Chronic Exposure to Proline Causes Aminoacidotoxicity and Impaired Beta-Cell Function: Studies in vitro

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Abstract
The Review of Diabetic Studies,2016,13,1,66-78.
Published:May 2016
Type:Original Article
Authors:
Author(s) affiliations:

Zhenping Liu1,3, Per B. Jeppesen1, Søren Gregersen1, Lotte Bach Larsen2, and Kjeld Hermansen1

1Department of Medicine and Endocrinology, Aarhus University Hospital, Tage-Hansens Gade 2, DK-8000 Aarhus C, DENMARK.

2Department of Food Science, Faculty of Agricultural Sciences, Aarhus University, DK-8230 Tjele, DENMARK.

3Department of Endocrinology, PLA 309 Hospital, 17 Heishanhu Road, Haidian District, 100091, Beijing, P. R. CHINA. 

Abstract:

Background: Pancreatic islet-cell dysfunction is a hallmark in the development of diabetes, but the reasons for the primary β-cell defect are still elusive. Elevated circulating proline levels have been found in subjects with insulin resistance, obesity, and type 2 diabetes. Therefore, we assessed β-cell function, gene expressions, and cell death after longterm exposure of pancreatic β-cells to excess proline in vitro. Methods: Isolated mouse islets and INS-1E cells were incubated with and without excess proline. After 72 h, we examined: (1) β-cell function, including basal insulin secretion (BIS) and glucose-stimulated insulin secretion (GSIS), (2) transcription factors related to insulin gene expression and enzymes involved in the tricarboxylic acid cycle and cholesterol biogenesis, (3) cellular triglycerides (TG) and cholesterol content, (4) the death of INS-1E cells and 3H thymidine incorporation, and (5) protein expression of INS- 1E cells in response to proline by proteomics. Results: We found that high doses of proline increased BIS and decreased GSIS in both isolated mouse islets and INS-1E cells. MafA, insulin 1, and the cytochrome c oxidase subunit VIa polypeptide 2 mRNA expressions were all downregulated, indicating that proline impaired insulin gene transcription and mitochondrial oxidative phosphorylation. In contrast, mevalonate decarboxylase gene expression was upregulated, and simultaneously, cholesterol content in INS-1E cells was enhanced. Protein profiling of INS-1E cells revealed that cytosolic non-specific dipeptidase and α enolase were differentially expressed. Conclusion: Our results indicate that proline-induced insulin transcription and mitochondrial oxidative phosphorylation impairment may contribute to the β-cell dysfunction observed in type 2 diabetes. Caution should be applied in interpreting the pathophysiological role of proline since very high proline concentrations were used in the experiments.