As you know well, serum includes numerous cytokines and growth factgors, some of which function as the agonist or antanogist for estrogen nuclear receptors. That is why the absence of serum in the medium makes difference in the response to the blockage to the ER signal pathway. 

I agree with J. Yoshida.

Thanks to Joshida Sensai and Dr. Avissar for your quick feed back, however it is not the case with BT474 which is also ER+ cell line, I wonder...