Translational regulation is critical in cancer development and progression. Translation sustains tumor growth and development of a tumor vasculature, a process known as angiogenesis, which is activated by hypoxia. Here we first demonstrate that a majority of large advanced breast cancers overexpress translation regulatory protein 4E-BP1 and initiation factor eIF4G. Using model animal and cell studies, we then show that overexpressed 4E-BP1 and eIF4G orchestrate a hypoxia-activated switch from cap-dependent to cap-independent mRNA translation that promotes increased tumor angiogenesis and growth at the level of selective mRNA translation. Elevated levels of 4E-BP1 trigger hypoxia inhibition of cap-dependent mRNA translation at high-oxygen levels and, with eIF4G, increase selective translation of mRNAs containing internal ribosome entry sites (IRESs) that include key proangiogenic, hypoxia, and survival mRNAs. The switch from cap-dependent to cap-independent mRNA translation facilitates tumor angiogenesis and hypoxia responses in animal models.

The tumor microenvironment is a crucial player in the ability of cancer cells to acquire the ability to survive under the hypoxic environment and promote migration and invasion. Translational regulation is an essential part of cancer development and progression. Protein synthesis consumes considerable cellular metabolic energy and is therefore highly regulated, in turn controlling tumor cell proliferation and survival in extreme tumor-host conditions. Protein synthesis is typically downregulated by hypoxia, impairing cell proliferation and migration. Here, we show that breast cancer cells expressing integrin avb3, when engaging the extracellular matrix (ECM) protein vitronectin, strongly upregulate both mTOR activity and capdependent mRNA translation, which overrides their inhibition by hypoxia and facilitates tumor cell invasion. Interaction of vitronectin with integrin avb3 results in the continued activation of the kinase mTOR despite hypoxia through a mechanism that is dependent on integrin-linked kinase but is independent of focal adhesion kinase. Continuous activation of mTOR despite hypoxia involves release of translation initiation factor eIF4E from its repressor protein 4E-BP1, which is required for vitronectin-mediated tumor cell invasion. As integrin avb3 is associated with breast cancer cell invasion and metastasis to bone, we propose that the interaction with specific ECM proteins can influence cancer cell invasion, in part, by hyperactivation of mTOR, thereby promoting and sustaining protein synthesis under hypoxic conditions.

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