PPARγ Activation Attenuates Glycated-Serum Induced Pancreatic Beta-Cell Dysfunction through Enhancing Pdx1 and Mafa Protein Stability

Yunxia Zhu,Hongxiu Zhang,Chaojun Li,Ai Ma,Rebecca Berdeaux

doi:10.1371/journal.pone.0056386

Abstract

Pancreatic-duodenal homeobox-1 (Pdx1) and v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (Mafa) play important roles in sustaining the pancreatic beta-cell differentiation phenotype. Peroxisome proliferator-activated receptor-γ (PPARγ) is also a regulator of cell differentiation. Our previous study revealed that glycated serum (GS) causes beta-cell dedifferentiation by down-regulating beta-cell specific genes, such as insulin and Pdx1. Here, we show that GS enhanced the cellular accumulation of ubiquitin-conjugated proteins, including Pdx1 and Mafa, in pancreatic beta-cells. Pharmacologic inhibition of proteolytic activity restored the protein levels of Pdx1 and Mafa, whereas inhibition of de novo protein synthesis accelerated their degradation. These findings suggest that both Pdx1 and Mafa are regulated at the post-transcriptional level. We further show that activation of PPARγ could restore GS-induced reduction of Pdx1 and Mafa protein levels, leading to improved insulin secretion and synthesis. Moreover, ectopic expression of Bcl-xl, a mitochondrial regulator, also restored Pdx1 and Mafa protein levels, linking mitochondrial function to Pdx1 and Mafa stability. Taken together, our results identify a key role of PPARγ in regulating pancreatic beta-cell function by improving the stability of Pdx1 and Mafa proteins.

Highlights

Advanced glycation end products (AGEs) are formed by nonenzymatic glycation and oxidation of proteins, lipids and nucleic acids, normally during aging, inflammation, renal failure and diabetes [1,2]
To determine whether the observed changes in the mRNA level of insulin genes would correspond to defective insulin production, the insulin content was detected after treatment of INS-1 cells with different concentrations of glycated serum (GS) for 24 h
We revealed that insulin synthesis and release was dramatically inhibited in a time- and dose-dependent manner by GS in INS-1 cells

Summary

Introduction

Advanced glycation end products (AGEs) are formed by nonenzymatic glycation and oxidation of proteins, lipids and nucleic acids, normally during aging, inflammation, renal failure and diabetes [1,2]. AGEs contribute to the deterioration in beta-cell function by inhibition of insulin gene transcription, degranulation of beta-cells and eventually abatement in beta-cell mass [9,10,11]. Mafa and Neurod are transcription factors that directly bind to the insulin gene promotor and serve as key regulators in pancreatic beta-cell differentiation and mature beta-cell function. Glucotoxicity, lipotoxicity and cytotoxic cytokines are well-known factors for progressive loss of beta-cell function and mass, and regardless of which signaling pathway is considered, compromising protein levels of Pdx, Mafa or Neurod are involved to some extent [12,13,14].

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