Regulation of IMP3 by EGFR signaling and repression by ERβ: implications for triple-negative breast cancer

Insulin-like growth factor II (IGF-II) mRNA-binding protein 3 (IMP3) is emerging as a useful indicator of the progression and outcome of several cancers. IMP3 expression is associated with triple-negative breast carcinomas (TNBCs), which are aggressive tumors associated with poor outcome. In this study, we addressed the hypothesis that signaling pathways, which are characteristic of TNBCs, impact the expression of IMP3 and that IMP3 contributes to the function of TNBCs. The data obtained reveal that IMP3 expression is repressed specifically by estrogen receptor beta (ER beta) and its ligand 3 beta A-diol but not by ER alpha. Epidermal growth factor receptor (EGFR) signaling and consequent activation of the mitogen-activated protein kinase pathway induce IMP3 transcription and expression. Interestingly, we discovered that the EGFR promoter contains an imperfect estrogen response element and that ER beta represses EGFR transcription. These data support a mechanism in which ER beta inhibits IMP3 expression indirectly by repressing the EGFR. This mechanism relates to the biology of TNBC, which is characterized by diminished ER beta and increased EGFR expression. We also demonstrate that IMP3 contributes to the migration and invasion of breast carcinoma cells. Given that IMP3 is an mRNA-binding protein, we determined that it binds several key mRNAs that could contribute to migration and invasion, including CD164 (endolyn) and MMP9. Moreover, expression of these mRNAs is repressed by ER beta and enhanced by EGFR signaling, consistent with our proposed mechanism for the regulation of IMP3 expression in breast cancer cells. Our findings show that IMP3 is an effector of EGFR-mediated migration and invasion and they provide the first indication of how this important mRNA-binding protein is regulated in cancer.