Reevesioside A, a Cardenolide Glycoside, Induces Anticancer Activity against Human Hormone-Refractory Prostate Cancers through Suppression of c-myc Expression and Induction of G1 Arrest of the Cell Cycle

Abstract

In the past decade, there has been a profound increase in the number of studies revealing that cardenolide glycosides display inhibitory activity on the growth of human cancer cells. The use of potential cardenolide glycosides may be a worthwhile approach in anticancer research. Reevesioside A, a cardenolide glycoside isolated from the root of Reevesia formosana, displayed potent anti-proliferative activity against human hormone-refractory prostate cancers. A good correlation (r2 = 0.98) between the expression of Na+/K+-ATPase α3 subunit and anti-proliferative activity suggested the critical role of the α3 subunit. Reevesioside A induced G1 arrest of the cell cycle and subsequent apoptosis in a thymidine block-mediated synchronization model. The data were supported by the down-regulation of several related cell cycle regulators, including cyclin D1, cyclin E and CDC25A. Reevesioside A also caused a profound decrease of RB phosphorylation, leading to an increased association between RB and E2F1 and the subsequent suppression of E2F1 activity. The protein and mRNA levels of c-myc, which can activate expression of many downstream cell cycle regulators, were dramatically inhibited by reevesioside A. Transient transfection of c-myc inhibited the down-regulation of both cyclin D1 and cyclin E protein expression to reevesioside A action, suggesting that c-myc functioned as an upstream regulator. Flow cytometric analysis of JC-1 staining demonstrated that reevesioside A also induced the significant loss of mitochondrial membrane potential. In summary, the data suggest that reevesioside A inhibits c-myc expression and down-regulates the expression of CDC25A, cyclin D1 and cyclin E, leading to a profound decrease of RB phosphorylation. G1 arrest is, therefore, induced through E2F1 suppression. Consequently, reevesioside A causes mitochondrial damage and an ultimate apoptosis in human hormone-refractory prostate cancer cells.

Targeting Na+/K+‐translocating adenosine triphosphatase in cancer treatment

“…The steroid may then be transported to the nucleus and play a role in the regulation of protein transcription. Cardiac glycosides have also been shown to affect the downregulation of tumour necrosis factor (TNF)‐ α and, along with c‐Myc, the NF‐ κ B pathway that leads to cell cycle arrest . The inhibition of the Na + /K + ‐ATPase can also play a role in the downregulation of multiple drug‐resistant proteins that allow cancer cells to resist to chemotherapy …”

“…Cardiac glycosides have also been shown to affect the downregulation of tumour necrosis factor (TNF)-a and, along with c-Myc, the NF-jB pathway that leads to cell cycle arrest. 59 The inhibition of the Na + /K + -ATPase can also play a role in the downregulation of multiple drug-resistant proteins that allow cancer cells to resist to chemotherapy. 60 The inhibition of cardiac glycosides creates the endocytosis of the Na + /K + -ATPase.…”

Non-immunosuppressive triazole-based small molecule induces anticancer activity against human hormone-refractory prostate cancers: the role in inhibition of PI3K/AKT/mTOR and c-Myc signaling pathways

“…c-Myc may collaborate with several growth factors and kinases in regulating cyclin D1 expression. Therefore, it is suggested that targeting c-Myc and cyclin D1 may be a good anticancer strategy [26, 28]. However, the role of c-Myc on cyclin D1 expression needs further clarification (please see below).…”

“…c-Myc may collaborate with several receptors of growth factors, Ras/Raf mitogen-activated protein kinases cascade and PI3K/Akt through coordination in regulating cyclin D1 expression [26, 28, 34]. To determine the functional roles of c-Myc and Akt, plasmid transfection was performed and the effects on protein levels were examined using Western blot analysis.…”

“…Recent studies have documented the functional link between c-Myc and mTOR in regulating protein synthesis, suggesting that mTOR-dependent phosphorylation of 4E-BP1 is necessary to cancer cell survival in c-Myc-dependent tumor initiation and maintenance [50]. Furthermore, in responding to oncogenic stimulation c-Myc may collaborate with PI3K/Akt in regulating cyclin D1 expression [26, 28, 34]. Because coordination of c-Myc and PI3K/Akt not only accelerates tumor formation but also renders its progression to a more aggressive phenotype, targeting these molecules is considered as good strategy for cancer therapy.…”