Jeong Eunshil, Koo Jung Eun, Yeon Sang Hyeon, Kwak Mi-Kyoung, Hwang Daniel H, Lee Joo Young
Integrated Research Institute of Pharmaceutical Sciences, College of Pharmacy, The Catholic University of Korea, Bucheon, Korea.
Mol Carcinog. 2014 Nov;53(11):926-37. doi: 10.1002/mc.22144. Epub 2014 Mar 9.
Peroxisome proliferator-activated receptor (PPAR) δ is highly expressed in colon epithelial cells and closely linked to colon carcinogenesis. However, the role of PPARδ in colon cancer cells in a hypoxic tumor microenvironment is not fully understood. We found that expression of the tumor-promoting cytokines, IL-8 and VEGF, induced by hypoxia (<1% O2) and deferoxamine (a hypoxia mimetic) was significantly attenuated in PPARδ-deficient HCT116 colon cancer cells. Consequently, PPARδ-knockout colon cancer cells exposed to hypoxia and deferoxamine failed to stimulate endothelial cell vascularization and macrophage migration/proliferation, whereas wild-type cells were able to induce angiogenesis and macrophage activation in response to hypoxic stress. Hypoxic stress induced transcriptional activation of PPARδ, but not its protein expression, in HCT116 cells. Exogenous expression of p300 potentiated deferoxamine-induced PPARδ transactivation, while siRNA knockdown of p300 abolished hypoxia- and deferoxamine-induced PPARδ transactivation. PPARδ associated with p300 upon hypoxic stress as demonstrated by coimmunoprecipitation studies. PI3K inhibitors or siRNA knockdown of Akt suppressed the PPARδ transactivation induced by hypoxia and deferoxamine in HCT116 cells, leading to decreased expression of IL-8 and VEGF. Collectively, these results reveal that PPARδ is required for hypoxic stress-mediated cytokine expression in colon cancer cells, resulting in promotion of angiogenesis, macrophage recruitment, and macrophage proliferation in the tumor microenvironment. p300 and the PI3K/Akt pathway play a role in the regulation of PPARδ transactivation induced by hypoxic stress. Our results demonstrate the positive crosstalk between PPARδ in tumor cells and the hypoxic tumor microenvironment and provide potential therapeutic targets for colon cancer.
过氧化物酶体增殖物激活受体（PPAR）δ在结肠上皮细胞中高表达，且与结肠癌发生密切相关。然而，PPARδ在缺氧肿瘤微环境中的结肠癌细胞中的作用尚未完全明确。我们发现，在PPARδ缺陷的HCT116结肠癌细胞中，缺氧（<1% O2）和去铁胺（一种缺氧模拟物）诱导的促肿瘤细胞因子IL-8和VEGF的表达显著减弱。因此，暴露于缺氧和去铁胺环境下的PPARδ基因敲除结肠癌细胞无法刺激内皮细胞血管生成以及巨噬细胞迁移/增殖，而野生型细胞能够响应缺氧应激诱导血管生成和巨噬细胞激活。缺氧应激在HCT116细胞中诱导了PPARδ的转录激活，但未诱导其蛋白表达。p300的外源性表达增强了去铁胺诱导的PPARδ反式激活，而p300的小干扰RNA敲低则消除了缺氧和去铁胺诱导的PPARδ反式激活。免疫共沉淀研究表明，在缺氧应激下PPARδ与p300相互关联。PI3K抑制剂或Akt的小干扰RNA敲低抑制了HCT116细胞中缺氧和去铁胺诱导的PPARδ反式激活，导致IL-8和VEGF表达降低。总体而言，这些结果表明，PPARδ是结肠癌细胞中缺氧应激介导的细胞因子表达所必需的，从而促进了肿瘤微环境中的血管生成、巨噬细胞募集和巨噬细胞增殖。p300和PI3K/Akt信号通路在缺氧应激诱导的PPARδ反式激活调节中发挥作用。我们的结果证明了肿瘤细胞中的PPARδ与缺氧肿瘤微环境之间存在正向串扰，并为结肠癌提供了潜在的治疗靶点。
