2124-P: The Tissue Plasminogen Activator (tPA)/Plasmin System Reduces Amyloid Formation by Cleaving Human Islet Amyloid Polypeptide (hIAPP)

Type 2 diabetes is characterized by islet amyloid deposition, which is associated with β-cell loss. hIAPP is the unique peptide component of these deposits. We previously found that expression of tPA, the initiator of fibrinolysis, is upregulated in islets by amyloid formation in vitro. As plasmin, the proteolytic product of tPA activity, cleaves Aβ and attenuates amyloid deposition in Alzheimer’s disease, we hypothesized that the increase in islet tPA by amyloid deposition serves a protective function by generating plasmin to limit further accumulation of hIAPP. We therefore sought to determine whether the tPA/plasmin system reduces amyloid formation by hIAPP and, if so, what islet cell type produces tPA. Using a thioflavin T assay, we found 4 µM plasmin abrogated 20 µM hIAPP fibril formation by 91.6±0.9% (n=5; p<0.01). To determine whether this could be by plasmin proteolysis of hIAPP, 20 µM hIAPP was incubated with 4 µM plasmin or buffer and then mass spectrometry was performed. After 10 minutes of incubation with plasmin, intact hIAPP was absent and expected cleavage products were present. As tPA is produced by macrophages and endothelial cells, we next determined whether tPA expression is increased by hIAPP in either or both of these cell types. In rat bone-marrow-derived macrophages treated for 24 h with 10 µM hIAPP, non-amyloidogenic 10 µM rat IAPP (rIAPP) or vehicle, tPA expression was increased only by hIAPP (4.2±1.0 vs. 1.5±0.6 vs. 1.0±0.5 fold, respectively; n=4; p<0.05 for hIAPP vs. rIAPP and vehicle). In contrast, in primary rat islet endothelial cells, tPA expression was not increased by 20 µM hIAPP (0.7±0.3 vs. 1.0±0.4 fold; n=3; p=0.2 for hIAPP vs. vehicle).

In summary, plasmin rapidly degrades hIAPP, curbing its aggregation into amyloid. As tPA is expressed in macrophages, enhancing its activity or its production by islet macrophages may represent a novel strategy to generate plasmin, cleave hIAPP and thus reduce islet amyloid formation and β-cell loss.