Islet macrophage depletion does not prevent amyloid-induced upregulation of tissue plasminogen activator (tPA)

Background and Aims: Islet amyloid deposition and islet inflammation are pathogenic hallmarks of type 2 diabetes. Aggregation of human islet amyloid polypeptide (hIAPP) increases pro-inflammatory gene expression in islet macrophages and results in beta-cell loss. We previously found the fibrinolysis activator tissue plasminogen activator (tPA) is specifically upregulated in islets with amyloid deposition and reduces amyloid fibril formation through plasmin generation. Since macrophages can produce tPA, we determined whether they contribute to hIAPP-induced tPA upregulation.
Materials and Methods: To determine whether hIAPP increases tPA expression in macrophages, tPA mRNA levels were quantified in rat bone-marrow-derived macrophages (BMDMs) treated for 24 h with 10 µM hIAPP, non-amyloidogenic 10 µM mouse IAPP (mIAPP) or vehicle. Further, to determine the contribution of islet macrophages to tPA upregulation by amyloid deposition, isolated islets from hIAPP transgenic mice (that have the propensity to form islet amyloid) and wild-type mIAPP mice (that do not form islet amyloid) were treated for 48 h with clodronate-containing liposomes (CLOD-lip) to deplete macrophages or PBS-containing liposomes (PBS-lip) as control. Islets were then cultured for 48 h in 16.7 mM glucose, which induces amyloid deposition in hIAPP, but not mIAPP islets. Islet macrophage markers (Emr1 and Itgam) and tPA mRNA levels were quantified.
Results: hIAPP but not mIAPP increased tPA gene expression in BMDMs (4.21.0 vs 1.50.6 vs 1.00.5 fold, hIAPP vs mIAPP vs vehicle respectively; n=4; p<0.05 hIAPP vs mIAPP and vehicle; p=0.25 mIAPP vs vehicle). As expected, tPA gene expression significantly increased by 2.1-fold in PBS-lip-treated hIAPP islets vs mIAPP islets treated with PBS-lip (Table 1). In both hIAPP and mIAPP islets, CLOD-lip treatment significantly abrogated Emr1 and Itgam gene expression vs PBS-lip treatment, confirming islet macrophages depletion in both genotypes (Table 1). CLOD-lip treatment also significantly reduced tPA expression (vs PBS-lip) in both hIAPP and mIAPP islets; however, tPA expression was still increased by 2.3-fold in CLOD-lip treated hIAPP islets vs mIAPP islets (Table 1).
Conclusion: hIAPP treatment induces tPA expression in macrophages and islet macrophage depletion decreases, but does not prevent, upregulation of tPA in amyloid-laden islets. Thus, our data suggests that, in addition to macrophages, other islet cell types produce tPA in response to amyloid deposition.