Emodin negatively affects the phosphoinositide 3-kinase/AKT signalling pathway: a study on its mechanism of action

The development of selective cell-permeable inhibitors of protein kinases whose aberrant activation contributes to cell transformation is a promising approach in cancer treatment. Emodin is a natural anthraquinone derivative that exhibits anti-proliferative effects in various cancer cell lines by efficient induction of apoptosis. The phosphoinositide 3-kinase (PI3K)/AKT pathway has been shown to be central in the promotion of cell survival since the alteration of this signalling cascade is a frequent event in human malignancies. Previous published results indicated that treatment of cells with inhibitors of protein kinase CK2, such as emodin, induces apoptosis and that the anti-apoptotic effect of CK2 is partially mediated by target phosphorylation and up-regulation of AKT by CK2. In the present study, a screening with selected CK2 inhibitors induced a variable response with respect to AKT down-regulation, emodin being the most effective, suggesting that other mechanisms other than the inhibition of CK2 were responsible for the emodin-mediated modulation of AKT. We found that emodin does not directly affect AKT kinase. Furthermore, we show that the down-regulation of AKT is due to the emodin-mediated target inhibition of components of the PI3K pathway, which directly or indirectly affect AKT activity, i.e. the mammalian target of rapamycin and the phosphatase and tensin homolog deleted on chromosome 10, but not the phosphoinositide-dependent kinase 1. Taken together, our results highlight a new mechanism by which emodin exerts anti-cancer activity and suggest the further investigation of plant polyphenols, such as emodin, as therapeutic and preventive agents for cancer therapy.