a-134974 and ortho-topolin-riboside

The naturally occurring cytokinin, ortho-topolin riboside (oTR), has been recently reported to have a strong anticancer effect. However, the molecular mechanism has not been elucidated. From our research we found that oTR strongly inhibited the proliferation of SMMC-7721 cells inducing apoptosis. After oTR treatment, up-regulation of the protein levels of pro-apoptotic Bax and the down-regulation of the anti-apoptotic proteins, Bcl-2 and Bcl-xL was observed, leading to the loss of mitochondrial membrane potential, the release of cytochrome c from the mitochondria into the cytosol, the downstream activation of caspase-9 and caspase-3, as well as the cleavage of poly ADP-ribose-polymerase (PARP), the effect of apoptosis could be blocked by the pan-specific caspase inhibitor z-VAD-fmk and caspase-9-specific inhibitor z-LEHD-fmk. Moreover, oTR was shown to inhibit the activation of the extracellular signal-regulated kinase-1/2 (ERK(1/2)) as well as the Akt pathway. These results suggest that oTR interferes with the mitogen-activated protein kinase (MAPK) and Akt pathways and induces the apoptosis of human SMMC-7721 cells through the activation of intrinsic mitochondria-mediated pathways. However, the apoptosis was completely prevented when cells were treated with A-134974, an inhibitor of adenosine kinase, it indicated that the intracellular phosphorylation of oTR is necessary for its cytotoxic effects to SMMC-7721 cells.

Topics: Adenosine Kinase; Amino Acid Chloromethyl Ketones; Apoptosis; Blotting, Western; Caspase Inhibitors; Cell Line, Tumor; Cell Survival; Cytochromes c; Cytokinins; Flow Cytometry; Humans; MAP Kinase Signaling System; Membrane Potentials; Mitochondrial Membranes; Mitogen-Activated Protein Kinases; Nucleosides; Oligopeptides; Oncogene Protein v-akt; Proto-Oncogene Proteins c-bcl-2; Signal Transduction