Laboratory of Biochemistry, Faculty of Medicine, School of Health Sciences, University of Thessaly, Biopolis, Larissa 41110, Greece.
Int J Biochem Cell Biol. 2013 Nov;45(11):2359-68. doi: 10.1016/j.biocel.2013.07.025. Epub 2013 Aug 16.
Hypoxia-inducible factors (HIFs) are transcriptional regulators that mediate the cellular response to low oxygen. Although HIF-1 is usually considered as the principal mediator of hypoxic adaptation, several tissues and different cell types express both HIF-1 and HIF-2 isoforms under hypoxia or when treated with hypoxia mimetic chemicals such as cobalt. However, the similarities or differences between HIF-1 and HIF-2, in terms of their tissue- and inducer-specific activation and function, are not adequately characterized. To address this issue, we investigated the effects of true hypoxia and hypoxia mimetics on HIF-1 and HIF-2 induction and specific gene transcriptional activity in two hepatic cancer cell lines, Huh7 and HepG2. Both hypoxia and cobalt caused rapid induction of both HIF-1α and HIF-2α proteins. Hypoxia induced erythropoietin (EPO) expression and secretion in a HIF-2-dependent way. Surprisingly, however, EPO expression was not induced when cells were treated with cobalt. In agreement, both HIF-1- and HIF-2-dependent promoters (of PGK and SOD2 genes, respectively) were activated by hypoxia while cobalt only activated the HIF-1-dependent PGK promoter. Unlike cobalt, other hypoxia mimetics such as DFO and DMOG activated both types of promoters. Furthermore, cobalt impaired the hypoxic stimulation of HIF-2, but not HIF-1, activity and cobalt-induced HIF-2α interacted poorly with USF-2, a HIF-2-specific co-activator. These data show that, despite similar induction of HIF-1α and HIF-2α protein expression, HIF-1 and HIF-2 specific gene activating functions respond differently to different stimuli and suggest the operation of oxygen-independent and gene- or tissue-specific regulatory mechanisms involving additional transcription factors or co-activators.
缺氧诱导因子（HIFs）是转录调节因子，介导细胞对低氧的反应。虽然 HIF-1 通常被认为是低氧适应的主要介导者，但在低氧或用缺氧模拟化学物质（如钴）处理时，几种组织和不同的细胞类型都表达 HIF-1 和 HIF-2 同工型。然而，HIF-1 和 HIF-2 在其组织和诱导剂特异性激活和功能方面的相似性或差异性尚未得到充分描述。为了解决这个问题，我们研究了真正的缺氧和缺氧模拟物对两种肝癌细胞系 Huh7 和 HepG2 中 HIF-1 和 HIF-2 诱导和特定基因转录活性的影响。缺氧和钴都能迅速诱导 HIF-1α 和 HIF-2α 蛋白的表达。缺氧以 HIF-2 依赖的方式诱导促红细胞生成素（EPO）的表达和分泌。然而，令人惊讶的是，当细胞用钴处理时，EPO 的表达并没有被诱导。同样，HIF-1 和 HIF-2 依赖性启动子（分别为 PGK 和 SOD2 基因的启动子）都被缺氧激活，而钴只激活 HIF-1 依赖性 PGK 启动子。与钴不同，其他缺氧模拟物，如 DFO 和 DMOG，激活了这两种类型的启动子。此外，钴损害了 HIF-2 的低氧刺激，但不损害 HIF-1 的活性，并且钴诱导的 HIF-2α 与 HIF-2 特异性共激活因子 USF-2 相互作用不良。这些数据表明，尽管 HIF-1α 和 HIF-2α 蛋白表达的诱导相似，但 HIF-1 和 HIF-2 特定基因激活功能对不同的刺激有不同的反应，并表明涉及额外的转录因子或共激活因子的氧非依赖性和基因或组织特异性调节机制的运作。
