title: Chronic Exposure to Proline Causes Aminoacidotoxicity and Impaired Beta-Cell Function: Studies In Vitro.
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 long-term 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.
  
  
  CONCLUSIONS
  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.
authors: Department of Medicine and Endocrinology, Aarhus University Hospital, Tage-Hansens Gade 2, DK-8000 Aarhus C, Denmark.
authors: Zhenping
authors: Z
authors: Liu
authors: Department of Medicine and Endocrinology, Aarhus University Hospital, Tage-Hansens Gade 2, DK-8000 Aarhus C, Denmark.
authors: Per B
authors: PB
authors: Jeppesen
authors: Department of Medicine and Endocrinology, Aarhus University Hospital, Tage-Hansens Gade 2, DK-8000 Aarhus C, Denmark.
authors: Søren
authors: S
authors: Gregersen
authors: Department of Food Science, Faculty of Agricultural Sciences, Aarhus University, DK-8230 Tjele, Denmark.
authors: Lotte
authors: L
authors: Bach Larsen
authors: Department of Medicine and Endocrinology, Aarhus University Hospital, Tage-Hansens Gade 2, DK-8000 Aarhus C, Denmark.
authors: Kjeld
authors: K
authors: Hermansen
mesh_terms: D000818:Animals; D016923:Cell Death / Q000187:drug effects*; D002460:Cell Line; D003576:Electron Transport Complex IV / Q000378:metabolism; D005260:Female; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000378:metabolism; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D010766:Phosphorylation / Q000187:drug effects; D011392:Proline / Q000494:pharmacology*
chemical_list: D007328:Insulin; D011392:Proline; D003576:Electron Transport Complex IV; D005947:Glucose