UCSF

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.