AMP-activated protein kinase (AMPK) functions as an energy sensor to provide metabolic adaptations under the ATP-deprived conditions such as hypoxia. In the present study, we considered a role of AMPK in the adaptive response to hypoxia by examining whether AMPK is involved in the regulation of hypoxia-inducible factor-1 (HIF-1), a heterodimeric transcription factor that is critical for hypoxic induction of physiologically important genes. We demonstrate that hypoxia or CoCl 2 rapidly activated AMPK in DU145 human prostate cancer cells, and its activation preceded the induction of HIF-1␣ expression. Under these conditions, blockade of AMPK activity by a pharmacological or molecular approach significantly attenuated hypoxia-induced responses such as HIF-1 target gene expression, secretion of vascular endothelial growth factor, glucose uptake, and HIF-1-dependent reporter gene expression, indicating that AMPK is critical for the HIF-1 transcriptional activity and its target gene expression. Its functional requirement for HIF-1 activity was also demonstrated in several different cancer cell lines, but AMPK activation alone was not sufficient to stimulate the HIF-1 transcriptional activity. We further present data showing that AMPK transmits a positive signal for HIF-1 activity via a signaling pathway that is independent of phosphatidylinositol 3-kinase/AKT and several mitogen-activated protein kinases. Taken together, our results suggest that AMPK is a novel and critical component of HIF-1 regulation, implying its new roles in oxygen-regulated cellular phenomena.The energy status of the cell plays a crucial role for cell survival, and exposure of eukaryotic cells to metabolic stresses that accompany the depletion of intracellular ATP triggers specific and systemic adaptive responses. AMP-activated protein kinase (AMPK), 1 a heterotrimeric enzyme consisting of a catalytic subunit (␣) and two regulatory subunits (␤ and ␥), plays a critical role as an energy sensor in these responses (reviewed in Refs. 1-3). In response to nutritional or environmental stress factors that deplete intracellular ATP, AMPK is activated by allosteric binding of AMP (4, 5) and by phosphorylation by a still uncharacterized upstream AMPK kinase (6). Once activated, AMPK minimizes further ATP consumption by suppressing ATP-consuming anabolic pathways as well as activating ATP-generating catabolic pathways. The physiological or stress conditions known to activate AMPK include exercise (7-9), nutritional starvation (10), heat shock (11), oxidative stress (12), and ischemia/hypoxia (3, 13-15). Similar to the intracellular energy status, cellular oxygen concentration is precisely regulated in mammals to maintain cellular function and integrity. The reduced oxygen availability also initiates a series of adaptive responses, and many of these are mediated by HIF-1, which trans-activates several dozens of target genes whose protein products function to increase oxygen delivery and to enhance metabolic adaptation to anaerobic conditions (reviewed in Re...

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Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in human cancers. Therefore, STAT3 is a therapeutic target of cancer drug discovery. We previously reported that natural products inhibited constitutively activated STAT3 in human prostate tumor cells. We used a dual-luciferase assay to screen 200 natural products isolated from herbal medicines and we identified ginkgetin obtained from the leaves of Ginkgo biloba L. as a STAT3 inhibitor. Ginkgetin inhibited both inducible and constitutively activated STAT3 and blocked the nuclear translocation of p-STAT3 in DU-145 prostate cancer cells. Furthermore, ginkgetin selectively inhibited the growth of prostate tumor cells stimulated with activated STAT3. Ginkgetin induced STAT3 dephosphorylation at Try705 and inhibited its localization to the nucleus, leading to the inhibition of expression of STAT3 target genes such as cell survival-related genes (cyclin D1 and survivin) and anti-apoptotic proteins (Bcl-2 and Bcl-xL). Therefore, ginkgetin inhibited the growth of STAT3-activated tumor cells. We also found that ginkgetin inhibited tumor growth in xenografted nude mice and downregulated p-STAT3Tyr705 and survivin in tumor tissues. This is the first report that ginkgetin exerts antitumor activity by inhibiting STAT3. Therefore, ginkgetin is a good STAT3 inhibitor and may be a useful lead molecule for development of a therapeutic STAT3 inhibitor.

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