The Role of Calreticulin in Endoplasmic Reticulum Stress and Apoptosis

One of the many purposes of the endoplasmic reticulum (ER) is to facilitate proper protein folding and the transport of properly synthesized proteins to other subcellular structures. ER “chaperone” proteins like calreticulin work to ensure that newly synthesized proteins cannot bind to each other to create dysfunctional compounds under stress conditions. In this way, they act as chaperones or regulators that assure only properly folded proteins are transported from the ER to other cellular structures.¹

Under heat shock, hypoxic and oxidative stress, as well as toxic conditions, the ER activates a complex set of signaling pathways collectively known as the unfolded (or improperly folded) protein response (UPR), which includes the synthesis of chaperone molecules. This is a cell’s normal response, which activates defense mechanisms designed to repair misfolded proteins in order to resume normal cellular function. The UPR induces cells to produce proapoptotic proteins as a front-line response. At the same time, cells generate stress granules that mitigate the potential damage from apoptosis by sequestering proapoptotic proteins. When ER stress is acute and/or persists over time, cells trigger the caspase cascade leading to apoptosis. In cancer, apoptosis stalls and cells accumulate as tumors.²

The precise molecular mechanisms that facilitate the switch between damage control and cell death are not completely understood, and multiple potential participants have been investigated. However, we do know that the formation of stress granules on exposure to hypoxia or oxidative stress leads to tumor cell resistance to apoptosis.³ As a result, many research teams have worked hard to elucidate the biochemical processes linked to apoptosis and ER stress.

One suspected pathway is the calcium chaperone, calreticulin. Calreticulin resides mainly in the ER and binds to calcium, functioning as both buffer and chaperone, binding to misfolded proteins, and preventing them from being exported from the ER. In stress conditions where calcium is lacking, calreticulin acts as a proapoptotic protein.

Researchers in Argentina have recently determined that, in its argynilated form, calreticulin expresses at cell membranes and contributes to the downregulation of calcium in cells under ER stress conditions. Calreticulin (R-CRT) is a posttranslational isoform created when argynilated by arginyl-tRNA protein transferase (ATE) as part of the unfolded protein response. R-CRT is recruited to stress granules that inactivate R-CRT and other proapoptotic proteins by sequestering them, thus interrupting the signaling cascade that leads to apoptosis.⁴

Presumably, if the stressor continues or is acute, the stress granules release R-CRT and apoptosis proceeds. In cancer, however, the stress granules do not release R-CRT and thus interrupt apoptosis, leading to the wild cell growth associated with tumors. There are two ways that R-CRT-based therapies may be directed toward cancer. First, destabilizing the argynilation of calreticulin to form R-CRT would interrupt the sequestration of calreticulin and thus free it to perform its role in apoptosis. This may involve ATE antagonists. Second, calreticulin agonists directed at cancer cells may overcome the UPR and allow enough freely circulating calreticulin to contribute to apoptosis.

In addition to cancers, ER stress-associated apoptosis pathways have been recognized in diabetes, hypoxia, ischemia/reperfusion injury, Alzheimer’s disease, Parkinson’s disease, and bipolar disorder. As a result, researchers are working hard to understand the basic molecular mechanisms of ER stress mediators in the unfolded protein response. Calreticulin is one of many.¹

References

1. Raghubir, R., et al. (2011) ‘Endoplasmic reticulum stress in brain damage‘, Methods in Enzymology, 2011 (489), (pp. 259-275)

2. Sambrooks, C.L., et al. (2012) ‘Arginylated calreticulin at plasma membrane increases susceptibility of cells to apoptosis‘, Journal of Biological Chemistry Papers, 287 (26), (pp. 22043-22054)

3. Jeffery, E., et al. (2011) ‘The polypeptide binding conformation of calreticulin facilitates its cell-surface expression under conditions of endoplasmic reticulum stress‘, Journal of Biological Chemistry, 286 (4), (pp. 2402-2415)

4. Carpio, M., et al. (2010) ‘The arginylation-dependent association of calreticulin with stress granules is regulated by calcium‘, Biochemical Journal, 429 (1), (pp. 63-72)