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(Circulation. 2003;107:2522.)
© 2003 American Heart Association, Inc.

Editorial

Role For Chymase in Heart Failure

Angiotensin II-Dependent or -Independent Mechanisms?

Jorma O. Kokkonen, MD, PhD; Ken A. Lindstedt, PhD; Petri T. Kovanen, MD, PhD

From the Wihuri Research Institute and the Division of Cardiology, Helsinki University Central Hospital, Helsinki, Finland.

Correspondence to Petri T. Kovanen, Wihuri Research Institute, Kalliolinnantie 4, FIN-00140, Helsinki, Finland. E-mail petri.kovanen{at}wri.fi

Key Words: Editorials • heart failure • angiotensin

One of the major questions regarding the role of angiotensin II (Ang II) in the pathophysiology of heart failure has been whether other enzymes, in addition to angiotensin-converting enzyme (ACE), could contribute to the local production of Ang II in the heart. Specifically, there is controversy as to whether the major Ang II-forming enzyme within the heart is ACE or chymase, a chymotrypsin-like serine protease that is synthesized and stored in the cardiac mast cells and is not affected by ACE inhibitors.

See p 2555

Formation of Ang II in the heart has been studied both in vitro and in vivo, but the results of these experiments are inconsistent. In vitro experiments with human^1,2 or animal heart extracts,^3,4 derived from either normal or failing hearts, have demonstrated unequivocally that the major Ang II-forming enzyme, responsible for 80% to 90% of the Ang II-forming capacity in the heart extracts, is chymase.

In striking contrast, experiments in vivo with normal^5,6 or failing dog hearts^7,8 have demonstrated that most (>70%) of the Ang II formation can be inhibited by an ACE inhibitor, indicating that, under these conditions, the major Ang II-forming enzyme is ACE. Moreover, in normal human hearts, Zisman et al⁹ have demonstrated that, in vivo, the major Ang II-forming enzyme is ACE, as 90% of the Ang II formation could be inhibited by an ACE inhibitor. However, no experiments with intact failing human hearts have been performed.

The obvious explanation for the discrepancy between the in vitro and in vivo studies is that the chymase-mediated Ang II formation is subjected to local regulation, a fact that has been overlooked in the studies performed in vitro. Thus, chymase activity is known to be regulated by at least 2 factors: Those that lead to stimulation and degranulation of mast cells, and those that inhibit chymase activity, ie, the natural protease inhibitors present in the interstitial fluid.

Firstly, chymase is normally not secreted, but is stored intracellularly in the secretory granules of mast cells, and for chymase to exert its action on the heart, the cardiac mast cells must have been stimulated to degranulate. Mast cells are expected to be stimulated to degranulate in chronic inflammatory states. Indeed, because some of the immunoreactivity of chymase has been found in the extracellular matrix of the failing human heart, at least some of the myocardial mast cells must have degranulated and released chymase.¹⁰ Moreover, the mast cell numbers were found to be increased both in failing human¹¹ and failing dog¹² hearts. It is important to realize that during preparation of heart extracts for in vitro experiments, the mast cells present in the myocardial sample are all disrupted and, as a result, all the originally intracellular chymase will obtain access to its potential substrates, eg, Ang I. This step corresponds to total degranulation of the mast cells, a situation only approached in vivo in severe allergic anaphylaxis. Thus, in these in vitro tests, the contribution of chymase to Ang II formation is heavily overestimated.

Secondly, the heart chymase-mediated conversion of Ang II takes place in the interstitial fluid. This contains high concentrations of protease inhibitors with the potential to inhibit chymase-mediated Ang-II formation that are lost when heart extracts are prepared. To mimic the actual conditions of angiotensin metabolism in the heart interstitium, we incubated human heart extracts with Ang I in vitro in the presence of interstitial fluid. We found that human heart chymase is highly sensitive to the protease inhibitors naturally present in the interstitial fluid.² Thus, despite the artificially increased amounts of chymase accessible to Ang I in the heart extracts, human heart chymase was strongly (>95%) inhibited in the presence of interstitial fluid. Accordingly, in this more physiological experimental system, the major Ang II-forming enzyme was ACE. Similarly, human chymase is likely to be strongly inhibited in vivo, as the heart interstitium is continuously bathed by interstitial fluid containing protease inhibitors in concentrations sufficiently high to inhibit this enzyme efficiently. Furthermore, in large clinical trials, the use of Ang II type-1 receptor blockers, which selectively inhibit the Ang II-mediated effects irrespective of the enzyme responsible for its generation, have not proven superior to ACE inhibitors.^13,14 Taken together, these results suggest that the local production of Ang II in the heart is largely regulated by ACE rather than by chymase.

