Abstract
As rates of non-communicable disease such as diabetes mellitus raise the questions regarding how these diseases arise becomes more pertinent. One of the major causes of these types of disease is the effects of the modern diet which is typically high in nutrients including sugars, fats, purines and salt. Of these nutrients, the effects of high purines have been underrepresented in the current body of research. Increased dietary purines result in a commensurate increase in the synthesis of uric acid elevating serum uric acid level. This increase induces hyperuricemia and has been correlated through epidemiological studies to the development and progression of degenerative diseases including diabetes mellitus.
In an attempt to shed light on the mechanism that underpins this effect we have elected to investigate the effects of hyperuricemic conditions on the viability and growth of pancreatic β-cells through the use of live cell assays to evaluate: cellular metabolism, cell count, autophagy, and apoptosis. To explain any changes in pancreatic β-cell viability and proliferation, we also investigated how the stability of a cellular growth and viability regulating protein, DEPTOR, may change under hyperuricemic conditions. We did this by directly measuring DEPTOR-bound ubiquitin though the use of co-immunoprecipitation. To explain any observed changes in DEPTOR ubiquitination we also investigated the expression of proteins responsible for DEPTOR ubiquitination, β-TrCP, and DEPTOR deubiquitination, USP3. Finally, we investigated if there was any direct interaction between DEPTOR and uric acid using co-immunoprecipitation to test for any potential bound uric acid as it may provide insights into any changes in DEPTOR ubiquitination, β-TrCP expression, or USP3 expression.
As a result of this investigation, we found that hyperuricemic conditions are sufficient to cause significant reductions in pancreatic β-cell metabolism and cell mass and a significant increase in iii
autophagy and apoptosis. This was paired with an increase in DEPTOR stability arising from the observed reduction in DEPTOR-bound ubiquitin. This reduction in DEPTOR-bound ubiquitin was caused by a significant shift in the expression of β-TrCP and USP3 which resulted in a net shift in the balance if ubiquitination to favour the removal of ubiquitin over its binding. Additionally, our attempts to probe for potential DEPTOR-bound uric acid were successful and found that uric acid did in fact bind to DEPTOR and that this binding may explain what initiated the shifted balance of DEPTOR ubiquitination.
These results have presented some interesting questions as they are the first to indicate: uric acid is capable of modifying DEPTOR ubiquitination, USP3 was identified as a new major regulator of DEPTOR stability within the pancreatic β-cell, uric acid as a possible new regulator of cell function that has as yet gone unnoticed, and finally the differences between mouse and human responses to hyperuricemia may provide new insights into the potential mechanism that underpins uric acid’s role in the regulation of ubiquitination.
In summary, in our attempts to better understand the causes of uric acid driven increases in diabetes mellitus risk we have found that hyperuricemic conditions are sufficient to reduce the function and mass of pancreatic β-cells and that this effect is caused by a previously undescribed uric acid-driven change in regulatory protein levels.