Stress-activated pathways mediate PFAS effects on human placental syncytiotrophoblast cells

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental pollutants associated with placenta-mediated pregnancy complications, including preeclampsia, fetal growth restriction, and preterm birth. The syncytiotrophoblast (STB), which forms the placental barrier at the maternal-fetal interface and is directly exposed to maternal blood, is a primary site of PFAS exposure. Although PFAS induce STB Apoptosis, the upstream stress-signaling pathways involved remain poorly defined. Here, we investigated stress-responsive signaling mechanisms mediating PFAS-induced STB cell death. STB differentiated from human trophoblast stem cells were exposed to vehicle or an environmentally relevant mixture of 5 PFAS (perfluorooctanoic acid, perfluorooctanesulfonic acid, perfluorohexane sulfonate, perfluorononanoic acid, and perfluorodecanoic acid; 0.0138 to 34.5 µM) for 3 or 6 h. Cytotoxicity, Apoptosis, mitochondrial membrane potential, and stress-signaling pathway activation were assessed by Lactate Dehydrogenase release, immunoblotting, JC-10 assay, and reverse transcription-quantitative PCR. PFAS mixtures did not induce cytotoxicity at 3 h but significantly increased cytotoxicity at 6 h at 34.5 µM, coinciding with the induction of cleaved Caspase-3, cleaved poly(ADP-ribose) polymerase, and NOXA. The pan-caspase inhibitor z-VAD-FMK prevented cytotoxicity, indicating caspase-dependent Apoptosis. PFAS exposure reduced mitochondrial membrane potential and activated the integrated stress response (ISR), as evidenced by eukaryotic initiation factor 2α phosphorylation, activating transcription factor 4 (ATF4) induction, and increased ATF4 target gene expression. In parallel, c-Jun N-terminal kinase (JNK) signaling was activated, as evidenced by JNK phosphorylation and induction of immediate-early genes (JUN, FOS, EGR1). Pharmacologic inhibition of the ISR modestly attenuated PFAS-induced cytotoxicity, whereas pharmacologic inhibition of JNK rescued cytotoxicity and apoptotic signaling. Together, these findings identify JNK-driven stress signaling as the dominant mediator of PFAS-induced STB Apoptosis, with a secondary contribution from the ISR.