Abstract
Uric acid or urate is an organic anion produced in the liver from alcohol, fructose and protein metabolism. Urate levels in humans are much higher than that of other animals, sometimes by a factor of 10. The most commonly thought of disease associated with high urate levels, or hyperuricemia, is gout. Historically gout was colloquially known as the ‘kings disease’, whereby only the kings could consume large quantities of meat and alcohol thereby raising urates levels. Nowadays, many people are able to ‘eat like kings’, in that high calorific foods are readily available and cheap. As a consequence urate levels around the world have increased and are still rising. In parallel with this treat is the much well publicised disease of diabetes mellitus. Type 2 Diabetes mellitus (T2DM) is considered, by many, as the biggest health issue of the 21st century. With that in mind, this project sets out to provide evidence that urate may be a contributing factor in T2DM. The two characteristics of T2DM that this project will focus around are a reduction in insulin secretion and reduced β-cell mass.
AMP-activated protein kinase (AMPK) is considered the major cellular energy sensor. The protein is responsive to changes in ATP:ADP ratio within the cell and able to coordinate a wide range of downstream pathways once activated/phosphorylated. It is a key target in the treatment of diabetes whereby its activation in liver, adipose and skeletal muscle lower blood glucose levels and increase insulin sensitivity. However, it has counterproductive effects in pancreatic β-cells whereby its activation inhibits insulin secretion and may even promote apoptosis. The other target in this project is the microRNA miR-34a. MiR-34a has negative effects on cell survival such as inhibiting cell cycle progression and promoting apoptosis. What’s interesting about this microRNA, and particularly relevant for this project, is that miR-34a has been shown to be the one of the most consistent microRNAs the expression of which increases under diabetic conditions. Its elevated expression in circulating plasma of pre-diabetes has even been suggested as a marker for type-2 diabetes development.
In this study I tested whether hyperuricemic conditions influence the expression of AMPK and miR-34a in cultured pancreatic β-cells and I postulate that the urate transporter GLUT9 is the transporter responsible for the urate-induced effects. Hyperuricemic conditions increased the expression of the α1-AMPK subunit and miR-34a after chronic exposures. Hyperuricemia also activated apoptotic cascades and reduced cell viability, which was attenuated with urate transport inhibition. Lastly, siRNA knockdown of GLUT9 significantly reduced the increase in miR-34a expression under hyperuricemic conditions providing evidence for GLUT9 as the primary urate transporter in β-cells. Because the above experiments were carried out in both mouse and human cells this may reflect that urate regulates basic (conserved) mechanisms in β-cells. In light of this, pathological hyperuricmia may contribute to reduced insulin secretin and β-cell death and therefore be implicated in the development of T2DM.