Cardiovascular Division, Department of Medicine Brigham & Women's Hospital and Harvard Medical School Cambridge, Massachusetts, USA jliao@rics.bwh.harvard.edu
The finding that cleavage and shedding of the membrane-bound heparin-binding epidermal growth factor by metalloproteases contribute to the hypertrophic process offers new insights for the treatment of cardiac hypertrophy and progression to heart failure. (pages 35−40)
Cardiac hypertrophy is an adaptive physiological response to increases in blood pressure that preserves myocardial wall stress, chamber size and contractile function. Despite these initial advantages, cardiac hypertrophy is also an independent risk factor for cardiovascular disease and, if left untreated, it frequently progresses to heart failure1. The hypertrophic process is mediated, in part, by signaling through G protein−coupled receptors (GPCRs). Until now, the precise downstream signaling mechanisms that link GPCRs to hypertrophic responses, such as activation of phospholipase C (PLC) and protein kinase C (PKC), induction of immediate-early genes, re-expression of embryonic genes, and increased synthesis of contractile proteins2 were unknown. In this issue, Asakura et al.3 provide tantalizing evidence that cell-sur-face cleavage and ecto-domain shedding of the myocardial hep-arin-binding epidermal growth factor (HB-EGF) is integrally responsible for eliciting the hypertrophic response. Their findings suggest a novel signaling paradigm for cardiac hypertrophy and offer potential therapeutic strategies for preventing the development of cardiac hypertrophy and, possibly, its progression to heart failure.
According to the new study, a key proteolytic enzyme in the GPCR signaling cascade is a disintegrin and metalloprotease, ADAM12, which cleaves membrane-bound HB-EGF in response to GPCR stimulation or exposure to pressure overload. The subsequent shedding of HB-EGF and transactivation of the EGF receptor (EGFR) are necessary to produce the hypertrophic response. When the function of ADAM12 was blocked in mice with the inhibitor KB-R7785 or in vitro by overexpression of a metalloprotease-deficient mutant protein of ADAM12, the processing of HB-EGF was greatly attenuated and the hypertrophic response was almost completely abolished (Fig. 1). The authors conclude that GPCR-mediated cardiac hypertrophy involves an autocrine/paracrine loop, which requires the shedding of membrane-bound HB-EGF and subsequent transactivation of the EGFR. The receptor subtype of EGFR and the downstream signaling pathway responsible for the hypertrophic response were not identified. Nevertheless, the clinical implication of this study is that agents such as KB-R7785, which target ADAM12, may be clinically useful in preventing the development of cardiac hypertrophy (although the beneficial effects of KB-R7785 in pre-existing cardiac hypertrophy and heart failure have not been established).
Figure 1. Processing and shedding of HB-EGF is necessary to elicit the myocardial hypertrophic response.
Stimulation of seven transmembrane GPCRs by hypertrophic stimuli such as angiotensin II (AngII), endothelin-1 (ET-1) and phenylephrine (PE) activate Gq-mediated signal transduction pathways. A prominent downstream effector of Gq is PLC, which generates diacylglyerol (DAG) and inositol 1,4,5- triphosphate (IP3) from polyphosphatidylinositides. DAG is a potent activator of PKC, whose suggested association with ADAM12 somehow leads to the processing and shedding of membrane-bound HB-EGF. The untethered HB-EGF is then able to bind and transactivate EGFR, which stimulates cellular growth leading to cardiac hypertrophy.
The ADAMs are widely distributed zinc-dependent metalloproteases, which possess a disintegrin, a cysteine-rich domain, and an EGF-like domain for binding, activation and proteolytic function4. Asakura et al. identified ADAM12 through its association with PKC-. Indeed, PKC isoforms are known to be involved in cardiac hypertrophy, and have been shown to mediate the shedding of HB-EGF in other tissues5. It is not known, however, how PKC- activates ADAM12, whether ADAM12 is membrane-bound or secreted, and what the relative specificity of KB-R7785 for ADAM12 is compared with other ADAMs. Regardless, ADAM12 is probably neither the only target of KB-R7785 nor the only metalloprotease that participates in cardiac remodeling. For example, other members of this extended metalloprotease family such as ADAM17, which has recently been identified as the tumor necrosis factor- (TNF-) converting enzyme (TACE)6, may also be involved. Indeed, TNF- levels are elevated in cardiac hypertrophy and the downstream inflammatory effects of TNF- are believed to contribute to cardiac decompensation and heart failure7. It seems likely that many ADAMs participate in the remodeling process associated with cardiac hypertrophy and heart failure.
An important question, crucial to the design of effective therapies, is whether preventing an adaptive physiological response, such as cardiac hypertrophy, can be beneficial, especially in the presence of sustained hemodynamic load. For example, in experimental models of left-ventricular pressure overload, inhibition of Gq signaling and cardiac hypertrophy by cardiac-specific overexpression of a specific regulator of G protein signaling, RGS-4, caused the rapid development of dilated cardiomyopathy and heart failure8. Although targeted disruption of cardiac Gq and G11 block the hypertrophic response9, it is not known whether progression to heart failure would have been prevented in these mice. Similarly, the results of Asakura et al. should be viewed with some caution, considering the long-term benefits and risks of blocking cardiac hypertrophy in response to GPCR activation or pressure overload. For example, it is quite possible that chronic inhibition of HB-EGF processing could actually lead to a more rapid cardiac decompensation by bypassing the adaptive stage of cardiac hypertrophy. Indeed, a monoclonal antibody, trastuzumab (Herceptin), which blocks the EGFR subtype, EGFR-2, caused increased incidence of unexplained cardiomyopathy in clinical trials for breast cancer10. This raises the question as to whether blocking EGFR signaling by antagonizing ADAM12 could actually prevent heart failure. However, antagonism of ADAM12 may be more tissue-specific, and may therefore yield different clinical outcomes compared with therapies that directly block EGFR transactivation.
Finally, for this novel HB-EGF shedding pathway to be physiologically relevant, it must be put into context of existing pathways, which are also known to elicit the hypertrophic response. For example, stimulation of GPCRs or exposure to pressure overload induces the activation of small G proteins of the Ras and Rho family3. These small G proteins activate mitogen-activated protein (MAP) kinases and mediate the generation of reactive oxygen species (ROS)11, processes that are necessary for the hypertrophic response. In particular, the Rho family of small G proteins is thought to have critical roles in the development of cardiac hypertrophy by regulating cell morphology and contractile elements.
What then, is the relationship between HB-EGF shedding and small G proteins? In some cellular systems, heterotrimeric G proteins activate small G proteins through the autocrine/paracrine transactivation of tyrosine kinase receptors such as EGFR (ref. 12). Thus, it is likely that small G proteins are activated downstream of HB-EGF shedding. But then, what about the activation of MAP kinases and the generation of intracellular ROS, processes which are also dependent on small G proteins? Do they function upstream of HB-EGF shedding by activating ADAM12 or some other metalloprotease? Or do they also work downstream of EGFR transactivation? Also, where does the calcium-dependent calcineurin/CaM kinase signaling pathway fit into all of this? Does it have a role in HB-EGF shedding or vice versa? Questions such as these remind us of just how little we really understand about the hypertrophic process.
This fascinating study by Asakura et al. provokes more questions than it answers. But the finding that HB-EGF shedding is integral to the hypertrophic process may help sort out some of the complex hierarchical relationships between the various signaling pathways that lead to cardiac hypertrophy.
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. Indeed, PKC isoforms are known to be involved in cardiac hypertrophy, and have been shown to mediate the shedding of HB-EGF in other tissues
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