It is widely accepted that the hypoxic nature of solid tumors contribute to their resistance to radiation therapy. There is increasing evidence that cyclooxygenase-2 (COX-2) contributes to increased resistance of tumors to radiation therapy. Several studies demonstrate that combination of COX-2 selective inhibitors with radiation therapy selectively enhances radio responsiveness of tumor cells. However, the majority of these studies utilised suprapharmacological concentrations under normoxic conditions only. Furthermore, the mechanism by which these agents act remain largely unclear. Therefore, the aim of this study was to determine the impact of COX-2 selective inhibitors on both normoxic and hypoxic radiosensitivity in vitro and the mechanisms underlying this. Because of the close, reciprocal relationship between COX-2 and p53 we investigated their contribution to radioresistance. To achieve this we exposed HeLa, MCF-7 and MeWo cells to the COX-2 selective inhibitor, NS398 (10μM). NS398 (10μM) selectively sensitized hypoxic HeLa and MCF-7 but not MeWo cells to ionising radiation (5 Gy). Furthermore, while knockdown of COX-2 with siRNA did not affect either normoxic radiosensitivity in HeLa cells, the radiosensitisation observed with NS398 was lost suggesting both COX-2 dependent and independent mechanisms. We also show that ionising radiation at 5 Gy results in phosphorylation of p53 at serine 15, a key phosphorylation site for p53-mediated apoptosis, and that hypoxia attenuates this phosphorylation. Attenuated phosphorylation of p53 under hypoxic conditions may therefore contribute to hypoxic radioresistance. We also show that NS398 selectively phosphorylates p53 under hypoxic conditions following irradiation at 5 Gy. p53 phosphorylation could be an underlying mechanism by which this agent and other COX-2 inhibitors sensitize tumors to radiation therapy.

中文翻译：

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COX-2抑制剂NS398通过依赖和独立于COX-2的机制选择性地使低氧HeLa细胞对电离辐射敏感。

实体瘤的低氧特性有助于其对放射治疗的抵抗力，这一点已被广泛接受。越来越多的证据表明，环氧合酶2（COX-2）有助于增加肿瘤对放射治疗的抵抗力。几项研究表明，COX-2选择性抑制剂与放射疗法的结合可选择性增强肿瘤细胞的放射反应能力。然而，这些研究中的大多数仅在常氧条件下利用了超药理学浓度。此外，这些试剂起作用的机制在很大程度上还不清楚。因此，本研究的目的是确定COX-2选择性抑制剂在体外对常氧和低氧放射敏感性的影响以及其潜在机制。因为亲密 我们调查了COX-2和p53之间的相互关系，研究了它们对放射抗性的贡献。为此，我们将HeLa，MCF-7和MeWo细胞暴露于COX-2选择性抑制剂NS398（10μM）中。NS398（10μM）对低氧HeLa和MCF-7选择性敏感，但对MeWo细胞不敏感，对电离辐射（5 Gy）敏感。此外，虽然用siRNA敲低COX-2既不影响HeLa细胞中的常氧放射敏感性，却失去了用NS398观察到的放射敏感性，提示了COX-2依赖性和独立机制。我们还表明，在5 Gy处的电离辐射会导致丝氨酸15处p53的磷酸化，这是p53介导的细胞凋亡的关键磷酸化位点，缺氧会减弱这种磷酸化。因此，在低氧条件下p53的磷酸化减弱可能会导致低氧辐射抗性。我们还显示，在低氧条件下，在5 Gy辐射后，NS398选择性磷酸化p53。p53磷酸化可能是该药物和其他COX-2抑制剂使肿瘤对放射治疗敏感的潜在机制。