Signal transduction revisited: recent developments in angiotensin II signaling in the cardiovascular system

Time for primary review 31 days. Two years have passed since the appearance of the focused issue on signal transduction (Cardiovascular Research, volume 30, issue 4, October 1995). In that issue a selection of topics related to cardiovascular signal transduction were highlighted. Since then a wealth of new information has deepened our insight into the factors that govern the activity of the different signal transduction pathways. Moreover, the importance of cross-talk between signal transduction pathways is being recognized and has, at the same time, made us aware of the complexity of the signal transduction network. Finally, new players in the field of cardiovascular signal transduction have emerged, the most striking example being the Stress-Activated Protein Kinases. The purpose of this review is to summarize these new developments in this area of research. Considering the rate with which the various signal transduction pathways and their interrelationships are being deciphered the task of writing an update that pays tribute to all these new insights and developments is virtually impossible. Accordingly, as a major theme of this review the recent developments on angiotensin II-mediated signal transduction was chosen. This choice is not entirely arbitrary. In the October 1995 issue on cardiovascular signaling the role of angiotensin II (Ang II) was a main topic of various reviews already 1–3. More importantly Ang II, either systemic or locally produced, is considered to play a pivotal role in cardiovascular adaptation. Signaling via this peptide growth factor involves various receptor subtypes and results in the activation of a variety of signal transduction cascades, which have been unravelled to a large extent over the past couple of years. The major type of mechanical force to which the cardiac muscle is subjected is cyclic stretch. In contrast, the vascular endothelial cells face both cyclic stretch (pressure) and shear forces (blood flow). It is generally accepted that mechano-sensing is of paramount importance in the activation of signal transduction pathways in endothelial cells, vascular smooth muscle cells, and cardiac muscle cells, that enable the cardiovascular system to adjust to changing demands 4, 5. Nevertheless the precise mechanism of cellular mechano-sensing remains to be elucidated. To date three major hypotheses have been put forward to pinpoint the molecular basis of mechano-sensing. According to the first hypothesis mechanical stimuli are sensed by stretch-activated ion channels 6, 7. Patch clamp analysis has shown that these channels behave like non-selective cation channels that can be blocked by gadolinium. Second it has been proposed that the molecular components of the extracellular matrix (ECM) and cytoskeleton are responsible for mechano-sensing 8. In this context the role of the family of proteins referred to as integrins and so-called focal adhesion kinases (FAK) have received widespread attention 9, 10. The integrins have been thought to act as transmembrane mechanoreceptors. FAK possesses tyrosine kinase activity and becomes activated when a cell makes contact with the extracellular matrix 11. Whatever the nature of the mechano-receptors, there is ample evidence to support the notion that mechano-transduction provides the initial trigger for short-term (cardiac contractility, vascular tone) as well as long-term adaptive responses (cardiac and vascular remodeling) in the cardiovascular system. The third hypothesis is based on the experimental observation that applying a mechanical stimulus to most cell types leads to the synthesis of a whole range of autocrine and paracrine factors that, in turn, are able to modulate the activity of various signal transduction pathways. Hence, according to this hypothesis the question as to how mechanical signals are conveyed to the cells interior, can be narrowed down to the question as to how the synthesis of these growth factors is induced in response to mechanical stimuli. In this sense the latter hypothesis does not provide a true mechanistic explanation for mechano-sensing. Imposing shear stress to endothelial cells has been shown to lead to the formation of, amongst others, endothelin-1 (ET-1), basic fibroblast growth factor (bFGF), and Nitric Oxide (NO) 12, 13. Stretch of cardiac myocytes has been shown to induce the synthesis and release of Angiotensin II (Ang II) 7. In addition, increased synthesis of transforming growth factor-β (TGFβ) and endothelin-1 (ET-1), either directly as a result of the mechanical stimulus or secondary to the effects of stretch-induced Ang II production, have been reported 14, 15. Each of these factors has been shown to induce changes in gene expression in primary cultures of neonatal cardiac myocytes that are reminiscent of