Inhibition of 4E-BP1 Sensitizes U87 Glioblastoma Xenograft Tumors to Irradiation by Decreasing Hypoxia Tolerance
Received 11 August 2008; received in revised form 18 November 2008; accepted 2 December 2008.
Purpose
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 and Materials
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.
∗Department of Radiation Oncology (Maastro Laboratory), GROW–School for Oncology and Developmental Biology, Maastricht University, Maastricht, the Netherlands
†Department of Pathology, GROW–School for Oncology and Developmental Biology, University Hospital Maastricht, Maastricht, the Netherlands
‡Institute of Veterinary Physiology, Vetsuisse Faculty and Zurich Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland
§Department of Experimental Radiobiology and Oncology, University Hospital Leuven, Leuven, Belgium
‖Division of Signaling Biology, Ontario Cancer Institute, Princess Margaret Hospital, Toronto, ON, Canada
¶Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada
#Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada
∗∗Selective Therapeutics Program, Ontario Institute for Cancer Research, Toronto, Canada
Reprint requests to: Bradly G. Wouters, Ontario Cancer Institute, Princess Margaret Hospital, 610 University Ave, Office 10-116, Toronto, ON M5G 2M9, Canada. Tel: (416) 581-7840; Fax: (416) 581-7840
The first and second authors contributed equally to this work.
Conflict of interest: none.
Supplementary material for this article can be found at www.redjournal.org.
ABSTRACT