Institute of Environmental Medicine, Karolinska Institutet, Nobels väg 13, S-17177 Stockholm, Sweden.
Chem Biol Interact. 2010 Mar 30;184(3):466-73. doi: 10.1016/j.cbi.2010.01.038. Epub 2010 Feb 1.
UVB induces the expression of genes controlled by the aryl hydrocarbon receptor (AhR), a transcription factor that has been implicated in the UV stress response. In this study, we used the human hepatoma cell line HepG2 to investigate in more detail the effects of UVB irradiation on AhR activation and induction of cytochrome P450 1A1 (CYP1A1), a highly AhR-responsive gene. The CYP1A1 enzyme efficiently degrades 6-formylindolo[3,2-b]carbazole (FICZ), a high affinity ligand and suggested endogenous activator of the AhR. We show that physiologically relevant doses of UVB suppress CYP1A1 gene expression immediately after irradiation, but induce its expression later in an AhR-dependent manner. The initial repression phase of CYP1A1 transcription was mediated by another UVB-inducible transcription factor, the nuclear factor kappaB (NFkappaB). Crosstalk between AhR and NFkappaB signaling has earlier been implicated to control CYP1A1 expression following stimulation by xenobiotics and cytokines. Now, our findings clearly indicate a role of NFkappaB also in UVB-dependent AhR signaling. We also observed that UVB reduced the catalytic activity of the CYP1A1 enzyme. Thereby, UVB attenuated the clearance of FICZ, which led to prolonged AhR activation. We further noted that repeated irradiation with UVB or H(2)O(2) treatment shifted the cells into a refractory state in which AhR signaling could not be efficiently activated by UVB or H(2)O(2), but by ligands. Together, our results suggest that the NFkappaB-mediated initial suppression of CYP1A1 as well as the unresponsiveness of AhR signaling to repeated irradiation may be part of a protective cellular UV stress response.
中波紫外线（UVB）可诱导芳香烃受体（AhR）控制的基因表达，该转录因子与紫外线应激反应有关。在这项研究中，我们使用人肝癌细胞系 HepG2 更详细地研究了中波紫外线照射对 AhR 激活和细胞色素 P450 1A1（CYP1A1）诱导的影响，CYP1A1 是一种高度 AhR 反应基因。CYP1A1 酶可有效降解 6-甲氧基吲哚并[3,2-b]咔唑（FICZ），这是 AhR 的高亲和力配体和内源性激活剂。我们发现，生理相关剂量的中波紫外线照射后立即抑制 CYP1A1 基因表达，但以后以 AhR 依赖的方式诱导其表达。CYP1A1 转录的初始抑制阶段由另一种中波紫外线诱导的转录因子 NFkappaB（核因子 kappaB）介导。AhR 和 NFkappaB 信号之间的串扰早些时候被认为可以控制细胞因子和外源性刺激物刺激后 CYP1A1 的表达。现在，我们的研究结果清楚地表明，NFkappaB 也在中波紫外线依赖性 AhR 信号中起作用。我们还观察到，中波紫外线降低了 CYP1A1 酶的催化活性。因此，中波紫外线减弱了 FICZ 的清除，导致 AhR 持续激活。我们进一步注意到，用中波紫外线或 H2O2 重复照射会使细胞进入一种不易恢复的状态，在这种状态下，AhR 信号不能被中波紫外线或 H2O2 有效激活，但可以被配体激活。总之，我们的结果表明，NFkappaB 介导的 CYP1A1 的初始抑制以及 AhR 信号对重复照射的无反应性可能是细胞对紫外线应激反应的一种保护机制的一部分。