the hypertrophic response of the heart in situ 16, 17. Ang II is the biologically most active component of the renin angiotensin system. The physiological effects of circulating Ang II, such as its involvement in blood pressure control, aldosterone release and water balance, are relatively well known. Only recently the biological significance of locally produced Ang II is being acknowledged. It has become apparent that the cardiac tissue itself expresses all components of the renin angiotensin system, i.e. renin, angiotensinogen, and angiotensin converting enzyme and that the expression of these components alters under pathophysiological conditions, like cardiac hypertrophy 18. Induction of cardiac hypertrophy in rats in situ by supra-renal constriction of the aorta has been shown to increase circulating levels of Ang II, in addition to the upregulation of the expression of components of the renin angiotensin system in the heart, suggesting activation of the intracardiac renin angiotensin system 19. In addition, cyclic stretch of cardiac myocytes has been shown to upregulate the activity of the angiotensinogen promoter 20and elicits release of Ang II into the surrounding medium 7. These findings support the notion that the intracardiac renin angiotensin system is of functional importance. The biological significance of Ang II is further illustrated by the fact that addition of a specific type I Angiotensin receptor antagonist to the culture medium prevents the effects of mechanical stimuli on cardiomyocyte phenotype 21. Accordingly, when evaluating the effects of Ang II in situ both changes in circulating levels and local production have to be taken into account. Divergent effects of Ang II at the systemic and tissue level have been reported by the group of Delafontaine 22, 23, who showed that venous infusion of Ang II in rats reduces Insulin like Growth Factor 1 (IGF-1) levels in the circulation, while at the same time increasing IGF-1 mRNA levels in the heart. Ligand binding studies have revealed the existence of at least two Angiotensin II receptor subtypes, referred to as the angiotensin receptor type I (AT1) and type II (AT2), respectively. Recently the existence of another subtype in human cardiac fibroblasts has been postulated on the basis of its pharmacological profile in binding studies with AT1 and AT2 specific ligands 24. The AT1 and AT2 receptors have been cloned and appeared to be members of the G-protein-coupled seven-transmembrane-domain receptor superfamily, which also includes the α- and β-adrenergic receptors and the endothelin receptors. Both AT1 and AT2 receptors are present in the heart, although with respect to their relative densities species differences have to be appreciated. Within the rat heart cardiomyocytes and fibroblasts have been demonstrated to express the AT1 receptor subtype mainly, whereas endothelial cells possess AT1 as well as AT2 receptors 25. To date virtually all of the physiological effects of Ang II are ascribed to the AT1 receptor. As described in detail below the signaling cascades involving the AT1 receptor have been largely elucidated (see Berk and Corson 26for recent review). However, relatively little is known about the coupling of the AT2 receptor to intracellular signaling pathways. In this respect it is worth mentioning that the AT2 receptor is the predominant receptor subtype found in the human heart 27, 28. It has been demonstrated that various (pathological) stimuli lead to a shift in AT1 and AT2 receptor density. However, as far as the direction of this shift is concerned, the results reported are not very consistent. In cultured neonatal myocytes and fibroblasts incubation with Ang II resulted in a downregulation of AT1 mRNA levels 29. In another study 21it was demonstrated that stretching of neonatal myocytes, which supposedly also leads to Ang II secretion by these cells 14, is accompanied by a simultaneous upregulation of AT1 and AT2 receptor mRNA. Similarly, at the tissue level conflicting results have been reported. Right ventricular hypertrophy and failure in the canine heart was with local of the mRNA level of and the whereas the AT1 receptor level was rat of cardiac hypertrophy and failure various were to changes in AT1 or AT2 receptor mRNA levels whereas reported downregulation upregulation the AT1 receptor. in the human heart the AT1 receptor mRNA level was found to whereas that of the AT2 receptor was The are related to species differences and to differences in the experimental In addition, it is that changes in expression are also Whatever the these findings that the of angiotensin II receptor subtype and the activation of different signal transduction provides another level of with respect to Ang II complexity is to the system by receptor which has been shown to binding evidence that kinase be in the of receptors. 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