Crosstalk between Estrogen Receptor alpha 36, a Dominant Extra-Nuclear ER, and HER2 Promotes Tumor Cell Survival in Breast Cancer.

Abstract

Abstract The ER-α36 protein is a 36kDa novel splice variant of the full length 66kDa ER-α. ER-α36 lacks both transcriptional activation domains of ER-α66 but retains the DNA-binding domain and partial dimerization and ligand-binding domains. It has been shown that ER-α36 could trigger membrane-initiated signaling in ER-α36 over-expressing 293 cells, suggesting that ER-α36 might have functions linked with activation of intracellular signaling.To further explore the mechanism(s) of ER-α36 activation of intracellular signaling, in particular the possible relationship between ER-α36 and HER2 in breast cancer, we performed a pilot study. First, we examined the correlation between ER-α36 and HER2 by immunohistochemistry (IHC) using a specific anti-ER-α36 antibody designed against a unique epitope located in the C-tail of ER-a36 which is not present in ER-α66. The results showed that the expression of ER-α36 was highly associated with HER2 in a cohort of 249 breast tumors (P<0.05). Then, we examined whether ER-α36 physically interacts with HER2 in breast cancer cell lines using co-immunoprecipitation (Co-IP) and Western blotting (WB) experiments. We found that ER-α36 interacts with HER2 in both MCF7/HER2 and BT474 cell lines. Second, we explored whether ER-α36 influences the activation of HER2 and its downstream signaling by knocking down ER-α36 expression in both MCF7/HER2 and BT474 cell lines. We found that knockdown of ER-α36 by siRNA abrogated the activation of P-HER2, as well as Akt and MAPK, in both MCF7/HER2 and BT474 cells. Finally, we asked the key question whether ER-α36 was associated with cell proliferation/survival. Strikingly, we found that when both MCF7/HER2 and BT474 cells were deprived of ER-α36 by siRNA, they died quickly, whereas control cells grew normally.In conclusion, our preliminary data suggests that there might be a crosstalk between ER-α36 and HER2 signaling, and that this crosstalk might promote tumor cell survival through HER2/Akt and/or HER2/MAPK signaling pathways in breast cancer. Thus, ER-α36 may be a valuable biomarker and a novel therapeutic target in breast cancer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2121.