TIBS 17 -


INTRODUCTION The biochemical basis of signal transduction across the cell membrane has long been a subject of great interest. Considerable attention is now focused on an elaborate network of intracellular signalling systems that eventually lead to the regulation of physiological processes such as cell-cell communication, cell proliferation and differentiation. The principal architecture of the major signalling systems is now known. Many of these signalling pathways consist of a series of proteins, including specific receptors, GTP-binding proteins, second messengergenerating enzymes, protein kinases, target -functional proteins and regulatory proteins. The signalling pathways that utilize cyclic AMP (cAMP) and diacylglycerol (DAG) as second messengers are classic examples. Several growth factor receptors themselves have intrinsic protein kinase activities. The signalling pathway via nitric oxide-cyclic GMP has been clarified in detail. External signals may also be transmitted directly into the cell through ion channels or with the aid of chemicals such as steroids; the cellular responses are then mediated by effector proteins. Extensive molecular cloning analysis has revealed that almost all of these signalling proteins show enormous heterogeneity and differential tissue expression with specific intracellular localization. However, the biological significance of this heterogeneity has not always been clear. Furthermore, there are diverse interactions between signalling systems. These interactions include potentiation, cooperation, synergism and antagonism, as well as co-transmission, which is a common feature of neuronal tissues. In biological systems a positive signal is frequently followed by immediate negative feedback control, as occurs, for example, during receptor down-regulation and internalization. These interactions are all important for our understanding of the dynamic aspects of cellular regulation. This special issue of TIBS has been designed to summarize a few examples of such 'crosstalks' among major cell signalling systems at the level of the plasma membrane, second messenger generation and degradation, protein kinases and phosphatases and gene transcription and cell cycle control. The role of calcium ions (Ca2+) in cell function has been recognized for more

Signal transduction: crosstalk than one hundred years, but it is only recently that we have begun to understand the biochemical basis of its action in cellular regulation. Ca2÷ homeostasis is maintained by complex interactions between many signalling systems. For example, cAMP-mediated signals can interact with Ca2+, and examples of this type of crosstalk are increasing as our knowledge of the signalling systems expands. In situations where signals are mediated by phosphatidylinositol (Ptdlns) hydrolysis, various physiological responses can result from synergistic interactions between the Ca2+ signal and protein kinase pathways. Mobilization of Ca2÷ and hydrolysis of Ptdlns are closely interrelated and sometimes exhibit a reciprocal relationship. As the level of intracellular Ca2÷ increases, the requirement for phospholipid hydrolysis is diminished, whereas when phospholipid hydrolysis increases, less Ca2÷is required for activation of protein kinase C (PKC). At an early phase of the cellular response, the signal-induced increase in intracellular Ca2÷ concentration is downregulated by its own, and also by several other, signalling systems. Thus, the Ca2÷ signal is often transient. In most cells, however, physiological responses persist for longer than it takes for the Ca2÷ concentration to return to basal levels. This prolonged physiological response is thought to be due to sustained activation of some protein kinases. In many cells, activation of cellsurface receptors often results in regular oscillations in intracellular Ca2÷ concentration. Several techniques developed in recent years have provided a detailed picture of temporal and spatial changes in Ca2+ concentration within a single cell, and the biochemical mechanism as well as the physiological significance of this Ca2+ oscillation is being explored. Stimulation of receptors or opening of Ca2÷ channels initiates hydrolysis of Ptdlns

© 1992, Elsevier Science Publishers, (UK) 0376-5067/92/$05.00

by phospholipase C. The DAG produced activates PKC, which provides the link between extracellular signals and intracellular responses. Hydrolysis of PtdIns was once thought to be the only source of DAG. However, it is becoming evident that stimulation of the cell-surface receptor initiates a series of degradation cascades of several membrane phospholipids, and many of the degradation products may be directly involved in the control of cellular responses. In addition to the well-known metabolic products of the arachidonic acid cascade, for instance, several unsaturated fatty acids and lysophospholipids produced by phospholipase A2 action, together with DAG produced from phosphatidylcholine hydrolysis by phospholipase D action, are all effective in enhancing and sustaining PKC activation. The sustained aetivation of this enzyme appears to be a prerequisite for long-term physiological responses such as cell proliferation and differentiation. The biochemical interactions among various phospholipases mentioned above remains largely unexplored, but it is plausible that the coordinated degradation, both temporal and spatial, of various membrane phospholipids depends on interactions between several signalling systems. A protein kinase cascade is a well-known concept for amplifying one single signal to regulate a large number of cellular functions including glycogen metabolism. The regulation of gene transcription and cell cycle involves a network of such protein kinase cascades. From research in these fields, it is now clear that knowledge of interactions between signalling systems and spatiotemporal aspects of biochemical events occurring within cells are prerequisites for an understanding of biological regulation. Further research may complicate individual pathways but, if this additional information can be integrated between signalling systems, then a more precise picture of physiological and pathological cellular responses should emerge. The reviews published here are only the beginning of our understanding of the dynamic regulation which is fundamental to the physiology of all cells.



Signal transduction: crosstalk.

TIBS 17 - OCTOBER 1992 INTRODUCTION The biochemical basis of signal transduction across the cell membrane has long been a subject of great interest...
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