Cell type-specific association of hypoxia-inducible factor-1α (HIF-1α) protein accumulation and radiobiologic tumor hypoxia

Purpose: The transcription factor subunit hypoxia-inducible factor-1alpha (HIF-1alpha) is currently discussed as a potential endogenous marker of tumor hypoxia to select patients for modified treatment. Despite an association of immunohistochemical HIF-1alpha overexpression and poor prognosis after radiotherapy in many tumor entities, the reported pattern of HIF-1alpha staining was often not consistent with tumor hypoxia. To explain this discrepancy, we studied the in vitro conditions under which HIF-1alpha protein accumulates. Methods and Materials: FaDu human pharyngeal carcinoma and HT 1080 human fibrosarcoma cells were treated with different schedules of in vitro hypoxia at 5%, 1%, and 0.1% O-2 and reoxygenation. HIF-1alpha protein levels were determined in nuclear extracts. Cellular radiation sensitivity was assessed by clonogenic survival assay after single-dose irradiation at the above oxygen concentrations. Results: In both cell lines, weak HIF-1alpha expression was observed at 20% O-2 and after 10 min of hypoxia. Increased HIF-1alpha protein levels were observed at 1 h of hypoxia, remained stable over 24 h, and decreased to baseline within 15 min of reoxygenation. HIF-1alpha protein at 5% O-2 was half-maximal in FaDu but indistinguishable from 0.1% O-2 in HT 1080. A good correlation of HIF-1alpha protein level and hypoxic radiation resistance, with equal ranking of data points by both assays, was observed in FaDu cells but not in HT 1080 cells. Conclusions: The ability of HIF-1alpha to indicate radiobiologically relevant levels of tumor hypoxia seems to be cell type specific. This finding may explain the inconsistent results regarding the pattern of HIF-1alpha expression in tumor sections. (C) 2004 Elsevier Inc.