Targeting the eIF4A RNA helicase blocks translation of the MUC1-C oncoprotein.

The oncogenic MUC1 C-terminal subunit (MUC1-C) subunit is aberrantly overexpressed in most human breast cancers by mechanisms that are not well understood. The present studies demonstrate that stimulation of non-malignant MCF-10A cells with epidermal growth factor (EGF) or heregulin (HRG) results in marked upregulation of MUC1-C translation. Growth factor-induced MUC1-C translation was found to be mediated by PI3KAKT, and not by MEKERK1/2, signaling. We also show that activation of the mammalian target of rapamycin complex 1 (mTORC1)ribosomal protein S6 kinase 1 (S6K1) pathway decreases tumor suppressor programmed cell death protein 4 (PDCD4), an inhibitor of the eIF4A RNA helicase, and contributes to the induction of MUC1-C translation. In concert with these results, treatment of growth factor-stimulated MCF-10A cells with the eIF4A RNA helicase inhibitors, silvestrol and CR-1-31-B, blocked increases in MUC1-C abundance. The functional significance of the increase in MUC1-C translation is supported by the demonstration that MUC1-C, in turn, forms complexes with EGF receptor (EGFR) and promotes EGFR-mediated activation of the PI3KAKT pathway and the induction of growth. Compared with MCF-10A cells, constitutive overexpression of MUC1-C in breast cancer cells was unaffected by EGF stimulation, but was blocked by inhibiting PI3KAKT signaling. The overexpression of MUC1-C in breast cancer cells was also inhibited by blocking eIF4A RNA helicase activity with silvestrol and CR-1-31-B. These findings indicate that EGF-induced MUC1-C expression is mediated by the PI3KAKT pathway and the eIF4A RNA helicase, and that this response promotes EGFR signaling in an autoinductive loop. The findings also indicate that targeting the eIF4A RNA helicase is a novel approach for blocking MUC1-C overexpression in breast cancer cells.