Receptor Tyrosine Kinases: Legacy of the First Two Decades

Receptor tyrosine kinases (RTKs) and their cellular signaling pathways play important roles in normal development and homeostasis. Aberrations in their activation or signaling leads to many pathologies, especially cancers, motivating the development of a variety of drugs that block RTK signaling that have been successfully applied for the treatment of many cancers. As the current field of RTKs and their signaling pathways are covered by a very large amount of literature, spread over half a century, I am focusing the scope of this review on seminal discoveries made before tyrosine phosphorylation was discovered, and on the early days of research into RTKs and their cellular signaling pathways. I review the history of the early days of research in the field of RTKs. I emphasize key early findings, which provided conceptual frameworks for addressing the questions of how RTKs are activated and how they regulate intracellular signaling pathways.The family of cell-surface receptors designated receptor tyrosine kinases (RTK) received their name more that a decade after the same molecules were already known as the cell-surface receptors for insulin (insulin receptor), epidermal growth factor (EGFR), and many other growth factor receptors. Following the pioneering discoveries of nerve growth factor and epidermal growth factor (EGF; Levi-Montalcini and Booker 1960; Cohen 1962) and the establishment of the important roles of these two growth factors in the control of neuronal differentiation and cell proliferation in vivo and in vitro, it became clear that these cytokines bind specifically to cell-surface receptors. Insulin had already been discovered by this time, and had been applied successfully to treat diabetes patients since the early twentieth century. The resulting homogenous preparations of pure insulin enabled the quantitative characterization of insulin binding to its receptor on intact cells or to solubilized insulin receptor preparations using radiolabeled insulin (De Meyts et al. 1973). These studies greatly advanced understanding of the ligand binding characteristics of insulin receptor and, later on EGFR (Carpenter et al. 1975), including the establishment of negative cooperativity in insulin binding to its receptor expressed on the surface of living cells (De Meyts et al. 1973). Moreover, these studies shed important light on the dynamic nature of the cellular behavior of these receptors. The capacities of insulin receptor and EGFR to undergo ligand-dependent down-regulation and desensitization through receptor-mediated internalization and degradation (Carpenter and Cohen 1976; Gordon et al. 1978; Schlessinger et al. 1978a,b; Carpentier et al. 1979; Haigler et al. 1979) were also established well before the realization that growth factors receptors are endowed with intrinsic protein tyrosine kinase activities (Fig. 1).Open in a separate windowFigure 1.A time line of key findings during the history of RTKs, with emphasis on findings and discoveries that produced the conceptual framework in the development of the RTK field and its application for cancer therapy. References for the key findings are also presented in the text (Lee et al. 1985; Libermann et al. 1985; Margolis et al. 1990; Bottaro et al. 1991; Bae et al. 2009).Progress was also made in elucidating the role of growth factors in normal embryonic development, wound healing, and pathological conditions such as cancer. Early studies in the 1960s and 1970s showed that growth factors play an important role in oncogenesis induced by retroviruses and in the proliferation of tumor-derived cancer cells. Pioneering studies performed by Howard Temin (1966, 1967) showed that cancer cells need less insulin and serum growth factors for cell proliferation compared with normal cells, suggesting that cancer cells produce and use their own growth factors and/or use cellular processes that in normal cells are regulated by exogenously supplied growth factors; both predictions were subsequently confirmed. A variety of new polypeptide growth factors that stimulate cell proliferation by binding to receptors at the cell surface were subsequently discovered. Those include a growth factor isolated from human platelets designated platelet-derived growth factor (PDGF; Antoniades et al. 1979; Heldin et al. 1979), a growth factor isolated from bovine brain designated fibroblast growth factor (FGF; Gospodarowicz et al. 1978), a growth factor isolated from rat platelets that stimulates the proliferation of mature hepatocytes, designated hepatocyte growth factor (HGF; Nakamura et al. 1986). In addition to EGF, another growth factor that binds selectively to cells expressing EGFR was isolated from virally and chemically transformed cells, suggesting that this growth factor—designated transforming growth factor α—may play a role in oncogenesis by an autocrine mechanism (Roberts et al. 1980, 1982). This discovery provided further support to the earlier finding that transformation by murine and feline sarcoma viruses selectively interferes with EGF binding to EGFR in transformed cells (Todaro et al. 1976). Together with many other studies published since the 1980s, this work showed that growth factors and their receptors play numerous important roles during development and in many normal cellular processes as well as in pathologies such as cancer, diabetes, atherosclerosis, severe bone disorders, and tumor angiogenesis.Visualization of dynamic cellular redistribution of ligand/receptor complexes, and rapid receptor-mediated internalization of growth factors such as insulin or EGF, led to the proposal that cell-surface receptors for these ligands may play a passive role in delivering them to intracellular compartments in which internalized EGF or insulin molecules exert their actions (Vigneri et al. 1978; Podlecki et al. 1986; Jiang and Schindler 1990). In other words, according to this hypothesis, the biological signals induced by insulin or EGF were thought to be mediated by binding of the ligands themselves to intracellular target(s) in the cytoplasm or nucleus, with the role of the cell-surface receptor being to act as a “carrier” that delivers them directly to these targets. An alternative hypothesis was that insulin or EGF activates their cognate receptors at the cell surface, which in turn stimulate the production of an intracellular second messenger molecule analogous to cAMP in signaling by the G-protein-activating β-adrenergic receptor. Indeed, several potential second messengers that are generated in cells on stimulation with insulin or other growth factors were proposed before (and even after) it became clear that insulin receptor, EGFR, and other RTKs are endowed with intrinsic tyrosine kinase activity (Larner et al. 1979; Das 1980; Saltiel and Cuatrecasas 1986).A demonstration that anti-insulin receptor antibodies from the serum of certain diabetic patients could mimic cellular responses of insulin (Flier et al. 1977; Van Obberghen et al. 1979) provided the first conclusive answer to the question of whether the biological activity of growth factors is mediated directly or indirectly through their membrane receptors. This experiment ruled out the possibility that insulin receptor functions as a passive carrier that delivers insulin to an intracellular target to induce cellular responses. Studies showing that intact, bivalent antibodies against the insulin receptor can activate its signaling, whereas monovalent Fab fragments of the same antibodies cannot further argued that ligand-induced receptor dimerization or stimulation of a particular arrangement between two receptor molecules in a dimer can activate the insulin receptor (Kahn et al. 1978).A similar conclusion was reached using certain monoclonal antibodies that bind to the extracellular region of EGFR and block ligand binding (Schreiber et al. 1981). Whereas intact antibodies were able to mimic EGF in stimulating a variety of EGF-like responses including cell proliferation, monovalent Fab fragments of the same monoclonal EGFR antibodies failed to do so—and acted instead as EGFR antagonists (Schreiber et al. 1981, 1983). These experiments provided strong evidence both that EGFR plays a crucial role in mediating EGF-induced cellular responses and that EGFR is activated by ligand-induced receptor dimerization (Schreiber 1981, 1983).