ABSTRACT
The human epidermal growth factor receptor (HER/ErbB) system comprises the epidermal growth factor receptor (EGFR/HER1) and three other homologs, namely HERs 2-4. This receptor system plays a critical role in cell proliferation and differentiation and receptor overexpression has been associated with poor prognosis in cancers of the epithelium. Here, we examine the effect of coexpressing varying levels of HERs 1-3 on the receptor dimerization patterns using a detailed kinetic model for HER/ErbB dimerization and trafficking. Our results indicate that coexpression of EGFR with HER2 or HER3 biases signaling to the cell surface and retards signal downregulation. In addition, simultaneous coexpression of HERs 1-3 leads to an abundance of HER2-HER3 heterodimers, which are known to be potent inducers of cell growth and transformation. Our new approach to use parameter dependence analysis in experimental design reveals that measurements of HER3 phosphorylation and HER2 internalization ratio may prove to be especially useful for the estimation of critical model parameters. Further, we examine the effect of receptor dimerization patterns on biological response using a simple phenomenological model. Results indicate that coexpression of EGFR with HER2 and HER3 at low to moderate levels may enable cells to match the response of a high HER2 expresser.
The HER system of receptor tyrosine kinases plays an important role in growth, proliferation, and differentiation of epithelial cells. This receptor system consists of four members-HERl, which is also known as the epidermal growth factor receptor (EGFR); and HERs 2-4. These receptors are also known as ErbBs 1-4 (1). In addition to the important physiological role of the HER system, these receptors play a key role in transformation and tumor progression. For instance, overexpression of HER2 is associated with poor prognosis in breast cancers with 25-30% of tumors from this tissue displaying significant HER2 overexpression (2). Although the link between HER overexpression and tumorigenesis is well documented, a number of details regarding the molecular mechanisms that are involved in this process remain to be elucidated.
The importance of the HER system in physiology and pathology coupled with the scientific desire to understand the general principles underlying growth factor signaling have led to extensive research in this area (3-9). It is known that all members of this receptor family display significant homology, with each of these receptors having distinct properties such as ligand binding or receptor trafficking (6,10). For example, the EGFR is rapidly internalized and degraded upon binding its ligand EGF (11,12), whereas the other receptors in the family do not display significant ligand-induced internalization and/or recycle rapidly back to the cell surface after endocytosis (10,13-15). Further, ligand binding induces dimerization of HER family receptors where various combinations of homo- and heterodimeric species can be formed (16-18). Dimerized receptors undergo trans-phosphorylation, which activates downstream signaling pathways such as the MAPK, PI3K/Protein Kinase B, and PKC pathways via the binding of signaling adaptors to phosphotyrosine sites on the receptor cytoplasmic tails (19). There is considerable evidence suggesting that the types of receptor heterodimers that are formed and their trafficking properties are important determinants of the cellular response to HER family ligands (17,20-22).
The important role of dimer identity in driving the cellular response is exemplified by the HER2-HER3 heterodimer, which has been reported as being a potent mitogenic and oncogenic unit (22-26). This is despite the fact that HER3 has impaired tyrosine kinase activity (27) and HER2 is devoid of an activating ligand (28). The potency of this dimer is thought to stem from the trafficking properties of this dimer, which tend to prolong signaling (13,15), and from the unique ability of HER3 to efficiently engage the prosurvival PI3K/PKB pathway (29,30).
In general, it is clear that the molecular mechanisms, underlying the manner in which heterodimer identity controls the cellular response, are complex. It has been reported that the specific tyrosine sites on the cytoplasmic tail of a receptor that end up getting mmv-phosphorylated depend upon the specific HER member with which the receptor dimerizes (31). This would in turn cause qualitative changes in the signaling properties of the very same HER receptor depending upon its dimerization partner, and each dimer type may be capable of engaging a unique complement of cell-signaling pathways. In addition, since the heterodimers possess distinct trafficking properties, the spatial location (plasma membrane versus internal compartments) and the duration of the phosphorylation signal would also depend upon dimer identity. Therefore, knowledge of the types of heterodimers formed in cells expressing various levels of HER molecules may pave the way toward unraveling the connections between receptor expression levels and cell phenotype. We use the term "cell phenotype" to characterize such eventual biological responses of the cell as cell migration, proliferation, and transformation.