Breast Tumor Kinase (BRK) Signaling in Breast Cancer

摘要

We previously demonstrated that the breast tumor kinase BRK regulates the RNA-binding protein Sam68 that plays roles in RNA metabolism. Two novel Sam68-like mammalian proteins SLM1 and SLM2 were identified, and the purpose of our study was to determine the roles of these RNA-binding proteins in breast cancer signaling. Cotransfection of normal murine mammary gland (NMuMG) cells with different BRK expression constructs and OFP-SLM1 or GFP-SLM2 revealed a direct correlation between BRK activity and SLM1 and SLM2 phosphorylation. Mutation of a regulatory terminal tyrosine in BRK (Y-F) increased the level of SLM protein phosphorylation. Phosphotyrosine immunoreactivity was enhanced in the nuclei of NMuMG cells when active BRK was expressed. Fractionation of cells transfected with BRK expression constructs showed that BRK(Y-F) was highly enriched in the nuclear fraction. In functional studies in NMuMG cells, we determined that BRK mediated phosphorylation of Sam68, SLM1 and SLM2 led to dramatic inhibition of their RNA- binding activities. In expression studies, only Sam68 was strongly coexpressed with BRK in normal and malignant breast epithelial cells, and it appears to be the major identified BRK nuclear substrate in these cells. Induction of BRK in breast tumor cells may lead to inappropriate phosphorylation and inhibition of Sam68.