Epidermal growth factor receptor stimulation activates the RNA bindin protein CUG-BP1 and increases expression of C/EBPβ-LIP in mammary epithelial cells

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) is a key regulator of growth and differentiation in many tissues. C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, and LIP) whose expression is regulated by alternative translational initiation at downstream AUG start sites. The dominant-negative LIP isoform is predominantly expressed during proliferative cellular responses and is associated with aggressive tumors. In this study, we investigated a mechanism by which the LIP isoform is translationally regulated in mammary epithelial cells. We have demonstrated that LIP expression is increased in response to activation of the epidermal growth factor receptor (EGFR) signaling pathway and that the increased expression of LIP is regulated in part by an RNA binding protein referred to as CUG repeat binding protein (CUG-BP1). Our data demonstrate that EGFR signaling results in the phosphorylation of CUG-BP1 and this leads to an increase in the binding of CUG-BP1 to C/EBPbeta mRNA and elevated expression of the LIP isoform. Phosphorylation is necessary for the binding activity of CUG-BP1 and the consequent increase in LIP expression, as determined by binding assays and a cell free, transcription-coupled translation system. CUG-BP1 is thus a previously unidentified downstream target of EGFR signaling and represents a new translational regulator of LIP expression in human mammary epithelial cells.