Raja R, Kale S, Thorat D, Soundararajan G, Lohite K, Mane A, Karnik S, Kundu G C
Laboratory of Tumor Biology, Angiogenesis and Nanomedicine Research, National Center for Cell Science, Pune, India.
Grant Medical Foundation, Ruby Hall Clinic, Pune, India.
Oncogene. 2014 Apr 17;33(16):2053-64. doi: 10.1038/onc.2013.171. Epub 2013 Jun 3.
Hypoxia is a salient feature of most solid tumors, and hypoxic adaptation of cancer cells has crucial implications in propagation of malignant clonal cell population. Osteopontin (OPN) has been identified as a hypoxia-responsive gene, but the mechanistic and regulatory role of OPN under hypoxia is less characterized. The present study identifies the existence of a positive inter-regulatory loop between hypoxia and OPN. We have shown that hypoxia induces OPN expression in breast cancer cells; however, the expression was found to be HIF1α independent. OPN enabled transcriptional upregulation of HIF1α expression both under normoxia and hypoxia, whereas stability of HIF1α protein in breast cancer cells remained unaffected. Moreover, we have shown that OPN induces integrin-linked kinase (ILK)/Akt-mediated nuclear factor (NF)-κB p65 activation leading to HIF1α-dependent vascular endothelial growth factor (VEGF) expression and angiogenesis in response to hypoxia. These in vitro data are biologically important as OPN expressing cells induce greater tumor growth and angiogenesis through enhanced expressions of proangiogenic molecules as compared with control. Immunohistochemical analysis of human breast cancer specimens revealed significant correlation between OPN and HIF1α but not HIF2α. Elevated expression of HIF1α and OPN was observed in pre-neoplastic and early stage infiltrating ductal carcinoma implicating the role of these proteins in neoplastic progression of breast cancer. Together, our results substantiate the prime role of OPN in cellular adaptation through ILK and NF-κB-mediated HIF1α-dependent VEGF expression in response to hypoxia that ultimately controls breast cancer progression and angiogenesis. Our study reinforces the fact that targeting OPN and its regulated signaling network hold important therapeutic implications.
缺氧是大多数实体瘤的一个显著特征，癌细胞的缺氧适应在恶性克隆细胞群体的增殖中具有至关重要的意义。骨桥蛋白（OPN）已被确定为一种缺氧反应基因，但OPN在缺氧条件下的机制和调节作用尚不明确。本研究确定了缺氧与OPN之间存在正向相互调节环。我们发现缺氧可诱导乳腺癌细胞中OPN的表达；然而，该表达与缺氧诱导因子1α（HIF1α）无关。在常氧和缺氧条件下，OPN均能使HIF1α表达转录上调，而乳腺癌细胞中HIF1α蛋白的稳定性不受影响。此外，我们还发现OPN可诱导整合素连接激酶（ILK）/蛋白激酶B（Akt）介导的核因子（NF）-κB p65激活，从而导致缺氧时HIF1α依赖性血管内皮生长因子（VEGF）的表达和血管生成。这些体外数据具有重要的生物学意义，因为与对照组相比，表达OPN的细胞通过增强促血管生成分子的表达诱导更大程度的肿瘤生长和血管生成。对人乳腺癌标本的免疫组织化学分析显示，OPN与HIF1α之间存在显著相关性，但与HIF2α无关。在癌前病变和早期浸润性导管癌中观察到HIF1α和OPN表达升高，这表明这些蛋白在乳腺癌的肿瘤进展中发挥作用。总之，我们的结果证实了OPN在细胞适应中的主要作用，即通过ILK和NF-κB介导的缺氧时HIF1α依赖性VEGF表达，最终控制乳腺癌的进展和血管生成。我们的研究强化了这样一个事实，即靶向OPN及其调控的信号网络具有重要的治疗意义。
