Differential regulation of IRESs in the Aurora A mRNA by bFGF through the mTOR complex TORC2 modulates Aurora A kinase expression
Date of Graduation
Masters of Science (MS)
Leslie A. Krushel Ph.D.
Jessica Tyler Ph.D.
Pierre McCrea Ph.D.
Jill Schumacher Ph.D.
Gilber Cote Ph.D.
Identifying the mechanisms that contribute to tumorigenesis is a major area of focus in our fight against cancer. Epithelial malignant tumors, such as breast, colon, ovarian and pancreatic cancers have been shown to overexpress proteins that control cell mitosis, growth, and proliferation. One of those proteins is the Aurora A kinase. Aurora A kinase is a member of a small family of kinases that contribute to mitotic events such as centrosome duplication, separation, and maturation. Aurora A overexpression leads to genomic instability, which can contribute to tumorigenesis, on the other hand, inhibiting Aurora A expression leads to apoptosis, making it a target for anti-cancer drugs.
Recent studies have shown that protein synthesis can play a significant role in the regulation protein expression. The major mechanism by which translation is regulated is at the step of initiation. The canonical pathway utilizes the m7G cap structure at the 5’ end of the mRNA to recruit the translational machinery including the 40S ribosome which scans the mRNA for the AUG start codon. Alternatively, the 40S robsome is recruited downstream of the cap to internal ribosomal entry site (IRES). IRESs are found in certain classes of viruses and a subset of eukaryotic mRNAs. IRES dependent translation is thought to occur during times when the cap is down regulated such as during stress or cell proliferation.
Previously we found that the Aurora A mRNA contains an IRES and that IRES dependent translation appears to be a “major contributor” in the over-expression of Aurora A in cell lines that overexpress the protein. The elevation of Aurora A is due mainly to increased IRES translation. Further study demonstrates that the alternatively spliced aurora A 5’ UTR contains three independent IRESs localized in three different exons (exon 1b, exon 2, exon 2a). I examined whether growth factors which stimulate Aurora A expression, differentially stimulate different Aurora A IRESs. I found that addition of bFGF enhances Aurora A protein expression within 2 hours. Moreover, bFGF led to an increase in exon 1b and exon 2a but not exon 2 IRES activity. Further study showed that the translational response was mediated through the mTOR pathway. Cap-dependent translation is known to be mediated through one of the two mTOR complexes (mTORC1). Surprisingly my data reveal that the second complex, mTORC2, mediates bFGF induced IRES activity. This study is the first demonstration that growth factors differentially regulate IRESs in the same mRNA and that this is regulated through mTORC2. Importantly, this study identifies mTORC2 as a novel target for inhibition of Aurora A expression in cancer cells.
Aurora A kinase, mTOR, FGF, over-express, protein, regulation