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Ionic Signals in the Action of Growth Factors

Growth factors regulate the proliferation of cells in vivo and in culture by binding to specific high-affinity receptors on the cell surface. Receptor occupancy triggers a cascade of biochemical and physiological changes in the target cell, which ultimately (after 10-20 hours) leads to stimulation of DNA synthesis and cell division. Among the immediate consequences of growth factor-receptor interaction are ( a ) the activation of a protein tyrosine kinase activity that is intrinsic to the receptors for epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and insulin-like growth factor 1 (IGF-I), and so on; (6) the phospholipase-C-mediated breakdown of inositol phospholipids generating several signal molecules; and (c) the activation of various ion transport systems in the plasma membrane and changes in intracellular ionic composition.' This chapter will focus on the generation of ionic signals in the action of growth factors. Mitogen-induced alterations in the levels of cytoplasmic free Ca2+( [Ca2+Ii) and H+ are of special interest because these ions are thought to serve as second messengers in activated cells. During the past few years our understanding of the ionic changes in stimulated cells has increased dramatically. The molecular mechanisms underlying ionic signaling have been largely unraveled and much effort is being devoted to determining how rapid increases in [Ca2+Ii and pHi may contribute to a proliferative response.