Prestera T, Talalay P, Alam J, Ahn Y I, Lee P J, Choi A M
Department of pharmacology and molecular sciences, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Mol Med. 1995 Nov;1(7):827-37.
Heme oxygenase (HO; EC 1.14.99.3) catalyzes the conversion of heme to biliverdin, which is reduced enzymatically to bilirubin. Since bilirubin is a potent antioxidant and heme a pro-oxidant, HO may protect cells against oxidative damage. HO-1 is highly inducible by diverse chemical agents, resembling those evoking induction of phase 2 enzymes (i.e., Michael reaction acceptors, heavy metals, trivalent arsenicals, and sulfhydryl reagents). Phase 2 enzymes (glutathione transferases; NAD (P)H:quinone reductase; glucuronosyltransferases) are regulated by antioxidant-responsive elements (ARE), and their induction protects against chemical carcinogenesis. Is HO-1 regulated by chemical agents and enhancer elements similar to those controlling phase 2 enzymes?
Induction of HO-1 by phorbol ester and heavy metals is transcriptionally controlled through a 268-bp SX2 fragment, containing two phorbol ester-responsive (TRE) sites (TGAC/GT C/AA) which overlap ARE consensus sequences (TGACNNNGC). Therefore, mutations of the SX2 element designed to distinguish ARE from TRE were inserted into chloramphenicol acetyltransferase (CAT) reporter plasmids, and the response of the CAT activity of murine hepatoma cells stably transfected with these constructs was examined with a wide range of inducers of phase 2 enzymes.
All compounds raised HO-1 mRNA and CAT expression constructs containing wild-type SX2. When the SX2 region was mutated to alter TRE consensus sequences without destroying the ARE consensus, full inducibility was preserved. Conversely, when the ARE consensus was disturbed, inducibility was abolished.
Induction of heme oxygenase-1 is regulated by several chemically distinct classes of inducers (mostly electrophiles), which also induce phase 2 enzymes, and these inductions are mediated by similar AREs. These findings support the importance of HO-1 as a protector against oxidative damage and suggest that HO-1 induction is part of a more generalized protective cellular response that involves phase 2 enzymes.
血红素加氧酶（HO；EC 1.14.99.3）催化血红素转化为胆绿素，胆绿素经酶促还原为胆红素。由于胆红素是一种强效抗氧化剂，而血红素是一种促氧化剂，HO可能保护细胞免受氧化损伤。HO-1可被多种化学试剂高度诱导，这些试剂类似于那些能诱导Ⅱ相酶（即迈克尔反应受体、重金属、三价砷化合物和巯基试剂）的试剂。Ⅱ相酶（谷胱甘肽转移酶；NAD（P）H：醌还原酶；葡糖醛酸转移酶）受抗氧化反应元件（ARE）调控，它们的诱导可预防化学致癌作用。HO-1是否受与控制Ⅱ相酶的化学试剂和增强子元件类似的调控？
佛波酯和重金属对HO-1的诱导是通过一个268bp的SX2片段进行转录调控的，该片段包含两个佛波酯反应性（TRE）位点（TGAC/GTC/AA），它们与ARE共有序列（TGACNNNGC）重叠。因此，将旨在区分ARE和TRE的SX2元件突变体插入氯霉素乙酰转移酶（CAT）报告质粒中，并用多种Ⅱ相酶诱导剂检测稳定转染这些构建体的小鼠肝癌细胞的CAT活性反应。
所有化合物均可提高含有野生型SX2的HO-1 mRNA和CAT表达构建体的水平。当SX2区域发生突变以改变TRE共有序列而不破坏ARE共有序列时，仍保留完全诱导性。相反，当ARE共有序列受到干扰时，诱导性则被消除。
血红素加氧酶-1的诱导受几类化学性质不同的诱导剂（大多为亲电试剂）调控，这些诱导剂也可诱导Ⅱ相酶，且这些诱导作用由相似的ARE介导。这些发现支持了HO-1作为抗氧化损伤保护剂的重要性，并表明HO-1的诱导是涉及Ⅱ相酶的更广泛的细胞保护反应的一部分。
