Engrailed 2 triggers the activation of multiple phosphorylation-induced signaling pathways in both transcription-dependent and -independent manners

Homeodomain (HD)-containing proteins are typically recognized as transcription factors. Engrailed 2 (EN2) is an HD-containing protein that is highly expressed in various types of cancers, however, the mechanism underlying the biological function of EN2 is not fully understood. Here, we report a transcription-independent function of EN2 in addition to its role as a transcription factor. EN2 expression leads to the activation of multiple signaling pathways mediated by phosphorylation cascades. A phosphoproteomic analysis revealed that the phosphorylation status of numerous protein sites was altered after EN2 is expressed. Notably, EN2 was shown to interact with a myriad of proteins implicated in phosphorylation signaling cascades, as determined by immunoprecipitation-mass spectrometry (IP-MS). We validated the interaction between EN2 and B55 alpha, the regulatory subunit of the PP2A-B55 alpha complex, and confirmed that the phosphatase activity of the complex was suppressed by EN2 binding. To target EN2-induced malignancy, two kinds of small molecules were utilized to inhibit the EN2activated NF-kappa B and AKT signaling pathways. A clear synergistic effect was observed when the activation of the two pathways was simultaneously blocked. Collectively, the data show that EN2 functions in a transcriptionindependent manner in addition to its role as a transcription factor. This finding may have therapeutic implications in treating esophageal squamous cell carcinoma (ESCC).