Cerebral ischaemia is a powerful inducer of the UPR [37], and subjecting JEG-3 cells to hypoxia-reoxygenation causes phosphorylation of eIF2α
[25]. This situation may be made worse by changes in posture, which in the bipedal human can influence uterine blood flow [38], or heightened uterine contractility, as maternal placental blood is reduced during a contraction [39]. The intervening steps in vivo are unclear at present, but various possibilities exist. Episodes of ischaemia will deplete intracellular concentrations of glucose, which may restrict normal glycosylation within the ER, activating the UPR. Alternatively, ischaemia will reduce intracellular levels of ATP, compromising the functioning of the GRP chaperone proteins, buy Trametinib and possibly also the Ca2+-ATPase ionic pumps within the ER membrane. Ischaemia may also have a more direct effect on calcium release from the ER by altering the redox balance within the cell, affecting thiol groups on the calcium channel proteins [40]. Calcium imbalance may further result from competitive binding of GRP78 to misfolded proteins, for under normal conditions GRP78 serves to plug unoccupied translocons, preventing leakage. Loss
of calcium from the ER lumen will compound the situation by compromising the protein folding machinery, and by activating calcium dependent signalling pathways within the cytsol. Ultimately, these could lead to opening of the mitochondrial membrane transition pore, with subsequent loss of mitochondrial function and generation of ROS. We have previously demonstrated that hypoxia-reoxygenation of villous Lumacaftor mw explants leads to opening of the pore, and activation of apoptosis within the syncytiotrophoblast [41]. Further work is required to tease apart these various possibilities, but the complex interactions between oxidative and ER stress mean that once one is initiated the other is likely to follow soon after through
feed-forward mechanisms. In many isothipendyl instances pathological activation of the UPR is a one-off event, following for example stroke or myocardial infarction. As mentioned earlier, phosphorylation of eIF2α and inhibition of protein synthesis are usually transient events, for activation of ATF4 leads to upregulation of the phosphatase GADD34. However, the precipitating vascular insult to the placenta in pre-eclampsia is likely to be of a lower grade than that in stroke, and also of a repetitive nature. To mimic this in vitro we have exposed JEG-3 cells to repetitive cycles of hypoxia-reoxygenation and observed sustained phosphorylation of eIF2α and activation of the UPR. We predict therefore that the ER stress is of a chronic nature, dating most likely from the time of onset of the maternal circulation at the end of the first trimester. The consequences for placental function are manifold, and are just beginning to be explored [42].
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