The ErbB2
receptor tyrosine kinase is overexpressed in approximately 30% of
breast tumor cases and its overexpression correlates with an unfavorable prognosis. A major contributor for this course of the disease is the insensitivity of these
tumors toward
chemotherapy.
Monoclonal antibodies, inhibiting the
ligand-induced activation of the receptor and
tyrosine kinase inhibitors acting on the intrinsic enzymatic activity of the intracellular domain, have been developed as targeted drugs. Both have been shown to be beneficial for
breast cancer patients. We targeted a third aspect of receptor function: its association with intracellular signaling components. For this purpose, we selected
peptide aptamers, which specifically interact with defined domains of the intracellular part of the receptor. The
peptide aptamers were selected from a
random peptide library using a yeast two-hybrid system with the intracellular
tyrosine kinase domain of ErbB2 as a bait construct. The
peptide aptamer AII-7 interacts with high specificity with the ErbB2 receptor in vitro and in vivo. The aptamers colocalized with the intracellular domain of ErbB2 within cells. We investigated the functional consequences of the aptamer interaction with the ErbB2 receptor within
tumor cells. The aptamer sequences were either expressed intracellularly or introduced into the cells as recombinant aptamer
proteins. The phosphorylation of p42/44
mitogen-activated protein kinase was nearly unaffected and the activation of signal transducers and activators of transcription-3 was only modestly reduced. In contrast, they strongly inhibited the induction of AKT
kinase in MCF7
breast cancer cells treated with
heregulin, whereas AKT activation downstream of
insulin-like growth factor I or
epidermal growth factor receptor was not or only slightly affected. High AKT activity is responsible for the enhanced resistance of ErbB2-overexpressing
cancer cells toward chemotherapeutic agents.
Peptide aptamer interference with AKT activation resulted in the restoration of regular sensitivity of
breast cancer cells toward
Taxol.