Critical role for the Tsc1-mTORC1 pathway in β-cell mass in Pdx1-deficient mice

Regulation of Pdx1 by oxidative stress and Nrf2 in pancreatic beta-cells

The beta-cell identity gene, pancreatic duodenal homeobox 1 (Pdx1), plays critical roles in many aspects of the life of beta-cells including differentiation, maturation, function, survival and proliferation. High levels of reactive oxygen species (ROS) are extremely toxic to cells and especially to beta-cells due to their relatively low expression of antioxidant enzymes. One of the major mechanisms for beta-cell dysfunction in type-2 diabetes results from oxidative stress-dependent inhibition of PDX1 levels and function. ROS inhibits Pdx1 by reducing Pdx1 mRNA and protein levels, inhibiting PDX1 nuclear localization, and suppressing PDX1 coactivator complexes. The nuclear factor erythroid 2-related factor (Nrf2) antioxidant pathway controls the redox balance and allows the maintenance of high Pdx1 levels. Therefore, pharmacological activation of the Nrf2 pathway may alleviate diabetes by preserving Pdx1 levels.

Epigenetic Regulation of PDX-1 in Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a metabolic disease characterized by hyperglycemia which is caused by insufficient insulin secretion or insulin resistance. Interaction of genetic, epigenetic and environmental factors plays a significant role in the development of T2DM. Several environmental factors including diet and lifestyle, as well as age have been associated with an increased risk for T2DM. It has been demonstrated that these environmental factors may affect global epigenetic status, and alter the expression of susceptible genes, thereby contributing to the pathogenesis of T2DM. In recent years, a growing body of molecular and genetic studies in diabetes have been focused on the ways to restore the numbers or function of β-cells in order to reverse a range of metabolic consequences of insulin deficiency. The pancreatic duodenal homeobox 1 (PDX-1) is a transcriptional factor that is essential for the development and function of islet cells. A number of studies have shown that there is a significant increase in the level of DNA methylation of PDX-1 resulting in reduced activity in T2DM islets. The decrease in PDX-1 activity may be a critical mediator causing dysregulation of pancreatic β cells in T2DM. This article reviews the epigenetic mechanisms of PDX-1 involved in T2DM, focusing on diabetes and DNA methylation, and discusses some potential strategies for the application of PDX-1 in the treatment of diabetes.

Real Talk: The Inter-play Between the mTOR, AMPK, and Hexosamine Biosynthetic Pathways in Cell Signaling

O-linked N-acetylglucosamine, better known as O-GlcNAc, is a sugar post-translational modification participating in a diverse range of cell functions. Disruptions in the cycling of O-GlcNAc mediated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively, is a driving force for aberrant cell signaling in disease pathologies, such as diabetes, obesity, Alzheimer's disease, and cancer. Production of UDP-GlcNAc, the metabolic substrate for OGT, by the Hexosamine Biosynthetic Pathway (HBP) is controlled by the input of amino acids, fats, and nucleic acids, making O-GlcNAc a key nutrient-sensor for fluctuations in these macromolecules. The mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) pathways also participate in nutrient-sensing as a means of controlling cell activity and are significant factors in a variety of pathologies. Research into the individual nutrient-sensitivities of the HBP, AMPK, and mTOR pathways has revealed a complex regulatory dynamic, where their unique responses to macromolecule levels coordinate cell behavior. Importantly, cross-talk between these pathways fine-tunes the cellular response to nutrients. Strong evidence demonstrates that AMPK negatively regulates the mTOR pathway, but O-GlcNAcylation of AMPK lowers enzymatic activity and promotes growth. On the other hand, AMPK can phosphorylate OGT leading to changes in OGT function. Complex sets of interactions between the HBP, AMPK, and mTOR pathways integrate nutritional signals to respond to changes in the environment. In particular, examining these relationships using systems biology approaches might prove a useful method of exploring the complex nature of cell signaling. Overall, understanding the complex interactions of these nutrient pathways will provide novel mechanistic information into how nutrients influence health and disease.