Punicalagin promotes autophagy to protect primary human syncytiotrophoblasts from apoptosis

Punicalagin is a prominent polyphenol in pomegranate juice that protects cultured syncytiotrophoblasts from stress-induced apoptosis. Here, we test the hypothesis that punicalagin has this effect by inhibiting the mTOR kinase pathway to enhance autophagic turnover and limit apoptosis in cultured primary human syncytiotrophoblasts. In syncytiotrophoblasts, starvation, rapamycin, or punicalagin all decreased the expression of phosphorylated ribosomal protein S6, a downstream target of the mTOR kinase, and of the autophagy markers, LC3-II and p62. In contrast, in the presence of bafilomycin, an inhibitor of late stages of autophagy and degradation in the autophagolysosome, syncytiotrophoblasts exposed to starvation, rapamycin, or punicalagin all showed increased levels of LC3-II and p62. The number of LC3-II punctae also increased in punicalagin-treated syncytiotrophoblasts exposed to chloroquine, another inhibitor of autophagic degradation, and punicalagin increased the number of lysosomes. The apoptosis-reducing effect of punicalagin was attenuated by inhibition of autophagy using bafilomycin or knockdown of the autophagy related gene, ATG16L1. Collectively, these data support the hypothesis that punicalagin modulates the crosstalk between autophagy and apoptosis to promote survival in cultured syncytiotrophoblasts.