Roles of the serine-threonine kinases glycogen synthase kinase-3 beta and protein kinase CK2 in mammary tumorigenesis and epithelial to mesenchymal transition

Recent studies have implicated ectopic activation of the Wnt pathway in many human cancers, including breast cancer. Glycogen synthase kinase-3beta (GSK3beta) phosphorylation of the N-terminal domain of beta-catenin, a critical co-activator in this signaling pathway, targets it for ubiquitination and proteosomal degradation. We hypothesized that expression of kinase inactive (KI) GSK3beta in mammary glands would function in a dominant negative fashion, antagonizing the endogenous GSK3beta and promoting breast cancer development. We found that KI-GSK3beta stabilized beta-catenin expression, catalyzed its localization to the nucleus, and upregulated downstream target genes in vitro. Transgenic mice overexpressing the KI-GSK3beta developed mammary tumors with over-expression of beta-catenin and cyclin D1. Thus, antagonism of GSK3beta activity is oncogenic in the mammary epithelium, and GSK inhibitors should be used with caution in patients.; Most deaths from cancer are due to metastasis. Invasion and intravasation, which are early stages of metastasis, involve the process of epithelial to mesenchymal transition (EMT). One of the transcription factors that promotes EMT in cancer by direct transcriptional repression of E-cadherin is snail. GSK3beta is a major regulator of snail stability. Inactivation of GSK3beta inhibits snail nuclear export, protein degradation and contributes to EMT formation. Protein kinase CK2 (CK2) is a ubiquitous and highly conserved serine/threonine kinase. Transgenic mice overexpressing the catalytic subunit of CK2 (MMTV-CK2alpha mice) develop mammary tumors. Some of the tumors exhibit features of EMT. Since there are consensus CK2 sites that flank the GSK3beta sites in the snail sequence, we hypothesized that CK2 may play a role in regulating snail and altering the EMT in invasive cancer. We found that CK2 interacted with snail and phosphorylated it in vitro and in vivo . Mutations of the CK2 consensus sites in these regions stabilized snail protein and altered snail subcellular localization. Surprisingly, CK2 had an additive effect with GSK3beta on snail destabilization that was altered by mutations in the CK2 consensus site. These results suggest that while CK2 overexpression promotes tumor formation, reduction in CK2 expression may favor progression to metastasis by stabilizing snail and contributing to EMT.