RESUMO
Amyloidosis is a common pathological event in which proteins self-assemble into misfolded soluble and insoluble molecular forms, oligomers and fibrils that are often toxic to cells. Notably, aggregation-prone human islet amyloid polypeptide (hIAPP), or amylin, is a pancreatic hormone linked to islet ß-cells demise in diabetics. The unifying mechanism by which amyloid proteins, including hIAPP, aggregate and kill cells is still matter of debate. The pathology of type-2 diabetes mellitus (T2DM) is characterized by extracellular and intracellular accumulation of toxic hIAPP species, soluble oligomers and insoluble fibrils in pancreatic human islets, eventually leading to loss of ß-cell mass. This review focuses on molecular, biochemical and cell-biology studies exploring molecular mechanisms of hIAPP synthesis, trafficking and degradation in the pancreas. In addition to hIAPP turnover, the dynamics and the mechanisms of IAPP-membrane interactions; hIAPP aggregation and toxicity in vitro and in situ; and the regulatory role of diabetic factors, such as lipids and cholesterol, in these processes are also discussed.
Assuntos
Diabetes Mellitus Tipo 2/patologia , Polipeptídeo Amiloide das Ilhotas Pancreáticas/metabolismo , Pâncreas/patologia , Agregação Patológica de Proteínas/patologia , Animais , Diabetes Mellitus Tipo 2/metabolismo , Humanos , Polipeptídeo Amiloide das Ilhotas Pancreáticas/análise , Pâncreas/metabolismo , Agregados Proteicos , Agregação Patológica de Proteínas/metabolismo , Conformação Proteica , Dobramento de Proteína , Deficiências na Proteostase/metabolismo , Deficiências na Proteostase/patologiaRESUMO
Here, we investigated transcriptional and trafficking mechanisms of human islet amyloid polypeptide (hIAPP) in normal and stressed ß-cells. In high glucose-challenged human islets and rat insulinoma cells overexpressing hIAPP, cell fractionation studies revealed increased accumulation of hIAPP. Unexpectedly, a significant fraction (up to 22%) of hIAPP was found in the nuclear soluble and chromatin-enriched fractions of cultured human islet and rat insulinoma cells. The nucleolar accumulation of monomeric forms of hIAPP did not have any adverse effect on the proliferation of ß-cells nor did it affect nucleolar organization or function. However, intact nucleolar organization and function were essential for hIAPP expression under normal and ER-stress conditions as RNA polymerase II inhibitor, α-amanitin, reduced hIAPP protein expression evoked by high glucose and thapsigargin. Promoter activity studies revealed the essential role of transcription factor FoxA2 in hIAPP promoter activation in ER-stressed ß-cells. Transcriptome and secretory studies demonstrate that the biosynthetic and secretory capacity of islet ß-cells was preserved during ER stress. Thus, the main reason for increased intracellular hIAPP accumulation is its enhanced biosynthesis under these adverse conditions.