Prolonged exposure of pancreatic beta cells to raised glucose concentrations results in increased cellular content of islet amyloid polypeptide precursors

Abstract

Islet amyloid polypeptide (IAPP) is a 37 amino acid peptide that is produced by pancreatic beta cells following synthesis of (pre)pro-IAPP and proteolytic cleavage at dibasic lys-arg residues [1]. The peptide is the predominant component of the amyloid deposits which are often observed in pancreatic islets of non-insulin dependent diabetic patients [2,3]. Formation of amyloid fibrils appears dependent on an amyloidogenic region (amino acids 20±29) in the IAPP-amino acid sequence, known to be present in the human peptide [4,5]. Overproduction and increased secretion of IAPP have been suggested to be predisposing conditions [6,7] but their relation to amyloid formation is not yet understood [8]. Newly diagnosed patients with Type II (non-insulin-dependent) diabetes have increased circulating concentrations of IAPP-like immunoreactivity, containing high MW IAPP-like peptides [9]. Islet amyloid polypeptide precursors also seem to be present in the amyloid deposits, as documented by their immunocytochemical reactivity for a flanking peptide sequence of pro-IAPP [10]. It is still not clear to which extent precursor forms are present in islet beta cells and whether continuously raised glucose concentrations ± as occurring in non-insulin dependent diabetes ± can increase their abundance, intra-and extracellu- Diabetologia (1999) Summary Most non-insulin dependent diabetic patients have amyloid deposits in their pancreatic islets. It is not known whether chronic hyperglycaemia contributes to the formation of amyloid fibrils from the islet amyloid polypeptide that is produced by the pancreatic beta cells. Since islet amyloid exhibits islet amyloid polypeptide precursors immunoreactivity, we examined whether sustained in vitro exposure to raised glucose increases the abundance of these precursors in human beta cells. After 6 days stimulation with 20 mmol/l glucose the cellular content of insulin but not islet amyloid polypeptide was decreased leading to an increase in the ratio of the latter over insulin (3.0 ± 0.6 vs 1.8 ± 0.3 after 6 mmol/l glucose culture, p < 0.05). Similar changes occurred in rat beta cells cultured for 3 days in the presence of 20 mmol/1 glucose plus 3-isobutyl-1-methylxanthine. Western blot analysis of cellular islet amyloid polypeptide after prolonged exposure to high glucose indicated the presence of higher proportions of its precursor-and intermediate forms. In human beta cells cultured in 20 mmol/l glucose, the major form corresponds to an intermediate species which exhibits an immunoreactivity for the N-flanking peptide, as is also the case in islet amyloid. We concluded that prolonged in vitro exposure of beta cells to raised glucose concentrations increases the relative proportion of islet amyloid polypeptide over insulin, as well as of its precursors over the mature form of islet amyloid polypeptide. [Diabetologia (1999)

Amyloid fibril formation is progressive and correlates with beta-cell secretion in transgenic mouse isolated islets

“…Synthetic proIAPP has been shown to form fibrils in vitro [28]; if this mechanism occurs in vivo, intracellular fibrils could be initiated at any point inside the cellular organelles involved with the pathway of synthesis and post-translational processing of the precursor peptide. Immunoreactivity for proIAPP fragments has been detected in some amyloid deposits [29] and it has been suggested that there is an increase in pro-IAPP intermediates in cultured human islets [30]; proIAPP fibrils either inside or outside the cells could act as an initiator for conversion of IAPP to beta conformation and fibril formation. The IAPPimmunoreactive material in beta-cell granules of TM has been described as fibrils [8,31]; if this represents some form of insoluble IAPP or proIAPP accumulation, this could act as a nidus for fibril formation [32] at the surface of the cell when the granule is released.…”

Islet amyloid: a complication of islet dysfunction or an aetiological factor in Type 2 diabetes?

“…In T2DM [71] and in patients with insulinoma (both conditions associated with amyloid) there is increased secretion of incompletely processed proinsulin, [72,73]. Islet studies in vitro indicate increased production of N-terminal intact proIAPP under stimulated secretory conditions [74] and immunoreactivity for the N-terminal peptide of proIAPP has been identified in islet amyloid deposits in insulinomas and T2DM [75]. N-terminal intact proIAPP has been identified in a PC2 knockout, hIAPP transgenic mouse model [76].…”

The role of caspase-8 in amyloid-induced beta cell death in human and mouse islets

“…Three major factors contribute to human islet amyloid polypeptide (hIAPP) aggregation in type 2 diabetes: (1) presence of an amyloidogenic sequence in the hIAPP molecule [4]; (2) elevated beta cell production and secretion of hIAPP due to increased insulin demand [16,17]; and (3) impaired prohIAPP processing and/or trafficking due to beta cell dysfunction [17][18][19]. Impaired clearance of secreted hIAPP because of disrupted blood vessels in isolated islets may also potentiate amyloid formation.…”