Hypoxia-inducible factor-2alpha: effect on radiation sensitivity and differential regulation by an mTOR inhibitor.
OBJECTIVE
To determine the role of hypoxia-inducible factor-2alpha (HIF2alpha) on the sensitivity of renal cell carcinoma (RCC) cell lines to ionizing radiation and to determine if the mTOR antagonist, rapamycin, could decrease HIF2alpha protein levels.


METHODS
Cell lines expressing stable short-hairpin RNAs (shRNAs) encoding HIF2alpha shRNAs or an empty vector were transfected with a hypoxia responsive element (HRE)-driven firefly luciferase reporter gene. Two separate paired cell lines were assayed for their response to increasing doses of ionizing radiation. Proliferation and cell cycle kinetics were compared for cell lines expressing HIF2alpha shRNAs and empty vectors. The effect of an mTOR antagonist, rapamycin on HIF1alpha and HIF2alpha proteins levels was also assessed.


RESULTS
We confirmed that the 786-O RCC lines with stably integrated shRNAs against HIF2alpha had decreased activation of a plasmid with a HRE-driven firefly luciferase reporter gene. Lines from two separate cell clones with decreased HIF2alpha levels showed a significant increase in radiation sensitivity and an increase in G2 cell cycle arrest. Rapamycin, while effective in decreasing HIF1alpha protein levels, did not affect HIF2alpha levels in either of the RCC cell lines.


CONCLUSIONS
These results show that decreasing levels of HIF2alpha leads to an increased sensitivity to ionizing radiation. This finding may explain in part, the known resistance of RCC to radiation therapy. Although mTOR antagonists are approved for the treatment of RCC, these agents do not decrease HIF2alpha levels and therefore might not be effective in enhancing the radio-sensitivity of these tumours.
Bhatt
Rupal S
RS
Division of Cancer Biology, Beth Israel Deaconess Medical Center, Boston, MA, USA.
Landis
Daniel M
DM
Zimmer
Michael
M
Torregrossa
Joelle
J
Chen
Shaoyong
S
Sukhatme
Vikas P
VP
Iliopoulos
Othon
O
Balk
Steve
S
Bubley
Glenn J
GJ
D000903:Antibiotics, Antineoplastic / Q000627:therapeutic use*; D051792:Basic Helix-Loop-Helix Proteins / Q000187:drug effects / Q000378:metabolism / Q000494:pharmacology*; D015153:Blotting, Western; D002292:Carcinoma, Renal Cell / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy*; D016923:Cell Death / Q000187:drug effects; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D005434:Flow Cytometry; D016195:G2 Phase / Q000187:drug effects; D017930:Genes, Reporter; D006801:Humans; D007680:Kidney Neoplasms / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy*; D008156:Luciferases / Q000378:metabolism; D011494:Protein Kinases / Q000187:drug effects; D011836:Radiation Tolerance; D011838:Radiation-Sensitizing Agents / Q000378:metabolism / Q000494:pharmacology*; D020123:Sirolimus / Q000627:therapeutic use*; D058570:TOR Serine-Threonine Kinases; D000099162:Endothelial PAS Domain-Containing Protein 1
D000903:Antibiotics, Antineoplastic; D051792:Basic Helix-Loop-Helix Proteins; D008156:Luciferases; D011494:Protein Kinases; D011838:Radiation-Sensitizing Agents; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D000099162:Endothelial PAS Domain-Containing Protein 1; C546842:MTOR protein, human