Baicalein induces autophagic cell death through AMPK/ULK1 activation and downregulation of mTORC1 complex components in human cancer cells

Baicalein, a flavonoid and aglycon hydrolyzed from baicalin, has anticancer properties in several human carcinomas, but its molecular mechanisms of action remain unclear. Here, we show that baicalein leads to human cancer cell death by inducing autophagy rather than apoptosis, because cell death induced by baicalein was completely reversed by suppressing the expression levels of key molecules in autophagy such as Beclin1, vacuolar protein sorting34 (Vps34), autophagy-related (Atg)5 and Atg7, but not by pan-caspase inhibitor. Our data revealed that baicalein significantly increased the number of green fluorescence protein-cytosol-associated protein light chain3 (GFP-LC3)-containing puncta and LC3B-II expression levels, which were further enhanced by chloroquine treatment. Furthermore, a luciferase-based reporter assay showed that the ratio of RLuc-LC3wt/RLuc-LC3G120A was greatly reduced. The data suggested that baicalein induced not only autophagosome formation, but also autophagic flux. Experiments using short interfering RNAs and pharmacological inhibitors revealed that Beclin1, Vps34, Atg5, Atg7 and UNC-51 (Caenorhabditiselegans)-like kinase1 (ULK1) play pivotal roles in mediating baicalein-induced autophagy. Moreover, baicalein activated AMP-activated protein kinase (AMPK), leading to ULK1 activation through phosphorylation at Ser555, whereas both protein and mRNA levels of mammalian target of rapamycin (mTOR) and Raptor, upstream inhibitors of ULK1 and autophagy, were markedly downregulated by baicalein. Our data suggest that the anticancer effects of baicalein are mainly due to autophagic cell death through activation of the AMPK/ULK1 pathway and inhibition of mTOR/Raptor complex1 expression. These results provide new mechanistic insights into the anticancer functions of autophagy inducers, such as baicalein, which may be used as potential therapeutics for cancer treatment.