The accumulation of unfolded proteins in the endoplasmic reticulum (ER) represents a cellular stress induced by multiple stimuli and pathologic conditions. Recent evidence implicates endothelin-1 (ET-1) in the induction of placental ER stress in pregnancy disorders. ER stress has previously also been implicated in various other disease states, including neurodegenerative disorders, diabetes, and cardiovascular diseases, as has ET-1 in the pathophysiology of these conditions. However, to date, there has been no investigation of the link between ET-1 and the induction of ER stress in these disease states. Based on recent evidence and mechanistic insight into the role of ET-1 in the induction of placental ER stress, the following review attempts to outline the broader implications of ET-1-induced ER stress, as well as strategies for therapeutic intervention based around ET-1.
Endoplasmic Reticulum StressThe endoplasmic reticulum (ER) is a multifunctional organelle involved in the synthesis, folding, and post-translational modification of membrane and secretory proteins. In addition, the ER also serves as a reservoir of calcium ions (Ca 21 ) (Zhang and Kaufman, 2008). In the ER lumen, Ca 21 is buffered by calcium-binding proteins. Many of these proteins also serve as molecular chaperones involved in the folding and quality control of ER proteins, and their functional activity alters with changes in Ca 21 concentration (Michalak et al., 1998). Growth factors, hormones, and stimuli that perturb cellular energy levels, nutrient availability, or redox status all affect ER calcium storage. Loss of ER Ca 21 homeostasis suppresses posttranslational modifications of proteins in the ER. As a result, misfolded proteins accumulate, provoking ER stress response pathways known collectively as the unfolded protein response (UPR) (Brostrom and Brostrom, 2003).The UPR is induced to restore ER homeostasis. It reduces the burden of new proteins entering the ER lumen through attenuation of translation, enhances the protein folding capacity by increasing ER chaperone proteins [glucose-regulated protein78 (GRP78) and GRP94] and folding enzymes, and promotes degradation of remaining unfolded or misfolded proteins through increased capacity of the cytosolic ubiquitin-proteasome system (Brostrom and Brostrom,