Cell-permeable hypoxia-inducible factor-1 (HIF-1) antagonists function as tumor radiosensitizers

Hypoxia is a common phenomenon in human solid tumors and has been considered as an important, independent negative prognostic factor for response to treatment and survival of tumor patients. Hypoxia-inducible factor-1 (HIF-1) is the central transcription factor which is activated by hypoxia and modulates the expression of many genes involved in cell metabolism, proliferation, apoptosis, angiogenesis. Recently, it has been reported that HIF-1 contributes to tumor radioresistance by upregulating survivin expression under hypoxic conditions. Moreover, in hypoxic tumor cells, HIF-1 dependent signal transduction pathway is activated and could be further enhanced by radiation, thereby providing survival signals to adjacent vascular endothelial cells by upregulation of VEGF and bFGF and resulting in tumor radioresistance through vascular radioprotection. Recent research revealed that the stability of HIF-1α, one of the two subunits of HIF-1, determines the whole HIF-1 activity and the C-terminal transactivation domain of HIF-1α could reduce HIF-1 activity when overexpressed in tumor cells by disruption of the assembly of HIF-1 transcription complex. Therefore, we postulate that fusion with protein transduction domains would overcome the inability of C-terminal transactivation domain of HIF-1α to cross cellular membrane. Thus the recombinant fusion proteins could serve as cell-permeable HIF-1 antagonists, function as both inhibitors of tumor angiogenesis and tumor radiosensitizers, and would be widely used in clinical settings to improve tumor response to radiotherapy.