Esther Zwick

EGF receptor transactivation by G-protein-coupled receptors requires metalloproteinase cleavage of proHB-EGF.

Cross-communication between different signalling systems allows the integration of the great diversity of stimuli that a cell receives under varying physiological situations. The transactivation of epidermal growth factor receptor (EGFR)-dependent signalling pathways upon stimulation of G-protein-coupled receptors (GPCRs), which are critical for the mitogenic activity of ligands such as lysophosphatidic acid, endothelin, thrombin, bombesin and carbachol, provides evidence for such an interconnected communication network. Here we show that EGFR transactivation upon GPCR stimulation involves proHB-EGF and a metalloproteinase activity that is rapidly induced upon GPCR–ligand interaction. We show that inhibition of proHB-EGF processing blocks GPCR-induced EGFR transactivation and downstream signals. The pathophysiological significance of this mechanism is demonstrated by inhibition of constitutive EGFR activity upon treatment of PC3 prostate carcinoma cells with the metalloproteinase inhibitor batimastat. Together, our results establish a new mechanistic concept for cross-communication among different signalling systems.

Epidermal growth factor receptors: critical mediators of multiple receptor pathways

Recently, the receptor for epidermal growth factor (EGF) was identified as a downstream element in different signaling pathways. This expanded its classical function as a receptor for EGF-like ligands to a role as mediator of diverse signaling systems and as a switch point of a cellular communication network. In addition, several downstream targets, (e.g. Smad proteins and STATs) into which signals from synergistic and antagonistic signaling pathways converge, were identified.

Cell communication networks: epidermal growth factor receptor transactivation as the paradigm for interreceptor signal transmission

Communication between different cellular signaling systems has emerged as a common principle that enables cells to integrate a multitude of signals from its environment. Transactivation of the epidermal growth factor receptor (EGFR) represents the paradigm for cross-talk between G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). The recent identification of Zn2+-dependent metalloproteinases and transmembrane growth factor precursors as critical elements in GPCR-induced EGFR transactivation pathways has defined new components of a cellular communication network of rapidly increasing complexity. Further elucidation of the molecular details of the EGFR transactivation mechanism will provide new understanding of its relevance for normal physiological processes and their pathophysiological deviations.

The EGF receptor as central transducer of heterologous signalling systems

The cross-talk between heterologous signalling systems of the cell represents a new dimension of complexity in the molecular communication network that governs a great variety of physiological processes. In pathophysiologically transformed cells, key elements of this network could offer unique opportunities for pharmacological intervention. In this article, the current state of knowledge regarding the role of epidermal growth factor (EGF) in such a network is described and the recent advances made in the elucidation of the mechanism underlying EGF receptor transactivation are discussed.

Molecular targets for breast cancer therapy and prevention

The recent completion of the human genome sequence has raised great hopes for the discovery of new breast cancer therapies based on newly-discovered genes linked to breast cancer development and progression. Here we describe breast cancer therapies that have emerged from gene-based scientific efforts over the past 20 years and that are now approved for clinical testing or treatment.

Receptor tyrosine kinases as targets for anticancer drugs

Receptor tyrosine kinases (RTKs) are the primary mediators of the signaling network that transmit extracellular signals into the cell. Gene amplification and/or overexpression of RTK proteins or functional alterations caused by mutations in the corresponding genes or abnormal autocrine-paracrine growth factor loops contribute to constitutive RTK signaling, ultimately resulting in the manifestation of dysregulated cell growth and cancer. The mechanism of uncontrolled RTK signaling that leads to cancer has provided the rationale for anti-RTK drug development. Strategies towards the prevention and interception of RTK signaling include monoclonal antibodies, small-molecule inhibitors, immunotoxins and antisense oligonucleotides.

Critical Role of Calcium- dependent Epidermal Growth Factor Receptor Transactivation in PC12 Cell Membrane Depolarization and Bradykinin Signaling

PC12 cells respond to a variety of external stimuli such as growth factors, neurotransmitters, and membrane depolarization by activating the Ras/mitogen-activated protein kinase pathway. Here we demonstrate that both depolarization-induced calcium influx and treatment with bradykinin stimulate tyrosine phosphorylation of the epidermal growth factor receptor (EGFR). Using a tetracycline-controlled expression system in conjunction with a dominant-negative EGFR mutant, we demonstrate that depolarization and bradykinin triggered signals involve EGFR function upstream of SHC and MAP kinase. Furthermore, bradykinin-stimulated EGFR transactivation is critically dependent on the presence of extracellular calcium, and when triggered by ionophore treatment, calcium influx is already sufficient to induce EGFR tyrosine phosphorylation. Taken together, our results establish calcium-dependent EGFR transactivation as a signaling mechanism mediating activation of the Ras/mitogen-activated protein kinase pathway in neuronal cell types.

Distinct Calcium-dependent Pathways of Epidermal Growth Factor Receptor Transactivation and PYK2 Tyrosine Phosphorylation in PC12 Cells

Recently, we have demonstrated that in PC12 cells activation of the Ras/extracellular signal-regulated kinase pathway in response to membrane depolarization or bradykinin is mediated by calcium-dependent transactivation of the epidermal growth factor receptor (EGFR). Here we address the question whether Ca2+-calmodulin-dependent protein kinase (CaM kinase) has a role in the EGFR transactivation signal. Using compounds that selectively interfere with either CaM kinase activity or calmodulin function, we show that KCl-mediated membrane depolarization-triggered, but not bradykinin-mediated signals involve CaM kinase function upstream of the EGFR. Although both depolarization-induced calcium influx and bradykinin stimulation of PC12 cells were found to induce c-fos transcription through EGFR activation, the former signal is CaM kinase-dependent and the latter was shown to be independent. As PYK2 is also activated upon elevation of intracellular calcium, we investigated the potential involvement of this cytoplasmic tyrosine kinase in EGFR transactivation. Interestingly, we observed that inhibition of CaM kinase activity in PC12 cells abrogated tyrosine phosphorylation of PYK2 upon KCl but not bradykinin treatment. Nevertheless, PYK2 activation in response to both stimuli appeared to be mediated by pathways parallel to EGFR transactivation. Our data demonstrate the existence of two distinct calcium-dependent mechanisms leading either to EGFR-mediated extracellular signal-regulated activation or to PYK2 tyrosine phosphorylation. Both pathways either in concert or independently might contribute to the definition of biological responses in neuronal cell types.

HER2/neu: a target for breast cancer therapy.

The receptor tyrosine kinase HER2 and its rat homologue neu were independently identified as close relatives of erbB, the gene encoding the epidermal growth factor receptor (EGFR). Genomic analysis of primary tumors revealed HER2 gene amplification and overexpression in breast and ovarian adenocarcinomas and demonstrated strong correlation with poor prognosis. Since its validation as the first human oncogene, HER2 has been intensively investigated as a target for therapeutic intervention. Nevertheless, it is still poorly understood how HER2 overexpression and enhanced cellular signaling contributes to the development of human cancer. Here we summarize the signaling characteristics of HER2 in regard to its function in tumorigenesis and address recent advances towards the pharmacological intervention in HER2 signaling and anti-cancer therapy.