Two major pathophysiologic abnormalities underlie most cases of type 2 diabetes (T2D): insulin resistance and defects in pancreatic beta-cell function. These defects are initially compensated by an increase in insulin secretion and in the number of insulin secreting cells. However, with time, beta cell dysfunction and T2D develops, requiring intervention therapies. Eventually, there is significant beta cell loss and increased difficulty in the management of hyperglycemia.
It has been postulated that the increased rate of insulin synthesis induced by hyperglycemia could lead to beta cell failure. Insulin, initially synthesized as a larger polypeptide, is modified in the endoplasmic reticulum (ER) and post-translationally converted into insulin within the secretory/storage vesicles prior to secretion. Two proteases, the prohormone convertases (PC) PC2 and PC3/1, are responsible for this conversion. Deficiencies in insulin production could be due to defects in the processing of precursor insulin molecules in the ER and/or the secretory granules. Defects in the ER function unleash the unfolded protein response (UPR), which can be adaptive and recover normal cell function. Alternatively if the stress is chronic and severe, the UPR initiates cell death.
In pilot studies, using immunocytochemical techniques, we discovered that chronic pharmacological induction of insulin synthesis in normoglycemic white outbred CD-1 mice induced the expression, in all islets of 30% of mice treated (N=10), of a transcription factor that indicates severe ER stress. Significantly, islets of CHOP+ mice had a dramatic decrease in the levels of PC3/1 and PC2. Similar defects were found in beta cells of all diabetic C57BI6/Ks db/db mice examined.
The objective of this proposal is to ascertain the clinical relevance of our findings. We wish to determine whether increased demand for insulin synthesis in diabetic patients induce CHOP expression and/or abnormal low levels of the proprotein convertases PC3/1 and PC2 in pancreatic islets.
Addendum 1 Summary:
In this extension of the proposal we will test whether the heterogeneity of PC1/3 expression in human beta cells correlates with differences in the level of expression of mature insulin, proinsulin and of transcription factors involved in insulin synthesis, Second, whether there is a parallel between expression of proinsulin/mature insulin and that of proteins involved in insulin secretion. The third specific aim will evaluate the level of expression of proinsulin and of molecules involved in its processing in T2D islets. This analysis will allow ascertaining if the phenotypic variability in T2D reflects the presence, in each pancreas, of normal islets and of islets with different stages of beta cell dysfunction.
Addendum 2 Summary:
This proposal will test the hypothesis that beta cell death in Type 1 Diabetes occurs sequentiallyand is initiated with the immune mediated -ablation of the ProIN+PC1/3-cells. Comparison of celltype in pancreatic islets from autoantibody positive, newly diagnosed and long term T1D donors may reveal heterogeneity in response of the three cell types to the immunological attack.