Diabetes mellitus is a disease caused by a combination of insulin resistance and decline of &beta-cell function. There is a high demand on the pancreatic &beta-cell to synthesize and secrete appropriate amounts of insulin in response to glucose levels. When the protein folding demand exceeds the folding capacity of the endoplasmic reticulum (ER), this results in a condition known as ER stress. The cell responds to ER stress by activating an adaptive signaling pathway called the unfolded protein response (UPR). The UPR first tries to restore homeostasis to the cell by reducing the load of proteins to be folded as well as by increasing the folding capacity of the cell. However, if ER stress is chronic, the UPR switches from a homeostatic pathway to an apoptotic pathway. The molecular mechanisms that guide this switch from adaptation to death remain unclear. Elucidating the underlying mechanism of ER stress induced apoptosis would allow better insight for future development of drugs that could prevent cell death and treat diseases such as diabetes. By utilizing mutations in different domains of the ER stress sensor IRE1&alpha, we were able to induce expression of IRE1&alpha mutants in &beta-cells and look at effects on cell fate. By using flow cytometry and microarray analysis we found that chronic IRE1&alpha overexpression resulted in downregulation of hundreds of mRNAs that localize to the ER or secretory pathway, which we hypothesize destabilizes the ER and mediates apoptosis. We also found that IRE1&alpha overexpression alone results in apoptosis, but expression of either kinase dead or RNase dead mutants of IRE1&alpha did not result in downregulation of mRNAs nor apoptosis. These findings uncovered a novel mechanism whereby IRE1&alpha has not only a cytoprotective function, but a proapoptotic arm as well, which requires both its kinase and RNase activity. Continuing this work, we found that TXNIP is a downstream proapoptotic mediator of ER stress apoptosis induced by IRE1&alpha that promotes oxidative stress and cell death. Through these novel findings, we have gained further insight on the mechanism of ER stress mediated apoptosis and also found a convergence of the oxidative stress and ER stress pathways.

Type 2 Diabetes (T2D) is characterized by a combination of factors that ultimately lead to loss of glycemic control. These factors include insulin resistance in insulin-responsive tissues (liver, muscle, and adipose tissues), and a progressive decline in -cell mass and function. In addition, more than half of T2D patients are obese and more than 85% are overweight, which in turn contribute to the observed insulin resistance. It has been reported that obesity induces Endoplasmic Reticulum (ER) stress, which leads to peripheral insulin resistance and ultimately to T2D, yet the underlying mechanisms are not well understood. Using two different mouse models, we found that obesity and a high fat diet induces activation of the bifunctional ER transmembrane kinase/endoribonuclease (RNase)–IRE1– and subsequent insulin desensitization by blunting phosphorylation of AKT. A mono-selective kinase inhibitor that allosterically attenuates IRE1’s RNase activity–KIRA8– prevented hyperglycemia, restored systemic insulin sensitivity, improved blood glucose clearance, enhanced insulin response in the liver and adipose tissues, and preserved pancreatic β-cell mass. Using chemical genetics tools of IRE1α we demonstrate that hyperactivation of IRE1 is sufficient to disrupt the insulin receptor signaling pathway. Our results demonstrate the potential efficacious properties of targeting IRE1α to enhance the adaptive capacity of the ER and prevent both peripheral insulin resistance and pancreatic β-cell loss.