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Transactivation of the EGFR Signal in Human Cancer Cells

One characteristic commonality to all organisms is the dynamic ability to coordinate complex physiological processes with environmental changes. The capability of cells to communicate with their environment is achieved through a number of pathways that receive and process signals. Signals can originate from the external environment ranging from soluble endocrine and paracrine factors to signalling molecules located on neighbouring cells, but also from different intracellular regions. Integration of external stimuli with internal signal transduction pathways is essential for the ability of cells to respond correctly to the environment in order to achieve an appropriate biological response. This type of information transfer is an important part of the cellular repertoire of regulatory mechanisms. During normal embryonic development and in adult life, signalling needs to be precisely coordinated and integrated at all times. Deregulated signal transmission is now recognized as a cause of many human diseases such as cancer and diabetes (Hanahan and Weinberg 2000; Shawver et al. 2002). Receptors of the tyrosine kinase family play pivotal roles in the regulation of biological processes during development and in adulthood of multicellular organisms. Cellular signalling of these receptors is counterbalanced by protein tyrosine phosphates (PTPs) which therefore act as key regulatory components in directing and modulating signal transduction pathways. Defects in this highly complex regulatory system result in pathological disorders such as cancer. G-protein coupled receptors (GPCRs) are integral membrane proteins that, in response to activation by extracellular stimuli, regulate intracellular second messenger levels via their coupling to heterotrimeric G-proteins (guanin-nucleotide binding protein). They are dynamically regulated via phosphorylation by G-protein coupled receptor kinases (GPKs) which desensitize the receptor after prolonged or repeated exposure to agonists. GPCRs represent the largest family of signal transduction molecules known and their dysfunction is responsible for numerous diseases. Protein tyrosine kinases are important regulators of intracellular signal transduction pathways mediating aspects of multicellular communication and development in metazoans (Cohen 1 Introduction 8 2002). These enzymes catalyze the transfer of the γ-phosphate of ATP to hydroxyl groups of tyrosines on target proteins. Tyrosine kinases play an important role in the control of most fundamental cellular processes including the cell cycle, migration, metabolism and survival, as well as proliferation and differentiation. There are currently more than 90 known tyrosine kinase genes in the human genome; 58 encode transmembrane receptor tyrosine kinases (RTKs) distributed into 20 subfamilies based on their structural characteristics (Fig. 1), and 32 encode cytoplasmic non-receptor tyrosine …