Transcriptional repression of interferon-gamma-inducible gene by hypoxia

干扰素-γ(干扰素-γ)是一种多效性细胞因子，对多种肿瘤细胞具有抗肿瘤活性。干扰素γ诱导MIg/CXCL9和IP10/CXCL10是CXC趋化因子家族的成员，已被证明能诱导强大的淋巴细胞趋化作用，并在肿瘤排斥反应中发挥重要作用。在肿瘤微环境中，大多数实体肿瘤形成称为低氧压的低氧区域，这是由于氧供应和消耗失衡所致。我们先前已经证明，低氧抑制干扰素-γ诱导的人类肿瘤细胞株中趋化因子CXCL9和CXCL10的表达。低氧(1%O2)可显著抑制干扰素-γ诱导的口腔鳞癌细胞株CXCL9和CXCL10mRNAs的表达，但对酪氨酸磷酸化和核转位STAT1水平无明显影响。染色质免疫沉淀分析表明，尽管STAT1在低氧条件下被招募到CXCL9和CXCL10基因的启动子区域，但在低氧条件下，RNA聚合酶II和辅助激活因子SRC-1的招募受到抑制。在本研究中，我们进一步研究了低氧下调干扰素-γ诱导的CXCL9和CXCL10基因表达的机制。低氧条件下干扰素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STAT1 Ser727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有的现象。此外，将Ser727突变为Glu(模拟Ser727的磷酸化状态)的突变STAT1转染U3A细胞，也能抑制干扰素-γ诱导的CXCL9和CXCL10基因的表达。综上所述，这些结果表明，STAT1Ser727的抑制不参与抑制干扰素-γ诱导的基因表达。

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.