Controlled dimerization of ErbB receptors provides evidence for differential signaling by homo- and heterodimers

The four members of the ErbB family of receptor tyrosine kinases are involv ed in a complex array of combinatorial interactions involving homo- and het erodimers. Since most cell types express more than one member of the ErbB f amily, it is difficult to distinguish the biological activities of differen t homo- and heterodimers. Here we describe a method for inducing homo- or h eterodimerization of ErbB receptors by using synthetic ligands without inte rference from the endogenous receptors. ErbB receptor chimeras containing s ynthetic ligand binding domains (FK506-binding protein [FKBP] or FKBP-rapam ycin-binding domain [FRB]) were homodimerized with the bivalent FKBP ligand AP1510 and heterodimerized with the bifunctional FKBP-FRB ligand rapamycin . AP1510 treatment induced tyrosine phosphorylation of ErbB1 and ErbB2 homo dimers and recruitment of Src homology 2 domain-containing proteins (Shc an d Grb2). In addition, ErbB1 and ErbB2 homodimers activated downstream signa ling pathways leading to Erk2 and Akt phosphorylation. However, only ErbB1 homodimers were internalized upon AP1510 stimulation, and only ErbB1 homodi mers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 homodimers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 hom odimers were able to form foci; however, cells expressing ErbB2 homodimers displayed a five- to sevenfold higher focus-forming ability. Using rapamyci n-inducible heterodimerization we show that c-Cbl is unable to associate wi th ErbB1 in a ErbB1-ErbB2 heterodimer most likely because ErbB2 is unable t o phosphorylate the c-Cbl binding site on ErbB1. Thus, we demonstrate that ErbB1 and ErbB2 homodimers differ in their abilities to transform fibroblas ts and provide evidence for differential signaling by ErbB homodimers and h eterodimers. These observations also validate the use of synthetic ligands to study the signaling and biological specificity of selected ErbB dimers i n any cell type.