BACKGROUND & AIMS:
:The hypoxia-inducible factor 1alpha (HIF-1alpha) is the master regulator of the cellular response to hypoxia. A key regulator of HIF-1alpha is von Hippel-Lindau protein (pVHL), which mediates the oxygen-dependent, proteasomal degradation of HIF-1alpha in normoxia. Here, we describe a new regulator of HIF-1alpha, the hypoxia-associated factor (HAF), a novel E3-ubiquitin ligase that binds HIF-1alpha leading to its proteasome-dependent degradation irrespective of cellular oxygen tension. HAF, a protein expressed in proliferating cells, binds and ubiquitinates HIF-1alpha in vitro, and both binding and E3 ligase activity are mediated by HAF amino acids 654 to 800. Furthermore, HAF overexpression decreases HIF-1alpha levels in normoxia and hypoxia in both pVHL-competent and -deficient cells, whereas HAF knockdown increases HIF-1alpha levels in normoxia, hypoxia, and under epidermal growth factor stimulation. In contrast, HIF-2alpha is not regulated by HAF. In vivo, tumor xenografts from cells overexpressing HAF show decreased levels of HIF-1alpha accompanied by decreased tumor growth and angiogenesis. Therefore, HAF is the key mediator of a new HIF-1alpha-specific degradation pathway that degrades HIF-1alpha through a new, oxygen-independent mechanism.
背景与目标:
:缺氧诱导因子1alpha(HIF-1alpha)是细胞对缺氧反应的主要调节因子。 HIF-1alpha的关键调控因子是von Hippel-Lindau蛋白(pVHL),它介导常氧条件下HIF-1alpha的氧依赖性蛋白酶体降解。在这里,我们描述了一种新的HIF-1alpha调节因子,缺氧相关因子(HAF),一种新型的E3-泛素连接酶,它结合HIF-1alpha导致其蛋白酶体依赖性降解,而与细胞氧张力无关。 HAF是一种在增殖细胞中表达的蛋白质,在体外结合并泛素化HIF-1alpha,结合和E3连接酶活性均由HAF氨基酸654-800介导。此外,HAF的过表达降低了正常氧和低氧状态下HIF-1alpha的水平。具有pVHL能力的细胞和缺陷型细胞,而HAF敲低会增加常氧,低氧和表皮生长因子刺激下的HIF-1alpha水平。相反,HIF-2alpha不受HAF调控。在体内,来自过表达HAF的细胞的异种移植物显示HIF-1alpha的水平降低,并伴随着肿瘤的生长和血管生成的降低。因此,HAF是新的HIF-1alpha特异性降解途径的关键介体,该途径通过新的,与氧无关的机制降解HIF-1alpha。