The regulation of mRNA translation is a crucial player in the development and progression of cancer. eIF4E, the mRNA cap-binding protein, is frequently overexpressed in tumors of the breast, colon, prostate, and lung, and expression correlates with disease progression, increased tissue invasion and metastasis. It has been suggested that overexpression of eIF4E contributes to malignancy by selectively increasing translation of mRNAs containing significant secondary structure in the their 5’ noncoding regions. This group includes mRNAs that encode key proteins involved in cellular growth, survival, angiogenesis and malignancy. Nevertheless, many of the effects of increased levels of eIF4E in tumor cells, however, is still not well understood, nor is the role of its mTOR-regulated repressor, 4E-BP1. Here we demonstrate the surprising finding that overexpression of eIF4E acts to increase breast cancer cell invasion largely by increasing translation of ITGβ1 mRNA. eIF4E overexpression at levels found in breast cancers (3-4 fold) results in a 10-fold increased level of tumor cell invasion. We find increased read-through of the complex GC-rich 5’UTR of ITGβ1, and a shift of the mRNA to heavy polysomal fractions with eIF4E overexpression. This effect is blocked by co-overexpression of cap-dependent translational repressor 4E-BP1. The modest increase in β1 integrin protein levels is amplified by a disproportionately large increase in the formation of the cell surface heterodimer α3β1, an integrin frequently implicated in breast tumor invasion. These and other results can account for the large increase in invasive ability in eIF4E overexpressing breast cancer cells, suggesting that β1 integrin levels may be limiting in the cell.
Surprisingly, increased expression of eIF4E was also found to increase transcription and consequent protein levels of many canonical TGFβ-responsive genes, including ITGβ1. The TGFβ transcriptional effect was shown to be due to an increased rate of activation of latent TGFβ (typically present in serum or the stroma), mediated by the eIF4E-driven increased levels of β1 integrin and its subsequent heterodimerization with αV integrins. TGFβ is partially responsible for the increased invasion seen with eIF4E overexpression, although cells are still more invasive even in the absence of the cytokine. Co-overexpression of 4E-BP1 blocks all TGFβ effects on signaling. These and other results suggest a new model whereby eIF4E level is a regulator of the direction of TGFβ activity. Increased expression of eIF4E at typical human breast cancer levels mediates increased tumor cell invasion through the selective increased translation of ITGβ1 mRNA. Higher levels of ITGβ1 in turn promote increased heterodimerization with α integrins, increasing invasion directly, and by activating latent TGFβ present in the breast stroma, thereby directing TGFβ to pro-invasive function. Other mRNAs whose translation is increased by high eIF4E levels likely aid in the invasive process.
|Advisor:||Schneider, Robert J.|
|Commitee:||Levy, David, Mohr, Ian, Reiss, Michael, Zagzag, David|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-B 72/01, Dissertation Abstracts International|
|Keywords:||Beta 1 integrin, Breast cancer, TGF-beta, Tumor cell invasion, eIF4E|
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