Prolactin receptor (PRLR) and epidermal growth factor receptor (EGFR/ERBB1) have important roles in the physiology of the human breast and in the etiology and progression of breast cancer. Our present studies in MCF-7 cells revealed that EGF induces up-regulation of PRLR via activation of EGFR signalling pathways leading to activation of estrogen receptor α (ERα). EGF treatment of MCF-7 cells cultured in absence of estradiol induced expression of PRLR that was consistent with the activation of PRLR generic promoter (hPIII). These were abolished by ERα antagonist and siRNA, indicating involvement of ERα in EGF-induced hPIII promoter activity. MEK/MAPK and PI3K/AKT pathways participate in the phosphorylation of ERα induced by EGF/EGFR. PI3K and MEK inhibitors abolished EGF-induced PRLR promoter activity. Increased recruitment of non-DNA bound unliganded ERα to Sp1 and C/EBPβ bound to their sites at hPIII induced by EGF was abrogated by ERα siRNA demonstrating the requisite role of phospho-ERα in PRLR upregulation. EGF/EGFR, independent of endogenous prolactin induced phosphorylation of STAT5b with participation of c-SRC and recruitment of STAT5b:STAT5b to a GAS site at hPIII. STAT5b interaction with ERα was essential for stable phospho-ERα recruitment to the SP1/CEBPβ complex. These studies indicate a role for paracrine EGF via EGFR independent of estrogen and prolactin in the transcriptional activation of PRLR gene expression and its contribution to high levels of PRLRs in breast cancer. These by maximizing the actions of endogenous prolactin could have a role in cancer progression and resistance to endocrine therapy.
Keywords: EGF; EGFR/ERBB1; ER; STAT5b; prolactin receptor.