Activation of the PI3 Kinase Pathway By Retinoic Acid Mediates Sodium/Iodide Symporter Induction and Iodide Transport in MCF-7 Breast Cancer Cells

Iodide uptake in the thyroid and breast is mediated by the sodium/iodide symporter (NIS). NIS activation is used for radioiodide imaging and therapeutic ablation of thyroid carcinoma. NIS is expressed in >70% of breast cancers but at a level insufficient for radioiodine treatment. All-traps retinoic acid (tRA) induces NIS gene expression and functional iodide uptake in human breast cancer cell lines and mouse breast cancer models. tRA usually regulates gene expression by direct interaction of RA receptor (RAR) with a target gene, but it can also act through nongenomic pathways. We report a direct influence of tRA treatment on the phosphoinositide 3-kinase (PI3K) signal transduction pathway that mediates tRA-induced NIS expression in MCF-7 breast cancer cells. MCF-7 cells express all three RAR isoforms, alpha, beta, and gamma, and RXR alpha. We previously identified RAR beta and RXR alpha as important for NIS induction by tRA. Treatment with LY294002, the PI3K inhibitor, or p85 alpha knockdown with siRNA abolished tRA-induced NIS expression. Immunoprecipitation experiments and glutathione S-transferase pull-down assay showed a direct interaction between RAR beta 2, RXR alpha, and p85 alpha. RA also induced rapid activation of Akt in MCF-7 cells. Treatment with an Akt inhibitor or Akt knockdown with siRNA reduced NIS expression. These findings indicate that RA induction of NIS in MCF-7 cells is mediated, by rapid activation of the PI3K pathway and involves direct interaction with RAR and retinoid X receptor. Defining these mechanisms should lead to methods to further enhance NIS expression, as well as retinoid targets that influence growth and differentiation of breast cancer. [Cancer Res 2009;69(8):3443-50]