Department of Hygiene and Public Health I, Tokyo Women's Medical University, Tokyo, 162-8666, Japan.
Arch Toxicol. 2014 Feb;88(2):403-14. doi: 10.1007/s00204-013-1129-y. Epub 2013 Sep 22.
Cadmium exposure causes endoplasmic reticulum (ER) stress and accumulation of activating transcription factor 4 (ATF4), an ER stress marker. To elucidate the role of phosphatidylinositol-3-kinase (PI3K) signaling in this process, we examined the effects of PI3K signaling on cadmium chloride (CdCl2) exposure-induced ATF4 expression in HK-2 human renal proximal tubular cells. ATF4 knockdown by siRNA enhanced CdCl2-induced cellular damage, indicating a cytoprotective function of ATF4. Treatment with LY294002, a PI3K inhibitor, suppressed CdCl2-induced ATF4 expression and Akt phosphorylation at Thr308 with little effect on phosphorylation of eukaryotic translation initiation factor 2 subunit α at Ser51. Activation of PI3K signaling with epidermal growth factor treatment enhanced CdCl2-induced Akt phosphorylation and ATF4 expression. Suppression of CdCl2-induced ATF4 expression by LY294002 treatment was markedly blocked by cycloheximide, a translation inhibitor, but not by MG-132, a proteasome inhibitor, or actinomycin D, a transcription inhibitor. CdCl2 exposure also induced phosphorylation of mammalian target of rapamycin (mTOR) at Ser2448, glycogen synthase kinase-3α (GSK-3α) at Ser21, GSK-3β at Ser9, and 90 kDa ribosomal S6 kinase 2 (RSK2) at Ser227 in HK-2 cells. Treatment with rapamycin, an mTOR inhibitor, MK2206, an Akt inhibitor, and BI-D1870, a RSK inhibitor, partially suppressed CdCl2-induced ATF4 expression. Conversely, SB216763, a GSK-3 inhibitor, markedly inhibited the potency of LY294002 to suppress CdCl2-induced ATF4 expression. These results suggest that PI3K signaling diversely regulates the expression of ATF4 in a translation-dependent manner via downstream molecules, including mTOR, GSK-3α/β, and RSK2, and plays a role in protecting HK-2 cells from cadmium-induced damage.
镉暴露会引起内质网 (ER) 应激和激活转录因子 4 (ATF4) 的积累，ATF4 是 ER 应激的标志物。为了阐明磷脂酰肌醇-3-激酶 (PI3K) 信号通路在此过程中的作用，我们研究了 PI3K 信号通路对氯化镉 (CdCl2) 暴露诱导 HK-2 人肾小管近端细胞 ATF4 表达的影响。siRNA 敲低 ATF4 增强了 CdCl2 诱导的细胞损伤，表明 ATF4 具有细胞保护功能。PI3K 抑制剂 LY294002 的处理抑制了 CdCl2 诱导的 ATF4 表达和 Akt 在 Thr308 处的磷酸化，而对真核翻译起始因子 2 亚基 α 在 Ser51 处的磷酸化影响很小。表皮生长因子处理激活 PI3K 信号通路增强了 CdCl2 诱导的 Akt 磷酸化和 ATF4 表达。LY294002 处理抑制 CdCl2 诱导的 ATF4 表达被翻译抑制剂环己酰亚胺显著阻断，但被蛋白酶体抑制剂 MG-132 或转录抑制剂放线菌素 D 阻断不明显。CdCl2 暴露还诱导 HK-2 细胞中哺乳动物雷帕霉素靶蛋白 (mTOR) 在 Ser2448 处、糖原合成酶激酶-3α (GSK-3α) 在 Ser21 处、GSK-3β 在 Ser9 处和 90 kDa 核糖体 S6 激酶 2 (RSK2) 在 Ser227 处磷酸化。mTOR 抑制剂 rapamycin、Akt 抑制剂 MK2206 和 RSK 抑制剂 BI-D1870 的处理部分抑制了 CdCl2 诱导的 ATF4 表达。相反，GSK-3 抑制剂 SB216763 显著抑制了 LY294002 抑制 CdCl2 诱导的 ATF4 表达的效力。这些结果表明，PI3K 信号通路通过下游分子（包括 mTOR、GSK-3α/β 和 RSK2）以依赖翻译的方式多样化调节 ATF4 的表达，并在保护 HK-2 细胞免受镉诱导的损伤中发挥作用。
