Department of Intensive Care Medicine, Radboud University Medical Center, 6500 HB Nijmegen, The Netherlands; Department of Pharmacology and Toxicology, Radboud University Medical Center, 6500 HB Nijmegen, The Netherlands; Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, 6500 HB Nijmegen, The Netherlands.
Institute for Transfusion Medicine, Hannover Medical School, 30625 Hannover, Germany.
Free Radic Biol Med. 2015 Jan;78:224-32. doi: 10.1016/j.freeradbiomed.2014.10.579. Epub 2014 Nov 5.
Heme oxygenase (HO)-1 is the inducible isoform of the heme-degrading enzyme HO, which is upregulated by multiple stress stimuli. HO-1 has major immunomodulatory and anti-inflammatory effects via its cell-type-specific functions in mononuclear cells. Contradictory findings have been reported on HO-1 regulation by the Toll-like receptor (TLR) 4 ligand lipopolysaccharide (LPS) in these cells. Therefore, we reinvestigated the effects of LPS on HO-1 gene expression in human and murine mononuclear cells in vitro and in vivo. Remarkably, LPS downregulated HO-1 in primary human peripheral blood mononuclear cells (PBMCs), CD14(+) monocytes, macrophages, dendritic cells, and granulocytes, but upregulated this enzyme in primary murine macrophages and human monocytic leukemia cell lines. Furthermore, experiments with human CD14(+) monocytes revealed that activation of other TLRs including TLR1, -2, -5, -6, -8, and -9 decreased HO-1 mRNA expression. LPS-dependent downregulation of HO-1 was specific, because expression of cyclooxygenase-2, NADP(H)-quinone oxidoreductase-1, and peroxiredoxin-1 was increased under the same experimental conditions. Notably, LPS upregulated expression of Bach1, a critical transcriptional repressor of HO-1. Moreover, knockdown of this nuclear factor enhanced basal and LPS-dependent HO-1 expression in mononuclear cells. Finally, downregulation of HO-1 in response to LPS was confirmed in PBMCs from human individuals subjected to experimental endotoxemia. In conclusion, LPS downregulates HO-1 expression in primary human mononuclear cells via a Bach1-mediated pathway. As LPS-dependent HO-1 regulation is cell-type- and species-specific, experimental findings in cell lines and animal models need careful interpretation.
血红素加氧酶（HO）-1 是血红素降解酶 HO 的诱导同工酶，可被多种应激刺激上调。HO-1 通过其在单核细胞中的细胞特异性功能具有主要的免疫调节和抗炎作用。在这些细胞中，关于 Toll 样受体（TLR）4 配体脂多糖（LPS）对 HO-1 的调节作用已有矛盾的发现。因此，我们重新研究了 LPS 对体外和体内人源和鼠源单核细胞中 HO-1 基因表达的影响。值得注意的是，LPS 下调了原代人外周血单核细胞（PBMC）、CD14+单核细胞、巨噬细胞、树突状细胞和粒细胞中的 HO-1，但在原代鼠巨噬细胞和人单核白血病细胞系中上调了该酶。此外，用人 CD14+单核细胞进行的实验表明，包括 TLR1、-2、-5、-6、-8 和 -9 在内的其他 TLR 的激活降低了 HO-1 mRNA 表达。LPS 依赖性 HO-1 下调是特异性的，因为在相同的实验条件下，环加氧酶-2、NAD(P)H-醌氧化还原酶-1 和过氧化物酶-1 的表达增加。值得注意的是，LPS 上调了 Bach1 的表达，Bach1 是 HO-1 的关键转录抑制因子。此外，在单核细胞中敲低该核因子增强了基础和 LPS 依赖性的 HO-1 表达。最后，在接受实验性内毒素血症的人类个体的 PBMC 中证实了 LPS 对 HO-1 的下调。总之，LPS 通过 Bach1 介导的途径下调原代人单核细胞中的 HO-1 表达。由于 LPS 依赖性 HO-1 调节具有细胞类型和物种特异性，因此需要仔细解释细胞系和动物模型中的实验发现。
