Blockade of the epidermal growth factor receptor tyrosine kinase suppresses tumorigenesis in MMTV/Neu plus MMTV/TGF-alpha bigenic mice

Overexpression of ErbB-2/Neu has been causally associated with mammary epit helial transformation. Here we report that blockade of the epidermal growth factor receptor (EGFR) kinase with AG-1478 markedly delays breast tumor fo rmation in mouse mammary tumor virus (MMTV)/Neu + MMTV/transforming growth factor alpha bigenic mice. This delay was associated with inhibition of EGF R and Neu signaling, reduction of cyclin-dependent kinase 2 (Cdk2) and mito gen-activated protein kinase (MAPK) activities and cyclin D1, and an increa se in the levels of the Cdk inhibitor p27(Kip1). In addition, BrdUrd incorp oration into tumor cell nuclei was prevented with no signs of tumor cell ap optosis, These observations prompted us to investigate the stability of p27 , Recombinant p27 was degraded rapidly in vitro by untreated but not by AG- 1478-treated tumor lysates, Proteasome depletion of the tumor lysates, addi tion of the specific MEK1/2 inhibitor U-0126. or a T187A mutation in recomb inant p27 all prevented p27 degradation. Cdk2 and MAPK precipitates from un treated tumor lysates phosphorylated recombinant wild-type p27 but not the T187A mutant in vitro. Cdk2 and MAPK precipitates from AG-1478-treated tumo rs were unable to phosphorylate p27 in vitro. These data suggest that incre ased signaling by ErbB receptors up-regulates MAPK activity, which, in turn , phosphorylates and destabilizes p27, thus contributing to dysregulated ce ll cycle progression.