Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions

Abstract

Abstract.Recently, it has become clear that acute hypoxia affecting radioresistance exists widely in tumor tissues. Concurrently, hypoxia-inducible factor-1α (HIF-1α) is recognized as an essential transcriptional factor, enabling cells to survive through hypoxia. However, it is unclear as to whether HIF-1α plays a direct role in the radioresistance caused by acute hypoxia. Therefore, in this study, we investigated the in vitro response of the human lung adenocarcinoma cell line, A549, to ionizing radiation in an experimental model that imitates acute hypoxia in the presence and absence of HIF-1α expression, using the HIF-1α inhibitor 5-[1-(phenylmethyl)-1H-indazol-3-yl]-2-furanmethanol (YC-1). Cells were treated with or without 10 µM YC-1 for 2 h. Cells were exposed to either 95% N 2 and 5% CO 2 (hypoxic condition of <0.1 mmHg) or atmospheric air (normoxic condition) for 1 h, and irradiated with 2, 5 and 10 Gy. Western blot analysis revealed that, without YC-1, cells exposed to hypoxic conditions expressed increased levels of HIF-1α compared with those exposed to normoxic conditions. Under hypoxic conditions, HIF-1α expression was suppressed by YC-1 to the same extent as that observed in cells exposed to normoxic conditions without YC-1. Clonogenic survival assay revealed that under hypoxic conditions there was no significant difference between the surviving fraction of cells treated with YC-1 and without YC-1 at any dose point examined. The oxygen enhancement ratio at 10% surviving fraction was calculated as 2.7 and 2.6 in the presence and the absence of YC-1, respectively. These results indicate that HIF-1α itself is not an immediate cause of acute hypoxia-induced radioresistance in A549 cells. IntroductionHypoxia in the microenvironment of tumors is well recognized as one of the major factors affecting resistance against radiation therapy (1-4). In the clinical setting, a number of investigators have proven the existence of hypoxic cells in human cancers by directly measuring intratumoral oxygen partial pressure (pO 2 ) using special electrodes (5-7). We have also previously reported that low pretreatment intratumoral pO 2 correlates with poor clinical outcome in cervical cancer treated with photon therapy (7).It is recognized that hypoxia in tumors may result from two quite different mechanisms, namely 'acute hypoxia' and 'chronic hypoxia' (8). Acute hypoxia is the result of the abrupt temporary closing of a tumor blood vessel owing to the malformed vasculature of the tumor, which lacks smooth muscle functionality and often has an incomplete endothelial lining and basement membrane (9). Meanwhile, chronic hypoxia arises in regions distant from the blood vessels owing to the limited diffusion distance of oxygen through solid and massive tumor tissues. Although the compositional contribution of acute hypoxia to total hypoxia in tumor tissues has been considered to be low (10,11), recently, Cárdenas-Navia et al proved the pervasive presence of fluctuating pO 2 in rat tumors (12), indicating that acute hypoxia i...

Digoxin Downregulates NDRG1 and VEGF through the Inhibition of HIF-1α under Hypoxic Conditions in Human Lung Adenocarcinoma A549 Cells

“…Our data showed that under hypoxic conditions, digoxin reduced the viability of A549 cells in a concentration- and time-dependent manner (Figure 1), indicating the potent effect of digoxin on the suppression of tumor cell growth. Furthermore, our results, which are consistent with those of the previous study, showed that hypoxia slightly reduced A549 cell viability compared with the cells under normoxic conditions (Figure 1) [21]. …”

“…HIF-1α has been considered as a potential therapeutic target of several cancers [8,20]. A number of genes involved in cell proliferation, cell survival, angiogenesis, tumor cell immortalization, stem cell maintenance/de-differentiation, genetic instability, autocrine growth, invasion/metastasis, and treatment failure, are upregulated under hypoxic conditions through the activation of HIF-1 [7,8,21]. …”

Radio-sensitization effect of an mTOR inhibitor, temsirolimus, on lung adenocarcinoma A549 cells under normoxic and hypoxic conditions

“…The oxygen concentration was assessed in real time using an oxygen electrode (UOE-04 T, Unique Medical, Tokyo, Japan). The details are reported elsewhere [ 20 ].…”

Combination Therapy With Charged Particles and Molecular Targeting: A Promising Avenue to Overcome Radioresistance

“…Some HIF-1 inhibitors have been developed and tested in combination with conventional radiotherapy. The results for this combination-therapy have been controversial, with some studies observing no radiosensitizing effect and other studies observing a suppressed tumor recurrence after treatment (172)(173)(174)(175). Combining particle therapy and HIF-1 inhibitors has not been investigated so far.…”