PNNL

Abstract

Endocytic trafficking plays an important role in the regulation of the epidermal growth factor receptor (EGFR) family. Many cell types express multiple EGFR family members (including EGFR, HER2, HER3 and/or HER4) that interact to form an array of homo- and hetero-dimers. Differential trafficking of these receptors should strongly affect signaling through this system by changing substrate access and heterodimerization efficiency. Because of the complexity of these dynamic processes we used a quantitative, computational model to understand this system. As a test case, parameters characterizing EGFR and HER2 interactions were derived using experimental data obtained from mammary epithelial cells constructed to express different levels of HER2. With this data we were able to estimate receptor-specific internalization rate constants and dimer uncoupling rate constants. These parameters were not otherwise experimentally accessible due to the complex system interplay. Our models indicated that HER2:EGFR heterodimers traffic as single entities. Direct experiments using EGF and anti-HER2 and anti-EGFR antibodies using independently derived cell lines confirmed many of the predictions of the model. Furthermore, our model could predict the relationship between HER2 expression levels and the transient distribution of EGFR homodimers and heterodimers. Our results suggest that the levels of HER2 found on normal cells are barely at the threshold necessary to drive efficient heterodimerization. Thus, altering local HER2 concentrations in membrane microdomains could serve as an effective mechanism for regulating HER2 heterodimerization and could explain why HER2 overexpression found in some cancers have such a profound effect on cell physiology.