However, previous studies attempting to clarify the actual role of chymase in myocardial Ang II formation have suffered from lack of a chymase inhibitor suitable for in vivo experiments. Therefore, the development of specific inhibitors of chymase for animal and human studies is emerging as an important research tool for defining the role of mast cells and mast cell chymase in the pathophysiology of heart failure.

In the current issue of Circulation, Matsumoto et al¹⁵ take advantage of such a novel, highly specific chymase inhibitor, SUNC8257, to test the actual role of cardiac chymase in a dog model of tachycardia-induced heart failure. The use of a dog model in this experimental set-up is appropriate, because dog chymase, like human chymase, has the ability to convert Ang I to Ang II. Thus, positive results obtained with a specific chymase inhibitor in a dog model of heart failure would imply that chymase might have a pathophysiological role in the development of heart failure in humans, as well.

In their study, Matsumoto et al¹⁵ show that dogs, when subjected to ventricular pacing for 3 weeks while receiving vehicle, developed significant interstitial fibrosis and heart failure. In striking contrast, dogs treated with the chymase inhibitor during the pacing period had significantly less collagen deposition and their left ventricular diastolic function was better than that of the dogs that did not receive the inhibitor during the pacing. The authors suggest that the observed antifibrotic effect may depend on a reduction in cardiac Ang II formation. However, treatment with the chymase inhibitor had only a minor inhibitory effect on the cardiac Ang II levels (18%), whereas the collagen volume fraction, representing the level of fibrosis, was significantly reduced (60%). Thus, we can infer that chymase-mediated tissue fibrosis cannot be due solely to its Ang II-forming potential. This finding is in agreement with the concept that ACE, rather than chymase, is the major cardiac Ang II-forming enzyme, and further suggests that the chymase inhibitor-mediated antifibrotic effect occurs independently of a reduction in cardiac Ang II levels.

Interestingly, by using a mast cell-deficient mouse model (W/W^v) and its healthy counterpart, Hara et al¹⁶ have shown that mast cells play a key role in the progression of heart failure. Because mouse chymase is unable to produce Ang II, the adverse effects mediated by the cardiac mast cells, including fibrosis, must have occurred independently of chymase-mediated Ang II formation. As stated by Matsumoto et al,¹⁵ alternative mechanisms exist, for cardiac chymase may also be involved in the generation of active transforming growth factor-ß (TGF-ß),^17,18 another profibrotic molecule.

It seems evident that hemodynamic stress induces the infiltration or proliferation of mast cells in the myocardium, both in humans¹¹ and in experimental animals.¹² As observed in the study by Matsumoto et al,¹⁵ the number of chymase-positive mast cells in the myocardium of the vehicle group was significantly increased (13-fold) during the pacing period, whereas their number was much less increased (2.9-fold) in the group receiving chymase inhibitor therapy. Interestingly, it has been shown that mast cell chymase promotes mast cell migration by c-kit ligand (stem cell factor) activation.¹⁹ Therefore, it is possible that the observed reduction in mast cell number reflects the effect of the chymase inhibitor on mast cell infiltration. By blocking this infiltration, the chymase inhibitor also indirectly reduces the synthesis and secretion of other mast cell-derived profibrotic molecules, such as tryptase and basic fibroblast growth factor.

As stated above, there are signs of mast cell stimulation and the presence of chymase activity in the extracellular spaces of failing hearts.¹⁰ In a hamster model of heart failure, Sukenaga et al²⁰ have shown that inhibition of chymase activity with NK3201, another potential chymase inhibitor, suppresses mast cell degranulation in vivo. These results show that chymase also participates in mast cell degranulation, suggesting that SUNC8257 may be involved in mast cell stabilization. However, from data presented by Matsumoto et al,¹⁵ it is difficult to judge the amount of chymase present in the extracellular space, ie, whether or not SUNC8257 inhibits the degree of mast cell stimulation and degranulation.

In summary, the data by Matsumoto et al¹⁵ suggest that most of the potentially harmful effects mediated by chymase are related not to Ang II formation but to other chymase-mediated profibrotic mechanisms, such as promotion of mast cell infiltration, mast cell stimulation, and release of TGF-ß. However, we may still be far from understanding the exact cellular and molecular mechanisms by which chymase participates in the progression of heart failure. Therefore, chymase inhibitors may emerge as an important research tool for defining the role of mast cells and mast cell chymase in the pathophysiology of heart failure. To meet with the authors’ suggestion¹⁵ that chymase inhibition may become one strategy for preventing cardiac remodeling and the progression of human heart failure, more in vivo experimental work needs to be done and safe and effective chymase inhibitors for human use must be developed.

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

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This Article Extract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kokkonen, J. O. Articles by Kovanen, P. T. Search for Related Content PubMed PubMed Citation Articles by Kokkonen, J. O. Articles by Kovanen, P. T. Related Collections Congestive Remodeling Animal models of human disease Hypertrophy Acute myocardial infarction Related Article