Hepcidin links gluco-toxicity to pancreatic beta cell dysfunction by inhibiting Pdx-1 expression.

Xuhua Mao,Tingting Shu,Liangliang Wang,Hucheng Chen,Junmin Tang

doi:10.1530/ec-16-0115

Xuhua Mao, Tingting Shu + Show 3 more

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https://doi.org/10.1530/ec-16-0115

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Abstract

ObjectiveGluco-toxicity is a term used to convey the detrimental effect of hyperglycemia on β-cell function through impaired insulin synthesis. Although it is known that the expression and activity of several key insulin transcription regulators is inhibited, other molecular mechanisms that mediate gluco-toxicity are poorly defined. Our objective was to explore the role of hepcidin in β-cell gluco-toxicity.DesignWe first confirmed that high glucose levels inhibited hepcidin expression in the mouse insulinoma cell line, MIN6. The downregulation of hepcidin decreased Pdx-1 expression, which reduced insulin synthesis.MethodsMIN6 cells were exposed to high glucose concentrations (33.3 mmol/L). Glucose-stimulated insulin secretion (GSIS) and serum hepcidin levels were measured by ELISA. The mRNA levels of insulin1, insulin2, Pdx-1 and hepcidin were measured by real-time polymerase chain reaction. Western blot analysis was used to detect the changes in PDX-1 expression. Transient overexpression with hepcidin was used to reverse the downregulation of Pdx-1 and insulin synthesis induced by gluco-toxicity.ResultsExposure of MIN6 cells to high glucose significantly decreased GSIS and inhibited insulin synthesis as well as Pdx-1 transcriptional activity and expression at both the mRNA and protein levels. High glucose also decreased hepcidin expression and secretion. Hepcidin overexpression in MIN6 cells partially reversed the gluco-toxicity-induced downregulation of Pdx-1 and insulin expression and improved GSIS. The restoration of insulin synthesis by transfection of a hepcidin overexpression plasmid confirmed the role of hepcidin in mediating the gluco-toxic inhibition of insulin synthesis.ConclusionsOur observations suggest that hepcidin is associated with gluco-toxicity-reduced pancreatic β-cell insulin synthesis in type 2 diabetes by inhibiting Pdx-1 expression.

Highlights

Type 2 diabetes arises when the endocrine pancreas fails to secrete sufficient insulin to cope with the metabolic demand because of acquired insulin resistance and β-cell dysfunction (1)
In accordance with previous reports, the Glucose-stimulated insulin secretion (GSIS) response of MIN6 cells after exposure to 33.3 mmol/L glucose was significantly (P < 0.005) reduced when the glucose concentration was raised from 3.3 mmol/L to 16.7 mmol/L in the assay compared to that observed after exposure to 5 mmol/L glucose for 48 h (1.3-fold vs 2.8-fold; Fig. 1A)
Insulin synthesis is regulated by three β-cell-specific transcription factors: pancreas duodenum homeobox-1 (Pdx-1), neurogenic differentiation 1 (NeuroD1) and V-maf musculoaponeurotic fibrosarcoma oncogene homologue A (MafA)

Summary

Introduction

Type 2 diabetes arises when the endocrine pancreas fails to secrete sufficient insulin to cope with the metabolic demand because of acquired insulin resistance and β-cell dysfunction (1). Inhibition of insulin synthesis plays an important role in the development of β-cell dysfunction. It is known that the mechanism by which glucotoxicity inhibits insulin synthesis involves the loss of expression of pancreas duodenum homeobox-1 (Pdx-1), which acts as a critical regulator of insulin promoter activity (5). The restricted expression of Pdx-1 induced by gluco-toxicity decreases its DNA-binding activity to insulin promoters (6, 7, 8). The oxidative stress signaling pathway is thought to play an important role in the mechanism by which gluco-toxicity leads to depressed Pdx-1 expression, causing downregulated insulin synthesis; JNK, FoxO1 and NF-κB have been confirmed as target molecules involved in the regulation of Pdx-1 expression (6, 9)

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