Zhang Yongzhao, Furuyama Kazumichi, Kaneko Kiriko, Ding Yuanying, Ogawa Kazuhiro, Yoshizawa Miki, Kawamura Masaki, Takeda Kazuhisa, Yoshida Tadashi, Shibahara Shigeki
Department of Molecular Biology and Applied Physiology, Tohoku University School of Medicine, Sendai, Japan.
FEBS J. 2006 Jul;273(14):3136-47. doi: 10.1111/j.1742-4658.2006.05319.x. Epub 2006 Jun 19.
Heme oxygenase consists of two structurally related isozymes, heme oxygenase-1 and and heme oxygenase-2, each of which cleaves heme to form biliverdin, iron and carbon monoxide. Expression of heme oxygenase-1 is increased or decreased depending on cellular microenvironments, whereas little is known about the regulation of heme oxygenase-2 expression. Here we show that hypoxia (1% oxygen) reduces the expression levels of heme oxygenase-2 mRNA and protein after 48 h of incubation in human cell lines, including Jurkat T-lymphocytes, YN-1 and K562 erythroleukemia, HeLa cervical cancer, and HepG2 hepatoma, as judged by northern blot and western blot analyses. In contrast, the expression level of heme oxygenase-1 mRNA varies under hypoxia, depending on the cell line; it was increased in YN-1 cells, decreased in HeLa and HepG2 cells, and remained undetectable in Jurkat and K562 cells. Moreover, heme oxygenase-1 protein was decreased in YN-1 cells under the conditions used, despite the induction of heme oxygenase-1 mRNA under hypoxia. The heme oxygenase activity was significantly decreased in YN-1, K562 and HepG2 cells after 48 h of hypoxia. To explore the mechanism for the hypoxia-mediated reduction of heme oxygenase-2 expression, we showed that hypoxia shortened the half-life of heme oxygenase-2 mRNA (from 12 h to 6 h) in YN-1 cells, without affecting the half-life of heme oxygenase-1 mRNA (9.5 h). Importantly, the heme contents were increased in YN-1, HepG2 and HeLa cells after 48 h of incubation under hypoxia. Thus, the reduced expression of heme oxygenase-2 may represent an important adaptation to hypoxia in certain cell types, which may contribute to the maintenance of the intracellular heme level.
血红素加氧酶由两种结构相关的同工酶组成，即血红素加氧酶-1和血红素加氧酶-2，它们各自将血红素裂解形成胆绿素、铁和一氧化碳。血红素加氧酶-1的表达根据细胞微环境而增加或减少，而关于血红素加氧酶-2表达的调控知之甚少。在此我们表明，在人细胞系（包括Jurkat T淋巴细胞、YN-1和K562红白血病细胞、HeLa宫颈癌细胞和HepG2肝癌细胞）中孵育48小时后，低氧（1%氧气）会降低血红素加氧酶-2 mRNA和蛋白质的表达水平，这通过Northern印迹和Western印迹分析得以判断。相比之下，血红素加氧酶-1 mRNA的表达水平在低氧条件下因细胞系而异；在YN-1细胞中增加，在HeLa和HepG2细胞中减少，在Jurkat和K562细胞中未检测到。此外，在所使用的条件下，尽管低氧诱导了血红素加氧酶-1 mRNA，但YN-1细胞中的血红素加氧酶-1蛋白质却减少了。低氧48小时后，YN-1、K562和HepG2细胞中的血红素加氧酶活性显著降低。为了探究低氧介导的血红素加氧酶-2表达降低的机制，我们表明低氧缩短了YN-1细胞中血红素加氧酶-2 mRNA的半衰期（从12小时缩短至6小时），而不影响血红素加氧酶-1 mRNA的半衰期（9.5小时）。重要地是，低氧孵育48小时后，YN-1、HepG2和HeLa细胞中的血红素含量增加。因此，血红素加氧酶-2表达的降低可能代表了某些细胞类型对低氧的重要适应性反应，这可能有助于维持细胞内血红素水平。
