25-10-2012, 10:57 AM
Breast Tumor Kinase (BRK) Signaling in Breast Cancer
ABSTRACT
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.