Mitogenic and Migratory Signals from GPCRs and Tyrosine Kinases 1203 EGFR signal transactivation in cancer cells O.M. Fischer, S. Hart, A. Gschwind 1 and A. Ullrich 2 Max-Planck-Institute of Biochemistry, Department of Molecular Biology, Am Klopferspitz 18A, 82152 Martinsried, Germany Abstract The EGFR (epidermal growth factor receptor) plays a key role in the regulation of essential normal cellular processes and in the pathophysiology of hyperproliferative diseases such as cancer. Recent investigations have demonstrated that GPCRs (G-protein-coupled receptors) are able to utilize the EGFR as a downstream signalling partner in the generation of mitogenic signals. This cross-talk mechanism combines the broad diversity of GPCRs with the signalling capacities of the EGFR and has emerged as a general concept in a multitude of cell types. The molecular mechanisms of EGFR signal transactivation involve processing of transmembrane growth factor precursors by metalloproteases which have been recently identified as members of the ADAM (a d isintegrin a nd m etalloprotease) family of zinc-dependent proteases. Subsequently, the EGFR transmits signals to prominent downstream pathways, such as mitogen- activated protein kinases, the phosphoinositide 3-kinase/Akt pathway and modulation of ion channels. Analysis of GPCR-induced EGFR activation in more than 60 human carcinoma cell lines derived from different tissues has demonstrated the broad relevance of this signalling mechanism in cancer. Moreover, EGFR signal transactivation was linked to diverse biological processes in human cancer cells, such as cell proliferation, migration and anti-apoptosis. Together with investigations revealing the importance of this GPCR–EGFR cross- talk mechanism in cardiac hypertrophy, Helicobacter pylori -induced pathophysiological processes and cystic fibrosis, these findings support an important role for GPCR ligand-dependent EGFR signal transactivation in diverse pathophysiological disorders. Introduction Cellular signal transduction networks serve to co-ordinate the plethora of extracellular stimuli into biological responses of the cell. Within these communication networks inter- receptor cross-talk has emerged as a general signalling mech- anism combining and diversifying signal transduction pathways. Key components of these networks are cell-surface receptors which transduce external signals through the membrane into the cell. Receptor tyrosine kinases represent an important subclass of these transmembrane proteins, with the EGFR (epidermal growth factor receptor) being the most prominent representative. The EGFR controls a wide variety of biological responses such as proliferation, differentiation, migration and the modulation of apoptosis. Aberrant receptor signalling due to overexpression, mutation or autocrine signalling loops has been frequently implicated in hyperproliferative diseases such as cancer. Direct stimulation of the receptor by binding of a ligand to the receptor’s extracellular domain leads to dimerization and subsequent autophosphorylation of two receptor molecules, Key words: a disintegrin and metalloprotease (ADAM), cancer, cross-talk, epidermal growth factor receptor (EGFR), G-protein-coupled receptor (GPCR), transactivation. Abbreviations used: ADAM, a d isintegrin a nd m etalloprotease; ET-1, endothelin-1; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; ERK, extracellular-signal- regulated kinase; GPCR, G-protein-coupled receptor; HB-EGF, heparin-binding epidermal growth factor; LPA, lysophosphatidic acid; MAPK, mitogen-activated protein kinase; TGF-α, transforming growth factor α; TMPS, triple membrane-passing signal; TACE, tumour necrosis factor-α- converting enzyme; PKC, protein kinase C. 1 Present address: Genentech, Inc., Department of Immunology, South San Francisco, CA 94080, U.S.A. 2 To whom correspondence should be addressed (e-mail ullrich@biochem.mpg.de). thereby creating phosphotyrosine docking sites to activate intracellular signalling cascades. Eight ligands have been described so far for the EGFR: EGF (epidermal growth factor), HB-EGF (heparin-binding EGF), amphiregulin, TGF-α (transforming growth factor α), betacellulin, epi- regulin, epigen and crypto. All of them, except crypto, are synthesized as membrane-spanning precursor molecules that have to be proteolytically processed to become fully active. Several modes of indirect EGFR activation have been described. Stimulation of EGFR phosphorylation occurs after treatment with unphysiological stimuli, including hyperosmolarity, oxidative stress, mechanical stress, UV light and γ-irradiation. This effect has been predomin- antly attributed to the inactivation of phosphatases that antagonize the intrinsic receptor kinase activity, thereby shifting the equilibrium of basal autophosphorylation and dephosphorylation towards the activated state. Apart from unphysiological stimuli, receptor activation can also be induced by chemokines, cell-adhesion molecules and GPCRs (G-protein-coupled receptors). GPCRs represent the largest group of cell-surface re- ceptors and exert a wide variety of biological function, in- cluding neurotransmission, photoreception, chemoreception, metabolism, growth, differentiation and migration [1]. The finding that GPCR stimulation induces EGFR phosphoryla- tion combines the broad diversity of GPCRs with the potent signalling capacities of the EGFR and serves as a paradigm for inter-receptor cross-talk. Since increasing evidence impli- cates this so-called EGFR signal transactivation in diverse pathophysiological disorders, elucidation of the underlying C  2003 Biochemical Society