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.