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The molecular mechanisms of stress-induced amylin turnover in pancreatic beta cell Open Access

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Islet amyloid polypeptide (IAPP) or amylin is a pancreatic hormone produced and co-secreted with insulin from pancreatic islet β-cells that exerts local and system effects in the human body. The human form is a highly amyloidogenic and pathogenic due to its intrinsic property to aggregate in the pancreas. Despite high amino acid sequence complementarity with human amylin (hIAPP), rodent (rat and mouse) amylin do not aggregate in the pancreas and are non-toxic to islet cells due to the presence of proline residues. Type-2 diabetes mellitus (T2DM) is currently the most prevalent metabolic disease in the world. It is characterized by insulin resistance, impaired insulin secretion and β-cell loss. It is generally accepted that hIAPP-derived toxic oligomers and aggregates contribute to the β-cell loss. Thus, there is growing interest in the field to unravel the cellular mechanisms regulating hIAPP production and aggregation, which however still remains a mystery. Hence, my project aimed to elucidate the molecular mechanisms of hIAPP turnover in the islet β-cells under physiological and stress conditions relevant to T2DM. I found that functional proteasome complex is essential for the transcription of IAPP and its key transcription factor FoxA2. Accordingly, marked drop in hIAPP’s mRNA and protein levels as well as dysfunctional proteasome complex was detected in islets isolated from T2DM subjects suggesting a possible link between proteasome activity and hIAPP expression in the β-cells. This novel regulatory pathway could be a potential target to modulate toxic hIAPP levels and islet amyloidosis in T2DM. I also demonstrated that ER stress and hyperlipidemia, two hallmarks of T2DM, stimulate hIAPP production via transcription-dependent and-independent manner, respectively. In contrast, ER stress moderately stimulates insulin mRNA level, while free fatty acid significantly downregulates its transcription, inferring the existence of differential stress-induced regulation of hIAPP and insulin production in the β-cells. Furthermore, crucial requirement of functional FoxA2 binding site in the hIAPP promoter for ER stress stimulates hIAPP transcription highlighting the possibility that hIAPP specific transcription factor FoxA2 could be accounted for the differential ER stress response of hIAPP and insulin in the β-cell. While exploring the trafficking pathways of hIAPP under physiological and stress conditions relevant to T2DM, I discovered a novel intracellular (nuclear/nucleolar) site where hIAPP accumulates. This seminal finding reveals a parallel alternate trafficking pathway for hIAPP in the islet β-cells. Marked increase of nuclear/nucleolar hIAPP accumulation under diabetic and stress conditions is suggestive of a signaling role in cellular response to stress. Based on the well-known role of the nucleolus in the regulation of ribosomal biosynthesis and cell proliferation, I investigated whether nucleolar hIAPP accumulation relates to β-cell proliferation. Presence of both nuclear hIAPP positive proliferative and non-proliferative β-cells under high glucose condition supports its possible proliferative role and implies the existence of an additional proliferation-independent function of nucleolar hIAPP in human islet cells. Elucidation of the molecular mechanisms of hIAPP turnover and trafficking and their regulations under normal and diabetic conditions are crucial for identifying the physiological and pathophysiological functions of hIAPP and biology of amyloid diseases such as diabetes.

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