Regulation of the Transcription Factor Yin Yang 1 by Tyrosine Phosphorylation

Yin Yang 1 (YY1) is a multifunctional transcription factor that can activate or repress transcription depending on the promotor and/or the co-factors recruited. YY1 is phosphorylated in various signaling pathways and is critical for different biological functions including embryogenesis, apoptosis, proliferation, cell-cycle regulation and tumorigenesis. Here we report that YY1 is a substrate of two different tyrosine kinases. First, c-Abl kinase phosphorylates YY1 at conserved residue Y254 in the spacer region. Pharmacological inhibition of c-Abl kinase by imatinib, nilotinib and GZD824, knock-down of c-Abl using siRNA and the use of c-Abl kinase-dead drastically reduces tyrosine phosphorylation of YY1. Both radioactive and non-radioactive in vitro kinase assays, as well as co-immunoprecipitation in different cell lines, show that the target of c-Abl phosphorylation is tyrosine residue 254. c-Abl phosphorylation has little effect on YY1 DNA binding ability or cellular localization in asynchronous cells. However, functional studies revealed that c-Abl mediated phosphorylation of YY1 regulated YY1’s transcriptional ability in vivo. Secondly, we show that YY1 is phosphorylated by non-receptor tyrosine kinase Src and this phosphorylation is mediated by the receptor tyrosine kinase c-Kit signaling pathway at tyrosine residue 251. Computational prediction using GPS 3.0 identified Src as a possible kinase that could target YY1 for tyrosine phosphorylation. The use of a highly sensitive phospho-specific antibody against phosphorylated Y251 in combination with non-radioactive in vitro kinase assay show that Src phosphorylates YY1 in vitro. Pharmacological inhibition of both c-Kit and Src kinase caused a great reduction in tyrosine phosphorylation of YY1 at Y251. The use of SCF ligand to stimulate c-Kit kinase show that YY1 may be a target of Src kinase phosphorylation under the c-Kit signaling cascade at Y251. Ongoing research includes the generation of phospho-mutations at tyrosine 251 using the CRISPR/Cas9 genome editing tool to uncover the biological significance of this phosphorylation. In conclusion, we demonstrate the novel role of c-Abl kinase in regulation of YY1’s transcriptional activity, linking YY1 regulation with the c-Abl tyrosine kinase signaling pathways. We also link YY1 phosphorylation to the c-Kit receptor tyrosine kinase signaling pathway. Because errors in signaling result in cancer growth and other disease, understanding the dynamic cellular processes of YY1 phosphorylation by tyrosine kinases will lead to a better understanding of the signaling networks within the cell leading to more effective treatment for disease.