ErbB receptors are overexpressed in many types of human cancer and are therefore considered as important anti-tumor therapeutic targets. Heterodimeric forms of erbB receptors, especially those formed between the epidermal growth factor receptor (EGFR) and HER2/neu have been shown to have higher tumorogenic activity than the homodimeric forms. Here, we demonstrate that in addition to the dimeric forms, EGFR and HER2 can associate as homo- and hetero-tetramers in both transiently transfected and stable overexpressing cell lines. By using truncated mutants of HER2, we found that the stoichiometry of the formed heterotetrameric complexes between EGFR and HER2 is 2:2. Unexpectedly, EGF-induced phosphorylation of the tetramers was significantly lower than that of the dimeric forms, indicating that the observed tetrameric receptor complexes have an impaired signaling activity relative to the dimers. Herceptin is a clinically used anti-HER2 antibody that is believed to disable HER2 receptor activity by different mechanisms including downregulation and blocking of an enzymatic cleavage site. In this paper, we show that Herceptin promotes formation of tetrameric and oligomeric forms of erbB receptors suggesting that anti-tumor activity of this antibody may be at least partly mediated by its ability to shift the equilibrium from active dimeric to impaired tetra- and oligomeric receptor complexes. In conclusion, our study demonstrates existence of homotetrameric EGFR, homotetrameric HER2 and heterotetrameric EGFR-HER2 (2:2) complexes and showes that these complexes have a significantly decreased signaling activity compared to the dimeric forms. The study suggests a novel approach to disable erbB and potentially other cell surface receptors by agents capable of inducing receptor tetramerization.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]