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

Abstract

The transcription factor subunit hypoxia-inducible factor-1 alpha (HIF-1 alpha) is currently discussed as a potential endogenous marker of tumor hypoxia to select patients for modified treatment. Despite an association of immunohistochemical HIF-1 alpha overexpression and poor prognosis after radiotherapy in many tumor entities, the reported pattern of HIF-1 alpha staining was often not consistent with tumor hypoxia. To explain this discrepancy, we studied the in vitro conditions under which HIF-1 alpha protein accumulates. 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-1 alpha 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. In both cell lines, weak HIF-1 alpha expression was observed at 20% O(2) and after 10 min of hypoxia. Increased HIF-1 alpha protein levels were observed at 1 h of hypoxia, remained stable over 24 h, and decreased to baseline within 15 min of reoxygenation. HIF-1 alpha 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-1 alpha 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. The ability of HIF-1 alpha 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-1 alpha expression in tumor sections.