Background: Receptors belonging to the epidermal growth factor receptor (EGFR) family transfer extracellular signals by homotypic and heterotypic receptor interaction and cross-activation. Cell differentiation, death, and proliferation are regulated via these receptor-tyrosine-kinases. However, the initial mechanisms that lead to signal specificity and diversity, which cause a defined cellular response, are incompletely understood. We investigated the recruitment of receptor complexes in two c-erbB2-overexpressing breast carcinoma cell lines, SK-BR-3 and BT474, after ligand binding and its effects on intracellular signal transduction and cell cycle regulation.
Methods: In order to analyze the coaggregation of receptors on the cell surface induced by specific growth factor treatment, we used the flow cytometric Foerster-type fluorescence resonance energy transfer (FRET) technique. Cell cycle kinetics were monitored flow cytometrically via the anti-BrdU technique and acitivation of intracellular signal cascades was analyzed by Western blotting.
Results: After stimulation with EGF BT474, but not SK-BR-3, cells formed EGFR/c-erbB2 receptor complexes. Neither EGF nor heregulin (HRG) induced c-erbB2/c-erbB3 receptor complexes in BT474. However, SK-BR-3 cells exhibited a high amount of c-erbB2/c-erbB3 heterodimers even without growth factor stimulation which could be elevated after prolonged EGF and HRG treatment. In both cell lines, mitogen-activated protein kinase (MAPK) phosphorylation was detectable after short-term and prolonged EGF and HRG treatment. However, only SK-BR-3 cells showed a constitutive activation of both protein kinase B (PKB)/Akt and MAPK signaling pathways. Growth factor treatment caused an amplified PKB/Akt activation in this cell line. The induction of EGFR/c-erbB2 complexes in BT474 was associated with shortening of the G1-phase of the cell cycle. In contrast, the concurrent activation of MAPK and PKB/Akt by EGF treatment led to an inhibition of proliferation in SK-BR-3 and can be attributed to missing EGFR/c-erbB2 heterodimers. HRG was a strong stimulator of proliferation in both cell lines.
Conclusions: We show that in the presence of identical amounts of c-erbB2 receptors, the ligand-induced cellular response differs significantly. These differences were mediated by variances in signal transduction, most likely due to different recruitment of heterotypic receptor complexes. Overall, there is strong evidence that c-erbB2 receptor overexpression in breast cancer cells is an insufficient marker to determine cellular response in terms of cell proliferation. 2001.
Copyright 2001 Wiley-Liss, Inc.