptor protease-activated receptor (PAR)1 and PAR4 [ 39 ].…”

Role of complement in diabetes

“…Islet amyloid polypetide (IAP) has been implicated in the pathophysiology of T2D and reduced insulin secretion (Jurgens et al, 2011). C4b-binding protein appears to have a protective role by inhibiting IAP aggregation into protofibrils (Sjolander et al, 2012;Sjolander et al, 2016), therefore preserving -cell function. Another protective molecule that received interest is CD59, which protects against membrane attack complex (MAC)-induced tissue destruction.…”

Brazilin inhibits fibrillogenesis of human islet amyloid polypeptide, disassembles mature fibrils, and alleviates cytotoxicity

“…The INS-1 cell line has been widely used as a model in studies of cytotoxicity related to T2DM. 71 In order to rst determine if brazilin alone shows signicant cytotoxicity, it was added at different concentrations to the cell cultures to determine its maximal tolerance dose (Fig. S3 †).…”