Inhibition of Akt (ser473) Phosphorylation and Rapamycin-Resistant Cell Growth by Knockdown of Mammalian Target of Rapamycin with Small Interfering RNA in Vascular Endothelial Growth Factor Receptor-1-Targeting Vector

Previously we developed dicetyl phosphate-tetraethylenepentamine-based polycation liposomes (TEPA-PCL) for use in small interfering RNA (siRNA) therapy. In the present study, mammalian target of rapamycin (mTOR) expression in cancer cells was silenced with mTOR-siRNA (simTOR) formulated in TEPA-PCL modified with Ala-Pro-Arg-Pro-Gly (APRPG), a peptide having affinity for vascular endothelial growth factor receptor-1 (VEGFR-1). We investigated the effects of inhibition of mTOR, focusing on the differences between cells treated with simTOR and those with rapamycin in terms of Akt (ser473) phosphorylation and antiproliferative effects. Rapamycin treatment is known to induce Akt (ser473) phosphorylation which attenuates the antiproliferative effects of rapamycin. As a result, knockdown of mTOR did not alter or only slightly reduced Akt (ser473) phosphorylation in phosphatase and tensin homolog deleted from chromosome 10 (PTEN)-null (LNCaP and MDA-MB-468 cells) and PTEN-positive (DU 145 and MDA-MB-231) cells, although rapamycin induced Akt (ser473) phosphorylation of these cells. Rapamycin suppressed the growth of PTEN-null cells, in which the rapamycin-sensitive mTOR complex 1 (mTORC1) is excessively activated. On the other hand, rapamycin did not suppress the growth of PTEN-positive cells possibly through a negative feedback mechanism via the rapamycin-insensitive mTOR complex 2 (mTORC2) signaling pathway. In contrast, simTOR significantly suppressed the growth of cancer cells regardless of the presence of PTEN, possibly through inhibition of both mTORC1 and mTORC2. These results indicate that mTOR knockdown using APRPG-TEPA-PCL/simTOR is likely to be an effective strategy for cancer siRNA therapy.