ATF4 and breast carcinoma: Oleandrin, a cardiac glycoside isolated from the leaves of Nerium oleander, induced apoptosis in breast cancer, MCF7 and MDA-MB-231, cells through the activation of ER stress, by enhancing the expression of phospho-protein kinase-like ER kinase, phosphorylating sequential target proteins (eukaryotic translation initiation factor 2α and activating transcription factor 4), and upregulating the expression of the DNA damage-inducible transcript 3 protein [63].