Response to Letter Regarding Article, “Role of p90 Ribosomal S6 Kinase-Mediated Prorenin-Converting Enzyme in Ischemia and Diabetic Myocardium”
We appreciate the interest of Dr Krop et al and their support of our efforts to better understand prorenin-converting enzyme (PRECE) in patients with diabetes,1 and we welcome the opportunity to respond. First, we have to clarify that there is no equilibration period before detecting angiotensinogen levels in the hearts of wild-type p90 ribosomal S6 kinase (WT-p90RSK-Tg) and nontransgenic littermate control mice (NLC). Dr Danser’s group reported a half-time for angiotensinogen washout from buffer-perfused hearts of approximately 3 minutes.2 We detected angiotensinogen levels from not only perfusates but also from whole hearts. Therefore, there may not be a significant discrepancy between our data and the data of Danser et al regarding angiotensinogen levels. In addition, because our perfusion model used mouse hearts whereas Dr Danser’s group used rat hearts, differences in size and the volume of perfusates may be significant. We also suggested in our article that the role of the renin-angiotensin system (RAS) may be different as the result of the differences in expression of PRECE among different strains and species. Dr Danser’s group reported that in isolated rat hearts, the angiotensinogen level was 0.1 pmol/mL and the angiotensin I level was 15 fmol/mL in coronary effluent.2 Again, we detected the angiotensinogen level in whole heart, which includes extracellular space, membrane, and intercellular compartments. In addition, it contains not only cardiomyocytes but also numerous other cell types, including fibroblasts and vascular cells. We are aware that Danser et al believe that angiotensinogen is only limited to the extracellular fluid compartment and is not located inside the cells.3 However, many investigators have shown that angiotensinogen is synthesized in all regions of the heart and in cultured cardiac myocytes and fibroblasts in both mice and rats.4 It is also not clear how much circulating prorenin and renin, as well as angiotensinogen, can be taken up into the heart in mice.4,5 Because we found increased PRECE mRNA in WT-p90RSK-Tg mice, we believe that prorenin/renin and angiotensinogen also exist intracellularly and that the expression of PRECE in cardiomyocytes can increase angiotensinogen cleavage both inside and outside the cells. However, the cellular localization of PRECE needs to be investigated. We agree that characterizing the localization and compartmentalization of the RAS system is difficult and controversial because of the contamination from the circulation and that this needs further investigation.
In summary, we demonstrated the induction of PRECE mRNA and protein in WT-p90RSK-Tg mice and determined the cardio-protection by inhibition of p90RSK activity in dominant-negative-p90RSK-Tg mice.6 We believe that p90RSK-induced PRECE and the subsequent RAS activation in the heart play a significant role in regulating ischemia/reperfusion-mediated cardiac damage. Finally, we agree with Dr Krop et al that it is surely time to investigate the exact role and mechanism of PRECE in regulating the RAS system in the heart.
This work was supported by grants from the American Heart Association to Dr Itoh (postdoctoral fellowship 0325769T), Dr Yan (grant-in-aid 0455847T), and Dr Blaxall (scientist development grant 0435437T), and from the National Institutes of Health to Dr Berk (HL44721) and Dr Abe (HL-66919 and GM-071485-01A1).
Itoh S, Ding B, Shishido T, Lerner-Marmarosh N, Wang N, Maekawa N, Berk BC, Takeishi Y, Yan C, Blaxall BC, Abe J. Role of p90 ribosomal S6 kinase-mediated prorenin-converting enzyme in ischemic and diabetic myocardium. Circulation. 2006; 113: 1787–1798.
de Lannoy LM, Danser AH, van Kats JP, Schoemaker RG, Saxena PR, Schalekamp MA. Renin-angiotensin system components in the interstitial fluid of the isolated perfused rat heart: local production of angiotensin I. Hypertension. 1997; 29: 1240–1251.
Danser AH, Saris JJ, Schuijt MP, van Kats JP. Is there a local renin-angiotensin system in the heart? Cardiovasc Res. 1999; 44: 252–265.
Muller DN, Fischli W, Clozel JP, Hilgers KF, Bohlender J, Menard J, Busjahn A, Ganten D, Luft FC. Local angiotensin II generation in the rat heart: role of renin uptake. Circ Res. 1998; 82: 13–20.
Maekawa N, Abe J, Shishido T, Itoh S, Ding B, Sharma VK, Sheu SS, Blaxall BC, Berk BC. Inhibiting p90 ribosomal S6 kinase prevents (Na+)-H+ exchanger-mediated cardiac ischemia-reperfusion injury. Circulation. 2006; 113: 2516–2523.