Isocudraxanthone K Induces Growth Inhibition and Apoptosis in Oral Cancer Cells via Hypoxia Inducible Factor-1α

Isocudraxanthone K (IK) is a novel, natural compound from a methanol extract of the root bark of Cudrania tricuspidata. It has not been shown previously that IK possessed antitumor activity. We investigated the antitumor effects and molecular mechanism of IK and related signal transduction pathway(s) in oral squamous cell carcinoma cells (OSCCCs). The MTT assay revealed that IK had an antiproliferative effect on OSCCCs, in a dose- and time-dependent manner. IK induced apoptosis in OSCCCs, as identified by a cell-cycle analysis, annexin V-FITC and propidium iodide staining, and the nuclear morphology in cell death. IK caused time-dependent phosphorylation of Akt, p38, and ERK (extracellular signal-regulated kinase). In addition, IK increased the cytosolic to nuclear translocation of nuclear factor-κB (NF-κB) p65 and the degradation and phosphorylation of IκB-α in HN4 and HN12 cells. Furthermore, IK treatment downregulated hypoxia-inducible factor 1α (HIF-1α) and its target gene, vascular endothelial growth factor (VEGF). Cobalt chloride (CoCl(2)), a HIF-1α activator, attenuated the IK-induced growth-inhibiting and apoptosis-inducing effects, and blocked IK-induced expression of apoptosis regulatory proteins, such as Bax, Bcl-2, caspase-3, caspase-8, and caspase-9, and cytochrome c. Collectively, these data provide the first evidence of antiproliferative and apoptosis-inducing effects of IK as a HIF-1α inhibitor and suggest it may be a drug candidate for chemotherapy against oral cancer.