Transcriptional repression of interferon-gamma-inducible gene by hypoxia

干扰素-γ (IFN-γ)是一种多效细胞因子，对多种肿瘤细胞具有抗肿瘤活性。IFNγ诱导CXC趋化因子家族成员Mig/CXCL9和IP10/CXCL10，已被证明可诱导有效的淋巴细胞趋化并在肿瘤排斥反应中发挥重要作用。在肿瘤微环境中，大多数实体肿瘤发展为低氧张力区域，称为缺氧，这是由于氧气供应和消耗的不平衡。我们之前已经证明，缺氧抑制IFN-γ诱导的趋化因子CXCL9和CXCL10在人肿瘤细胞系中的表达。虽然缺氧（1% O2）显著抑制IFN-γ诱导的人口腔鳞癌细胞系中CXCL9和CXCL10 mrna的表达，但缺氧对酪氨酸磷酸化和核易位STAT1的水平没有影响。此外，染色质免疫沉淀实验表明，尽管在缺氧条件下STAT1被募集到CXCL9和CXCL10基因的启动子区域，但在缺氧条件下RNA聚合酶II和辅激活因子SRC-1的募集受到抑制。在本研究中，我们进一步探讨了缺氧下调ifn -y诱导的CXCL9和CXCL10基因表达的机制。缺氧条件下，ifn γ诱导的STAT1 （Ser727）磷酸化水平下调。然而，在HSC-3和T98G细胞中未观察到缺氧对磷酸化的抑制作用，提示缺氧对STAT1 Ser727的抑制是一种细胞类型特异性现象。此外，转染了Ser727突变为Glu的突变体STAT1的U3A细胞也被缺氧抑制了ifn -y诱导的CXCL9和CXCL10基因的表达，Glu模拟了Ser727的磷酸化状态。综上所述，这些结果表明STAT1 Ser727的抑制与IFN-γ诱导的基因表达的抑制无关。

Interferon-γ (IFN-γ) is a pleiotropic cytokine that exerts anti-tumor activity to various cancer cells. IFNγ induces Mig/CXCL9 and IP10/CXCL10, members of the CXC chemokine family, have been shown to induce potent lymphocyte chemotaxis and play an important role in tumor rejection. In the tumor microenvironment, most solid tumors develop regions of low oxygen tension called hypoxia, which is due to an imbalance in oxygen supply and consumption. We have previously shown that hypoxia inhibited the IFN-γ induced expression of chemokines CXCL9 and CXCL10 in human tumor cell lines. Although hypoxia (1% O2) markedly inhibited IFN-γ -induced expression of CXCL9 and CXCL10 mRNAs in human oral squamous carcinoma cell line, hypoxia had no effect on the levels of tyrosine-phosphorylated and nuclear-translocated STAT1. Furthermore,Chromatin immunoprecipitation assay demonstrated that although STAT1 was recruited to the promoter region of the CXCL9 and CXCL10 gene under hypoxia, recruitments of RNA polymerase II and coactivator SRC-1 was inhibited under hypoxia. In the present study, we further investigated the mechanisms by which hypoxia down-regulates the IFN-y-induced CXCL9 and CXCL10 gene expression. IFNγ-induced phosphorylation of STAT1 (Ser727) was down-regulated under the hypoxia in HSC-2 cell. However, the inhibitory effect of hypoxia on the phosphorylation was not observed in HSC-3 and T98G cells, suggesting that the inhibition of STAT1 Ser727 by hypoxia is a cell-type specific phenomenon. Furthermore, U3A cells transfected with mutant STAT1 in which Ser727 had been mutated to Glu, which mimics the phosphorylated status of Ser727, were also inhibited by hypoxia for the IFN-y-induced CXCL9 and CXCL10 gene expression. Taken together these results suggest that the inhibition of STAT1 Ser727 does not involved in the inhibition of IFN-γ-induced gene expression.