Placental Stress, Protein Synthesis Inhibition, and Growth Restriction.

Abstract

Intrauterine growth restriction (IUGR) affects 4-7% of births and poses a major clinical challenge. It is associated with an increased risk of stillbirth and neonatal morbidity and mortality, and also a predisposition to type II diabetes, obesity and cardiovascular disease in adult life. There are many potential causes, including genetic disorders, infection, and maternal undernutrition, but the majority of cases remain unexplained. Nonetheless, these cases are frequently associated with deficient physiological conversion of the maternal spiral arteries supplying the placenta, suggesting malperfusion may contribute to the aetiology. We hypothesise that the retention of smooth muscle within the walls of the arteries leads to spontaneous vasoconstriction, greater intermittent perfusion of the placenta and ischaemia-reperfusion-type stress. One of the potential consequences of malperfusion is endoplasmic reticulum (ER) stress, which can lead to protein synthesis inhibition through activation of the unfolded protein response. We tested for this by examining placentas from 6 cases of IUGR alone, 6 cases of IUGR complicated by early-onset preeclampsia (PE+IUGR) and 6 normal placentas. All placentas were collected with informed patient consent, were non-laboured and delivered by caesarean section. We observed multiple blocks to protein synthesis in the pathological placentas. Firstly, increased phosphorylation of eukaryotic initiation factor 2α (eIF2α) indicated suppression of translation initiation in the IUGR placentas compared to controls, and was further increased in PE+IUGR placentas. Consequently the levels of many proteins were reduced. In particular, AKT was reduced at the protein, but not mRNA, level and cyclin D1 protein was also lowered. These changes could be reproduced in trophoblast-like cell lines by induction of ER stress, and were associated with reduced proliferation. Phosphorylation of eIF2α blocks translation initiation by acting as a competitive inhibitor of the guanine-nucleotide-exchange factor, eIF2B. Activity of eIF2B can also be regulated through phosphorylation by kinases such as GSK-3β. We observed increased activity of GSK-3β and levels of P-eIF2B in the pathological placentas, representing a second block to protein synthesis. These blocks to translation initiation are sufficient to account for the smaller placental phenotype in IUGR pregnancies. Our findings are consistent with ER stress, which is one of at least four pathways that lead to phosphorylation of eIF2α. Indeed, we observed dilatation of the ER cisternae in the syncytiotrophoblast in the pathological placentas. Molecular evidence of ER stress was seen in IUGR placentas, but was more severe in PE+IUGR placentas where it was associated with activation of the pro-apoptotic protein CHOP. This may account for the deportation of placental apoptotic debris into the maternal circulation, which has been proposed as a causal factor in preeclampsia. To conclude, we provide the first evidence that ER stress may play a key role in limiting placental growth in complicated pregnancies. The severity of that stress may influence the balance between proliferation and apoptosis, and so determine the clinical outcome of the pregnancy. Supported by the Wellcome Trust (069027/Z/02/Z).