Does prorenin exert angiotensin-independent effects in vivo?

Thirteen years ago, Véniant et al1 made the intriguing observation that targeted expression of rat prorenin to the liver under the control of the human 1-antitrypsin promoter increased plasma prorenin levels 600-fold in male transgenic rats and caused cardiac hypertrophy, myocardial fibrosis, and severe renal lesions by 20 weeks of age. Renal renin content in these rats decreased, and plasma angiotensin (Ang) I–generating activity increased, suggesting that (local) prorenin activation had resulted in Ang II generation and, thus, a negative feedback effect on renal renin release. Yet, blood pressure was unaltered. The authors, therefore, concluded that long-term exposure to prorenin is vasculotoxic. Whether this toxic effect involved Ang II remained unexplored. The discovery of the “(pro)renin receptor” subsequently provided the putative missing link: apparently, prorenin (like renin) was able to bind to a receptor and to even exert direct, Ang-independent effects via this receptor. Such effects included the production of transforming growth factor1 and plasminogen-activator inhibitor 1 and, thus, possibly fibrosis.2 Moreover, blockade of this receptor with an antagonist called handle region peptide prevented these prorenininduced effects in vivo, even in Ang II type 1 receptor knockout animals.3,4 Clearly, therefore, the vasculotoxic effects of prorenin now had a firm basis and did not appear to involve Ang. Nevertheless, not all studies supported this view. The effect of handle region peptide on prorenin binding and signaling was questioned both in vitro and in vivo.5,6 Virtually all of the studies investigating prorenin’s (profibrotic) effects in vitro applied nanomolar concentrations of the enzyme, despite its picomolar concentrations in vivo. Thus, the physiological relevance of such findings is questionable. In addition, 2 recent studies in transgenic rodents7,8 displaying, respectively, 180and 13to 28-fold elevated levels of prorenin observed an increase in blood pressure, but no glomerulosclerosis or cardiac fibrosis, in full contrast to the results by Véniant et al.1 In such rodents, angiotensinconverting enzyme inhibition rapidly normalized blood pressure, whereas handle region peptide was without effect.8 Moreover, animals expressing active site-mutated prorenin (not capable of generating Ang I) showed no change in blood pressure.8 Thus, on the basis of these studies it appeared that prorenin acted through the generation of Ang to raise blood pressure. In support of enhanced Ang generation, the prorenin-expressing animals had low renin levels,7,8 suggestive of a negative feedback effect of Ang II on renal renin synthesis. The latter observation had been made by Véniant et al1 as well, although the rise in Ang II in that study, if present, had not resulted in a rise in blood pressure. In the present issue of Hypertension, Campbell et al9 have re-evaluated the transgenic rats of the 1996 article, focusing in particular on the mechanism of the phenotype. The parental line for these animals was the original 85-26 line described in the article by Véniant et al.1 Yet, in contrast to the initial report, the authors now find that the male transgenic rats were hypertensive by 3 months of age, developed only modest renal lesions and cardiac fibrosis after 6 months of age, and had a normal aortic wall thickness. Cardiac hypertrophy was present from the age of 3 months on, presumably as a consequence of the rise in blood pressure. In other words: these animals resembled the transgenic proreninoverexpressing animals reported recently by Peters et al7 and Mercure et al.8 Plasma prorenin levels in the current transgenic rats were 1000-fold higher than in wild-type littermates, whereas plasma and tissue (kidney) Ang II levels were no different from wild-type levels. Kidney renin levels were suppressed by 90%, in agreement with the negative feedback of prorenin-dependent Ang II generation on renal renin release. Indeed, the Ang I–generating activity of plasma was above normal, suggesting that somehow the exceptionally high prorenin levels were capable of keeping Ang I generation at a high level. Importantly, binephrectomy yielded the expected results in wild-type rats (a reduction in plasma renin activity by 80% and a reduction in plasma Ang II by 90% after 24 hours) but increased the plasma Ang I–generating activity and Ang II levels in transgenic rats. The latter probably relates to the acute phase response of the 1antitrypsin promoter of the prorenin transgene, evidenced by the 6-fold rise in prorenin after binephrectomy. The authors are to be complimented for their courage to publish these data after the initial, opposite report. Moreover, the detailed biochemical measurements, in particular, those of Ang II, provide important additional information that was not yet clear from the studies by Peters et al7 and Mercure et al.8 Now that we have all of the data, a more complete picture emerges on the role of prorenin. Campbell et al9 are unable to explain the difference in phenotype. Differences in genetic background, diet, and/or animal housing are briefly mentioned, as well as the use of The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Pharmacology, Vascular and Metabolic Diseases, Department of Internal Medicine, Erasmus MC, Rotterdam, The Netherlands. Correspondence to A.H. Jan Danser, Division of Pharmacology, Vascular and Metabolic Diseases, Department of Internal Medicine, Room EE1418b, Erasmus MC, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. E-mail [email protected] (Hypertension. 2009;54:1218-1220.) © 2009 American Heart Association, Inc.