We sought to determine whether
nucleolin, a bcl-2
mRNA-
binding protein, has a role in the regulation of bcl-2 mRNA stability in MCF-7 and MDA-MB-231
breast cancer cells. Furthermore, we examined the efficacy of the aptamer
AS1411 in targeting
nucleolin and inducing bcl-2 mRNA instability and cytotoxicity in these cells.
AS1411 at 5 micromol/L inhibited the growth of MCF-7 and MDA-MB-231 cells, whereas 20 micromol/L
AS1411 had no effect on the growth rate or viability of normal MCF-10A mammary epithelial cells. This selectivity of
AS1411 was related to a greater uptake of
AS1411 into the cytoplasm of MCF-7 cells compared with MCF-10A cells and to a 4-fold higher level of cytoplasmic
nucleolin in MCF-7 cells. Stable
siRNA knockdown of
nucleolin in MCF-7 cells reduced
nucleolin and bcl-2
protein levels and decreased the half-life of bcl-2
mRNA from 11 to 5 hours. Similarly,
AS1411 (10 micromol/L) decreased the half-life of bcl-2
mRNA in MCF-7 and MDA-MB-231 cells to 1.0 and 1.2 hours, respectively. In contrast,
AS1411 had no effect on the stability of bcl-2
mRNA in normal MCF-10A cells.
AS1411 also inhibited the binding of
nucleolin to the instability element AU-rich
element 1 of bcl-2
mRNA in a cell-free system and in MCF-7 cells. Together, the results suggest that
AS1411 acts as a molecular decoy by competing with bcl-2
mRNA for binding to cytoplasmic
nucleolin in these
breast cancer cell lines. This interferes with the stabilization of bcl-2
mRNA by
nucleolin and may be one mechanism by which
AS1411 induces
tumor cell death.