The ErbB receptor tyrosine family as signal integrators

ErbB receptor tyrosine kinases (RTKs) and their ligands have important role s in normal development and in human cancer. Among the ErbB receptors only ErbB2 has no direct ligand; however, ErbB2 acts as a co-receptor for the ot her family members, promoting high affinity ligand binding and enhancement of ligand-induced biological responses. These characteristics demonstrate t he central role of ErbB2 in the receptor family, which likely explains why it is involved in the development of many human malignancies, including bre ast cancer. ErbB RTKs also function as signal integrators, cross-regulating different classes of membrane receptors including receptors of the cytokin e family. Cross-regulation of ErbB RTKs and cytokines receptors represents another mechanism for controlling and enhancing tumor cell proliferation.