Inhibition of 4E-BP1 sensitizes U87 glioblastoma xenograft tumors to irradiation by decreasing hypoxia tolerance.
OBJECTIVE
Eukaryotic initiation factor 4E (eIF4E) is an essential rate-limiting factor for cap-dependent translation in eukaryotic cells. Elevated eIF4E activity is common in many human tumors and is associated with disease progression. The growth-promoting effects of eIF4E are in turn negatively regulated by 4E-BP1. However, although 4E-BP1 harbors anti-growth activity, its expression is paradoxically elevated in some tumors. The aim of this study was to investigate the functional role of 4E-BP1 in the context of solid tumors.


METHODS
In vitro and in vivo growth properties, hypoxia tolerance, and response to radiation were assessed for HeLa and U87 cells, after stable expression of shRNA specific for 4E-BP1.


RESULTS
We found that loss of 4E-BP1 expression did not significantly alter in vitro growth but did accelerate the growth of U87 tumor xenografts, consistent with the growth-promoting function of deregulated eIF4E. However, cells lacking 4E-BP1 were significantly more sensitive to hypoxia-induced cell death in vitro. Furthermore, 4E-BP1 knockdown cells produced tumors more sensitive to radiation because of a reduction in the viable fraction of radioresistant hypoxic cells. Decreased hypoxia tolerance in the 4E-BP1 knockdown tumors was evident by increased cleaved caspase-3 levels and was associated with a reduction in adenosine triphosphate (ATP).


CONCLUSIONS
Our results suggest that although tumors often demonstrate increases in cap-dependent translation, regulation of this activity is required to facilitate energy conservation, hypoxia tolerance, and tumor radioresistance. Furthermore, we suggest that targeting translational control may be an effective way to target hypoxic cells and radioresistance in metabolically hyperactive tumors.
Dubois
Ludwig
L
Department of Radiation Oncology (Maastro Laboratory), GROW-School for Oncology and Developmental Biology, Maastricht University, Maastricht, the Netherlands.
Magagnin
Michaël G
MG
Cleven
Arjen H G
AH
Weppler
Sherry A
SA
Grenacher
Beat
B
Landuyt
Willy
W
Lieuwes
Natasja
N
Lambin
Philippe
P
Gorr
Thomas A
TA
Koritzinsky
Marianne
M
Wouters
Bradly G
BG
D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000502:physiology*; D000818:Animals; D018797:Cell Cycle Proteins; D015687:Cell Hypoxia / Q000502:physiology*; D002470:Cell Survival / Q000502:physiology; D039561:Eukaryotic Initiation Factor-4E / Q000037:antagonists & inhibitors / Q000502:physiology*; D005260:Female; D055786:Gene Knockout Techniques; D005909:Glioblastoma / Q000098:blood supply / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy*; D006367:HeLa Cells / Q000502:physiology / Q000528:radiation effects; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D010750:Phosphoproteins / Q000235:genetics / Q000502:physiology*; D014176:Protein Biosynthesis; D011829:Radiation Dosage; D011836:Radiation Tolerance / Q000502:physiology*; D014183:Transplantation, Heterologous
D048868:Adaptor Proteins, Signal Transducing; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D039561:Eukaryotic Initiation Factor-4E; D010750:Phosphoproteins