Akt activity regulates the function of internal ribosome entry sites (IRES) in the VEGF 5'UTR

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We recently demonstrated that the mTOR inhibitor, CCI-779, curtailed growth of a subcutaneous challenge of multiple myeloma cells in immunodeficient mice. This anti-tumor effect was associated with prevention of cell proliferation, induction of apoptosis and inhibition of angiogenesis. Interestingly, myeloma tumors with heightened AKT activation were particularly sensitive to a CCI-779-induced anti-tumor response. Using isogenic multiple myeloma (MM) cell lines that only differ by their degree of AKT activity, we demonstrated that AKT activity significantly sensitized MM cells to the following inhibitory effects of mTOR inhibition: angiogenesis in vivo, VEGF expression in vitro and in vivo and VEGF translation (but not transcription). Since internal ribosome entry site (IRES)-mediated cap-independent translation is a salvage pathway for protein expression when mTOR is inhibited, we analyzed a possible regulatory role of AKT on VEGF IRES activity in the VEGF 5’-UTR. Our experiments demonstrated that elevated AKT activity inhibited VEGF IRES function in isogenic MM cells. Recent studies have shown that the 5’UTR region of VEGF contains at least 2 internal IRES sequences (IRES A and IRES B): IRES A has been shown to allow for the expression of VEGF translation under hypoxic conditions, whereas IRES B mediates the cap-independent translational initiation of a VEGF isoform (L-VEGF). However, it is not known if one or both of these IRES sequences are regulated by AKT activity in MM cells. In preliminary experiments, we are testing whether AKT activity inhibits the activity of either IRES A or IRES B activity and are studying the underlying mechanism by which these regulatory effects occur.