Werno Christian, Zhou Jie, Brüne Bernhard
Institute of Biochemistry I/ZAFES, Faculty of Medicine, Johann Wolfgang Goethe-University, Frankfurt, Germany.
J Cell Physiol. 2008 Jun;215(3):708-14. doi: 10.1002/jcp.21351.
Hypoxia inducible factor 1 (HIF-1) coordinates major responses to hypoxia, with the notion that its alpha subunit is degraded under normoxia but stable under hypoxia. Recently, calcium was shown to affect expression of HIF-1alpha. While lowering cytosolic calcium accumulates HIF-1alpha, a calcium increase by the ionophores A23187 or ionomycin as well as the endoplasmic reticulum (ER) Ca(2+)-ATPase inhibitor thapsigargin produced inconsistent results with reports to either increase or decrease HIF-1alpha protein. In the human hepatocyte cell line HepG2 only A23187, but neither ionomycin nor thapsigargin, accumulated HIF-1alpha protein under normoxia. However, A23187 does so independently of a calcium increase. A23187 not only accumulated HIF-1alpha protein but also mRNA of HIF-1alpha, with the notion that protein but not mRNA accumulation was attenuated by blocking mitogen-activated protein kinase (MAPK), suggesting that HIF-1alpha protein accumulation is not a direct consequence of its mRNA elevation. Indeed, protein stability determinations implied that A23187 enhanced translation of HIF-1alpha. Interestingly, ionomycin and thapsigargin also increased the HIF-1alpha mRNA content. Although not increasing the HIF-1alpha protein amount under normoxia, both compounds enhanced protein accumulation of HIF-1alpha under hypoxia. Taking into account that induction of ER stress by tunicamycin and brefeldin A, without altering intracellular calcium concentrations, also increased HIF-1alpha mRNA, suggests that ER stress pathways enhanced transcription of HIF-1alpha mRNA. We conclude that ER stress rather than calcium fluctuations increased HIF-1alpha mRNA content by established calcium liberating agents, which alone is insufficient for normoxic HIF-1alpha accumulation.
缺氧诱导因子1（HIF-1）协调机体对缺氧的主要反应，其α亚基在常氧条件下会降解，但在缺氧条件下稳定。最近，有研究表明钙会影响HIF-1α的表达。降低胞质钙会使HIF-1α积累，而离子载体A23187或离子霉素以及内质网（ER）Ca²⁺-ATP酶抑制剂毒胡萝卜素引起的钙增加，关于HIF-1α蛋白是增加还是减少的报道结果并不一致。在人肝细胞系HepG2中，只有A23187能在常氧条件下使HIF-1α蛋白积累，而离子霉素和毒胡萝卜素都不能。然而，A23187积累HIF-1α蛋白的作用与钙增加无关。A23187不仅使HIF-1α蛋白积累，还使HIF-1α的mRNA积累，有研究认为通过阻断丝裂原活化蛋白激酶（MAPK）可使蛋白积累减弱，但mRNA积累不受影响，这表明HIF-1α蛋白积累并非其mRNA升高的直接结果。实际上，蛋白质稳定性测定表明A23187增强了HIF-1α的翻译。有趣的是，离子霉素和毒胡萝卜素也增加了HIF-1α的mRNA含量。虽然在常氧条件下这两种化合物不会增加HIF-1α蛋白量，但在缺氧条件下它们都增强了HIF-1α的蛋白积累。考虑到衣霉素和布雷菲德菌素A诱导内质网应激，且不改变细胞内钙浓度，也会增加HIF-1α mRNA，这表明内质网应激途径增强了HIF-1α mRNA的转录。我们得出结论，内质网应激而非钙波动通过既定的钙释放剂增加了HIF-1α mRNA含量，而仅靠这些钙释放剂不足以在常氧条件下积累HIF-1α。
