The flavonoid quercetin induces hypoxia-inducible factor-1alpha (HIF-1alpha) and inhibits cell proliferation by depleting intracellular iron

Quercetin, a flavonoid with anti-oxidant, metal chelating, kinase modulating and anti-proliferative properties, can induce hypoxia-inducible factor-1alpha (HIF-1alpha) in normoxia, but its mechanism of action has not been determined. In this study we characterized the induction of HIF-1alpha and the inhibition of cell proliferation caused by quercetin in HeLa and ASM (airway smooth muscle) cells and examined the effect of iron on these processes. Furthermore, we investigated the relevance of the intracellular levels of quercetin to HIF-1alpha expression and cell proliferation. Our data demonstrate that quercetin depletes intracellular calcein-chelatable iron and that supplying additional iron from extracellular or intracellular pools abrogates the induction of HIF-1alpha by quercetin. Moreover, addition of iron reverses the quercetin-induced inhibition of DNA synthesis, cell proliferation and cycle progression, but to different extents, depending on cell type. We propose that quercetin stabilises HIF-1alpha and inhibits cell proliferation predominantly by decreasing the concentration of intracellular iron through chelation.     

Expression of hypoxia-inducible factor-1alpha (HIF-1alpha) in pituitary tumours

The present study was performed to investigate HIF-1alpha (hypoxia-inducible factor-1alpha) expression in a large number of immunohistochemically and ultrastructurally characterized surgically removed pituitary tumours. The potential relation of HIF-1alpha with outcome variables as well as the presence of HIF-1alpha expression in the tumours treated with dopamine agonists and octreotide, a long-acting somatostatin analogue was also investigated. HIF-1alpha immunoreactivity was confined to the nucleoplasm whereas the nucleoli were unconspicuous. The distribution of HIF-1alpha was evident in the tumours whereas normal adenohypophysial cells showed no HIF-1alpha staining. HIF-1alpha expression was detected not only in the tumour cells but also in endothelial cells lining the blood vessels within the tumour. ACTH producing adenomas showed the lowest level of HIF-1alpha expression whereas pituitary carcinomas and GH producing adenomas had the highest counts. The statistical study demonstrated no significant correlation between HIF-1alpha expression, patient age, gender, tumour, size, invasiveness, cell proliferation rate and vascularity. These results suggest that the behaviour of pituitary tumours does not primarily depend of HIF-1alpha expression. Our study demonstrated an increase HIF-1alpha expression in bromocriptine treated PRL producing pituitary adenomas compared with untreated tumours but no increase in octreotide treated tumours.     

Role of prolyl hydroxylation in oncogenically stabilized hypoxia-inducible factor-1alpha

Stabilization of the hypoxia-inducible factor-1 (HIF-1) protein is essential for its role as a regulator of gene expression under low oxygen conditions. Here, employing a novel hydroxylation-specific antibody, we directly show that proline 564 of HIF-1alpha and proline 531 of HIF-2alpha are hydroxylated under normoxia. Importantly, HIF-1alpha Pro-564 and HIF-2alpha Pro-531 hydroxylation is diminished with the treatment of hypoxia, cobalt chloride, desferrioxamine, or dimethyloxalyglycine, regardless of the E3 ubiquitin ligase activity of the von Hippel-Lindau (VHL) tumor suppressor gene. Furthermore, in VHL-deficient cells, HIF-1alpha Pro-564 and HIF-2alpha Pro-531 had detectable amounts of hydroxylation following transition to hypoxia, indicating that the post-translational modification is not reversible. The introduction of v-Src or RasV12 oncogenes resulted in the stabilization of normoxic HIF-1alpha and the loss of hydroxylated Pro-564, demonstrating that oncogene-induced stabilization of HIF-1alpha is signaled through the inhibition of prolyl hydroxylation. Conversely, a constitutively active Akt oncogene stabilized HIF-1alpha under normoxia independently of prolyl hydroxylation, suggesting an alternative mechanism for HIF-1alpha stabilization. Thus, these results indicate distinct pathways for HIF-1alpha stabilization by different oncogenes. More importantly, these findings link oncogenesis with normoxic HIF-1alpha expression through prolyl hydroxylation.     

Cobalt chloride-induced estrogen receptor alpha down-regulation involves hypoxia-inducible factor-1alpha in MCF-7 human breast cancer cells

The estrogen receptor (ER) is down-regulated under hypoxia via a proteasome-dependent pathway. We studied the mechanism of ERalpha degradation under hypoxic mimetic conditions. Cobalt chloride-induced ERalpha down-regulation was dependent on the expression of newly synthesized protein(s), one possibility of which was hypoxia-inducible factor-1alpha (HIF-1alpha). To examine the role of HIF-1alpha expression in ERalpha down-regulation under hypoxic-mimetic conditions, we used a constitutively active form of HIF-1alpha, HIF-1alpha/herpes simplex viral protein 16 (VP16), constructed by replacing the transactivation domain of HIF-1alpha with that of VP16. Western blot analysis revealed that HIF-1alpha/VP16 down-regulated ERalpha in a dose-dependent manner via a proteasome-dependent pathway. The kinase pathway inhibitors PD98059, U0126, wortmannin, and SB203580 did not affect the down-regulation. A mammalian two-hybrid screen and immunoprecipitation assays indicated that ERalpha interacted with HIF-1alpha physically. These results suggest that ERalpha down-regulation under hypoxia involves protein-protein interactions between the ERalpha and HIF-1alpha.     

Reactive oxygen species attenuate nitric-oxide-mediated hypoxia-inducible factor-1alpha stabilization

Tissue hypoxia/ischemia are major pathophysiological determinants. Conditions of decreased oxygen availability provoke accumulation and activation of hypoxia-inducible factor-1 (HIF-1). Recent reports demonstrate a crucial role of HIF-1 for inflammatory events. Regulation of hypoxic responses by the inflammatory mediators nitric oxide (NO) and reactive oxygen species (ROS) is believed to be of pathophysiolgical relevance. It is reported that hypoxic stabilization of HIF-1alpha can be antagonized by NO due to its ability to attenuate mitochondrial electron transport. Likely, the formation of ROS could contribute to this effect. As conflicting results emerged from several studies showing either decreased or increased ROS production during hypoxia, we used experiments mimicking hypoxic intracellular ROS changes by using the redox cycling agent 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), which generates superoxide inside cells. Treatment of A549, HEK293, HepG2, and COS cells with DMNQ resulted in a concentration-dependent raise in ROS which correlated with HIF-1alpha accumulation. By using a HIF-1alpha-von Hippel-Lindau tumor suppressor protein binding assay, we show that ROS produced by DMNQ impaired prolyl hydroxylase activity. When HIF-1alpha is stabilized by NO, low concentrations of DMNQ (<1 microM) revealed no effect, intermediate concentrations of 1 to 40 microM DMNQ attenuated HIF-1alpha accumulation and higher concentrations of DMNQ promoted HIF-1alpha stability. Attenuation of NO-induced HIF-1alpha stability regulation by ROS was mediated by an active proteasomal degradation pathway. In conclusion, we propose that scavenging of NO by ROS and vice versa attenuate HIF-1alpha accumulation in a concentration-dependent manner. This is important to fully elucidate HIF-1alpha regulation under inflammatory conditions.